JP6398210B2 - 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 divided, materials used in each technical field are required to have special and high performance. 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, the demand for thin-walled, high-formability, high-tensile cold-rolled steel sheets has been remarkably increasing in order to reduce the weight of the vehicle body and improve fuel efficiency in consideration of the global environment. 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 process.

Although these techniques improve the balance between strength and ductility in hot-rolled steel sheets, there is no description of a method for improving the press formability by refining the structure of cold-rolled steel sheets. According to the study by the present inventors, when ordinary 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 obtain a rolled steel sheet. In particular, in the production of a cold-rolled steel sheet having a microstructure including a low-temperature transformation generation phase and residual austenite that requires annealing in a high temperature range of Ac ₁ point or higher, coarsening of crystal grains is remarkable, and ductility It is not possible to enjoy the advantages of the cold-rolled steel sheet having a superior structure.

  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. In a steel sheet containing retained austenite in the metal structure, austenite becomes martensite during processing and exhibits a large elongation due to transformation-induced plasticity, but the hole expandability is impaired due to 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 forming is possible. It is hard to say that it has sex. 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, or to perform a solution treatment for a long time at a high temperature, The increase in manufacturing cost and the decrease in productivity are remarkable.

  Patent Document 6 discloses high-tensile molten zinc having excellent ductility, stretch flangeability, and fatigue resistance, in which retained austenite and low-temperature transformation phase are dispersed in ferrite and tempered martensite having an average grain size of 10 μm or less. A plated 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 producing martensite and secondary annealing for tempering martensite and obtaining retained austenite are necessary. The characteristics are greatly impaired.

  In Patent Document 7, immediately after hot rolling, it is quenched at a cooling rate exceeding 600 ° C./s to 720 ° C. or less and held in a temperature range of 600 to 720 ° C. for 2 seconds or more, and the obtained hot rolled steel sheet is cold rolled. And a method for producing a cold-rolled steel sheet in which retained austenite is finely dispersed in a structure in which a low-temperature transformation phase is a main phase and subjected to annealing is disclosed.

  In Patent Document 8, the primary cooling is started at a cooling rate exceeding 600 ° C./s after a predetermined time has elapsed after completion of hot rolling, and the secondary cooling is started after the cooling is temporarily stopped. It is possible to perform the manufacturing method, and a manufacturing method with improved productivity is disclosed. Further, the same document discloses a manufacturing method for introducing fine ferrite into a hot-rolled steel sheet structure by performing cooling for about 3 to 15 seconds in a temperature range of 600 to 680 ° C. after secondary cooling. Yes. Furthermore, it is disclosed that after this cooling, the cold-rolled steel sheet is wound up by tertiary cooling to a temperature range of 650 ° C. or less at a cooling rate of 30 to 100 ° C./second (paragraphs 0088 to 0092 and 0116 to 0118).

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 JP 2013-14821 A International Publication No. 2013/125399

  The technique disclosed in the above-mentioned Patent Document 7 is a rapid cooling immediately after the end of hot rolling, so as not to release the processing strain accumulated in the austenite, but to transform the processing strain as a driving force, thereby fine grain structure. In the annealing process after cold rolling, annealing is performed to suppress the coarsening of the structure, so that the steel sheet has high strength, good ductility, good work hardenability, and good stretch flangeability. It is an excellent invention for obtaining a rolled steel sheet. However, this method requires an excessive cooling facility, resulting in a problem that productivity is lowered and production cost is increased.

  Similarly to Patent Document 7, since Patent Document 8 requires an excessive cooling facility, there is a problem in that productivity is reduced and production cost is increased.

  The present invention has been made in order to solve such problems, and more specifically, the subject is high tensile strength having excellent ductility, work-hardening properties and stretch flangeability and a tensile strength of 780 MPa or more. An object of the present invention is to provide a production method having low cost, high productivity, and high stable productivity for obtaining a cold-rolled steel sheet.

