JP3350945B2 - High tensile hot rolled steel sheet with excellent ductility and corrosion resistance and manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a high-tensile hot-rolled steel sheet excellent in ductility and corrosion resistance, and suitable for a structural member formed into various shapes by press working, stretch flange working and the like, and a method for producing the same.

[0002]

2. Description of the Related Art The so-called "hot rolled steel sheet" manufactured by continuous hot rolling has been widely applied to various industrial equipment such as automobiles as a relatively inexpensive structural material. In recent years, demand for not only higher performance but also lighter weight of these machines and devices has been rapidly increasing. For this reason, many technologies have been developed to increase the strength of steel sheets used as materials in order to promote weight reduction.However, since increasing the strength of steel sheets generally involves deterioration of ductility, steel sheets that have both good workability and high strength have been developed. Manufacturing was very difficult.

However, recently, "After hot rolling a low carbon steel sheet to which Si and Mn are added in combination, a part of austenite is transformed into bainite and finally [ferrite + bainite + retained austenite] Hot rolled steel sheet (
This phenomenon has been used since the discovery of the phenomenon that "retained austenite undergoes strain-induced transformation during deformation during processing and exhibits large elongation." Attempts to produce high-ductility, high-strength hot-rolled steel sheets have also been made.

[0004] For example, Japanese Patent Application Laid-Open No. 55-145121 discloses that a steel having a C content of 0.35 to 0.85% (hereinafter, the percentage indicating the component ratio is referred to as weight%) is 380 from the austenite region.
A method is disclosed in which most of austenite is transformed into bainite by quenching to a temperature range of ４480 ° C. and holding at a constant temperature, thereby obtaining a residual γ hot-rolled steel sheet having high ductility and high strength.

However, in this method, since the C content in the steel sheet must be adjusted to a high value of 0.35 to 0.85%, the obtained steel sheet is inferior in weldability, and cannot be used as a steel sheet for automobiles. It was difficult.

[0006] In addition, even if a hot-rolled steel sheet having the above-mentioned mixed structure (residual γ hot-rolled steel sheet) shows good ductility in a tensile test, the formability in press working or the like is not necessarily good. It was pointed out that it was not satisfactory as a steel sheet for processing. For example,
When the mixed-structure steel sheet is processed, most of the retained austenite is transformed into high-carbon martensite by strain-induced transformation in a later stage of deformation, so that local ductility is extremely poor. This phenomenon is remarkable in the case of stretch flange processing such as “hole expansion”, and as a result, the hole expandability of the mixed structure steel sheet is inferior to that of the conventional low carbon steel sheet. This is when a hole is made by punching,
It is considered that high-carbon martensite generated by the strain-induced transformation is so hard that a crack is generated, and the crack propagates and propagates when the hole is subsequently expanded.

In the known manufacturing technique relating to the residual γ hot rolled steel sheet, it is necessary to change the C concentration in the steel when the strength level is changed. As a result, the volume fraction of austenite was reduced, and it was difficult to produce "a hot-rolled steel sheet containing a large amount of retained austenite in a relatively low strength region and exhibiting high ductility".

On the other hand, separately from this,
No. 7 discloses a method for obtaining high ductility by securing retained austenite under a low C content to obtain high ductility.
A method of obtaining a residual γ hot-rolled steel sheet having high ductility and high strength by subjecting a steel containing steel to a large pressure reduction in a low-temperature austenite region is disclosed.

However, the hot-rolled steel sheet obtained by the above method cannot be said to have sufficient formability at the time of press working or the like. In addition, since the hot deformation resistance of the steel in the austenite region rises significantly with a decrease in the rolling temperature, an excessive load is applied to the hot rolling mill in this method, and the thickness and flatness of the steel plate are increased. There was a problem that the control of the degree became difficult. That is, in the actual operation for producing a hot-rolled steel sheet, it is a major premise that "do not apply an excessive load to the hot rolling mill". Therefore, the first attention should be paid to the production of the residual γ hot-rolled steel sheet. Japanese Patent Application Laid-Open No. 63-4017 discloses that the component must be designed so that retained austenite can be stably obtained without applying an excessive load to the hot rolling mill. The method described was not practical.

