JP5648597B2 - 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, such press formability and high strength of the steel sheet are contradictory properties, and it is difficult to satisfy these properties at the same time.

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

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

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

  Regarding cold-rolled steel sheets having a microstructure, in Patent Document 4, residual austenite having an average crystal grain size of 5 μm or less is dispersed in ferrite having an average crystal grain size of 10 μm or less. An excellent high strength cold rolled steel sheet for automobiles is disclosed. A steel sheet containing retained austenite in the metal structure exhibits a large elongation due to transformation-induced plasticity (TRIP) generated by austenite becoming martensite during processing, but the hole expandability is impaired by the formation of hard martensite. In the cold-rolled steel sheet disclosed in Patent Document 4, the 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 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 obtaining retained austenite are required. Is greatly impaired.

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

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

  The technique disclosed in the above-mentioned Patent Document 7 does not release the processing strain accumulated in austenite after the end of hot rolling, and forms a fine grain structure by transforming ferrite using the processing strain as a driving force. This is excellent in that a cold-rolled steel sheet having improved properties and thermal stability can be obtained.

  However, in recent years, there has been a demand for cold-rolled steel sheets having high strength, good ductility, good work hardenability and good stretch flangeability at the same time due to the need for higher performance.

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

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

  A series of test steels are in mass%, C: 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 lower, air-cooled for 5 to 10 seconds, and then cooled to various temperatures at a cooling rate of 90 ° C./s or less. This cooling temperature is taken as the coiling temperature and is maintained at the same temperature. Then, after charging in an electric heating furnace and holding for 30 minutes, the furnace was cooled at a cooling rate of 20 ° C./h to simulate slow cooling after winding. The obtained hot-rolled steel sheet was heated to various temperatures and then cooled to obtain a hot-rolled annealed steel sheet. The obtained hot-rolled annealed steel sheet was pickled and cold-rolled to a thickness of 1.0 mm at a rolling rate of 50%. Using the continuous annealing simulator, the obtained cold rolled steel sheet was heated to various temperatures and held for 95 seconds, and then cooled to obtain an annealed steel sheet.

  Test specimens for structure observation were collected from hot-rolled steel sheets, hot-rolled annealed steel sheets, and annealed steel sheets, and the plates were removed 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 1/4 depth position of the thickness, 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 (E) 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, as the annealing temperature rises, the ductility and stretch flangeability of the cold-rolled steel sheet improve, but if the annealing temperature is too high, the austenite grains become coarser. The ductility and stretch flangeability of the annealed steel sheet may deteriorate rapidly.

  (B) When hot-rolled sheet steel heated to a temperature range of 300 ° C. or higher is applied to a hot-rolled steel sheet manufactured at a coiling temperature of less than 400 ° C., this may occur when annealing at a high temperature after cold rolling. The coarsening of austenite grains is suppressed. The reason for this is not clear, but (a) immediately after rapid cooling, the low-temperature transformation phase is refined in the metal structure of the hot-rolled steel plate. Therefore, when the hot-rolled steel plate is annealed, iron carbide precipitates finely in the low-temperature transformation phase. (B) Since iron carbide functions as a nucleation site in the reverse transformation to austenite during annealing after cold rolling, the nucleation frequency increases as iron carbide precipitates finely, and austenite becomes finer (C) It is estimated that the undissolved iron carbide is caused by austenite grain refinement in order to suppress grain growth of austenite.

  (C) 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) the iron carbide becomes finer and the number density increases as the Si content increases, and (b) this further increases the nucleation frequency in the reverse transformation to austenite. It is presumed that this is due to the increase in the amount of undissolved iron carbide and the austenite grain growth being further suppressed and the austenite further becoming finer.

  (D) When soaking and cooling at a high temperature while suppressing the coarsening of austenite grains, the coarse austenite contains fine residual austenite and fine polygonal ferrite in the second phase, with the fine low-temperature transformation generation phase as the main phase. A metal structure with few grains is obtained.

