JP5664482B2 - Hot-dip cold-rolled steel sheet - Google Patents

Description

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

  Now that the industrial technology field is highly fragmented, special and advanced performance is required for materials used in each technical field. For example, steel sheets used by press forming are also required to have better formability with the diversification of press shapes. In addition, high strength is required, and application of high-tensile 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 steel sheets has been remarkably increasing in order to reduce vehicle weight and improve fuel efficiency. In press molding, since the thinner the steel sheet used, the easier it is to crack and wrinkle, a steel sheet with better ductility and stretch flangeability is required. However, these press formability and high strength of the steel sheet are contradictory characteristics, and it is difficult to satisfy these characteristics at the same time.

Until now, as a method for improving the press formability of a high-tensile cold-rolled steel sheet, many techniques relating to micronization of the microstructure have been proposed. For example, Patent Document 1 discloses a method for producing an ultrafine-grained high-strength hot-rolled steel sheet that performs rolling with a total rolling reduction of 80% or more in a temperature range near the Ar ₃ point in a hot rolling process. Document 2 discloses a method for producing ultrafine-grained ferritic steel in which rolling at a reduction rate of 40% or more is continuously performed in the hot rolling step.

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

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

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

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

  Patent Document 6 discloses a high-tensile melt excellent in ductility, stretch flangeability, and fatigue resistance, in which retained austenite and low-temperature transformation product phase are dispersed in ferrite and tempered martensite having an average crystal grain size of 10 μm or less. A galvanized steel sheet is disclosed. Tempered martensite is an effective phase for improving stretch flangeability and fatigue resistance, and it is said that these properties will be further improved 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 the austenite after the hot rolling is completed, and forms a fine grain structure by transforming ferrite using the processing strain as a driving force. And it is excellent in that a cold-rolled steel sheet with improved thermal stability can be obtained.

  However, in recent years, there has been a demand for hot-dipped cold-rolled steel sheets that simultaneously have high strength, good ductility, good work hardenability, and good stretch flangeability due to the need for higher performance.

  The present invention has been made to meet such a demand. Specifically, an object of the present invention is to provide a high-tensile hot-dip cold-rolled steel sheet having a tensile strength of 750 MPa or more and having excellent ductility, work-hardening property, and stretch flangeability.

As a result of intensive studies on the effects of chemical composition and production conditions on the mechanical properties of high-tensile hot-dip cold-rolled steel sheets, the present inventors have obtained the knowledge described in (A) to (G) below.
(A) Hot-rolled steel sheet manufactured through a so-called quenching process immediately after water rolling, immediately after hot rolling, specifically, quenching to a temperature range of 720 ° C. or less within 0.40 seconds after completion of hot rolling When the hot-rolled steel sheet manufactured by cold rolling is annealed by cold rolling, the ductility and stretch flangeability of the cold-rolled steel sheet improve as the annealing temperature rises, but if the annealing temperature is too high, the austenite grains become coarser. The ductility and stretch flangeability of the annealed steel sheet may deteriorate rapidly.

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

(C) In the winding process immediately after the rapid cooling, if the winding temperature is increased, austenite grain coarsening that may occur when annealing at a high temperature after cold rolling is suppressed. Also, immediately after the rapid cooling, in the winding process, the hot rolled steel sheet wound at a lower winding temperature is annealed in a temperature range of 500 ° C. or higher and Ac ₁ point or lower, and then cold rolled and annealed at a high temperature. In the same manner, coarsening of austenite grains is suppressed. Although the reason for this is not clear, (a) immediately after the rapid cooling, the hot-rolled steel sheet becomes finer, so that the precipitation amount of iron carbide in the hot-rolled steel sheet increases remarkably as the coiling temperature rises. Alternatively, immediately after quenching and winding at a low temperature, fine martensite is formed in the metal structure, and by further annealing the hot-rolled steel sheet, fine iron carbide precipitates in the metal structure, (b ) Since iron carbide functions as a nucleation site in the transformation from ferrite to austenite during annealing, the nucleation frequency increases as the precipitation amount of iron carbide increases, and austenite becomes finer. It is estimated that the solid solution iron carbide is caused by the austenite becoming finer in order to suppress the austenite grain growth.

