KR20170102297A - High-strength cold-rolled steel sheet, high-strength plated steel sheet, and method for manufacture thereof - Google Patents

Abstract

A high strength cold rolled steel sheet having a tensile strength at the surface layer of 780 MPa or more and excellent moldability, and a method for producing the same. A specific component composition and an area ratio of the ferrite phase in the region from 1/4 to 3/4 of the plate thickness is 20% or more and 80% or less, and the sum of area ratios of the bainite phase, the martensite phase and the tempering martensite is , A steel structure having a C content concentration of not less than 20% and not more than 80% and having a C content of 0.10% by mass / mm in the thickness direction x from the surface of the steel sheet and represented by the following formula (1), and using a JIS No. 5 tensile test specimen Strength steel sheet having a tensile strength of 780 MPa or more when subjected to a tensile test.
d [% C _x ] / dx = ([% C _x ] - [% C _{x -10 탆} ]) /
[% C _x ] in the formula (1) is the C concentration in x, and x is 50 탆 or less.

Description

TECHNICAL FIELD [0001] The present invention relates to a high strength cold rolled steel sheet, a high strength cold rolled steel sheet, a high strength coated steel sheet,

The present invention relates to a high-strength cold-rolled steel sheet, a high-strength coated steel sheet, and a method for producing the same, which have high tensile strength (TS) of 780 MPa or more and excellent moldability, which are useful as materials for skeleton members for automobiles.

Recently, from the viewpoint of global environmental preservation, improvement of fuel efficiency of automobiles aiming at reduction of CO ₂ emissions is aimed at throughout the automobile industry. In order to improve the mileage of automobiles, weight reduction of automobiles by thinning of parts used is most effective. There are high-strength steel sheets as materials for automotive parts that can be thinned. Therefore, in recent years, the amount of high-strength steel sheet used as a material for automobile parts has been increasing.

On the other hand, in general, the steel sheet is deteriorated in moldability accompanied with high strength. When the formability is lowered, it is difficult to process the steel sheet. BACKGROUND OF THE INVENTION [0002] Automotive parts often have complicated shapes. Therefore, in lightening the weight of automobile parts and the like, there is a demand for a steel plate having good workability in addition to high strength.

From the above, development of a steel sheet having both high strength and bendability (also referred to as workability and moldability) is required. To date, various techniques have been proposed for a high-strength cold-rolled steel sheet and a hot-dip galvanized steel sheet which are notable for workability.

For example, Patent Document 1 discloses a hot-dip galvanized steel sheet having a hot-dip galvanized steel sheet on the surface of a steel sheet. The hot-dip galvanized steel sheet contains, by mass%, C: more than 0.02% 0.003 to 0.10%, S: 0.020% or less, Al: 0.001 to 1.0%, N: 0.0004 to 0.015% and Ti: 0.03 to 0.2%, the balance being Fe and inevitable impurities And the ferrite is contained in an area ratio of 30 to 95% and the remainder is composed of at least one of martensite, bainite, pearlite, cementite and retained austenite, and further contains martensite Wherein the steel sheet has a Ti-based carbonitride precipitate having an average particle-to-particle distance of 30 to 300 nm and an area ratio of martensite of 0 to 50% lt; RTI ID = 0.0 > TiN < / RTI > By satisfying these configurations, Patent Document 1 discloses that a high-yield-ratio high strength steel sheet excellent in bendability and notch fatigue resistance having a tensile strength of 620 MPa or more is obtained .

In Patent Document 2, it is preferable that the content of C is 0.05 to 0.20%, the content of Si is less than 0.01 to 0.6%, the content of Mn is 1.6 to 3.5%, the content of P is 0.05% or less, the content of S is 0.01% Al: not more than 1.5%, N: not more than 0.01%, and the balance of iron and inevitable impurities, wherein the steel has a polygonal ferrite structure and a low temperature transformation forming structure, the low temperature transformation forming structure contains at least bainite , And martensite may be further included. In Patent Document 2, the plate surface at a depth of 0.1 mm from the surface of the steel sheet is changed in the plate width direction, and a total of 20 fields of view are observed with a microscope, and image analysis is performed for a region of 50 탆 50 탆 in each field of view The maximum value and the minimum value of the area ratio of the polygonal ferrite and the maximum value of the area ratio of the martensite are determined. By satisfying these constitutions, Patent Document 2 discloses a hot-dip galvanized steel sheet having a tensile strength of 780 MPa or more and excellent bending workability and fatigue strength.

Japanese Laid-Open Patent Publication No. 2006-063360 Japanese Laid-Open Patent Publication No. 2010-209428

In the technique proposed in Patent Document 1, nothing is disclosed in the Examples on the influence of the composition of the components on the steel structure. For this reason, in Patent Document 1, it can not be said that improvement in consideration of the steel structure is sufficiently performed. The formability of the steel sheet described in Patent Document 1 does not satisfy the presently required formability.

Further, the technique proposed in Patent Document 2 does not consider the inclusion density or the morphology of the martensite phase. For this reason, in the technique described in Patent Document 2, the formability is insufficient.

The tensile strength of the surface layer, which is a region from the surface of the steel sheet to the thickness of 50 mu m, is important as the strength.

The present invention has been made in view of such circumstances, and it is an object of the present invention to provide a steel sheet obtained by adjusting the C concentration in the vicinity of the surface in order to adjust the tensile strength of the surface layer to a desired range, Or more) and high moldability, and a process for producing the same.

