JP5708320B2 - Cold rolled steel sheet - Google Patents

Description

  The present invention relates to a cold-rolled steel sheet. More specifically, the present invention relates to a high-tensile cold-rolled steel sheet that is excellent in ductility, work hardenability, and stretch flangeability.

  Now that the industrial technology field is highly fragmented, special and advanced performance is required for materials used in each technical field. For example, even cold-rolled steel sheets used by press forming are required to have better formability with the diversification of press shapes. In addition, high strength is required, and application of high-tensile cold-rolled steel sheets is being studied. In particular, regarding automotive steel sheets, in consideration of the global environment, the demand for thin-walled, high-formability, high-tensile cold-rolled steel sheets has been significantly increased in order to reduce the weight of the vehicle body and improve fuel efficiency. In press molding, since the thinner the steel sheet used, the easier it is to crack and wrinkle, a steel sheet with better ductility and stretch flangeability is required. However, such press formability and high strength of the steel sheet are contradictory properties, and it is difficult to satisfy these properties at the same time.

Until now, as a method for improving the press formability of a high-tensile cold-rolled steel sheet, many techniques relating to micronization of the microstructure have been proposed. For example, Patent Document 1 discloses a method for producing an ultrafine-grained high-strength hot-rolled steel sheet that performs rolling with a total rolling reduction of 80% or more in a temperature range near the Ar ₃ point in a hot rolling process. No. 2 discloses a method for producing ultrafine-grained ferritic steel in which rolling at a rolling reduction of 40% or more is continuously performed in the hot rolling step.

These techniques improve the balance between strength and ductility in a hot-rolled steel sheet, but there is no description in the above-mentioned patent document regarding a method for making a cold-rolled steel sheet finer and improving press formability. According to the study by the present inventors, when cold rolling and annealing are performed using a fine-grained hot-rolled steel sheet obtained by rolling under large rolling as a base material, the crystal grains are likely to be coarsened and have excellent press formability. It is difficult to get. In particular, in the manufacture of a cold-rolled steel sheet having a microstructure that includes a low-temperature transformation generation phase and residual austenite that needs to be annealed in a high temperature range of Ac ₁ point or higher, coarsening of crystal grains during annealing is remarkable. In addition, the advantage of the cold-rolled steel sheet having excellent ductility cannot be enjoyed.

  Patent Document 3 discloses a method for producing a hot-rolled steel sheet having ultrafine grains, in which a reduction in a dynamic recrystallization region is performed by a reduction pass of 5 stands or more in a hot rolling process. However, it is necessary to extremely reduce the temperature drop during hot rolling, and it is difficult to carry out with normal hot rolling equipment. Moreover, although the example which performed cold rolling and annealing after hot rolling is shown, the balance of tensile strength and hole expansibility is bad, and press formability is inadequate.

  Regarding cold-rolled steel sheets having a microstructure, in Patent Document 4, residual austenite having an average crystal grain size of 5 μm or less is dispersed in ferrite having an average crystal grain size of 10 μm or less. An excellent high strength cold rolled steel sheet for automobiles is disclosed. A steel sheet containing retained austenite in the metal structure exhibits a large elongation due to transformation-induced plasticity (TRIP) generated by austenite becoming martensite during processing, but the hole expandability is impaired by the formation of hard martensite. In the cold-rolled steel sheet disclosed in Patent Document 4, ductility and hole expandability are improved by refining ferrite and retained austenite, but the hole expansion ratio is 1.5 at most, and sufficient press It is hard to say that it has moldability. Further, in order to improve the work hardening index and improve the collision resistance safety, the main phase needs to be a soft ferrite phase, and it is difficult to obtain a high tensile strength.

  Patent Document 5 discloses a high-strength steel sheet excellent in elongation and stretch flangeability in which a second phase composed of retained austenite and / or martensite is finely dispersed in crystal grains. However, in order to refine the second phase to the nano size and disperse it in the crystal grains, it is necessary to contain a large amount of expensive elements such as Cu and Ni and to perform a solution treatment for a long time at a high temperature. There is a marked increase in cost and productivity.

  Patent Document 6 discloses a high-tensile melt excellent in ductility, stretch flangeability, and fatigue resistance, in which retained austenite and low-temperature transformation product phase are dispersed in ferrite and tempered martensite having an average crystal grain size of 10 μm or less. A galvanized steel sheet is disclosed. Tempered martensite is an effective phase for improving stretch-flangeability and fatigue resistance, and it is said that these properties will be further improved when tempered martensite is refined. However, in order to obtain a metal structure containing tempered martensite and retained austenite, primary annealing to generate martensite and secondary annealing to temper martensite and further obtain retained austenite are required. Productivity is greatly impaired.

  In Patent Document 7, in a fine ferrite, which is rapidly cooled to 720 ° C. or less immediately after hot rolling, held in a temperature range of 600 to 720 ° C. for 2 seconds or more, and subjected to cold rolling and annealing on the obtained hot rolled steel sheet. Discloses a method for producing a cold-rolled steel sheet in which retained austenite is dispersed.

JP 58-123823 A JP 59-229413 A Japanese Patent Laid-Open No. 11-152544 JP-A-11-61326 JP 2005-179703 A JP 2001-192768 A International Publication No. 2007/15541 Pamphlet

  The technique disclosed in the above-mentioned Patent Document 7 does not release the processing strain accumulated in the austenite after the hot rolling is completed, and transforms the ferrite using the processing strain as a driving force, thereby forming a fine grain structure. And it is excellent in that a cold-rolled steel sheet with improved thermal stability can be obtained.

However, in recent years, there has been a demand for cold-rolled steel sheets that simultaneously have high strength, good ductility, good work hardenability, and good stretch flangeability due to the need for higher performance.
The present invention has been made to solve such problems. Specifically, an object of the present invention is to provide a high-tensile cold-rolled steel sheet having excellent ductility, work-hardening properties, and stretch flangeability and having a tensile strength of 780 MPa or more.

  The present inventors conducted a detailed investigation on the influence of chemical composition and production conditions on the mechanical properties of high-tensile cold-rolled steel sheets. In this specification, “%” indicating the content of each element in the chemical composition means mass%.

  A series of test steels are in mass%, C: less than 0.30%, Si: 3.00% or less, Mn: more than 1.00%, 3.50% or less, P: 0.10% or less, S: 0.010% or less, sol. It had a chemical composition containing Al: 2.00% or less and N: 0.010% or less.

A slab having such a chemical composition is heated to 1200 ° C., then hot-rolled to a thickness of 2.0 mm in various reduction patterns in a temperature range of Ar ₃ or higher, and after hot rolling, various cooling conditions are applied. After cooling to a temperature range of 720 ° C. or less and air-cooling for 5 to 10 seconds, it was cooled to various temperatures at a cooling rate of 90 ° C./s or less, and this temperature was taken as the coiling temperature, and kept at the same temperature After charging in an electric heating furnace and holding for 30 minutes, the furnace was cooled at a cooling rate of 20 ° C./h to simulate slow cooling after winding. The obtained hot-rolled steel sheet was pickled and cold-rolled to a thickness of 1.0 mm at a rolling rate of 50%. The obtained cold-rolled steel sheet was heated to various temperatures using a continuous annealing simulator, held for 95 seconds, and then cooled to obtain an annealed steel sheet.

