JP2011174101A - High tensile strength cold rolled steel sheet, and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high tensile strength cold rolled steel sheet which has excellent formability, and a method for producing the same. <P>SOLUTION: A steel stock comprising, by mass, 0.0010 to 0.0080% C, regularly comprising Si, Mn, P and S, comprising &le;0.05% Al, 0.0060 to 0.0200% N or further comprising 0.001 to 0.050% Nb and/or &le;0.0020% B, and in which N/(Al+0.3Nb+2.5B) is &ge;0.2 is made into a cold rolled steel sheet having a strength-ductility balance, in which solid solution N is &ge;0.0040%, a ferritic phase is formed as the main phase, tensile strength in the rolling direction is &ge;600 MPa, total elongation is &ge;6%, and local elongation is &ge;6%. The production method includes: a hot rolling step of subjecting a sheet bar subjected to rough rolling at a heating temperature of &ge;1,000&deg;C to finish rolling at an outlet side temperature of &ge;800&deg;C, and coiling the same at a temperature of [700-10&times;ä(Al+0.3Nb)/N}]&deg;C or less; a cold rolling step at a draft of 50 to 95%; and a heat treatment step of performing heating to a temperature in the range of 300 to 650&deg;C and thereafter performing cooling. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to an inexpensive high-tensile cold-rolled steel sheet suitable for parts mainly used in the fields of electrical machinery, building materials, automobiles, and the like, and more particularly to improvement of formability. The “steel plate” here includes a steel plate and a steel strip.

  In recent years, in the electric field and building material field, with the intensifying sales competition, there has been a strong demand for inexpensive materials in order to reduce costs. Furthermore, weight reduction of materials and products is also required to reduce transportation costs. In the automobile field, in addition to cost reduction, there is a strong demand for improving the fuel efficiency of automobiles from the viewpoint of protecting the global environment, and recently, the weight reduction of automobile bodies has been promoted. As an effective measure for such a demand, in order to reduce the thickness of the steel sheet and reduce the weight, it is necessary to use a high-strength high-strength steel sheet, and to reduce the amount of alloy elements for cost reduction, First of all, the use of an inexpensive high-tensile steel plate is mentioned.

  Strengthening methods such as solid solution strengthening, precipitation strengthening, structure strengthening and work hardening are known as means for increasing the strength of steel sheets. However, the solid solution strengthening and precipitation strengthening methods require a large amount of alloying elements to be strengthened, and the structure strengthening method requires a large amount of alloying elements that improve hardenability and further quenching. It is necessary to perform a heat treatment such as this, which is accompanied by an increase in steel sheet manufacturing costs. Therefore, it is considered advantageous to increase the strength of the steel sheet by work hardening in the steel sheet for the electrical machinery field and the building material field where the cost reduction demand is strong. However, the increase in strength by work hardening has a problem in that the ductility is lowered as compared with other strengthening methods.

  For such a problem, for example, Patent Document 1 proposes a “method for manufacturing a steel plate for cans” that can be used as it is in cold rolling without using a continuous annealing step and can reduce costs. The technology described in Patent Document 1 is mainly intended for three-piece can materials, C: 0.0015% or less, Si: 0.020% or less, Mn: 0.10% or less, P: 0.010% or less, S: 0.005% or less N: 0.0030% or less, Al: 0.150% or less, Cr: 0.020-0.500%, Nb: 0.0020-0.0200%, Ti: 0.0050-0.0200%, B: 0.0002-0.0020% The continuous cast slab containing the above is re-heated to 1050 ° C or less and then hot-rolled, the finish rolling mill entry side temperature is set to 950 ° C or less, and the total rolling reduction is 40% or more, and the final rolling reduction is 25% or more. And the final hot-rolled mother board thickness is 1.2 mm or less, the steel sheet is wound at a temperature of 500 to 750 ° C., and after normal pickling, the steel sheet for cans is subjected to cold rolling at a reduction rate of 50 to 98%. It is a manufacturing method. In the technique described in Patent Document 1, a steel slab having a composition with a reduced amount of alloy elements is used, the hot rolling temperature and the rolling reduction are adjusted, and the rolling reduction during cold rolling is adjusted to the minimum. The steel plate for cans is soft and has high ductility.

Japanese Unexamined Patent Publication No. 08-176674

However, the cold-rolled steel sheet manufactured by the technique described in Patent Document 1 has a high slab composition, and the thickness after hot rolling needs to be as thin as 1.2 mm or less, resulting in a high manufacturing cost. There is a problem.
An object of the present invention is to advantageously solve the problems of the prior art and to provide a high-tensile cold-rolled steel sheet that is inexpensive and excellent in formability and a method for producing the same. The sheet thickness of the high-tensile cold-rolled steel sheet of the present invention is preferably from the viewpoint of suppressing cost increase in the cold rolling process due to excessive thinning and effectively utilizing work hardening by cold rolling, preferably 0.4: From mm to 2.0 mm.

Here, the “high-tensile cold-rolled steel sheet” refers to a cold-rolled steel sheet having a tensile strength TS in the rolling direction of 600 MPa or more. “Excellent formability” means that JIS13B test pieces (GL: 25mm) with only the parallel part length shortened in accordance with JIS Z 2201 and JIS Z 2241 are used. The total elongation El _T obtained by conducting a tensile test is 6% or more, and the local elongation El _L is 6% or more.

  Conventionally, soft steel sheets with a tensile strength of TS: 270 MPa have been mainly used as materials for the electric field and building materials field. Typical processing includes bending processing of about 90 °, mild hole expansion processing for screw holes, and mild overhanging processing for securing rigidity and attaching parts. If it is a process of this grade, it can often be processed into the member of a desired shape, without using the steel plate which was excellent in workability as a raw material by optimizing a processing method. For example, conventionally, hot-rolled sheets for soft cold-rolled steel sheets are cold-rolled, and further subjected to hot-dip galvanizing treatment, as hot-dip galvanized steel sheets having a tensile strength TS of about 500 to 600 MPa, A part having a desired shape may be obtained by utilizing a processing technique such as roll forming.

