WO2012002566A1 - High-strength steel sheet with excellent processability and process for producing same - Google Patents

Abstract

Provided are a high-strength steel sheet which is thin and has excellent processability and a process for producing the steel sheet. The process comprises: a hot rolling step in which a steel material having a composition containing, in terms of mass%, 0.08-0.15% C, 0.5-1.5% Si, 0.5-1.5% Mn, 0.01-0.1% Al, and up to 0.005% N is hot-rolled to obtain a hot-rolled sheet; annealing in which the hot-rolled sheet is pickled and then held for 5-400 seconds in the first temperature range of from the A_c1 transformation point to the A_c3 transformation point in a continuous annealing line, without being cold-rolled; and cooling in which the sheet is cooled from the first temperature range to 700ºC at an average cooling rate of 5 ºC/sec or higher and held for 30-400 seconds in the second temperature range of from 700ºC to 400ºC. Thus, a structure comprising a ferrite phase and a second phase comprising pearlite can be obtained, the ferrite phase and the pearlite accounting for 75-90% by area and 10-25% by area, respectively, of the whole structure. The pearlite accounts for 70% by area or more of the whole second phase and has an average grain diameter of 5 µm or less. As a result, a high-strength steel sheet which has a strength (TS) as high as 540 MPa or above, excellent stretchability and stretch flangeability, and excellent processability is obtained.

Description

High-strength steel sheet with excellent workability and method for producing the same

The present invention relates to a high-strength steel sheet suitable for use as a strength member for automobile parts, which requires excellent workability (stretch flangeability), and a method for producing the same.

In recent years, improving fuel efficiency of automobiles has become an important issue from the viewpoint of global environmental conservation. For this reason, the movement to increase the strength of the material to be used, to reduce the thickness of the member, and to reduce the weight of the vehicle body has been activated. As a material to be used, a high strength steel plate having a tensile strength of 540 MPa or more is particularly required. However, increasing the strength of a steel sheet causes a decrease in workability, so a high-strength steel sheet having excellent workability is demanded, and this demand is particularly high for a thin steel sheet (thin steel sheet).

In response to such demands, there are various types such as a duplex steel sheet (DP steel sheet) having a two-phase structure composed of a ferrite phase and a martensite phase, and a steel sheet having a composite structure including a ferrite phase, a martensite phase, and a bainite phase. Composite steel sheets have been proposed.
For example, in Patent Document 1, C: 0.08 to 0.30%, Si: 0.1 to 2.5%, Mn: 0.5 to 2.5%, P: 0.01 to 0.15 the cold-rolled steel sheet of percent composition containing, recrystallization annealing at a _c1 point or higher, then, after forced cooling to a temperature range in the range of a _r1 point to 600 ° C., the cooling rate of more than 100 ° C. / s Is rapidly cooled to form a composite structure composed of a ferrite phase and a low-temperature transformation generation phase, and thereafter, the ratio of the low-temperature transformation generation phase hardness Hv (L) to the ferrite hardness Hv (α) obtained by a predetermined relational expression, Hv Production of high-strength cold-rolled steel sheet with excellent local ductility in which overaging is performed at a temperature in the range of 350 to 600 ° C. so that (L) / Hv (α) satisfies 1.5 to 3.5. A method is described. In the technique described in Patent Document 1, the quenching start temperature is increased to increase the volume ratio of the low-temperature transformation generation phase, and then an overaging treatment is performed at 350 to 600 ° C. to precipitate C in the ferrite, and the low temperature The transformation generation phase is softened to reduce Hv (L) / Hv (α) and improve local elongation.

In Patent Document 2, C: 0.02 to 0.25%, Si: 2.0% or less, Mn: 1.6 to 3.5%, P: 0.03 to 0.20%, S : 0.02% or less, Cu: 0.05 to 2.0%, sol. A steel slab containing Al: 0.005 to 0.100% and N: 0.008% or less is hot-rolled to form a hot-rolled coil. After pickling, the hot-rolled coil is 720 to 950 ° C. in a continuous annealing line. Describes a method for producing a low-yield ratio, high-tensile hot-rolled steel sheet excellent in corrosion resistance that is annealed at a temperature of 5 ° C. According to the technique described in Patent Document 2, a high-tensile hot-rolled steel sheet having a composite structure that maintains a low yield ratio, high ductility, and good hole expandability, and has excellent corrosion resistance can be manufactured.

