CN107208222B - High strength cold rolled steel plate, high-intensitive coated steel sheet and its manufacturing method - Google Patents

Abstract

Provided are a high-strength cold-rolled steel sheet, a high-strength plated steel sheet, a high-strength plated steel sheet, and a method for producing the same, which have a surface layer tensile strength of 780 MPa or more and good formability. The high-strength cold-rolled steel sheet has a specific composition and steel structure as follows, the area percentage of the ferrite phase in the region from 1/4 to 3/4 of the plate thickness in the steel structure is 20% or more and 80% or less, Bainian The total area percentage of bulk phase, martensite phase, and tempered martensite phase is 20% or more and 80% or less. The amount of trace is 0.10% by mass/mm or more; the tensile strength in a tensile test using a JIS No. 5 tensile test piece is 780 MPa or more. d[%C _x ]/dx=([%C _x ]-[%C _x-10μm ])/0.01(1), [%C _x ] in formula (1) is the C concentration at x, and x is Below 50μm.

Description

High-strength cold-rolled steel sheet, high-strength plated steel sheet, and manufacturing method thereof

technical field

The present invention relates to a high-strength cold-rolled steel sheet having a tensile strength (TS) of 780 MPa or more and excellent formability, which are useful as raw materials for automobile frame parts, and a high-strength plated steel sheet and a method for producing the same.

Background technique

From the viewpoint of global environmental protection in recent years, the entire automobile industry is seeking to improve the fuel efficiency of automobiles for the purpose of reducing CO ₂ emissions. The weight reduction of automobiles by thinning the parts used is the most effective for improving the fuel efficiency of automobiles. High-strength steel sheets are included as raw materials for automotive parts that can be thinned. For this reason, in recent years, the amount of high-strength steel sheets used as raw materials for automobile parts has gradually increased.

On the other hand, in general, as the strength of a steel sheet increases, its formability decreases. If the formability is lowered, it becomes difficult to process the steel sheet. Since many automobile parts have complicated shapes, steel sheets having high strength and good workability are required in addition to reducing the weight of automobile parts and the like.

In view of the above circumstances, it is necessary to develop a steel sheet having both high strength and bendability (also referred to as workability and formability). So far, various technologies have been proposed for high-strength cold-rolled steel sheets and hot-dipped steel sheets focusing on workability.

For example, in Patent Document 1, a hot-dip galvanized steel sheet having a hot-dip galvanized layer on the surface of the steel sheet has the following composition: in mass %, C: more than 0.02% and 0.20% or less, Si: 0.01% ～2.0%, Mn: 0.1～3.0%, P: 0.003～0.10%, S: 0.020% or less, Al: 0.001～1.0%, N: 0.0004～0.015%, Ti: 0.03～0.2%, the rest is Fe and not allowed Avoid impurities; and have the following steel structure: 30% to 95% ferrite by area percentage, and the rest is one of martensite, bainite, pearlite, cementite and retained austenite Or two or more kinds of structures, and when martensite is contained, the area percentage of martensite is 0 to 50%; the steel plate contains Ti-based carbonitriding precipitates with a particle size of 2 to 30 nm at an average interparticle distance of 30 to 300 nm, In addition, crystal precipitation system TiN (crystallizing TiN) having a particle diameter of 3 μm or more is contained at an average inter-particle distance of 50 to 500 μm. By satisfying the above configuration, in Patent Document 1, a high-yield-ratio high-strength steel sheet having a tensile strength of 620 MPa or more, excellent in bending workability and notch fatigue resistance can be obtained.

In Patent Document 2, the steel sheet contains C: 0.05% to 0.20%, Si: 0.01% to less than 0.6%, Mn: 1.6% to 3.5%, P: 0.05% or less, S: 0.01% or less, sol.Al( Sol Al): 1.5% or less, N: 0.01% or less, the rest is composed of iron and unavoidable impurities, and has a polygonal ferrite structure and a low-temperature transformation structure. The low-temperature transformation formation structure contains at least bainite, which can be It further includes martensite. In addition, in Patent Document 2, for a plate surface 0.1 mm deep from the surface of the steel plate, a total of 20 fields of view are observed with a microscope while changing the position in the width direction of the plate, and image analysis is performed on a region of 50 μm×50 μm in each field of view. , the maximum and minimum values of the area percentage of polygonal ferrite and the maximum value of the area percentage of martensite are specified. By satisfying the above configuration, in Patent Document 2, a hot-dip galvanized steel sheet having excellent bending workability and fatigue strength and a tensile strength of 780 MPa or more can be obtained.

prior art literature

patent documents

Patent Document 1: Japanese Patent Laid-Open No. 2006-063360

Patent Document 2: Japanese Unexamined Patent Publication No. 2010-209428

Contents of the invention

The problem to be solved by the invention

In the technology proposed in Patent Document 1, there is nothing disclosed in the examples regarding the influence of the composition on the steel structure. Therefore, in Patent Document 1, it cannot be said that the steel structure was sufficiently considered and improved. The formability of the steel sheet described in Patent Document 1 as described above does not satisfy the formability currently required.

In addition, in the technology proposed in Patent Document 2, the density of inclusions and the form of the martensite phase are not considered. Therefore, in the technique described in Patent Document 2, moldability is not sufficient.

In addition, as the strength, the tensile strength of the surface layer portion from the surface of the steel sheet to 50 μm in the thickness direction is important.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a steel sheet having a tensile strength (using a JIS No. Measured tensile strength: 780 MPa or more), and a high-strength cold-rolled steel sheet with good formability, a high-strength plated steel sheet, and a manufacturing method thereof.

means of solving problems

The inventors of the present application paid attention to the stress gradient with respect to the sheet thickness direction during bending in the process of studying the constituent requirements of a steel sheet having a surface layer tensile strength within a desired range and having good formability. During bending, the stress in the vicinity of the central portion in the thickness direction becomes infinitely small, but the stress in the surface layer portion becomes remarkably large in the thickness direction from the surface of the steel sheet to a region of 50 μm. The above-mentioned stress changes continuously in the thickness direction of the plate. As a result of observation of cracks (cracks) in the bent portion, it was found that the pore density in the surface layer portion was extremely high compared to the pore density at a position deeper than 50 μm in the thickness direction.