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

(E) When soaking and cooling at a high temperature while suppressing coarsening of austenite grains, a metal structure containing a fine low-temperature transformation-forming phase as a main phase and a fine residual austenite in the second phase is obtained.
(F) By suppressing the formation of coarse retained austenite grains having a grain size of 1.2 μm or more, the stretch flangeability of a steel sheet having a low-temperature transformation generation phase as a main phase is improved. The reason for this is not clear, but (a) the retained austenite is transformed into hard martensite by processing. However, if the retained austenite grains are coarse, the martensite grains become coarse, stress concentration increases, (B) Since coarse retained austenite grains are martensite in the initial stage of processing, they tend to crack more than fine retained austenite grains. It is estimated that

  (G) As the annealing temperature rises, the fraction of the low-temperature transformation generation phase increases and the work hardening tends to deteriorate, but the formation of coarse residual austenite grains having a particle size of 1.2 μm or more is suppressed. Thus, it is possible to prevent the work hardenability from deteriorating in the steel sheet whose main phase is the low temperature transformation generation phase. The reason for this is not clear, but (a) coarse residual austenite grains become martensite at the initial stage of processing when the strain is less than 5%, and therefore contributes almost to the increase of the n value when the strain is 5 to 10%. It is presumed that (b) when the formation of coarse retained austenite grains is suppressed, the fine retained austenite grains that become martensite in a high strain region of 5% or more increase.

  From the above results, the steel containing a certain amount or more of Si is hot-rolled after increasing the final reduction amount, and then immediately quenched and wound in a coil shape at a high temperature, or wound at a low temperature and a predetermined temperature. After the hot-rolled sheet annealing is performed, the main phase is a low-temperature transformation generation phase and the second phase contains residual austenite, and the grain size is 1.2 μm or more. It has been found that a hot-dip cold-rolled steel sheet having a metal structure with few coarse austenite grains and excellent ductility, work hardening characteristics and stretch flangeability can be obtained.

The present invention completed based on the above knowledge is as follows.
(1) A hot-dip cold-rolled steel sheet having a hot-dip plated layer on the surface of the cold-rolled steel sheet, wherein the cold-rolled steel sheet is in mass%, C: more than 0.10% and less than 0.25%, Si: 0.00. More than 50% and less than 2.0%, Mn: more than 1.50% and 3.0% or less, P: less than 0.050%, S: 0.010% or less, sol. Al: 0.50% or less and N: 0.010% or less, the remainder having a chemical composition consisting of Fe and impurities, the main phase is a low-temperature transformation product phase, and the retained austenite and polygonal in the second phase comprising a metallic structure made of ferrite, the retained austenite volume fraction relative to the total tissue of 4.0 percent less than 25.0%, an average particle size of less than 0.80 .mu.m, among the retained austenite grain size 1 the number density of the residual austenite grains is .2μm or 3.0 × ^{10 -2} cells / [mu] m ² Ri der hereinafter the polygonal ferrite Ru der volume ratio is less than 2.0 percent 40 percent relative to the total tissue A hot-rolled cold-rolled steel sheet.

  (2) The chemical composition is selected from the group consisting of Ti: less than 0.040%, Nb: less than 0.030%, and V: 0.50% or less in mass% instead of part of Fe. The hot-dip cold-rolled steel sheet according to (1), containing one or more kinds.

  (3) The chemical composition is selected from the group consisting of Cr: not more than 1.0%, Mo: less than 0.20% and B: not more than 0.010% in mass%, instead of part of Fe. The hot-rolled cold-rolled steel sheet according to (1) or (2), containing one or more kinds.

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

  According to the present invention, a high-tensile hot-dip galvanized cold-rolled steel sheet having sufficient ductility, work-hardening property, 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.

  In the hot-dip cold-rolled steel sheet according to the present invention, the metal structure and chemical composition of the cold-rolled steel sheet, and rolling, annealing in a manufacturing method capable of efficiently, stably and economically manufacturing the cold-rolled steel sheet and the hot-dip steel sheet, The plating conditions and the like will be described in detail below.

1. Metallic structure The cold-rolled steel sheet, which is the plating base of the hot-dip cold-rolled steel sheet according to the present invention, has a main phase of a low-temperature transformation generation phase and a residual austenite in the second phase, and the residual austenite has a volume ratio with respect to the entire structure. More than 4.0% and less than 25.0%, the average particle size is less than 0.80 μm, and among the retained austenite, the number density of the retained austenite grains having a particle size of 1.2 μm or more is 3.0 × 10 ^{− It} has a metal structure of ² / μm ² or less.

The main phase means a phase or structure having the largest volume fraction, and the second phase means a phase and structure other than the main phase.
The low temperature transformation generation phase refers to a phase and structure generated by low temperature transformation such as martensite and bainite. Bainitic ferrite is mentioned as a low temperature transformation production phase other than these. Bainitic ferrite is distinguished from polygonal ferrite because of its high dislocation density, and bainitic because it does not precipitate iron carbide inside or at its boundary. The bainitic ferrite means so-called lath or plate bainitic ferrite and bulk granular bainitic ferrite. This low-temperature transformation generation phase may contain two or more phases and structures, specifically, 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.