(E) 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 a steel containing a certain amount or more of Si, immediately after quenching, coiled at a low temperature coiled, annealed hot-rolled steel sheet, cold-rolled, high-temperature cold-rolled steel sheet By cooling after annealing with, the main phase is a low-temperature transformation generation phase, the second phase contains fine residual austenite and polygonal ferrite, and has a metal structure with few coarse austenite grains, ductility, work hardenability and It was found that a cold-rolled steel sheet having excellent stretch flangeability 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) to (D), wherein the main phase is a low-temperature transformation generation phase and the second phase has a metal structure containing retained austenite and polygonal ferrite: :
(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. The slab containing a chemical composition containing Al: 2.00% or less and N: 0.010% or less, with the balance being Fe and impurities, the final one-pass reduction amount is more than 15% and 55% in terms of sheet thickness reduction rate. And hot rolling to complete rolling in a temperature range of Ar ₃ points or more and less than 950 ° C. to form a hot rolled steel sheet, and the hot rolled steel sheet is 780 ° C. or lower within 0.4 seconds after completion of the rolling. A hot rolling step of cooling to a temperature range of less than 400 ° C. and winding in a temperature range of less than 400 ° C .;
(B) A hot-rolled sheet annealing step in which the hot-rolled steel sheet is subjected to hot-rolled sheet annealing to a temperature range of 300 ° C. or higher and lower than 750 ° C. to obtain a hot-rolled annealed steel sheet;
(C) Cold rolling step of cold rolling the hot-rolled annealed steel sheet to obtain a cold-rolled steel sheet; and (D) (Ac ₃ points−40 ° C.) or more (Ac ₃ points + 100 ° C.) on the cold rolled steel sheet. was subjected to a soaking treatment for holding less than 150 seconds in a temperature range of less than, 500 ° C. and cooled to a temperature range of not lower than below 300 ° C., annealing step of holding at that temperature range for more than 30 seconds.

(2) in the step (D), the applied in a temperature range of less than soaking (Ac ₃ point -40 ° C.) or higher (Ac ₃ point + 50 ° C.), a manufacturing method of a cold-rolled steel sheet according to (1) .
(3) In the step (D), after the soaking, the method for producing a cold-rolled steel sheet according to (1) or (2), wherein cooling is performed at 50 ° C. or more at a cooling rate of less than 10.0 ° C./s. .

(4) 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 manufacturing method of the cold rolled steel sheet in any one of said (1) to said (3) containing 1 type or 2 types or more.
(5) 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 manufacturing method of the cold rolled steel sheet in any one of said (1) to said (4) containing 1 type or 2 types or more.
(6) 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 manufacturing method of the cold-rolled steel plate in any one of said (1) to said (5) containing 1 type or 2 types or more selected from the group which consists of the following.

  According to the present invention, a high-tensile cold-rolled steel sheet having sufficient ductility, work-hardening properties, and stretch flangeability that can be applied to processing such as press forming can be obtained. Therefore, the present invention greatly contributes to industrial development, such as being able to contribute to solving global environmental problems through weight reduction of automobile bodies.

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

1. Metal structure The cold-rolled steel sheet of the present invention has a metal structure in which the main phase is a low-temperature transformation generation phase and the second phase contains residual austenite and polygonal ferrite. 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 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%. This volume fraction is more preferably 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 having a metal structure containing a low-temperature transformation-forming phase as the main phase and residual austenite in the second phase, the fineness of ductile, work hardenability and stretch flangeability is significantly improved when the residual austenite is refined. The average particle size 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 preferably, it is not more than μm ² .

  In order to further improve the ductility and work hardening, 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%.

  Moreover, since the effect of improving ductility and work-hardenability increases as the polygonal ferrite is 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 the stretch flangeability, the volume ratio of tempered martensite contained in the low-temperature transformation generation phase that is the main phase is preferably less than 50.0% with respect to the entire structure. More preferably, it is less than 35.0%, particularly preferably less than 10.0%.

