RU2625357C1 - Hot straight thick-gauge plate formed by pumping the product and method of manufacturing formed by stamping the product - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to the field of metallurgy, namely hot-stamped plate steel, suitable for producing a stamped article. Steel contains wt %: C: 0.15 to 0.5, Si: 0.2 to 3, Mn: 0.5 to 3, P: 0.05 or less (except 0), S: 0.05 or less (except 0), Al: 0.01 to 1, B: of 0.0002 to 0.01, N: 0.001 to 0.01, Ti: in the amount equal to or greater than 3.4[N]+0.01 and equal to or less than 3.4[N]+0.1, where [N] is the content (wt %) N, the rest are iron and unavoidable impurities. The average diameter of the equivalent circle of Ti-containing inclusions having an equivalent circle diameter of 30 nm or less among the Ti-containing inclusions contained in the steel sheet is 6 nm or more. The amount of Ti in the inclusions and the total amount of Ti in the steel satisfy the ratio: (the amount of Ti in the inclusions (wt %)-3.4[N]) < 0.5[×(Total amount of Ti (wt %)) - 3.4[N]], where [N] is the content (wt %) N in the steel. The share of ferrite in the metallographic microstructure is 30% or more.

EFFECT: products have excellent characteristics of softening resistance in the heat affected zone and the required ratio between high strength and elasticity.

6 cl, 1 dwg, 16 tbl, 2 ex

Description

FIELD OF THE INVENTION

[0001] The present invention relates to a hot stamping steel sheet used for an automotive structural component and suitable for hot stamping, a stamping molded product obtained from a hot stamping steel sheet, and a method for manufacturing a stamped product. More specifically, the present invention relates to a steel sheet for hot stamping, which is suitable when a preheated steel sheet (billet) is subjected to molding to a predetermined shape, for processing by hot stamping to shape, and heat treatment to obtain a predetermined strength, stamped product, and to the method applicable for the manufacture of such a stamped product.

BACKGROUND

[0002] As one of the measures aimed at improving the fuel efficiency of automobile fuels due to global environmental problems, a vehicle body weight is being reduced, and, in turn, the strength of the steel sheet used for automobiles should be increased as much as possible . On the other hand, when the strength of the steel sheet increases, the accuracy of the mold decreases during stamping.

[0003] For this reason, a component (a stamped product) is manufactured using a hot stamping method, where the steel sheet is heated to a predetermined temperature (eg, temperature to form an austenitic phase) to reduce strength, and then molded using a mold with temperature ( for example, room temperature), lower than the temperature of the formed steel sheet, and perform heat treatment with rapid cooling (hardening) using the temperature difference between them, to ensure spine after molding. Such a hot stamping method has various names, such as a hot forming method, a hot stamping method, a hot die forging method, and a stamp hardening method, in addition to a hot stamping method.

FIG. 1 is a schematic explanatory view showing a configuration of a mold for carrying out the above hot stamping method. In FIG. 1, position code number 1 represents the punch, position code number 2 represents the matrix, position code number 3 represents the workpiece clamp, position code 4 position represents the steel sheet (workpiece), “BHF” represents the force to press the workpiece, “rp” represents the edge radius of the punch, “rd” represents the radius of the edge of the die, and “CL” represents the gap between the punch and the die. Among these parts, the plunger 1 and the die 2 are configured so that channels 1a and 2a are formed in the respective inner regions to allow the flow of the cooling medium (eg, water), and that the parts cool when the cooling medium passes through the channel.

[0005] When hot stamping (for example, hot deep drawing) is performed using such a mold, molding begins in a state where the steel sheet (preform) 4 softens as a result of heating at a temperature of the two-phase zone (from the transformation point Ac ₁ to the Ac point ₃ transformations) or the temperature of a single-phase zone equal to or greater than the point of Ac ₃ transformations. More specifically, in a state in which the high temperature steel sheet 4 is sandwiched between the die 2 and the work clamp 3, the steel sheet 4 is pressed into the die hole 2 (between 2 and 2 in FIG. 1) by the punch 1 and deformed to a shape, corresponding to the outer profile of the punch 1, while reducing the outer diameter of the steel sheet 4. In addition, heat is removed from the steel sheet 4 to the mold (punch 1 and die 2) by cooling the punch and die simultaneously with molding, and hardening the material (steel of the sheet) is carried out with additional holding and cooling of the steel sheet at the bottom dead center of the molding (at the point where the punch head is located in the deepest part: the state shown in FIG. 1). When carrying out such a molding method, a stamped product of strength class 1500 MPa can be obtained with high dimensional accuracy, and, moreover, the molding force can be reduced in comparison with the molding situation of a component with the same strength class in cold working conditions, so that the required volume stamping press can be small.

[0006] As a steel sheet for hot stamping, which is widely used at present, a steel sheet using steel 22MnB5 as a material is known. This steel sheet has a tensile strength of 1500 MPa and an elongation of approximately 6 to 8%, and is used for impact-resistant parts (parts that undergo as little deformation as possible during a collision and do not collapse). However, its use for a component to be deformed, such as an energy absorbing part, is difficult due to the low elongation (ductility).

[0007] With respect to a hot stamping steel sheet that exhibits good elongation, methods have also been proposed according to, for example, Patent Documents 1-4. In these methods, the carbon content in the steel sheet is set to a value in various ranges to control the base strength class of the respective steel sheets, and elongation is increased by introducing ferrite having high deformability and reducing the average particle diameters of ferrite and martensite. The above methods are effective for increasing the elongation, but in terms of increasing the elongation in accordance with the strength of the steel sheet, this is still not enough. For example, the elongation EL of a steel sheet having a tensile strength of TS 1470 MPa or more is a maximum of about 10.2%, and further improvement is required.

[0008] On the other hand, a low-strength molded product compared to hot-formed products that have been investigated so far, for example, a molded product having a tensile strength TS of class 980 MPa or class 1180 MPa, also poses a problem of accuracy molding during cold stamping, and as a measure of improvement there is a need for hot stamping with low strength. In this case, the energy absorption characteristics of the molded product should be significantly improved.

[0009] In particular, in recent years, a method for differentiating strength within a single component has been developed. As such a technology, a method has been proposed for imparting high strength to a portion that should prevent deformation (high strength side: side of the shockproof portion), and messages of low strength and high ductility in a portion that should absorb energy (low strength side: side of the energy absorbing portion) . For example, in a medium or higher class passenger car, both functional sections with resistance to shock and energy absorption are sometimes provided in the middle pillar component or the rear side part, taking into account compatibility during side collision and rear impact (protection function also of the opposite side when a small car is involved in the collision). For the manufacture of such a part, for example, (a) a method was proposed where a steel sheet having low strength, even when heated / cooled in a die at the same temperature, is attached to a normal steel sheet for hot stamping (body welding in a pattern: TWB) , (b) a method where the cooling rate in the mold is made different to create a difference in strength within the respective areas of the steel sheet, (c) a method where different temperatures of the heating for different tion strength.

[0010] In these methods, a strength class of 1500 MPa (side of the shock-resistant portion) is achieved on the high strength side, but the low strength side (energy-absorbing side) remains with a maximum tensile strength of 700 MPa and an elongation of EL of about 17%, and so to further improve energy absorption performance, higher strength and higher ductility are required.

[0011] In addition, in order to provide a complicated shape by hot stamping, the applicability of the approach is required in which stamping is performed at room temperature to form to a certain extent, and then hot stamping is performed, or since the steel sheet is used in stamping under conditions hot stamping is cut, the strength of the steel sheet for hot stamping should also not be excessively high.

[0012] Meanwhile, the automotive component should be connected mainly by spot welding, but it is known that the strength in the heat affected zone (HAZ) is significantly reduced and the weld undergoes a decrease in strength (softening) (for example, Non-Patent Document 1).

State of the art

PATENT DOCUMENT

[0013] Patent Document 1: JP-A-2010-65292

Patent Document 2: JP-A-2010-65293

Patent Document 3: JP-A-2010-65294

Patent Document 4: JP-A-2010-65295

NON-PATENT DOCUMENT

[0014] Non-Patent Document 1: Hirosue et al., Nippon Steel Technical Report, No. 378, pp. 15-20 (2003)

SUMMARY OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

[0015] The present invention has been completed in these circumstances, and its purpose is to provide a hot stamping steel sheet that enables easy molding or processing before hot stamping to produce a stamped product capable of achieving a high level balance between high strength and elongation, when uniform characteristics are required in the molded product, achieve a high level balance between high strength and elongation asno respective portions, when the single molded article is required portions corresponding portion shock resistance and energy-absorbing portion, and moreover, improve softening resistance characteristic in HAZ; a stamped product exhibiting the above properties; and a method suitable for the manufacture of such a stamped product.