  The reason why the metal structure of the cold-rolled steel sheet as the plating substrate is limited as described above will be described next. Here, the cold-rolled steel sheet includes both a cold-rolled steel sheet obtained by cold-rolling a hot-rolled steel sheet obtained by hot rolling, and an annealed cold-rolled steel sheet that has been annealed thereafter.

  The reason why the main phase is a low-temperature transformation generation phase and the second phase is a structure containing residual austenite is that it is suitable for improving ductility, work hardenability and stretch flangeability while maintaining tensile strength. . 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 volume ratio of the retained austenite to the entire structure is more than 4.0% and less than 25.0%. If the volume fraction of retained austenite is 4.0% or less, the ductility becomes insufficient, and if it is 25.0% or more, the stretch flangeability is significantly deteriorated. The volume fraction of retained austenite 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. Accordingly, 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%.

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

In a hot-dipped steel sheet based on a cold-rolled steel sheet having a metal structure containing a low-temperature transformation-forming phase as a main phase and residual austenite in the second phase, the grain size of the retained austenite is less than 0.80 μm. When there are many coarse residual austenite grains having a diameter of 1.2 μm or more, work hardening and stretch flangeability are impaired. Therefore, the number density of residual austenite grains having a grain size of 1.2 μm or more is set to 3.0 × 10 ⁻² particles / μm ² or less. The number density of residual austenite grains having a particle size of 1.2 μm or more is preferably 2.0 × 10 ⁻² particles / μm ² or less. The number density is more preferably 1.8 × 10 ⁻² pieces / μm ² or less, particularly preferably 1.6 × 10 ⁻² pieces / μm ² or less.

  In order to further improve the balance between ductility and stretch flangeability, the average carbon concentration of retained austenite is preferably 0.80% or more. More preferably, it is 0.84% or more. On the other hand, if the average carbon concentration of retained austenite is excessive, stretch flangeability deteriorates. Therefore, the average carbon concentration of retained austenite is preferably less than 1.7%. More preferably, it is less than 1.6%, more preferably less than 1.4%, and particularly preferably less than 1.2%.

  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. It is preferable that the volume ratio of the polygonal ferrite with respect to the entire structure exceeds 2.0%. On the other hand, when the volume fraction of polygonal ferrite becomes excessive, stretch flangeability deteriorates. Accordingly, the volume fraction of polygonal ferrite is preferably less than 40.0%. Further preferably, it is less than 30%, more preferably less than 24.0%, particularly preferably less than 20.0%, and most preferably less than 18.0%.

  In order to increase the tensile strength and work hardening, 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 1.0%. More preferably, it is more than 2.0%. 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%. More preferably it is less than 10.0%, particularly preferably less than 8.0%, and most preferably less than 6.0%.

  The metal structure of the base cold-rolled steel sheet of the hot-dip 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 was determined by taking a test piece from a hot-dip plated steel sheet, polishing a longitudinal section parallel to the rolling direction, and subjecting it to a corrosion treatment with nital. The metal structure is observed using a SEM at a 1/4 depth position of the plate thickness from the interface with the steel plate, and the following is also measured, and by image processing, the area ratio of the low-temperature transformation generation phase and polygonal ferrite is measured, Each area ratio is obtained assuming that the area ratio is equal to the volume ratio.

  The volume fraction of retained austenite and the average carbon concentration were obtained by taking a test piece from a hot dip plated steel plate, chemically polishing the rolled surface from the steel plate surface to a 1/4 depth position of the plate thickness, and using XRD, respectively. Determined by measuring intensity and diffraction angle.

  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 collected from a hot-dip plated steel sheet, 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 ¼ depth from the steel sheet surface. . The region surrounded by the parent phase is observed as a phase composed of a face-centered cubic type crystal structure (fcc phase), and the number density (per unit area) of the remaining austenite grains is obtained by image processing. The number of grains) and the area ratio of the individual retained austenite grains. The circle equivalent diameter of each austenite grain is determined from the area occupied by each retained austenite grain in the field of view, and the average value thereof is taken as the average grain size of the retained austenite.

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

In this invention, the above-mentioned metal structure is prescribed | regulated in the 1/4 depth position of the plate | board thickness of the steel plate which is a base material from the boundary of the steel plate which is a base material, and a plating layer.
As a mechanical property that can be realized based on the above characteristics on the metal structure, the hot-dip cold-rolled steel sheet according to the present invention has a tensile strength (TS) in a direction orthogonal to the rolling direction in order to ensure shock absorption. The pressure is preferably 750 MPa or more, more preferably 850 MPa or more, and particularly preferably 950 MPa or more. On the other hand, in order to ensure ductility, it is preferable that TS is less than 1180 MPa.