  Samples for structure observation were collected from hot-rolled steel sheets and annealed steel sheets, and ¼ of the plate thickness from the steel sheet surface using a scanning electron microscope (SEM) equipped with an optical microscope and an electron beam backscattering pattern analyzer (EBSP). While observing the metal structure at the depth position, the volume fraction of retained austenite was measured at the 1/4 depth position from the steel sheet surface of the annealed steel sheet using an X-ray diffractometer (XRD). In addition, a tensile test piece is taken from the annealed steel sheet along a direction orthogonal to the rolling direction, a tensile test is performed, ductility is evaluated by total elongation, and work hardening index is a work hardening index having a strain range of 5 to 10% ( n value). Furthermore, a 100 mm square hole expansion test piece was sampled from the annealed steel sheet, a hole expansion test was performed, and stretch flangeability was evaluated. In the hole expansion test, a punching hole having a diameter of 10 mm with a clearance of 12.5% is formed, and the punching hole is expanded with a conical punch having a tip angle of 60 °. (Expansion rate) was measured.

As a result of these preliminary tests, the following findings (A) to (H) were obtained.
(A) A hot-rolled steel sheet manufactured through a so-called immediate quenching process in which water quenching is performed immediately after hot rolling, specifically, quenching to a temperature range of 720 ° C. or less within 0.40 seconds after completion of hot rolling. When the hot-rolled steel sheet manufactured by cold-rolling is annealed by cold rolling, the ductility and stretch flangeability of the cold-rolled steel sheet improve as the annealing temperature rises, but if the annealing temperature is too high, the austenite grains become coarse, The ductility and stretch flangeability of an annealed steel sheet may deteriorate rapidly.

  (B) When the final reduction amount of hot rolling is increased, coarsening of austenite grains that may occur when annealing at high temperature after cold rolling is suppressed. The reason for this is not clear, but (a) the higher the final reduction amount, the higher the ferrite fraction in the metal structure of the hot-rolled steel sheet, and the more fine the ferrite, (b) the higher the final reduction amount, the hot rolling. Coarse low-temperature transformation formation phase in the metallographic structure of the steel sheet decreases. (C) The ferrite grain boundary functions as a nucleation site in the transformation from ferrite to austenite during annealing. It is presumed that the frequency increases and the austenite becomes finer, and (d) the coarse low-temperature transformation generation phase becomes coarse austenite grains during annealing.

  (C) In the winding process immediately after the rapid cooling, if the winding temperature is increased, austenite grain coarsening that may occur when annealing at a high temperature after cold rolling is suppressed. Although the reason for this is not clear, (a) immediately after the rapid cooling, the hot-rolled steel sheet becomes finer, so that the precipitation amount of iron carbide in the hot-rolled steel sheet increases remarkably as the coiling temperature rises. (B) Since the iron carbide functions as a nucleation site in the transformation from ferrite to austenite during annealing, the nucleation frequency increases as the precipitation amount of iron carbide increases, and the austenite becomes finer, (c ) It is estimated that the undissolved iron carbide is caused by the austenite becoming finer in order to suppress the austenite grain growth.

  (D) As the Si content in the steel increases, the effect of preventing coarsening of austenite grains becomes stronger. The reason for this is not clear, but (a) as the Si content increases, the iron carbide becomes finer and its number density increases. (B) Thereby, the nucleation frequency in the transformation from ferrite to austenite is increased. It is presumed to be caused by the further increase and (c) the increase in undissolved iron carbide further suppresses the grain growth of austenite and further refines the austenite.

  (E) When cooling by soaking at a high temperature while suppressing the coarsening of austenite grains, a metal structure containing a fine low-temperature transformation generation phase as a main phase and fine residual austenite in the second phase is obtained.

  FIG. 1 shows that the final reduction amount is 42% in terms of sheet thickness reduction rate, the rolling completion temperature is 900 ° C., the quenching stop temperature is 660 ° C., and the time from the completion of rolling to the quenching stop is 0.16 seconds. It is a graph which shows the result of having investigated the particle size distribution of a retained austenite in the annealing steel plate obtained by cold-rolling a hot-rolled steel plate at 520 degreeC, and annealing at a soaking temperature of 850 degreeC. FIG. 2 shows the results of investigating the particle size distribution of retained austenite in an annealed steel sheet obtained by hot rolling a slab having the same chemical composition immediately after being subjected to hot rolling by a conventional method without performing rapid cooling. It is a graph to show. From the comparison of FIGS. 1 and 2, in the annealed steel sheet (FIG. 1) manufactured through an appropriate immediate quenching process, the formation of coarse retained austenite grains having a grain size of 1.2 μm or more is suppressed, and the retained austenite becomes finer. It can be seen that they are dispersed.

(F) By suppressing the formation of coarse residual austenite grains having a grain size of 1.2 μm or more, the stretch flangeability of a steel sheet having a low-temperature transformation generation phase as a main phase is improved.
FIG. 3 is a graph showing the relationship between TS ^1.7 × λ and the number density (N _R ) of coarse residual austenite grains having a grain size of 1.2 μm or more. TS is the tensile strength, λ is the hole expansion rate, and TS ^1.7 × λ is an index for evaluating the hole expansion property from the balance between the strength and the hole expansion rate. As shown in the figure, TS ^1.7 × λ has a correlation with N _R, and it can be seen that the lower the N _R , the better the hole expandability. The reason for this is not clear, but (a) retained austenite changes to hard martensite by processing, but if retained austenite grains are coarse, the martensite grains also become coarse, stress concentration increases, (B) Since coarse retained austenite grains are martensite in the initial stage of processing, they tend to crack more than fine retained austenite grains. It is estimated that

  (G) As the annealing temperature rises, the fraction of the low-temperature transformation generation phase increases and the work hardening tends to deteriorate, but the formation of coarse residual austenite grains having a particle size of 1.2 μm or more is suppressed. Thus, it is possible to prevent the work hardenability from deteriorating in the steel sheet whose main phase is the low temperature transformation generation phase.

FIG. 4 is a graph showing the relationship between TS × n value and N _R. The TS × n value is an index for evaluating work hardening from the balance between strength and work hardening index. As shown in the figure, the TS × n value has a correlation with N _R, and it can be seen that the lower the N _R , the better the work hardenability. The reason for this is not clear, but (a) coarse residual austenite grains are martensiticized at the initial stage of processing when the strain is less than 5%, so that the increase of the n value is almost in the range of 5 to 10%. This is presumably due to the fact that no contribution is made and (b) when the formation of coarse residual austenite grains is suppressed, fine residual austenite grains that become martensite in a high strain region of 5% or more increase.

  (H) Grain having a bcc (body-centered cubic) structure and grain having a bct (body-centered tetragonal) structure surrounded by a grain boundary having an orientation difference of 15 ° or more (hereinafter, these two kinds of grains are collectively referred to as “bcc grains”. )), The ductility, work hardenability and stretch flangeability of a steel sheet having a metal structure containing a low-temperature transformation generation phase as a main phase and residual austenite in the second phase are improved. The reason for this is not clear, but it is due to the fact that (a) the arrangement of retained austenite is optimized by making the bcc grains finer, and (b) the crack extension is suppressed by making the bcc grains finer. It is estimated that.

  From the above results, after hot rolling the steel containing a certain amount or more of Si, increasing the final reduction amount, immediately after quenching, coiled at high temperature, coiled cold, and annealed at high temperature , By cooling, the main phase is a low-temperature transformation generation phase, the second phase contains residual austenite, there are few coarse residual austenite grains having a grain size of 1.2 μm or more, and bcc grains are fine grains. It has been found that a cold-rolled steel sheet having excellent ductility, work hardening characteristics and stretch flangeability can be produced.

