JP5644703B2 - Cold rolled steel sheet manufacturing method - Google Patents

Description

  The present invention relates to a method for manufacturing a cold-rolled steel sheet. More specifically, the present invention relates to a method for producing a cold-rolled steel sheet that is formed and used in various shapes by press working or the like, and in particular, a high-tensile cold-rolled steel sheet that is excellent in ductility, work-hardening property, 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, these press formability and high strength of the steel sheet are contradictory characteristics, and it is difficult to satisfy these characteristics 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.

Although these techniques improve the balance between the strength and ductility of the hot-rolled steel sheet, there is no description in the above-mentioned patent document regarding a method for making the cold-rolled steel sheet finer and improving the 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 if tempered martensite is refined. However, in order to obtain a metal structure containing tempered martensite and retained austenite, primary annealing for generating martensite and secondary annealing for tempering martensite and further obtaining retained austenite are required. The characteristics are greatly impaired.

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

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 meet such a demand. Specifically, an object of the present invention is to provide a method for producing a high-tensile cold-rolled steel sheet having a tensile strength of 780 MPa or more having excellent ductility, work-hardening properties, and stretch flangeability.

  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: more than 0.020% and less than 0.30%, Si: more than 0.10% and less than 3.00%, Mn: more than 1.00% and less than 3.50%, 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, air-cooled for 5 to 10 seconds, and then cooled to various temperatures at a cooling rate of 90 ° C./s or less, and this temperature is taken as the coiling temperature. After charging in a 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 (F) 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 780 ° 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) By controlling the hot rolling conditions, grains having a bcc (body-centered cubic) structure and grains having a bct (body-centered tetragonal) structure in the hot-rolled steel sheet (hereinafter collectively referred to as “bcc grains”) ), The austenite grain coarsening that may occur when annealing at a high temperature after cold rolling is suppressed. The reason for this is not clear, but the grain boundaries of the bcc grains function as nucleation sites for austenite due to transformation during annealing. Therefore, by making the bcc grains fine, the nucleation frequency increases and the annealing temperature is high. Even so, it is presumed that the coarsening of the austenite grains is suppressed.

(C) When the final reduction amount of hot rolling is increased, bcc grains in the hot-rolled steel sheet are refined, and coarsening of austenite grains that can occur when annealing is performed at a high temperature after cold rolling is suppressed.
(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) bcc grains in the hot-rolled steel sheet become finer as the Si content increases, and (b) iron carbide in the hot-rolled steel sheet as the Si content increases. (C) Since iron carbide functions as a nucleation site in the transformation to austenite, the frequency of nucleation in the transformation to austenite further increases, and (d) unsolidified It is presumed that due to the increase in dissolved iron carbide, austenite grain growth is suppressed and austenite is further refined.

  (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 the retained austenite in the annealed 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 result of investigating the grain size distribution of retained austenite in an annealed steel sheet obtained by hot rolling a slab having the same chemical composition by a conventional method without immediately quenching, cold rolling and annealing. It is a graph which shows. From the comparison between FIG. 1 and FIG. 2, it can be seen that in the annealed steel sheet (FIG. 1) manufactured through an appropriate immediate quenching process, the formation of coarse retained austenite grains is suppressed and the retained austenite is finely dispersed.

(F) A cold-rolled steel sheet having such a metal structure exhibits good ductility, good work hardenability and good stretch flangeability while having high strength.
From the above results, after hot-rolling steel containing a certain amount or more of Si, the hot-rolled steel sheet with bcc grains refined was cold-rolled and then annealed at a high temperature. Thereafter, by cooling, the main phase is a low-temperature transformation generation phase, the second phase contains fine retained austenite, and has a metal structure with few coarse austenite grains, and is excellent in ductility, work hardenability and stretch flangeability. It has been found that rolled steel sheets can be produced.

The present invention completed based on the above knowledge is as follows.
(1) A method for producing a cold-rolled steel sheet comprising the following steps (A) and (B), wherein the main phase is a low-temperature transformation generation phase and the second phase includes a metal structure containing retained austenite:
(A) 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: 0.0. 10% or less, S: 0.010% or less, sol. Grains having a bcc structure containing Al: 2.00% or less and N: 0.010% or less, the balance having a chemical composition composed of Fe and impurities, and surrounded by grain boundaries with an orientation difference of 15 ° or more, and a cold rolling step in which a hot rolled steel sheet having an average grain size of grains having a bct structure is 6.0 μm or less is subjected to cold rolling to obtain a cold rolled steel sheet; and (C) the cold rolled steel sheet (Ac ₃ points) −40 ° C.) or higher (Ac ₃ points + 100 ° C.) , and after soaking for 15 seconds to less than 150 seconds , cooling to 500 ° C. or lower and 300 ° C. or higher is performed. An annealing process for holding for at least 2 seconds.