Patent Document 3 discloses that C: 0.03 to 0.17%, Si: 1.0% or less, Mn: 0.3 to 2.0%, P: 0.010% or less, S: 0.0. A composition containing 010% or less, Al: 0.005 to 0.06%, satisfying C (%)> (3/40) × Mn, and a second phase mainly composed of bainite or pearlite and a ferrite phase. A high-strength cold-rolled steel sheet having a structure and satisfying (second-phase Vickers hardness) / (ferrite-phase Vickers hardness) less than 1.6 and having an excellent strength-stretch flangeability balance is described. The high-strength cold-rolled steel sheet described in Patent Document 3 is obtained by hot rolling a steel (slab) having the above composition, winding it at a temperature of 650 ° C. or lower, pickling it, and then cold rolling, , _{a 1} point or more, _{(a 3} point + 50 ° C.) soaking at a temperature, then slowly cooled below 20 ° C. / s until temperature _{T 1} of between the range of 750 ~ 650 ° C., then, from _{T 1} It is said that it can be obtained by performing an annealing treatment of cooling to 500 ° C. at a rate of 20 ° C./s or more, and subsequently overaging at a temperature of 500 to 250 ° C.

JP-A-63-293121 JP 05-111282 A Japanese Patent Laid-Open No. 10-60593

However, the technique described in Patent Document 1 requires a continuous annealing facility capable of rapid cooling (quenching) after recrystallization annealing and suppresses a rapid strength decrease due to an overaging treatment at a high temperature. There is a problem that a large amount of alloying element is required.
In addition, in the technique described in Patent Document 2, it is essential to add a large amount of P and Cu in combination. However, a large amount of Cu decreases hot workability, and a large amount of P is contained. Makes the steel brittle. Moreover, P has a strong tendency to segregate in the steel, and this segregated P has a problem of causing embrittlement of the welded part in addition to lowering the stretch flangeability of the steel sheet.

Further, the high-strength cold-rolled steel sheet described in Patent Document 3 is excellent in stretch flangeability, but in the case of a high strength of 540 MPa or more, the elongation is less than 26%, and the desired excellent workability can be maintained. There is a problem that sufficient growth cannot be secured.
An object of the present invention is to solve such problems of the prior art and to provide a high-strength steel sheet having a thin plate thickness of about 1.0 to 1.8 mm and excellent workability, and a method for producing the same. Here, “high strength” refers to a case where the tensile strength TS is 540 MPa or more, preferably 590 MPa or more, and “excellent workability” means elongation El: 30% or more. (When a JIS No. 5 test piece is used), the hole expansion rate λ in a hole expansion test in accordance with Japan Iron and Steel Federation standard JFST 1001-1996 is 80% or more.

In order to achieve the above-mentioned object, the present inventors conducted extensive research on the influence of the composition and the microstructure on the strength and workability. As a result, the hot-rolled sheet with the alloy element amount adjusted to an appropriate range is subjected to an annealing process and an appropriate cooling process, which are heated to an appropriate two-phase temperature range, without performing cold rolling. The main phase and the second phase can be made mainly of fine pearlite, which can ensure the desired high strength, greatly improve the workability, and achieve the desired elongation and desired hole expansion. It was found that a high-strength steel sheet excellent in workability that combines the efficiency can be obtained.

Although the detailed mechanism for significantly improving the workability by subjecting the hot-rolled sheet to the appropriate annealing treatment directly by omitting cold rolling is not clear so far, the present invention They think as follows.
When the hot-rolled sheet is subjected to an annealing treatment for heating to a two-phase temperature range without performing cold rolling, only α → γ transformation occurs during annealing heating, and no new recrystallization occurs. . In this case, only α → γ transformation preferentially occurs at a location where the C concentration is high, so that a more uniform structure can be obtained, and C having a high diffusion rate is converted into α and γ up to the equilibrium composition during annealing treatment. Redistributed. For this reason, it is considered that the precipitation of film-like cementite at the grain boundaries is suppressed, and that it has an advantageous effect on the improvement of stretch flangeability. On the other hand, if the annealing process is performed after cold rolling the hot-rolled sheet, recrystallization and α → γ transformation occur at the time of annealing and heating, which tends to result in a non-uniform structure and significant processing. It is difficult to expect improvement in sex.

The present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows.
(1) In mass%, C: 0.08 to 0.15%, Si: 0.5 to 1.5%, Mn: 0.5 to 1.5%, P: 0.1% or less, S: 0.01% or less, Al: 0.01 to 0.1%, N: 0.005% or less, the composition composed of the balance Fe and inevitable impurities, the ferrite phase as the main phase, and at least pearlite The ferrite phase is 75 to 90%, the pearlite is 10 to 25%, and the average particle size of the pearlite is 5 μm or less. Furthermore, the high-strength steel sheet excellent in workability, wherein the pearlite is 70% or more in terms of the area ratio with respect to the total area of the second phase.

(2) In (1), in addition to the above composition, further, in mass%, Cr: 0.05 to 0.5%, V: 0.005 to 0.2%, Mo: 0.005 to 0.2 % High-strength steel sheet characterized by containing one or more selected from the group%.