  Under these circumstances, the present inventors have further improved the steel sheet while maintaining a ductility at least equal to or higher than the ductility of the steel sheet currently used (local elongation: 5% or more, total elongation: 5% or more). If high strength can be achieved at a low cost, it can be used as a material for the electric field, building materials field, or automobile field, avoiding the deterioration of formability associated with high strength, and having higher strength than conventional and ductility equivalent to or higher than conventional. As a high-strength steel sheet having excellent formability, it was thought that it can contribute to weight reduction of parts and can also contribute to cost reduction.

  In the fields of electrical machinery, building materials and automobiles, unlike beverage cans and food cans, use a continuous hot dip galvanizing line or dip in a hot dip galvanizing bath to galvanize the steel sheet (substrate) surface. It is common. By such hot dip galvanizing treatment, the steel sheet is inevitably heated in the vicinity of the melting point of zinc, that is, the steel sheet is heat-treated at a temperature in the vicinity of 400 ° C.

Therefore, the present inventors have come up with the idea of actively using heat treatment at a temperature in the vicinity of 400 ° C., which has not been actively used so far, in order to achieve the above-described problems. And steel plate was manufactured by changing various steel plate composition and manufacturing conditions, and many material evaluation was implemented. As a result, C: 0.0010 to 0.0080 mass%, Si: 0.4 mass% or less, Mn: 0.1 to 1.0 mass%, P: 0.08 mass% or less, S: 0.05 mass% or less, Al: 0.05 mass% or less, N : A steel slab containing 0.0060 to 0.0200% by mass with N / Al being 0.2 or more and having a high N content is subjected to a hot rolling process under a predetermined condition and a cold rolling process under a predetermined condition to obtain a cold rolled sheet Furthermore, by performing heat treatment at a temperature in the vicinity of 400 ° C., the amount of solute N becomes as high as 0.0040 mass% or more, the work-hardened ferrite is further strengthened, and the tensile strength TS in the rolling direction is 600 MPa or more. Further, it was found that a high-tensile cold-rolled steel sheet having an overall elongation El _T of 6% or more and a local elongation El _L of 6% or more can be obtained at low cost and excellent in formability.

First, a description will be given of experimental results performed by the present inventors and serving as the basis of the present invention.
0.002% C-0.01% Si-0.2% Mn-0.01% P-0.01% S-0.03% Al based on the basic component, N is 0.003% (low N material) and 0.012% (high N material) Two types of sheet bars having a composition changed to two levels of) were produced. These sheet bars are heated to 1250 ° C and soaked, and after that, three passes of hot rolling are performed so that the finish rolling finish temperature (finish rolling exit temperature) is 900 ° C. It was a sheet. In addition, the heat processing (600 degreeC x 1 h) equivalent to the heat retention of the coil winding process at 600 degreeC was performed after completion | finish of finish rolling. Subsequently, the hot-rolled sheet was pickled and then cold-rolled with a reduction ratio of 60% to obtain a cold-rolled sheet with a sheet thickness of 1.2 mm. Subsequently, these cold-rolled plates were heated to a temperature of 250 to 450 ° C. and held for 60 seconds, and then cooled by blowing gas.

Test specimens were collected from the obtained cold-rolled steel sheets and subjected to a tensile test to measure tensile properties (tensile strength TS, total elongation El _T , local elongation El _L ). The tensile test was carried out in accordance with the provisions of JIS Z 2241 by collecting JIS 13 B test pieces (GL: 25 mm) in which only the parallel part length was shortened so that the rolling direction was the tensile direction.
The obtained results are shown in FIGS. FIG. 1 shows the relationship between TS, El _T, El _L and the heat treatment temperature. FIG. 2 shows the strength-ductility balance.

From FIG. 1, it can be seen that the strength of the high N material (0.012N material) is higher than that of the low N material (0.003N material), and TS increases significantly due to the large amount of N contained. In addition, it can be seen that the elongation El _{T and} El _L are almost the same between the low N material and the high N material, and almost the same ductility can be ensured regardless of the presence or absence of N. It can also be seen that the elongation El _{T and} El _L are remarkably increased by performing a heat treatment that is heated and maintained at a temperature of 250 to 450 ° C., which is equivalent to the hot dip galvanizing treatment after cold rolling. Note that the decrease in TS by this heat treatment is slight, and both the low N material and the high N material have the same amount of reduction.

From FIG. 2, it can be seen that the elongation of the high N material, particularly the local elongation El _L, is remarkably superior to that of the low N material as compared at the same strength level. That is, the strength-ductility balance of the high N material is superior to the low N material. If the heat treatment temperature is optimized, the elongation, particularly the local elongation increases, and the steel sheet has an excellent strength-ductility balance.
For this reason, a strength increase due to work hardening is achieved by subjecting a cold-rolled steel sheet containing a large amount of N and an appropriate range of N / Al to an appropriate heat treatment in the vicinity of 400 ° C. corresponding to hot dip galvanizing. In addition, an increase in strength due to solid solution strengthening of N can be added, and it is possible to achieve higher strength at a lower cost with high ductility, particularly high local ductility, and to obtain a cold-rolled steel sheet having an excellent strength-ductility balance. I found out that I can do it.

The details of the cause of such a significant increase in strength are currently unknown, but the present inventors consider as follows.
The significant increase in strength due to N is thought to be due to the fact that N having a high solid solution strengthening ability can be effectively utilized by adjusting the A1 content and the N content within appropriate ranges. Moreover, the reason why the local ductility of the high N material is superior to the low N material of the same strength is because of the high strength using the work hardening in which the hard portion exists locally, the precipitate harder than the parent phase, the second phase, etc. This is thought to be because the strength was increased by solid solution strengthening with N, which can be strengthened with a uniform microstructure. Work hardening, precipitates harder than the parent phase, the second phase, and the like are factors that reduce local elongation (ductility).