From the viewpoint of press formability, the total elongation (El ₀ ) in the direction perpendicular to the rolling direction is converted to a total elongation equivalent to a plate thickness of 1.2 mm based on the following formula (1): El, Japanese Industrial Standard JIS Z2253 The work hardening index calculated by using 2 points of nominal strain of 5% and 10% and the test force corresponding to these, with the strain range of 5 to 10% in accordance with JIS, is in accordance with Japan Iron and Steel Federation Standard JFST1001 When the hole expansion ratio measured in this manner is λ, the value of TS × El is 18000 MPa% or more, the value of TS × n value is 150 MPa or more, the value of TS ^1.7 × λ is 450000 MPa ^1.7 % or more, (TS × El) The value of × 7 × 10 ³ + (TS ^1.7 × λ) × 8 is preferably 180 × 10 ⁶ or more.

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

TS × El is an index for evaluating ductility from the balance between strength and total elongation, and TS × n value is an index for evaluating work curability from the balance between strength and work hardening index. TS ^1.7 × λ Is an index for evaluating hole expandability from the balance between strength and hole expansion rate. (TS × El) × 7 × 10 ³ + (TS ^1.7 × λ) × 8 is an index for evaluating formability in which elongation and hole expandability are combined, so-called stretch flange formability.

TS × El value is 20000 MPa% or more, TS × n value is 160 MPa or more, TS ^1.7 × λ value is 5500000 MPa ^1.7 % or more, (TS × El) × 7 × 10 ³ + (TS ^1.7 × λ) × More preferably, the value of 8 is 190 × 10 ⁶ or more. Particularly preferably, the value of (TS × El) × 7 × 10 ³ + (TS ^1.7 × λ) × 8 is 200 × 10 ⁶ 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 in thickness are likely to occur. From the viewpoint of shape freezeability, 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 0.10% and less than 0.25% When the C content is 0.10% or less, it is difficult to obtain the above metal structure. Therefore, the C content is more than 0.10%. Preferably it is more than 0.12%, more preferably more than 0.14%, particularly preferably more than 0.16%. On the other hand, when the C content is 0.25% 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 0.25%. It is preferably 0.23% or less, more preferably 0.21% or less, and particularly preferably 0.19% or less.

Si: more than 0.50% and less than 2.0% Si has an effect of improving ductility, work hardenability and stretch flangeability through suppressing 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 0.50% or less, it is difficult to obtain the effect by the above action. Therefore, the Si content is more than 0.50%. Preferably it is more than 0.70%, more preferably more than 0.90%, particularly preferably more than 1.20%. On the other hand, when the Si content is 2.0% or more, the surface properties of the steel sheet deteriorate. Furthermore, the plating property is significantly deteriorated. Therefore, the Si content is less than 2.0%. It is preferably less than 1.8%, more preferably less than 1.6%, and particularly preferably less than 1.4%.

  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 1.50% and not more than 3.0% 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 1.50% or less, it is difficult to obtain the above metal structure. Therefore, the Mn content is more than 1.50%. Preferably it is more than 1.60%, more preferably more than 1.80%, particularly preferably more than 2.0%. 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 3.0% or less. Preferably it is less than 2.70%, more preferably less than 2.50%, particularly preferably less than 2.30%.

P: Less than 0.050% 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 less than 0.050%. Preferably it is less than 0.030%, 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: 0.50% 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. That is, it may be at the impurity level. When it is contained for the purpose of promoting deoxidation, sol. It is preferable to contain 0.0050% or more as Al. Further preferred sol. The Al content is more than 0.020%. Al, like Si, has the effect of increasing the stability of austenite and is an effective element for obtaining the above metal structure. Therefore, Al can be contained for this purpose. In this case, sol. The Al content is preferably more than 0.040%, more preferably more than 0.050%, particularly preferably more than 0.060%. On the other hand, sol. If the Al content is too high, not only surface flaws are likely to occur due to alumina, but the transformation point is greatly increased, 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 0.50% or less. Preferably it is less than 0.30%, 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, Most preferably, it is 0.003% 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 0.040%, Nb: less than 0.030% 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 these elements are contained excessively, the effect of the above action is saturated and uneconomical. In addition, the recrystallization temperature during annealing increases, the metal structure after annealing becomes non-uniform, and stretch flangeability is also impaired. Furthermore, the precipitation amount of carbide or nitride increases, the yield ratio increases, and the shape freezing property also deteriorates. Accordingly, the Ti content is less than 0.040%, the Nb content is less than 0.030%, and the V content is 0.50% or less. The Ti content is preferably less than 0.030%, more preferably less than 0.020%, the Nb content is preferably less than 0.020%, more preferably less than 0.012%, and the V content is Preferably it is 0.30% or less, More preferably, it is less than 0.050%. The Nb + Ti × 0.2 value is preferably less than 0.030%, and more preferably less than 0.020%.