In addition, as described above, the stress decreases continuously in the range from the surface of the steel plate to the center of the plate thickness. Therefore, it has been found that if the hardness of the surface layer, which is the region from the surface of the steel plate to 50 μm in the thickness direction of the steel sheet, which is the location where cracks occur, is not continuously changed, stress concentration occurs and causes cracks.

Using the following findings obtained from the above studies, a steel plate having both high strength and good formability has been completed.

(1) Forming a bainite phase, a martensite phase, and a tempered martensite phase in an appropriate range contributes to the tensile strength of the surface layer portion being in a desired range. As will be described later, the hardness of the above-mentioned bainite phase, martensite phase, and tempered martensite phase is determined by the contents of C, Si, Mn, etc., but the amount of C has the greatest influence.

(2) Continuously changing the hardness of the surface layer in the thickness direction can be achieved by continuously changing the C concentration in the surface layer. Specifically, in order to improve the formability, when the differential amount of the C concentration in the surface layer portion is 0.10% by mass/mm or more, desired formability can be obtained.

(3) Continuously changing the C concentration can be realized by controlling the furnace atmosphere, dew point, and heating temperature of a continuous annealing line or a continuous coating line.

The present invention was completed based on the above findings, and the gist thereof is as follows.

[1] A high-strength cold-rolled steel plate having the following composition and steel structure,

The composition contains C: 0.06% to 0.20%, Si: 0.01% to 2.0%, Mn: 1.8% to 5.0%, P: 0.06% or less, and S: 0.005% or less in mass %. , Al: 0.08% or less, N: 0.008% or less, the rest is composed of Fe and unavoidable impurities;

In the steel structure, the area percentage of the ferrite phase in the region from 1/4 to 3/4 of the plate thickness is not less than 20% and not more than 80%, and the bainite phase, martensite phase and tempered martensite phase The sum of the area percentages of bulk phases is not less than 20% and not more than 80%,

The differential amount of the C concentration represented by the following formula (1) at x μm from the surface of the steel plate in the thickness direction is 0.10% by mass/mm or more;

When using JIS No. 5 tensile test piece to carry out the tensile test, the tensile strength of the high-strength cold-rolled steel plate is more than 780MPa,

d[% _Cx ]/dx=([% _Cx ]-[% _Cx-10μm ])/0.01 (1)

[%C _x ] in formula (1) represents the C concentration at x, where x represents the distance in the thickness direction from the surface of the steel sheet, and its value is 50 μm or less.

[2] The high-strength cold-rolled steel sheet described in [1], wherein the composition further contains Mo: 0.01% to 0.5%, Cr: 0.01% to 0.9%, and Ni: 0.01% in mass % 1 type or 2 or more types of 0.2% or more.

[3] The high-strength cold-rolled steel sheet described in [1] or [2], wherein the composition further contains Ti: 0.01% to 0.15%, Nb: 0.01% to 0.1%, in mass %, V: 0.01% to 0.5% of one or more types, the density of inclusions with a particle size of 0.2 μm or more in the steel structure in the region from the steel plate surface to 50 μm in the thickness direction 500 pieces/mm ² or less.

[4] The high-strength cold-rolled steel sheet according to any one of [1] to [3], wherein the component composition further contains B: not less than 0.0002% and not more than 0.0030% in mass%.

[5] A high-strength plated steel sheet comprising the high-strength cold-rolled steel sheet according to any one of [1] to [4], and a plating layer formed on the high-strength cold-rolled steel sheet.

[6] The high-strength plated steel sheet described in [5], wherein the coating is a hot-dip coating or an alloyed hot-dip coating.

[7] The high-strength plated steel sheet described in [5] or [6], wherein the plated layer contains Fe: 5.0 to 20.0%, Al: 0.001% to 1.0%, and further contains a total of 0 to 3.5%. One or two or more selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, REM, the rest is composed of Zn and unavoidable impurities constitute.

[8] A method of manufacturing a high-strength cold-rolled steel plate, comprising:

Hot rolling process, heating the steel raw material having the composition according to any one of claims 1 to 4 to 1150°C or higher, performing rough rolling, finishing rolling at a finishing temperature of 800°C or higher, at 350°C or higher And coiling at a coiling temperature below 720°C;

A cold-rolling process, after the hot-rolling process, cold-rolling the hot-rolled steel sheet; and

In the annealing process, after the cold rolling process, in a continuous annealing line or a continuous coating line, using a burner, the air ratio is 1.05 to 1.30 in the temperature range of 580°C or higher and T°C or lower, 730°C or higher Under the condition that the dew point in the temperature range is -40 to -15°C, the cold-rolled steel plate is heated to the highest temperature above 730°C, and then the average cooling rate from 700°C to 550°C is below -10°C/s It is cooled to a cooling stop temperature of 25 to 530°C under conditions, and then heated as necessary to maintain in a temperature range of 200°C to 530°C, where 580<T≦730.

[9] The method for producing a high-strength cold-rolled steel sheet described in [8], wherein in the hot rolling step, the atmosphere when heating the steel raw material satisfies the formula (2), and the temperature of the steel raw material is 1150°C or higher. Stay in the temperature zone for more than 30 minutes,

(pCO+pCO ₂ +pCH ₄ )/(pCO+2pCO ₂ +pH ₂ O+2pO ₂ )≤1 (2)

pCO, pCO ₂ , pCH ₄ , pH ₂ O, and pO ₂ in formula (2) are partial pressures (Pa) of CO, CO ₂ , CH ₄ , H ₂ O, and O ₂ , respectively.

[10] A method for producing a high-strength plated steel sheet, comprising a plating step of plating a high-strength cold-rolled steel sheet produced by the production method described in [8] or [9].

[11] The method for producing a high-strength plated steel sheet described in [10], comprising an alloying step of performing an alloying treatment after the plating step.