(3) In (1) or (2), in addition to the above composition, it was further selected by mass%, from Ti: 0.01 to 0.1%, Nb: 0.01 to 0.1% A high-strength steel sheet characterized by containing one or two kinds.

(4) A high-strength steel sheet according to any one of (1) to (3), further containing B: 0.0003 to 0.0050% in mass% in addition to the above composition.

(5) In any one of (1) to (4), in addition to the above-mentioned composition, it is further possible to use mass% of Ni: 0.05 to 0.5% and Cu: 0.05 to 0.5%. A high-strength steel sheet characterized by containing one or two selected materials.

(6) In any one of (1) to (5), in addition to the above-mentioned composition, further, in mass%, Ca: 0.001 to 0.005%, REM: 0.001 to 0.005% A high-strength steel sheet characterized by containing one or two selected materials.

(7) In mass%, C: 0.08 to 0.15%, Si: 0.5 to 1.5%, Mn: 0.5 to 1.5%, P: 0.1% or less, S: Hot-rolled sheet by hot rolling a steel material having a composition comprising 0.01% or less, Al: 0.01 to 0.1%, N: 0.005% or less, the balance being Fe and inevitable impurities and a hot rolling step of, after having been subjected to pickling the hot rolled sheet, the heat-rolled sheet in a continuous annealing line, at a first temperature range of transformation point a _c1 ~ a _c3 transformation point 5 to inter 400s An annealing treatment to be held, and after the annealing treatment, cooling from the first temperature range to 700 ° C. at an average cooling rate of 5 ° C./s or more, and further in a second temperature range of 700 ° C. to 400 ° C. A method for producing a high-strength steel sheet having excellent workability, characterized by performing a continuous annealing step for performing a cooling treatment with a residence time of 30 to 400 s.

(8) In (7), after the said hot rolling process heats the said steel raw material to the temperature of the range of 1100-1280 degreeC, it hot-rolls by hot rolling which makes hot rolling completion temperature: 870-950 degreeC. A method for producing a high-strength steel sheet, comprising the step of winding the hot-rolled sheet at a winding temperature of 350 to 720 ° C. after completion of the hot rolling.

(9) The method for producing a high-strength steel sheet according to (7) or (8), wherein a cooling time in a temperature range of 700 to 550 ° C. of the second temperature range is 10 s or more.

(10) In any one of (7) to (9), in addition to the above composition, further, in mass%, Cr: 0.05 to 0.5%, V: 0.005 to 0.2%, Mo: A method for producing a high-strength steel sheet, comprising one or more selected from 0.005 to 0.2%.

(11) In any one of (7) to (10), in addition to the above composition, further, in mass%, Ti: 0.01 to 0.1%, Nb: 0.01 to 0.1% A method for producing a high-strength steel sheet, comprising one or two selected materials.

(12) In any one of (7) to (11), in addition to the above-described composition, the method further comprises mass: B: 0.0003 to 0.0050%. .

(13) In any one of (7) to (12), in addition to the above-mentioned composition, mass%, Ni: 0.05 to 0.5%, Cu: 0.05 to 0.5% A method for producing a high-strength steel sheet, comprising one or two selected materials.

(14) In any one of (7) to (13), in addition to the above composition, further, in mass%, Ca: 0.001 to 0.005%, REM: 0.001 to 0.005% A method for producing a high-strength steel sheet, comprising one or two selected materials.

According to the present invention, there is provided a high-strength steel sheet excellent in workability, which has a high strength of tensile strength TS: 540 MPa or more, an elongation of El: 30% or more, and an elongation flangeability of λ: 80% or more. It can be manufactured easily and inexpensively, and has a remarkable industrial effect. In addition, the present invention has an effect that cold rolling can be omitted, and the manufacturing cost can be reduced and productivity can be greatly improved. In addition, if the steel sheet according to the present invention is applied particularly to automobile body parts, it can greatly contribute to weight reduction of the automobile body.

First, the reason for limiting the composition of the steel sheet of the present invention will be described. Hereinafter, unless otherwise specified, mass% is simply expressed as%.
C: 0.08 to 0.15%
C is an element that contributes to an increase in the strength of the steel sheet and effectively acts on the formation of a composite structure composed of a ferrite phase and a second phase other than the ferrite phase. In the present invention, the desired tensile strength: In order to ensure a high strength of 540 MPa or more, it is necessary to contain 0.08% or more. On the other hand, if the content exceeds 0.15%, spot weldability is lowered, and workability such as ductility is further lowered. Therefore, C is limited to the range of 0.08 to 0.15%. Note that the content is preferably 0.10 to 0.15%.

