Thin steel sheets and methods for manufacturing them.

Abstract

DEPCT64 Thin steel sheets with a tensile strength of 980 megapascals or higher and Well-forming conditions have been prepared, and methods for producing them have been established. Thin steel sheets have specific chemical alloys and a particular microstructure. Ferrite is present in a proportion of 4 percent or less (which includes 0 percent), martensite. As hardening occurs in a proportion of 10 percent or less of the area (including 0 percent). Residual austenite is present in amounts of 7 percent or more and 20 percent or more. Less binite, upper binite, lower binite, and temper martensite were present in the overall quantity. 71 percent and less than 93 percent; BCC steel which has a misalignment of 1 degree or less and Surrounding the remaining austenite, which has a circular diameter equivalent to 1 micrometer or less. Appears in area proportions of 4 percent or more and 50 percent or less; and BCC steel. This means that a deviation of more than 1 degree appears in 25 percent or more of the area. 85 percent or less -----------------------------------------------------------

Description

Thin Steel Sheet and Method for Producing the Same

[0001] The present invention relates to a thin steel sheet and a method for producing the same. The thin steel sheet of the present invention has a tensile strength (TS) of 980 MPa or more and excellent workability. Therefore, the thin steel sheet of the present invention is suitable as a material for automobile seat parts.

[0002] In recent years, from the perspective of global environmental conservation, the entire automotive industry is aiming to improve the fuel efficiency of automobiles for the purpose of regulating CO ２ emissions. To improve the fuel efficiency of automobiles, weight reduction of automobiles by thinning the used parts is the most effective. Therefore, in recent years, the amount of high-strength steel sheets used as materials for automobile parts has been increasing.

[0003] Generally, as the strength of steel sheets increases, the formability tends to deteriorate. Therefore, in order to further expand the spread of high-strength steel sheets, improvement of formability is essential. Therefore, the need for materials with both formability is increasing more and more.

[0004] Regarding TRIP steel sheets that utilize retained austenite as a method for improving formability, various techniques have been known so far.

[0005] For example, in Patent Document 1, ferrite has an average crystal grain size of 3 μm or less and a volume fraction of 5% or less, retained austenite has a volume fraction of 10 to 20%, martensite has an average crystal grain size of 4 μm or less and a volume fraction of 20% or less, and the balance contains bainite and / or tempered martensite. In a plate thickness cross-section parallel to the rolling direction of the steel sheet, 100 μm ２ By precipitating cementite particles with an average particle number of 30 or more cementite particles with a particle size of 0.1 μm or more per hit, elongation is obtained, and a steel sheet having excellent elongation flangeability and a high yield ratio of 1180 MPa or more is obtained.

[0006] In Patent Documents 2 and 3, the ferrite fraction is 5% or less or the ferrite fraction is more than 5% and 50% or less, respectively, and the amount of retained austenite is 10% or more. Then, by refining MA, which is a composite structure of retained austenite and martensite, and increasing the retained austenite with a size of 1.5 μm or more, a steel sheet having excellent elongation, hole expansion property, and deep drawing property is obtained.

[0007] Japanese Patent Publication No. WO2015-115059, Japanese Patent Publication No. 2017-214648, Japanese Patent Publication No. 2017-214647

[0008] The technology proposed in Patent Document 1 states that if cementite is not precipitated, the hardness of tempered martensite and bainite increases, and the ductility of the flange deteriorates. In other words, the strength and formability of the steel sheet inevitably change depending on the state of cementite precipitation. For this reason, the technology proposed in Patent Document 1 cannot obtain a steel sheet with stable mechanical properties.

[0009] The technologies proposed in Patent Documents 2 and 3 state that if the carbon-enriched region is too large, the MA becomes coarse, which worsens hole-expanding properties and reduces the hole-expanding ratio. However, while the ductility of TRIP steel increases with increasing carbon concentration in the retained austenite, there is a problem in that the TRIP effect is not fully obtained from the viewpoint of achieving both ductility and ductility.

[0010] In all the technologies proposed in the patent documents, there is a need to achieve excellent formability and high strength at an even higher level. In view of this requirement, the present invention aims to provide a thin steel sheet having a tensile strength of 980 MPa or more and good formability, and a method for manufacturing the same.