TROUBLESHOOTING MEANS

[0016] The hot stamping steel sheet of the present invention, which can achieve the above purpose, comprises:

C: from 0.15 to 0.5% (wt.%; Hereinafter the same applies to the chemical component composition),

Si: 0.2 to 3%,

Mn: 0.5 to 3%,

P: 0.05% or less (excluding 0%),

S: 0.05% or less (excluding 0%),

Al: from 0.01 to 1%,

B: 0.0002 to 0.01%,

Ti: in an amount equal to or greater than 3.4 [N] + 0.01% and equal to or less than 3.4 [N] + 0.1% (wherein [N] denotes the content (wt.% ) N), and

N: from 0.0010 to 0.01%, with the remainder of iron and unavoidable impurities, in which

the average equivalent diameter of the Ti-containing inclusions having an equivalent circle diameter of 30 nm or less among the Ti-containing inclusions contained in the steel sheet is 6 nm or less, the amount of Ti in the inclusions and the total amount of Ti in the steel satisfies the ratio according to the following formula (1), and the area fraction of ferrite in the metallographic microstructure is 30% or more. Here, “equivalent circle diameter” represents the diameter of a circle having the same area as the size (area) of Ti-containing inclusions (eg, TiC) when the precipitated phase is converted into a circle (“average equivalent circle diameter” represents its average value).

The amount of Ti in the inclusions (wt.%) - 3.4 [N] <0.5 × [(total amount of Ti (wt.%)) - 3.4 [N]] ... (1)

(in the formula (1) [N] denotes the content (wt.%) N in steel).

[0017] In the steel sheet for hot stamping in the present invention, if desired, the content, as another (s) element (s), of at least one of the following (a) to (c) is also useful. The properties of the stamped product are further improved in accordance with the type of element that is contained as necessary.

(a) One or more types selected from the group consisting of V, Nb and Zr, in an amount of 0.1% or less (excluding 0%) in total

(b) One or more types selected from the group consisting of Cu, Ni, Cr and Mo, in an amount of 1% or less (excluding 0%) in total

(c) One or more types selected from the group consisting of Mg, Ca and REM (rare earth metals) in an amount of 0.01% or less (excluding 0%) in total

[0018] In the method of manufacturing a stamped product according to the present invention, which can achieve the above purpose, the hot stamping steel sheet according to the present invention is heated at a temperature equal to or greater than the transformation point Ac ₁ + 20 ° C and equal to or less than the Ac point ₃ transformations -20 ° C, then the hot stamping of the steel sheet is started, and the steel sheet is cooled to a temperature equal to or lower than the temperature 100 ° C below the temperature Bs of the onset of bainitic transformation, while ensuring an average cooling rate of 20 ° C / sec or more in the mold during molding and after completion of molding.

[0019] In the molded product according to the present invention, the metallographic microstructure of the molded product includes residual austenite: from 3 to 20% by area, ferrite: from 30 to 80% by area, bainitic ferrite: less than 30% by area (except 0% by area area), and martensite: 31% by area or less (except 0% by area), and the average diameter of the equivalent circle of Ti-containing inclusions having an equivalent circle diameter of 30 nm or less, among the Ti-containing inclusions contained in the stamped product is 10 nm or thereof, the amount of Ti in the inclusions and the total amount of Ti in the steel satisfies the ratio according to the following formula (1), and a high level balance between high strength and elongation as uniform characteristics can be achieved in the molded product.

The amount of Ti in the inclusions (wt.%) - 3.4 [N] <0.5 × [(total amount of Ti (wt.%)) - 3.4 [N]] ... (1)

(in the formula (1) [N] denotes the content (wt.%) N in steel).

[0020] On the other hand, in another method of manufacturing a stamped product according to the present invention, which can achieve the above objectives, use the above steel sheet for hot stamping, the heating region of the steel sheet is divided into at least two zones, one zone of them is heated at the temperature of the Ac ₃ conversion point or more, and 950 ° C or less, another zone of them is heated at a temperature equal to or greater than the Ac ₁ transformation point + 20 ° C and equal to or lower than the Ac ₃ transformation point-20 ° C, then begin stamping of both zones, and the steel sheet is cooled to a temperature equal to or lower than the temperature Ms of the onset of martensitic transformation, while at the same time providing an average cooling rate of 20 ° C / s or more in the mold in both zones during molding and after completion molding.

[0021] Another stamped product according to the present invention is a stamped product of a steel sheet having the above chemical component composition, and the stamped product has a first zone having a metallographic microstructure including residual austenite: from 3 to 20% by area, and martensite: 80% by area or more, and a second zone with a metallographic microstructure, including residual austenite: from 3 to 20% by area, ferrite: from 30 to 80% by area, bainitic ferrite: less than 30% by area (per lawsuit by 0% by area), and martensite: 31% by area or less (excluding 0% by area), and the average diameter of the equivalent circle of Ti-containing inclusions having an equivalent circle diameter of 30 nm or less, among Ti-containing inclusions, contained in the steel of the second zone is 10 nm or less, and the amount of Ti in the inclusions and the total amount of Ti in the steel satisfies the ratio according to the following formula (1). In this stamped product, a high level balance between high strength and elongation can be achieved depending on the respective zones, and areas corresponding to the impact-resistant section and the energy-absorbing section are present in a single molded product, and, moreover, when spot welding is performed in the second zone, the softening resistance characteristic in HAZ is improved.

The amount of Ti in the inclusions (wt.%) - 3.4 [N] <0.5 × [(total amount of Ti (wt.%)) - 3.4 [N]] ... (1)

(in the formula (1) [N] denotes the content (wt.%) N in steel).

Advantage of the Invention

[0022] According to the present invention, a steel sheet is used where the chemical component composition is strictly defined and the size of the Ti-containing inclusions is controlled, and where the degree of Ti release in the non-TiN forming inclusion is controlled, and the ferrite fraction is adjusted with respect to the metallographic microstructure so that by hot stamping a steel sheet under predetermined conditions, the strength-elongation balance in a stamped product can be brought to a high-level balance. In addition, when hot stamping is performed under various conditions in multiple zones, a shock-resistant portion and an energy-absorbing portion can be formed in a single molded product, and a high level balance between high strength and elongation can be achieved in the respective zones, and in addition, the performance is improved softening resistance in HAZ.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] [FIG. 1] A schematic explanatory view showing the configuration of a mold for hot stamping.

MODE FOR CARRYING OUT THE INVENTION

[0024] The inventors of the present invention have studied a variety of aspects for making a steel sheet for hot stamping, which ensures that in the manufacture of a stamped product by heating the steel sheet at a predetermined temperature and then hot stamping the steel sheet, a stamped product exhibiting good ductility (elongation) is obtained while providing high strength after forming by stamping.

[0025] As a result, it was found that when the chemical composition of the hot stamping steel sheet is strictly predetermined, the size of the Ti-containing inclusions as well as the amount of Ti in the inclusions are regulated, and when an appropriate metallographic microstructure is created and the steel sheet is subjected to hot stamping under predetermined conditions, after stamping, a predetermined amount of residual austenite is provided, and a stamped product having increased intrinsic plasticity is obtained nost (residual ductility). The present invention has been completed based on these findings.

[0026] In the steel sheet for hot stamping according to the present invention, the chemical component composition must be strictly defined, and the rationale for limiting the range of each chemical component is as follows.

[0027] (C: 0.15 to 0.5%)

Carbon (C) is an important element for achieving a high level balance between high strength and elongation when uniform properties are required in a stamped product, or to provide residual austenite, especially in a low strength / high ductility zone, when areas in a single molded product require corresponding to the shockproof portion and the energy absorbing portion. In addition, austenite is enriched in carbon (C) during heating during hot stamping, so that after quenching, residual austenite can be formed. In addition, this element helps to increase the amount of martensite and increases strength. To exhibit such effects, the C content should be 0.15% or more.

[0028] However, if the C content is too large and exceeds 0.5%, the two-phase zone of the heating region becomes narrow, and when uniform properties are required in the molded product, the balance between high strength and elongation is not achieved at a high level, or when in the only molded product requires areas corresponding to the shock-resistant section and the energy-absorbing section, it is difficult to regulate the metallographic microstructure (microstructure where the preload is provided but predetermined amounts of ferrite, bainitic ferrite and martensite), especially necessary in the zone of low strength / high ductility. The lower limit of the C content is preferably 0.17% or more (more preferably 0.20% or more), and the upper limit is preferably 0.45% or less (more preferably 0.40% or less).

[0029] (Si: 0.2 to 3%)

Silicon (Si) has an effect consisting in the formation of residual austenite by preventing the release of martensite during cooling of the mold, quenching with the formation of cementite, or by suppressing the decomposition of unreformed austenite. To exhibit such an effect, the Si content should be 0.2% or more. If the Si content is too high and exceeds 3%, during cooling after hot rolling, ferrite conversion is stimulated, and coarse-grained TiС can form in the formed ferrite, and as a result, the softening resistance characteristic in HAZ is not achieved. The lower limit of the Si content is preferably 0.5% or more (more preferably 1.0% or more), and the upper limit is preferably 2.5% or less (more preferably 2.0% or less).

[0030] (Mn: 0.5 to 3%)

Manganese (Mn) is an element that effectively increases hardenability during quenching and inhibits the formation of a different microstructure (e.g., ferrite, perlite, bainite) than martensite and residual austenite during cooling during quenching in a die. In addition, Mn is an element capable of stabilizing austenite and an element promoting an increase in the amount of residual austenite. To exhibit such effects, Mn must be present in an amount of 0.5% or more. If only characteristics are taken into account, then the Mn content is preferably higher, but since the cost of the dopant increases, the upper limit is adjusted to 3% or less. The lower limit of the Mn content is preferably 0.7% or more (more preferably 1.0% or more), and the upper limit is preferably 2.5% or less (more preferably 2.0% or less).