Furthermore, when the amount of Si added to steel is large, eutectic reaction occurs between SiO ₂ and FeO during slab heating in the hot rolling step, resulting in non-uniform low-melting scale. There is also a problem that unevenness occurs on the surface of the hot-rolled sheet and the appearance is impaired.

In addition, a steel sheet reinforced with a transformation structure generally has a problem that its corrosion resistance is inferior to that of a steel sheet reinforced by solid solution. This problem is considered to be caused by the fact that a so-called "local cell" is easily formed in a composite structure steel sheet composed of a plurality of structures having different corrosion potentials, which leads to corrosion.

In view of the above, an object of the present invention is to provide excellent workability such as ductility, and excellent strength of corrosion resistance and appearance in which the strength level can be adjusted without a large change in the C content. The aim was to establish means that could provide stable high-strength hot-rolled steel sheets.

[0013]

The inventor of the present invention has made intensive studies to achieve the above object, and has obtained the following findings. (A) First, the effect of Si and Al on the amount of retained austenite in a hot-rolled sheet with a composition of 0.15% C-1.5% Mn as a standard composition was investigated. Thus, even if the hot-rolling completion temperature is high or the cooling rate from completion of hot-rolling to winding is not so strictly controlled, the residual oil can be relatively easily formed.
As long as the austenite can be ensured and the amounts of addition are equal, almost the same volume percentage of retained austenite can be obtained in the steel sheet to which both Si and Al are added. Although the total elongation was slightly smaller than that of the Si-added steel sheet, the local elongation obtained by subtracting the uniform elongation from the total elongation was conversely large, indicating that the hole had good performance in terms of hole expandability.

This is presumably because in the steel sheet to which Al is added, since the retained austenite is stable, it hardly undergoes strain-induced transformation until it reaches a high strain range, and transforms after reaching a large deformation range. Although the cause of such a difference is unknown, it is assumed that the distribution form of the retained austenite changes.

(B) Further, the C concentration in the steel sheet and [Si + A
l), it is possible to arbitrarily change the tensile strength of the steel sheet while maintaining the same volume ratio of retained austenite by changing the balance between Si (%) and Al (%). is there.

(C) However, if the amount of Si is reduced by positively adding Al as a component for securing retained austenite, a smooth surface state can be secured on the hot-rolled steel sheet,
The appearance does not deteriorate.

(D) Mn and Cr are also elements known as stabilizing components of austenite. However, addition of Mn and Cr to the Al-added steel sheet or the Si-added steel sheet is the same as that of the residual austenite. Although it exerts a favorable effect on the securing and contributes to the improvement of the workability, when Cr is added, the effect that the corrosion resistance of the steel sheet is improved can also be secured.

(E) When manufacturing a hot-rolled steel sheet from a steel in which Cr is contained in the Al- or Si-added steel to enhance the corrosion resistance, devising cooling conditions and winding conditions after hot rolling. When hardened, it becomes much easier to secure the amount of retained austenite which is favorable for ductility, and the productivity of a corrosion-resistant high-strength hot-rolled steel sheet excellent in workability including ductility is extremely stabilized.

The present invention has been completed on the basis of the above findings and the like, and describes that "Hot-rolled steel sheet is made of C, Si, Mn, P,
Content of S, Cr, Al and N: C: 0.05-0.3%, Si: 2.0% or less, Mn: 0.05
To 4.0%, P: 0.1% or less, S: 0.1% or less, Cr: 0.5
5.0%, Al: 0.1% to 2.0%, N: 0.01% or less, and satisfy Si (%) + Al (%) ≧ 0.5, 7.0 ≧ Mn (%) + Cr (%) ≧ 1.0, and the balance is Fe and By having a composition composed of unavoidable impurities and having a structure containing retained austenite in a volume ratio of 5% or more, it has both high strength and excellent ductility and corrosion resistance. In addition, the content of C, Si, Mn, P, S, Cr, Al and N is as follows: C: 0.05 to 0.3%, Si: 2.0% or less, Mn: 0.05
To 4.0%, P: 0.1% or less, S: 0.1% or less, Cr: 0.5
5.0%, Al: 0.1% to 2.0%, N: 0.01% or less, and satisfy Si (%) + Al (%) ≧ 0.5, 7.0 ≧ Mn (%) + Cr (%) ≧ 1.0, and the balance is Fe and A steel slab having a composition of unavoidable impurities is hot-rolled, and hot-rolled in a temperature range of at least ₃ points of Ar, and then cooled at a cooling rate of 10 to 100 ° C / s.
Cooling and winding to a temperature range of 50 to 550 ° C, or 600 to 600 ° C at a cooling rate of 10 ° C / s or more after finishing hot rolling.
After cooling to a temperature range of 700 ° C.,
Air cooling for 2 seconds, then at a cooling rate of 20 ° C / s or more for 300 ~
By cooling to 550 ° C and winding, 5% by volume
% High-strength hot-rolled steel sheet which contains high-strength, excellent ductility, and corrosion resistance by containing at least 1.0% of retained austenite ".

