KR20170096197A - High-strength plated steel sheet and production method for same - Google Patents

Abstract

A high strength hot-dip coated steel sheet having a tensile strength of 780 MPa or more and excellent in workability and a method for producing the same.
A high strength hot-dip coated steel sheet having a steel sheet and a plated layer formed on the steel sheet, the steel sheet having a specific component composition and comprising a ferrite phase in an area ratio of not more than 20% (including 0%), a bainite phase in an area ratio of 35% Or more and 90% or less, the martensite phase is contained in an area ratio of 10% or more and 65% or less, the number density of inclusions having a circle equivalent diameter exceeding 5.0 占 퐉 is 400 / mm ² or less, And the maximum length of the martensite is 5.0 m or less. The surface hardness of the steel sheet is 1 / 4t from the surface of the steel sheet in the thickness direction (t is the thickness of the steel sheet Thickness) position is set to 100%, it is 95% or less.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high strength coated steel sheet,

The present invention relates to a high strength plated steel sheet and a method for producing the same. The high strength plated steel sheet of the present invention has a high tensile strength (TS) of 780 MPa or more and excellent formability. Therefore, the high strength plated steel sheet of the present invention is suitable for the material of structural parts for automotive.

Recently, in order to reduce CO ₂ emissions from the viewpoint of global environmental preservation, improvement of fuel efficiency of automobiles is aimed at the automobile industry as a whole. In order to improve the fuel efficiency of automobiles, the weight reduction of automobiles due to the thinning of parts used is most effective. 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 has poor moldability due to its high strength, which makes processing difficult. Therefore, in lightening the weight of automobile parts and the like, the steel sheet is required to have good workability in addition to high strength.

From the above, it is required to develop a steel sheet having high strength and bendability (also referred to as workability and moldability), and various techniques have been proposed for a high-strength cold-rolled steel sheet and a coated steel sheet.

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% of P, 0.020% or less of S, 0.001 to 1.0% of Al, 0.0004 to 0.015% of N, 0.03 to 0.2% of Ti, or 0.1% or less of Nb , The remainder being Fe and an impurity, and the ferrite is contained in an area ratio of 30 to 95%, and the second phase of the remainder contains at least one of martensite, bainite, pearlite, cementite and retained austenite By weight of martensite, the martensite having an area ratio of 0 to 50% when martensite is contained, and a steel sheet having a grain size of 2 to 30 nm, A distance of 30 to 300 nm, and a diameter of 3 占 퐉 or more It is supposed that a high strength steel sheet having excellent bending workability and bending fatigue fatigue property having a tensile strength of 620 MPa or more can be obtained by containing a crystallized TiN having an average grain distance of 50 to 500 탆.

In Patent Document 2, it is preferable that 0.05 to 0.20% of C, less than 0.01 to 0.6% of Si, 1.6 to 3.5% of Mn, 0.05% or less of P, 0.01 or less of S, , N: 0.01% or less and the balance of iron and inevitable impurities, the steel sheet having a polygonal ferrite structure and low-temperature transformed phases, wherein the low-temperature transformation forming structure comprises at least bainite And a martensite may additionally be contained. By changing the position in the plate width direction with respect to the plate surface 0.1 mm deep from the surface of the steel sheet, a total of 20 field of view is observed with a microscope, and 50 탆 50 When the image analysis was performed on the area of 占 퐉, the maximum value and the minimum value of the area ratio of polygonal ferrite and the maximum value of the area ratio of the martensite were determined, whereby a tensile strength of 780 MPa excellent in bending workability and fatigue strength Is a hot-dip galvanized steel sheet is be obtained.

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

In the technique proposed in Patent Document 1, the influence of the composition of the components on the steel structure is not disclosed at all in the examples, and the improvement due to consideration of the steel structure is insufficient, none.

Also, in the technique proposed in Patent Document 2, factors to be considered for realizing improvement in formability due to high strain hardenability required in the present invention are not fully understood.

SUMMARY OF THE INVENTION The present invention has been made in view of such circumstances, and an object thereof is to provide a high strength plated steel sheet having a tensile strength of 780 MPa or more and excellent workability and a method for producing the same.