As a result of intensive studies to solve the problems of the rapid cooling method as in Patent Document 7 and Patent Document 8 described above, after hot rolling at ₃ or more points of Ar, 0% from the end temperature of the rolling to a temperature of 50 ° C. or less. We found it important to cool within 30 seconds. If the condition is satisfied, a hot-rolled sheet having a sufficiently fine crystal structure can be obtained even if an air cooling period of more than 0.3 seconds and less than 3.0 seconds is provided at a temperature exceeding 750 ° C. Such a hot-rolled sheet is cold-rolled and annealed to suppress the coarsening of austenite grains, so that the fine low-temperature transformation phase is the main phase and the phase containing fine residual austenite is the second phase. Since it has a metal structure, a high-tensile cold-rolled steel sheet having good ductility, good work hardenability and good stretch flangeability can be obtained. Further, since excessive rapid cooling is not necessary, it is possible to reduce the variation in cooling temperature, greatly reduce the amount of water required, and reduce equipment costs. Furthermore, using the air-cooled section, the thickness, shape, and temperature of the plate can be measured immediately after rolling, and accelerated rolling is possible, thereby improving productivity and stable productivity.

  In addition, the amount of reduction in the final rolling pass of hot rolling and the last immediately preceding rolling pass of the preceding stage and the pass time from the completion of rolling of the immediately preceding rolling pass to the start of rolling of the final rolling pass are set within a predetermined range. The organization can be further refined. For this reason, excessive rapid cooling becomes unnecessary, and the required amount of water can be greatly reduced and the equipment cost can be reduced.

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) to (F), wherein the main phase is a low-temperature transformation generation phase and the second phase has a metal structure containing residual 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. Hot rolling process in which Al: 2.00% or less and N: 0.010% or less are included, and the slab having a chemical composition composed of Fe and impurities is subjected to hot rolling in a temperature range of _three or more points of Ar. ;
(B) The hot-rolled steel sheet obtained by the hot rolling step is heated at an average cooling rate of 200 ° C./s to 600 ° C./s (hot rolling completion temperature) within 0.30 seconds after the hot rolling is completed. A first water-cooling step of cooling to −50 ° C.)
(C) About the hot-rolled steel sheet obtained by the first water-cooling step, a water-cooling stop step for continuously stopping the water-cooling in a temperature range higher than 750 ° C. for 0.3 seconds or more and less than 3.0 seconds;
(D) A second water cooling step of cooling the hot-rolled steel sheet obtained by the water cooling stop step to a temperature range of 750 ° C. or less within 6.0 seconds after completion of hot rolling. (E) Obtained by the second water cooling step. A cold rolling step in which the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet; and (F) the cold-rolled steel sheet is subjected to soaking in a temperature range of (Ac ₃ points-40 ° C) or higher. The annealing process which cools to the temperature range of 500-300 degreeC, and hold | maintains for 30 seconds or more in this temperature range.

  (2) The chemical composition is selected from the group consisting of Ti: less than 0.05%, Nb: less than 0.050%, and V: 0.50% or less 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% in place 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:

  (5) In the hot rolling step, the time t between passes, which is the time from the completion of rolling of the last rolling pass performed immediately before the final rolling to the start of rolling of the final rolling pass, satisfies the following formula. The method for producing a cold-rolled steel sheet according to any one of (1) to (4) above, wherein the amount of reduction in each of the immediately preceding rolling pass and the final rolling pass is 15% or more.

0.002 / exp (−6080 / (T + 273)) ≦ t ≦ 2.0
In the formula, t: time (seconds) between passes from the completion of rolling of the last rolling pass immediately before the final rolling pass to the start of rolling of the final rolling pass, T: immediately before the final rolling pass. The rolling completion temperature (° C.) of the rolling pass.

  According to the present invention, it is possible to obtain a high-tensile cold-rolled steel sheet having sufficient ductility, work-hardening property and stretch flangeability that can be applied to processing such as press molding without providing excessive cooling equipment. The present invention greatly contributes to the development of industries, such as contributing to the solution of global environmental problems through weight reduction of automobile bodies.