The inventors of the present invention paid attention to the stress gradient in the plate thickness direction at the time of bending in studying the constituent requirements of a steel sheet having a desired range of tensile strength at the surface layer portion and having good moldability. In the bending process, the stress in the vicinity of the central portion in the plate thickness direction is made endlessly small, but the stress in the surface layer portion, which is a region of 50 mu m in the plate thickness direction from the surface of the steel sheet, becomes remarkably large. The stress continuously changes in the thickness direction. As a result of observing cracks (cracks) in the bent portion, it was found that the void density at the surface layer portion was much larger than the void density at a position deeper than 50 占 퐉 in the plate thickness direction.

In addition, as described above, the stress continuously decreases from the surface of the steel sheet to the center of the thickness of the steel sheet. Therefore, unless the hardness of the surface layer portion, which is an area up to 50 m in the plate thickness direction, from the surface of the steel sheet as a crack generation portion is continuously changed, a stress concentrated portion is generated and becomes a cause of cracking.

Using the following recognition obtained from the above, a steel sheet having high strength and good moldability was completed.

(1) Production of a bainite phase, a martensite phase and a tempering martensite phase in an appropriate range contributes to the tensile strength of the surface layer being in a desired range. The hardness of the bainite phase, the martensite phase and the tempering martensite phase is determined by the content of C, Si, Mn and the like as described later, but the effect of C amount is greatest.

(2) In order to continuously change the hardness in the surface layer portion in the thickness direction, it can be achieved by continuously changing the C concentration in the surface layer portion. Concretely, in order to obtain good formability, desired formability is obtained when the differential quantity of C concentration in the surface layer portion is 0.10 mass% / mm or more.

(3) To continuously change the C concentration, it is possible to achieve by controlling the atmosphere in the furnace, the dew point and the heating temperature in the continuous annealing line or the continuous plating line.

The present invention has been completed on the basis of the above recognition, and its gist is as follows.

The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains at least 0.06% of C, 0.20% or less of Si, 0.01 to 2.0% of Si, 1.8% By mass or less, N: 0.008% or less, the balance being Fe and inevitable impurities, and the area ratio of the ferrite phase in the region from 1/4 to 3/4 of the sheet thickness is 20% or more and 80% , The bainite phase, the martensite phase, and the tempering martensite phase area ratio is 20% or more and 80% or less, and the amount of the C concentration represented by the following formula (1) at x m in the plate thickness direction from the surface of the steel sheet Having a steel structure of not less than 0.10 mass% / mm and having a tensile strength of 780 MPa or more when subjected to a tensile test using a JIS No. 5 tensile test specimen.

d [% C _x ] / dx = ([% C _x ] - [% C _{x -10 탆} ]) /

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

[2] The steel sheet according to any one of [1] to [3], wherein the composition further comprises one or more of Mo: 0.01 to 0.5%, Cr: 0.01 to 0.9% The high strength cold rolled steel sheet according to [1].

[3] The steel sheet according to any one of [1] to [3], wherein the composition further comprises one or more of Ti: 0.01 to 0.15%, Nb: 0.01 to 0.1%, V: 0.01 to 0.5% The high strength cold rolled steel sheet according to [1] or [2], wherein the inclusion density of the steel structure in a region from the surface of the steel sheet to the thickness direction of 50 mu m in the thickness direction is 0.2 mu m or more.

[4] The high strength cold rolled steel sheet according to any one of [1] to [3], wherein the composition further contains 0.0002% or more and 0.0030% or less of B by mass%.

[5] A high strength coated steel sheet having a 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 coated steel sheet according to [5], wherein the plating layer is a hot-dip coating layer or an alloyed hot-dip coating layer.

The plating layer contains, in mass%, Fe: 5.0 to 20.0% and Al: 0.001 to 1.0%, and further contains Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, The method according to [5] or [6], wherein the total amount of one or more elements selected from Ca, Cu, Li, Ti, Be, Bi and REM is 0 to 3.5% in total and the remainder is Zn and inevitable impurities High strength galvanized steel sheet.

[8] A steel material having the composition described in any one of [1] to [4], which is heated to a temperature of not lower than 1150 캜, subjected to rough rolling, finish rolling with a finish rolling finish temperature of 800 캜 or higher A cold rolling step of cold rolling the hot-rolled steel sheet after the hot-rolling step; and a cold rolling step of forming a continuous annealing line or a continuous (580 < T &amp;le; 730) having an air ratio of 1.05 to 1.30 and a dew point in a temperature range of 730 DEG C or higher of -40 to -15 ° C, the cold-rolled steel sheet is heated to the maximum attained temperature of 730 ° C or higher, and then cooled to a cooling-stop temperature of 25 to 550 ° C under the condition that the average cooling rate from 700 ° C to 550 ° C is -10 ° C / s or lower , Then as needed Column and method of manufacturing a high strength cold rolled steel sheet having an anneal step of maintaining (holding) in the temperature range of 200 ℃ to 530 ℃.

[9] The method for producing a steel material according to [8], wherein in the hot rolling step, the atmosphere satisfies the formula (2) when the steel material is heated and the time for which the steel material stays at a temperature range of 1150 ° C or more is 30 minutes or more A method for producing a high strength cold rolled steel sheet.

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

Expression pCO, pCO ₂ in the _{(2), pCH 4, pH} 2 O and pO ₂ refers to the partial pressure (Pa), _{respectively, CO, CO 2, CH 4} , H 2 O and O _2.

[10] A method for manufacturing a high strength coated steel sheet having a plating process for plating a high-strength cold-rolled steel sheet produced by the manufacturing method according to [8] or [9].

[11] The method for producing a high strength plated steel sheet according to [10], which further comprises an alloying step of performing an alloying treatment after the plating step.