The present invention has been completed by further study based on the above-described findings, and the gist of the present invention is as follows.
(1) By mass%, C: 0.0010 to 0.0080%, Si: 0.4% or less, Mn: 0.1 to 1.0%, P: 0.08% or less, S: 0.05% or less, Al: 0.05% or less, N: 0.0060 to 0.0200 %, And N and Al are contained so that the ratio of N content to Al content, N / Al is 0.2 or more, and further, solid solution N is contained 0.0040% or more, the remainder Fe and inevitable impurities And a structure having a ferrite phase as a main phase, a tensile strength in the rolling direction TS: 600 MPa or more, a total elongation El _T of 6% or more, and a local elongation El _L of 6% or more. A high-tensile cold-rolled steel sheet.

(2) By mass%, C: 0.0010 to 0.0080%, Si: 0.4% or less, Mn: 0.1 to 1.0%, P: 0.08% or less, S: 0.05% or less, Al: 0.05% or less, N: 0.0060 to 0.0200 Nb: 0.001 to 0.050% and / or B: 0.0020% or less, and N and Al, Nb, B, the ratio of the N content to the contents of Al, Nb, B, N / (Al + 0.3Nb + 2.5B) is contained so as to be 0.2 or more, and further contains a solid solution N of 0.0040% or more, a composition comprising the balance Fe and inevitable impurities, and a structure having a ferrite phase as a main phase. A high-tensile cold-rolled steel sheet having a tensile strength TS in the rolling direction of 600 MPa or more, a total elongation El _T of 6% or more, and a local elongation El _L of 6% or more.

(3) A high-tensile cold-rolled steel sheet having a hot-dip galvanized layer, an alloyed hot-dip galvanized layer or an electrogalvanized layer as a plated layer on the steel sheet surface in (1) or (2).
(4) A hot rolling process in which the steel material is heated and hot-rolled to form a hot-rolled sheet; a cold-rolling process in which the hot-rolled sheet is cold-rolled to form a cold-rolled sheet; In the method of manufacturing a high-tensile cold-rolled steel sheet, which is sequentially subjected to a heat treatment step of heating and heat-treating the rolled sheet to obtain a high-tensile cold-rolled steel sheet, the steel material is, in mass%, C: 0.0010 to 0.0080%, Si : 0.4% or less, Mn: 0.1 to 1.0%, P: 0.08% or less, S: 0.05% or less, Al: 0.05% or less, N: 0.0060 to 0.0200%, N and Al, N content and Al The ratio to the content, N / Al is contained so as to be 0.2 or more, and further includes a solid solution N containing 0.0040% or more, and a steel material having a composition composed of the remaining Fe and unavoidable impurities, and the hot rolling step, A steel material is heated to a heating temperature of 1000 ° C. or higher, roughly rolled into a sheet bar, and then the finish rolling is performed on the sheet bar to a finish rolling temperature of 800 ° C. or higher. Taking temperature: It is set as the process made into a hot-rolled sheet by winding up below {700-10x (Al / N)} ° C, and the cold rolling process is pickled on the hot-rolled sheet, and the rolling reduction: 50-95%. High-strength cooling, characterized in that a cold-rolled sheet is formed by performing cold rolling, and the heat-treating process is a step of cooling the cold-rolled sheet after heating to a temperature in the range of 300 to 650 ° C. A method for producing rolled steel sheets.

  (5) A hot rolling process in which the steel material is heated and hot-rolled to form a hot-rolled sheet; a cold-rolling process in which the hot-rolled sheet is cold-rolled to form a cold-rolled sheet; In the method of manufacturing a high-tensile cold-rolled steel sheet, which is sequentially subjected to a heat treatment step of heating and heat-treating the rolled sheet to obtain a high-tensile cold-rolled steel sheet, the steel material is, in mass%, C: 0.0010 to 0.0080%, Si : 0.4% or less, Mn: 0.1 to 1.0%, P: 0.08% or less, S: 0.05% or less, Al: 0.05% or less, N: 0.0060 to 0.0200%, and Nb: 0.001 to 0.050% and / or B: 0.0020% or less, and N and Al, Nb, B, ratio of N content to Al, Nb, B content, N / (Al + 0.3Nb + 2.5B) is 0.2 or more The steel material is composed of the balance Fe and unavoidable impurities, and the hot rolling step is performed by heating the steel material to a heating temperature of 1000 ° C. or more, roughly rolling it into a sheet bar, Finishing and rolling the sheet bar at a finish rolling exit temperature of 800 ° C or higher and winding it up to a temperature of [700-10 x {(Al + 0.3Nb) / N}] ° C or less The cold rolling step is a step of subjecting the hot-rolled sheet to pickling and cold rolling at a reduction ratio of 50 to 95% to form a cold-rolled plate, and the heat treatment step is performed to change the cold-rolled plate to 300 A method for producing a high-tensile cold-rolled steel sheet, characterized by comprising a step of cooling after heating to a temperature in the range of ~ 650 ° C.

(6) In (4) or (5), in place of the heat treatment step, a heat treatment for heating the cold-rolled sheet to a temperature in the range of 450 to 650 ° C. and further hot dip galvanizing are performed, and the surface of the steel sheet is molten zinc. A method for producing a high-tensile cold-rolled steel sheet, characterized by comprising a hot-dip galvanizing treatment step for performing a hot-dip galvanizing treatment for forming a plating layer.
(7) In (4) or (5), in place of the heat treatment step, a heat treatment for heating the cold-rolled sheet to a temperature in the range of 450 to 650 ° C. and further hot-dip galvanizing are performed, and a hot-dip zinc is applied to the steel sheet surface. A galvanizing treatment for forming a plated layer and an alloying treatment for alloying the galvanized layer to form an alloyed galvanized layer on the surface of the steel sheet. A method for producing a high-tensile cold-rolled steel sheet.