  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 embodiment has a 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. If the main phase is polygonal ferrite, it is difficult to ensure tensile strength and stretch flangeability. The low temperature transformation generation phase refers to a phase and structure produced by low temperature transformation such as martensite and bainite. Besides these, bainitic ferrite and tempered martensite are exemplified. Bainitic ferrite is distinguished from polygonal ferrite from the point of low dislocation density and from bainite from the point of low iron carbide in the grains. This low temperature transformation product phase may contain two or more phases and structures, such as martensite and bainitic ferrite. The main phase means a phase or structure having the maximum volume fraction, and the second phase means a phase and structure other than the main phase. 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 6.0%. More preferably, it is over 8.0%, particularly preferably over 10.0%. On the other hand, if the volume ratio of retained austenite is excessive, stretch flangeability deteriorates. Therefore, the volume ratio of retained austenite is preferably less than 18.0%. More preferably, it is less than 16.0%, and particularly preferably less than 14.0%.

  In cold-rolled steel sheets that have a low-temperature transformation generation phase as the main phase and a retained austenite in the second phase, reducing the retained austenite remarkably improves ductility, work hardening and stretch flangeability, so the average grain size of retained austenite is reduced. The thickness 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 cold-rolled steel sheets that have a low-temperature transformation generation phase as the main phase and 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 will be It is easily damaged. Therefore, the number density of the retained austenite grains having a grain size of 1.2 μm or more is preferably 3.0 × 10 ⁻² particles / μm ² or less. 2.0 × 10 ⁻² pieces / μm ² or less is more preferable, 1.5 × 10 ⁻² pieces / μm ² or less is particularly preferable, and 1.0 × 10 ⁻² pieces / μm ² or less. Most preferred.

  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 7.0%, particularly preferably more than 11.0%. 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 over 6.0%, particularly preferably over 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 an arbitrary position of the steel sheet, polishing a longitudinal section parallel to the rolling direction, and a position on the inner side of the sheet thickness from the steel sheet surface. In FIG. 5, the metal structure is observed using SEM, 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 determined 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.

  The volume fraction of retained austenite is obtained by collecting a test piece from an arbitrary position of the steel plate, chemically polishing the rolled surface from the steel plate surface to a position inside 1/4 of the plate thickness, and measuring the X-ray diffraction intensity using XRD. Ask.

  The particle size of retained austenite grains and the average particle size of retained austenite are measured as follows. That is, a specimen is taken from an arbitrary position of a steel plate, a longitudinal section parallel to the rolling direction is electrolytically polished, and a metal structure is formed using an SEM equipped with EBSP at a position ¼ inside the plate thickness from the steel plate surface. Observe. An area observed as a phase (fcc phase) having a face-centered cubic crystal structure (fcc phase) and surrounded by a parent phase is defined as one residual austenite grain, and by image processing, the number density of residual austenite grains (number of grains per unit area) ) 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 the electron beam in 0.1 μm increments in the region of 50 μm or more in the plate thickness direction and 100 μm or more in the rolling direction. Of 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. Further, in order to prevent the residual austenite particle size from being excessively evaluated due to measurement noise, the average particle size is calculated using only the retained austenite particles having an equivalent circle diameter particle size of 0.15 μm or more as effective particles.

  In the present embodiment, in the case of a cold-rolled steel sheet, the position of a quarter depth of the sheet thickness from the surface of the steel sheet, and in the case of a plated steel sheet, the steel sheet that is the base material from the boundary between the steel sheet that is the base material and the plating layer. The above-described metal structure is defined at a position of a quarter depth of the plate thickness.

As a mechanical property that can be realized based on the above-described characteristics on the metal structure, the steel plate of the present embodiment preferably has a tensile strength (TS) of 780 MPa or more in order to ensure shock absorption. 950 MPa or more is more preferable. Moreover, 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, Japanese Industrial Standard. When the work hardening index calculated in accordance with JIS Z2253 and the strain range is 5 to 10% is n, and the hole expansion ratio measured in accordance with Japan Iron and Steel Federation Standard JFST1001 is λ, the value of TS × El is It is preferable that the value of 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. 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 ductility from the balance between strength and work hardening index, and TS ^1.7 × λ Is an index for evaluating hole expandability from the balance between strength and hole expansion rate. The work hardening index is expressed as an n value with respect to a strain 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%.