The present invention completed based on the above knowledge is as follows.
(1) By mass%, C: more than 0.020% and less than 0.30%, Si: more than 0.10% and 3.00% or less, Mn: more than 1.00% and 3.50% or less, P: 0.00. 10% or less, S: 0.010% or less, sol. A steel plate having a chemical composition comprising Al: 2.00% or less and N: 0.010% or less, the balance being Fe and impurities, the main phase being a low-temperature transformation product phase, and the second phase being retained austenite The residual austenite has a volume ratio of more than 4.0% to less than 25.0%, an average particle size of less than 0.80 μm, and a particle size of 1.2 μm or more. The number density of austenite grains is 3.0 × 10 ⁻² particles / μm ² or less, and the average grain size of grains having a bcc structure and grains having a bct structure surrounded by grain boundaries having an orientation difference of 15 ° or more is 7 cold-rolled steel sheet .0μm Ri der below, tensile strength, characterized in der Rukoto least 780 MPa.

  (2) The chemical composition is selected from the group consisting of Ti: less than 0.050%, Nb: less than 0.050%, and V: 0.50% or less, in mass%, instead of part of Fe. The cold-rolled steel sheet according to the above (1), which contains one type or two or more types.

  (3) The chemical composition is selected from the group consisting of Cr: 1.0% or less, Mo: 0.50% or less, and B: 0.010% or less in mass% instead of a part of Fe. The cold-rolled steel sheet according to (1) or (2) above, which contains one kind or two or more kinds.

  (4) The chemical composition is mass% instead of part of Fe, Ca: 0.010% or less, Mg: 0.010% or less, REM: 0.050% or less, and Bi: 0.050% The cold-rolled steel sheet according to any one of (1) to (3) above, which contains one or more selected from the group consisting of:

  ADVANTAGE OF THE INVENTION According to this invention, the high tension cold-rolled steel plate which has sufficient ductility applicable to processes, such as press molding, work hardening property, and stretch flangeability, is provided. Therefore, the present invention greatly contributes to industrial development, such as being able to contribute to solving global environmental problems through weight reduction of automobile bodies.

It is a graph which shows the particle size distribution of the retained austenite in the annealed steel plate manufactured through the rapid cooling process immediately after. It is a graph which shows the particle size distribution of the retained austenite in the annealed steel plate manufactured without passing through a rapid cooling process immediately after. TS ^1.7 × lambda and particle size is a graph showing the relationship between the number density of the residual austenite is 1.2μm or more (N _R). It is a graph which shows the relationship between the TS * n value and the number density (N _R ) of retained austenite having a particle size of 1.2 μm or more.

  The metallographic structure and chemical composition of the high-tensile cold-rolled steel sheet according to the present invention and the rolling and annealing conditions in the production method capable of producing the steel sheet efficiently, stably and economically will be described in detail below.

1. Metal structure The cold-rolled steel sheet of the present invention has a main phase that is a low-temperature transformation generation phase, and the second phase contains residual austenite. The residual austenite has a volume ratio of more than 4.0% to 25.0% with respect to the entire structure. The number density of residual austenite grains having an average particle size of less than 0.80 μm and a particle size of 1.2 μm or more is 3.0 × 10 ⁻² particles / μm ² or less, and the orientation difference is 15 ° or more. The grains having the bcc structure surrounded by the grain boundaries and the grains having the bct structure have a metal structure having an average grain size of 7.0 μm or less.

The main phase means a phase or structure having the largest volume fraction, and the second phase means a phase and structure other than the main phase.
The reason why the main phase is a low-temperature transformation generation phase and the second phase is a structure containing residual austenite is that it is suitable for improving ductility, work hardenability and stretch flangeability while maintaining tensile strength. . When the main phase is polygonal ferrite that is not a low-temperature transformation generation phase, it is difficult to ensure tensile strength and stretch flangeability.

  The low temperature transformation generation phase refers to a phase and structure generated by low temperature transformation such as martensite and bainite. Examples of low-temperature transformation generation phases other than these include bainitic ferrite and tempered martensite. Bainitic ferrite is distinguished from polygonal ferrite in that it has a lath-like or plate-like form and a high dislocation density, and is distinguished from bainite in that iron carbide is not present inside and at the interface. This low temperature transformation product phase may contain two or more phases or structures, for example, martensite and bainitic ferrite. When the low-temperature transformation generation phase includes two or more kinds of phases or structures, the total volume ratio of these phases and structures is defined as the volume ratio of the low-temperature transformation generation phase.

  The bcc phase is a phase having a body-centered cubic lattice (bcc lattice) type crystal structure, and examples thereof include polygonal ferrite, bainitic ferrite, bainite, and tempered martensite. On the other hand, the bct phase is a phase having a body-centered tetragonal lattice (bct) type crystal structure, and is exemplified by martensite. A grain having a bcc structure is a region surrounded by a boundary having an orientation difference of 15 ° or more in the bcc phase. Similarly, a grain having a bct structure is a region surrounded by a boundary having an orientation difference of 15 ° or more in the bct phase. Hereinafter, the bcc phase and the bct phase are collectively referred to as a bcc phase.

  The volume ratio of the retained austenite to the entire structure is more than 4.0% and less than 25.0%. If the volume ratio of the retained austenite to the entire structure is 4.0% or less, the ductility becomes insufficient. Therefore, the volume ratio of the retained austenite with respect to the whole structure | tissue shall be over 4.0%. This volume fraction is preferably more than 6.0%, more preferably more than 9.0%, particularly preferably more than 12.0%. On the other hand, when the volume ratio of the retained austenite with respect to the entire structure is 25.0% or more, the stretch flangeability is significantly deteriorated. Accordingly, the volume ratio of the retained austenite with respect to the entire structure is set to less than 25.0%. Preferably it is less than 18.0%, more preferably less than 16.0%, particularly preferably less than 14.0%.

  The average particle size of retained austenite is less than 0.80 μm. In a cold-rolled steel sheet having a metal structure containing a low-temperature transformation generation phase and a secondary austenite in the second phase, if the average grain size of the residual austenite is 0.80 μm or more, ductility, work hardenability and stretch flangeability Deteriorates significantly. The average particle size of retained austenite is preferably less than 0.70 μm, and more preferably less than 0.60 μm. The lower limit of the average particle size of the retained austenite is not particularly limited, but in order to make it finer to 0.15 μm or less, it is necessary to make the final reduction amount of hot rolling very high, and the production load is remarkably increased. Therefore, the lower limit of the average particle size of retained austenite is preferably more than 0.15 μm.

In a cold-rolled steel sheet having a metal structure including a low-temperature transformation generation phase as a main phase and a residual austenite in the second phase, the particle size is 1.2 μm or more even if the average particle size of the residual austenite is less than 0.80 μm. If there are many coarse residual austenite grains, work hardening and stretch flangeability are impaired. Therefore, the number density of residual austenite grains having a grain size of 1.2 μm or more is set to 3.0 × 10 ⁻² particles / μm ² or less. The number density of residual austenite grains having a particle size of 1.2 μm or more is preferably 2.0 × 10 ⁻² particles / μm ² or less. 1.5 × 10 ⁻² pieces / μm ² or less is more preferable, and 1.0 × 10 ⁻² pieces / μm ² or less is particularly preferable.