According to the present invention, it is possible to obtain a high strength cold rolled steel sheet and a high strength coated steel sheet having tensile strength in the surface layer in a desired range and having good formability. INDUSTRIAL APPLICABILITY The present invention is suitable for use as a structural member of an automobile and the like, and the effect is remarkable by reducing the weight of automobile parts and improving the reliability thereof.

(Mode for carrying out the invention)

Hereinafter, an embodiment of the present invention will be described. The present invention is not limited to the following embodiments.

<High strength cold rolled steel plate>

A high strength cold rolled steel sheet according to the present invention is characterized by containing 0.06 to 0.20% of C, 0.01 to 2.0% of Si, 1.8 to 5.0% of Mn, 0.06% or less of P and 0.005% or less of S , Al: 0.08% or less, and N: 0.008% or less.

The composition of the high-strength cold-rolled steel sheet of the present invention may further contain, by mass%, Mo: 0.01 to 0.5%, Cr: 0.01 to 0.9%, Ni: 0.01 to 0.2% One or more of them may be contained.

The composition of the high-strength cold-rolled steel sheet of the present invention may further contain not less than 0.01% and not more than 0.15% of Ti, not less than 0.01% and not more than 0.1% of Nb, and not less than 0.01% and not more than 0.5% of V, One or more of them may be contained.

The high-strength cold-rolled steel sheet of the present invention may further contain, by mass%, B: 0.0002% or more and 0.0030% or less.

The remainder other than the above are Fe and inevitable impurities. Each component will be described below. In the following description, "%" of the content of the component means "% by mass".

C: not less than 0.06% and not more than 0.20%

C has the effect of increasing the hardness on the bainite phase, martensite phase and tempering martensite, which are responsible for the strength. It is necessary to set the C content to 0.06% or more so that the tensile strength of the surface layer portion in the region from the surface of the steel sheet in the plate thickness direction to 50 m is in a desired range. On the other hand, C has a property of suppressing ferrite phase formation. When the C content exceeds 0.20%, the area ratio of the ferrite phase is lower than 20% in the region from 1/4 to 3/4 of the plate thickness And the ductility and bendability are lost, making practical use difficult. Therefore, the C content is 0.06% or more and 0.20% or less. The preferable C content is 0.07% or more and 0.18% or less.

Si: not less than 0.01% and not more than 2.0%

Si is an element contributing to the enhancement of strength by solid solution strengthening. On the other hand, Si slows the diffusion of C and makes it difficult to impart a concentration gradient of C to it. Therefore, there is an optimum value for the Si content. If the Si content exceeds 2.0%, a desired C concentration gradient can not be obtained in the surface layer portion. Therefore, the upper limit of the Si content was set to 2.0%. On the other hand, since about 0.01% of Si is inevitably incorporated into the steel, the lower limit of the Si content is set to 0.01%. The Si content is preferably 0.02% or more and 1.6% or less.

Mn: 1.8% or more and 5.0% or less

Mn contributes to enhancement of strength by solid solution strengthening and is an element which inhibits ferrite phase formation. In order to set the tensile strength of the surface layer to a desired range, it is necessary to set the Mn content to 1.8% or more. On the other hand, if the Mn content exceeds 5.0%, the area ratio of the ferrite phase is less than 20% in the region of 1/4 to 3/4 of the plate thickness, and the bending workability deteriorates. Therefore, the upper limit of the Mn content was set to 5.0%. The preferable range of the Mn content is 1.9% or more and 3.5% or less.

P: not more than 0.06%

P is an element which is segregated at the grain boundaries and serves as a starting point of cracking at the time of bending, thereby causing an adverse effect on moldability. Therefore, it is preferable that the P content is reduced as much as possible. In the present invention, the P content is set to 0.06% or less in order to avoid the above problem. The preferable P content is 0.03% or less. The P content is preferably reduced as much as possible, but 0.002% is inevitably incorporated in the production.

S: not more than 0.005%

S exists as an inclusion such as MnS in the steel. This inclusion is formed into a wedge shape by hot rolling and cold rolling. With such a form, inclusions tend to be a starting point of void formation, and moldability is deteriorated. Therefore, in the present invention, it is preferable to reduce the S content as much as possible, and it is made 0.005% or less. The preferable S content is 0.003% or less. It is preferable that the S content is reduced as much as possible, but 0.0005% of the S content is inevitably incorporated.

Al: 0.08% or less

When Al is added as a deoxidizer at the stage of steelmaking, the Al content is 0.02% or more. On the other hand, if the Al content exceeds 0.08%, the formability deteriorates due to the influence of coarse inclusions such as alumina. Therefore, the Al content should be 0.08% or less. The preferable Al content is 0.07% or less.

N: not more than 0.008%

In the present invention, N is precipitated as a coarse Ti-based nitride by binding with Ti. Since this coarse Ti-based nitride causes an adverse effect on the formability, it is necessary to reduce the N content as much as possible, and the upper limit amount is made 0.008%. The preferable N content is 0.006% or less. It is preferable that the N content is reduced as much as possible, but 0.0005% is inevitably incorporated in the production.

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

Mo: 0.01 to 0.5%, Cr: 0.01 to 0.9%, Ni: 0.01 to 0.2%

Mo, Cr and Ni are elements which have an effect of promoting the formation of a bainite phase, a martensite phase and a tempering martensite phase, in addition to solid solution strengthening. Therefore, these elements substantially contribute to the enhancement of the strength. On the other hand, when these elements are contained excessively, the moldability is deteriorated. In view of the above, Mo: 0.01 to 0.5%, Cr: 0.01 to 0.9%, Ni: 0.01 to 0.2%.