El = El ₀ × (1.2 / t ₀ ) ^0.2 (1)
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 total elongation corresponding to the case where the plate thickness is 1.2 mm.

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-described action. Therefore, the Si content is more than 0.10%. Preferably it is 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, it is less than 1.80%, Most preferably, it is 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 3.50% or less Mn has an effect of improving the hardenability of steel and is an effective element for obtaining the above metal structure. When 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%. It is preferably more than 1.50%, more preferably more than 1.80%, particularly preferably more than 2.10%. If the Mn content is excessive, 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. , 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 steel as an impurity, and segregates at 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 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 a function 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. 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 over 0.020%. Moreover, Al has the effect | action which improves the stability of austenite similarly to Si, and since it is an element effective in obtaining said metal structure, it can be contained. 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. The Al content is 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 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 embodiment 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, it has the effect of increasing the working strain and refining the 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.

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 0.0030% 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 0.0020% or less, the Mg content is 0.0020% or less, the REM content is 0.0020% 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 (1) Slab The steel having the above-described chemical composition is melted by a known means and then made into a steel ingot by a continuous casting method, or is made into a steel ingot by an arbitrary casting method and then divided into pieces. It is made into a steel piece by a rolling method or the like. 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 Ar ₃ points or more as described later.

(2) Hot rolling process Hot rolling is completed in a temperature range of ₃ or more points of Ar in order to refine the 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 points or higher and higher than 850 ° C., and particularly preferably Ar ₃ points 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 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 ₃ points or higher and higher than 780 ° C., more preferably ₃ points or higher and higher 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 temperature fluctuation over the entire length of the rough rolled material at the start of finish rolling to 140 ° C. or lower by heating so that the rear end of the rough rolled material is higher than the front end. 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.

  As for the reduction amount of hot rolling, it is preferable that the reduction amount in the final rolling pass and the immediately preceding final rolling pass in the preceding stage 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 reduction amount of the final rolling pass and the rolling roll immediately before the final rolling is more preferably more than 25%, particularly preferably more than 30%. 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 and the preceding 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.

  It is preferable that the time between passes from the completion of rolling in the last rolling pass to the start of rolling in the final rolling pass satisfies the following formula (5).

0.002 / exp (−6080 / (T + 273)) ≦ t ≦ 2.0 (5)
Here, the meaning of each symbol is t (seconds) between passes from the completion of rolling of the last rolling pass immediately before the final rolling pass to the start of rolling of the final rolling pass, and T is the final rolling pass. Is the rolling completion temperature (° C.) of the last immediately preceding rolling pass carried out one before. By satisfying the above formula (5), the recrystallization of austenite is promoted and the austenite is promoted between the passes from the completion of rolling of the last rolling pass immediately before the final rolling pass to the start of rolling of the final rolling pass. Therefore, the recrystallized austenite grains are reduced in size during rolling, and it becomes easier to obtain a fine hot-rolled steel sheet structure. In formula (5), the rolling completion temperature T of the last rolling pass immediately before the final rolling pass is at least the hot rolling completion temperature (of the final rolling pass) in order to obtain the above-described reduction amount at the time of final rolling. It is preferable that the temperature is equal to or higher than the temperature at the completion of rolling. Further, the inter-pass time t satisfying the formula (5) is preferably 1.5 seconds or less, and more preferably 1.0 seconds or less, in order to obtain a finer structure of the hot-rolled steel sheet.