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

  The metal structure of the cold rolled steel sheet according to the present invention is measured as follows. That is, the volume ratio of the low-temperature transformation generation phase and polygonal ferrite is obtained by taking a test piece from a steel plate, polishing a longitudinal section parallel to the rolling direction, and subjecting it to a corrosion treatment with nital. The metal structure is observed using the SEM at the depth position, and the area ratios of the low-temperature transformation generation phase and the polygonal ferrite are measured by image processing, and the respective volume ratios are obtained assuming that the area ratio is equal to the volume ratio. The 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, and obtaining the equivalent circle 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 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. 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 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. 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. 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 that is the base material from the boundary between the steel sheet that is the base material and the plating layer in the case of the plated steel sheet in the case of a galvanized steel sheet. The above-mentioned metal structure is defined at the 1/4 depth position.

As a mechanical property that can be realized based on the above characteristics on the metal structure, the 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. Preferably 950 MPa or more. Moreover, in order to ensure ductility, it is preferable that TS is less than 1180 MPa. In addition, from the viewpoint of press formability, El is a value obtained by converting the total elongation (El ₀ ) in the direction orthogonal to the rolling direction into a total elongation equivalent to a plate thickness of 1.2 mm based on the following formula (1). N value, which is a work hardening index calculated using two nominal strains of 5% and 10% and test force corresponding to these, with a strain range of 5 to 10% in accordance with the standard JIS Z2253, Japan Iron and Steel Federation When λ is the hole expansion rate measured in accordance with the standard JFST1001,
-The value of TS x El is 15000 MPa% or more,
It is preferable that the value of TS × n value is 150 MPa or more, and that the value of TS ^1.7 × λ is 4500000 MPa ^1.7 % or more.

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 the total elongation corresponding to the case where the plate thickness is 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. The value of TS × El is 19000MPa% or more, the value of TS × n value is more than 160 MPa, and even more preferably a value of TS ^1.7 × lambda is 5500000MPa ^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 as close to 0% as possible. When it is contained for the purpose of promoting deoxidation, sol. It is preferable to contain 0.0050% or more as Al. Further preferred sol. The Al content is more than 0.020%. Al, like Si, has the effect of increasing the stability of austenite and is an effective element for obtaining the above metal structure. Therefore, Al can be contained for this purpose. In this case, sol. The Al content is preferably more than 0.040%, more preferably more than 0.050%, particularly preferably more than 0.060%.

  On the other hand, sol. If the Al content is too high, not only surface flaws are likely to occur due to alumina, but the transformation point is greatly increased, and it becomes difficult to obtain a metal structure having a low-temperature transformation generation phase as a main phase. Therefore, sol. Al content shall be 2.00% or less. Preferably it is less than 0.60%, more preferably less than 0.20%, particularly preferably less than 0.10%.

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

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

  Therefore, the Ti content is less than 0.050%, the Nb content is less than 0.050%, and the V content is 0.50% or less. The Ti content is preferably less than 0.040%, more preferably less than 0.030%, the Nb content is preferably less than 0.040%, more preferably less than 0.030%, and the V content is Preferably it is 0.30% or less, More preferably, it is less than 0.050%. In order to more reliably obtain the effect of the above action, it is preferable to satisfy any of Ti: 0.005% or more, Nb: 0.005% or more, and V: 0.010% or more. When Ti is contained, the Ti content is more preferably 0.010% or more, and when Nb is contained, the Nb content is more preferably 0.010% or more, and V is When contained, the V content is more preferably set to 0.020% or more.

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

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

3. Manufacturing conditions The steel having the above-mentioned chemical composition is melted by a known means and then made into a steel ingot by a continuous casting method, or a method of rolling into pieces after making it into an ingot by any casting method, etc. It is made into a billet. In the continuous casting process, in order to suppress the occurrence of surface defects due to inclusions, it is preferable to cause an external additional flow such as electromagnetic stirring in the molten steel in the mold. The steel ingot or steel slab may be reheated once it has been cooled and subjected to hot rolling. The steel ingot in the high temperature state after continuous casting or the steel slab in the high temperature state after partial rolling is used as it is. Alternatively, it may be kept hot or subjected to auxiliary heating for hot rolling. In the present specification, such steel ingots and steel slabs are collectively referred to as “slabs” as materials for hot rolling.