Invention effect

According to the present invention, a high-strength cold-rolled steel sheet and a high-strength plated steel sheet having a tensile strength of the surface layer within a desired range and having good formability can be obtained. The present invention is suitable for applications such as structural parts of automobiles, and has remarkable effects of improving the weight reduction and reliability of automobile parts.

Detailed ways

Embodiments of the present invention will be described below. In addition, this invention is not limited to the following embodiment.

<High-strength cold-rolled steel sheet>

In terms of mass%, the high-strength cold-rolled steel sheet of the present invention contains C: 0.06% to 0.20%, Si: 0.01% to 2.0%, Mn: 1.8% to 5.0%, P: 0.06% or less, S: 0.005% or less, Al: 0.08% or less, N: 0.008% or less.

In addition to the above-mentioned components, the composition of the high-strength cold-rolled steel sheet of the present invention may contain Mo: 0.01% to 0.5%, Cr: 0.01% to 0.9%, and Ni: 0.01% in mass %. 1 type or 2 or more types of 0.2% or more.

In addition to the above-mentioned components, the composition of the high-strength cold-rolled steel sheet of the present invention may further contain Ti: 0.01% to 0.15%, Nb: 0.01% to 0.1%, and V: 0.01% in mass %. 1 type or 2 or more types of 0.5% or more.

In addition, the high-strength cold-rolled steel sheet of the present invention may further contain B: not less than 0.0002% and not more than 0.0030% in mass %.

The remainder other than the above is Fe and unavoidable impurities. Hereinafter, each component is demonstrated. In the following description, "%" of content of a component is "mass %".

C: 0.06% to 0.20%

C has the effect of increasing the hardness of the bainite phase, martensite phase, and tempered martensite phase responsible for strength. In order to make the tensile strength of the surface layer from the surface of the steel sheet to 50 μm in the thickness direction, that is, the surface layer, within a desired range, the C content needs to be 0.06% or more. On the other hand, C has hardenability to inhibit the formation of ferrite phase. If the C content exceeds 0.20%, the area percentage of ferrite phase will be less than 20% in the area from 1/4 to 3/4 of the plate thickness. , thus losing ductility and bendability, and becoming practically difficult. Therefore, the C content is made 0.06% or more and 0.20% or less. A preferable C content is 0.07% or more and 0.18% or less.

Si: 0.01% to 2.0%

Si is an element that contributes to high strength by solid solution strengthening. On the other hand, Si retards the diffusion of C so that the concentration gradient of C is less likely to occur. Therefore, there is an optimum value for the Si content. If the Si content exceeds 2.0%, the desired C concentration gradient cannot be obtained in the surface layer. Therefore, the upper limit of the Si content is set to 2.0%. On the other hand, about 0.01% of Si is inevitably mixed into steel, so the lower limit of the Si content is made 0.01%. A preferable Si content is 0.02% or more and 1.6% or less.

Mn: 1.8% to 5.0%

Mn is an element that contributes to high strength through solid solution strengthening and suppresses the formation of a ferrite phase. In order to bring the tensile strength of the surface layer within a desired range, the Mn content needs to be 1.8% or more. On the other hand, if the Mn content exceeds 5.0%, the area percentage of the ferrite phase is less than 20% in the region from 1/4 to 3/4 of the plate thickness, and bending workability deteriorates. Therefore, the upper limit of the Mn content is set to 5.0%. A preferable range of Mn content is 1.9% or more and 3.5% or less.

P: less than 0.06%

P is an element that segregates at grain boundaries and becomes a starting point of cracks during bending forming, and thus adversely affects formability. Therefore, it is preferable to reduce the P content as much as possible. In the present invention, in order to avoid the above problems, the P content is set to 0.06% or less. A preferable P content is 0.03% or less. Although it is preferable to reduce the P content as much as possible, 0.002% is unavoidably mixed in many cases in terms of production.

S: 0.005% or less

S exists in steel in the form of inclusions such as MnS. The above-mentioned inclusions are formed into a wedge shape by hot rolling and cold rolling. In such a form, the inclusions tend to become the origin of void formation, and the formability decreases. Therefore, in the present invention, it is preferable to reduce the S content as much as possible to 0.005% or less. The preferable S content is 0.003% or less. It is preferable to reduce the S content as much as possible, but in many cases, 0.0005% is unavoidably mixed in.

Al: less than 0.08%

When adding Al as a deoxidizer at the stage of steel production, 0.02% or more of Al is contained. On the other hand, if the Al content exceeds 0.08%, formability will deteriorate due to the influence of coarse inclusions such as alumina. Therefore, the Al content is made 0.08% or less. A preferable Al content is 0.07% or less.

N: 0.008% or less

In the present invention, N combines with Ti to precipitate as a coarse Ti-based nitride. Since this coarse Ti-based nitride adversely affects formability, it is necessary to reduce the N content as much as possible, and the upper limit is made 0.008%. A preferable N content is 0.006% or less. Although it is preferable to reduce the N content as much as possible, 0.0005% is unavoidably mixed in many cases in terms of production.

As mentioned above, in addition to the above-mentioned components, the following components may be contained.

One or more of Mo: 0.01% to 0.5%, Cr: 0.01% to 0.9%, Ni: 0.01% to 0.2%

Mo, Cr, and Ni are elements that have the effect of promoting the formation of a bainite phase, a martensite phase, and a tempered martensite phase in addition to solid solution strengthening. Therefore, the above elements substantially contribute to high strength. On the other hand, when the above-mentioned elements are excessively contained, formability deteriorates. From the above considerations, Mo: 0.01% to 0.5%; Cr: 0.01% to 0.9%; Ni: 0.01% to 0.2%.