  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: less than 0.20%, and B: 0.010% or less Cr, Mo and B improve the hardenability of steel. It is an element effective in obtaining the above metal structure. Therefore, you may contain 1 type, or 2 or more types of these elements. However, even if these elements are contained excessively, the effect of the above action is saturated and uneconomical. Therefore, the Cr content is 1.0% or less, the Mo content is less than 0.20%, and the B content is 0.010% or less. The Cr content is preferably 0.50% or less, the Mo content is preferably 0.10% 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 these elements are contained excessively, the effect of the above action is saturated and 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. Hot-dip plating layer 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-melt Zn-Al-Mg-Si alloy plating, etc. Is exemplified. For example, when the plating layer is alloyed hot dip galvanizing, the Fe concentration in the plating film is preferably 7% or more and 15% or less. Examples of the molten Zn—Al alloy plating include molten Zn-5% Al alloy plating and molten Zn-55% Al alloy plating.

The amount of plating adhesion is not particularly limited, and may be the same as the conventional one. For example, it may be 25 g / m ² or more and 200 g / m ² or less per side. When the plating layer is alloyed hot dip galvanizing, it is preferably 25 g / m ² or more and 60 g / m ² or less per side from the viewpoint of suppressing powdering.

  For the purpose of further improving corrosion resistance and paintability, after plating, a single layer or multiple layers after treatment selected from chromic acid treatment, phosphate treatment, silicate non-chromium chemical conversion treatment, resin coating, etc. Also good.

4). Manufacturing conditions First, a cold-rolled steel sheet having the metal structure and chemical composition to be a base material is manufactured.
Specifically, after the steel having the above-described chemical composition is melted by a known means, it is made into a steel ingot by a continuous casting method, or it is made into a steel ingot by an arbitrary casting method and then subjected to block rolling. Use steel slabs, etc. In the continuous casting process, in order to suppress the occurrence of surface defects due to inclusions, it is preferable to cause an external additional flow such as electromagnetic stirring in the molten steel in the mold. The steel ingot or steel slab may be reheated once it has been cooled and subjected to hot rolling. The steel ingot in the high temperature state after continuous casting or the steel slab in the high temperature state after partial rolling is used as it is. Alternatively, it may be kept hot or subjected to auxiliary heating for hot rolling. In the present specification, such steel ingots and steel slabs are collectively referred to as “slabs” as materials for hot rolling.

The temperature of the slab subjected to hot rolling is preferably less than 1250 ° C. and more preferably 1200 ° C. or less in order to prevent coarsening of austenite. The lower limit of the temperature of the slab to be subjected to hot rolling is not particularly limited, and is a temperature at which hot rolling can be completed in a temperature range of (Ar ₃ point + 30 ° C.) or higher and higher than 880 ° C. as will be described later. I just need it.

Hot rolling is completed in a temperature range of (Ar ₃ point + 30 ° C.) or more and more than 880 ° C. 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 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. In addition, work hardenability and stretch flangeability tend to deteriorate. Therefore, completion temperature of the hot rolling is made (Ar ₃ point + 30 ° C.) or higher and 880 ° C. greater. 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 viewpoint, it is preferable that the completion temperature of hot rolling is (Ar ₃ point + 50 ° C.) or more and more than 900 ° C.

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

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

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

  After hot rolling, it is rapidly cooled to a temperature range of 720 ° C. or less within 0.40 seconds after completion of rolling. This suppresses the release of work strain introduced into austenite by rolling, transforms austenite using the work strain as a driving force, refines the structure of the hot-rolled steel sheet, and coarsens the metal structure after cold rolling and annealing. This is to suppress the formation of retained austenite grains and to refine the polygonal ferrite. Preferably, it is rapidly cooled to a temperature range of 720 ° C. or less within 0.30 seconds after completion of rolling, and more preferably, it is rapidly cooled to a temperature range of 720 ° C. or less within 0.20 seconds after completion of rolling. .

  Since the structure of the hot-rolled steel sheet becomes finer as the temperature at which rapid cooling is stopped is lower, it is preferable to rapidly cool to a temperature range of 700 ° C. or lower after completion of rolling, and to cool to a temperature range of 680 ° C. or lower after completion of rolling. Is more preferable. In addition, since release of processing strain is suppressed as the average cooling rate during rapid cooling increases, the average cooling rate during rapid cooling is preferably set to 300 ° C./s or more, thereby further increasing the structure of the hot-rolled steel sheet. It can be miniaturized. The average cooling rate during quenching is more preferably 400 ° C./s or more, particularly preferably 600 ° C./s or more, and most preferably 800 ° C./s or more. 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, (1) the steel sheet is wound in a temperature range of more than 500 ° C., or (2) after the rapid cooling stop, the steel sheet is wound in a temperature range of less than 200 ° C. Ac is annealed in a temperature range of ₁ point or less.