  (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 method for producing a cold-rolled steel sheet according to (1) above, which contains one or more kinds.

  (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 method for producing a cold-rolled steel sheet according to (1) or (2) above, which contains one 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 method for producing a cold-rolled steel sheet according to any one of (1) to (3) above, which comprises one or more selected from the group consisting of:

  ADVANTAGE OF THE INVENTION According to this invention, the high-tensile cold-rolled steel plate which has sufficient ductility applicable to processes, such as press molding, work-hardening property, and stretch flangeability can be manufactured. Therefore, the present invention greatly contributes to the development of industry, 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.

  Rolling in the manufacturing method of the cold-rolled steel sheet according to the present invention, which can produce the metal structure and chemical composition in the high-tensile cold-rolled steel sheet manufactured by the method according to the present invention, and the steel sheet efficiently, stably and economically, The annealing conditions and the like will be described in detail below.

1. Metal structure The cold-rolled steel sheet produced according to the present invention has a metal structure in which the main phase is a low-temperature transformation generation phase and the second phase contains residual austenite. This is because 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 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 low temperature transformation generation phase refers to a phase and structure generated by low temperature transformation such as martensite and bainite. Examples of the low temperature transformation generation phase other than these include bainitic ferrite and tempered martensite. Bainitic ferrite is distinguished from polygonal ferrite in that it has a lath or plate-like form and a high dislocation density, and is distinguished from bainite in that there is no iron carbide inside and at the interface. This low temperature transformation product phase may contain two or more phases and structures, such as martensite and bainitic ferrite. When the low temperature transformation product phase includes two or more phases and structures, the sum of the volume fractions of these phases and tissues is defined as the volume fraction of the low temperature transformation product phase.

  In order to improve the ductility, the volume ratio of the retained austenite with respect to the entire structure is preferably more than 4.0%. More preferably, it is more than 6.0%, particularly preferably more than 9.0%, and most preferably more than 12.0%. On the other hand, if the volume ratio of retained austenite is excessive, stretch flangeability deteriorates. Accordingly, the volume ratio of retained austenite is preferably less than 25.0%. More preferably it is less than 18.0%, particularly preferably less than 16.0%, and most preferably less than 14.0%.

  In cold-rolled steel sheets with a low-temperature transformation generation phase as the main phase and a metal structure containing residual austenite in the second phase, reducing the residual austenite will significantly improve ductility, work hardening and stretch flangeability. The average particle size of austenite is preferably less than 0.80 μm. It is more preferable that the thickness be less than 0.70 μm, and it is particularly preferable that the thickness be 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 containing a low-temperature transformation-forming phase as a main phase and a residual austenite in the second phase, even if the average grain size of residual austenite is small, if there are many coarse residual austenite grains, work hardening and Stretch flangeability is easily impaired. Therefore, it is preferable that the number density of residual austenite grains having a grain size of 1.2 μm or more is 3.0 × 10 ⁻² particles / μm ² or less. The number density is more preferably 2.0 × 10 ⁻² pieces / μm ² or less, particularly preferably 1.5 × 10 ⁻² pieces / μm ² or less, and 1.0 × 10 ⁻² pieces / μm ^2. Most preferred is ² or less.

  In order to further improve the ductility and work hardenability, it is preferable that the second phase contains polygonal ferrite in addition to retained austenite. The volume ratio of the polygonal ferrite 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%.

  Further, since the effect of improving ductility and work hardening increases as the granular ferrite becomes finer, the average crystal grain size of the polygonal ferrite is preferably less than 5.0 μm. More preferably, it is less than 4.0 micrometers, Most preferably, it is less than 3.0 micrometers.

  In order to further improve 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 determined by taking a specimen from a steel plate, polishing a longitudinal section parallel to the rolling direction, and subjecting it to corrosion treatment with nital or picric acid, The metal structure is observed using a SEM at a 1/4 depth position, and the area ratios of the low-temperature transformation generation phase and 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 average particle diameter of polygonal ferrite is obtained by dividing the area occupied by the entire polygonal ferrite in the field of view by the number of crystal grains of polygonal ferrite to obtain the equivalent circle diameter, which is taken as the average particle diameter.