  In order to further improve ductility, work hardenability and stretch flangeability, bcc grains (as described above, bcc grains have bcc structures and bct structures surrounded by grain boundaries having an orientation difference of 15 ° or more. The average particle diameter of the general term “grain” is 7.0 μm or less. The average particle size of the bcc particles is more preferably 6.0 μm or less, and particularly preferably 5.0 μm or less.

  In order to further improve the ductility and work hardenability, the volume fraction of polygonal ferrite with respect to the entire structure is preferably more than 2.0%. More preferably, it is more than 8.0%, particularly preferably more than 13.0%. On the other hand, when the volume fraction of polygonal ferrite becomes excessive, stretch flangeability deteriorates. Therefore, the volume fraction of polygonal ferrite is preferably less than 27.0%. More preferably, it is less than 24.0%, particularly preferably less than 18.0%.

  In order to further improve the stretch flangeability, the volume ratio of tempered martensite contained in the low-temperature transformation generation phase is preferably less than 50.0% with respect to the entire structure. More preferably, it is less than 35.0%, particularly preferably less than 10.0%.

  In order to increase the tensile strength, the low temperature transformation generation phase preferably contains martensite. In this case, the volume ratio of the martensite to the entire structure is preferably more than 4.0%. More preferably, it is more than 6.0%, particularly preferably more than 10.0%. On the other hand, when the volume ratio of martensite becomes excessive, stretch flangeability deteriorates. For this reason, it is preferable that the volume ratio of martensite in the whole structure is less than 15.0%.

  The metal structure of the cold rolled steel sheet according to the present invention is measured as follows. That is, the volume ratio of the low-temperature transformation generation phase and polygonal ferrite is obtained by taking a test piece from a steel plate, polishing a longitudinal section parallel to the rolling direction, and subjecting it to a corrosion treatment with nital. The metal structure is observed using the SEM at the depth position, and the area ratios of the low-temperature transformation generation phase and the polygonal ferrite are measured by image processing, and the respective volume ratios are obtained assuming that the area ratio is equal to the volume ratio.

The volume fraction of retained austenite is obtained by taking a test piece from a steel plate, chemically polishing the rolled surface from the steel plate surface to a 1/4 depth position of the plate thickness, and measuring the X-ray diffraction intensity using XRD.
The particle size distribution of retained austenite grains and the average particle size of retained austenite are measured as follows. That is, a test piece is taken from a steel plate, a longitudinal section parallel to the rolling direction is electropolished, and the metal structure is observed using an SEM equipped with EBSP at a position of a depth of the plate thickness from the steel plate surface. Observed as a phase consisting of a face-centered cubic crystal structure (fcc phase), and the region surrounded by the parent phase is defined as one retained austenite grain, and by image processing, the number density of retained austenite grains (grains per unit area) Number) and the area ratio of individual retained austenite grains. The circle equivalent diameter of each austenite grain is determined from the area occupied by each retained austenite grain in the field of view, and the average value thereof is taken as the average grain size of the retained austenite.

  In the structure observation by EBSP, the phase is determined by irradiating an electron beam in increments of 0.1 μm in a region having a size of 50 μm or more in the plate thickness direction and 100 μm or more in the rolling direction. Among the obtained measurement data, those having a reliability index (Confidence Index) of 0.1 or more are used as effective data for the particle size measurement. In order to prevent the residual austenite grain size from being excessively evaluated due to measurement noise, the average grain size is calculated using only the retained austenite grains having an equivalent circle diameter of 0.15 μm or more as effective grains.

The particle size of bcc grains is measured as follows. That is, a test piece is taken from a steel plate, a longitudinal section parallel to the rolling direction is electropolished, and the metal structure is observed using an SEM equipped with EBSP at a position of a depth of the plate thickness from the steel plate surface. A region observed as a bcc phase and surrounded by a boundary having an orientation difference of 15 ° or more is defined as one bcc particle, and a value calculated according to the definition of the following formula (1) is defined as an average particle size. Where N is the crystal grain numbers subsumed average particle size evaluation area, A _i is the i-th (i = 1,2, ··, N ) grain area, d _i is the i th grain circle Each equivalent diameter is shown.

  In the present invention, the grains having the bcc structure and the grains having the bct structure are treated as one body. Since the lattice constant is not considered in the metal structure evaluation by EBSP, grains having a bcc structure (for example, polygonal ferrite, bainitic ferrite, bainite, tempered martensite) and grains having a bct structure (for example, martensite). ) Is difficult to distinguish.

  In the structure observation by EBSP at this time, the phase is determined by irradiating an electron beam in increments of 0.1 μm in an area of 50 μm in the plate thickness direction and 100 μm in the rolling direction. Of the obtained measurement data, those having a reliability index of 0.1 or more are used for the particle size measurement as effective data. Further, in order to prevent underestimation of the particle size due to measurement noise, in the evaluation of the bcc phase, unlike the case of the above-mentioned retained austenite, only the bcc phase having a particle size of 0.47 μm or more is regarded as an effective particle. Perform the calculation. When the structure is a mixed grain structure in which fine grains and coarse grains are mixed, the influence of coarse grains may be underestimated when evaluated by a cutting method generally used for evaluating the crystal grain size of metal structures. is there. In the present invention, as a method for calculating the crystal grain size in consideration of the influence of coarse grains, the above-described formula (1) obtained by multiplying the area of each crystal grain by a weight is used.

  In the present invention, in the case of a cold-rolled steel sheet, the thickness of the steel sheet that is the base material from the boundary between the steel sheet that is the base material and the plating layer in the case of the plated steel sheet in the case of a galvanized steel sheet. The above-mentioned metal structure is defined at the 1/4 depth position.

  As a mechanical property that can be realized based on the above-described features on the metal structure, the cold-rolled steel sheet according to the present invention has a tensile strength (TS) of 780 MPa or more in a direction orthogonal to the rolling direction in order to ensure shock absorption. It is preferable that it is 950 MPa or more. Moreover, in order to ensure ductility, it is preferable that TS is less than 1180 MPa.

From the viewpoint of press formability, El is a value obtained by converting the total elongation (El ₀ ) in the direction orthogonal to the rolling direction into a total elongation equivalent to a plate thickness of 1.2 mm based on the following formula (2): El, Japan Industrial Standard JIS Z2253 Measured in accordance with JFST1001, n-value, work hardening index calculated using 5% and 10% nominal strain and 5% and 10% nominal strain and corresponding test force It is preferable that the value of TS × El is 20000 MPa% or more, the value of TS × n value is 160 MPa or more, and the value of TS ^1.7 × λ is 5500000 MPa ^1.7 % or more, where λ is the hole expansion rate.

El = El ₀ × (1.2 / t ₀ ) ^0.2 (2)
Here, El ₀ in the formula represents an actual measurement value of the total elongation measured using a JIS No. 5 tensile test piece, and t ₀ represents a plate thickness of the JIS No. 5 tensile test piece subjected to the measurement. Is a converted value of the total elongation corresponding to the case where the plate thickness is 1.2 mm.

  The work hardening index is expressed as an n value with respect to a strain range of 5 to 10% in a tensile test because a strain generated when press molding an automobile part is about 5 to 10%. Even if the total elongation of the steel sheet is high, if the n value is low, the strain propagation property becomes insufficient in press forming of automobile parts, and forming defects such as local reduction of the plate thickness are likely to occur. Further, from the viewpoint of shape freezing property, the yield ratio is preferably less than 80%, more preferably less than 75%, and particularly preferably less than 70%.