Hereinafter, the reason why the composition of the steel sheet (slab) and the manufacturing conditions of the steel sheet are limited as described above in the present invention will be described together with the operation thereof.

[Action]

A) Component composition C is the most powerful austenite stabilizing element. To stabilize austenite at room temperature, it is necessary that C contains 1% or more of austenite. If the cooling pattern after hot rolling is optimized, if the C content in the steel sheet is 0.05% or more, sufficient C is concentrated in the austenite. By adding a larger amount of C, a hot-rolled steel sheet with higher strength can be manufactured. However, if the content exceeds 0.3%, the steel becomes too hard, and it is necessary to process it into a hot-rolled steel sheet in a normal sheet making process. Can not be done. Therefore, the C content is limited to 0.05 to 0.3%, but is preferably
Adjust to 0.1-0.2%. Further, considering the weldability, 0.1 to 0.15% is most preferable.

Si Si is a ferrite stabilizing element, and has an effect of precipitating ferrite in a cooling process after hot rolling and increasing the C concentration of the austenite phase to be equilibrated. Also with this
Si also has the effect of strengthening ferrite. However, if the content of Si exceeds 2.0%, the surface quality is significantly degraded due to the high Si scale peculiar to the Si-added steel sheet. Therefore, the Si content is set to 2.0% or less. Note that the content of Si must be controlled in relation to Al, which is the same ferrite stabilizing element. In order to obtain the desired effect in the above operation, the value of [Si (%) + Al (%)] must be adjusted. It is necessary to adjust it to be 0.5 or more.

Mn Mn is an austenite stabilizing element. From this viewpoint, the content of Mn is regulated by the sum of the content of Cr having the same effect and the content of [Mn (%) + Cr (%)] It needs to be adjusted so that the value is 1.0 or more. That is, the value of [Mn (%) + Cr (%)]
If it is less than 1.0, austenite is not stabilized. Just M
n content exceeds 4.0% or the value of [Mn (%) + Cr (%)] is 7.
If it exceeds 0, the steel sheet may be too hard and may not have sufficient ductility, so the upper limit of the Mn content is 4.0%.
And the value of [Mn (%) + Cr (%)] was in the range of 1.0 to 7.0. On the other hand, since Mn also has the effect of fixing S in steel as MnS and preventing hot brittleness, it is necessary to secure a content of at least 0.05% in order to ensure a desired effect on the effect. There is. FIG. 1 relates to the hot-rolled steel sheet of the present invention.
4 is a graph illustrating the content ranges of Mn and Cr.

[0023] P P is an element which is inevitably contained in steel as an impurity, who only low it is preferable. In particular, when the content exceeds 0.1%, the ductility of the hot-rolled steel sheet becomes remarkable, so the P content is set to 0.1% or less.

S S is also an element inevitably contained in steel as an impurity, and it is preferable that S is also lower. In particular, when the content exceeds 0.1%, the precipitation amount of MnS becomes conspicuous, not only impairs the ductility of the hot-rolled steel sheet, but also consumes Mn added as an austenite stabilizing element as the precipitate. Therefore, the S content was determined to be 0.1% or less.