Means for Solving the Problems In order to solve the above problems, the present inventors have studied extensively on the requirements of a steel sheet having a tensile strength of 780 MPa and a good workability. As a result, in order to obtain a high-strength steel sheet, it has been noted that a soft ferrite phase is reduced as much as possible and a low temperature transformation phase such as a bainite phase or a martensite phase is utilized. On the other hand, when the ferrite phase rich in formability is reduced in the prior art, good formability can not be obtained. Therefore, a means for improving the formability of a steel sheet not containing a ferrite phase in large amounts has been studied. As a result, it has been found that when the fine granular martensite is in a martensite phase dispersed on the bainite, the uniform deformation of the bainite phase is promoted, and as a result, the workability of hardening is increased, and the formability is improved . It was found that it is effective to disperse the fine martensite on the bainite by suppressing the coarsening of the austenite grain size under annealing after finely dispersing the cementite in the structure before the annealing process. On the other hand, as the austenite grain size under annealing is miniaturized, the area of the austenite grain boundary that becomes the nucleation site of the ferrite transformation is increased, so that the ferrite phase is liable to come out. It has been found that it is important to reduce the inclusion density of 5.0 탆 or more which becomes the ferrite nucleation site after improving the hardenability of the steel sheet by proper element addition in order to suppress the ferrite transformation after the martensite phase is refined.

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

[1] A high strength coated steel sheet having a steel sheet and a plated layer formed on the steel sheet, wherein the steel sheet has a composition of C: 0.06 to 0.18%, Si: 0.50%, Mn: 1.9 to 3.2% 0.004% or less, P: not more than 0.03%, S: not more than 0.005%, Al: not more than 0.08%, N: not more than 0.006%, B: not less than 0.002% And the remainder is made of Fe and inevitable impurities, and the steel structure of the steel sheet has a ferrite phase in an area ratio of 20% or less (including 0%), a bainite phase in an area ryulro containing not more than 65% or more and 10% or more 35% 90% or less, maten Xi-like in the area ratio, and also the inclusion of circle equivalent diameter exceeds 5.0 ㎛ in number density and containing less than 400 / ㎜ ^2, wherein The average particle diameter of the granular martensite constituting the martensitic phase is 4.0 占 퐉 or less, The maximum length between the tensile strengths is not more than 5.0 占 퐉 and the surface hardness of the steel sheet is not more than 95% when the hardness at a position of 1 / 4t (t is the thickness of the steel sheet) High strength steel plate.

[% N] -14 [% Ti] / 48? 0 (One)

[% N] in the formula (1) means the N content, and [% Ti] means the Ti content.

0.001% to 0.3% of Mo, 0.001% to 0.3% of Mo, and 0.001% or more and 0.3% or less of Mo, , And a composition of one or more of W: 0.001% or more and 0.2% or less and Hf: 0.001% or more and 0.3% or less.

[3] The composition according to [1] or [2], further comprising one or more of REM, Mg and Ca in a total mass ratio of not less than 0.0002% and not more than 0.01% High strength galvanized steel sheet.

[4] The high strength coated steel sheet according to any one of [1] to [3], wherein the plating layer is any one of a hot-dip coating layer and an alloyed hot-dip coating layer.

[5] A steel material having a composition according to any one of [1] to [3], which is heated at a temperature of 1000 ° C to 1200 ° C and finishes at a finish rolling temperature of 800 ° C or more, , A cold rolling step of cooling the hot rolled sheet after the hot rolling step and a cold rolling step of cold rolling the hot rolled sheet after the hot rolling step, heating the cold-rolled sheet to at least the Ac ₃ point maximum attained temperature, that the heated cold-rolled sheet after and the cooling rate up to 580 ℃ cooling under the conditions at least 5 ℃ / s, the heating and the at least Ac ₃ point in the cooling And the dew point at the Ac ₃ point or higher is set at -45 캜 to -20 캜, and at the cooling of 440 캜 to 530 캜, the cold- The current time is a pre-heating step, the pre-heating a cold-rolled sheet after the step from 100 ℃ (Ac ₃ point -10) average heating rate of 3.0 ℃ / s or more conditions to ℃ than the maximum attained temperature of more than 20 seconds The cold-rolled sheet heated to the maximum reaching temperature is cooled under the condition that the average cooling rate up to 560 ° C is 10 ° C / s or more. In the heating and cooling thereof, the cold-rolled sheet remains at (Ac ₃ point -10) The annealing step is carried out such that the time for which the cold-rolled sheet is retained at 440 DEG C or higher and 530 DEG C or lower for cooling the steel sheet is 20 seconds or longer and 180 seconds or shorter; and after the annealing step, And a plating step of forming a plating layer on the plating layer.