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 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 electrolytically polished, and the metal structure is observed using an SEM equipped with EBSP at a position of a quarter 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.

  In the present invention, in the case of a cold-rolled steel sheet, the thickness of the steel sheet is ¼ depth position from the surface of the steel sheet. In the case of a plated steel sheet, the thickness of the steel sheet as the base material is 1 In the / 4 depth position, the above-mentioned metal structure is defined.

  As a mechanical property that can be realized based on the above characteristics on the metal structure, the cold-rolled steel sheet of 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 to have 950 MPa or more. On the other hand, in order to ensure ductility, it is preferable that TS is less than 1180 MPa.

Further, from the viewpoint of press formability, the value obtained by converting the total elongation (El ₀ ) in the direction orthogonal to the rolling direction to the total elongation equivalent to a plate thickness of 1.2 mm based on the following formula (1) is El, Nippon Kogyo. According to the standard JIS Z2253, the strain range is 5 to 10%, and the work hardening index calculated by using the nominal strain of 2 points of 5% and 10% and the test force corresponding to these is n value, Japan Iron and Steel Federation Standard When the hole expansion rate measured according to JFST1001 is λ, the value of TS × El is 15000 MPa% or more, the value of TS × n value is 150 MPa or more, and the value of TS ^1.7 × λ is 4500000 MPa ^1.7 % or more. Preferably there is.

El = El ₀ × (1.2 / t ₀ ) ^0.2 (1)
Here, El ₀ in the formula represents an actual measurement value of total elongation measured using a JIS No. 5 tensile test piece, t ₀ represents a plate thickness of a JIS No. 5 tensile test piece subjected to measurement, and El represents a plate thickness. It is a converted value of total elongation corresponding to the case of 1.2 mm.

TS × El is an index for evaluating ductility from the balance between strength and total elongation, and TS × n value is an index for evaluating work curability from the balance between strength and work hardening index. TS ^1.7 × λ Is an index for evaluating hole expandability from the balance between strength and hole expansion rate. More preferably, the value of TS × El is 19000 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.

  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 (2), more preferably satisfies the following formula (3), and particularly preferably satisfies the following formula (4).

Si + sol. Al> 0.60 (2)
Si + sol. Al> 0.90 (3)
Si + sol. Al> 1.20 (4)
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 at the impurity level. 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, making it 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 any of these elements is contained in excess, the effect of the above action is saturated and 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. When Nb is contained, the Nb content is more preferably 0.010% or more, and V is contained. In some cases, the V content is more preferably 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 these elements are contained excessively, the effect of the above action is saturated and 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 (cold rolling process)
In the cold rolling process, grains having the above-described chemical composition and having a bcc structure surrounded by grain boundaries having an orientation difference of 15 ° or more and grains having a bct structure (as described above, these grains are referred to as bcc grains). A hot-rolled steel sheet having a mean particle diameter of 6.0 μm or less is cold-rolled to obtain a cold-rolled steel sheet.

  Here, the average particle diameter of the bcc grains is calculated by the following method. 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 surrounded by a boundary having an orientation difference of 15 ° or more observed as a grain having a bcc structure or a bct structure is defined as one grain, and a value calculated according to the following formula (5) is defined as an average grain diameter. Here, N is the number of crystal grains included in the average grain size evaluation region, Ai is the area of the i-th (i = 1, 2,..., N) crystal grain, and di is equivalent to the circle of the i-th crystal grain. Each diameter is shown.

  Here, the reason why the grains having the bcc structure and the grains having the bct structure are treated as one is because the lattice constant is not taken into account in the metal structure evaluation by EBSP, and thus the grains having the bcc structure (for example, polygonal ferrite, bainitic). This is because it is difficult to distinguish between ferrite (bainite, tempered martensite) and grains having a bct structure (for example, martensite).

  In the structure evaluation by EBSP here, the phase is determined by controlling the electron beam in increments of 0.1 μm in a region having a size of 50 μm in the plate thickness direction and 100 μm in the rolling direction (direction perpendicular to the plate thickness direction). Do. Among the obtained measurement data, those having a reliability index of 0.1 or more are used as effective data for the particle size measurement. Further, in order to prevent underestimation of the particle size due to measurement noise, in the evaluation of the bcc phase, unlike the above-described case of retained austenite, only the bcc particles having a particle size of 0.47 μm or more are used as effective particles. Perform the calculation.