Si: 0.5 to 1.5%
Si is an element that dissolves in steel and effectively acts to strengthen the ferrite, and also contributes to the improvement of ductility. In order to secure a desired tensile strength: 540 MPa or more, the content of Si is 0.8. It needs to contain 5% or more. On the other hand, an excessive content exceeding 1.5% promotes the generation of red scale and the like, lowers the surface properties of the steel sheet, and lowers the chemical conversion treatment property. Moreover, excessive inclusion of Si is accompanied by an increase in electrical resistance during resistance welding, and impedes resistance weldability. For this reason, Si was limited to the range of 0.5 to 1.5%. In addition, Preferably it is 0.7 to 1.2%.

Mn: 0.5 to 1.5%
Mn is an element that contributes to an increase in the strength of the steel sheet and that effectively acts in the formation of a composite structure. In order to obtain such an effect, the Mn content needs to be 0.5% or more. On the other hand, a content exceeding 1.5% tends to form a martensite phase in the cooling process during annealing, and causes deterioration in workability, particularly stretch flangeability. For this reason, Mn was limited to the range of 0.5 to 1.5%. In addition, Preferably it is 0.7 to 1.5%.

P: 0.1% or less P is an element that has the effect of increasing the strength of the steel sheet by solid solution in steel, but has a strong tendency to segregate to the grain boundary, lowering the bonding strength of the grain boundary, and processing In addition to causing a decrease in the properties, it concentrates on the surface of the steel sheet to reduce chemical conversion properties, corrosion resistance, and the like. Such an adverse effect of P becomes remarkable when the content exceeds 0.1%. For this reason, P was limited to 0.1% or less. In order to avoid such adverse effects of P, P is preferably 0.1% or less and preferably reduced as much as possible. However, excessive reduction leads to an increase in manufacturing cost, so it should be about 0.001% or more. Is preferred.

S: 0.01% or less S forms sulfides (inclusions) such as MnS mainly in steel and lowers the workability of the steel sheet, particularly the local elongation. In addition, the presence of sulfide (inclusions) also reduces weldability. Such an adverse effect of S becomes remarkable when the content exceeds 0.01%. For this reason, S was limited to 0.01% or less. In order to avoid such an adverse effect of S, S is preferably 0.01% or less, and is preferably reduced as much as possible. However, excessive reduction leads to an increase in manufacturing cost, so it should be about 0.0001% or more. Is preferred.

Al: 0.01 to 0.1%
Al acts as a deoxidizer and is an essential element for improving the cleanliness of the steel sheet, and also effectively acts for improving the yield of carbide forming elements. In order to acquire such an effect, 0.01% or more of content is required. If the content is less than 0.01%, the removal of Si-based inclusions that are the starting point of delayed fracture becomes insufficient, and the risk of delayed fracture increases. On the other hand, even if the content exceeds 0.1%, the above-described effect is saturated, an effect commensurate with the content cannot be expected, and it becomes economically disadvantageous, workability is reduced, and surface defects tend to occur. Increase. For this reason, Al was limited to the range of 0.01 to 0.1%. The content is preferably 0.01 to 0.05%.

N: 0.005% or less N is an element that is essentially harmful in the present invention, and it is desirable to reduce it as much as possible, but up to 0.005% is acceptable. For this reason, N was limited to 0.005% or less. In addition, since excessive reduction of N causes a rise in manufacturing cost, it is preferable to make it about 0.0001% or more.

The above components are basic components. In addition to the basic components, Cr: 0.05 to 0.5%, V: 0.005 to 0.2%, Mo: 0.005 to One or more selected from 0.2% and / or one selected from Ti: 0.01 to 0.1% and Nb: 0.01 to 0.1% Or 2 and / or B: 0.0003 to 0.0050% and / or Ni: 0.05 to 0.5%, Cu: 0.05 to 0.5% 1 type or 2 types and / or 1 or 2 types selected from Ca: 0.001 to 0.005% and REM: 0.001 to 0.005% are selected and contained. Can do.

One or more selected from Cr: 0.05 to 0.5%, V: 0.005 to 0.2%, Mo: 0.005 to 0.2% Cr, V, and Mo are Any of them is an element that increases the strength of the steel sheet and contributes to the formation of the composite structure, and can be selected as necessary and contained in one or more kinds. In order to acquire such an effect, it is desirable to contain Cr: 0.05% or more, V: 0.005% or more, and Mo: 0.005% or more. On the other hand, excessive content exceeding Cr: 0.5%, V: 0.2%, Mo: 0.2%, respectively, makes it difficult to produce a desired amount of pearlite during the cooling treatment after the annealing treatment, A desired composite structure cannot be secured, stretch flangeability is lowered, and workability is lowered. For this reason, when it is contained, it may be limited to the ranges of Cr: 0.05 to 0.5%, V: 0.005 to 0.2%, and Mo: 0.005 to 0.2%, respectively. preferable.