[0011] To solve the above problems, the inventors investigated the requirements for improving formability. The main target of this invention is sheet components, which require extremely high bendability. In this case, since there is an effect of unbending before the final process, it is necessary to suppress the reduction in sheet thickness of the processed part under bending-unbending conditions, and in addition to normal bendability, it is necessary to have high uniform elongation and work hardening. To achieve this, it was found that it is effective to include BCC iron, which has a small disorder in its crystal structure, at a certain fraction or higher. Furthermore, it was found that in order to suppress the generation of voids when tension and compression are repeated, it is necessary to refine the size of the hard phase. To suppress the disorder of the BCC iron crystal and refine the size of the hard phase, it was found that it is effective to allow sufficient reverse transformation to austenite during annealing, hold it at about 450°C, and then rapidly cool it. The sheet thickness of the thin steel sheet targeted in this invention is 0.4 mm to 2.6 mm.

[0012] The present invention was completed by diligently pursuing manufacturing conditions for steel sheet components and steel sheet structures that satisfy the above requirements. The gist of the invention is as follows: [1] A component composition consisting of, in mass%, C: 0.10% or more and 0.23% or less, Si: 1.30% or more and 2.20% or less, Mn: 2.0% or more and 3.2% or less, P: 0.05% or less, S: 0.005% or less, Al: 0.005% or more and 0.100% or less, N: 0.0060% or less, with the remainder being Fe and unavoidable impurities, and a ferrite area ratio of 4% or less (including 0%), and an as-quenched martensite area ratio Thin steel sheet having a steel structure in which 10% or less (including 0%) retained austenite is 7% to 20%, upper bainite, lower bainite, and tempered martensite together account for more than 71% but less than 93%, and furthermore, the area ratio of BCC iron with an orientation difference of 1° or less surrounding retained austenite with an equivalent circle diameter of 1 μm or less is 4% to 50%, and the area ratio of BCC iron with an orientation difference of more than 1° is 25% to 85%. [2] In mass%, A thin steel sheet having a composition consisting of C: 0.10% to 0.23%, Si: 1.30% to 2.20%, Mn: 2.0% to 3.2%, P: 0.05% or less, S: 0.005% or less, Al: 0.005% to 0.100%, N: 0.0060% or less, with the remainder being Fe and unavoidable impurities, and a steel structure having a ferrite area ratio of 4% or less (including 0%), an as-quenched martensite area ratio of 10% or less (including 0%), retained austenite of 7% to 20%, upper bainite, lower bainite, and tempered martensite totaling more than 71% but less than 93%, and further having a BCC iron area ratio of 5% to 50% with an orientation difference of 1° or less surrounding retained austenite with an equivalent circle diameter of 1 μm or less, and a BCC iron area ratio of 25% to 85% with an orientation difference of more than 1°. [3] The thin steel sheet according to [1] or [2], wherein the component composition further contains, by mass%, Sb: 0.001% or more and 0.050% or less. [4] The thin steel sheet according to any one of [1] to [3], wherein the component composition further contains, by mass%, one or more of the following: Ti: 0.001% or more and 0.1% or less, Nb: 0.001% or more and 0.1% or less, V: 0.001% or more and 0.3% or less, Ni: 0.01% or more and 0.1% or less, Cr: 0.01% or more and 1.0% or less, and B: 0.0002% or more and 0.0050% or less.[5] The component composition further contains, by mass%, one or more of the following: Cu: 0.01% to 0.2%, Mo: 0.01% to 1.0%, REM: 0.0002% to 0.050%, Mg: 0.0002% to 0.050%, and Ca: 0.0002% to 0.050%, in any one of [1] to [4]. A method for manufacturing a thin steel sheet, comprising: a cold rolling step in which a hot-rolled steel sheet having the component composition described in any of [6] [1] to [5] is cold-rolled with a cold rolling ratio of 46% or more; and an annealing step in which, after the cold rolling step, the sheet is heated and kept at 815°C or higher for 130 seconds or more, then cooled to a temperature range of 420°C to 520°C with an average cooling rate of 8°C / s or more from 800°C to 520°C, kept in that temperature range for 12 seconds to 60 seconds, cooled to a cooling stop temperature of 200°C to 350°C with an average cooling rate of 8°C / s or more in the temperature interval from 420°C to 300°C, kept in a temperature range of ±50°C from the cooling stop temperature for 2 seconds to 25 seconds, then heated to a temperature of 300°C to 500°C, and kept in that temperature range for 480 seconds to 1800 seconds.

[0013] According to the present invention, the material possesses high strength with a tensile strength (TS) of 980 MPa or higher, and excellent formability. Applying the thin steel sheet of the present invention to automobile parts will enable further weight reduction of automobile parts.

[0014] Figures 1(a) to 1(c) are schematic diagrams illustrating the definition of BCC iron in the present invention, which surrounds retained austenite with an equivalent circular diameter of 1 μm or less and an orientation difference of 1° or less.

[0015] The embodiments of the present invention will be described in detail below.