[0031] (P: 0.05% or less (excluding 0%))

Phosphorus (P) is an element inevitably contained in steel, but it impairs ductility, and therefore, the content of P is preferably reduced as much as possible. However, the marginal decrease leads to an increase in the cost of steel production, and is difficult in terms of manufacturing with a decrease in the content to 0%. On this basis, the upper limit is adjusted to 0.05% or less (with the exception of 0%). The upper limit of the content of P is preferably 0.045% or less (more preferably 0.040% or less).

[0032] (S: 0.05% or less (excluding 0%))

Sulfur (S), like P, is an element that is inevitably contained in steel and impairs ductility, and therefore, the content of S is preferably reduced as much as possible. However, excessive removal causes an increase in the cost of steel production, and is difficult in terms of manufacturing with a decrease in the content to 0%. For this reason, the upper limit is adjusted to 0.05% or less (with the exception of 0%). The upper limit of the S content is preferably 0.045% or less (more preferably 0.040% or less).

[0033] (Al: 0.01 to 1%)

Aluminum (Al) is applicable as a deoxidizing element and allows to bind dissolved N in the form of AlN present in steel, which is useful for increasing ductility. For the effective manifestation of such an effect, the Al content should be 0.01% or more. However, if the Al content is too high and exceeds 1%, Al ₂ O _{3 is} excessively formed, impairing ductility. The lower limit of the Al content is preferably 0.02% or more (more preferably 0.03% or more), and the upper limit is preferably 0.8% or less (more preferably 0.6% or less).

[0034] (B: 0.0002 to 0.01%)

Boron (B) is an element whose action is to suppress ferrite transformation, pearlite transformation and bainitic transformation on the side of the high-strength section, and therefore helps to prevent the formation of ferrite, perlite and bainite during cooling after heating at a temperature of a two-phase zone (from Ac ₁ transformation to the point of Ac ₃ transformation), and provides the formation of residual austenite. For the manifestation of such effects, B must be contained in an amount of 0.0002% or more, but even if this element is contained in excess in excess of 0.01%, the effects are saturated. The lower limit of B content is preferably 0.0003% or more (more preferably 0.0005% or more), and the upper limit is preferably 0.008% or less (more preferably 0.005% or less).

[0035] (Ti: in an amount equal to or greater than 3.4 [N] + 0.01% and equal to or less than 3.4 [N] + 0.1%: [N] represents the content ( wt.%) N)

Titanium (Ti) exhibits an action consisting in improving hardenability during quenching by binding N and maintaining B in a solid solution state. For the manifestation of such an effect, it is important that this element is contained in a greater amount than the stoichiometric ratio of Ti to N (3.4-fold content of N), at the level of 0.01% or more. In addition, when Ti is added in excess with respect to N, this causes it to be present in the solid solution state in the hot stamped product, and the precipitated compound is finely dispersed, the decrease in strength in HAZ due to dispersion hardening due to the formation, in the form of TiC dissolved in solid, can be suppressed. Ti material during welding of a hot stamped product, or due to an effect such as a delayed increase in the dislocation density, due to the action of TiС, which prevents dislocation movement. However, if the Ti content is too high and exceeds 3.4 [N] + 0.1%, the resulting Ti-containing inclusions (eg, TiN) become coarse, impairing the ductility of the steel sheet. The lower limit of the Ti content is preferably 3.4 [N] + 0.02% or more (more preferably 3.4 [N] + 0.05% or more), and the upper limit is preferably 3.4 [N] +0 09% or less (more preferably 3.4 [N] + 0.08% or less).

[0036] (N: from 0.001 to 0.01%)

Nitrogen (N) is an impurity element inevitably present, and its content is preferably reduced as much as possible, but its removal in the real process is limited, and therefore the lower limit is adjusted to 0.001%. If the N content is too large, the resulting Ti-containing inclusions (for example, TiN) are coarsened, and these inclusions act as a fracture source, impairing the ductility of the steel sheet. For this reason, the upper limit is adjusted to 0.01%. The upper limit of the N content is preferably 0.008% or less (more preferably 0.006% or less).

[0037] The main chemical components in the hot stamping steel sheet according to the present invention are as described above, and the remainder are iron and unavoidable impurities (eg, O, H) other than P, S and N. In steel An additional content of at least one of the following (a) to (c), if desired, is also useful for the hot stamping sheet of the present invention. The properties of the hot stamping steel sheet (i.e., the stamped product) are further improved according to the type of element that is contained as necessary. In the case of the content of such an element, the preferred range and rationale for limiting the range are as follows.

(a) One or more selected from the group consisting of V, Nb and Zr in an amount of 0.1% or less (excluding 0%) in total

(b) One or more selected from the group consisting of Cu, Ni, Cr and Mo, in an amount of 1% or less (excluding 0%) in total

(c) One or more selected from the group consisting of Mg, Ca and REM, in an amount of 0.01% or less (excluding 0%) in total

[0038] (One or more selected from the group consisting of V, Nb and Zr in an amount of 0.1% or less (excluding 0%) in total)

Vanadium (V), niobium (Nb) and zirconium (Zr) exhibit an action consisting in the formation of fine-grained carbide and grinding of the microstructure as a result of the pinning effect. To exhibit such an effect, these elements are preferably contained in an amount of 0.001% or more in total. However, if the content of these elements is too high, coarse carbide is formed and acts as a source of destruction, impairing ductility. On this basis, the content of these elements is preferably 0.1% or less in total. The lower limit of the content of these elements is more preferably 0.005% or more (even more preferably 0.008% or more) in total, and the upper limit is more preferably 0.08% or less (even more preferably 0.06% or less) in total.

[0039] (One or more selected from the group consisting of Cu, Ni, Cr, and Mo, in an amount of 1% or less (excluding 0%) in total)

Copper (Cu), nickel (Ni), chromium (Cr) and molybdenum (Mo) inhibit ferrite conversion, pearlite transformation and bainitic transformation, and therefore act effectively in preventing the formation of ferrite, perlite and bainite during cooling after heating, and provide the formation residual austenite. To exhibit such an effect, they are preferably contained in an amount of 0.01% or more in total. If only properties are taken into account, the content is preferably higher, but since the cost of the dopant increases, the content is preferably 1% or less in total. In addition, these elements act so as to significantly increase the strength of austenite and cause an increased load during hot rolling, making it difficult to manufacture a steel sheet. Therefore, also for manufacturing reasons, the content is preferably 1% or less. The lower limit of the content of these elements is more preferably 0.05% or more (even more preferably 0.06% or more) in total, and the upper limit is more preferably 0.5% or less (even more preferably 0.3% or less ) in total.

[0040] (One or more selected from the group consisting of Mg, Ca and REM (rare earth element) in an amount of 0.01% or less (excluding 0%) in total)

These elements grind inclusions and therefore act effectively to increase ductility. To exhibit such an effect, these elements are preferably contained in an amount of 0.0001% or more in total. When only properties are taken into account, the content is preferably higher, but since the effect is saturated, the content is preferably 0.01% or less in total. The lower limit of the content of these elements is more preferably 0.0002% or more (even more preferably 0.0005% or more) in total, and the upper limit is more preferably 0.005% or less (even more preferably 0.003% or less) in total.

[0041] In the steel sheet for hot stamping according to the present invention, the following requirements are also important: (A) the average diameter of the equivalent circle of Ti-containing inclusions having an equivalent circle diameter of 30 nm or less among the Ti-containing inclusions contained in the steel sheet is 6 nm or less, (B) the ratio “amount of Ti in inclusions (wt.%) - 3.4 [N] <0.5 × [(total amount of Ti (wt.%)) - 3.4 [N] is satisfied ] ”(Ratio according to formula (1)), and (C) the fraction of the ferrite area in the metallographic microstructure is 30% and whether more.

[0042] The Ti-containing inclusions and the ratio in formula (1) are controlled to prevent softening of the HAZ, and such regulation is initially the control needed in the molded product, but these values do not change much between the states before and after hot stamping. Therefore, regulation should be carried out already at the stage before molding (in a steel sheet for hot stamping). When an excess amount of Ti relative to N in the steel sheet prior to molding causes it to be in the solid solution or finely divided state, the Ti-containing inclusions can be maintained in the solid solution or finely divided state during heating during hot stamping. As a result of this, the amount of Ti released in the stamped product into inclusions can be adjusted to a value no more than a predetermined amount, and softening in the HAZ can be prevented, whereby the properties of the welded joint can be improved.

[0043] For these reasons, Ti-containing inclusions should be finely dispersed, and for this purpose, the average diameter of the equivalent circle of Ti-containing inclusions having an equivalent circle diameter of 30 nm or less, among Ti-containing inclusions contained in the steel sheet should be 6 nm or less (above requirement (A)). Here, the diameter of the equivalent circle of these Ti-containing inclusions is set to 30 nm or less, since it is necessary to adjust the Ti-containing inclusions, with the exception of TiN, which is coarse-grained at the stage of melting, and then does not affect changes in the microstructure or characteristics. The size (average equivalent circle diameter) of the Ti-containing inclusions is preferably 5 nm or less, more preferably 3 nm or less. Examples of Ti-containing inclusions in the present invention include TiC, TiN and other Ti-containing inclusions such as TiVC, TiNbC, TiVCN and TiNbCN.