One or more of Ti: 0.01% to 0.15%, Nb: 0.01% to 0.1%, V: 0.01% to 0.5%

Ti, Nb, and V are elements that combine with carbon to form precipitates. Strengthening by the precipitates strengthens the tensile strength of the steel sheet over the entire thickness direction. On the other hand, softening near the surface is more difficult than the bainite phase, martensite phase, and tempered martensite phase. That is, when the amount of Ti, Nb, and V exceeds the upper limit specified in the present invention, the degree of strengthening by the above-mentioned precipitates becomes excessively large. As a result, even when the differential amount of C concentration from the surface to 50 μm in the plate thickness direction is set to 0.10 mass %/mm or more as described later, the surface layer from the surface to 50 μm in the plate thickness direction The formability of the part also deteriorates, and the favorable bending workability (formability) that is characteristic of the present invention cannot be obtained. Therefore, Ti: not less than 0.01% and not more than 0.15%, Nb: not less than 0.01% and not more than 0.1%, and V: not less than 0.01% and not more than 0.5%. In this case, formability may be deteriorated due to the influence of formation of coarse carbonitrides containing Ti, Nb, and V. From this point of view, when the above-mentioned elements are contained in the above-mentioned content, the density of inclusions with a particle size of 0.2 μm or more in the region from the steel plate surface to 50 μm in the thickness direction, that is, the surface layer is set to 500/ ^mm2 or less. It should be noted that the preferred upper limit of the inclusion density is 350 inclusions/mm ² or less. On the other hand, it is preferably 50 cracks/mm ² or more from the viewpoint of promoting the formation of cracks on the shear plane during press working.

B may be contained: not less than 0.0002% and not more than 0.0030%.

B has the effect of segregating the grain boundaries of austenite before transformation, significantly delaying the nucleation of the ferrite phase, and has the effect of suppressing the formation of the ferrite phase. In order to obtain the above effects, B needs to be contained in an amount of 0.0002% or more. On the other hand, if it exceeds 0.0030%, the effect of hardenability will not only be saturated, but also will have a bad influence on ductility. From the above considerations, the B content is set to be 0.0002% or more and 0.0030% or less. A preferable B content is 0.0005% or more and 0.0020% or less.

Components other than the above are Fe and unavoidable impurities. In addition, when the said arbitrary component is contained below a lower limit value, it considers that the said element is contained as an unavoidable impurity.

Next, the steel structure of the high-strength cold-rolled steel sheet of the present invention will be described. In the steel structure of the high-strength cold-rolled steel sheet of the present invention, the area percentage of the ferrite phase in the region of 1/4 to 3/4 of the plate thickness is not less than 20% and not more than 80%, and the bainite phase, martensite The total area percentage of the tempered martensite phase and the tempered martensite phase is 20% to 80%, and the C represented by the above formula (1) at a distance of x μm (where x is 50 μm or less) from the steel plate surface in the thickness direction The trace amount of the concentration is 0.10% by mass/mm or more. In the present invention, the area percentage of the ferrite phase in the region of 1/4 to 3/4 of the plate thickness is not less than 20% and not more than 80%, and the bainite phase, martensite phase and tempered martensite phase The total of the area percentages is set to 20% or more and 80% or less. In addition, by setting the differential amount of the C concentration represented by the above formula (1) at x μm (where x is 50 μm or less) from the surface of the steel sheet in the thickness direction to 0.10 mass %/mm or more, the first pass The invention achieves the expected bending workability.

Ferrite phase in the region of 1/4 to 3/4 of the plate thickness

The tensile strength obtained by a tensile test using a JIS No. 5 tensile test piece is determined by the structure of a region of 1/4 to 3/4 with respect to the plate thickness direction. The ferrite phase is a soft structure, and if the area percentage of the ferrite phase exceeds 80%, the tensile strength is lower than 780MPa. In addition, if the above-mentioned tensile strength is lower than 780 MPa by making the area percentage of the above-mentioned ferrite phase more than 80%, the content of the ferrite phase will increase even in the surface layer, so the tensile strength of the surface layer cannot be reduced. The tensile strength is adjusted to the desired range. On the other hand, since the ferrite phase is a structure that improves the formability, if the above-mentioned area percentage is less than 20%, the formability will be significantly reduced and the ductility will be lost. From this point of view, the area percentage of the ferrite phase is set to be 20% or more and 80% or less. A preferable area percentage of the ferrite phase is 30% or more and 70% or less.

Total area percentage of bainite phase, martensite phase and tempered martensite phase in the range of 1/4 to 3/4 plate thickness

Since the bainite phase, martensite phase, and tempered martensite phase are harder than the ferrite phase, metal structures containing these phases are suitable for high strength. In order to set the tensile strength of the surface layer within a desired range, the area percentages of these metallic structures need to be 20% or more in total. On the other hand, the ductility of these phases is insufficient, and the presence of these phases generally lowers the formability. In the present invention, attention has been paid to the fact that the hardness of these metal structures largely depends on the C content, and that the formability is improved by continuously reducing the C concentration in the surface layer. However, if the total area percentage of the bainite phase, martensite phase, and tempered martensite phase exceeds 80%, the desired formability cannot be obtained even if the C concentration in the surface layer is changed. The total area percentage of the tissue is set to be 80% or less. The preferable range of the total area percentage of the bainite phase, the martensite phase, and the tempered martensite phase is 30% or more and 70% or less.

The differential amount of C concentration expressed by formula (1) at xμm from the steel plate surface in the thickness direction

As described above, the strength of the high-strength cold-rolled steel sheet of the present invention largely depends on the C concentration. Therefore, the C concentration in the vicinity of the surface, that is, the above-mentioned C concentration, has a correlation with the tensile strength of the surface layer. The combination of adjusting the concentration of C and having a tensile strength of 780 MPa or more measured using a JIS No. 5 test piece enables the tensile strength of the surface layer to exhibit a desired range. In addition, when the C concentration is partially reduced, the processability of the portion becomes good. In addition, in bending, since the stress changes continuously with respect to the thickness direction of the plate, for example, even if the hardness of the steel plate surface is only lowered, cracks due to lack of ductility and Cracks caused by poor hardness. In order to avoid the above-mentioned adverse effects, the differential amount of the C concentration represented by the following formula (1) at x μm from the surface of the steel sheet in the thickness direction (where x is 50 μm or less) may be 0.10% by mass/mm or more. On the other hand, when the amount of the above-mentioned differential exceeds 6.5% by mass/mm, the difference in hardness with respect to the plate thickness direction becomes large, and there is a possibility of causing cracks. Therefore, it is preferable to set the differential amount of the C concentration to 6.5% by mass/mm or less.