The present invention does not contain a large amount of expensive alloy elements, has a tensile strength TS in the rolling direction of 600 MPa or more, a total elongation El _T of 6% or more, and a local elongation El _L of 6% or more. High-strength cold-rolled steel sheet that is superior to the above can be manufactured easily and inexpensively, and has a remarkable industrial effect. In addition, according to the present invention, there is an effect that parts having sufficient characteristics for electrical machinery, building materials, automobiles, and the like can be manufactured at low cost.

It is a graph which shows the relationship between a tensile characteristic and heat processing temperature. It is a graph which shows the relationship of intensity-ductility balance.

First, the reason for limiting the composition of the cold-rolled steel sheet of the present invention will be described. Hereinafter, unless otherwise specified, mass% is simply expressed as%.
C: 0.0010 to 0.0080%
C is an element that increases the strength of the steel sheet, and needs to contain O.0010% or more in order to secure a desired strength. On the other hand, if the content exceeds 0.0080%, cracks and rough skin are likely to occur on the outside of the bent part during close contact bending, and the ductility tends to decrease slightly, and for parts that require a high degree of bending workability. Is a problem. For this reason, C was limited to the range of 0.0010 to 0.0080%. It should be noted that, in the case of use in which extremely high local ductility and bending workability are required, it is preferably 0.0050% or less, or more preferably 0.0040% or less.

Si: 0.4% or less
Si is a useful strengthening element that can increase the strength of a steel sheet without significantly reducing the ductility of the steel. In order to acquire such an effect, it is desirable to contain 0.01% or more. On the other hand, a content exceeding 0.4% adversely affects the surface aesthetics such as surface properties and chemical conversion treatment properties, and in order to suppress these adverse effects, it is inevitable that the pickling treatment and the like will be prolonged, resulting in significant costs. Incurs an increase. For this reason, Si was limited to a range of O.4% or less. In addition, it is preferable to set it as 0.3% or less in the use for which the outstanding surface beauty is calculated | required.

Mn: 0.1-1.0%
Mn is an element that improves hardenability and greatly contributes to an increase in steel sheet strength. Further, Mn is an effective element for preventing hot cracking due to S, and is preferably contained according to the amount of S contained. Such an effect is recognized when the content is 0.1% or more. On the other hand, when it contains exceeding 1.0%, ductility will fall remarkably. For this reason, Mn was limited to the range of O.1 to 1.O%. In applications where better moldability is required, it is desirable that the content be O.6% or less.

P: 0.08% or less P has an action of strengthening steel, and in order to obtain this effect, it is desirable to contain 0.001% or more according to the desired strength in the present invention. However, if it exceeds 0.08%, the weldability and the low temperature toughness after processing are lowered. For this reason, P was limited to O.08% or less. In addition, when more excellent weldability and low temperature toughness are required, P is preferably 0.05% or less.

S: 0.05% or less S is present as an inclusion in the steel sheet, and is an element that causes deterioration of the ductility and formability of the steel sheet, in particular, stretch flange formability. However, if the content is reduced to 0.05% or less, the adverse effect on stretch flangeability can be ignored. Therefore, in the present invention, S is limited to 0.05% or less. When more excellent stretch flange formability or weldability is required, S is preferably 0.03% or less.

Al: 0.05% or less
Al is a useful element that acts as a deoxidizer for steel and improves the cleanliness of the steel. Al also has an effect of refining crystal grains, and is an element that is desirably contained for refining the structure of steel. In order to obtain such an effect, Al is preferably contained in an amount of 0.001% or more. However, if the content exceeds 0.05%, the surface properties are deteriorated and the solid solution N is significantly reduced, and a good strength-ductility balance is obtained. It becomes difficult to obtain. For this reason, Al was limited to 0.05% or less. In addition, from a viewpoint of material stability, it is preferable to set it as 0.001 to 0.04%. In addition, there is a concern that the reduction of the Al content may lead to the coarsening of the crystal grains, but in the present invention, by limiting the other alloy elements to the optimum amount and making the annealing conditions in the optimum range, Prevent coarsening.

N: 0.0060-0.0200%
N is a solid solution that increases the strength of the steel and contributes to securing excellent formability in the present invention, and such an effect can be stably obtained with a content of approximately 0.0060% or more. On the other hand, when the content exceeds 0.0200%, the occurrence of slab cracking during continuous casting becomes remarkable, and the internal defect occurrence rate of the steel sheet increases. For this reason, N was limited to the range of O.O060 to O.0200%. Preferably, it is 0.0060 to 0.0170% from the viewpoint of improving the stability and yield of the material in consideration of the entire manufacturing process. N also has an effect of lowering the transformation point of steel, and the inclusion of N is effective in situations where it is not desired to perform rolling with a thin material that greatly cuts the transformation point. If N is contained within the scope of the present invention, weldability and the like are not adversely affected.

N / Al: 0.2 or more
Al is an element that strongly fixes N. From the viewpoint of securing a desired amount of dissolved N, in the present invention, the ratio of N content to Al content, N / Al is limited to 0.2 or more. did. Thereby, the amount of solid solution N of O.O040% or more can be stably secured, and desired excellent moldability can be secured. N / Al is preferably 0.3 or more.

Solid solution N: 0.0040% or more N in the solid solution state contributes to strengthening of the ferrite phase. In order to solid-solution strengthen the work-hardened ferrite phase to secure a desired high strength as a cold-rolled steel sheet, it is necessary to secure N in a solid solution state of 0.040% or more. Here, N in the solid solution state is a value obtained by subtracting the amount of precipitated N (stopped by the dissolution method by electrolytic extraction) from the total N amount in the steel. As the amount of precipitated N, a value obtained by applying a dissolution method by electrolytic extraction using a constant potential electrolysis method is used. Various methods have been examined for the analysis of the precipitate N, and the method of applying the dissolution method by electrolytic extraction using the constant potential electrolysis method is based on the fact that it best corresponds to the change in material. As the electrolytic solution, it is preferable to use an acetylacetone-based electrolytic solution. The residue extracted by the dissolution method by electrolytic extraction using a constant potential electrolysis method was chemically analyzed to determine the amount of N in the residue, and this was defined as the amount of precipitated N. In addition, when it is necessary to obtain higher strength more stably, it is effective to set the solid solution N to 0.0060% or more.