[0016] The thin steel sheet of the present invention will be described in the following order: its component composition and its structural composition. In the following description of the component composition, "%" representing the content of a component means "mass percent".

[0017] C: 0.10% to 0.23% C contributes to increasing the strength of the steel sheet and also has the effect of improving workability by promoting the formation of retained austenite. In order to obtain the tensile strength of 980 MPa or more and the desired hardness of the molten metal part required in this invention, the C content must be at least 0.10%. Preferably it is 0.11% or more. On the other hand, if the C content exceeds 0.23%, BCC iron with less crystalline disorder and fine retained austenite cannot be obtained, so the workability deteriorates. For the above reasons, the C content was set to 0.23% or less. Preferably it is 0.22% or less.

[0018] Si: 1.30% or more and 2.20% or less. Si increases the elongation of steel sheets. Therefore, the Si content is set to 1.30% or more. Preferably, it is 1.35% or more. On the other hand, if Si is added excessively, the chemical treatment properties deteriorate, making it unsuitable as an automotive component. From this viewpoint, the Si content is set to 2.20% or less. Preferably, it is 2.10% or less.

[0019] Mn: 2.0% to 3.2% Mn is an austenite-stabilizing element and is necessary to suppress the persistence of the ferrite phase and obtain a retained austenite area ratio. Therefore, the Mn content is set to 2.0% or more. Preferably, it is 2.1% or more. On the other hand, if the Mn content is excessive, the above effect will saturate and problems will arise with castability and rollability. For the above reasons, the Mn content is set to 3.2% or less. Preferably, it is 3.0% or less.

[0020] P: 0.05% or less. P is a harmful element that reduces weldability. Therefore, it is preferable to reduce the P content as much as possible. In this invention, a P content of up to 0.05% is acceptable. Preferably, it is 0.02% or less. For use under more severe welding conditions, it is even preferable to suppress it to 0.01% or less. On the other hand, due to manufacturing processes, 0.002% may inevitably be mixed in.

[0021] S: 0.005% or less. S forms coarse sulfides in steel, which expand during hot rolling to form wedge-shaped inclusions, adversely affecting weldability. Therefore, since S is also a harmful element, it is preferable to reduce it as much as possible. In this invention, since up to 0.005% is acceptable, the S content is set to 0.005% or less. Preferably, it is 0.003% or less, but for use under more severe welding conditions, it is even more preferable to suppress it to 0.001% or less. Due to manufacturing processes, 0.0002% may inevitably be present.

[0022] Al: 0.005% or more and 0.100% or less. Al is added as a deoxidizing agent during the steelmaking process. For this purpose, the Al content is set to 0.005% or more. On the other hand, if the Al content exceeds 0.100%, the deoxidizing effect becomes saturated and castability deteriorates. From this viewpoint, the Al content is set to 0.100% or less. Preferably, it is 0.085% or less.

[0023] N: 0.0060% or less. N is a harmful element that negatively affects moldability because it worsens room-temperature aging and causes unexpected cracking. Therefore, it is desirable to reduce N as much as possible. In this invention, up to 0.0060% is acceptable. Preferably, it is 0.0050% or less. It is desirable to reduce the N content as much as possible, but due to manufacturing processes, 0.0005% may inevitably be mixed in.

[0024] The thin steel sheet of the present invention contains the above-mentioned basic components, and the remainder of the basic components consists of Fe (iron) and unavoidable impurities. Preferably, the thin steel sheet of the present invention contains the above-mentioned basic components, and the remainder consists of Fe and unavoidable impurities.

[0025] In addition to the basic components described above, the component composition of the present invention may also include the following elements as optional elements.

[0026] The material may contain Sb at a concentration of 0.001% to 0.050% by mass. Sb is an element that helps suppress surface decarburization of steel sheets during high-temperature annealing and ensures stable mechanical properties. To obtain this effect, it is necessary to include at least 0.001% Sb. On the other hand, if the Sb content exceeds 0.050%, the above effect saturates. Therefore, the Sb content was set to 0.050% or less.

[0027] In addition to the above, it may also contain one or more of the following: Ti: 0.001% to 0.1%, Nb: 0.001% to 0.1%, V: 0.001% to 0.3%, Ni: 0.01% to 0.1%, Cr: 0.01% to 1.0%, and B: 0.0002% to 0.0050%.

[0028] Ti and Nb are elements that contribute to increased strength. On the other hand, excessive amounts of these elements inhibit the formation of BCCs with less disorder in the crystal structure due to their pinning effect. Therefore, the Ti and Nb content is preferably 0.001% to 0.1% and 0.001% to 0.1%, respectively.