  In the embodiment of (1), the steel sheet is wound in a temperature range higher than 500 ° C. When the winding temperature is 500 ° C. or lower, iron carbide is not sufficiently precipitated in the hot-rolled steel sheet, and cold rolling is performed. This is because coarse retained austenite grains are generated in the metal structure after annealing, and polygonal ferrite is coarsened. The winding temperature is preferably above 550 ° C. On the other hand, if the coiling temperature is too high, ferrite becomes coarse in the hot-rolled steel sheet, and coarse residual austenite grains are generated in the metal structure after cold rolling and annealing. For this reason, the winding temperature is preferably less than 650 ° C, and more preferably less than 620 ° C.

On the other hand, in the embodiment (2), the steel sheet is wound in a temperature range of less than 200 ° C., and the hot rolled steel sheet is annealed in a temperature range of 500 ° C. or more and Ac ₁ point or less. It is because the production | generation of a martensite will become inadequate that it is 200 degreeC or more. Further, if the annealing temperature is less than 500 ° C., iron carbide is not sufficiently precipitated, and if it exceeds the Ac ₁ point, the ferrite becomes coarse, and coarse residual austenite grains are generated in the metal structure after cold rolling and annealing.

  In the case of the embodiment of (2), the hot-rolled steel sheet that has been hot-rolled and wound is subjected to a treatment such as degreasing according to a known method, if necessary, and then annealed. 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 holding time in hot-rolled sheet annealing need not be particularly limited. A hot-rolled steel sheet produced through a suitable immediately-cooling process does not have to be held for a long time because the metal structure is fine. 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 conditions from the rapid cooling stop to the winding are not particularly defined, but after the rapid cooling stop, it is preferable to hold at a temperature range of 720 to 600 ° C. for 1 second or more. Thereby, the production | generation of a fine ferrite is accelerated | stimulated. On the other hand, if the holding time becomes too long, the productivity is impaired. Therefore, the upper limit of the holding time in the temperature range of 720 to 600 ° C. is preferably within 10 seconds. After holding in the temperature range of 720 to 600 ° C., it is preferable to cool to the coiling temperature at a cooling rate of 20 ° C./s or more in order to prevent the generated ferrite from becoming coarse.

  A hot-rolled steel sheet obtained by hot rolling in which the winding is performed according to the embodiment of (1) or (2) is descaled by pickling or the like and then cold-rolled according to a conventional method. In cold rolling, in order to promote recrystallization, uniformize the metal structure after cold rolling and annealing, and further improve stretch flangeability, the cold pressure ratio (rolling ratio in cold rolling) is 40% or more. It is preferable to do. If the cold pressure ratio is too high, the rolling load increases and rolling becomes difficult, so the upper limit of the cold pressure ratio is preferably less than 70%, and more preferably less than 60%.

The steel sheet after cold rolling is annealed after being subjected to a treatment such as degreasing according to a known method, if necessary. The lower limit of the soaking temperature in annealing is more than Ac ₃ points. 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. However, if the soaking temperature becomes too high, the austenite becomes excessively coarse and the ductility, work hardenability and stretch flangeability tend to deteriorate. For this reason, the upper limit of the soaking temperature is preferably less than (Ac ₃ points + 100 ° C.). It is more preferable to be less than (Ac ₃ point + 50 ° C.), and it is particularly preferable to be less than (Ac ₃ point + 20 ° C.).

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

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

  In the cooling process after soaking in annealing, it is preferable to cool a temperature range of 650 to 500 ° C. at a cooling rate of 15 ° C./s or more in order to obtain a metal structure having a low-temperature transformation generation phase as a main phase. 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 20 ° C./s or more, and particularly preferably 40 ° C./s or more. 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.

  When promoting the formation of fine polygonal ferrite and improving ductility and work hardenability, it is preferable to cool from the soaking temperature to 50 ° C. or more at a cooling rate of less than 5.0 ° C./s. The cooling rate after soaking is more preferably less than 3.0 ° C./s. Particularly preferably, it is less than 2.0 ° C./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 5.0 ° C./s, more preferably at least 100 ° C., It is particularly preferable to cool at 120 ° C. or higher.

  Moreover, in order to ensure the amount of retained austenite, it hold | maintains for 15 seconds or more in a 450-340 degreeC temperature range. In order to improve the stability of retained austenite and further improve the ductility, work hardenability and stretch flangeability, the holding temperature range is preferably 430 to 360 ° C. Further, since the stability of retained austenite increases as the holding time is increased, the holding time is set to 30 seconds or more. Preferably, it is 50 seconds or more.