The above-described components are basic components, and in the present invention, Nb: 0.001 to 0.050% and / or B: 0.0020% or less may be further contained in addition to the basic components.
Nb: 0.001 to 0.050%
Nb is an element partly precipitated as carbide NbC or nitride NbN during the manufacturing process of the steel sheet, and has an action of suppressing grain growth, recrystallization, and the like, and can be contained as necessary. In the present invention, it is added in the heat treatment step when it is necessary to suppress a decrease in strength accompanying an increase in the heat treatment temperature. By containing Nb, the strength change accompanying the change in the heat treatment temperature is reduced, and a cold-rolled steel sheet having high strength can be manufactured stably. Such an effect becomes remarkable when the content is approximately 0.001% or more. On the other hand, the content exceeding 0.050% leads to a decrease in ductility and a significant decrease in the amount of dissolved N, and it becomes difficult to ensure a good strength-ductility balance. For this reason, when it contains, it is preferable to limit Nb to 0.001 to 0.050% of range. In addition, More preferably, it is 0.005-0.03%.

B: 0.0020% or less B is an element that partially segregates at the grain boundaries during the manufacturing process of the steel sheet and has the action of suppressing grain growth, recrystallization, and the like, and can be contained as necessary. In the present invention, B, like Nb, is added in the heat treatment step when it is necessary to suppress a decrease in strength accompanying an increase in the heat treatment temperature. By containing B, the strength change accompanying the change in the heat treatment temperature is reduced, and a cold-rolled steel sheet having high strength can be produced stably. Since such an effect becomes remarkable when the content is generally 0.0001% or more, it is desirable to contain 0.0001% or more. On the other hand, the content exceeding 0.0020% leads to a decrease in ductility and a significant decrease in the amount of dissolved N, and it is difficult to ensure a good strength-ductility balance. For this reason, when it contains, it is preferable to limit B to 0.0020% or less. In addition, More preferably, it is 0.0002 to 0.0015%.

  When Nb and B are contained, N / (Al + 0.3Nb + 2.5B) is used instead of N / Al, and N / (Al + 0.3Nb + 2.5B): is adjusted to satisfy 0.2 or more. Here, N represents N content (mass%), Al represents Al content (mass%), and Nb and B represent Nb content (mass%) and B content (mass%), respectively. When Nb or B is not added, the value of N / (Al + 0.3Nb + 2.5B) is calculated assuming that the amount of elements not contained is zero.

The balance other than the above components is Fe and inevitable impurities. Inevitable impurities include, for example, Sb, Sn, Zn, Co and the like, and Sb: 0.01% or less, Sn: 0.1% or less, Zn: 0.01% or less, Co: O.1% or less are acceptable.
Next, the reason for limiting the structure of the cold-rolled steel sheet of the present invention will be described.
The cold-rolled steel sheet of the present invention has a structure whose main phase is a ferrite phase. The “main phase” here refers to a case where the volume ratio with respect to the entire structure is 97% or more. The second phase other than the main phase is cementite or the like. The second phase is 3% or less by volume ratio. When the amount of the second phase exceeds 3%, cracks and rough skin are likely to occur on the outer side of the bent part during close contact bending, and the ductility tends to decrease slightly. For parts that require a high degree of bending workability. Is a problem. In applications where better bending workability is required, the content is preferably 2% or less. In the present invention, the ferrite phase that is the main phase is a ferrite phase that is work-hardened by cold rolling. For example, the half width of the diffraction peak of the (211) plane by X-ray diffraction is 0.20 ° or more and 0.40 ° or less. It is a phase to become.

Next, a preferred method for producing the cold-rolled steel sheet of the present invention will be described.
In the present invention, a hot rolling process in which the steel material is heated and subjected to hot rolling to form a hot rolled sheet, a cold rolling process in which the hot rolled sheet is subjected to cold rolling to form a cold rolled sheet, A heat treatment step of heating the cold-rolled plate and performing a heat treatment is sequentially performed to obtain a high-tensile cold-rolled steel plate.
The steel material to be used is in mass%, C: 0.001 to 0.0080%, Si: 0.4% or less, Mn: 0.1 to 1.0%, P: 0.08% or less, S: 0.05% or less, in the same manner as the steel plate composition described above. , Al: 0.05% or less, N: 0.0060 to 0.0200% included, or Nb: 0.001 to 0.050% and / or B: 0.0020% or less, N and Al or further Nb and / or B, N content Al content ratio, N / Al or N content to Al, Nb, B content ratio, N / (Al + 0.3Nb + 2.5B) is contained so as to be 0.2 or more, the remainder Fe and inevitable impurities A steel material having a composition consisting of

The manufacturing method of the steel material is not particularly limited, but the molten steel having the above composition is melted by a conventional melting method such as a converter, and a steel material such as a slab by a conventional casting method such as a continuous casting method. It is preferable that The casting method of the steel material is desirably a continuous casting method in order to prevent macro segregation of components, but there is no problem even with the ingot forming method or the thin slab casting method.
The obtained steel material is then subjected to a hot rolling process. In addition to the method of once cooling to room temperature and then reheating, the heat for hot rolling was not cooled to room temperature, but was charged in a heating furnace as it was, or a little heat was retained. Energy saving processes such as direct feed rolling and direct rolling, which are rolled immediately afterwards, can be applied without any problem.