[0044] As described later, the average diameter of the equivalent circumference of Ti-containing inclusions in the stamped product is set to 10 nm or less, while before molding (steel sheet for hot stamping) is set to 6 nm or less. The rationale for why the size of the precipitated phase is set larger in the molded product than in the steel sheet is that Ti is present in the steel sheet as a finely divided precipitated phase or in the state of a solid solution, and, being heated at a temperature of about 800 ° C for 15 minutes or more, Ti-containing inclusions slightly enlarged. In order to ensure properties similar to those in a molded product, the average diameter of the equivalent circumference of Ti-containing inclusions should be 10 nm or less, and to achieve this state of precipitated phases in a hot stamped product, it is necessary that, in the state of a steel sheet for hot stamping, the average diameter of the equivalent circumference of finely divided phases of 30 nm or less has been adjusted to 6 nm or less, and most Ti should be present in the solid solution state.

[0045] In addition, in the hot stamping steel sheet, the main amount of Ti, with the exception of Ti used to form inclusions and N binding, must be present in the solid solution state or in the finely divided state. On this basis, the amount of Ti present in the form of inclusions other than TiN (that is, the amount of Ti in the inclusions (wt.%) Is 3.4 [N]) should be less than half the residue after subtraction of Ti, which forms TiN from the total amount of Ti (that is, 0.5 × [(total amount of Ti (wt.%)) - 3.4 [N]] (above requirement (B)). The value is “the amount of Ti in inclusions (wt. .%) - 3.4 [N] "is preferably 0.4 × [(total amount of Ti (wt.%)) - 3.4 [N]] or less, more preferably 0.3 × [(total amount of Ti (wt.%)) - 3.4 [N]] or less.

[0046] The steel material must necessarily be processed before hot stamping, and sometimes stamped, in which case a predetermined amount of ferrite as a soft microstructure must be provided. From this point of view, the proportion of ferrite in the hot stamping steel sheet should be 30% by area or more (above requirement (C)). The ferrite fraction is preferably 50% by area or more, more preferably 70% by area or more.

[0047] In the hot stamping steel sheet, the remaining metallographic microstructure is not particularly limited, but includes, for example, at least one of perlite, bainite, martensite and residual austenite.

[0048] For the manufacture of a steel sheet (hot stamping steel sheet) according to the present invention, a slab obtained by melting a steel material having the above chemical component composition can be hot rolled at a heating temperature of 1100 ° C or more (preferably 1150 ° C or more), and 1300 ° C or less (preferably 1250 ° C or less), and finish rolling at a temperature of 850 ° C or more (preferably 900 ° C or more), and 1050 ° C or less (preferably 1000 ° C or less), and immediately thereafter can be cooled (by rapid cooling) at an average cooling rate of 20 ° C / sec or more (preferably 30 ° C / sec or more) until a temperature of 650 ° C or less (preferably 625 ° C or less) is reached, and then it can be cooled at a speed of 10 ° C / sec or less (preferably 5 ° C / sec) from a temperature of 620 ° C to 580 ° C, and then it can be cooled at an average cooling rate of 10 ° C / sec or more, and then it can be wound into a roll at a temperature of 350 ° C or more (preferably 380 ° C or more), and 450 ° C or less (preferably 430 ° C or less).

[0049] In the above method (1), rolling is stopped in the temperature range where the dislocation embedded in austenite during hot rolling remains, (2) rapid cooling is carried out immediately afterwards to ensure the formation of Ti-containing inclusions on the dislocation, such as finely divided TiС , and (3) then perform two-stage cooling, followed by winding into a roll, which regulates the course of ferrite transformation, while at the same time providing the number of Ti-containing inclusions.

[0050] A hot stamping steel sheet that has the above chemical component composition, metallographic microstructure, and Ti state in the inclusions can be directly used for hot stamping, or can be cold rolled with a reduction rate of 60% or less when rolling (preferably 40% or less) after decapitation, and then used for hot stamping. In a steel sheet for hot stamping according to the present invention, its microstructure can be created during the heat treatment of the hot rolled material in a continuous annealing furnace or in a production line for continuous hot dip galvanizing. In short, while the required characteristics, such as the metallographic microstructure and the state of Ti in the inclusions, are satisfied, the steel sheet corresponds to the steel sheet for hot stamping in the present invention.

[0051] Using the above hot stamping steel sheet, the steel sheet is heated at a temperature equal to or higher than the Ac ₁ transformation point + 20 ° C (Ac ₁ + 20 ° C), and equal to or less than the Ac ₃ transformation point-20 ° C ( Ac ₃ -20 ° C), and after the beginning of stamping, the steel sheet is cooled to a temperature equal to or lower than the temperature 100 ° C below the temperature Bs of the onset of bainitic transformation (Bs-100 ° C), while providing an average cooling rate of 20 ° C / sec or more in the mold during molding and also after completed forming, as a result of which the optimal microstructure, as in a molded product with low strength and high ductility, can be created in a stamped product having a uniform property (hereinafter sometimes referred to as "single-zone molded product"). The rationale for accurately specifying each requirement in this molding method is as follows.

[0052] In a steel sheet containing a predetermined amount of ferrite, in order to cause a partial conversion to austenite, while allowing part of the ferrite to remain, it is necessary to adjust the heating temperature to a predetermined range. If the heating temperature of the steel sheet is lower than the transformation point Ac ₁ + 20 ° C, a sufficient amount of austenite cannot be obtained during heating, and a predetermined amount of residual austenite in the final microstructure (microstructure of the molded product) cannot be provided. If the heating temperature of the steel sheet exceeds the Ac ₃ transformation point of -20 ° C, the degree of conversion to austenite increases excessively during heating, and a predetermined amount of ferrite in the final microstructure (microstructure of the molded product) cannot be provided.

[0053] In order to ensure that the austenite formed in the above heating step is converted to a desired microstructure, while preventing the formation of a microstructure such as ferrite or perlite, it is necessary to properly control the average cooling rate during molding, as well as after molding, and temperature end of cooling. For such reasons, it is necessary that the average cooling rate during molding is 20 ° C / s or more, and the temperature of the end of cooling is equal to or lower than the temperature 100 ° C below the temperature Bs of the onset of bainitic transformation. The average cooling rate during molding is preferably 30 ° C / s or more (more preferably 40 ° C / s or more). When the temperature of the end of cooling is equal to or lower than the temperature 100 ° C below the temperature Bs of the onset of bainitic transformation, the austenite present during heating turns into bainite or martensite, while preventing the formation of such a microstructure as ferrite or perlite, so that finely divided austenite between the bats of bainite or martensite, and a predetermined amount of residual austenite is provided, along with the conservation of bainite and martensite.

[0054] If the temperature of the end of cooling exceeds a temperature that must be 100 ° C lower than the temperature Bs of the onset of bainitic transformation, or the average cooling rate is less than 20 ° C / s, a microstructure such as ferrite and perlite is formed, and a predetermined the amount of residual austenite, leading to a deterioration in elongation (ductility) in the molded product. The temperature of the end of cooling is not particularly limited as long as it is equal to or lower than the temperature 100 ° C below Bs, and the temperature of the end of cooling can be, for example, equal to or lower than the temperature Ms of the onset of martensitic transformation.

[0055] After reaching a temperature equal to or lower than the temperature 100 ° C lower than the temperature Bs of the onset of bainitic transformation, the regulation of the average cooling rate is in principle not necessary, but the steel sheet can be cooled to room temperature with an average cooling rate, for example, 1 ° C / sec more, and 100ºС / sec or less. Monitoring the average cooling rate during molding as well as after molding is completed can be achieved by, for example, (a) controlling the temperature of the molding mold (using the cooling medium shown in FIG. 1), or (b) controlling the thermal conductivity Press forms.

[0056] In a stamped article (single-zone molded article) made by hot stamping described above, the metallographic microstructure includes residual austenite: from 3 to 20% by area, ferrite: from 30 to 80% by area; bainitic ferrite: less than 30% by area (excluding 0% by area) and martensite: 31% by area or less (except 0% by area), and a high-level balance between high strength and elongation can be achieved in a molded product as uniform characteristics. The rationale for regulating the range of each requirement (base microstructure) in such a hot stamped product is as follows.

[0057] Residual austenite has an effect consisting in increasing the degree of strain hardening (induced by the plasticity transformation) and enhancing the plasticity of a stamped product as a result of transformation into martensite occurring during plastic deformation. To exhibit this effect, the fraction of residual austenite should be 3% or more. Plasticity improves even more as the proportion of residual austenite increases. In the composition used for automotive steel sheet, the provided residual austenite is limited, and the upper limit is about 20% by area. The lower limit of residual austenite is preferably 5% by area or more (more preferably 7% by area).

[0058] When the basic microstructure is a finely divided ferrite having high ductility, ductility (elongation) of the stamped article can be increased. From this position, the area fraction of ferrite is 30% or more. However, if this proportion exceeds 80% by area, the strength of the molded product cannot be ensured. The lower limit of the ferrite fraction is preferably 35% by area or more (more preferably 40% by area), and the upper limit is preferably 75% by area or less (more preferably 70% by area or less).

[0059] Bainitic ferrite is a microstructure effective in increasing the strength of the molded product, but is a structure having slightly less ductility, and therefore, when present in large quantities, it worsens the elongation. For these reasons, the area fraction of bainitic ferrite is less than 30%. The upper limit of the area fraction of bainitic ferrite is preferably 25% or less (more preferably 20% by area or less).