In addition, since the above-mentioned differential amount becomes smaller as it goes deeper in the plate thickness direction, when x=50 μm, if it is 0.10 mass %/mm or more, the above-mentioned differential amount can be said to be within the scope of the present invention.

In addition, the above-mentioned differential amount at x<20 μm is the region where the strain caused by bending is the largest, so the differential amount is preferably large, and the above-mentioned differential amount at x=20 μm is preferably 3.5 to 6.5% by mass/mm. When it is less than 3.5% by mass/mm, the softening of the surface layer portion is insufficient, and cracks extending from the surface layer in the plate thickness direction tend to occur during molding. On the other hand, when it exceeds 6.5% by mass/mm, the deformability gradient of the material becomes too large, and the deformation with respect to the sheet thickness direction becomes uneven, and tends to occur in a direction parallel to the sheet surface direction during molding. Extended cracks. It should be noted that, in fact, although x is 0 μm or more and less than 20 μm is the region with the largest strain, but the measurement of the C concentration in the region of 0 μm or more and less than 10 μm has a large error due to the influence of pollution, so in the present invention Among them, it is preferable to specify the differential amount of the C concentration in the region where x is 20 μm or more.

x = 30 μm to 50 μm. As x increases, the differential amount of C concentration decreases stepwise, so that strain during bending can be appropriately absorbed in the sheet thickness direction, and as a result, cracks during bending can be suppressed. That is, it is preferable that the above-mentioned fraction amount at x=30 μm is 2.0 to 3.5 mass %/mm. It is preferable that the above-mentioned fractional amount at x=40 μm is 0.5 to 2.0 mass %/mm. It is preferable that the above-mentioned fraction amount of x=50 micrometers is 0.12-0.5 mass %/mm.

<High-strength plated steel sheet>

The high-strength plated steel sheet of the present invention is composed of the above-mentioned high-strength cold-rolled steel sheet and a plated layer formed thereon.

Plating will be described. In the high-strength plated steel sheet of the present invention, the components constituting the plated layer are not particularly limited, and may be general components. For example, in terms of mass %, the plating layer contains Fe: 5.0-20.0%, Al: 0.001%-1.0%, and further contains 0-3.5% of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr , Co, Ca, Cu, Li, Ti, Be, Bi, REM, or one or more of them, and the remainder consists of Zn and unavoidable impurities. In addition, the coating can also be a hot-dipped coating or an alloyed coating.

<Manufacturing method of high-strength cold-rolled steel sheet>

Next, a method for producing the high-strength cold-rolled steel sheet of the present invention will be described. The method for producing a high-strength cold-rolled steel sheet of the present invention includes a hot-rolling step, a cold-rolling step, and an annealing step. Hereinafter, each step will be described. In addition, in the following description, unless otherwise specified, the temperature is a surface temperature. In addition, the average cooling rate is ((surface temperature after cooling−surface temperature before cooling)/cooling time).

The so-called hot rolling process is a process of heating the steel raw material having the above-mentioned composition to 1150°C or higher, performing rough rolling, finishing rolling at a finishing temperature of 800°C or higher, and coiling at 350°C or higher and 720°C or lower. temperature for coiling.

The smelting method used for the production of the steel raw material is not particularly limited, and known smelting methods such as a converter and an electric furnace can be used. In addition, secondary refining can also be performed in a vacuum degassing furnace. Thereafter, from the viewpoint of productivity and quality, it is preferable to produce a slab (steel raw material) by a continuous casting method. In addition, the slab can be produced by a known casting method such as an ingot-bloom rolling method and a thin slab continuous casting method.

The raw steel material obtained in the above manner was heated under the following conditions.

Heating temperature of steel raw material: above 1150°C

In the present invention, it is necessary to heat the steel raw material before rough rolling so that the steel structure of the steel raw material becomes a substantially homogeneous austenite phase. In addition, in order to suppress the formation of coarse inclusions, it is important to control the heating temperature. If the heating temperature is lower than 1150°C, hot rolling cannot be completed at a finish rolling temperature of 800°C or higher. On the other hand, if the heating temperature exceeds 1400°C, scale will be formed excessively and the yield will decrease, so the heating temperature is preferably 1400°C or lower.

In order to stably impart a C concentration gradient to the surface layer, it is preferable to control the heating atmosphere, temperature, and residence time of the steel raw material. _{Stable _ _ _ _} A concentration gradient of C is given to the surface layer. Here, pCO, pCO ₂ , pCH ₄ , pH ₂ O, and pO ₂ are partial pressures (Pa) of CO, CO ₂ , CH ₄ , H ₂ O, and O ₂ , respectively.

Rough rolling conditions for the above-mentioned rough rolling after heating are not particularly limited.

Finishing temperature: above 800°C

If the finish rolling temperature is lower than 800° C., ferrite transformation starts during finish rolling to form a structure in which ferrite grains are stretched, and a mixed crystal structure in which ferrite grains are partially grown. Therefore, a finish rolling temperature of less than 800° C. adversely affects the thickness accuracy during cold rolling. Therefore, the finish rolling temperature is set to 800° C. or higher. The preferred finish rolling temperature is 820°C or higher. In addition, the finish rolling temperature is preferably 940° C. or lower for the reason of deterioration of the surface properties due to scale biting. After finish rolling, cooling is preferably performed under the condition that the average cooling rate from the finish rolling temperature to 560°C is -30°C/s or less. In the case where the average cooling rate exceeds -30°C/s, it is difficult to perform winding at a low temperature due to the limitation of the length of the runout table. The cooling stop temperature in the cooling after the above finish rolling may or may not be the same as the coiling temperature. In case of inconsistency, further, cooling or heating to winding temperature is required.