In the hot rolling process, the steel material is heated to a heating temperature of 1000 ° C. or higher, roughly rolled into a sheet bar, and then subjected to finish rolling to a finish rolling exit temperature of 800 ° C. or higher, and the coiling temperature : {700-10 × (Al / N)} ° C. or less, and when Nb and / or B is contained, [700−10 × {(Al + 0.3Nb) / N}] ° C. or less, and rolled hot rolled sheet And
Heating temperature: 1000 ° C. or higher The heating temperature for hot rolling is preferably 1000 ° C. or higher. If the heating temperature is less than 1000 ° C, it becomes difficult to ensure the desired solid solution N in the cold-rolled steel sheet and the lower limit (800 ° C) of the finish rolling exit temperature after hot rolling, and rolling during hot rolling. The increase in load becomes significant, and rolling may be difficult. The upper limit of the heating temperature is not particularly limited. However, when the temperature exceeds 1280 ° C. and the temperature becomes high, loss due to oxidation increases and yield decreases. For this reason, the heating temperature for hot rolling is preferably about 1280 ° C. or less.

Finishing rolling exit temperature: 800 ° C. or more The finishing rolling exit temperature of hot rolling is preferably 800 ° C. or more. Thereby, the structure of a hot-rolled sheet can be made into a uniform fine structure. If the finish rolling exit temperature is less than 800 ° C., the structure of the hot-rolled sheet to be obtained becomes non-uniform, and the non-uniformity of the structure does not disappear even if the subsequent cold rolling process and heat treatment (annealing) process are performed. For this reason, the danger of generating various malfunctions at the time of press molding increases. In addition, when the finish rolling exit temperature is less than 800 ° C., if a high coiling temperature is employed to avoid the remaining of the processed structure, coarse grains are generated and the same problem occurs. For this reason, it is preferable that the finish rolling outlet temperature is limited to 800 ° C. or higher. In order to further improve the mechanical properties, the finish rolling outlet temperature is more preferably 820 ° C. or higher. The upper limit of the finish rolling exit temperature is not particularly limited. However, when rolling at an excessively high finish rolling exit temperature, scale wrinkles are likely to occur, so the upper limit of the finish rolling exit temperature. Is preferably about 1O00 ° C.

Winding temperature: {700-10 × (Al / N)} ° C. or lower or [700-10 × {(Al + 0.3Nb) / N}] ° C. or lower After finishing rolling, the hot rolled sheet is then wound in a coil shape Taken. The precipitation of nitrides such as AlN and NbN is greatly affected by the coiling temperature. When the coiling temperature becomes high, AlN, NbN, etc. are likely to be generated, and Al / N or (Al + 0.3Nb) / N is large. In addition, a compound of Al, Nb and N (AlN, NbN, etc.) is likely to precipitate, and it becomes difficult to ensure a predetermined amount of solid solution N. For this reason, in the present invention, the winding temperature is preferably set to {700-10 × (Al / N)} ° C. or lower or [700-10 × {(Al + 0.3Nb) / N}] ° C. or lower. The relational expression between the coiling temperature and Al / N or (Al + 0.3Nb) / N is an empirical formula obtained by various studies by the inventors in order to obtain a predetermined solid solution N amount. When the coiling temperature exceeds {700-10 × (Al / N)} ° C. or [700-10 × {(Al + 0.3Nb) / N}] ° C., AlN is likely to be generated, and the desired amount of dissolved N Cannot be secured. The lower limit of the coiling temperature is not particularly limited, but is preferably 200 ° C. or higher. When the temperature is lower than 200 ° C., the shape of the steel sheet is significantly disturbed, and the risk of causing problems in actual use increases. In addition, the uniformity of the material tends to decrease. When higher material uniformity is required, the lower limit of the coiling temperature is desirably 300 ° C. or higher.

The hot rolled sheet is then subjected to a cold rolling process.
The cold rolling process is a process in which a hot rolled sheet is pickled and cold rolled with a reduction ratio of 50 to 95% to obtain a cold rolled sheet.
In addition, although it is preferable to perform pickling according to a usual method, if a scale is a very thin state, you may perform a cold rolling process directly, without performing pickling.

Cold rolling reduction: 50-95%
In the present invention, in order to increase the strength by utilizing work hardening by cold rolling, the rolling reduction of cold rolling is set to 50% or more. If it is less than 50%, the desired high strength cannot be ensured. On the other hand, when the rolling reduction exceeds 95%, the cold rolling time becomes long, and a significant increase in the cold rolling process cost is inevitable. For this reason, it is preferable to limit the rolling reduction of cold rolling to 50 to 95%. In order to obtain high strength more stably, the rolling reduction is desirably 55% or more.

The cold rolled sheet is then subjected to a heat treatment step.
The heat treatment step is a step in which the cold-rolled sheet is heated to a temperature in the range of 300 to 650 ° C. and then cooled.
Heat treatment temperature: 300-650 ° C
The heat treatment applied to the cold-rolled sheet is preferably performed at a temperature in the range of 300 ° C to 650 ° C. When the heat treatment temperature is less than 300 ° C., the increase in ductility is small, and particularly low local ductility can be obtained. On the other hand, at a temperature exceeding 650 ° C., the recovery of the work phase hardened by cold rolling proceeds so much that the desired tensile strength TS cannot be ensured. Therefore, the heat treatment temperature is preferably limited to a temperature in the range of 300 to 650 ° C. More preferably, it is 300 degreeC or more and 600 degrees C or less, More preferably, it is 550 degrees C or less. What is necessary is just to cool appropriately temperature after heating to predetermined temperature. The holding time in the above temperature range is preferably 600 s or less. If the holding time exceeds 600 s, the heat treatment time becomes too long, leading to an increase in steel sheet manufacturing cost. The holding time may be 0 s, but the holding time is preferably 10 s or more in order to further improve ductility, particularly local elongation. Here, the holding time of 0 s means that the cooling is started immediately after reaching the predetermined temperature. Subsequent to this heat treatment step, an electrogalvanizing treatment can be applied to form an electrogalvanized layer on the steel sheet surface to obtain an electrogalvanized steel sheet.