The temperature of the slab to be subjected to hot rolling is preferably less than 1250 ° C. and more preferably 1200 ° C. or less in order to prevent coarsening of austenite. The lower limit of the temperature of the slab to be subjected to hot rolling is not particularly limited as long as it is a temperature at which hot rolling can be completed at an Ar ₃ point or higher as described later.

Hot rolling is completed in a temperature range of Ar ₃ or higher in order to refine the metal structure of the hot-rolled steel sheet by transforming austenite after completion of rolling. If the 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 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.

  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 processing strain introduced into austenite, refines the metal structure of the hot-rolled steel sheet, finely precipitates iron carbide in the hot-rolled annealed steel sheet, and refines the metal structure after cold rolling and annealing. This is to improve ductility, work hardening and stretch flangeability. 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 plate and rolling is performed with a reduced friction coefficient.

  After hot rolling, it is rapidly cooled to a temperature range of 780 ° C. or less within 0.40 seconds after the completion of rolling. This suppresses the release of processing strain introduced into austenite by rolling, transforms austenite using the processing strain as a driving force, refines the metal structure of the hot-rolled steel sheet, and refines the iron carbide in the hot-rolled annealed steel sheet. This is because the metal structure after precipitation, cold rolling and annealing is refined 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, the steel sheet is wound in a temperature range below 400 ° C. This is because if the coiling temperature is 400 ° C. or higher, iron carbide cannot be finely precipitated in the hot-rolled sheet annealing step, and the metal structure after cold rolling and annealing becomes coarse. The winding temperature is preferably less than 300 ° C. More preferably, it is less than 200 degreeC, and it is especially preferable that it is less than 100 degreeC.

  The hot-rolled steel sheet is annealed after being subjected to treatment such as degreasing according to a known method as necessary. Annealing performed on a hot-rolled steel sheet is called hot-rolled sheet annealing, and a steel sheet after hot-rolled sheet annealing is called a hot-rolled annealed steel sheet. Before hot-rolled sheet annealing, descaling may be performed by pickling or the like. The minimum of the heating temperature in hot-rolled sheet annealing shall be 300 degreeC or more. This is because iron carbide is finely precipitated to refine the metal structure after cold rolling and annealing. As the heating temperature is higher, Mn and Cr are concentrated in the iron carbide, and the effect of preventing the coarsening of the austenite grains by the iron carbide is increased. If it exceeds 500 ° C., it is more preferable, and if it exceeds 600 ° C., it is particularly preferable. On the other hand, if the heating temperature is too high, the iron carbide is coarsened and re-dissolved, and the effect of preventing the austenite grains from coarsening is impaired. Therefore, the heating temperature is preferably less than 750 ° C. If it is less than 700 degreeC, it is still more preferable, and if it is less than 650 degreeC, it is especially preferable.

  The holding time in hot-rolled sheet annealing need not be limited. A hot-rolled steel sheet manufactured through an appropriate immediate quenching process does not have to be held for a long time because the metal structure is fine and there are many precipitation sites of iron carbide, and iron carbide precipitates quickly. Since the productivity deteriorates when the holding time becomes long, the upper limit of the holding time is preferably less than 20 hours. If it is less than 10 hours, it is more preferable, and if it is less than 5 hours, it is especially preferable.

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

The steel sheet after cold rolling is annealed after being subjected to a treatment such as degreasing according to a known method, if necessary. The lower limit of the soaking temperature in the annealing is (Ac ₃ points−40 ° C.) or more. This is to obtain a metal structure in which the main phase is a low-temperature transformation generation phase and the second phase contains residual austenite. In order to increase the volume ratio of the low-temperature transformation generation phase and improve stretch flangeability, the soaking temperature is preferably more than (Ac ₃ point−20 ° C.), more preferably more than Ac ₃ point. However, if the soaking temperature becomes too high, the austenite becomes excessively coarse, and ductility, work hardenability and stretch flangeability tend to deteriorate. For this reason, it is preferable that the soaking temperature is less than (Ac ₃ point + 100 ° C.). The soaking temperature is more preferably less than (Ac ₃ point + 50 ° C.), and particularly preferably less than (Ac ₃ point + 20 ° C.). In order to promote the formation of fine polygonal ferrite and improve ductility and work hardening, the upper limit of the soaking temperature is preferably less than (Ac ₃ point + 50 ° C.), and (Ac ₃ point + 20 ° C.). ) Is more preferable.