[0029] Because V has high solubility in steel, it can dissolve to some extent during the high-temperature annealing process targeted by this invention. On the other hand, if added excessively, the pinning effect, similar to that of Ti and Nb, prevents the acquisition of BCC with less disorder in the crystal structure. Therefore, a V content of 0.001% to 0.3% is preferable. More preferably, the lower limit of the total Ti, Nb, and V content is 0.005% or more, and more preferably, the total Ti and Nb content is 0.1% or less.

[0030] Ni, Cr, and B increase hardenability, making it easier to obtain BCC iron with an orientation difference of 1° or less surrounding retained austenite with an equivalent circle diameter of 1 μm or less, as described later. On the other hand, excessive inclusion of these elements can prevent the acquisition of fine retained austenite or saturate the hardenability effect. Therefore, a range of Ni: 0.01% to 0.1%, Cr: 0.01% to 1.0%, and B: 0.0002% to 0.0050% is preferred.

[0031] Furthermore, in addition to the above, one or more of the following elements may be included: Cu: 0.01% to 0.2%, Mo: 0.01% to 1.0%, REM: 0.0002% to 0.050%, Mg: 0.0002% to 0.050%, and Ca: 0.0002% to 0.050%. These elements are used for strength adjustment, inclusion control, etc., but the effects of the present invention are not impaired even if these elements are included within the above ranges.

[0032] Components other than those listed above are Fe and unavoidable impurities. Furthermore, if any of the above-mentioned arbitrary elements are present in amounts below the lower limit, the effects of the present invention are not impaired by the arbitrary elements present in amounts below the lower limit; therefore, any arbitrary elements present in amounts below the lower limit are considered to be unavoidable impurities.

[0033] Next, the structural framework of the thin steel sheet of the present invention will be described.

[0034] Ferrite area ratio: 4% or less (including 0%) In this invention, after allowing sufficient reverse transformation to austenite during annealing, BCC iron with low crystalline disorder containing fine retained austenite is generated in an appropriate fraction by holding at around 450°C, and then rapidly cooled to generate a fine low-temperature transformation phase. For this reason, if the ferrite phase is excessively generated, the formation of the desired steel structure during the holding process will be delayed. Furthermore, since the ferrite generated during annealing is soft, voids are easily formed at the interface with the adjacent hard phase, reducing the bendability. The allowable range in which such effects can be suppressed is 4%, so the ferrite area ratio is set to 4% or less. Preferably it is 3% or less. The ferrite of this invention is polygonal ferrite, and the target structure is one in which corrosion marks or second-phase structures are not included in the grains.

[0035] If the area ratio of as-quenched martensite is 10% or less (including 0%), as-quenched martensite is extremely hard, and during bending, the grain boundaries near the surface become crack initiation points, significantly reducing the bendability. To obtain the bendability required in this invention, the area ratio of as-quenched martensite must be 5% or less. Preferably, it is 3% or less. The lower the area ratio of as-quenched martensite, the better, and it may even be 0%.

[0036] Retained austenite: 7% to 20% Retained austenite improves formability, and to obtain the tensile properties required in this invention, it is necessary to generate 7% or more retained austenite. Therefore, the area ratio of retained austenite was set to 7% or more. Preferably, it is 8% or more. On the other hand, excess retained austenite worsens the delayed fracture properties, so the amount of retained austenite was set to 20% or less. Preferably, it is 17% or less.

[0037] The total proportion of upper bainite, lower bainite, and tempered martensite should be between 71% and 93%. Areas other than those described above should ideally be composed mainly of upper bainite, lower bainite, and tempered martensite. When the steel sheet base is primarily composed of these low-temperature transformation structures, it becomes easier to obtain the desired strength, and the distribution of hardness within the steel structure is narrowed, mitigating localized stress concentration during bending and improving bendability. To effectively achieve these effects, the total proportion should be between 71% and 93%.