[0025] Cr is an element having a function of stabilizing austenite like Mn, and thus Cr is added for this purpose. At the same time, Cr is essential for ensuring the desired corrosion resistance of the hot-rolled steel sheet. It is also a component of If the Cr content is less than 0.5%, the desired corrosion resistance cannot be imparted to the hot-rolled steel sheet.
If the Cr content exceeds 5.0%, it contributes to the stabilization of ferrite, thereby destabilizing austenite and, at the same time, it becomes extremely difficult to pickle the steel sheet. Therefore,
The Cr content is determined to be 0.5 to 5.0%, preferably 2 to 3%.
It is better to adjust to about%. The Cr content is 7.0 in total with the Mn content in order to ensure high ductility in the hot-rolled steel sheet.
% As described above.

Al As described above, Al is a ferrite stabilizing element like Si, and has the effect of precipitating ferrite in the cooling process after hot rolling and increasing the C concentration of the equilibrium austenite phase. I have. However, compared to Si, it has a stronger effect of precipitating ferrite and also has the effect of stabilizing austenite. If a content of 0.1% or more is secured, the effect of improving the local ductility of hot-rolled steel sheet can be obtained. . on the other hand,
If the Al content exceeds 2.0%, the inclusions in the steel sheet increase, leading to a decrease in ductility. Therefore, although the Al content is set to 0.1 to 2.0%, in order to secure the desired effect as a ferrite stabilizing element, the value of [Si (%) + Al (%)] must be 0.5 or more together with Si. Need to adjust. FIG. 2 is a graph illustrating the content ranges of Si and Al related to the hot-rolled steel sheet of the present invention.

[0027] N N is also an element which is inevitably contained in steel as an impurity, the content thereof is preferably low. When the N content exceeds 0.01%, the amount of Al consumed as AlN is large.
Since not only the effect of Al addition is reduced but also the ductility deterioration due to AlN becomes noticeable, the upper limit of the N content is set to 0.01%.

B) Volume fraction of retained austenite The ductility of the hot-rolled steel sheet of the present invention depends on the volume fraction of the retained austenite contained therein, and when the volume fraction is less than 5%, ductility due to strain-induced transformation of austenite. Cannot be expected to improve. In addition, since the ductility of the hot-rolled steel sheet increases with an increase in the retained austenite, the volume fraction of the retained austenite is preferably set to 10% or more.

C) Manufacturing Conditions The hot rolling may be carried out on a normal rolling schedule after slab heating under normal conditions. However, it is important to specifically control the cooling and winding conditions after finishing hot rolling. This is one of the major features of the method for manufacturing a hot-rolled steel sheet according to the present invention. That is, in order to secure the desired effect aimed at by the present invention, the hot rolling conditions described below must be satisfied.

The hot rolling conditions are as follows: after finishing hot rolling with Ar ₃ or more, 350 to 5 at a cooling rate of 10 to 100 ° C./s.
It is a condition of cooling to 50 ° C. and winding. In this case, 10 ° C
Cooling at a cooling rate of less than / s or winding at a temperature exceeding 550 ° C. not only increases the volume fraction of ferrite, but also transforms austenite into pearlite, leaving a sufficient volume fraction remaining. Austenite cannot be obtained. on the other hand,
If the cooling is performed at a cooling rate exceeding 100 ° C./s, ferrite is not precipitated at a sufficient volume ratio, so that C is not sufficiently concentrated in the retained austenite, and is transformed into martensite in the subsequent cooling process. Similarly, when wound at a temperature lower than 350 ° C., bainite transformation does not occur after winding, and residual
Since C is not sufficiently concentrated in the austenite,
Stenite transforms into martensite.