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 produce precipitates. This strengthening by the precipitate strengthens the tensile strength of the steel sheet in the entire thickness direction. On the other hand, the softening near the surface layer is more difficult than the bainite phase, the martensite phase and the tempering martensite phase. That is, when the Ti, Nb and V contents exceed the upper limit specified in the present invention, the degree of strengthening by these precipitates becomes excessively large. As a result, even when the amount of the C concentration in the thickness direction from the surface to the thickness direction of 50 mu m is 0.10 mass% / mm or more as described later, the formability of the surface layer portion from the surface to the thickness direction And thus the excellent bending workability (moldability) characteristic of the present invention can not 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%. At this time, the formability may deteriorate due to the influence of the formation of coarse carbonitrides including Ti, Nb and V. From this viewpoint, when these elements are included in the above content, the inclusion density of the grain size of 0.2 mu m or more in the surface layer portion in the region from the surface of the steel sheet up to 50 mu m in the thickness direction is set to 500 pieces / mm2 or less. Further, the upper limit of the inclusion density is preferably 350 pieces / mm &lt; 2 &gt; or less. On the other hand, from the viewpoint of promoting the generation of cracks in the front end face at the time of punching processing, it is preferably not less than 50 / mm &lt; 2 &gt;.

B: not less than 0.0002% and not more than 0.0030%.

B is segregated at grain boundaries of austenite before transformation to remarkably retard nucleation of ferrite phase and has an effect of inhibiting ferrite phase formation. To obtain this effect, B should be contained in an amount of 0.0002% or more. On the other hand, when it exceeds 0.0030%, not only the effect of the saturation is saturated but also adversely affects the ductility. From the above, the B content is 0.0002% or more and 0.0030% or less. The preferable content of B is 0.0005% or more and 0.0020% or less.

The other components are Fe and inevitable impurities. When the arbitrary component is included below the lower limit value, it is assumed that these elements are included as inevitable impurities.

Next, the steel structure of the high-strength cold-rolled steel sheet of the present invention will be described. The steel structure of the high strength cold rolled steel sheet of the present invention is characterized in that the area ratio of the ferrite phase in the region from 1/4 to 3/4 of the plate thickness is 20% or more and 80% or less, the bainite phase, the martensite phase and the tempering martensite phase (1) is less than 0.10 mass% in the thickness direction x from the surface of the steel sheet in the sheet thickness direction (where x is 50 탆 or less) and the total area ratio is 20% or more and 80% / Mm. In the present invention, the area ratio of the ferrite phase in the region from 1/4 to 3/4 of the plate thickness is 20% or more and 80% or less, and the sum of the area ratios of the bainite phase, martensite phase and tempered martensite phase is 20 % Or more and 80% or less. In addition, by setting the amount of the C concentration represented by the formula (1) to 0.10 mass% / mm in the thickness direction x from the surface of the steel sheet in the thickness direction (x is 50 탆 or less) Bending workability is achieved.

The ferrite phase in the region from 1/4 to 3/4 plate thickness

The tensile strength obtained in the tensile test using the JIS No. 5 tensile test specimen is determined by the structure of the area from 1/4 to 3/4 with respect to the plate thickness direction. The ferrite phase is a soft structure, and when the area ratio of the ferrite phase is more than 80%, the tensile strength is less than 780 MPa. When the tensile strength is less than 780 MPa because the area ratio of the ferrite phase is more than 80%, the content of the ferrite phase is increased even in the surface layer portion, so that the tensile strength of the surface layer portion is not adjusted to a desired range. On the other hand, since the ferrite phase is a structure improving the formability, when the area ratio is less than 20%, the formability is significantly lowered and the ductility is also impaired. From this point of view, the area ratio of the ferrite phase was 20% or more and 80% or less. The area ratio of the ferrite phase is preferably 30% or more and 70% or less.

The sum of the area ratios of the bainite phase, the martensite phase and the tempering martensite in the plate thickness range of 1/4 to 3/4

Since the bainite phase, the martensite phase and the tempering martensite phase are harder than the ferrite phase, the metal structure containing these phases is suitable for high strength. In order to set the tensile strength of the surface layer to a desired range, it is necessary to set the area ratio of these metal structures to 20% or more in total. On the other hand, these phases are deficient in ductility, and the content of these phases generally lowers the moldability. The present invention is characterized in that the hardness of these metal structures is greatly dependent on the C content, and the moldability is improved by continuously lowering the C concentration in the surface layer portion. However, if the total area ratio of the bainite phase, the martensite phase and the tempering martensite exceeds 80%, the desired formability can not be obtained even if the C concentration in the surface layer portion is changed. Therefore, . The preferable range of the total area ratio of the bainite phase, the martensite phase and the tempered martensite phase is 30% or more and 70% or less.

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

As described above, the strength of the high-strength cold-rolled steel sheet of the present invention depends on the C concentration. Therefore, the C concentration, which is the C concentration in the vicinity of the surface, is related to the tensile strength at the surface layer portion. The combination of the adjustment of the C concentration and the tensile strength measured using the JIS No. 5 test specimen of not less than 780 MPa can indicate that the tensile strength of the surface layer portion is within a desired range. Further, when the C concentration is locally lowered, the workability of the portion becomes good. Further, in the bending process, the stress is continuously changed with respect to the thickness direction. For example, even if only the hardness of the surface of the steel sheet is lowered, cracks due to lack of ductility or cracks due to hardness difference do. In order to avoid these adverse effects, the amount of the C concentration represented by the following formula (1) in the thickness direction x in the thickness direction from the surface of the steel sheet (x is 50 탆 or less) may be 0.10 mass% / mm or more. On the other hand, when the amount of the fine particles exceeds 6.5 mass% / mm, the difference in hardness with respect to the plate thickness direction becomes large, which may cause cracks. Therefore, it is preferable that the minute amount of the C concentration is 6.5 mass% / mm or less.