[0060] Martensite (quenching martensite) is a microstructure that is effective in increasing the strength of the formed product, but is a structure that does not have ductility, and therefore, when present in large quantities, it degrades elongation. From this position, the martensite area fraction is less than 31%. The upper limit of the martensite area fraction is preferably 25% or less (more preferably 20% by area or less).

[0061] A different microstructure than those described above is not specifically limited, and perlite, etc., may be contained as the rest of the microstructure, but such a microstructure is worse than other microstructures in terms of contributing to strength or promoting ductility, and, in principle, it is preferable that such a microstructure is not contained (may even be 0% by area).

[0062] In the above stamped product (single-zone molded product), the average diameter of the equivalent circle of Ti-containing inclusions having an equivalent circle diameter of 30 nm or less among Ti-containing inclusions present in the stamped product (that is, in the steel sheet constituting the stamped product) is 10 nm or less. When this requirement is met, a stamped product capable of achieving a high level balance between high strength and elongation can be obtained. The average equivalent circle diameter of the Ti-containing inclusions is preferably 8 nm or less, more preferably 6 nm or less.

[0063] In addition, in a stamped article (single-zone molded article), the amount of Ti present in a different precipitated phase than TiN (that is, the amount of Ti in inclusions is 3.4 [N]) is less than half the residue after subtracting Ti that forms TiN from the total amount of Ti (that is, less than 0.5 × [total amount of Ti (%) - 3.4 [N]]). When this requirement is met, Ti dissolved in the solid material during welding forms a finely divided precipitated phase in the HAZ, or existing finely divided Ti-containing inclusions inhibit reduction, etc. dislocation, and as a result, softening in HAZ is prevented, and weldability is improved. The value “amount of Ti in inclusions-3.4 [N]” is preferably 0.4 × [(total amount of Ti (wt.%)) - 3.4 [N]] or less, more preferably 0.3 × [( total Ti (wt.%)) 3.4 [N]] or less.

[0064] When a steel sheet for hot stamping according to the present invention is used, properties such as strength and elongation of the stamped product can be controlled by appropriately controlling the stamping conditions (heating temperature and cooling rate), and, moreover, a stamped product having a high ductility (residual ductility), providing the possibility of its use in place (for example, as an energy-absorbing part), for which a standard stamped product is hardly It can be applied. This is very useful for expanding the use of the stamped product. In addition to the single-zone molded product described above, in the manufacture of a stamped product by stamping a steel sheet using a stamping mold, when the heating temperature and conditions in each region during molding are properly controlled, and thereby the microstructure of each region is controlled, a stamped product is obtained , showing a balance of strength and ductility depending on the respective areas (hereinafter sometimes referred to as "multi-zone molded product").

[0065] In the manufacture of a multi-zone molded product, as described above, using a steel sheet for hot stamping according to the present invention, the manufacture can be performed so that the heated region of the steel sheet is divided into at least two zones, one zone is heated (hereinafter referred to as the first zone) at a temperature _{of the} transformation point Ac ₃ or more, and 950 ° C or less, heat another zone (hereinafter referred to as the second zone) at a temperature equal to or greater than the transformation point Ac ₁ + 20 ° C, and explicit or less than the point _of transformation ₃ Ac-20 ° C, then stamping of both the first and second zones is started, and the steel sheet is cooled to a temperature equal to or lower than the temperature Ms of the beginning of the martensitic transformation, while at the same time providing an average cooling rate of 20 ° C / sec or more in the mold of both the first and second zones during molding and also after molding is completed.

[0066] In the above method, the heated region of the steel sheet is divided into at least two zones (high strength side zone and low strength side zone), and the manufacturing conditions are controlled according to the respective zones, whereby a stamped article is obtained that exhibits a balance of strength and ductility in depending on the respective zones. Of the two zones, the second zone corresponds to the low-side side zone, and the manufacturing conditions, microstructure and properties in this zone are in principle the same as in the above-described single-zone molded product. The following describes the manufacturing conditions for the formation of the first zone (corresponding to the zone of the high strength side). Here, when this manufacturing method is performed, regions differing in heating temperature should be formed in a single steel sheet, but the temperature can be adjusted to withstand a temperature change boundary portion of 50 mm or less using an existing heating furnace (e.g., a furnace with infrared radiation in the far infrared region, electric furnace + screen).

[0067] (Manufacturing conditions of the first zone / zone of the high strength side)

In order to properly adjust the microstructure of the stamped product, the heating temperature must be controlled to a predetermined range. With proper control of this heating temperature, in the subsequent cooling process, the transformation of the microstructure, mainly including martensite, can be stimulated, while at the same time providing a predetermined amount of residual austenite, and the desired microstructure can be created on the site of the final hot stamped product. If the heating temperature of the steel sheet in this zone is less than the Ac ₃ transformation point, a sufficient amount of austenite cannot be obtained during heating, and a predetermined amount of residual austenite in the final microstructure (microstructure of the molded product) cannot be provided. If the heating temperature of the steel sheet exceeds 950 ° C, the size of austenite grains increases during heating, the temperature of the onset of martensitic transformation (point Ms) and the temperature of the end of martensitic transformation (point Mf) increase, residual austenite cannot be provided during quenching, and good formability is not achieved . The heating temperature of the steel sheet is preferably an Ac ₃ transformation point of + 50 ° C or more, and 930 ° C or less.

[0068] In order to ensure that the austenite formed in the above heating step is converted to the desired microstructure, while preventing the formation of a microstructure such as ferrite or perlite, the average cooling rate during molding, as well as after molding, and temperature must be properly controlled. end of cooling. For such reasons, it is necessary that the average cooling rate during molding is 20 ° C / s or more, and the temperature of the end of cooling is equal to or lower than the temperature of the onset of martensitic transformation (point Ms). The average cooling rate during molding is preferably 30 ° C / s or more (more preferably 40 ° C / s or more). When the temperature of the end of cooling is equal to or lower than the temperature of the onset of martensitic transformation (point Ms), the austenite present during heating is converted to martensite, while at the same time preventing the formation of a microstructure such as ferrite or perlite, which ensures martensite. More specifically, the temperature of the end of cooling is 400 ° C or less, preferably 300 ° C or less.

[0069] In a stamped product obtained in this way, the metallographic microstructure, precipitated phase, etc., are different between the first zone and the second zone. In the first zone, the metallographic microstructure includes residual austenite: from 3 to 20% by area (the action and effect of residual austenite are the same as described above), and martensite: 80% by area or more. The second zone satisfies the metallographic microstructure and state of Ti (for example, the average diameter of the equivalent circle of Ti-containing inclusions, the value of "the number of Ti in inclusions (wt.%) - 3.4 [N]"), which are the same as in the above single-zone molded product.

[0070] When the basic microstructure of the first zone is a high-strength martensite containing a predetermined amount of residual austenite, ductility of the stamped product in a given region and high strength can be ensured. From this point of view, the proportion of martensite area should be 80% by area or more. The proportion of martensite is preferably 85% by area or more (more preferably 90% by area or more). The first zone may partially contain ferrite, perlite, bainite, etc., as the rest of the microstructure.

[0071] The effects of the present invention are more specifically described below with reference to Examples, but the present invention is not limited to the following Examples, and all configuration changes made within the scope of the above described invention or later are included in the technical field of the present invention.

EXAMPLES

[0072] [Example 1]

Steel materials (steels Nos. 1-16 and 18-32) having the chemical component composition shown in Tables 1 and 2 below were melted in a vacuum to obtain an experimental slab, then they were hot rolled to obtain a steel sheet, followed by cooling and exposure processing simulating winding into a roll (sheet thickness: 1.6 mm or 3.0 mm). Regarding the method for simulating winding into a processing roll, the sample was cooled to the winding temperature and placed in a furnace heated to the winding temperature, followed by aging for 30 minutes and then cooling in the oven. The manufacturing conditions of the steel sheets are shown below in Tables 3 and 4. Here, in Tables 1 and 2, the transformation point Ac _{1, the} transformation point Ac ₃ , the point Ms and the point Bs were determined using the following formulas (2) - (5) (see , for example, the publication "The Physical Metallurgy of Steels", authors Leslie, Maruzen, (1985)). In addition, treatments (1) and (2) shown in the Notes to Table 3 mean that each treatment described below (rolling, cooling, and alloying) was performed.

[0073] The Ac ₁ conversion point (ºC) = 723 + 29.1 × [Si] -10.7 × [Mn] + 16.9 × [Cr] -16.9 [Ni] ..... (2 )

Transformation point Ac ₃ (ºC) = 910-203 × [C] ^1/2 + 44.7 × [Si] -30 × [Mn] + 700 × [P] + 400 × [Al] + 400 × [Ti] + 104 × [V] -11 × [Cr] + 31.5 × [Mo] -20 × [Cu] -15.2 × [Ni] ..... (3)

Point Ms (ºC) = 550-361 × [C] -39 × [Mn] -10 × [Cu] -17 × [Ni] -20 × [Cr] -5 × [Mo] + 30 × [Al]. ....(four)

Point Bs (ºC) = 830-270 × [C] -90 × [Mn] -37 × [Ni] -70 × [Cr] -83 × [Mo] ...... (5)

moreover, [C], [Si], [Mn], [P], [Al], [Ti], [V], [Cr], [Mo], [Cu] and [Ni] represent levels of content (wt. %) of the elements C, Si, Mn, P, Al, Ti, V, Cr, Mo, Cu, and Ni, respectively. In the case where the element shown in each term of formulas (2) - (5) is not contained, the calculation is performed taking into account that the term is not present.