  The crystal grain size is defined with the grain boundary having an orientation difference of 15 ° or more as an effective grain boundary. The grain boundary having an orientation difference of 15 ° or more becomes an effective nucleation site of reverse transformed austenite grains and is annealed. This is because the coarsening of the austenite grains at the time is suppressed and the workability of the cold-rolled steel sheet is greatly improved. Moreover, when the structure of the hot-rolled steel sheet is a mixed grain structure in which fine grains and coarse grains are mixed, the coarse grain portion is easily coarsened after cold rolling and annealing, and ductility and stretch flangeability are lowered. When the grain size of such a mixed grain structure is evaluated by a cutting method generally used for evaluating the crystal grain diameter of a metal structure, the influence of coarse grains may be underestimated. In order to avoid this, in the present invention, the above-described equation (5) obtained by multiplying the area of each crystal grain as a weight is used as a method for calculating the crystal grain size in consideration of the influence of coarse grains.

  When the average particle diameter of bcc grains calculated by the above method for hot-rolled steel sheets to be subjected to cold rolling exceeds 6.0 μm, the metal structure after cold rolling and annealing becomes coarse, resulting in ductility, work hardenability and stretch flange. Sexuality is impaired. Therefore, the average particle diameter of the bcc grains of the hot rolled steel sheet is 6.0 μm or less. Preferably it is 4.0 micrometers or less, More preferably, it is 3.5 micrometers or less.

  The types and volume ratios of the phases and structures constituting the hot-rolled steel sheet are not particularly specified, and are selected from the group consisting of polygonal ferrite, acicular ferrite, bainitic ferrite, bainite, pearlite, retained austenite, and martensite. 1 type or 2 types or more may be mixed. However, it is preferable that the hot-rolled steel sheet is soft in that the cold rolling load is reduced and the annealing structure can be made finer by further increasing the cold rolling rate.

The hot-rolled steel sheet having the above-described average particle diameter of bcc grains is preferably produced as follows.
That is, after melting the steel having the above-described chemical composition by a known means, it is made into a steel ingot by a continuous casting method, or it is made into a steel ingot by an arbitrary casting method and then rolled by a method such as a method of carrying out a block rolling. A piece. 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 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 temperature at the completion of rolling is too low, a coarse low-temperature transformation generation phase that extends in the rolling direction occurs in the metal structure of the hot-rolled steel sheet, and the metal structure after cold rolling and annealing becomes coarse, resulting in ductility and work hardening. And stretch flangeability tends 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 amount of reduction in hot rolling is preferably such that the amount of reduction in the final pass is more than 15% in terms of sheet thickness reduction rate. This increases the amount of work strain introduced into austenite, refines the metal structure of hot-rolled steel sheets, refines the metal structure after cold rolling and annealing, and improves ductility, work hardenability and stretch flangeability. It is. The rolling amount in the final pass is more preferably more than 25%, particularly preferably more than 30%, and most 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 780 ° C. or less within 0.4 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 sheets, and refines the metal structure after cold rolling and annealing. This is to improve ductility, work hardening and stretch flangeability. Since the release of processing strain is suppressed as the time until the rapid cooling is stopped is shorter, the time until the rapid cooling is stopped after the completion of rolling is preferably within 0.30 seconds, and within 0.20 seconds. More preferably. Since the metallographic structure of the hot-rolled steel sheet becomes finer as the temperature at which quenching is stopped is lower, it is preferably quenched to a temperature range of 760 ° C. or less after completion of rolling, and is rapidly cooled to a temperature range of 740 ° C. or less after completion of rolling. More preferably, it is particularly preferable to rapidly cool to a temperature range of 720 ° C. or lower 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, it is preferable to wind the steel sheet in a temperature range exceeding 400 ° C. This is because if the coiling temperature exceeds 400 ° C., iron carbide is sufficiently precipitated in the hot-rolled steel sheet, and this iron carbide has an effect of suppressing the coarsening of the metal structure after cold rolling and annealing. The winding temperature is more preferably more than 500 ° C. It is particularly preferably higher than 550 ° C, and most 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 the metal structure after cold rolling and annealing becomes coarse. For this reason, the coiling 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.