2. Chemical composition of steel C: more than 0.020% and less than 0.30% When the C content is 0.020% or less, it is difficult to obtain the above metal structure. Therefore, the C content is more than 0.020%. Preferably it is more than 0.070%, more preferably more than 0.10%, particularly preferably more than 0.14%. On the other hand, if the C content is 0.30% or more, not only the stretch flangeability of the steel sheet is impaired, but also the weldability deteriorates. Therefore, the C content is less than 0.30%. Preferably it is less than 0.25%, more preferably less than 0.20%, particularly preferably less than 0.17%.

Si: more than 0.10% and not more than 3.00% Si has an effect of improving ductility, work hardenability and stretch flangeability through suppression of austenite grain growth during annealing. Moreover, it is an element which has the effect | action which improves the stability of austenite and is effective in obtaining said metal structure. When the Si content is 0.10% or less, it is difficult to obtain the effect by the above action. Therefore, the Si content is more than 0.10%. It is preferably more than 0.60%, more preferably more than 0.90%, particularly preferably more than 1.20%. On the other hand, if the Si content exceeds 3.00%, the surface properties of the steel sheet deteriorate. Furthermore, chemical conversion property and plating property are remarkably deteriorated. Therefore, the Si content is 3.00% or less. Preferably it is less than 2.00%, more preferably less than 1.80%, and particularly preferably less than 1.60%.

  In the case of containing Al described later, the Si content and sol. The Al content preferably satisfies the following formula (3), more preferably satisfies the following formula (4), and particularly preferably satisfies the following formula (5).

Si + sol. Al> 0.60 (3)
Si + sol. Al> 0.90 (4)
Si + sol. Al> 1.20 (5)
Here, Si in the formula represents the Si content in steel, sol. Al represents the acid-soluble Al content in mass%.

Mn: more than 1.00% and not more than 3.50% Mn has an effect of improving the hardenability of steel and is an effective element for obtaining the above metal structure. If the Mn content is 1.00% or less, it is difficult to obtain the above metal structure. Therefore, the Mn content is more than 1.00%. Preferably it is more than 1.50%, more preferably more than 1.80%, particularly preferably more than 2.10%. When the Mn content is excessive, in the metal structure of the hot-rolled steel sheet, a coarse low-temperature transformation generation phase stretched in the rolling direction occurs, and coarse residual austenite grains increase in the metal structure after cold rolling and annealing, Work hardenability and stretch flangeability deteriorate. Therefore, the Mn content is 3.50% or less. Preferably it is less than 3.00%, more preferably less than 2.80%, particularly preferably less than 2.60%.

P: 0.10% or less P is an element contained in the steel as an impurity, and segregates at the grain boundaries to embrittle the steel. For this reason, the smaller the P content, the better. Therefore, the P content is 0.10% or less. Preferably it is less than 0.050%, more preferably less than 0.020%, particularly preferably less than 0.015%.

S: 0.010% or less S is an element contained in steel as an impurity, and forms sulfide inclusions to deteriorate stretch flangeability. For this reason, the smaller the S content, the better. Therefore, the S content is set to 0.010% or less. Preferably it is less than 0.005%, more preferably less than 0.003%, particularly preferably less than 0.002%.

sol. Al: 2.00% or less Al has an action of deoxidizing molten steel. In the present invention, since Si having a deoxidizing action is contained in the same manner as Al, Al is not necessarily contained. That is, it may be as close to 0% as possible. When it is contained for the purpose of promoting deoxidation, sol. It is preferable to contain 0.0050% or more as Al. Further preferred sol. The Al content is more than 0.020%. Al, like Si, has the effect of increasing the stability of austenite and is an effective element for obtaining the above metal structure. Therefore, Al can be contained for this purpose. In this case, sol. The Al content is preferably more than 0.040%, more preferably more than 0.050%, particularly preferably more than 0.060%. On the other hand, sol. If the Al content is too high, not only surface flaws are likely to occur due to alumina, but the transformation point is greatly increased, and it becomes difficult to obtain a metal structure having a low-temperature transformation generation phase as a main phase. Therefore, sol. Al content shall be 2.00% or less. Preferably it is less than 0.60%, more preferably less than 0.20%, particularly preferably less than 0.10%.

N: 0.010% or less N is an element contained in steel as an impurity, and deteriorates ductility. For this reason, the smaller the N content, the better. Therefore, the N content is set to 0.010% or less. Preferably it is 0.006% or less, More preferably, it is 0.005% or less.

The steel plate according to the present invention may contain the elements listed below as optional elements.
One or more selected from the group consisting of Ti: less than 0.050%, Nb: less than 0.050% and V: 0.50% or less Ti, Nb and V are recrystallized in the hot rolling process By suppressing the above, there is an effect of increasing the working strain and refining the metal structure of the hot-rolled steel sheet. Moreover, it precipitates as a carbide | carbonized_material or nitride, and has the effect | action which suppresses the coarsening of the austenite during annealing. Therefore, you may contain 1 type, or 2 or more types of these elements. However, even if it contains excessively, the effect by the said effect | action will be saturated and it will become uneconomical. In addition, the recrystallization temperature during annealing increases, the metal structure after annealing becomes non-uniform, and stretch flangeability is also impaired. Furthermore, the precipitation amount of carbide or nitride increases, the yield ratio increases, and the shape freezing property also deteriorates. Therefore, the Ti content is less than 0.050%, the Nb content is less than 0.050%, and the V content is 0.50% or less. The Ti content is preferably less than 0.040%, more preferably less than 0.030%, the Nb content is preferably less than 0.040%, more preferably less than 0.030%, and the V content is Preferably it is 0.30% or less, More preferably, it is less than 0.050%. In order to more reliably obtain the effect of the above action, it is preferable to satisfy any of Ti: 0.005% or more, Nb: 0.005% or more, and V: 0.010% or more. When Ti is contained, the Ti content is more preferably 0.010% or more, and when Nb is contained, the Nb content is more preferably 0.010% or more, and V is When contained, the V content is more preferably set to 0.020% or more.

One or more selected from the group consisting of Cr: 1.0% or less, Mo: 0.50% or less and B: 0.010% or less Cr, Mo and B improve the hardenability of steel. It is an element effective in obtaining the above metal structure. Therefore, you may contain 1 type, or 2 or more types of these elements. However, even if it contains excessively, the effect by the said effect | action will be saturated and it will become uneconomical. Therefore, the Cr content is 1.0% or less, the Mo content is 0.50% or less, and the B content is 0.010% or less. The Cr content is preferably 0.50% or less, the Mo content is preferably 0.20% or less, and the B content is preferably 0.0003% or less. In order to more reliably obtain the effect of the above action, it is preferable to satisfy any of Cr: 0.20% or more, Mo: 0.05% or more, and B: 0.0010% or more.

Ca, Mg and REM are selected from the group consisting of Ca: 0.010% or less, Mg: 0.010% or less, REM: 0.050% or less, and Bi: 0.050% or less. By adjusting the shape of the inclusions, Bi has the effect of improving stretch flangeability by refining the solidified structure. Therefore, you may contain 1 type, or 2 or more types of these elements. However, even if it contains excessively, the effect by the said effect | action will be saturated and it will become uneconomical. Therefore, the Ca content is 0.010% or less, the Mg content is 0.010% or less, the REM content is 0.050% or less, and the Bi content is 0.050% or less. Preferably, the Ca content is 0.0001% or less, the Mg content is 0.000020% or less, the REM content is 0.000020% or less, and the Bi content is 0.010% or less. In order to obtain the above action more reliably, it is preferable to satisfy any of Ca: 0.0005% or more, Mg: 0.0005% or more, REM: 0.0005% or more, and Bi: 0.0010% or more. . Note that REM means a rare earth element and is a generic name for a total of 17 elements of Sc, Y and lanthanoid, and the REM content is the total content of these elements.