[0074] Processing (1): the hot-rolled steel sheet was cold rolled (sheet thickness: 1.6 mm), then heated at a temperature of 800 ° C to simulate continuous annealing in a heat treatment simulator, held for 90 seconds, cooled to a temperature of 500 ° C at medium cooling rates of 20 ° C / sec, and held for 300 seconds.

Processing (2): hot-rolled steel sheet was cold rolled (sheet thickness: 1.6 mm), then heated at a temperature of 860 ° C to simulate a continuous dip galvanizing production line in a heat treatment simulator, cooled to a temperature of 400 ° C at an average cooling rate of 30 ° C / s, kept, additionally kept at a temperature of 500 ° C for 10 seconds to simulate immersion in a clad bath and fusion treatment, and then cooled to room temperature with an average cooling rate wait 20ºС / sec.

[0075]

[0076]

[0077]

[0078]

[0079] Regarding the obtained steel sheets (steel sheets for hot stamping), an analysis of the state of Ti in the inclusions and examination of the metallographic microstructure (fractions of each microstructure) was performed as follows. In addition, the tensile strength (TS) of each steel sheet was measured by the method described later. The results obtained are shown below in Tables 5 and 6 together with a calculated value of 0.5 × [total amount of Ti (wt.%) - 3.4 [N]] [designated as 0.5 × [total amount of Ti-3.4 [ N]].

[0080] (Analysis of the state of Ti in the inclusions in the steel sheet)

An extraction replica sample was prepared, and a TEM image of the Ti-containing inclusions was photographed using a transmission electron microscope (100,000x magnifications) using a transmission electron microscope. At this time, Ti-containing inclusions (having an equivalent circle diameter of 30 nm or less) were identified by composition analysis precipitated phases using an energy dispersive x-ray spectrometer (EDX). At least 100 sections of Ti-containing inclusions were measured by area using image analysis, the diameter of the equivalent circle was determined from it, and its average value was derived as the size of the precipitated phases (average diameter of the equivalent circle of Ti-containing inclusions). Regarding the value “the amount of Ti in the inclusions (wt.%) - 3.4 [N]]” (the amount of Ti present in the form of inclusions), the extraction residue was analyzed using a mesh having a mesh size of 0.1 μm (during the extraction treatment could also measure the finely divided precipitated phase formed by aggregation of inclusions), and determine the “amount of Ti in the inclusions (wt.%) - 3.4 [N]” (indicated in Tables 5 and 6 as “The amount of Ti in the inclusions - 3.4 [N] "). In the case where the Ti-containing inclusions partially contain V or Nb, the content levels of these inclusions were also measured.

[0081] (Inspection of the metallographic microstructure (fractions of each microstructure))

(1) With respect to the microstructure of ferrite, bainitic ferrite, perlite and martensite in the steel sheet, the steel sheet was etched with nital, and after distinguishing ferrite, bainitic ferrite, perlite and martensite from each other using SEM examination (magnification: 1000 times or 2000 times ) determined the fraction (area fraction) of each microstructure.

(2) The fraction of residual austenite in a steel sheet was measured by X-ray diffraction after the steel sheet was ground to 1/4 of the thickness and then chemically polished (for example, according to ISJJ Int., Volume 33, (1933), No. 7 , p. 776).

[0082]

Table 5 Steel No. Stamping steel sheet The amount of Ti in the inclusions is 3.4 [N] (wt.%) 0.5 × [total amount of Ti-3.4 [N] (wt.%) The average diameter of the equivalent circle of Ti-containing inclusions (nm) Ferrite fraction
(% by area) The rest of the microstructure * Tensile Strength (MPa) one 0.008 0.015 3.8 40 B 799 2 0.007 0.015 2.7 38 B 808 3 0.007 0.015 3.0 43 P + B 785 four 0.007 0.015 2.7 43 P + B 787 5 0.001 0.003 3.2 45 B 776 6 0.008 0.015 2.7 36 B 822 7 0.018 0.015 9.2 39 B 807 8 0.007 0.015 3,7 0 B 1150 9 0.010 0.015 7.2 42 B 789 10 0.008 0.015 2,8 40 B 799 eleven 0.008 0.015 2,8 40 B 799 12 0.008 0.015 2,8 40 B 799 13 0.008 0.015 2,8 40 B 799 fourteen 0.008 0.015 2,8 40 B 799 fifteen 0.008 0.015 2,8 40 B 799 16 0.006 0.015 3,1 43 B 787 * B: bainitic ferrite, P: perlite.

[0083]

Table 6 Steel No. Stamping steel sheet The amount of Ti in the inclusions is 3.4 [N] (wt.%) 0.5 × [total amount of Ti-3.4 [N] (wt.%) The average diameter of the equivalent circle of Ti-containing inclusions (nm) Ferrite fraction
(% by area) The rest of the microstructure * Tensile Strength (MPa) eighteen 0.007 0.015 3.9 0 B + m 1277 19 0.009 0.015 3,1 45 P + B 777 twenty 0.007 0.015 2.1 40 B + m 798 21 0,025 0,043 2.6 39 B 804 22 0.166 0,093 12.8 44 B 780 23 0.007 0.015 3.4 43 B 785 24 0.007 0.015 3,7 44 B 779 25 0.008 0.015 2.1 42 B 789 26 0.009 0.015 2.9 38 B 809 27 0.008 0.015 2,3 41 B 796 28 0.008 0.015 3,5 41 B 794 29th 0.007 0.015 2,8 42 B 780 thirty 0.007 0.015 2.7 40 B 785 31 0.007 0.015 2.9 40 P + B 790 32 0.007 0.015 2.7 42 P + B 785 * B: bainitic ferrite, P: perlite, M: martensite.

[0084] Each of the above steel sheets (1.6 mm (thickness t) × 150 mm × 200 mm) (sheet thickness t, other than for processing (1) and (2), was adjusted to 1.6 mm by hot rolling ) was heated at a predetermined temperature in a heating furnace, followed by stamping and cooling using a hat-shaped mold (FIG. 1) to obtain a molded product. The stamping conditions (heating temperature, average cooling rate, and the end temperature of rapid cooling during stamping) are shown below in Table 7.

[0085]

Table 7 Steel No. Stamping conditions Heating Temperature (ºC) Average cooling rate (ºC / s) Quick Cooling End Temperature (ºC) one 810 40 300 2 810 40 300 3 760 40 300 four 770 40 300 5 800 40 300 6 810 40 300 7 810 40 300 8 810 40 300 9 810 40 300 10 810 40 300 eleven 810 40 300 12 900 40 300 13 810 5 300 fourteen 810 40 600 fifteen 810 40 one hundred 16 840 40 300 eighteen 770 40 300 19 810 40 300 twenty 780 40 300 21 820 40 300 22 840 40 300 23 850 40 300 24 810 40 300 25 810 40 300 26 800 40 300 27 800 40 300 28 810 40 300 29th 810 40 300 thirty 810 40 300 31 810 40 300 32 810 40 300

[0086] In relation to the obtained stamped products, measurement of tensile strength (TS), elongation (total elongation EL), examination of the metallographic microstructure (fractions of each microstructure), and measurement of the degree of hardness reduction after heat treatment were carried out by the following methods, and the state released Ti phases were analyzed as described above.

[0087] (Measurement of tensile strength (TS) and elongation (total elongation EL)

Tensile testing was performed using test sample JIS No. 5, and tensile strength (TS) and elongation (EL) were measured. At this time, the strain rate in the tensile test was set to 10 mm / s. In the present invention, the test piece was evaluated as “passed the test” when the tensile strength (TS) of 980 MPa or more and the elongation (EL) of 16% or more and the strength-elongation balance (TS × EL) were satisfied ) was 16,000 (MPa⋅%) or more.

[0088] (Investigation of a metallographic microstructure (fraction of each microstructure))

(1) With respect to the microstructure of ferrite, bainitic ferrite and perlite in the steel sheet, the steel sheet was etched with nital, and after distinguishing between ferrite, bainitic ferrite and perlite from each other (including the distinction between ferrite and needle ferrite) by SEM examination ( magnification: 1000 times or 2000 times) determined the proportion (area ratio) of each microstructure.

(2) The fraction of residual austenite in a steel sheet was measured by X-ray diffraction after the steel sheet was ground to 1/4 of the thickness and then chemically polished (for example, according to ISJJ Int., Volume 33, (1933), No. 7 , p. 776).

(3) Regarding the fraction of martensite (quenching martensite), after etching the steel sheet with LePera reagent, the fraction of white contrast area was measured, which was considered as a mixed microstructure from quenching martensite and residual austenite, and the fraction of residual austenite found by X-ray diffraction was subtracted from it. As a result, the martensite fraction was calculated.