[0038] Area ratio of BCC iron with an orientation difference of 1° or less surrounding retained austenite with an equivalent circular diameter of 1 μm or less: 4% to 50%. BCC iron with less crystalline disorder is highly ductile and increases the amount of dislocation strengthening associated with deformation, thereby increasing the amount of work hardening and uniform elongation. One of the features of the present invention is that the BCC iron surrounds retained austenite with an equivalent circular diameter of 1 μm or less, that is, it produces BCC iron with little crystalline disorder that contains fine retained austenite. Here, "surrounding" means surrounding 90% or more of the outer circumference of retained austenite with an equivalent circular diameter of 1 μm or less, as confirmed by the method described in the examples. By adopting this steel structure, in deformations with small strain amounts, BCC iron with small crystalline disorder preferentially deforms, and as dislocations accumulate, the BCC iron hardens, and the retained austenite undergoes plastic-induced transformation, resulting in a high amount of work hardening in the deformation region with high strain amounts, and thus obtaining high resistance to bending and unbending. Furthermore, as the retained austenite transforms into martensite and hardens, the BCC iron with small crystalline disorder surrounding it mitigates local stress concentration due to the hardness difference between different phases, improving bendability. It has been found that if the area ratio of BCC iron surrounding the fine retained austenite is 4%, local stress concentration due to the hardness difference between different phases can be mitigated, and good bendability can be ensured. Therefore, in order to obtain such properties, the area ratio of BCC iron surrounding the fine retained austenite needs to be 4% or more. Preferably it is 5% or more, more preferably 7% or more, and even more preferably 10% or more. On the other hand, if the area ratio exceeds 50%, the desired steel sheet strength cannot be obtained. Therefore, the area ratio of BCC iron with an orientation difference of 1° or less surrounding fine retained austenite was set to 50% or less. Preferably, it was 45% or less. Furthermore, if the equivalent circular diameter of this fine retained austenite exceeds 1 μm, the retained austenite undergoes plastic-induced transformation at a relatively low strain level, and the desired work hardening characteristics cannot be obtained. Therefore, the equivalent circular diameter of retained austenite surrounded by the above-mentioned BCC iron was set to 1 μm or less. By satisfying the steel structure of the present invention, the formation of BCC iron surrounding retained austenite with an equivalent circular diameter exceeding 1 μm is suppressed, and the desired effect is obtained.

[0039] In measuring the area ratio of BCC iron within 1° of orientation difference surrounding retained austenite with an equivalent circle diameter of 1 μm or less, the region with a KAM value of 1° or less is specified by EBSD (electron backscatter diffraction method), and then the region with an average value of the equivalent circle diameter of 1 μm or less is extracted. When the equivalent circle diameter exceeds 1 μm, it is excluded even if the KAM value of BCC iron is 1° or less. When excluding, it is within the range of blocks having the same orientation. Thus, the "orientation difference" can be represented by the "KAM value" measured by the method described in the examples.

[0040] Area ratio of BCC iron with an orientation difference exceeding 1°: 25% or more and 85% or less. The structures with an orientation difference exceeding 1° include lower bainite, martensite, and tempered martensite, etc. They not only contribute to the high strength of the steel sheet, but also further develop fine lower structures within the crystal grains, making the microscopic interfaces a barrier to the propagation of cracks generated during bending. Thus, in addition to the effect of forming the above-mentioned hard and uniform structure, there is a synergistic effect of improving the bendability. To fully obtain such an effect, the area ratio of BCC iron with an orientation difference exceeding 1° needs to exceed 25%. On the other hand, since such structures are inferior in plastic deformation ability, if the area ratio exceeds 85%, the desired formability cannot be obtained. Therefore, the area ratio of BCC iron with an orientation difference exceeding 1° is set to 25% or more and 85% or less. The preferred range is 35% or more and 75% or less.

[0041] The remaining structure is not particularly defined, but as long as the above-mentioned steel structure is achieved, the effects of the invention will not be impaired even if other structures are mixed.

[0042] Next, the manufacturing method of the thin steel sheet of the present invention will be described. The manufacturing method of the thin steel sheet of the present invention includes a hot rolling process, a cold rolling process, and an annealing process. Hereinafter, each process will be described.

[0043] The hot rolling process is a process of hot rolling a steel material having the above component composition.

[0044] The melting method for manufacturing the steel material is not particularly limited, and known melting methods such as a converter, an electric furnace, etc. can be adopted. Further, secondary refining may be performed in a vacuum degassing furnace. Thereafter, it is preferable to obtain a slab (steel material) by a continuous casting method in view of productivity and quality. Also, it may be made into a slab by a known casting method such as an ingot-block rolling method, a thin slab continuous casting method, etc.

[0045] When hot rolling the above steel material, the hot rolling conditions are not particularly limited and may be set as appropriate. For example, the coiling temperature after hot rolling may be 580°C or lower, and more preferably 530°C or lower, from the viewpoint of the shape of the cold rolling coil.

[0046] The cold rolling process is a process of pickling and then cold rolling after the above hot rolling process. In cold rolling, in order to distribute the nucleation of reverse transformation in a high density during the subsequent heating process and promote the reverse transformation to austenite, the cold rolling rate needs to be 46% or more. Preferably it is 50% or more. Although no upper limit is provided, it is substantially 75% or lower for the convenience of the cold rolling load. Here, the pickling conditions are not particularly limited and may be set by a normal method.