In the cold rolling process, for example, a hot rolled steel sheet having a metal structure with an average particle diameter of bcc grains of 6.0 μm or less, which can be obtained by the above-described hot rolling, is descaled by pickling or the like if necessary. Then, it cold-rolls 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%.
(Annealing process)
The steel sheet after cold rolling is annealed after performing a treatment such as degreasing according to a known method as 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, the austenite becomes excessively coarse and the ductility, work hardenability and stretch flangeability tend to deteriorate. For this reason, the upper limit of the soaking temperature is preferably less than (Ac ₃ points + 100 ° C.). It is more preferable to be less than (Ac ₃ point + 50 ° C.), and it is particularly preferable to be 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, homogenize the metal structure after annealing, and improve stretch flangeability. It is preferable. 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 fine polygonal ferrite and improve ductility and work hardenability, the soaking temperature is 50 ° C. or more at a cooling rate of less than 10.0 ° C./s. It may be cooled. The cooling rate after soaking is preferably less than 5.0 ° C./s. More preferably, it is less than 3.0 degreeC / s, Most preferably, it is less than 2.0 degreeC / s. In order to further increase the volume fraction of polygonal ferrite, it is preferable to cool at 80 ° C. or higher from the soaking temperature at a cooling rate of less than 10.0 ° C./s. Cooling at 100 ° C. or higher is more preferable, and cooling at 120 ° C. or higher is particularly preferable.

  In order to obtain a metal structure whose main phase is a low temperature transformation generation phase, it is preferable to cool a temperature range of 650 to 500 ° C. at a cooling rate of 15 ° 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.

  In order to obtain retained austenite, the temperature is maintained at 500 to 300 ° C. for 30 seconds or more. In order to improve the stability of retained austenite and improve ductility, work hardening and stretch flangeability, the holding temperature range is preferably 475 to 320 ° C. It is more preferable to set it as 450-340 degreeC, and it is especially preferable to set it as 430-360 degreeC. 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 500 to 300 ° 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 improve ductility, work hardening and stretch flangeability, the holding temperature range is preferably 475 to 320 ° C. It is more preferable to set it as 450-340 degreeC, and it is especially preferable to set it as 430-360 degreeC. 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. These steel ingots were made into steel pieces having a thickness of 30 mm by hot forging. The steel slab was heated to 1200 ° C. using an electric heating furnace and held 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 to 720 ° C. under various cooling conditions using a water spray, then allowed to cool for 5 to 10 seconds, and then cooled to various temperatures at a cooling rate of 60 ° C./s. The temperature is taken up as a coiling temperature, charged in an electric heating furnace maintained at the same temperature and held for 30 minutes, and then cooled to room temperature at a cooling rate of 20 ° C./h and gradually cooled after winding. The hot rolled steel sheet was obtained by simulating.

EBSP measurement specimens were collected from the obtained hot-rolled steel sheet, and after electropolishing the longitudinal section parallel to the rolling direction, the metal structure was observed from the steel sheet surface at a 1/4 depth position, and image analysis was performed. Was used to measure the average particle size of the bcc particles. Specifically, OSL ^TM 5 manufactured by TSL is used for the EBSP measuring device, and the measurement data obtained by irradiating with an electron beam at a pitch of 0.1 μm in an area of 50 μm in the thickness direction and 100 μm in the rolling direction. Of these, bcc grains were determined by using data having a reliability index of 0.1 or more as valid data. The area surrounded by the grain boundaries with an orientation difference of 15 ° or more, observed as bcc grains, is taken as one bcc grain, the circle equivalent diameter and area of each bcc grain are obtained, and the average grain is determined according to the equation (5) described above. The diameter 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. As described above, since the lattice constant is not considered in the metal structure evaluation by EBSP, grains having a bct (body-centered tetragonal lattice) structure such as martensite are also measured. Therefore, the bcc grains include both bcc structure grains and bct structure grains.

  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. The obtained cold-rolled steel sheet was heated to 550 ° C. at a heating rate of 10 ° C./s using a continuous annealing simulator, and then heated to various temperatures shown in Table 2 at a heating rate of 2 ° C./s. Soaked for 2 seconds. Then, it cooled to the various cooling stop temperature shown by Table 2 by making the average cooling rate from 700 degreeC into 60 degreeC / s, and hold | maintained to the temperature for 330 second, Then, it cooled to room temperature and obtained the 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, the metal structure at the 1/4 depth position of the sheet thickness was observed from the steel sheet surface, and low-temperature transformation was performed by image processing. The volume fraction of the product phase and polygonal ferrite was measured. Further, the area occupied by the entire polygonal ferrite was divided by the number of crystal grains of the polygonal ferrite, and the average particle diameter (equivalent circle diameter) of the polygonal ferrite was determined.