Further, more preferable hot rolling conditions are as follows: after finishing hot rolling is performed with Ar ₃ or more, the hot rolling is performed at a cooling rate of 10 ° C./s or more.
This is a condition in which the film is cooled to a temperature range of 00 to 700 ° C, air-cooled in the temperature range for 2 to 10 seconds, and then further cooled to 300 to 550 ° C at a cooling rate of 20 ° C / s or more, and wound. By providing the air cooling section, the formation of ferrite is promoted, so that the austenite stabilization due to the C concentration becomes remarkable, and the ductility is further improved. To ensure this effect,
The air cooling time needs to be at least 2 seconds or more. In this case, the cooling rate after the finish hot rolling is less than 10 ° C./s, and thereafter, the temperature is maintained at a temperature exceeding 700 ° C. (air cooling), or the temperature is maintained between 600 and 700 ° C. (air cooling).
If the time exceeds 10 seconds, or the cooling rate after holding (air cooling) between 600 and 700 ° C. is less than 20 ° C./s, or if winding is performed at a temperature exceeding 550 ° C., the volume fraction of ferrite becomes Not only does it increase too much, but the austenite is transformed into pearlite, and a sufficient volume fraction of retained austenite cannot be obtained. On the other hand, after finishing hot rolling, 6
When rapidly cooled to a temperature range of less than 00 ° C., ferrite does not precipitate at a sufficient volume ratio, so that C is not sufficiently concentrated in the retained austenite, and is transformed into martensite in the subsequent cooling process. Similarly, when wound at a temperature lower than 300 ° C., bainite transformation does not occur after winding, and residual
Since C is not sufficiently concentrated in the austenite,
Stenite transforms into martensite.

Next, the effects of the present invention will be described more specifically with reference to examples.

EXAMPLES First, steels having the respective component compositions shown in Table 1 were melted in a 50 kg vacuum melting furnace, and these were formed into slabs having a thickness of 60 mm by hot forging. Next, after reheating the slab to 1200 ° C., a hot-rolled sheet having a thickness of 2 mm was manufactured under the conditions shown in Tables 2 and 3. In addition, the Ar ₃ points of these steel types were measured by thermal expansion, and all were lower than 800 ° C.

[0033]

[Table 1]

[0034]

[Table 2]

[0035]

[Table 3]

Next, JIS No. 5 tensile test pieces were sampled from the obtained hot-rolled steel sheets and "mechanical properties" were examined. Also, 9
Using a test piece having a size of 5 mm × 95 mm, “punch hole expandability” was also examined by a test in which a 14 mm φ punch hole was expanded to the occurrence of a through-thickness crack with a 30 mm φ cylindrical punch. The hole expansion rate was calculated by the following equation (1). Hole expansion rate = [(hole diameter when through-thickness cracking occurs)-(base hole diameter)] / base hole diameter… (1)

Further, a test piece for an X-ray test was taken from the center of each hot-rolled steel sheet, and the "remaining austenite amount" was measured from the X-ray reflection intensity. In addition, a "corrosion resistance" survey was conducted for each hot-rolled steel sheet. Corrosion resistance is measured by applying a cross-cut after applying a polyester resin coating to the steel sheet.
Annual exposure to the atmosphere was performed. At this time, the evaluation was made based on the maximum width of a portion where red rust was generated in the cross cut portion and the coating film was peeled off. The results are shown in Tables 2 and 3.

The following can be seen from the results shown in Tables 2 and 3. That is, each of the hot-rolled steel sheets manufactured according to the method of the present invention has a retained austenite having a volume fraction of 5% or more, and therefore has a high ductility in which “product of tensile strength and elongation” exceeds 2,500, and “ The product of the tensile strength and the hole expansion ratio "also exceeds 3000 and shows good hole expansion. Also,
Also excellent in corrosion resistance.

On the other hand, the hot-rolled steel sheets according to Test Nos. 1 and 22 made of steel whose Al addition amount is lower than the lower limit of the range of the present invention have a residual austenite of 5% or more and have a ductility. Shows a high value, but a low value of the hole spreadability. The hot rolled steel sheets according to Test Nos. 2 and 23 using this steel type and having a high hot rolling finish temperature had residual Au.
The amount of stenite is significantly reduced and the ductility is only low.

On the other hand, the hot-rolled steel sheets according to Test Nos. 9 and 35 in which the hot-rolling finishing temperature was lower than the _three points of Ar had significantly reduced ductility and hole-expandability because the processed ferrite remained. Then, the hot-rolled steel sheets related to Test Nos. 10, 36 and 39 having slow cooling rates after hot rolling, the hot-rolled steel sheets related to Test Nos. 11 and 40 having high winding temperatures, and the start of air cooling during cooling after hot rolling The hot-rolled steel sheet of Test No. 37 with a high temperature and the hot-rolled steel sheet of Test No. 38 with a long air-cooling time have a sufficient amount of austenite to achieve good ductility due to progress of pearlite transformation. Does not remain.