Winding temperature: above 350°C and below 720°C

In the case of setting the winding temperature to a temperature lower than 350° C., it is difficult due to the limitation of the output roller table length. On the other hand, if the coiling temperature is lower than 350°C, the shape of the sheet will deteriorate and cold rolling will become difficult. On the other hand, if the winding temperature exceeds 720° C., the winding machine will be damaged due to the influence of the high temperature, which will adversely affect the equipment. From the above considerations, the winding temperature is set to be 350°C or higher and 720°C or lower. Preferably, it is 450°C or more and 680°C or less.

The subsequent cold rolling step is a step of cold rolling the hot-rolled steel sheet after the hot rolling step. In order to obtain a desired plate thickness, it is necessary to cold-roll the hot-rolled steel sheet after the hot-rolling step. There is no limitation on the cold rolling rate, but it is preferable to set the cold rolling rate to 30% or more and 80% or less in view of the limitation of the production line.

The subsequent annealing process is carried out after the cold rolling process, in the continuous annealing production line or continuous coating production line, using a burner, and the air ratio is 1.05 to 1.30 in the temperature range of 580～T℃ (580<T≤730). Conditions, under the condition that the dew point in the temperature range above 730°C is -40 to -15°C, the cold-rolled steel sheet is heated to the maximum temperature above 730°C, and then the average cooling rate between 700°C and 550°C is -10°C. The process of cooling to a cooling stop temperature of 25 to 530°C under the condition of °C/s or less, followed by heating as necessary, and holding in a temperature range of 200°C to 530°C.

Air ratio at 580～T℃: 1.05～1.30

When the air ratio is 1.05 or more, it becomes an oxidizing atmosphere, and a C concentration gradient on the steel sheet surface is generated by the reaction of C on the steel sheet surface with oxygen. The reaction speed at this time varies with the temperature. In the continuous annealing production line or continuous coating production line, in order to obtain a sufficient reaction speed, it is necessary to use a burner with an air ratio of 1.05 or more and use a burner to heat at a temperature range of 580-T°C. On the other hand, if the air ratio exceeds 1.30, the bad influence of the formability reduction due to grain boundary cracking due to grain boundary oxidation becomes significant, so the air ratio of the burner in the above temperature range is set to 1.30 or less. The preferable range of the said air ratio is 1.07-1.26.

Since the above adverse effects may occur even when heated at a high temperature, T°C is made 730°C or lower. In addition, as long as the air ratio at the time of heating in the temperature range of 580 to T°C by the burner is within the above range, T is close to 580°C (when the air ratio is controlled to be narrow in the range of 1.05 to 1.30). It is within the scope of the present invention, but T°C is preferably 600 to 700°C in order to sufficiently obtain the effect of controlling the air ratio. It should be noted that the annealing furnace at this time can consider the oxidation furnace in the direct heating furnace or the oxidation furnace in the oxygen-free furnace, as long as the heating method using the direct flame burner is sufficient, it can Be applicable.

Dew point in the temperature range above 730°C: -40 to -15°C

The reaction of water vapor in the furnace with C in the surface layer of the steel sheet also produces a gradient in the C concentration in the surface layer of the steel sheet. In order to exhibit this effect, the dew point in the temperature region of 730°C or higher during heating is set to -40 to -15°C. If the dew point in the above temperature range exceeds -15°C, the bad influence of the reduction in formability due to grain boundary cracking due to the influence of grain boundary oxidation becomes significant. On the other hand, when the dew point is lower than -40°C in the above temperature range, water vapor in the furnace does not react with C on the surface layer of the steel sheet, and a desired C concentration gradient cannot be obtained. The preferred range of dew point is -35 to -20°C. In addition, in order to prevent water vapor from reacting insufficiently with C in the surface layer of the steel sheet, it is necessary to control the dew point at least in a temperature range of 730° C. or higher. In addition, the dew point was made into the said range up to the highest attained temperature.

Maximum reaching temperature: above 730°C

As mentioned above, if the maximum attained temperature is less than 730° C., the C concentration gradient may not be properly formed, and a desired steel structure may not be obtained. From the viewpoint of formation of a C concentration gradient, the highest attained temperature is preferably 750° C. or higher. It should be noted that the upper limit of the maximum attained temperature is not particularly limited, but if the maximum attained temperature is too high, heat-induced damage will be brought to the equipment of the annealing furnace, or the dye consumption rate will be reduced. The upper limit of the highest attained temperature is preferably 880°C or lower.

Average cooling rate from 700°C to 550°C: below -10°C/s

700°C to 550°C is the temperature range for ferrite transformation, and if the average cooling rate in this temperature range exceeds -10°C/s, the area percentage of ferrite phase will exceed 80%, and the desired steel plate strength. Therefore, the average cooling rate from 700°C to 550°C is set to -10°C/s or less. Preferably, the average cooling rate in the temperature range from 700°C to 550°C is -20°C/s or less.

Cooling stop temperature: 25～530℃

In order to sufficiently obtain the bainite phase and the martensite phase, it is necessary to cool to 530° C. or lower. The cooling method may be any one of jet cooling, water cooling, and the like as long as the above-mentioned conditions of the average cooling rate are satisfied. In order to stably set the total area percentage of the bainite phase, the martensite phase, and the tempered martensite phase at 20% or more, it is preferable to set the cooling stop temperature at 520° C. or less. In the present invention, since cooling to a temperature lower than room temperature is unnecessary, the lower limit of the cooling stop temperature is set to 25° C. corresponding to room temperature.

Holding temperature: 200°C to 530°C

The martensite phase obtained in the previous step is tempered or maintained at 200° C. to 530° C. to promote bainite transformation. When one of the above-mentioned cooling stop temperatures is lower than the holding temperature, heating is required, and the heating method can use IH heating, a gas heating device, or the like. Below 200°C, the martensite cannot be sufficiently tempered, and the ductility of the steel sheet will be lost. On the other hand, when the temperature exceeds 530° C., ferrite transformation occurs and desired steel sheet strength cannot be obtained. The range of preferable holding temperature is 250 degreeC or more and 520 degreeC or less. In addition, the above-mentioned holding is not limited to constant-temperature holding as long as the temperature of the steel sheet is in the above-mentioned temperature range. In addition, the retention time is not particularly limited, but from the viewpoint of the above purpose, the retention time is preferably 20 to 1200 seconds.