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, Co, 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. A method of manufacturing a plated steel sheet.

[7] The method of producing a high strength coated steel sheet according to [5] or [6], wherein the plating treatment is any one of a hot dip galvanizing treatment and an alloying hot dip galvanizing treatment.

According to the present invention, the high-strength plated steel sheet of the present invention has a high tensile strength (TS) of 780 MPa or more and excellent moldability. When the high-strength plated steel sheet of the present invention is applied to an automobile part, the weight of the automobile part is further reduced.

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

<High Strength Plated Steel Sheet>

The high strength coated steel sheet of the present invention has a steel sheet and a plating layer formed on the steel sheet. A steel sheet, and a plated layer.

The steel sheet has a composition of C: 0.06 to 0.18%, Si: less than 0.50%, Mn: 1.9 to 3.2%, P: 0.03% or less, S: 0.005% 0.006% or less of N, 0.0002% or more and 0.0030% or less of B, 0.007% or more and 0.030% or less of Nb, and a composition containing Ti so as to satisfy the above formula (1). The following components are described below. In the following description, "%" representing the content of the component means "% by mass".

C: not less than 0.06% and not more than 0.18%

C has hardenability to increase the hardness of martensite and suppress ferrite transformation. In order to obtain a steel sheet having a tensile strength of 780 MPa or more, it is necessary to set the C content at least 0.06%. On the other hand, if the C content exceeds 0.18%, the area ratio of the martensite exceeds 65%, resulting in loss of ductility and moldability. Therefore, the C content is 0.06% or more and 0.18% or less. , Preferably not less than 0.07% and not more than 0.18%.

Si: less than 0.50%

Si is an element contributing to the enhancement of strength by solid solution strengthening. On the other hand, Si increases the transformation point (Ac ₃ point) from the ferrite phase to the austenite phase, making it difficult to remove the ferrite phase during the annealing. Also, since Si lowers the wettability of the plating layer and the surface of the steel sheet, the excessive content of Si causes defects such as plating. In the present invention, Si content is permissible in the range of less than 0.50%. Preferably less than 0.30%. The lower limit is not particularly limited, but Si of 0.01% may inevitably be incorporated into the steel.

Mn: 1.9% or more and 3.2% or less

Mn contributes to enhancement of strength by solid solution strengthening and also has an effect of lowering the Ac ₃ transformation point to make it easy to remove the ferrite phase during annealing and to improve the hardenability of the steel sheet. In order to obtain a desired steel structure, it is necessary to set the Mn content to 1.9% or more. On the other hand, if the Mn content exceeds 3.2%, the bainite transformation does not proceed and consequently the area ratio of the martensite exceeds 65%. Therefore, the upper limit of the Mn content is set to 3.2%. The preferable range of the Mn content is 2.0% or more and 3.0% or less.

P: not more than 0.03%

P is an element that is segregated at the grain boundaries and becomes a starting point of cracking at the time of molding, and thus has an adverse effect on moldability. Therefore, it is preferable to reduce the P content as much as possible. In the present invention, the P content is set to 0.03% or less in order to avoid the above problem. It is preferably 0.02% or less. It is preferable to reduce the amount as much as possible, but 0.002% of the production amount is inevitably incorporated.