In addition, since the above-mentioned minute amount becomes smaller as it gets deeper in the plate thickness direction, when the value x becomes 50% or more, when it becomes 0.10% by mass / mm or more, it can be said that the above-mentioned minute amount is within the scope of the present invention.

In addition, since the above-mentioned minute amount at x < 20 [micro] m is the region where the deformation by bending is greatest, it is preferable that the minute amount is large, and the minute amount at x = 20 [micro] m is 3.5 to 6.5% . When the content is less than 3.5% by mass / mm, softening of the surface layer is insufficient and cracks extending from the surface layer to the plate thickness direction are likely to occur at the time of molding. On the other hand, if it exceeds 6.5 mass% / mm, the strain gradient of the material becomes excessively large, and the deformation of the material in the sheet thickness direction becomes uneven, so that cracks are likely to occur in the direction parallel to the sheet surface direction during molding . Actually, the deformation at x is more than 0 占 퐉 and less than 20 占 퐉 is the largest region. However, since the measurement of the C concentration in the region of 0 占 퐉 to less than 10 占 퐉 has a large error due to contamination or the like, In the present invention, it is preferable to define a minute amount of C concentration in a region where x is 20 mu m or more.

In the range of x = 30 μm to 50 μm, since x is increased and the amount of the C concentration is decreased stepwise, the deformation during bending can be suitably absorbed in the thickness direction, Can be suppressed. That is, it is preferable that the minute amount at x = 30 占 퐉 is 2.0 to 3.5 mass% / mm. The amount of the fine particles at x = 40 占 퐉 is preferably 0.5 to 2.0 mass% / mm. The amount of the fine particles at x = 50 탆 is preferably 0.12 to 0.5 mass% / mm.

<High Strength Plated Steel Sheet>

The high strength coated steel sheet of the present invention comprises the high strength cold rolled steel sheet and a plating layer formed thereon.

The plating layer will be described. In the high-strength plated steel sheet of the present invention, the component constituting the plating layer is not particularly limited and may be a general component. For example, the plating layer contains 5.0 to 20.0% of Fe and 0.001 to 1.0% of Al in terms of mass%, and further contains Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Ca, Cu, Li, Ti, Be, Bi and REM in a total amount of 0 to 3.5% and the balance of Zn and inevitable impurities. The plating layer may be a hot-dip coating layer or an alloyed plating layer.

&Lt; Production method of high strength cold rolled steel sheet >

Next, a method of manufacturing the high-strength cold-rolled steel sheet of the present invention will be described. The method for manufacturing 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 the following description, the temperature is the surface temperature unless otherwise specified. The average cooling rate is ((surface temperature after cooling-surface temperature before cooling) / cooling time).

The hot rolling step is a step in which a steel material having the above composition is heated to 1150 占 폚 or higher and subjected to finish rolling at a finish rolling finish temperature of 800 占 폚 or higher and winding at a coiling temperature of 350 占 폚 to 720 占 폚 to be.

The solvent method for producing the steel material is not particularly limited, and a known solvent method such as a converter and an electric furnace may be employed. Further, secondary refining may be performed in a vacuum degassing furnace. Thereafter, it is preferable to make a slab (steel material) by a continuous casting method from the viewpoint of productivity and quality. The slab may be formed by a known casting method such as an ingot-bloom rolling method, a thin slab continuous casting method, or the like.

The steel material thus obtained is heated under the following conditions.

Heating temperature of steel material: 1150 ℃ or more

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

In order to stably provide the C concentration gradient to the surface layer portion, it is preferable to control the heating atmosphere, temperature and residence time of the steel material. If the atmosphere in the heating furnace _{(pCO + pCO 2 + pCH 4} ) / (pCO + 2pCO 2 + pH 2 O + 2pO 2) ≤1, the temperature is above 1150 ℃, the residence time is 30 minutes or more, the concentration gradient of C can be stably given to the surface layer . _{Here, pCO, pCO 2, pCH 4} , pH 2 O and pO ₂ refers to the partial pressure (Pa), _{respectively, CO, CO 2, CH 4} , H 2 O and O _2.

The conditions of the rough rolling of the rough rolling after the heating are not particularly limited.

Finishing rolling temperature: 800 ℃ or more

When the finish rolling temperature is lower than 800 캜, ferrite transformation starts during finish rolling, and the ferrite lips become a stretched structure and a ferrite grain partially grows into a coarse grain structure. For this reason, the finishing rolling temperature of less than 800 ° C adversely affects the plate thickness precision during cold rolling. Therefore, the finishing rolling temperature should be 800 ° C or higher. The preferred finish rolling temperature is 820 DEG C or higher. Further, the finishing rolling temperature is preferably 940 占 폚 or less because the surface properties of the scale due to biting are deteriorated. After finishing rolling, it is preferable to cool under the condition that the average cooling rate from the finish rolling temperature to 560 캜 is -30 캜 / s or less. When the average cooling rate exceeds -30 DEG C / s, it is difficult to wind up at a low temperature due to the constraint of the runout table length. The cooling stop temperature in the cooling after the finish rolling may or may not coincide with the winding temperature. If they do not coincide, further cooling or heating is required up to the coiling temperature.