<Manufacturing method of high-strength plated steel sheet>

The next plating step is a step of performing plating after the above-mentioned annealing step to form a plating layer on the annealed sheet. For example, as a coating treatment, in the case of performing hot-dip coating, which is often used for steel sheets for automobiles, the above-mentioned annealing is performed in a continuous hot-dip coating production line, and the cooling after annealing is followed by immersion in a hot-dip coating bath, thereby It is sufficient to form a plating layer on the surface. In addition, after the above-mentioned plating step, an alloying step of performing an alloying treatment may be provided as necessary. In addition, the type of plating is preferably galvanized.

Example 1

A steel material having a composition shown in Table 1 and a thickness of 250 mm was subjected to a hot rolling process under the conditions shown in Table 2 to produce a hot-rolled steel sheet, and a cold rolling process was performed under the cold rolling conditions shown in Table 2. Thus, a cold-rolled steel sheet having a thickness of 1.0 mm to 2.0 mm is produced. Then, under the conditions shown in Table 2, an annealing step was implemented in a continuous annealing line or a continuous hot-dipping line, respectively. Thereafter, a plating treatment and, if necessary, an alloying treatment are performed. Here, the temperature of the plating bath (plating composition: Zn-0.13 mass% Al) dipped in the continuous hot-dipping production line is 460°C, and the amount of plating is 460°C. Hot-dip-coated steel sheets) are 45 to 65 g/m ² per one side, and the amount of Fe contained in the plating layer is in the range of 6 to 14% by mass.

Test pieces were collected from the cold-rolled steel sheets, hot-dip-coated steel sheets, or alloyed hot-dip-coated steel sheets obtained above, and evaluated by the following method.

(i) Tissue observation image

The area percentage of each phase was evaluated by the following method. Cut out a plate thickness section parallel to the rolling direction from the steel plate in such a way that it becomes the observation surface, etch the observation surface with 1% nital to expose it, and use a scanning electron microscope to magnify 2000 times to measure the thickness of the plate with 10 fields of view. /4 to 3/4 of the area to take pictures. The ferrite phase is a structure in which corrosion marks and cementite are not observed in crystal grains, and the bainite phase is a structure in which corrosion marks and large carbides are recognized in crystal grains. The martensite phase is a structure in which carbides are not recognized in the crystal grains and are observed with white contrast. The tempered martensite is a structure in which corrosion marks are recognized in the crystal grains and fine carbides are recognized in the laths. These were separated into a ferrite phase, a bainite phase, a martensite phase, and a tempered martensite phase by image analysis, and the area percentage was calculated|required with respect to the observation field of view. The results are shown in Table 3.

(ii) Determination of inclusion density

The inclusion density measurement was evaluated by the following method. Cut out a plate thickness section parallel to the rolling direction from the steel plate so as to become an observation surface, and use a scanning electron microscope to magnify 2000 times to observe a region of 1 ^mm2 from the surface layer of the steel plate to 50 μm in the direction of the plate thickness. For 0.5 μm The number of the above inclusions is counted, and the inclusion density is determined. The results are shown in Table 3.

(iii) X-ray measurement

In the plate thickness section parallel to the rolling direction, the area of 1/4 to 3/4 of the plate thickness in the plate thickness direction from the surface of the steel plate is ground, and the chemical polishing of 200 μm or more is performed, and the X-ray diffraction of the ground surface is used. Strength quantifies the amount of retained austenite. The incident ray source was measured from the peaks of (200) _α , (211) _α , (200) _γ , (220) _γ , and (311) _γ , using MoKα ray, and the volume percentage was obtained. It should be noted that, in the present invention, the obtained volume percentage is regarded as the area percentage.

(iv) Measurement of C concentration in the surface layer of the steel sheet

The surface of the steel plate is ground successively in the thickness direction, and the exposed surface is subjected to spark discharge emission spectroscopic analysis to obtain the C concentration on the surface. By thus performing a plurality of measurements according to the amount of polishing, it is possible to measure the C concentration distribution in the plate thickness direction.

First, in order to remove error causes such as contamination on the surface of the steel sheet, 10 μm grinding is performed. Next, the measurement was performed 5 times at a depth of 10 μm to 50 μm at intervals of 10 μm, and the differential amount of C concentration (d[%C _x ]/dx) was derived. Here, [%C _x ] is the C concentration at a position at a depth (x μm) from the surface in the sheet thickness direction. Here, (d[%C ₂₀ ]/dx), (d[%C ₃₀ ]/dx), (d[%C ₄₀ ]/dx), (d[%C ₅₀ ]/dx) are obtained. The results are shown in Table 3.

It should be noted that, when an approximate expression can be derived with an error range of ±1% or less relative to each measurement point, the above-mentioned C concentration from the surface of the steel plate to a depth of 50 μm can be obtained by differentiating the approximate expression. of the differential amount.

In order to discharge C from the surface layer of the steel sheet, the value of (d[%C ₂₀ ]/dx) is the largest and the value of (d[%C ₅₀ ]/dx) is the smallest in many cases.

(v) Tensile test

A JIS No. 5 tensile test piece (which is the original thickness, and the test piece includes the surface layer) is produced from the obtained steel plate in a direction perpendicular to the rolling direction, and the tensile test according to JISZ2241 (2011) is carried out for 5 Then, the average yield strength (YS), tensile strength (TS), and total elongation (E1) were obtained. The crosshead speed for the tensile test was 10 mm/min. When the yield strength is continuous yield, read the value of 0.2% yield point, and when it is discontinuous yield, read the lower yield point. In Table 3, tensile strength: 780 MPa or more and total elongation: 8% or more are properties of steel sheets sought in the present invention. Here, the reason why the total elongation is set at 8% or more is that when it is less than 8%, the formability deteriorates, and problems such as cracking due to insufficient ductility of the steel sheet during press working, etc., are not practical. . In addition, the yield strength is preferably 490 MPa or more.