  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. This heating rate is more preferably 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 the ductility and work hardenability, the soaking temperature is reduced to 50 ° C. at a cooling rate of less than 10.0 ° C./s. It is preferable to perform cooling as described above. 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 650-500 degreeC with the cooling rate of 15 degrees C / s or more. It is more preferable to cool the temperature range of 650 to 450 ° C. at a cooling rate of 15 ° C./s or more. The higher the cooling rate, the higher the volume ratio of the low temperature transformation product phase. Therefore, the cooling rate is more preferably 30 ° C./s, and particularly preferably 50 ° C./s. On the other hand, if the cooling rate is too high, the shape of the steel sheet is impaired, so the cooling rate in the temperature range of 650 to 500 ° C. is preferably 200 ° C./s or less. More preferably, it is less than 150 ° C./s, and particularly preferably less than 130 ° C./s.

  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 22 to 42% in terms of plate 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.

  The obtained hot-rolled steel sheet was heated to various heating temperatures shown in Table 2 at a heating rate of 50 ° C./h, and kept at room temperature at a cooling rate of 20 ° C./h after or without being held for various times. To obtain a hot-rolled annealed steel sheet. Hot-rolled sheet annealing was omitted for some hot-rolled steel sheets.

  The obtained hot-rolled annealed steel sheet or hot-rolled steel sheet is pickled to form a cold-rolled base material, which is cold-rolled at a reduction ratio of 50 to 60%, and has a thickness of 1.0 to 1.2 mm. Got. 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. Thereafter, primary cooling is performed to various temperatures shown in Table 2, and secondary cooling is performed from the primary cooling stop temperature to various temperatures shown in Table 2 at an average cooling rate of 60 ° C./s, and maintained at that temperature for 330 seconds. Then, it was cooled to room temperature to obtain an annealed steel plate.

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

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

Furthermore, after taking a specimen for EBSP measurement from the annealed steel sheet and electrolytically polishing the longitudinal section parallel to the rolling direction, the metal structure was observed at the 1/4 depth position of the sheet thickness from the steel sheet surface, and by image analysis, The particle size distribution of retained austenite grains and the average particle size of retained austenite were measured. Specifically, TSL OIM5 was used for the EBSP measuring device, and an electron beam was irradiated at a pitch of 0.1 μm in an area of 50 μm in the plate thickness direction and 100 μm in the rolling direction. The fcc phase was determined with valid data having a reliability index of 0.1 or more. 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. 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 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 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%. 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, underlined numerical values or symbols mean that they are outside the scope of the present invention.

As for the test results (test numbers 6-11, 13, 15-24) of the steel plates manufactured by the manufacturing method according to the present invention, the TS × El value is 16000 MPa% or more, and the TS × n value is It was 150 or more, and the value of TS ^1.7 × λ was 5700000 MPa ^1.7 % or more, and showed good ductility, work hardening property and stretch flangeability.

The test results (test numbers 7 to 11, 13, 15 to 24) in which the rolling reduction in the final one pass of hot rolling is more than 25% and the cooling stop temperature after annealing is 340 ° C. or higher are all TS × The El value was 19000 MPa% or more, the TS × n value was 160 or more, and the TS ^1.7 × λ value was 5700000 MPa ^1.7 % or more, and particularly good ductility, work hardening property and stretch flangeability were exhibited.

The rolling reduction in the final pass of hot rolling is more than 25%, and the soaking temperature in annealing is (Ac ₃ point −40 ° C.) or more and less than (Ac ₃ point + 50 ° C.), and after the soaking treatment, 10.0 ° C. / The test result (cooling number 7-11, 13, 15, 16, 18, 19, 21, 24) which cools 50 degreeC or more from soaking temperature with the cooling rate below s, and a secondary cooling stop temperature is 340 degreeC or more Each has a TS × El value of 20000 MPa% or more, a TS × n value of 165 or more, and a TS ^1.7 × λ value of 5700000 MPa ^1.7 % or more, and has extremely good ductility, work hardening and elongation. It showed flanging.