S: not more than 0.005%

S is present as an inclusion such as MnS in the steel. This inclusion is formed into a wedge shape by hot rolling and cold rolling. Such a form tends to be a starting point of void formation, which also has an adverse effect on moldability. 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. It is preferably 0.003% or less. The S content is preferably reduced as much as possible, but 0.0005% in the production may be inevitably incorporated.

Al: 0.08% or less

When Al is added as a deoxidizer at the stage of steelmaking, it is preferable that Al is contained in an amount of 0.02% or more. On the other hand, if the Al content exceeds 0.08%, ferrite transformation is promoted by the influence of inclusions such as alumina and the tensile strength is lower than 780 MPa. Therefore, the Al content should be 0.08% or less. Preferably 0.07% or less.

N: not more than 0.006%

In the present invention, N is precipitated as a coarse Ti-based nitride by binding with Ti. Since this coarse Ti-based nitride becomes the nucleation site of the ferrite transformation, it is necessary to reduce the N content as much as possible, and the upper limit is set to 0.006%. The preferable N content is 0.005% or less. Although it is preferable to reduce the N content as much as possible, 0.0005% in the production may be inevitably incorporated.

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

B is segregated at the austenitic grain boundaries before transformation to have an effect of significantly retarding the nucleation of the ferrite phase and has the effect of inhibiting the formation of the ferrite phase. In order to obtain this effect, the B content needs to be 0.0002% or more. On the other hand, if the B content exceeds 0.0030%, the effect of quenching is saturated and the ductility is adversely affected. From the above, the B content is 0.0002% or more and 0.0030% or less. Preferably not less than 0.0005% and not more than 0.0020%.

Nb: 0.007% or more and 0.030% or less

Nb is an important element for suppressing the coarsening of the austenite grains under annealing. If the Nb content is excessive, the coarse carbonitrides containing Nb (collectively carbides, nitrides and carbonitrides, hereinafter the same in the present invention) are precipitated, so that the area ratio of the ferrite phase is increased. In order to inhibit the coarsening of the austenitic grains, the Nb content needs to be 0.007% or more. On the other hand, if the Nb content exceeds 0.030%, coarse Nb-based carbonitrides are precipitated under the production conditions specified in the present invention. Therefore, the upper limit of the Nb content is set to 0.030%. The preferable Nb content is 0.012% or more and 0.027% or less.

Ti: [% N] -14 [% Ti] / 48? 0

When the Ti content does not satisfy the above inequality and [N] -14 [% Ti] / 48> 0 is satisfied, N is bonded to B, so the quenchability is lowered and the area ratio of the ferrite phase is more than 20% do. When [% N] -14 [% Ti] / 48? 0, N is bonded to Ti, so that the hardenability of the steel sheet is not lost. On the other hand, if Ti is contained excessively, it is bonded with C to form carbide. This carbide precipitates in the potential and significantly inhibits the movement of dislocations, which is a factor of deteriorating the formability. From this viewpoint, the left side of the expression (1) is preferably -0.010 or more. More preferably -0.006 or more.

The high strength steel sheet according to the present invention may further contain 0.001 to 0.9% of Cr, 0.001 to 0.5% of Ni, 0.001 to 0.3% of V, 0.001 to 0.3% of Mo, W: 0.001% or more and 0.2% or less, and Hf: 0.001% or more and 0.3% or less.

Cr, Ni, V, Mo, W and Hf have an effect of retarding the initiation of ferrite transformation. In addition to the effect of quench-hardening by B, if the effect of these elements is effective, a desired steel structure can be stably obtained. On the other hand, if the Cr content exceeds 0.9%, the plating performance is adversely affected. If the content of Ni is 0.5%, the content of V is 0.3%, the content of Mo is 0.3%, the content of W is 0.2% and the content of Hf is more than 0.3%, the effect of quenching is saturated. From the above, it is preferable that Cr: 0.001 to 0.9%, Ni: 0.001 to 0.5%, V: 0.001 to 0.3%, Mo: 0.001 to 0.3% 0.001% or more and 0.3% or less.

The high-strength plated steel sheet of the present invention may further contain 0.0002% or more and 0.01% or less in total of one or more of REM, Mg and Ca in terms of mass%.