(3) 1st cooling process After hot rolling, it cools to the temperature range below (hot rolling completion temperature -50 degreeC) within 0.30 second 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 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. The release of processing strain is suppressed as the time until cooling is shorter and the temperature is lower. Therefore, the time required for cooling to a temperature range of (hot rolling completion temperature −50 ° C.) or less is preferably within 0.25 seconds, more preferably within 0.20 seconds, and 0.15 seconds. If it is within, it is especially preferable. The time zone until the start of cooling is an uncooled zone such as air cooling or standing cooling.

  Moreover, it is preferable to cool to the temperature range below (hot rolling completion temperature-75 degreeC), and it is further more preferable to cool to the temperature range below (hot rolling completion temperature-100 degreeC). The average cooling rate does not need to be specified in particular. However, the higher the average cooling rate, the more the processing strain is released, so the average cooling rate is preferably 200 ° C./s or more, and 300 ° C./s or more. More preferred. Thereby, the structure of the hot-rolled steel sheet can be further refined. On the other hand, in order to increase the average cooling rate, an excessive cooling device is required and the productivity and operation stability are deteriorated. Therefore, the average cooling rate is preferably 600 ° C./s or less, and 500 ° C./s or less. More preferably.

  Equipment for cooling is not particularly specified, but industrially, it is preferable to use a water spray device with a high water density, and a water spray header is arranged between the rolling plate conveyance rollers, and sufficient from the top and bottom of the rolling plate. A method of injecting high-pressure water having a water density is exemplified.

(4) Cooling stop step, second water cooling step (Hot rolling completion temperature-50 ° C) After cooling to a temperature range of less than or equal to 750 ° C, the time is 0.3 seconds or more and less than 3.0 seconds, An uncooled zone such as air cooling or standing cooling is provided, and the temperature is cooled to 750 ° C. or less within 6.0 seconds after the end of rolling. This reduces the temperature variation of the cooling and improves the uniformity of the material characteristics. In addition, by using this air cooling section, it is possible to measure the plate thickness, plate shape, and plate temperature immediately after rolling, enabling accelerated rolling, and productivity is dramatically increased.

  If the stop time of the uncooled zone is less than 0.3 seconds, nucleation of ferrite grains becomes insufficient and a mixed grain structure tends to be formed, the structure is not sufficiently refined, and good mechanical properties cannot be obtained. Furthermore, it becomes difficult to sufficiently reduce the temperature variation in cooling, and it becomes difficult to improve the uniformity of the characteristics of the material. Moreover, it becomes difficult to measure the plate thickness, plate shape, plate temperature, and the like using this water cooling stop section, and it is difficult to desire improvement in productivity by accelerated rolling. Accordingly, the water cooling stop time is set to 0.3 seconds or more. Preferably it is 0.4 second or more.

  Here, the “mixed grain structure” refers to a structure in which fine crystal grains and coarse crystal grains are mixed.

  If the time of the uncooled zone is long, the processing strain introduced into the austenite is released and the structure becomes coarse, so the time of the uncooled zone is set to less than 3.0 seconds. The time of the non-cooling zone is preferably less than 2.5 seconds, and more preferably less than 2.0 seconds.

  Further, when the temperature of the non-cooling zone is 750 ° C. or lower, or when the time required for cooling to 750 ° C. or lower exceeds 6.0 seconds, coarsening similarly occurs and the characteristics of the final product deteriorate. . The time required for cooling to 750 ° C. or lower is preferably within 5.0 seconds, more preferably within 4.0 seconds, and particularly preferably within 3.0 seconds. The cooling condition of 750 ° C. or lower is not particularly specified, but it is preferable to hold it for 1 second or more in a temperature range of 750 ° C. to 600 ° C. 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 750 to 600 ° C. is preferably within 10 seconds. From the viewpoint of suppressing ferrite growth and further miniaturizing the structure, the water cooling performed after the water cooling stop is more preferably cooled at a cooling rate of 30 ° C./s or more, and 50 ° C./s. The above is more preferable, and 80 ° C./s or more is particularly preferable.

(5) Third water cooling step After holding in the temperature range of 750 to 600 ° C., it is possible 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. preferable.