Coiling temperature: 350 ° C or more and 720 ° C or less

It is difficult to set the coiling temperature to a temperature below 350 DEG C due to the constraint of the run-out table length. If the coiling temperature is less than 350 占 폚, the shape of the plate is deteriorated and cold rolling becomes difficult. On the other hand, if the coiling temperature exceeds 720 占 폚, the coiler is damaged due to the high temperature, which adversely affects the equipment. From the above, the coiling temperature was set to 350 ° C or higher and 720 ° C or lower. And preferably 450 DEG C or more and 680 DEG C or less.

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

The annealing step to be performed next is a condition in which the temperature range of 580 to T 占 폚 (580 < T? 730) in the continuous annealing line or the continuous plating line after the cold rolling step is set to a condition that the air ratio is 1.05 to 1.30, The cold-rolled steel sheet is heated to a maximum attainable temperature of 730 ° C or higher under the condition that the dew point in the temperature range of -40 ° C to -15 ° C is -40 ° C to -15 ° C, and then the average cooling rate from 700 ° C to 550 ° C is -10 ° C / To a cooling stop temperature of 25 to 530 캜, and then, if necessary, heating to maintain the temperature in the range of 200 캜 to 530 캜.

Air ratio from 580 to T ° C: 1.05 to 1.30

When the air ratio is 1.05 or more, an oxidizing atmosphere occurs, and a C gradient of the surface of the steel sheet occurs due to the reaction of C with oxygen on the surface of the steel sheet. In order to obtain a sufficient reaction rate in the continuous annealing line or the continuous plating line, it is necessary to heat the temperature range of 580 to T ° C by using a combustion burner at an air ratio of 1.05 or more . On the other hand, if the air ratio exceeds 1.30, the adverse effect of decrease in formability due to intergranular cracking due to grain boundary oxidation becomes evident, so that the air ratio of the combustion burner at the above temperature range is 1.30 or less. The preferred air ratio range is 1.07 to 1.26.

This adverse effect may occur even at high temperature heating, so T ° C is set to 730 ° C or less. Further, the air ratio at the time of heating the temperature range of 580 to T 占 폚 by the combustion burner may be within the above range, and when T is close to 580 占 폚 (when the area where the air ratio is controlled to 1.05 to 1.30 is narrow) But T ° C is preferably 600 to 700 ° C in order to sufficiently obtain the effect of controlling the air ratio. The annealing furnace at this time may be an oxidizing furnace in a direct heating furnace or an oxidizing furnace in a non-oxygen furnace. However, in a heating method using a flame burner , &Lt; / RTI &gt;

Dew point in the temperature range of 730 ° C or higher: -40 to -15 ° C

The water vapor in the furnace reacts with the C in the surface layer portion of the steel sheet to cause a gradient of the C concentration in the surface layer portion of the steel sheet. In order to exhibit this effect, the dew point in the temperature range of 730 ° C or more at the time of heating should be -40 to -15 ° C. If the dew point in the above-mentioned temperature range exceeds -15 占 폚, the adverse effect of the deterioration of the formability due to intergranular cracking becomes present due to the influence of the grain boundary oxidation. On the other hand, when the temperature is lower than -40 ° C at the dew point, the water vapor in the furnace does not react with C in the surface layer portion of the steel sheet, and a desired C concentration gradient is not obtained. The preferred range of the dew point is -35 to -20 占 폚. Further, in order to prevent sufficient reaction of water vapor and C in the surface layer of the steel sheet, it is necessary to control the dew point at a temperature range of at least 730 캜. Further, the dew point is set in the above range up to the maximum reaching temperature.

Maximum reaching temperature: 730 ℃ or more

When the maximum reaching temperature is less than 730 캜, there is a case where a C concentration gradient can not be appropriately formed or a desired steel structure can not be obtained as described above. The maximum attained temperature is preferably 750 DEG C or higher from the viewpoint of formation of the C concentration gradient. The upper limit of the maximum attained temperature is not particularly limited. However, if the maximum attained temperature is excessively high, thermal damage to the facilities of the annealing furnace may occur, the fuel consumption rate may decrease, Therefore, the temperature is preferably 880 DEG C or less.

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

When the average cooling rate in this temperature range exceeds -10 DEG C / s, the area ratio of the ferrite phase exceeds 80% and the desired steel sheet strength is not obtained . Therefore, the average cooling rate from 700 ° C to 550 ° C is -10 ° C / s or less. The average cooling rate in the temperature range of 700 to 550 占 폚 is preferably -20 占 폚 / 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 it to 530 캜 or lower. The cooling method may be any of a gas jet cooling device and water cooling as long as the condition of the average cooling rate is satisfied. Stably, in order to make the sum of the area ratios of the bainite phase, the martensite phase and the tempered martensite phase 20% or more, it is preferable to set the cooling stop temperature to 520 캜 or lower. In the present invention, the lower limit of the cooling stop temperature is 25 占 폚, which corresponds to the room temperature, since it is not necessary to cool down to a temperature below the room temperature.

Holding temperature: 200 ° C to 530 ° C

The martensite phase obtained in the previous step is maintained at 200 ° C to 530 ° C for the purpose of tempering or promoting bainite transformation. When the cooling stop temperature is lower than the holding temperature, it is necessary to heat. The heating method may be IH heating, gas heating, or the like. Martensite is not sufficiently tempered at a temperature of 200 DEG C or lower, and ductility of the steel sheet is impaired. On the other hand, when the temperature exceeds 530 占 폚, the ferrite transformation progresses and the desired steel sheet strength can not be obtained. The preferable range of the holding temperature is 250 deg. C or higher and 520 deg. C or lower. In addition, the above-mentioned firing is not limited to the constant-temperature holding as long as the steel sheet temperature is in the above temperature range. The holding time is not particularly limited, but from the viewpoint of the above purpose, the holding time is preferably 20 to 1200 seconds.