[0089] (The degree of decrease in hardness after heat treatment)

As a thermal history based on spot welding, we measured the degree of decrease in hardness (ΔHV) relative to the initial hardness (Vickers hardness) after heating to a temperature of 700 ° C with an average heating rate of 50 ° C / s in a heat treatment simulator, and then cooling with an average cooling rate of 50 ° C / sec The softening resistance characteristic in HAZ was rated as good when the degree of hardness reduction (ΔHV) was 50 HV or less.

[0090] The results of the examination (the state of the released Ti phases, the fraction of each microstructure, and the amount of Ti in the inclusions is 3.4 [N]) of the metallographic microstructure are shown in Tables 8 and 9 below. In addition, mechanical characteristics are shown in Table 10 below (limit tensile strength TS, elongation EL, TS × EL, and degree of reduction in hardness ΔHv) of the molded product. Here, the “amount of Ti in inclusions-3.4 [N]” in the molded product is slightly different from the value of “amount of Ti in inclusions-3.4 [N]” in the steel sheet for stamping, but this is a measurement error.

[0091]

Table 8 Steel No. The metallographic microstructure of the stamped product Ferrite fraction
(% by area) The proportion of bainitic ferrite
(% by area) Martensite share (% by area) The proportion of residual austenite
(% by area) The amount of Ti in the inclusions is 3.4 [N] (wt.%) The average diameter of the equivalent circle of Ti-containing inclusions (nm) Other one 54 eighteen twenty 8 0.010 3,7 - 2 64 twenty 8 8 0.012 4.0 - 3 46 fifteen 39 0 0.009 3.9 - four fifty 16 thirty four 0.013 3.9 - 5 54 16 23 7 0,000 3.6 - 6 46 17 29th 8 0.011 2.2 - 7 47 17 29th 7 0,022 8.9 - 8 53 eighteen 21 8 0.010 3.4 From ferrite, needle ferrite: 48% 9 51 eighteen 24 7 0.014 11.0 - 10 fifty eighteen 26 6 0.009 3.4 - eleven 52 16 26 6 0.008 3.4 - 12 0 0 95 5 0.002 3.4 - 13 82 0 10 8 0.009 2.2 - fourteen 65 0 10 0 0.010 2.5 Perlite: 25% fifteen fifty 16 27 7 0.009 2,3 - 16 46 eighteen 27 9 0.012 3.2 -

[0092]

Table 9 Steel No. The metallographic microstructure of the stamped product Ferrite fraction
(% by area) The proportion of bainitic ferrite
(% by area) Martensite share (% by area) The proportion of residual austenite
(% by area) The amount of Ti in the inclusions is 3.4 [N] (wt.%) The average diameter of the equivalent circle of Ti-containing inclusions (nm) Other eighteen 8 0 80 12 0.009 2,8 - 19 62 eighteen 12 8 0.011 2,8 - twenty 46 17 thirty 7 0.011 3.0 - 21 47 eighteen 28 7 0,035 2,3 - 22 51 19 22 8 0.175 15.3 - 23 45 17 31 7 0.011 2.0 - 24 54 16 23 7 0.013 3.0 - 25 51 17 25 7 0.011 2.2 - 26 48 twenty 25 7 0.011 3.3 - 27 46 19 28 7 0.011 2.0 - 28 54 19 twenty 7 0.012 2.6 - 29th 54 eighteen twenty 8 0.008 3.8 - thirty 52 twenty twenty 8 0.009 4.0 - 31 fifty eighteen 24 8 0.009 4.0 - 32 53 19 twenty 8 0.009 3.8 -

[0093]

Table 10 Steel No. The mechanical characteristics of the molded product Tensile Strength TS (MPa) Elongation EL (%) TS × EL (MPa⋅%) The degree of decrease in hardness ΔHV (HV) one 1031 17.1 17630 25 2 1065 17.7 18851 25 3 981 10.8 10595 22 four 983 19.3 18972 22 5 1003 18.7 18756 26 6 1029 18.2 18728 23 7 1070 19.0 20330 57 8 1006 22.2 22333 24 9 1003 19,4 19458 55 10 1057 17.5 18498 25 eleven 1050 17.6 18480 25 12 1500 10.3 15450 23 13 889 19.8 17602 25 fourteen 811 15,2 12327 23 fifteen 1028 18.1 18607 23 16 1015 18.9 19184 22 eighteen 1682 6.5 10933 24 19 1044 17.9 18688 24 twenty 1038 18.1 18788 24 21 1077 17.7 19063 28 22 1043 18,4 19191 62 23 1076 17.6 18938 22 24 1063 17.6 18709 23 25 1070 18.1 19367 25 26 1071 17.6 18850 25 27 1006 19.1 19215 26 28 1048 17.7 18550 23 29th 1028 18.9 19429 22 thirty 1030 19,2 19776 19 31 1050 18.3 19215 eighteen 32 1050 18.8 19740 19

[0094] These results allow us to state the following consideration. It was found that in the case of Steels No. 1, 2, 4-6, 10, 11, 15, 16, 19-21, and 23-32, which are Examples meeting the requirements specified in the present invention, a component having good balance "strength-ductility" and a good characteristic of resistance to softening.

[0095] On the other hand, in the case of Steels No. 3, 7-9, 12-14, 18, and 22, which are Comparative Examples that do not satisfy any of the requirements specified in the present invention, any of the properties is impaired. More specifically, in the case of Steel No. 3, where a stamping steel sheet is used that has a low Si content, the proportion of residual austenite is not provided in the stamped product, and due to the low elongation, the strength-elongation balance worsens. In the case of Steel No. 7, where the temperature of the finish rolling is low in the manufacture of the steel sheet, Ti-containing inclusions in the steel sheet for stamping are enlarged, the ratio according to formula (1) is not satisfied both at the stage of the steel sheet for stamping and at the stage of the stamped product, and the softening resistance characteristic deteriorates.

[0096] In the case of Steel No. 8, where the cooling rate is high in the range of 620 ° C to 580 ° C in the manufacture of the steel sheet, the ferrite transformation is not sufficient, not providing a proportion of ferrite in the steel sheet for stamping, and it is believed that the strength is increased, making it difficult molding or processing before stamping. In the case of Steel No. 9, where the temperature of winding into a roll is high in the manufacture of a steel sheet, Ti-containing inclusions in the steel sheet for stamping are enlarged, and when stamping with this steel sheet is performed, the softening resistance performance deteriorates despite the proper stamping conditions and good balance "strength-ductility".

[0097] In the case of Steel No. 12, where the heating temperature is high during stamping, ferrite does not form, although martensite is formed, and the elongation decreases, and the strength-elongation balance (TS × EL) worsens. In the case of Steel No. 13, where the cooling rate is low during stamping, the proportion of ferrite at the stage of the stamped product increases, and the strength decreases.

[0098] In the case of Steel No. 14, where the temperature of the end of cooling during stamping is high, perlite is formed, residual austenite is not provided, and strength and elongation are reduced, worsening the strength-elongation balance (TS × EL). In the case of Steel No. 18, where a stamping steel sheet is used that has an excessively high C content, due to the fact that the ferrite fraction in the steel sheet is not provided, the ferrite fraction in the stamped product cannot be ensured, and as a result, only a low relative EL elongation, and the strength-elongation balance (TS × EL) worsens. In the case of Steel No. 22, where a steel sheet for stamping is used, which has an excessively high Ti content, the ratio according to formula (1) is not satisfied both at the stage of the steel sheet for stamping and at the stage of the stamped product, and not only are Ti-containing inclusion, but also worsens the characteristic of resistance to softening.

[0099] [Example 2]

Steel materials (Steels No. 33-37) having the chemical component composition shown in Table 11 below were melted in a vacuum to make an experimental slab, and then hot rolled, followed by cooling and winding into a roll (sheet thickness: 3.0 mm ) The manufacturing conditions for the steel sheets are shown below in Table 12.

[0100]

[0101]

[0102] Regarding the obtained steel sheets (stamping steel sheets), the analysis of the state of Ti in the inclusions, the examination of the metallographic microstructure (fractions of each microstructure), and the measurement of tensile strength were performed in the same manner as in Example 1. The results are shown below in Table 13.

[0103]

Table 13 Steel No. Stamping steel sheet The amount of Ti in the inclusions is 3.4 [N] (wt.%) 0.5 × [total amount of Ti-3.4 [N] (wt.%) The average diameter of the equivalent circle of Ti-containing inclusions (nm) Ferrite fraction
(% by area) The rest of the microstructure * Tensile Strength (MPa) 33 0.006 0.015 3.2 36 B 789 34 0,023 0.015 3.4 37 B 830 35 0.009 0.015 3.6 42 B 702 36 0.008 0.015 3.4 37 B 830 37 0.008 0.015 3.4 37 B 830 * B: bainitic ferrite.

[0104] Each of the above steel sheets (3.0 mm (thickness t) × 150 mm × 200 mm) was heated at a predetermined temperature in a heating furnace, and then subjected to stamping and cooling using a hat-shaped mold (FIG. 1 ) to obtain a molded product. At this time, the steel sheet was placed in an infrared oven, and a temperature difference was created by directing the infrared radiation directly to the section intended to create high strength (the section of the steel sheet corresponding to the first zone) so that the section could be heated at high temperature and placing the lid in the area intended to create low strength (the area of the steel sheet corresponding to the second zone), so that the area can be heated at low temperature by blocking part of the infrared of radiation.