One or two selected from Ti: 0.01 to 0.1% and Nb: 0.01 to 0.1% Ti and Nb are both elements that increase the steel sheet strength by precipitation strengthening. These can be selected as necessary and can be contained alone or in combination. In order to obtain such an effect, it is desirable to contain Ti: 0.01% or more and Nb: 0.01% or more, respectively, but Ti: 0.1% and Nb: more than 0.1%, respectively. Containment reduces processability and shape freezing property. For this reason, when it contains, it is preferable to limit in the range of Ti: 0.01-0.1% and Nb: 0.01-0.1%, respectively.

B: 0.0003 to 0.0050%
B is an element that segregates at the austenite grain boundary and has an action of suppressing the formation and growth of ferrite from the grain boundary, and can be contained as necessary. In order to acquire such an effect, it is desirable to contain 0.0003% or more, but inclusion exceeding 0.0050% reduces workability. For this reason, when contained, B is preferably limited to a range of 0.0003 to 0.0050%. In addition, in order to acquire the effect of B as mentioned above, it is necessary to suppress the production | generation of BN, and it is preferable to make it contain with Ti.

One or two selected from Ni: 0.05 to 0.5% and Cu: 0.05 to 0.5% Ni and Cu both have an action of increasing the strength of the steel sheet, It is an element that also has an effect of promoting oxidation and improving plating adhesion, and can be selected and contained as necessary. In order to obtain such an effect, it is desirable to contain Ni: 0.05% or more and Cu: 0.05% or more, respectively, but Ni: 0.5% and Cu: 0.5% are exceeded. Containment makes it difficult to produce a desired amount of pearlite during the cooling treatment after the annealing treatment, making it impossible to secure a desired composite structure, lowering stretch flangeability, and lowering workability. For this reason, when it contains, it is preferable to limit to Ni: 0.05-0.5% and Cu: 0.05-0.5%.

Ca: 0.001 to 0.005%, REM: One or two selected from 0.001 to 0.005% Ca and REM are elements that contribute to sulfide morphology control. It has the effect of suppressing the adverse effect on the workability of sulfides, particularly the stretch flangeability, by making the shape of sulfides spherical. In order to obtain such an effect, it is desirable to contain Ca: 0.001% or more, REM: 0.001% or more, respectively, but Ca: 0.005%, REM: 0.005%, respectively. Excessive inclusion causes an increase in inclusions, resulting in frequent occurrence of surface defects and internal defects. For this reason, when it contains, it is preferable to limit to Ca: 0.001-0.005% and REM: 0.001-0.005%.

The balance other than the components described above consists of Fe and inevitable impurities.
The steel sheet of the present invention has the above-described composition and a structure composed of a ferrite phase as a main phase and a second phase containing at least pearlite.
In the steel sheet of the present invention, the area ratio of the ferrite phase that is the main phase is 75 to 90% in terms of the area ratio with respect to the entire structure. If the area ratio of the ferrite phase is less than 75%, the desired elongation and the desired hole expansion rate cannot be ensured, and the workability deteriorates. On the other hand, if the area ratio of the ferrite phase exceeds 90%, the area ratio of the second phase decreases, and a desired high strength cannot be ensured. For this reason, the area ratio of the ferrite phase as the main phase is limited to a range of 75 to 90%. A preferable area ratio of the ferrite phase is 80 to 90%.

In the steel sheet of the present invention, the second phase contains at least pearlite. The area ratio of pearlite is the area ratio with respect to the whole structure, and is 10 to 25%. If the area ratio of pearlite is less than 10%, a desired hole expansion rate cannot be ensured, the stretch flangeability is lowered, and the workability is lowered. On the other hand, when the area ratio of pearlite exceeds 25%, the interface between the ferrite phase and pearlite increases, voids are likely to be generated during processing, stretch flangeability decreases, and workability decreases.

In addition, pearlite is a fine particle having an average particle size of 5 μm or less. When the average grain size of pearlite exceeds 5 μm and becomes coarse, stress concentrates on the pearlite grains (interface) during the processing of the steel sheet, and microvoids are generated, so that stretch flangeability is lowered and workability is lowered. For this reason, the average particle size of pearlite was limited to 5 μm or less. In addition, Preferably it is 4.0 micrometers or less.

The second phase in the structure of the steel sheet of the present invention is a phase mainly composed of pearlite that contains at least pearlite, and the pearlite has an area ratio of 70% or more with respect to the total area of the second phase. If the pearlite is less than 70% in area ratio with respect to the total area of the second phase, the hard martensite phase, bainite phase, or residual γ increases too much, and the workability tends to be lowered. For this reason, pearlite was limited to 70% or more in the area ratio with respect to the total area of a 2nd phase. Preferably, it is 75 to 100%.