(vi) Bending test (formability evaluation)

The No. 3 test piece described in JISZ2248 was taken so that the direction perpendicular to the thickness direction of the steel plate was the longitudinal direction of the test piece, and a bending test was performed by the V-block method. The critical bending radius (R/t) was obtained by dividing the radius of the front end of the press fitting when a crack was recognized on the bending ridgeline by the plate thickness. Actually, the bendability obtained varies with the strength. Therefore, when (R/t)/TS is 2.0×10 ^-3 or less, it is evaluated as "○" as a very good result, and when it is more than 2.0×10 ^-3 and 2.5×10 ^-3 or less, it is evaluated as a good result is "Δ", and these are taken as the range sought by the present invention. When (R/t)/TS exceeds 2.5×10 ^-3 , the evaluation is "×", and it is out of the scope of the present invention.

Table 3 shows the results obtained above.

In the examples of the present invention, it can be seen that all have a tensile strength TS: 780 MPa or more and good moldability was obtained. On the other hand, in Comparative Examples outside the range of the present invention, when the metal structure was outside the range of the present invention, the tensile strength was lower than 780 MPa, or the desired formability could not be obtained. In particular, when the C concentration gradient in the surface layer portion is out of the range of the present invention, the moldability is low as a result.

Claims (12)

1. A high-strength cold-rolled steel plate has the following composition and steel structure, The composition contains C: 0.06% to 0.20%, Si: 0.01% to 2.0%, Mn: 1.8% to 5.0%, P: 0.06% or less, and S: 0.005% or less in mass %. , Al: 0.08% or less, N: 0.008% or less, the rest is composed of Fe and unavoidable impurities; In the steel structure, the area percentage of the ferrite phase in the region from 1/4 to 3/4 of the plate thickness is not less than 20% and not more than 80%, and the bainite phase, martensite phase and tempered martensite phase The sum of the area percentages of bulk phases is not less than 20% and not more than 80%, The differential amount of the C concentration represented by the following formula (1) at x μm from the surface of the steel plate in the thickness direction is 0.10% by mass/mm or more; When using JIS No. 5 tensile test piece to carry out the tensile test, the tensile strength of the high-strength cold-rolled steel plate is more than 780MPa, d[% _Cx ]/dx=([% _Cx ]-[% _Cx-10μm ])/0.01 (1) [%C _x ] in formula (1) represents the C concentration at x, where x represents the distance in the thickness direction from the surface of the steel sheet, and its value is 50 μm or less. 2. The high-strength cold-rolled steel sheet according to claim 1, wherein the composition further includes Mo: 0.01% to 0.5%, Cr: 0.01% to 0.9%, and Ni: 0.01% in mass % 1 type or 2 or more types of 0.2% or more. 3. The high-strength cold-rolled steel sheet according to claim 1 or 2, wherein the composition further includes Ti: 0.01% to 0.15%, Nb: 0.01% to 0.1%, V: 0.01% to 0.5% of one or more types, In the steel structure, the density of inclusions having a grain size of 0.2 μm or more in the region from the surface of the steel plate to 50 μm in the thickness direction is 500 inclusions/mm ² or less. 4. The high-strength cold-rolled steel sheet according to claim 1 or 2, wherein the component composition further contains B: not less than 0.0002% and not more than 0.0030% in mass %. 5 . The high-strength cold-rolled steel sheet according to claim 3 , wherein the component composition further contains B: not less than 0.0002% and not more than 0.0030% by mass %. 6. A high-strength plated steel sheet comprising the high-strength cold-rolled steel sheet according to any one of claims 1 to 5, and a plating layer formed on the high-strength cold-rolled steel sheet. 7. The high-strength plated steel sheet according to claim 6, said coating is a hot-dip coating or an alloyed hot-dip coating. 8. The high-strength plated steel sheet according to claim 6 or 7, wherein the coating layer contains Fe: 5.0-20.0%, Al: 0.001%-1.0%, and further contains 0-3.5% of selected from One or two or more of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM, and the remainder consists of Zn and unavoidable impurities. 9. A method for manufacturing a high-strength cold-rolled steel plate, comprising: Hot rolling process, heating the steel raw material having the composition according to any one of claims 1 to 5 to 1150°C or higher, performing rough rolling, finishing rolling at a finishing temperature of 800°C or higher, at 350°C or higher And coiling at a coiling temperature below 720°C; A cold-rolling process, after the hot-rolling process, cold-rolling the hot-rolled steel sheet; and In the annealing process, after the cold rolling process, in a continuous annealing line or a continuous coating line, using a burner, the air ratio is 1.05 to 1.30 in the temperature range of 580°C or higher and T°C or lower, 730°C or higher Under the condition that the dew point in the temperature range is -40 to -15°C, the cold-rolled steel plate is heated to the highest temperature above 730°C, and then the average cooling rate from 700°C to 550°C is below -10°C/s It is cooled to a cooling stop temperature of 25 to 530°C under conditions, and then heated as necessary to maintain in a temperature range of 200°C to 530°C, where 580<T≦730. 10. The manufacturing method of high-strength cold-rolled steel sheet according to claim 9, in the hot rolling process, the atmosphere when heating the steel raw material satisfies the formula (2), and the steel raw material is at a temperature above 1150°C Stay in the temperature zone for more than 30 minutes, (pCO+pCO ₂ +pCH ₄ )/(pCO+2pCO ₂ +pH ₂ O+2pO ₂ )≤1 (2) pCO, pCO ₂ , pCH ₄ , pH ₂ O, and pO ₂ in formula (2) are the partial pressures of CO, CO ₂ , CH ₄ , H ₂ O, and O ₂ respectively, and the unit is Pa. 11. A method of manufacturing a high-strength plated steel sheet, comprising a plating step of plating the high-strength cold-rolled steel sheet produced by the manufacturing method according to claim 9 or 10. 12. The method of manufacturing a high-strength plated steel sheet according to claim 11, comprising an alloying step of performing an alloying treatment after the plating step.