3. Manufacturing conditions The steel having the above-mentioned chemical composition is melted by a known means and then made into a steel ingot by a continuous casting method, or a method of rolling into pieces after making it into an ingot by any casting method, etc. It is made into a billet. In the continuous casting process, in order to suppress the occurrence of surface defects due to inclusions, it is preferable to cause an external additional flow such as electromagnetic stirring in the molten steel in the mold. The steel ingot or steel slab may be reheated once it has been cooled and subjected to hot rolling. The steel ingot in the high temperature state after continuous casting or the steel slab in the high temperature state after partial rolling is used as it is. Alternatively, it may be kept hot or subjected to auxiliary heating for hot rolling. In the present specification, such steel ingots and steel slabs are collectively referred to as “slabs” as materials for hot rolling. The temperature of the slab to be subjected to hot rolling is preferably less than 1250 ° C. and more preferably 1200 ° C. or less in order to prevent coarsening of austenite. The lower limit of the temperature of the slab to be subjected to hot rolling is not particularly limited as long as it is a temperature at which hot rolling can be completed at an Ar ₃ point or higher as described later.

Hot rolling is completed in a temperature range of Ar ₃ or higher in order to refine the metal structure of the hot-rolled steel sheet by transforming austenite after completion of rolling. If the rolling completion temperature is too low, a coarse low-temperature transformation phase that extends in the rolling direction occurs in the metal structure of the hot-rolled steel sheet, and coarse residual austenite grains increase in the metal structure after cold rolling and annealing. Further, work hardenability and stretch flangeability are liable to deteriorate. Therefore, completion temperature of the hot rolling is preferably at least the Ar ₃ point and 820 ° C. greater. More preferably, it is Ar ₃ point or higher and higher than 850 ° C., and particularly preferably Ar ₃ point or higher and higher than 880 ° C. On the other hand, if the temperature at the completion of rolling is too high, accumulation of processing strain becomes insufficient, and it becomes difficult to refine the metal structure of the hot-rolled steel sheet. For this reason, it is preferable that the completion temperature of hot rolling is less than 950 degreeC, and it is further more preferable in it being less than 920 degreeC. Moreover, in order to reduce manufacturing load, it is preferable to raise the completion temperature of hot rolling and to reduce rolling load. From this point of view, it is preferable that the hot rolling completion temperature is not less than Ar ₃ point and more than 780 ° C., more preferably not less than Ar ₃ point and more than 800 ° C.

  In addition, when hot rolling consists of rough rolling and finish rolling, in order to complete finish rolling at the said temperature, you may heat a rough rolling material between rough rolling and finish rolling. At this time, it is desirable to suppress the fluctuation of the temperature over the entire length of the rough rolled material at the start of finish rolling to 140 ° C. or less by heating so that the rear end of the rough rolled material is higher than the tip. Thereby, the uniformity of the product characteristic in a coil improves.

  The heating method of the rough rolled material may be performed using a known means. For example, a solenoid induction heating device is provided between the rough rolling mill and the finish rolling mill, and the heating temperature rise is controlled based on the temperature distribution in the longitudinal direction of the rough rolled material on the upstream side of the induction heating device. May be.

  The reduction amount of the hot rolling is such that the reduction amount of the final one pass is more than 25% in terms of sheet thickness reduction rate. This increases the amount of processing strain introduced into the austenite, refines the metal structure of the hot-rolled steel sheet, suppresses the formation of coarse retained austenite grains in the metal structure after cold rolling and annealing, and fines the bcc grains. This is because of The amount of reduction in the final pass is preferably more than 30%, more preferably more than 40%. If the amount of reduction is too high, the rolling load increases and rolling becomes difficult. Therefore, the amount of reduction in the final one pass is preferably less than 55%, and more preferably less than 50%. In order to reduce the rolling load, so-called lubricated rolling may be performed in which rolling oil is supplied between a rolling roll and a steel sheet to reduce the friction coefficient and perform rolling.

  After hot rolling, it is rapidly cooled to a temperature range of 720 ° C. or less within 0.40 seconds after completion of rolling. This suppresses the release of processing strain introduced into austenite by rolling, transforms austenite using processing strain as a driving force, refines the metal structure of hot-rolled steel sheet, and coarsens the metal structure after cold rolling and annealing. This is to suppress generation of excessive retained austenite grains and to refine bcc grains. Preferably, it is rapidly cooled to a temperature range of 720 ° C. or less within 0.30 seconds after completion of rolling, and more preferably, it is rapidly cooled to a temperature range of 720 ° C. or less within 0.20 seconds after completion of rolling. . In addition, since the release of processing strain is suppressed as the average cooling rate during rapid cooling is increased, the average cooling rate during rapid cooling is preferably set to 300 ° C./s or more. Further miniaturization can be achieved. The average cooling rate during the rapid cooling is more preferably 400 ° C./s or more, and particularly preferably 600 ° C./s or more. Note that the time from the completion of rolling to the start of rapid cooling and the cooling rate during that time do not need to be specified.

  The equipment for rapid cooling is not particularly defined, but industrially, it is preferable to use a water spray device with a high water density, and a water spray header is disposed between the rolling plate conveyance rollers, and sufficient from above and below the rolling plate. A method of injecting high-pressure water having a water density is exemplified.

  After the rapid cooling stop, the steel sheet is wound in a temperature range exceeding 500 ° C. This is because when the coiling temperature is 500 ° C. or less, iron carbide is not sufficiently precipitated in the hot-rolled steel sheet, coarse residual austenite grains are formed in the metal structure after cold rolling and annealing, and bcc grains are coarse. It is because it granulates. The winding temperature is preferably higher than 550 ° C, and more preferably higher than 580 ° C. On the other hand, if the coiling temperature is too high, ferrite becomes coarse in the hot-rolled steel sheet, and coarse residual austenite grains are generated in the metal structure after cold rolling and annealing. For this reason, the winding temperature is preferably less than 650 ° C, and more preferably less than 620 ° C. The conditions from the rapid cooling stop to the winding are not particularly defined, but after the rapid cooling stop, it is preferable to hold at a temperature range of 720 to 600 ° C. for 1 second or more. Thereby, the production | generation of a fine ferrite is accelerated | stimulated. On the other hand, if the holding time becomes too long, the productivity is impaired. Therefore, the upper limit of the holding time in the temperature range of 720 to 600 ° C. is preferably within 10 seconds. After holding in the temperature range of 720 to 600 ° C., it is preferable to cool to the coiling temperature at a cooling rate of 20 ° C./s or more in order to prevent the generated ferrite from becoming coarse.

  The hot-rolled steel sheet is descaled by pickling or the like and then cold-rolled according to a conventional method. In cold rolling, in order to promote recrystallization, uniformize the metal structure after cold rolling and annealing, and further improve stretch flangeability, the cold pressure ratio (rolling ratio in cold rolling) is 40% or more. It is preferable to do. If the cold pressure ratio is too high, the rolling load increases and rolling becomes difficult, so the upper limit of the cold pressure ratio is preferably less than 70%, and more preferably less than 60%.