In addition to pearlite, the second phase may contain bainite, martensite, retained austenite (residual γ), etc., but in particular, bainite and martensite are hard phases, and residual γ is transformed during processing. It transforms into martensite, which degrades workability. For this reason, it is desirable that these bainite, martensite and retained austenite be as small as possible, and the total area ratio with respect to the entire structure is preferably 5% or less. More preferably, the total content is 3% or less.

Below, the preferable manufacturing method of this invention steel plate is demonstrated.
A steel material having the above composition is used as a starting material. The method for producing the steel material is not particularly limited, but the molten steel having the above composition is melted by a conventional melting method such as a converter or an electric furnace, and a slab or the like is obtained by a conventional casting method such as a continuous casting method. It is preferable to use a steel material from the viewpoint of productivity. It is also possible to apply an ingot-bundling rolling method, a thin slab casting method, or the like.

The steel material having the above composition is subjected to a hot rolling process to obtain a hot rolled sheet. In the hot rolling step, the steel material is heated to a temperature in the range of 1100 to 1280 ° C., and then hot rolled to a hot rolling finish temperature of 870 to 950 ° C. to form a hot rolled sheet, The hot-rolled sheet is preferably a step of winding at a winding temperature of 350 to 720 ° C.
If the heating temperature of the steel material is less than 1100 ° C., the deformation resistance becomes too high, the rolling load becomes excessive, and hot rolling may be difficult. On the other hand, if it exceeds 1280 ° C., the crystal grains become too coarse, and it becomes difficult to secure a desired fine steel sheet structure even if hot rolling is performed. For this reason, the heating temperature for hot rolling is preferably set to a temperature in the range of 1100 to 1280 ° C. More preferably, it is less than 1280 degreeC.

Also, when the hot rolling finish temperature is less than 870 ° C., ferrite (α) and austenite (γ) are generated during rolling, and a band-like structure is easily generated on the steel sheet. This band-like structure remains even after annealing, and may cause anisotropy in the obtained steel sheet characteristics or cause a decrease in workability. On the other hand, when the hot rolling finish temperature exceeds 950 ° C., the hot rolled sheet structure becomes coarse, and a desired structure may not be obtained even after annealing. Therefore, the hot rolling end temperature is preferably 870 to 950 ° C.

In addition, when the coiling temperature after the hot rolling is less than 350 ° C., bainitic ferrite, bainite, martensite, etc. are generated, and it is easy to form a hard and non-sized hot rolled structure, and in the subsequent annealing treatment In some cases, it inherits the hot-rolled structure, tends to become a non-sized structure, and cannot secure the desired workability. On the other hand, at a high temperature exceeding 720 ° C., it is difficult to ensure uniform mechanical properties over the entire length direction and width direction of the steel sheet. For this reason, the winding temperature is preferably set to a temperature in the range of 350 to 720 ° C. More preferably, the temperature is 500 to 680 ° C.

In order to remove the scale formed on the surface of the steel sheet, the hot-rolled sheet obtained through the hot-rolling process is subjected to pickling according to a conventional method, and then cold-rolling the hot-rolled sheet. However, the continuous annealing process which performs an annealing process and a subsequent cooling process in a continuous annealing line directly is performed.
The annealing process is a process of holding for 5 to 400 s in the first temperature range from the A _c1 transformation point to the A _c3 transformation point.

When the temperature (heating temperature) in the first temperature range of the annealing treatment is less than the _Ac1 transformation point, or the holding time (annealing time) in the first temperature range is less than 5 s, hot rolling Since the carbide in the plate does not dissolve sufficiently, or the α → γ transformation does not occur or is insufficient, the desired composite structure cannot be secured by the subsequent cooling treatment, so the desired elongation and hole expansion rate are satisfied. A steel sheet having ductility and stretch flangeability cannot be obtained. On the other hand, when the heating temperature of the annealing process becomes higher than the _Ac3 transformation point, coarsening of the austenite grains becomes remarkable, the structure generated by the subsequent cooling process becomes coarse, and workability may be lowered. In addition, if the holding time (annealing time) in the first temperature range exceeds 400 s, the processing time becomes long, the energy consumption increases, and the manufacturing cost increases. For this reason, the annealing treatment is limited to a treatment for holding for 5 to 400 s in the first temperature range from the A _c1 transformation point to the A _c3 transformation point.

The A _c1 transformation point of each steel plate is the following equation (1), and the A _c3 transformation point is the value calculated by the following equation (2). In addition, when there is an element not contained in the element in the formula, the element is calculated as zero.
A _c1 transformation point (° C.) = 723 + 29.1Si-10.7Mn-16.9Ni + 16.9Cr + 6.38W + 290As (1)
Ac ₃ transformation point (° C.) = 910−203√C + 44.7Si-30Mn + 700P + 400Al-15.2Ni-11Cr-20Cu + 31.5Mo + 104V + 400Ti + 13.1W + 120As (2)
Here, C, Si, Mn, Ni, Cr, W, As, C, P, Al, Cu, Mo, V, Ti: Content of each element (mass%)
The cooling treatment after the annealing treatment is performed by cooling from the first temperature range to 700 ° C. at an average cooling rate of 5 ° C./s or more, and further in the second temperature range of 700 ° C. to 400 ° C. The residence time is set to 30 to 400 s.