  The winding temperature is preferably over 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 winding temperature is preferably less than 650 ° C, and more preferably less than 620 ° C.

(6) Cold rolling process 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 to make the metal structure after cold rolling and annealing uniform and further improve stretch flangeability, the cold pressure ratio is preferably set to 40% or more. 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%.

(7) Annealing process The steel sheet after cold rolling is annealed after being subjected to a treatment such as degreasing according to a known method as necessary. The lower limit of the soaking temperature in 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 (Ac ₃ points−20 ° C.), more preferably Ac ₃ points. 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 points + 50 ° C.), and it is particularly preferable to be less than (Ac ₃ points + 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 hardening, cooling at 50 ° C. or more from the soaking temperature at a cooling rate of less than 10.0 ° C./s. You may do it. 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.

  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 650-450 degreeC with the cooling rate of 15 degrees C / s or more. It is more preferable to cool the temperature range of 650 to 500 ° 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 more preferably less than 130 ° C./s.

  Moreover, in order to obtain a retained austenite, it hold | maintains for 30 second or more in a 500-300 degreeC temperature range. 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, it is preferable to hold the holding time for 60 seconds or more. 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.

(8) Plating process When producing an electroplated steel sheet, the cold-rolled steel sheet produced by the above-described method may be electroplated according to a conventional method, and the chemical composition of the plating film is 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, it is preferable to hold the holding time for 60 seconds or more. 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 of the plating film is not limited, and the types of hot dip plating are hot dip galvanizing, alloying 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. are illustrated.

  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 is heated to 1200 ° C. using an electric heating furnace and held for 60 minutes, and then hot rolling is performed for 6 passes in a temperature range of Ar ₃ or more using an experimental hot rolling mill, A 2-3 mm hot-rolled sheet was produced. The hot rolling conditions and the cooling conditions after hot rolling were the conditions shown in Table 2. In addition, when the cooling stop temperature in the primary cooling was low, the secondary cooling and the tertiary cooling were omitted. The primary cooling corresponds to the first water cooling step in the present invention, and the secondary cooling corresponds to the second water cooling step in the present invention. In addition, after cooling, after charging in an electric heating furnace maintained at the same temperature as the coiling temperature and holding for 30 minutes, the furnace is cooled to room temperature at a cooling rate of 20 ° C./h and gradually cooled after winding. Was simulated.

  The obtained hot-rolled steel sheet was pickled to form a cold-rolled base material, 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 3 at a heating rate of 2 ° C./s. Soaked for 2 seconds. Then, it cooled to 700 degreeC which is the primary cooling stop temperature, it cooled to the various secondary cooling stop temperature shown in Table 3 by making the average cooling rate from 700 degreeC into 60 degreeC / s, and hold | maintained at that temperature for 330 seconds 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.

  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. Among them, the fcc phase was determined by using data with Confidence 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 particle diameter 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 obtained by conducting a tensile test on a JIS No. 5 tensile test specimen taken along the direction perpendicular 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 is 1.2 mm was obtained. The work hardening index (n value) was calculated by performing 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%.

  Stretch flangeability was evaluated by measuring the hole expansion rate (λ) by the following method. A 100 mm square square base plate was sampled from the annealed steel plate, a punched hole with a diameter of 10 mm was formed with a clearance of 12.5%, and the punched hole was expanded from the sag side with a conical punch with a tip angle of 60 °, and the crack penetrated the plate thickness. 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.

The test results (test numbers 5 to 9, 11, 13 to 15, and 17 to 27) for the steel sheets within the range defined by the present invention all have a TS × El value of 16500 MPa% or more, and TS × n. The value of the value was 150 or more and the value of TS ^1.7 × λ was 4700000 MPa or more and ^1.7 % or more, and good ductility, work hardenability and stretch flangeability were exhibited.

  The test results (test numbers 1-4, 10, 12, 16) for steel sheets whose steel composition or manufacturing method deviates from the range specified by the present invention are any or all of ductility, work hardenability and stretch flangeability. It was inferior.