Accordingly, the molded product has sections that differ in strength in a single component. The stamping conditions (heating temperature, average cooling rate and the end temperature of rapid cooling of each section during stamping) are shown below in Table 14.

[0105]

Table 14 Steel No. Plot Stamping conditions Heating Temperature (ºC) Average cooling rate (ºC / s) Quick Cooling End Temperature (ºC) 33 Low strength side 810 40 300 High strength side 920 40 300 34 Low strength side 800 40 300 High strength side 920 40 300 35 Low strength side 820 40 300 High strength side 920 40 300 36 Low strength side 800 40 300 High strength side 920 40 300 37 Low strength side 800 40 300 High strength side 850 40 300

[0106] In relation to the obtained stamped products, tensile strength (TS), elongation (total elongation EL), examination of the metallographic microstructure (fraction of each microstructure), and the degree of decrease in hardness (ΔHv) in each section were determined in the same way as in Example 1.

[0107] The results of the examination (fractions of each microstructure) of the metallographic microstructure and the analysis of the state of the released Ti phases are shown below in Table 15. In addition, Table 16 below shows the mechanical characteristics (tensile strength TS, elongation EL, TS × EL, and the degree of reduction in hardness ΔHv) of the stamped product. Here, the test piece was rated as “passed the test” when the tensile strength (TS) of 1470 MPa or more and the elongation (EL) of 8% or more satisfied the high strength side and the strength-elongation balance (TS × EL ) was 14000 (MPa⋅%) or more (criteria for evaluating the low strength side are the same as in Example 1).

[0108]

Table 15 Steel No. Plot The metallographic microstructure of the stamped product Ferrite fraction
(% by area) The proportion of bainitic ferrite
(% by area) Martensite share (% by area) The proportion of residual austenite
(% by area) The amount of Ti in the inclusions is 3.4 [N] (wt.%) The average diameter of the equivalent circle of Ti-containing inclusions (nm) Other 33 Low strength side 54 eighteen 21 7 0.013 3.0 - High strength side 0 0 94 6 0.010 3.3 - 34 Low strength side fifty 16 27 7 0.011 3.0 - High strength side 0 0 95 5 0.009 3.0 - 35 Low strength side 54 16 22 8 0.010 4.0 - High strength side 0 0 94 6 0.010 3.2 - 36 Low strength side fifty 16 27 7 0.011 3.0 - High strength side 0 0 95 5 0.010 3.4 - 37 Low strength side fifty 16 27 7 0.011 3.2 - High strength side 25 0 70 5 0.010 3.3 -

[0109]

Table 16 Steel No. Plot The mechanical characteristics of the molded product Tensile Strength TS (MPa) Elongation EL (%) TS × EL (MPa⋅%) The degree of decrease in hardness ΔHv (Hv) 33 Low strength side 1078 17.6 18973 25 High strength side 1511 10,2 15412 37 34 Low strength side 1192 18.1 21575 26 High strength side 1820 10.3 18746 38 35 Low strength side 1043 18,4 19191 26 High strength side 1499 10.1 15140 38 36 Low strength side 1035 17,2 17802 26 High strength side 1520 10,2 15504 44 37 Low strength side 1052 17,2 18094 26 High strength side 1302 11.8 15364 41

[0110] These results allow us to make the following judgment. It was found that in the case of Steels No. 33-36, which are Examples that satisfy the requirements specified in the present invention, a stamped product having a good balance of "strength-ductility" in each section is obtained. On the other hand, in the case of Steel No. 37, where the heating temperature during stamping is low, the proportion of martensite on the high strength side is insufficient, and the strength on the high strength side is reduced (the difference in strength with the low strength side is less than 300 MPa).

INDUSTRIAL APPLICABILITY

[0111] In the present invention, the steel sheet has a predetermined chemical component composition, the average equivalent circle diameter of Ti-containing inclusions having an equivalent circle diameter of 30 nm or less, among the Ti-containing inclusions contained in the steel sheet, is 6 nm or less, the amount of Ti in the inclusions and the total amount of Ti in the steel satisfies a predetermined ratio, and the proportion of ferrite in the metallographic microstructure is 30% by area or more, due to which it can be created n a hot stamping steel sheet that is suitable for forming a stamped product with a good softening resistance characteristic in HAZ and ensures that molding or processing before hot stamping is facilitated, a stamped product capable of achieving a high level balance between high strength and elongation can be obtained when uniform properties are required in the molded product, and the stamped product can achieve a high level balance between high strength and in relative elongation according to the respective areas, when in a single molded article is required areas corresponding CRASHWORTHINESS site and energy-absorbing region.

DESCRIPTION OF CODE NUMBERS OF POSITIONS AND DESIGNATIONS

[0112] 1: Punch

2: Matrix

3: Workpiece clamp

4: Steel sheet (blank).

Claims (26)

1. Steel sheet for hot stamping, containing, in wt.%: C: 0.15 to 0.5 Si: 0.2 to 3, Mn: 0.5 to 3, P: 0.05 or less (excluding 0), S: 0.05 or less (excluding 0), Al: 0.01 to 1, B: 0.0002 to 0.01, Ti: in an amount equal to or greater than 3.4 [N] +0.01, and equal to or less than 3.4 [N] +0.1, where [N] denotes the content (wt.%) N, N: from 0.001 to 0.01, and the rest is iron and inevitable impurities in which the average diameter of the equivalent circle of Ti-containing inclusions having an equivalent circle diameter of 30 nm or less among Ti-containing inclusions contained in the steel sheet is 6 nm or less, the amount of Ti in the inclusions and the total amount of Ti in the steel satisfies the ratio according to the following expression (1), and the area fraction of ferrite in the metallographic microstructure is 30% or more: the amount of Ti in the inclusions (wt.%) - 3.4 [N] <0.5 × [(total amount of Ti (wt.%)) - 3.4 [N]] ... (1), where [N] means the content (wt.%) N in steel. 2. A steel sheet for hot stamping according to claim 1, further comprising at least one of the following elements from (a) to (c): (a) one or more selected from the group consisting of V, Nb and Zr in an amount of 0.1% or less (excluding 0) in total; (b) one or more selected from the group consisting of Cu, Ni, Cr and Mo, in an amount of 1% or less (excluding 0) in total; and (c) one or more selected from the group consisting of Mg, Ca and REM, in an amount of 0.01% or less (excluding 0) in total. 3. A method of manufacturing a molded stamped product, in which the steel sheet for hot stamping according to claim 1 or 2 is heated at a temperature equal to or greater than the transformation point Ac ₁ + 20 ° C and equal to or less than the transformation point Ac ₃ 20 ° C, then stamping of the steel sheet begins, and the steel sheet is cooled to a temperature equal to or less than 100 ° C below the temperature Bs of the onset of bainitic transformation, while at the same time providing an average cooling rate of 20 ° C / sec or more in mold during mold tions and after molding is complete. 4. A stamped product from a steel sheet having a chemical component composition according to claim 1 or 2, in which the metallographic microstructure of the stamped product includes residual austenite: from 3 to 20% by area, ferrite: from 30 to 80% by area, bainitic ferrite: less than 30% (except 0) by area and martensite: 31% (except 0) by area or less, and the average diameter of the equivalent circle of Ti-containing inclusions having an equivalent circle diameter of 30 nm or less among Ti-containing inclusions contained in pc the amplified product is 10 nm or less, and the amount of Ti in the inclusions and the total amount of Ti in the steel satisfies the ratio according to the following expression (1): The amount of Ti in the inclusions (wt.%) - 3.4 [N] <0.5 × [(total amount of Ti (wt.%)) - 3.4 [N]] ... (1), where in the expression (1) [N] denotes the content (wt.%) N in steel. 5. A method of manufacturing a molded stamped product, which uses a steel sheet for hot stamping according to claim 1 or 2, the heating region of the steel sheet is divided into at least two zones, one zone of them is heated at a temperature _of transformation point Ac ₃ or more, and 950 ° C or less, another zone of them is heated at a temperature equal to or greater than the Ac ₁ transformation point + 20 ° C and equal to or less than the Ac ₃ transformation point-20 ° C, and then forming by stamping of both zones is started and the steel sheet is cooled to a temperature equal to or lower than the temperature Ms of the onset of martensitic transformation, while at the same time providing an average cooling rate of 20 ° C / sec or more in the mold in both zones during molding and after completion of molding. 6. A stamped product from a steel sheet having a chemical component composition according to claim 1 or 2, which has a first zone having a metallographic microstructure including residual austenite: from 3 to 20% by area, and martensite: 80% by area or more, and a second zone having a metallographic microstructure, including residual austenite: from 3 to 20% by area, ferrite: from 30 to 80% by area, bainitic ferrite: less than 30% (except 0) by area and martensite: 31% (for except 0) in area or less, and the average diameter is equivalent to a circumference Ti Ti-containing inclusions having an equivalent circle diameter of 30 nm or less, among Ti-containing inclusions contained in the steel of the second zone, is 10 nm or less, and the amount of Ti in the inclusions and the total amount of Ti in the steel satisfy the ratio according to the following expression ( one): The amount of Ti in the inclusions (wt.%) - 3.4 [N] <0.5 × [(total amount of Ti (wt.%)) - 3.4 [N]] ... (1), where in the expression (1) [N] denotes the content (wt.%) N in steel.

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