The steel sheet after cold rolling is annealed after being subjected to a treatment such as degreasing according to a known method, if necessary. The lower limit of the soaking temperature in the annealing is (Ac ₃ points−40 ° C.) or more. This is to obtain a metal structure in which the main phase is a low-temperature transformation generation phase and the second phase contains residual austenite. In order to increase the volume ratio of the low-temperature transformation generation phase and improve stretch flangeability, the soaking temperature is preferably more than (Ac ₃ point−20 ° C.), more preferably more than Ac ₃ point. However, if the soaking temperature becomes too high, austenite becomes excessively coarse, the metal structure after annealing becomes coarse, and ductility, work hardenability and stretch flangeability deteriorate. Therefore, the upper limit of the soaking temperature is less than (Ac ₃ points + 50 ° C.). The upper limit of the soaking temperature is preferably less than (Ac ₃ point + 20 ° C.).

  The holding time at the soaking temperature (soaking time) is not particularly limited, but in order to obtain stable mechanical properties, it is preferably more than 15 seconds, and more preferably more than 60 seconds. On the other hand, if the holding time is too long, the austenite becomes excessively coarse, and ductility, work hardenability and stretch flangeability tend to deteriorate. For this reason, the holding time is preferably less than 150 seconds, and more preferably less than 120 seconds.

  In the heating process in annealing, the heating rate from 700 ° C. to the soaking temperature is set to less than 10.0 ° C./s in order to promote recrystallization, uniformize the metal structure after annealing, and further improve stretch flangeability. It is preferable to do. More preferably, it is less than 8.0 ° C./s, and particularly preferably less than 5.0 ° C./s.

  In the cooling process after soaking in annealing, in order to promote the formation of polygonal ferrite and improve ductility and work hardening, cooling is performed at 50 ° C. or more from the soaking temperature at a cooling rate of less than 5.0 ° C./s. It is preferable. The cooling rate at this time is more preferably less than 3.0 ° C./s, and particularly preferably less than 2.0 ° C./s. In order to further increase the volume fraction of polygonal ferrite, cooling at 80 ° C. or higher is more preferable, cooling at 100 ° C. or higher is particularly preferable, and cooling at 120 ° C. or higher is most preferable.

  Moreover, in order to obtain the metal structure which makes a low temperature transformation production | generation phase a main phase, it is preferable to cool the temperature range of 650-500 degreeC with the cooling rate of 15 degrees C / s or more. It is more preferable to cool the temperature range of 650 to 450 ° C. at a cooling rate of 15 ° C./s or more. The higher the cooling rate, the higher the volume ratio of the low temperature transformation product phase. Therefore, the cooling rate is more preferably 30 ° C./s, and particularly preferably 50 ° C./s. On the other hand, if the cooling rate is too high, the shape of the steel sheet is impaired, so the cooling rate in the temperature range of 650 to 500 ° C. is preferably 200 ° C./s or less. More preferably, it is less than 150 ° C./s, and particularly preferably less than 130 ° C./s.

  Moreover, in order to ensure the amount of retained austenite, it hold | maintains for 30 seconds or more in a 450-340 degreeC temperature range. In order to improve the stability of retained austenite and further improve the ductility, work hardenability and stretch flangeability, the holding temperature range is preferably 430 to 360 ° C. Moreover, since the stability of retained austenite increases as the holding time is lengthened, the holding time is preferably 60 seconds or longer. It is more preferable to set it for 120 seconds or more, and it is especially preferable to set it for more than 300 seconds.

  In the case of producing an electroplated steel sheet, the cold-rolled steel sheet produced by the above-described method is subjected to a known pretreatment for surface cleaning and adjustment as necessary, and then electroplated according to a conventional method. The chemical composition and adhesion amount of the plating film are not limited. Examples of types of electroplating include electrogalvanizing and electro-Zn-Ni alloy plating.

  When manufacturing a hot-dip plated steel sheet, the annealing process is performed by the above-described method, and after holding at a temperature range of 450 to 340 ° C. for 30 seconds or more, the steel sheet is heated as necessary and then immersed in a plating bath. Apply hot dip plating. In order to improve the stability of retained austenite and further improve the ductility, work hardenability and stretch flangeability, the holding temperature range is preferably 430 to 360 ° C. Moreover, since the stability of retained austenite increases as the holding time is lengthened, the holding time is preferably 60 seconds or longer. It is more preferable to set it for 120 seconds or more, and it is especially preferable to set it for more than 300 seconds. The alloying treatment may be performed by reheating after hot dipping. The chemical composition and the amount of adhesion of the plating film are not limited. Examples of hot dip plating include hot dip galvanizing, alloyed hot dip galvanizing, hot dip aluminum plating, hot dip Zn-Al alloy plating, hot dip Zn-Al-Mg alloy plating, hot dip Zn-Al-Mg-Si alloy plating, etc. The

  In order to further improve the corrosion resistance, the plated steel sheet may be subjected to an appropriate chemical conversion treatment after plating. The chemical conversion treatment is preferably carried out using a non-chromium chemical conversion treatment solution (for example, silicate-based, phosphate-based, etc.) instead of the conventional chromate treatment.

  The cold-rolled steel sheet and the plated steel sheet thus obtained may be subjected to temper rolling according to a conventional method. However, when the elongation rate of temper rolling is high, ductility is deteriorated, and therefore, the elongation rate of temper rolling is preferably 1.0% or less. A more preferable elongation is 0.5% or less.

The present invention will be described more specifically with reference to examples.
Steel having the chemical composition shown in Table 1 was melted and cast using a laboratory vacuum melting furnace. Each obtained steel ingot was made into a steel piece having a thickness of 30 mm by hot forging. The steel slab was heated to 1200 ° C. using an electric heating furnace and kept at this temperature for 60 minutes, and then hot rolled under the conditions shown in Table 2.

Specifically, using an experimental hot rolling mill, 6-pass rolling was performed in a temperature range of ₃ or more points of Ar, and finished to a thickness of 2 to 3 mm. The rolling reduction rate in the final pass was 12 to 42% in terms of sheet thickness reduction rate. After hot rolling, it is cooled to 650-720 ° C. under various cooling conditions using water spray, allowed to cool for 5-10 seconds, then cooled to various temperatures at a cooling rate of 60 ° C./s, The temperature is set as the coiling temperature, charged in an electric heating furnace maintained at the same temperature, held for 30 minutes, cooled to room temperature at a cooling rate of 20 ° C./h, and gradually cooled after winding. A hot-rolled steel sheet was obtained by simulating.

  The obtained hot-rolled steel sheet was pickled to form a cold-rolled base metal, and cold-rolled at a reduction rate of 50 to 60% to obtain a cold-rolled steel sheet having a thickness of 1.0 to 1.2 mm. Using the continuous annealing simulator, the obtained cold-rolled steel sheet was heated to 550 ° C. at a heating rate of 10 ° C./s, and then heated to various temperatures shown in Table 2 at a heating rate of 2 ° C./s. Soaked for 95 seconds. Thereafter, the primary cooling is performed to the temperature shown in Table 2, and the secondary cooling is further performed from the primary cooling stop temperature to various cooling stop temperatures shown in Table 2 at an average cooling rate of 60 ° C./s. After being held, it was cooled to room temperature to obtain an annealed steel plate.