[0047] After the cold rolling process and before the annealing process described later, it is more preferable to perform a heat treatment process of heating to 480°C or higher and 650°C or lower and retaining for 1 hour or more in this temperature range. By performing the above heat treatment process, fine cementite precipitates, and based on this as a nucleus, the reverse transformation further proceeds, and it becomes easier to obtain a desired structure.

[0048] The annealing process is a process of heating after the cold rolling process, retaining for 130 seconds or more at 815°C or higher, then cooling from 800°C to 520°C at an average cooling rate of 8°C / s or higher and cooling to 420°C or higher and 520°C or lower and retaining for 12 seconds or more and 60 seconds or less, cooling from 420°C to 300°C at an average cooling rate of 8°C / s or higher and cooling to a cooling stop temperature of 200°C or higher and 350°C or lower, retaining for 2 seconds or more and 25 seconds or less in a temperature range of ±50°C from the cooling stop temperature, then heating to a temperature of 300°C or higher and 500°C or lower, and retaining for 480 seconds or more and 1800 seconds or less in this temperature range.

[0049] Heating temperature: 815°C or higher Residence time: 130 seconds or higher This heating and residence process allows the reverse transformation to austenite to proceed sufficiently, creating a base for forming BCC iron with an orientation difference of 1° or less surrounding the retained austenite and BCC iron with an orientation difference greater than 1° in an appropriate balance. If the reverse transformation to austenite does not proceed sufficiently, the formation of BCC iron with an orientation difference of 1° or less surrounding the retained austenite will be insufficient, and the fraction of BCC iron with an orientation difference greater than 1° will also decrease, resulting in poor resistance to bending and unbending. To obtain the desired austenite, it is necessary to reside at 815°C or higher for 130 seconds or more. Preferably, reside at 830°C or higher for 130 seconds or more, and more preferably at 850°C or higher for 140 seconds or more. There is no particular upper limit to the heating temperature, but it is preferable to keep it below 900°C due to the risk of thermal damage to the heating furnace. There is no particular upper limit to the residence time, but it is preferable to keep it below 350 seconds from the viewpoint of productivity.

[0050] Average cooling rate from 800°C to 520°C: 8°C / s or more Cooling stop temperature: 420°C to 520°C After heating, it is necessary to suppress the formation of polygonal ferrite. If polygonal ferrite is formed during this period, BCC iron with small crystalline disorder containing fine retained austenite cannot be obtained, and the desired steel sheet properties cannot be obtained. From this viewpoint, the average cooling rate from 800°C to 520°C, which is the polygonal ferrite formation region, is set to 8°C / s or more. Preferably it is 10°C / s or more. There is no particular upper limit set for the above average cooling rate, but it is substantially 150°C / s or less.

[0051] To suppress polygonal ferrite formation and produce BCC iron with minimal crystalline disorder surrounding fine retained austenite, cooling to between 420°C and 520°C is necessary. Below 420°C, martensitic transformation proceeds, increasing crystalline disorder and preventing the acquisition of the desired steel microstructure. Therefore, the cooling stop temperature is set to 420°C or higher, preferably 450°C or higher. Above 520°C, the formation of polygonal ferrite prevents the acquisition of fine retained austenite. Therefore, the cooling stop temperature is set to 520°C or lower.

[0052] Residence time in the temperature range of 420°C to 520°C: 12 seconds to 60 seconds. Residence time of 12 seconds to 60 seconds in the temperature range of 420°C to 520°C generates BCC iron with small crystalline disorder surrounding the fine retained austenite. If the residence temperature falls below 420°C, or if the residence time in the temperature range of 420°C to 520°C falls below 12 seconds, sufficient BCC iron with small crystalline disorder surrounding the fine retained austenite cannot be obtained. Preferably, it is 15 seconds or more. On the other hand, if it exceeds 520°C, the desired retained austenite cannot be obtained. If the residence time in the temperature range of 420°C to 520°C exceeds 60 seconds, excessive BCC iron with small crystalline disorder is generated, and the desired tensile strength of 980 MPa cannot be obtained. The preferred range is residence time of 20 seconds to 55 seconds at a temperature of 430°C to 505°C. Furthermore, during this retention period, temperature changes are acceptable as long as they remain within the above temperature range, or isothermal maintenance is also acceptable.

[0053] Average cooling rate from 420°C to 300°C: 8°C / s or higher Cooling stop temperature: 200°C to 350°C In order to refine the steel structure generated during the cooling process and promote the formation of BCC iron with an orientation difference of more than 1°, it is necessary to cool from 420°C to 300°C at an average cooling rate of 8°C / s or higher. If it is below 8°C / s, the refinement of the lower structure will be suppressed, and the formation of BCC iron with an orientation difference of more than 1° will be insufficient. Preferably it is 10°C / s or higher. There is no particular upper limit to the average cooling rate.