  In addition, a specimen for XRD measurement was collected from the annealed steel sheet, and the rolled surface was chemically polished from the steel sheet surface to a 1/4 depth position of the sheet thickness, and then an X-ray diffraction test was performed to determine the volume fraction of retained austenite. It was measured. 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.

Further, a specimen for EBSP measurement was collected from the annealed steel sheet, and after electropolishing the longitudinal section parallel to the rolling direction, the metal structure was observed at a 1/4 depth position from the steel sheet surface, and image analysis was performed. The particle size distribution of retained austenite grains and the average particle size of retained austenite were measured. Specifically, OSL ^TM 5 manufactured by TSL was used for the EBSP measuring device, and an electron beam was 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. Of these, the fcc phase was determined with valid data having a reliability index of 0.1 or more as valid data. The region observed as the fcc phase and surrounded by the parent 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. In addition, the number density (N _R ) per unit area of residual 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 obtained by conducting a tensile test on a JIS No. 5 tensile test specimen taken along the direction orthogonal to the rolling direction, and using the obtained actual measurement value (El ₀ ), based on the above formula (1), A conversion value corresponding to the case where the plate thickness was 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 the metal structure observation and performance evaluation of the cold-rolled steel sheet after annealing. In Tables 1 to 3, a numerical value marked with * means outside the scope of the present invention.

The test results (test numbers 5-9, 11, 12, 14, 16-28) of the cold-rolled steel sheets manufactured according to the conditions specified in the present invention all have a TS × El value of 17000 MPa% or more, TS × The value of n value was 150 or more, and the value of TS ^1.7 × λ was 4700000 MPa ^1.7 % or more, and good ductility, work hardenability and stretch flangeability were exhibited. In particular, the test results (test number 5 to 5), the rolling reduction of the final pass of hot rolling is over 25%, the coiling temperature is over 500 ° C., and the secondary cooling stop temperature after annealing is 340 ° C. or more. 9, 11, 12, 18-28), TS × El value is 19000 MPa% or more, TS × n value is 160 or more, TS ^1.7 × λ value is 6000000 MPa ^1.7 % or more, Particularly good ductility, work hardenability and stretch flangeability were exhibited.

  The test results (test numbers 1 to 4, 10, 13, and 15) for steel sheets whose steel composition or manufacturing method deviates from the range defined by the present invention are inferior in any or all of ductility, work hardenability and stretch flangeability. It was.

Specifically, in the test using steel A (test number 1), since the Si content in the steel is small, the average particle size of bcc grains of the hot-rolled steel sheet is large, and the average particle size of retained austenite is large. Further, the volume ratio of retained austenite is low, and the work hardenability and stretch flangeability are poor. The test using Steel B (Test No. 2), the test using Steel C (Test No. 4) and the test using Steel H (Test No. 10) have a long time from completion of hot rolling to quenching stop, Since the average particle diameter of the bcc grains of the hot-rolled steel sheet is large, N _R is large, and work hardenability and stretch flangeability are poor. In the test using Steel B (Test No. 3) and the test using Steel K (Test No. 15), since the soaking temperature during annealing is too low, a metal structure whose main phase is a low-temperature transformation phase is obtained. It is not stretched and the flangeability is poor. In the test using steel K (test number 13), since the average particle diameter of bcc grains of the hot-rolled steel sheet is large, N _R is large, and work hardenability and stretch flangeability are poor.

Claims (4)

A method for producing a cold-rolled steel sheet, comprising the following steps (A) and (B), wherein the main phase is a low-temperature transformation generation phase and has a metal structure containing residual austenite in the second phase:
(A) 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: 0.0. 10% or less, S: 0.010% or less, sol. Grains having a bcc structure containing Al: 2.00% or less and N: 0.010% or less, the balance having a chemical composition composed of Fe and impurities, and surrounded by grain boundaries with an orientation difference of 15 ° or more, and a cold rolling step of cold rolling a hot rolled steel sheet having an average grain size of bct structure of 6.0 μm or less to form a cold rolled steel sheet; and (B) the cold rolled steel sheet (Ac ₃ points) −40 ° C.) or higher (Ac ₃ points + 100 ° C.) , and after soaking for 15 seconds to less than 150 seconds , cooling to 500 ° C. or lower and 300 ° C. or higher is performed. An annealing process for holding for at least 2 seconds.   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 method for producing a 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 method for producing a cold-rolled steel sheet according to claim 1 or 2, comprising 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 method for producing a cold-rolled steel sheet according to any one of claims 1 to 3, comprising one or more selected from the group.

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