The test results (test numbers 1 to 5, 12, and 14) for steel sheets whose steel composition or manufacturing method deviated from the range defined by the present invention were inferior in ductility, work hardenability and stretch flangeability. . Specifically, in the test using Steel A (Test No. 1), since the Si content in the steel is small, the average grain size of retained austenite is large, the volume fraction of retained austenite is low, and polygonal The average grain size of ferrite is large, and work hardenability and stretch flangeability are poor. In the test using steel B (test number 2), since the time from the completion of hot rolling to the quenching stop is too long, N _R is large, the average grain size of polygonal ferrite is large, work hardening and Stretch flangeability is poor. In the test using steel J (Test No. 12), since the time from the completion of hot rolling to the rapid cooling stop is too long, N _R is large, and work hardenability and stretch flangeability are poor. In the test using Steel C (Test No. 4), since the heating temperature of hot-rolled sheet annealing is too low, N _R is large, the average grain size of polygonal ferrite is large, work hardening and stretch flangeability Is bad. In the test using steel C (test number 5), since hot-rolled sheet annealing was not performed, N _R was large, and ductility, work hardenability and stretch flangeability were poor. In the test using steel K (test number 14), since hot-rolled sheet annealing was not performed, N _R was large and work hardenability and stretch flangeability were poor. In the test using Steel B (Test No. 3), since the soaking temperature during annealing is too low, a metal structure having a low-temperature transformation generation phase as a main phase is not obtained, and stretch flangeability is poor.

Claims (6)

A method for producing a cold-rolled steel sheet comprising the following steps (A) to (D), wherein the main phase is a low-temperature transformation generation phase and the second phase has a metal structure containing retained austenite and polygonal ferrite:
(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. The slab containing a chemical composition containing Al: 2.00% or less and N: 0.010% or less, with the balance being Fe and impurities, the final one-pass reduction amount is more than 15% and 55% in terms of sheet thickness reduction rate. And hot rolling to complete rolling in a temperature range of Ar ₃ points or more and less than 950 ° C. to form a hot rolled steel sheet, and the hot rolled steel sheet is 780 ° C. or lower within 0.4 seconds after completion of the rolling. A hot rolling step of cooling to a temperature range of less than 400 ° C. and winding in a temperature range of less than 400 ° C .;
(B) A hot-rolled sheet annealing step in which the hot-rolled steel sheet obtained in the step (A) is subjected to hot-rolled sheet annealing to a temperature range of 300 ° C. or higher and lower than 750 ° C. to obtain a hot-rolled annealed steel sheet;
(C) Cold rolling step of cold rolling the hot-rolled annealed steel sheet to obtain a cold-rolled steel sheet; and (D) (Ac ₃ points−40 ° C.) or more (Ac ₃ points + 100 ° C.) on the cold rolled steel sheet. was subjected to a soaking treatment for holding less than 150 seconds in a temperature range of less than, 500 ° C. and cooled to a temperature range of not lower than below 300 ° C., annealing step of holding at that temperature range for more than 30 seconds. The method for producing a cold-rolled steel sheet according to claim 1, wherein, in the step (D), the soaking is performed in a temperature range of (Ac ₃ points-40 ° C) or more and less than (Ac ₃ points + 50 ° C).   The method for producing a cold-rolled steel sheet according to claim 1 or 2, wherein, in the step (D), after the soaking, cooling is performed at 50 ° C or more at a cooling rate of less than 10.0 ° C / s.   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 manufacturing method of the cold rolled steel plate in any one of Claims 1-3 containing 2 or more types.   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 manufacturing method of the cold-rolled steel plate in any one of Claims 1-4 containing 2 or more types.   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 manufacturing method of the cold-rolled steel plate in any one of Claims 1-5 containing 1 type, or 2 or more types selected from a group.