[0054] After cooling, the cooling is stopped in the temperature range of 200°C to 350°C. Preferably, it is between 230°C and 330°C. If the cooling stop temperature falls below 200°C, the austenite present in the steel sheet will undergo martensitic transformation, making it impossible to obtain the desired amount of retained austenite.

[0055] The material remains in a temperature range of ±50°C from the cooling stop temperature for 2 to 25 seconds. In addition, lower bainite transformation progresses in the temperature range up to 50°C lower than the cooling stop temperature. This lower bainite transformation reduces the amount of untransformed austenite, which in turn reduces the amount of as-quenched martensite and improves flexibility. To obtain this effect, it is necessary to remain in the temperature range of ±50°C from the cooling stop temperature for 2 to 25 seconds, from the time of cooling to the cooling stop temperature of 200°C to 350°C until reheating. If it is less than 2 seconds, the lower bainite transformation will not progress sufficiently and the desired effect will not be obtained, and if it exceeds 25 seconds, the effect will not only saturate but will also cause fluctuations in the reheating effect of the next process, resulting in large fluctuations in material, especially strength. Preferably, it is 3 to 20 seconds.

[0056] Heating temperature: 300°C to 500°C Residence time in the temperature range of 300°C to 500°C: 480 seconds to 1800 seconds The purpose of residence in the temperature range of 300°C to 500°C is to concentrate carbon in the retained austenite, to remain as retained austenite when cooled to room temperature, and to temper the portion that underwent martensitic transformation during heating. If the residence temperature is below 300°C or the residence time is below 480 seconds, the retained austenite will not be concentrated, and the thermally unstable austenite will undergo martensitic transformation when cooled to room temperature, so the desired amount of retained austenite will not be obtained. Furthermore, the tempering of the hard, as-quenched martensite will not proceed sufficiently. On the other hand, if the residence temperature is above 500°C or the residence time is above 1800 seconds, cementite will precipitate and decompose in the austenite, so the desired amount of retained austenite will not be obtained. Furthermore, if tempering proceeds excessively, the desired strength will be obtained. Therefore, during reheating after cooling from 200°C to 350°C, the mixture was kept at a temperature of 300°C to 500°C for 480 seconds to 1800 seconds.

[0057] Steel materials with a thickness of 250 mm having the component composition shown in Table 1 were subjected to hot rolling, pickling, and cold rolling. After annealing in a continuous annealing furnace under the conditions shown in Table 2, temper rolling was performed to an elongation rate of 0.2% to 0.4% to produce steel sheets for evaluation. Some materials underwent a heat treatment process in a box-type annealing furnace before cold rolling or before the annealing process. The resulting steel sheets were then evaluated using the following methods.

[0058] (i) Microstructure observation (area ratio of metallic microstructure) A section of the steel plate was cut out so that the cross-section parallel to the rolling direction was the observation surface. The center of the plate thickness was etched with 1% nital, and 10 fields of view were photographed at a depth of 1 / 4 of the plate thickness from the surface of the steel plate (hereinafter simply referred to as the 1 / 4t portion) using a scanning electron microscope at 2000x magnification. Ferrite is a microstructure in which no corrosion traces or second phase structures are observed within the grains. Upper bainite is a microstructure in which corrosion traces and second phase structures are observed within the grains, while tempered martensite and lower bainite are microstructures in which lath structures and fine second phase structures are observed within the grains. The sum of the microstructures of upper bainite, lower bainite, and tempered martensite was calculated as the sum of the area ratios of all of the above.

[0059] For measuring BCC iron surrounding retained austenite with a circular equivalent diameter of 1 μm or less, EBSD was used on the same cross-section as observed by SEM. Specifically, 1 × 10 in the 1 / 4t thickness section. ３ μm ２ The above regions were measured with a measurement step of 0.1 μm. For crystalline structure disorder, BCC iron with a KAM (Kernel Average Misorientation) value of 1° or less was determined by the KAM method, and retained austenite was identified using a phase map.