If the average cooling rate from the first temperature range to 700 ° C. is less than 5 ° C./s, the amount of ferrite produced increases too much, the desired composite structure cannot be obtained, the workability decreases, and the desired tensile strength is further reduced. The strength (540 MPa or more) may not be ensured. For this reason, the cooling rate from the first temperature range to 700 ° C. was limited to 5 ° C./s or more on average. The temperature is preferably 20 ° C./s or less, more preferably 5 to 15 ° C./s.

Also, the residence time in the second temperature range of 700 ° C. to 400 ° C. is an important factor for the formation of pearlite contained in the second phase. Here, the “residence time” means the time of staying in the above-mentioned second temperature range, and when holding at the specific temperature of the second temperature range, The case where it cools with a cooling rate and the case where it cools with the pattern which mixed them are included. If the residence time in the second temperature range is less than 30 s, pearlite transformation does not occur or the amount of pearlite produced is insufficient, so a desired composite structure cannot be ensured. On the other hand, when the residence time in the second temperature range is longer than 400 s, productivity is lowered. For this reason, the residence time in the second temperature range is limited to the range of 30 to 400 s. In addition, Preferably it is 150 s or less. In the second temperature range, the cooling time in the temperature range of 700 to 550 ° C. is 10 s or more, that is, the cooling rate in the temperature range of 700 to 550 ° C. is 15 ° C./s or less on average. It is preferable for securing a desired amount of pearlite. If the cooling time in the temperature range of 700 to 550 ° C. is less than 10 s, the formation of pearlite becomes insufficient, the desired composite structure cannot be obtained, and the desired workability may not be ensured.

Hereinafter, the present invention will be described more specifically based on examples. The present invention is not limited to these examples.

Molten steel having the composition shown in Table 1 was melted and used as a steel material by a conventional method. These steel materials are hot-rolled at the heating temperature and hot rolling end temperature shown in Table 2 to form a 1.6 mm thick hot rolled sheet, and after hot rolling, coiled at the winding temperature shown in Table 2 Rolled up. Thereafter, pickling was performed. Some hot-rolled sheets (thickness: 3.2 mm) were pickled and then cold-rolled at a reduction ratio of 50% to obtain 1.6 mm-thick cold-rolled sheets as comparative examples. .

The obtained hot-rolled sheet or cold-rolled sheet is further heated to the temperature in the first temperature range under the conditions shown in Table 2, and the annealing treatment to be held, from the temperature in the first temperature range to 700 ° C, Cooling is performed at the average cooling rate shown in Table 2, and 700 to 550 ° C. of the second temperature range is further cooled at the cooling rate (cooling time) shown in Table 2, and then the second temperature of 700 to 400 ° C. The region residence time is shown in Table 2 and the residence time is used. Cooling treatment is performed, and a continuous annealing step is performed to obtain an annealing plate. In addition, the transformation point of each steel plate shown in Table 2 is a value calculated using the above-described equations (1) and (2).

Specimens were collected from the obtained annealed plate and subjected to a structure observation, a tensile test, and a hole expansion test. The test method was as follows.

(1) Microstructure observation From the obtained annealed plate, a specimen for microstructural observation was collected, a cross section parallel to the rolling direction (L cross section) was polished, corroded with a nital solution, and a scanning electron microscope (magnification: 3000 times). 3), the tissue was observed for 3 fields or more, and imaged, and the type of tissue and the area ratio of each phase with respect to the entire tissue were measured, and the area ratio of the total area of the second phase with respect to the entire tissue was calculated. The average crystal grain size of pearlite contained in the second phase was also calculated. The average crystal grain size of pearlite is determined by measuring the area of each pearlite grain, calculating the equivalent circle diameter from the area, arithmetically averaging the equivalent circle diameter of each obtained grain, It was. The measured number of pearlite particles was 20 or more. Moreover, the area ratio with respect to the total area of the second phase of pearlite was also calculated.

(2) Tensile test From the obtained annealed plate, a JIS No. 5 test piece was sampled so that the tensile direction coincided with the direction perpendicular to the rolling direction, and a tensile test was performed in accordance with the provisions of JIS Z 2241. Tensile properties (yield point YP, tensile strength TS, elongation El) were determined.

(3) Hole expansion test A 100 mm square hole expansion test piece was collected from the obtained annealed plate. Then, a hole expansion test was performed in accordance with the provisions of the Japan Iron and Steel Federation standard JFST 1001-1996, and the hole expansion ratio λ (%) was obtained.