  The annealed cold-rolled steel sheet manufactured in this way is hot-dip plated. In the hot dipping, the cold rolling steel sheet is annealed by the above-described method, and the hot steel sheet is reheated as necessary, and then the hot dipping process is performed. As the conditions for the hot dipping process, the conditions that are usually applied may be adopted depending on the hot dipping type. For example, when the hot dip plating is hot dip galvanization or hot dip Zn-Al alloy plating, the hot dip plating is performed in the temperature range of 450 ° C. or higher and 620 ° C. or lower in the same manner as the conditions performed in a normal hot dip plating line. A hot dip galvanized layer or a hot dip Zn—Al alloy plated layer may be formed on the steel plate surface.

  Further, after the hot dip galvanizing treatment, an alloying treatment for alloying the hot dip galvanized layer may be performed. In this case, the Al concentration in the plating bath is preferably controlled to 0.08 to 0.15%. In addition to Zn and Al, the plating bath contains 0.1% or less of Fe, V, Mn, Ti, Nb, Ca, Cr, Ni, W, Cu, Pb, Sn, Cd, Sb, Si, and Mg. There is no particular hindrance. Moreover, it is preferable that alloying process temperature shall be 470 degreeC or more and 570 degrees C or less. This is because when the alloying treatment temperature is lower than 470 ° C., the alloying rate is remarkably reduced, and the time required for the alloying treatment is increased, which may lead to a decrease in productivity. On the other hand, when the alloying treatment temperature exceeds 570 ° C., the alloying speed of the plated layer is remarkably increased, and the alloyed hot-dip galvanized layer may be embrittled. More preferably, it is 550 degrees C or less. After hot dipping, the composition of the coating on the surface of the cooled steel sheet generally has a slightly higher Fe concentration than the plating bath composition because element interdiffusion occurs between the steel and the molten metal during immersion and cooling. The alloyed hot dip galvanizing actively utilizes this mutual diffusion, and the Fe concentration in the coating is 7 to 15%.

The amount of plating adhesion is not particularly limited, but generally it is preferably 25 to 200 g / m ² per side. In the case of alloyed hot dip galvanizing, there is concern about powdering, so the amount of plating is preferably 25 to 60 g / m ² per side. Although the hot dipping is typically double-sided plating, it can also be single-sided plating.

  The galvanized cold-rolled 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 in 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, rolling was performed for 6 passes in a temperature range of Ar ₃ point + 30 ° C. or higher and 880 ° C. or higher, and finished to a thickness of 2 mm. The rolling reduction rate in the final pass was 12 to 42% in terms of sheet thickness reduction rate. After hot rolling, it is cooled to 650-720 ° C. under various cooling conditions using a water spray, allowed to cool for 5-10 seconds, and then cooled to various temperatures at a cooling rate of 60 ° C./s. Was the coiling temperature. Except for those with a coiling temperature of room temperature, after being charged in an electric heating furnace maintained at the coiling temperature and held for 30 minutes, the furnace was cooled to room temperature at a cooling rate of 20 ° C./h and wound up A hot-rolled steel sheet was obtained by simulating the slow cooling. In addition, for the case where the coiling temperature is set to room temperature, except for a part, a hot-rolled sheet that is heated from room temperature to 600 ° C. at a temperature rising rate of 50 ° C./h and then cooled to room temperature at a cooling rate of 20 ° C./h. Annealed.

  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 obtain a cold-rolled steel sheet having a thickness of 1.0 mm. Using the continuous annealing simulator, the obtained cold-rolled steel sheet was heated to 550 ° C. at a heating rate of 10 ° C./s, then heated to various temperatures shown in Table 2 at a heating rate of 2 ° C./s, and 95 Soaked for 2 seconds. Thereafter, it is cooled to various primary cooling stop temperatures shown in Table 2 at a cooling rate of 2 ° C./s, cooled to various secondary cooling stop temperatures shown in Table 2 at a cooling rate of 40 ° C./s, Next, after maintaining the secondary cooling stop temperature for 60 to 330 seconds and performing a heat treatment corresponding to the annealing step, a heat treatment corresponding to immersion in a hot dip galvanizing bath at 460 ° C. and a heat treatment corresponding to alloying treatment at 500 to 520 ° C. And cooled to room temperature to obtain an annealed steel sheet that had undergone a heat treatment equivalent to galvannealed alloying after annealing.