&Lt; Method of Manufacturing High Strength Plated Steel Sheet >

Subsequently, the plating step is a step of performing plating after the annealing step and forming a plating layer on the annealing plate. For example, in the case of performing hot-dip coating for automobile steel sheets, the annealing is performed in a continuous hot-dip plating line, followed by cooling after annealing, followed by dipping in a hot-dip coating bath to form a plating layer on the surface . After the plating step, if necessary, an alloying step for performing alloying treatment may be provided. The kind of plating is preferably zinc plating.

Example 1

A steel material having a composition shown in Table 1 having a thickness of 250 mm was subjected to a hot rolling step under the conditions shown in Table 2 to form a hot rolled steel sheet and subjected to a cold rolling step under the cold rolling conditions shown in Table 2, A cold-rolled steel sheet having a thickness of 2.0 mm or less. Annealing was performed on the continuous annealing line or continuous hot-dip plating line under the conditions shown in Table 2, respectively. Thereafter, a plating treatment and, if necessary, an alloying treatment were carried out. Here, the temperature of the plating bath (plating composition: Zn-0.13 mass% Al) immersed in the continuous hot-dip coating line was 460 占 폚, and the amount of plating adhered to both the GI material (hot-dip galvanized steel sheet) and GA material (galvannealed steel sheet) 45 to 65 g / m &lt; 2 &gt; per one side, and the amount of Fe contained in the plating layer is in the range of 6 to 14 mass%.

A test piece was taken from the cold-rolled steel sheet, the hot-dip coated steel sheet or the alloyed hot-dip galvanized steel sheet obtained as described above and evaluated by the following technique.

(I) an observation image,

The area ratio of each phase was evaluated by the following technique. From the steel sheet, the plate thickness cross section parallel to the rolling direction was cut out to be the observation surface, and the observation surface was corroded with 1% or nital, and enlarged to 2000 times with a scanning electron microscope to obtain plate thicknesses of 1/4 to 3 / 4 area was photographed in 10 fields of view. The ferrite phase is a structure in which no trace of corrosion or cementite is observed in the particles, and a bainite phase is a structure in which corrosion marks or large carbides are observed in the particles. The martensite phase is a structure in which no carbides are observed in the particles and is observed with a white contrast. Tempering martensite is a structure in which corrosion marks are observed in the particles, and fine carbides are observed between the laths. These were subjected to image analysis to separate a ferrite phase, a bainite phase, a martensite phase, and a tempering martensite phase to obtain an area ratio with respect to the observation visual field. The results are shown in Table 3.

(Ii) Inclusion density measurement

The inclusion density was measured by the following technique. From the steel sheet, the plate thickness cross-section parallel to the rolling direction was cut out to be the observation plane, and the area was enlarged to 2000 times with a scanning electron microscope to observe a region of up to 50 mu m in the plate thickness direction from the surface layer of the steel sheet, , And the inclusion density was measured. The results are shown in Table 3.

(Iii) X-ray measurement

In the plate thickness cross-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 steel plate surface was subjected to grinding and chemical polishing at 200 占 퐉 or more, and the X-ray diffraction intensity To thereby determine the amount of retained austenite. The incident sources were measured from the peaks of (200) _α , (211) _α , (200) _γ , (220) _γ and (311) _γ using MoKα line to obtain volume ratios. In the present invention, the obtained volume ratio is treated as an area ratio.

(Iv) C concentration measurement in the surface layer of the steel sheet

The surface of the steel sheet was grinded in the direction of the thickness of the plate sequentially, and the surface exhibited was subjected to spark discharge emission spectral analysis to determine the C concentration on the surface. Accordingly, the C concentration distribution in the plate thickness direction can be measured by performing measurement a plurality of times in accordance with the grinding amount.

First, a 10 탆 grinding process was performed to remove the cause of errors such as dirt on the surface of the steel sheet. Subsequently, the amount of the C concentration (d [% C _x ] / dx) was determined five times from 10 탆 to 50 탆 at intervals of 10 탆. Here, [% C _x ] is the C concentration at the position of the depth (x m) in the plate thickness direction from the surface. In this case, d [% C ₂₀ ] / dx, d [% C ₃₀ ] / dx, d [% C ₄₀ ] / dx and d [% C ₅₀ ] / dx are obtained. The results are shown in Table 3.

When an approximate expression can be derived with an error range of 占 1% or less with respect to each measurement point, a minute amount of the C concentration at a depth of 50 占 퐉 from the surface of the steel sheet may be obtained by differentiating the approximate expression.

The value of d [% C ₂₀ ] / dx is the largest, and the value of d [% C ₅₀ ] / dx is the smallest value, because C is discharged from the surface layer of the steel sheet.

(V) tensile test

JIS No. 5 tensile test specimens were prepared from the obtained steel sheet in the direction perpendicular to the rolling direction (the original thickness was the same and the test specimen contained the surface layer), and the tensile test was performed five times in accordance with JIS Z 2241 (2011) The yield strength (YS), tensile strength (TS) and total elongation (El) of the average were obtained. The crosshead speed of the tensile test was 10 mm / min. The yield strength was read as 0.2% proof stress for continuous yield and lower yield point for discontinuous yield. In Table 3, the tensile strength is 780 MPa or more, and the total elongation is 8% or more. Here, when the total elongation is 8% or more, when it is less than 8%, the formability is bad and practically useless due to problems such as cracking due to lack of ductility of the steel sheet in press working or the like. The yield strength is preferably 490 MPa or more.