In addition, when forming a hot dip galvanized layer instead of the electrogalvanized layer on the steel sheet surface, instead of the above-described heat treatment step, using a hot dip galvanizing line, Further, a hot dip galvanizing treatment step in which the heat treatment and the hot dip galvanizing treatment are continuously performed may be performed.
The hot dip galvanizing treatment step is preferably a step of performing a heat treatment for heating the cold-rolled sheet that has undergone the cold rolling step to a temperature in the range of 450 to 650 ° C., and then performing a hot dip galvanizing treatment.

  In the hot dip galvanizing process, the heat treatment temperature is preferably 450 ° C. or higher. This is because the temperature of the hot dip galvanizing bath used for the hot dip galvanizing treatment is approximately 460 ° C., and thus 450 ° C. was selected as a temperature that does not adversely affect the hot dip galvanizing treatment. On the other hand, when the heat treatment temperature exceeds 650 ° C., the recovery of work-hardened ferrite proceeds and the tensile strength is significantly reduced. For this reason, it is preferable to limit the heat processing temperature in a hot dip galvanization process to the temperature of the range of 450-650 degreeC. Further, the holding time within the range of the heat treatment temperature is preferably 600 s or less. If the holding time exceeds 600 s, the annealing time becomes longer, the productivity is lowered, and the heat treatment cost is increased. The holding time may be 0 s, but is preferably 10 s or longer from the viewpoint of improving local ductility. In the hot dip galvanizing process, following the heat treatment, a hot dip galvanizing process is performed according to a conventional method. It is preferable that the hot dip galvanizing treatment is performed continuously after heating and cooling to the heat treatment temperature using a continuous hot dip galvanizing line.

  Moreover, it replaces with the above-mentioned hot dip galvanizing process, and the alloy which heats a hot dip galvanized layer to the temperature of 550 degrees C or less, and alloys the hot dip galvanized phase following the above-mentioned heat treatment-hot dip galvanizing process. It is good also as an alloying hot-dip galvanization process process which performs an alloying process continuously. In addition, the effect of the present invention can be obtained by so-called “spotting plating” in which a steel sheet is immersed in a hot dip galvanizing bath and subjected to a plating process instead of a continuous process.

  Molten steel having the composition shown in Table 1 was melted in a converter and made into a steel material (slab: thickness 260 mm) by a continuous casting method. These steel materials were made into a hot-rolled steel strip (hot-rolled sheet) having a thickness of 4.0 mm in the hot-rolling process under the conditions shown in Table 2. Next, these hot-rolled sheets were pickled, and then cold-rolled sheets having a thickness of 1.2 mm were formed in the cold-rolling step under the conditions shown in Table 2. The obtained cold-rolled sheet was subjected to a heat treatment step, a hot dip galvanizing treatment step, or an alloying hot dip galvanizing treatment step under the conditions shown in Table 2 using a continuous annealing line or a hot dip galvanizing line. Note that the hot dip galvanizing treatment was performed by continuously immersing the steel sheet in a hot dip galvanizing bath at a plating bath temperature of 460 ° C. following the heat treatment. The alloying hot dip galvanizing treatment was a treatment of alloying the hot dip galvanized layer at an alloying treatment temperature of 500 ° C. or 520 ° C. after the hot dip galvanizing treatment.

In addition, after the heat treatment process, the hot dip galvanizing process, and the alloying hot dip galvanizing process, temper rolling with an elongation of 0.1% was performed for shape correction.
From the obtained steel plate (cold-rolled steel plate or plated steel plate), test pieces were collected and subjected to structure observation, tensile test, bending test, and plating test. The test method was as follows.
(1) Microstructure observation From the obtained cold-rolled steel sheet and plated steel sheet, a specimen for microstructural observation is collected so that a cross section parallel to the rolling direction (L cross section) becomes the observation surface, polished, and subjected to nital corrosion, optical Using a microscope (magnification: 400 times) or a scanning electron microscope (magnification: 1000 times), each phase constituting the tissue is identified, and the tissue fraction (area ratio) of each phase is image analysis apparatus And measured. In addition, about the ferrite phase, the peak of (211) plane was calculated | required with the X ray diffraction method, and the half value width was calculated | required. The X-ray used was a Co-Kα ray (wavelength 1.79Å).
(2) Tensile test J1S13 No. B test piece (GL: 25mm) was sampled from the obtained cold-rolled steel sheet and plated steel sheet so that the test direction was the rolling direction, and tensioned according to the provisions of J1S Z 2241 Tests were conducted to determine tensile properties (yield strength YS, tensile strength TS, total elongation (El _T ), local elongation (El _L )). Here, J1S13 No. B test piece (GL: 25mm) is the same shape as J1S13 No. B test piece (Gl: 50mm), only the length is shortened, and the gauge distance is halved to 25mm It is a piece.
(3) Bending test From the obtained cold-rolled steel sheet and plated steel sheet, a bending test piece is taken so that the ridge line after bending is in the rolling direction, and an adhesion bending test is performed in accordance with the provisions of JIS Z 2248. Carried out. After the test, for the test piece, the outside of the bent portion was visually observed to observe the presence or absence of cracks, and the bending workability was evaluated with x when there was a crack even at one place and ◯ when there was no crack.
(4) Plating property test About the obtained plated steel plate, the steel plate surface was observed visually and the presence or absence of the non-plating defect was observed. The case where there was no unplating was evaluated as good (◯), and the case where unplating was observed even at one location was evaluated as defective (x), and the plating property was evaluated.

  The obtained results are shown in Table 3.

Each of the examples of the present invention has high tensile strength of 650 MPa or more, total elongation El _{T of} 6% or more, and local elongation El _{L of} 6% or more without containing a large amount of expensive alloy elements. In addition, it is a cold-rolled steel sheet or a plated steel sheet that has excellent formability and an excellent balance between strength and ductility. Furthermore, in all of the examples of the present invention, cracks are not observed outside the bent portion even in a severe process such as close contact bending, and a cold-rolled steel sheet or a plated steel sheet having excellent bending workability is obtained. On the other hand, in the comparative example outside the scope of the present invention, the strength is insufficient, or the strength-ductility balance is deteriorated, or further, cracks may be observed outside the bent portion, and the bending workability is deteriorated. ing.