Specifically, in the test using steel A (Trial No. 1), since the Si content in the steel is small, the average grain size of retained austenite is large, the volume ratio of retained austenite is low, ductility, Work hardening and stretch flangeability are poor. Tests using Steel B (Trial No. 2), tests using Steel C (Trial No. 4), and tests using Steel H (Trial No. 10) for cooling is slow, the average particle size of the residual austenite is large, also, N _R is large, ductility, poor work hardening properties and stretch flangeability. Test using a steel I (Run No. 12), in order uncooled time after the primary cooling is long, the average particle size of the residual austenite is large, large N _R, work hardenability and stretch flangeability Is bad.

  In the test using Steel B (Trial No. 3) and the test using Steel K (Trial No. 16), since the soaking temperature during annealing was too low, a metal structure having a low-temperature transformation generation phase as the main phase was obtained. It is not stretched and the flangeability is poor.

Claims (5)

It has the following steps (A) to (F), the main phase is a low-temperature transformation generation phase, the second phase has a metal structure containing residual austenite, and has a tensile strength (TS) of 780 MPa or more. The value obtained by converting the total elongation (El ₀ ) in the direction orthogonal to the rolling direction into the total elongation equivalent to a plate thickness of 1.2 mm based on the following formula (i) is El, and the strain range is based on Japanese Industrial Standard JIS Z2253. When the work hardening index calculated as 5 to 10% is n value, and the hole expansion ratio measured in accordance with Japan Iron and Steel Federation Standard JFST1001 is λ, the value of TS × El is 15000 MPa% or more, TS × n value values above 150 ^MPa, the value of ^{TS 1.7} × lambda is Ru der least ^1.7% 4500000MPa, method for producing cold-rolled steel sheet:
(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. Hot rolling process in which Al: 2.00% or less and N: 0.010% or less are included, and the slab having a chemical composition composed of Fe and impurities is subjected to hot rolling in a temperature range of _three or more points of Ar. ;
(B) The hot-rolled steel sheet obtained by the hot rolling step is heated at an average cooling rate of 200 ° C./s to 600 ° C./s within 0.30 seconds after completion of the hot rolling (hot rolling completion temperature). A first water-cooling step of cooling to −50 ° C.)
(C) About the hot-rolled steel sheet obtained by the first water-cooling step, a water-cooling stop step for continuously stopping water cooling for 0.3 seconds or more and less than 3.0 seconds in a temperature range exceeding 750 ° C .;
(D) a second water cooling step of cooling the hot rolled steel sheet obtained by the water cooling stop step to a temperature range of 750 ° C. or less within 6.0 seconds after completion of hot rolling;
(E) a cold rolling process in which the hot-rolled steel sheet obtained by the second water-cooling process is cold-rolled to obtain a cold-rolled steel sheet; and (F) the cold-rolled steel sheet (Ac ₃ points-40 ° C) or higher. After performing soaking in the temperature range, it is cooled to a temperature range of 500 to 300 ° C. and kept in the temperature range for 30 seconds or more.
El = El ₀ × (1.2 / t ₀ ) ^0.2 (i)
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 total elongation corresponding to the case where the plate thickness is 1.2 mm.   The chemical composition is one type selected from the group consisting of Ti: less than 0.05%, 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 a group. In the hot rolling step, the time t between passes, which is the time from the completion of rolling of the last rolling pass immediately before the final rolling to the start of rolling of the final rolling pass, satisfies the following formula (1). The method for producing a cold-rolled steel sheet according to any one of claims 1 to 4, wherein the amount of reduction in each of the immediately preceding rolling pass and the final rolling pass is 15% or more.
0.002 / exp (−6080 / (T + 273)) ≦ t ≦ 2.0 (1)
In the formula (1), t: time between passes (seconds) from the completion of rolling of the last rolling pass immediately before the final rolling pass to the start of rolling of the final rolling pass, T: one before the final rolling pass It is the rolling completion temperature (° C.) of the rolling roll immediately before the final rolling.