The hot-rolled steel sheet according to Test No. 12 having a low winding temperature undergoes martensitic transformation and has low ductility and hole-expandability. In the hot-rolled steel sheets according to Test Nos. 13 and 41 in which the amount of Al exceeds the upper limit of the range of the present invention, the inclusions increased and the ductility and the hole-expandability decreased. Hot-rolled steel sheets according to Test Nos. 17 and 45 in which the total amount of Mn and Cr exceeds the upper limit of the present invention range, and hot-rolled steel sheets according to Test Nos. 21 and 49 having a C content exceeding the upper limit of the present invention range, However, it is difficult to work because of its high tensile strength, and the hole expandability is low. The hot rolled steel sheets according to Test Nos. 18 and 46 in which the C content is lower than the lower limit of the range of the present invention have insufficient austenite stabilization and a small amount of retained austenite. Further, the hot rolled steel sheets according to Test Nos. 14 and 42 having a small Cr content have sufficient workability but are inferior in corrosion resistance.

[0042]

[Summary of Effects] As described above, according to the present invention, a high-tensile hot-rolled steel sheet having excellent ductility, good workability such as good hole expandability, and also excellent corrosion resistance can be stably obtained. Thus, industrially extremely useful effects are obtained.

[Brief description of the drawings]

FIG. 1 is a graph illustrating a content range of Mn and Cr according to the hot-rolled steel sheet of the present invention.

FIG. 2 is a graph illustrating the content ranges of Si and Al according to the hot-rolled steel sheet of the present invention.

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C22C 38/00 301 C21D 8/02 C22C 38/18

Claims (3)

(57) [Claims] 1. The content of C, Si, Mn, P, S, Cr, Al and N in terms of weight ratio is as follows: C: 0.05 to 0.3%, Si: 2.0% or less, Mn: 0.05
To 4.0%, P: 0.1% or less, S: 0.1% or less, Cr: 0.5
5.0%, Al: 0.1 to 2.0%, N: 0.01% or less, and satisfy Si (%) + Al (%) ≧ 0.5, 7.0 ≧ Mn (%) + Cr (%) ≧ 1.0, and the balance is Fe and A high-strength hot-rolled steel sheet having excellent ductility and corrosion resistance, having a component composition of unavoidable impurities and having a structure containing retained austenite in a volume ratio of 5% or more. . 2. The content of C, Si, Mn, P, S, Cr, Al and N in terms of weight ratio is as follows: C: 0.05 to 0.3%, Si: 2.0% or less, Mn: 0.05
To 4.0%, P: 0.1% or less, S: 0.1% or less, Cr: 0.5
5.0%, Al: 0.1 to 2.0%, N: 0.01% or less, and satisfy Si (%) + Al (%) ≧ 0.5, 7.0 ≧ Mn (%) + Cr (%) ≧ 1.0, and the balance is Fe and A steel slab having a component composition consisting of unavoidable impurities is hot-rolled, and hot-rolled in a temperature range of at least ₃ points of Ar, then cooled at a cooling rate of 10 to 100 ° C./s.
A method for producing a high-tensile hot-rolled steel sheet containing 5% or more by volume of retained austenite and having excellent ductility and corrosion resistance, wherein the sheet is cooled to a temperature range of 50 to 550 ° C and wound. 3. The content of C, Si, Mn, P, S, Cr, Al and N in terms of weight ratio is as follows: C: 0.05 to 0.3%, Si: 2.0% or less, Mn: 0.05
To 4.0%, P: 0.1% or less, S: 0.1% or less, Cr: 0.5
5.0%, Al: 0.1 to 2.0%, N: 0.01% or less, and satisfy Si (%) + Al (%) ≧ 0.5, 7.0 ≧ Mn (%) + Cr (%) ≧ 1.0, and the balance is Fe and A steel slab having a component composition consisting of unavoidable impurities is hot-rolled, and hot-rolled at a temperature range of _three or more points of Ar.
After cooling to a temperature range of ~ 700 ° C,
Air cooling for 0 seconds, followed by 300 ° C at a cooling rate of 20 ° C / s or more
A method for producing a high-tensile hot-rolled steel sheet containing 5% or more of retained austenite by volume and having excellent ductility and corrosion resistance, wherein the sheet is cooled to -550 ° C and wound.