Table 3 shows the obtained results.

All of the examples of the present invention have high tensile strength TS: 540 MPa or more, elongation El: high ductility of 30% or more, and excellent stretch flangeability of hole expansion ratio λ: 80% or more. It is a high-strength steel sheet with excellent properties. On the other hand, in the comparative example that is out of the scope of the present invention, the desired high strength is not obtained, the desired elongation is not obtained, or the desired hole expansion ratio λ is not obtained. , Workability is degraded.

Claims (14)

mass%,
C: 0.08 to 0.15%, Si: 0.5 to 1.5%,
Mn: 0.5 to 1.5%, P: 0.1% or less,
S: 0.01% or less, Al: 0.01-0.1%,
N: 0.005% or less, having a composition composed of the remaining Fe and inevitable impurities, a structure composed of a ferrite phase as a main phase, and a second phase including at least pearlite, and an area with respect to the entire structure The ferrite phase is 75 to 90%, the pearlite is 10 to 25%, and the average particle size of the pearlite is 5 μm or less, and the pearlite is an area ratio relative to the total area of the second phase. A high-strength steel sheet excellent in workability characterized by being 70% or more. In addition to the above-mentioned composition, it is 1% selected from mass%, Cr: 0.05 to 0.5%, V: 0.005 to 0.2%, Mo: 0.005 to 0.2% The high-strength steel sheet according to claim 1, comprising seeds or two or more kinds. In addition to the above composition, the composition further comprises one or two selected from the group consisting of Ti: 0.01 to 0.1% and Nb: 0.01 to 0.1% in mass%. The high-strength steel sheet according to claim 1 or 2. The high-strength steel sheet according to any one of claims 1 to 3, further comprising B: 0.0003 to 0.0050% in mass% in addition to the composition. In addition to the above composition, the composition further comprises one or two selected from the group consisting of Ni: 0.05 to 0.5% and Cu: 0.05 to 0.5% in mass%. The high-strength steel sheet according to any one of claims 1 to 4. In addition to the above composition, the composition further comprises one or two selected from Ca: 0.001 to 0.005% and REM: 0.001 to 0.005% in mass%. The high-strength steel plate according to any one of claims 1 to 5. mass%,
C: 0.08 to 0.15%, Si: 0.5 to 1.5%,
Mn: 0.5 to 1.5%, P: 0.1% or less,
S: 0.01% or less, Al: 0.01-0.1%,
N: A hot rolling process in which a steel material containing 0.005% or less and having the balance Fe and inevitable impurities is hot-rolled to form a hot-rolled sheet, and pickling the hot-rolled sheet After that, the hot-rolled sheet is held in the continuous annealing line for 5 to 400 s in the first temperature range from the A _c1 transformation point to the A _c3 transformation point, and after the annealing treatment, the first temperature range A continuous annealing step in which a cooling treatment is performed at an average cooling rate of 5 ° C./s or higher from 700 ° C. to 700 ° C., and further a cooling treatment with a residence time in the second temperature range of 700 ° C. to 400 ° C. being 30 to 400 s The manufacturing method of the high strength steel plate excellent in workability characterized by performing these. In the hot rolling step, the steel material is heated to a temperature in the range of 1100 to 1280 ° C., and then hot rolled to a hot rolling finish temperature of 870 to 950 ° C. to obtain a hot rolled sheet, 8. The method for producing a high-strength steel sheet according to claim 7, wherein the hot-rolled sheet is wound at a winding temperature of 350 to 720 ° C. after completion. The method for producing a high-strength steel sheet according to claim 7 or 8, wherein a cooling time in a temperature range of 700 to 550 ° C in the second temperature range is set to 10 s or more. In addition to the above-mentioned composition, it is 1% selected from mass%, Cr: 0.05 to 0.5%, V: 0.005 to 0.2%, Mo: 0.005 to 0.2% The method for producing a high-strength steel sheet according to any one of claims 7 to 9, comprising seeds or two or more kinds. In addition to the above composition, the composition further comprises one or two selected from the group consisting of Ti: 0.01 to 0.1% and Nb: 0.01 to 0.1% in mass%. The manufacturing method of the high strength steel plate in any one of Claim 7 thru | or 10. The method for producing a high-strength steel sheet according to any one of claims 7 to 11, further comprising B: 0.0003 to 0.0050% in mass% in addition to the composition. In addition to the above composition, the composition further comprises one or two selected from the group consisting of Ni: 0.05 to 0.5% and Cu: 0.05 to 0.5% in mass%. The manufacturing method of the high strength steel plate in any one of Claim 7 thru | or 12. In addition to the above composition, the composition further comprises one or two selected from Ca: 0.001 to 0.005% and REM: 0.001 to 0.005% in mass%. The manufacturing method of the high strength steel plate in any one of Claim 7 thru | or 13.