(Vi) Bending test (formability evaluation)

No. 3 test piece described in JIS Z2248 was sampled so that the direction perpendicular to the thickness direction of the steel sheet was the longitudinal direction of the test piece, and a bending test was conducted by the V-block method. The limit bending radius (R / t) was obtained by dividing the radius of the tip of the mandrel when the crack was confirmed in the bending ridge by the plate thickness. Actually, the bendability determined by the strength is different. Accordingly, (R / t) / TS is 2.0 × 10 ^-3 or less is a very good result "○", as more than 2.0 × 10 ^-3 2.5 × 10 ^-3 or less is preferred, and as a result the evaluation of the "△" To the extent required by the invention. (R / t) / TS exceeded 2.5 x 10 &lt; ^-3 &gt;, it was evaluated as "x &quot;, and was outside the range required by the present invention.

Table 3 shows the results obtained by the above.

All of the examples of the present invention have a tensile strength TS of 780 MPa or more and good moldability is obtained. On the other hand, in the comparative example deviating from the scope of the present invention, when the metal structure was out of the range, the tensile strength was lower than 780 MPa or the desired formability was not obtained. Particularly, when the C concentration gradient in the surface layer portion was out of the range of the present invention, the moldability was low.

Claims (11)

Si: not less than 0.01% and not more than 2.0%, Mn: not less than 1.8% and not more than 5.0%, P: not more than 0.06%, S: not more than 0.005%, Al: not more than 0.08%, N : 0.008% or less, the balance being Fe and inevitable impurities,
The area ratio of the ferrite phase in the region from 1/4 to 3/4 of the plate thickness is 20% or more and 80% or less, the total area ratio of the bainite phase, the martensite phase and the tempering martensite phase is 20% or more and 80% ego,
The steel sheet has a steel structure in which the minute amount of C concentration represented by the following formula (1) at x m in the plate thickness direction from the steel sheet surface is 0.10 mass% / mm or more,
A high strength cold rolled steel sheet having a tensile strength of 780 MPa or more when subjected to a tensile test using a JIS No. 5 tensile test piece.
d [% C _x ] / dx = ([% C _x ] - [% C _{x -10 탆} ]) /
[% C _x ] in the formula (1) represents the C concentration in x, and x represents the distance in the plate thickness direction from the surface of the steel sheet, and its value is 50 μm or less. The method according to claim 1,
The steel sheet according to any one of claims 1 to 3, wherein the composition of the steel sheet further comprises one or more of Mo: 0.01 to 0.5%, Cr: 0.01 to 0.9%, Ni: 0.01 to 0.2% . 3. The method according to claim 1 or 2,
Wherein said composition further comprises one or more of Ti: 0.01 to 0.15%, Nb: 0.01 to 0.1%, V: 0.01 to 0.5%
A high strength cold rolled steel sheet having an inclusion density of not more than 500 pieces / mm &lt; 2 &gt; in a range from a surface of a steel sheet to a thickness of 50 mu m in a thickness direction of the steel sheet. 4. The method according to any one of claims 1 to 3,
Wherein said composition further contains, by mass%, B: 0.0002% or more and 0.0030% or less. A high strength cold rolled steel sheet as set forth in any one of claims 1 to 4,
And a plating layer formed on the high-strength cold-rolled steel sheet. 6. The method of claim 5,
Wherein the plating layer is a hot-dip coating layer or an alloyed hot-dip coating layer. The method according to claim 5 or 6,
Wherein the plating layer contains Fe in an amount of 5.0 to 20.0% by weight and Al in an amount of 0.001 to 1.0% by weight and further contains Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, , Li, Ti, Be, Bi, and REM in a total amount of 0 to 3.5% and the balance of Zn and inevitable impurities. A steel material having a composition according to any one of claims 1 to 4 is subjected to finish rolling at a temperature of not lower than 1150 캜 and subjected to rough rolling and finish rolling at a finish temperature of 800 캜 or higher, Rolled at a coiling temperature of < RTI ID = 0.0 &gt;
A cold rolling step of performing cold rolling on the hot-rolled steel sheet after the hot-rolling step,
After the cold rolling step, a continuous annealing line or a continuous plating line is heated to a temperature of 580 DEG C or more and T DEG C or less under a condition of an air ratio of 1.05 to 1.30 (580 &lt; T & The cold-rolled steel sheet is heated to a maximum attained temperature of 730 DEG C or higher under the condition that the dew point in the temperature range is -40 to -15 DEG C, and then the average cooling rate from 700 DEG C to 550 DEG C is -10 DEG C / And cooling the steel sheet to a cooling stop temperature of 25 to 530 占 폚, and then heating the steel sheet as necessary to maintain the steel sheet at a temperature in the range of 200 占 폚 to 530 占 폚. 9. The method of claim 8,
The method of manufacturing a high-strength cold-rolled steel sheet according to any one of claims 1 to 3, wherein the steel material satisfies the formula (2) when the steel material is heated and the steel material stays at a temperature of 1150 占 폚 or more for 30 minutes or longer.
_{(pCO + pCO 2 + pCH 4} ) / (pCO + 2pCO 2 + pH 2 O + 2pO 2) ≤1 (2)
Expression pCO, pCO ₂ in the _{(2), pCH 4, pH} 2 O and pO ₂ refers to the partial pressure (Pa), _{respectively, CO, CO 2, CH 4} , H 2 O and O _2. A method of manufacturing a high strength coated steel sheet having a plating process for plating a high strength cold rolled steel sheet produced by the manufacturing method according to claim 8 or 9. 11. The method of claim 10,
And an alloying step of performing an alloying treatment after the plating step.