Claims (7)

% By mass
C: 0.0010 to 0.0080%, Si: 0.4% or less,
Mn: 0.1 to 1.0%, P: 0.08% or less,
S: 0.05% or less, Al: 0.05% or less,
N: 0.0060-0.0200%
N and Al are contained so that the ratio of N content to Al content, N / Al is 0.2 or more, and further contains solid solution N 0.0040% or more, and the balance Fe and unavoidable impurities And tensile structure TS in the rolling direction: 600 MPa or more, total elongation El _T is 6% or more, and local elongation El _L is 6% or more. A high-tensile cold-rolled steel sheet characterized by % By mass
C: 0.0010 to 0.0080%, Si: 0.4% or less,
Mn: 0.1 to 1.0%, P: 0.08% or less,
S: 0.05% or less, Al: 0.05% or less,
N: 0.0060-0.0200%
Nb: 0.001 to 0.050% and / or B: 0.0020% or less, and N and Al, Nb, B, the ratio of N content to the content of Al, Nb, B, N /(Al+0.3Nb+2.5B) is contained so as to be 0.2 or more, and further contains 0.0040% or more of solute N, the composition comprising the balance Fe and inevitable impurities, and the structure having the ferrite phase as the main phase. And a tensile strength TS in the rolling direction: 600 MPa or more, a total elongation El _T of 6% or more, and a local elongation El _L of 6% or more.   The high-tensile cold-rolled steel sheet according to claim 1 or 2, comprising a hot-dip galvanized layer, an alloyed hot-dip galvanized layer or an electrogalvanized layer as a plated layer on the surface of the steel plate. A hot rolling process in which the steel material is heated and rolled into a hot rolled sheet, a cold rolling process in which the hot rolled sheet is cold rolled into a cold rolled sheet, and the cold rolled sheet In the manufacturing method of the high-tensile cold-rolled steel sheet, which is sequentially subjected to a heat treatment step of heating and heat-treating to obtain a high-tensile cold-rolled steel sheet,
The steel material in mass%,
C: 0.0010 to 0.0080%, Si: 0.4% or less,
Mn: 0.1 to 1.0%, P: 0.08% or less,
S: 0.05% or less, Al: 0.05% or less,
N: 0.0060-0.0200%
And N and Al, a ratio of N content to Al content, N / Al is contained so as to be 0.2 or more, and a steel material having a composition comprising the balance Fe and inevitable impurities,
In the hot rolling step, the steel material is heated to a heating temperature of 1000 ° C. or higher, roughly rolled into a sheet bar, and then subjected to finish rolling to a finish rolling outlet temperature of 800 ° C. or higher. Taking temperature: {700-10 × (Al / N)}
The cold rolling step is a step of subjecting the hot-rolled sheet to pickling and cold rolling at a reduction ratio of 50 to 95% to obtain a cold-rolled sheet,
The method for producing a high-tensile cold-rolled steel sheet, characterized in that the heat treatment step is a step of heating the cold-rolled plate to a temperature in the range of 300 to 650 ° C and then cooling. A hot rolling process in which the steel material is heated and rolled into a hot rolled sheet, a cold rolling process in which the hot rolled sheet is cold rolled into a cold rolled sheet, and the cold rolled sheet In the manufacturing method of the high-tensile cold-rolled steel sheet, which is sequentially subjected to a heat treatment step of heating and heat-treating to obtain a high-tensile cold-rolled steel sheet,
The steel material in mass%,
C: 0.0010 to 0.0080%, Si: 0.4% or less,
Mn: 0.1 to 1.0%, P: 0.08% or less,
S: 0.05% or less, Al: 0.05% or less,
N: 0.0060-0.0200%
Nb: 0.001 to 0.050% and / or B: 0.0020% or less, and N and Al, Nb, B, the ratio of N content to the content of Al, Nb, B, N /(Al+0.3Nb+2.5B) is contained so as to be 0.2 or more, and the steel material is composed of the balance Fe and inevitable impurities, and the hot rolling process is performed by heating the steel material to a heating temperature of 1000 ° C. or more. Then, rough rolling into a sheet bar, and then finishing rolling to a finish rolling exit temperature of 800 ° C. or higher is performed on the sheet bar, and a winding temperature: [700-10 × {(Al + 0.3Nb) / N}] It is a process to make a rolled hot rolled sheet at a temperature below ℃,
The cold rolling step is a step of subjecting the hot-rolled sheet to pickling and cold rolling at a reduction ratio of 50 to 95% to obtain a cold-rolled sheet,
The method for producing a high-tensile cold-rolled steel sheet, characterized in that the heat treatment step is a step of heating the cold-rolled plate to a temperature in the range of 300 to 650 ° C and then cooling.   Instead of the heat treatment step, a heat treatment for heating the cold-rolled sheet to a temperature in the range of 450 to 650 ° C. and a hot dip galvanizing treatment for forming a hot dip galvanized layer on the surface of the steel sheet are performed. The method for producing a high-tensile cold-rolled steel sheet according to claim 4 or 5, wherein a hot-dip galvanizing process is performed.   Instead of the heat treatment step, a heat treatment for heating the cold-rolled sheet to a temperature in the range of 450 to 650 ° C., a hot-dip galvanizing, and a hot-dip galvanizing treatment for forming a hot-dip galvanized layer on the steel sheet surface; 6. An alloying hot dip galvanizing treatment step of forming an alloy hot dip galvanizing layer on a steel sheet surface by performing an alloying treatment for alloying the hot dip galvanized layer. Manufacturing method of high-tensile cold-rolled steel sheet.

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