[0060] For the measurement of area fractions, the sectioning method was employed for both SEM and EBSD images. Twenty horizontal and twenty vertical lines, each with a real length of 30 μm, were drawn in a grid pattern on the obtained images. The microstructure at the intersections was identified, and the ratio of the number of intersections for each microstructure to the total number of intersections was defined as the area fraction of each microstructure. Here, for each measurement point, BCC iron with a KAM value of 1° or less that does not cross large-angle grain boundaries with an orientation difference of 15° or more, does not cross BCC iron with a KAM value exceeding 1°, surrounds retained austenite with an equivalent circular diameter of 1 μm or less, or is in contact with more than 90% of the circumference of retained austenite, was identified as BCC iron with an orientation difference of 1° or less surrounding retained austenite with an equivalent circular diameter of 1 μm or less. According to the above definition, items that meet (a) and (b) below are outside the range of BCC iron with an orientation difference of 1° or less surrounding retained austenite with an equivalent circular diameter of 1 μm or less, as defined above, and only BCC iron that meets (c) below is within the range of the above definition. (a) BCC iron in which retained austenite with an equivalent circle diameter of 1 μm or less is in contact with two BCC iron crystal grains across a large-angle grain boundary with an orientation difference of 15° or more, and the length of the boundary between the BCC iron in the two regions and the retained austenite with an equivalent circle diameter of 1 μm or less is greater than 10% of the total circumference of the retained austenite with an equivalent circle diameter of 1 μm or less. (b) BCC iron containing retained austenite with an equivalent circle diameter of 1 μm or less and adjacent BCC iron crystal grains with a KAM value of 1° or more. (c) BCC iron in which retained austenite with an equivalent circle diameter of 1 μm or less is in contact with two BCC iron crystal grains across a large-angle grain boundary with an orientation difference of 15° or more, but the length of the boundary between the BCC iron in the two regions and the retained austenite with an equivalent circle diameter of 1 μm or less is greater than 10% of the total circumference of the retained austenite with an equivalent circle diameter of 1 μm or less. Schematic diagrams of (a) to (c) above are shown in Figure 1. Furthermore, when calculating the area ratio of BCC iron with an orientation difference of more than 1° surrounding retained austenite with a circular equivalent diameter of 1 μm or less, the formula should be 100% - (area ratio of BCC iron with an orientation difference of 1° or less surrounding retained austenite with a circular equivalent diameter of 1 μm or less + area ratio of blocks surrounding retained austenite with a circular equivalent diameter of 1 μm or more + area ratio of retained austenite or volume ratio obtained by XRD).

[0061] (ii) Measurement of retained austenite fraction by XRD After polishing the steel plate to a position 1 / 4 of its thickness, the surface was further polished by 0.1 mm by chemical polishing. Using an X-ray diffractometer with Mo Kα rays, the integrated reflectance intensities of the (200), (220), and (311) faces of FCC iron (austenite) and the (200), (211), and (220) faces of BCC iron (ferrite) were measured. The percentage of austenite obtained from the intensity ratio of the integrated reflectance intensity from each face of FCC iron (austenite) to the integrated reflectance intensity from each face of BCC iron (ferrite) was defined as the retained austenite fraction.

[0062] (iii) Tensile Test A JIS No. 5 tensile test specimen was prepared from the obtained steel sheet perpendicular to the rolling direction, and a tensile test was performed five times in accordance with the provisions of JIS Z 2241 (2011). The average tensile strength (TS), uniform elongation (U-El), and total elongation (El) were determined. The crosshead speed for the tensile test was set to 10 mm / min. In Table 3, the mechanical properties of the steel sheet obtained from the present invention were defined as having a tensile strength of 980 MPa or more and a product of TS and U-El ​​of 12000 MPa·% or more.

[0063] Furthermore, to achieve good formability, it is effective to prevent necking when subjected to harsh processing and to suppress necking and cracking by distributing the strain. In this invention, as a material that can withstand harsh processing such as bending and unbending, such as roll forming, the preferred range for suppressing necking and cracking is a product of uniform elongation and tensile strength of 12,000 MPa·% or more, and a value of dσ / dε divided by tensile strength at 80% of ε satisfying the plastic instability condition (dσ / dε = 0) on the true stress (σ)-true strain (ε) curve is 1.3 or more.

[0064] (iv) Bending Test To investigate the bendability, strip-shaped samples measuring 100 mm in width and 35 mm in length were cut out, and bending tests were conducted using the V-block method with a 90° apex angle in accordance with JIS Z 2248. The minimum die diameter (R) at which cracking did not occur was determined, and the limit bending radius (R / t) was calculated by dividing this by the plate thickness (t). The preferred range for this was set to 1.5 or less.

[0065] All of the examples of the present invention demonstrated a tensile strength (TS) of 980 MPa or higher, indicating good formability. Furthermore, the examples of the present invention in which the area ratio of BCC iron surrounding fine retained austenite was 4% or higher showed good uniform elongation (U-El), total elongation (El), work hardening amount, and bendability even at a tensile strength (TS) of 980 MPa or higher. On the other hand, the comparative examples that fell outside the scope of the present invention either did not reach a tensile strength of 980 MPa or did not obtain the work hardening amount or bendability required by the present invention.

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