  A specimen for SEM observation was collected from the annealed steel sheet, and after polishing the longitudinal section parallel to the rolling direction, it was corroded with nital, and the metal structure at the 1/4 depth position of the plate thickness was observed from the steel sheet surface. The volume fraction of the low temperature transformation product phase and polygonal ferrite was measured by the treatment. Further, the area occupied by the entire polygonal ferrite was divided by the number of crystal grains of the polygonal ferrite to obtain an average particle diameter (equivalent circle diameter) of the polygonal ferrite.

  In addition, a specimen for XRD measurement is collected from the annealed steel sheet, and the rolled surface is chemically polished from the steel sheet surface to a 1/4 depth position of the sheet thickness, and then an X-ray diffraction test is performed to measure the volume fraction of retained austenite. did. Specifically, RINT 2500 made by Rigaku is used for the X-ray diffractometer, Co-Kα rays are incident, and α phase (110), (200), (211) diffraction peaks and γ phase (111), (200) The integrated intensity of the (220) diffraction peak was measured to determine the volume fraction of retained austenite.

  Furthermore, after taking a specimen for EBSP measurement from the annealed steel sheet and electrolytically polishing the longitudinal section parallel to the rolling direction, the metal structure was observed at the 1/4 depth position of the sheet thickness from the steel sheet surface, and by image analysis, The average particle diameter of the bcc grains, the grain size distribution of the retained austenite grains, and the average grain diameter of the retained austenite were measured. Specifically, TSL OIM5 is used for the EBSP measuring device, and the electron beam is irradiated at a pitch of 0.1 μm in a region of 50 μm in the plate thickness direction and 100 μm in the rolling direction. The bcc phase and the fcc phase were determined with valid data having an index of 0.1 or more.

  A region that is observed as a bcc phase and surrounded by a grain boundary with an orientation difference of 15 ° or more is defined as one bcc grain, the circle equivalent diameter and area of each bcc grain are obtained, and the average is calculated according to the definition of the above-described formula (1). The particle size was calculated. In calculating the average particle diameter, bcc grains having an equivalent circle diameter of 0.47 μm or more were determined as effective bcc grains. Strictly speaking, the martensite crystal structure is a body-centered tetragonal lattice (bct). However, since the lattice constant is not taken into account in the metal structure evaluation by EBSP, martensite is also handled as a bcc phase.

Further, the region surrounded by the parent phase, which was observed as the fcc phase, was defined as one retained austenite grain, and the equivalent circle diameter of each retained austenite grain was determined. The average grain size of the retained austenite was calculated as the average value of the equivalent circle diameters of the individual effective retained austenite grains, with the retained austenite grains having an equivalent circle diameter of 0.15 μm or more as effective retained austenite grains. Further, the number density (N _R ) per unit area of the retained austenite grains having a grain size of 1.2 μm or more was determined.

Yield stress (YS) and tensile strength (TS) were determined by collecting JIS No. 5 tensile specimens from an annealed steel sheet along the direction perpendicular to the rolling direction and conducting a tensile test at a tensile speed of 10 mm / min. The total elongation (El) is based on the above formula (2) using a measured value (El ₀ ) obtained by conducting a tensile test with a JIS No. 5 tensile test specimen taken along the direction orthogonal to the rolling direction. The conversion value corresponding to the case where the plate thickness is 1.2 mm was obtained. The work hardening index (n value) was obtained by conducting a tensile test on a JIS No. 5 tensile specimen taken along the direction perpendicular to the rolling direction and setting the strain range to 5 to 10%. Specifically, it was calculated by a two-point method using test forces for nominal strains of 5% and 10%.

  Stretch flangeability was evaluated by measuring the hole expansion rate (λ) by the following method. A 100 mm square plate is taken from the annealed steel sheet, a punched hole with a diameter of 10 mm is formed with a clearance of 12.5%, and the punched hole is expanded from the sag side with a conical punch with a tip angle of 60 °. The hole enlargement ratio was measured when this occurred, and this was defined as the hole expansion ratio.

  Table 3 shows the results of metal structure observation and performance evaluation of the cold-rolled steel sheet after annealing. In Tables 1 to 3, numerical values or symbols marked with * mean outside the scope of the present invention.

As for the test results (test numbers 6 to 10, 12, 14, 17 to 27) for the steel plates within the range defined by the present invention, the value of TS × El is 20000 MPa% or more, and the value of TS × n The value was 165 or more, and the value of TS ^1.7 × λ was 6000000 MPa ^1.7 % or more, and good ductility, work hardenability and stretch flangeability were exhibited.

  The test results (test numbers 1 to 5, 11, 13, 15, 16) of the steel sheets in which the metal structure of the steel sheets deviates from the range defined by the present invention are any or all of ductility, work hardenability and stretch flangeability. Was inferior.

Specifically, in the test using Steel A (Test No. 1), since the Si content in the steel is small, the average particle size of retained austenite is large, the volume fraction of retained austenite is low, and Average particle size is large, and ductility, work hardenability and stretch flangeability are poor. In the test using Steel B (Test No. 2), the test using Steel C (Test No. 4), and the test using Steel H (Test No. 11), the time from hot rolling completion to quenching stop is too long. Therefore, N _R is large, the average particle size of bcc grains is large, and ductility, work hardenability and stretch flangeability are poor. In the test using Steel C (Test No. 5) and the test using Steel I (Test No. 13), since the coiling temperature is too low, N _R is large, and the average particle size of bcc grains is large. , Work hardenability and stretch flangeability are poor. In the test using Steel B (Test No. 3) and the test using Steel K (Test No. 16), since the soaking temperature during annealing is too low, a metal structure having a low-temperature transformation generation phase as a main phase is obtained. It is not stretched and the flangeability is poor. In the test using steel K (test number 15), since the rolling reduction in the final one pass of hot rolling is low, N _R is large and the average particle diameter of bcc grains is large, so that ductility, work hardenability and elongation are high. Bad flangeability.

Claims (4)

In mass%, C: more than 0.020% and less than 0.30%, Si: more than 0.10% and 3.00% or less, Mn: more than 1.00% and 3.50% or less, P: 0.10% or less , S: 0.010% or less, sol. A steel plate having a chemical composition comprising Al: 2.00% or less and N: 0.010% or less, the balance being Fe and impurities, the main phase being a low-temperature transformation product phase, and the second phase being retained austenite The retained austenite has a volume ratio of more than 4.0% to less than 25.0%, an average particle size of less than 0.80 μm, and a particle size of 1.2 μm or more. The number density of residual austenite grains is 3.0 × 10 ⁻² particles / μm ² or less, and the average grain size of grains having a bcc structure and grains having a bct structure surrounded by grain boundaries having an orientation difference of 15 ° or more is cold-rolled steel sheet 7.0μm Ri der below, tensile strength, characterized in der Rukoto least 780 MPa.   The chemical composition may be one selected from the group consisting of Ti: less than 0.050%, Nb: less than 0.050%, and V: 0.50% or less in mass%, instead of part of Fe. The cold-rolled steel sheet according to claim 1, comprising two or more kinds.   The chemical composition may be one selected from the group consisting of Cr: 1.0% or less, Mo: 0.50% or less, and B: 0.010% or less in mass%, instead of part of Fe. The cold-rolled steel sheet according to claim 1 or 2, which contains two or more kinds.   The chemical composition is, in place of part of Fe, in mass%, Ca: 0.010% or less, Mg: 0.010% or less, REM: 0.050% or less, and Bi: 0.050% or less. The cold-rolled steel sheet according to any one of claims 1 to 3, which contains one or more selected from the group.

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