  A specimen for SEM observation was collected from the annealed steel sheet, and after polishing a longitudinal section parallel to the rolling direction, it was subjected to corrosion treatment 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 generation phase and polygonal ferrite was measured by image processing. 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 was collected from the annealed steel sheet, and the rolled surface was chemically polished from the steel sheet surface to a ¼ depth position of the sheet thickness, and then an X-ray diffraction test was performed to determine the volume fraction of retained austenite and Average carbon concentration 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, the lattice constant dγ (Å) is obtained from the diffraction angle of the γ phase (111), (200), (220) diffraction peaks, and the average carbon concentration Cγ (mass%) of the retained austenite is obtained by the following conversion formula. It was.

Cγ = (dγ−3.572 + 0.000015 × Si−0.0012 × Mn) /0.033
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, OSL5 manufactured by TSL is used for the EBSP measuring apparatus, and an electron beam is 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. In addition, the number density (N _R ) per unit area of residual austenite grains having a grain size of 1.2 μm or more was determined.

Yield stress (YS) and tensile strength (TS) were determined by collecting JIS No. 5 tensile specimens from an annealed steel sheet along the direction perpendicular to the rolling direction and conducting a tensile test at a tensile speed of 10 mm / min. The total elongation (El) is obtained by conducting a tensile test on a JIS No. 5 tensile test specimen taken along the direction orthogonal to the rolling direction, and using the obtained actual measurement value (El ₀ ), based on the above formula (1), A conversion value corresponding to the case where the plate thickness was 1.2 mm was obtained. The work hardening index (n value) was 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 conducting a hole expansion test specified in Japan Iron and Steel Federation Standard JFST1001 and measuring the hole expansion ratio (λ). 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, numerical values or symbols marked with * are outside the scope of the present invention.

Test results for steel sheets within the range defined by the present invention (Test Nos. 1 to 27 all have a TS × El value of 18000 MPa% or more, a TS × n value of 150 or more, TS ^1.7 The value of × λ is 4500000 MPa ^1.7 % or more, and the value of (TS × E1) × 7 × 10 ³ + (TS ^1.7 × λ) × 8 is 180 × 10 ⁶ or more, and good ductility, work hardenability and stretch flange Showed sex.

  The test results (test numbers 28 to 33) for the steel sheet in which the metal structure of the steel sheet deviated from the range specified by the present invention was inferior in ductility, work hardenability and stretch flangeability.

Specifically, in the test using steel U (test number 28), since the Si content in the steel is small, the average particle size of retained austenite is large, the volume fraction of retained austenite is low, and polygonal The average grain size of ferrite is large, and ductility, work hardenability and stretch flangeability are poor. Test using a test (Test No. 29) and steel W used steel V (Test No. 31), for time to quench stop from hot rolling completion is too long, N _R is large, ductility and the stretch flangeability The nature is bad. In the test using Steel V (Test No. 30), the soaking temperature during annealing was too low, so a metal structure having a low-temperature transformation generation phase as a main phase was not obtained, and stretch flangeability was poor. In the test using steel W (test number 32), since the coiling temperature is too low and hot-rolled sheet annealing is not performed, N _R is large, and ductility and stretch flangeability are poor. In the test using steel F (test number 33), since the amount of reduction in the final one pass of hot rolling is low, N _R is large and ductility, work hardenability and stretch flangeability are poor.

Claims (4)

A cold-plated cold-rolled steel sheet having a hot-dip plated layer on the surface of the cold-rolled steel sheet, the cold-rolled steel sheet being in mass%, C: more than 0.10% and less than 0.25%, Si: more than 0.50% Less than 2.0%, Mn: more than 1.50%, 3.0% or less, P: less than 0.050%, S: 0.010% or less, sol. Al: 0.50% or less and N: 0.010% or less, the balance is a chemical composition consisting of Fe and impurities, the main phase is a low-temperature transformation product phase and the second phase is retained austenite and polygonal ferrite The retained austenite has a volume ratio of more than 4.0% to less than 25.0% and an average particle size of less than 0.80 μm with respect to the entire structure. the number density of the residual austenite grains is 2μm or 3.0 × ^{10 -2} cells / [mu] m ² Ri der hereinafter the polygonal ferrite der Rukoto volume ratio is less than 2.0 percent 40 percent relative to the total tissue Hot-dip cold-rolled steel sheet characterized by   The chemical composition may be one selected from the group consisting of Ti: less than 0.040%, Nb: less than 0.030%, and V: 0.50% or less in place of part of Fe The hot-rolled cold-rolled steel sheet according to claim 1, containing two or more kinds.   The chemical composition may be one selected from the group consisting of Cr: 1.0% or less, Mo: less than 0.20%, and B: 0.010% or less in mass% instead of a part of Fe. The hot-dip cold-rolled steel sheet according to claim 1 or 2, containing 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 hot-rolled cold-rolled steel sheet according to any one of claims 1 to 3, comprising one or more selected from the group.