REM (REM: lanthanoid elements of atomic numbers 57 to 71), Mg and Ca spheronize cementite precipitated in bainite. As a result, the stress concentration around the cementite is lowered and the moldability is improved. On the other hand, if the total content of REM, Mg and Ca exceeds 0.01%, the effect of the shape change of the cementite becomes saturated and the ductility is adversely affected. From the above, it is preferable that they contain one or more of REM, Mg and Ca in a total amount of not less than 0.0002% and not more than 0.01%. Preferably, the total amount of one or more of REM, Mg and Ca is 0.0005% or more and 0.005% or less.

The components other than the above components are Fe and inevitable impurities.

Next, the steel structure of the high-strength plated steel sheet of the present invention will be described. The steel structure of the high strength plated steel sheet of the present invention has a ferritic phase in an area ratio of 20% or less (including 0%), a bainite phase in an area ratio of 35% to 90%, a martensite phase in an areal ratio of 10 % Or more and 65% or less, and the number density of inclusions having a circle-equivalent diameter exceeding 5.0 mu m is 400 / mm ² or less. The mean particle diameter of the granular martensite constituting the martensite phase is 4.0 占 퐉 or less and the maximum length between the martensite particles is 5.0 占 퐉 or less.

Ferrite phase

The ferrite phase is a soft texture, and when the content of the ferrite phase exceeds 20%, the tensile strength is lower than 780 MPa. Further, since the solubility of the ferrite phase is small, if the content of the ferrite phase is excessive, the arrangement of the finely dispersed cementite in the pre-annealing structure is changed and a fine martensite phase is not obtained. Therefore, it is preferable to reduce the content of the ferrite phase as much as possible, and it is necessary to suppress the content of the ferrite phase in the present invention to 20% or less. And preferably 15% or less.

Bainite Awards

The bainite phase is more effective than the ferrite phase because it has a higher hardness and produces a martensite phase finely. In order to obtain a desired steel structure, the content of the bainite phase needs to be 35% or more. On the other hand, when the content of the bainite phase exceeds 90%, the maximum length (maximum distance) of the interval between the martensite exceeds 5.0 m, and good moldability can not be obtained. The content of the preferable bainite phase is 40% or more and 80% or less by area ratio.

Martensite

The content of the martensite phase and the morphology of the martensite have a great influence on the strength and moldability. If the content of the martensitic phase is less than 10% by area, the tensile strength is less than 780 MPa. On the other hand, if the content of the martensite phase exceeds 65% by area ratio, ductility and formability are lost. The content of the martensite phase is preferably 20% or more and 55% or less by area ratio.

In the high-strength coated steel sheet of the present invention, the martensitic phase is composed of granular martensite. When the average particle diameter of martensite exceeds 4.0 탆, deformation in the vicinity of coarse martensite is restrained and the steel sheet is unevenly deformed during molding. In this case, cracks are liable to occur in the first deformed portion, so that good moldability can not be obtained. The average particle diameter of the martensite is preferably 3.0 m or less. The lower limit value of the average particle diameter of the martensite is not particularly limited, but it is preferably not less than 0.5 m from the viewpoint of a martensite fraction stably not less than 10%.

Further, the maximum length of the interval between the martensite is 5.0 占 퐉 or less. When the maximum length of the interval between the martensite is within this range, most of the bainite phase comes into contact with the martensite phase. The bainite phase in contact with the martensitic phase is liable to generate dislocations and work hardening. As a result, the work hardening exponent is increased and uniformly deformed, so that good moldability is obtained. The maximum interval length (maximum length) between the martensite is preferably 4.0 m or less. The lower limit value of the maximum interval length (maximum length) between the martensite is not particularly limited, but when the distance between the martensite is excessively close, the dislocation is introduced near the martensite due to the transformation transformation caused by the martensitic transformation As a result, generation of new dislocations between martensite is inhibited and work hardening becomes difficult, and therefore, it is preferable to be 1.0 m or more.

Inclusion

In the steel structure of the high strength plated steel sheet of the present invention, the number density of inclusions having a diameter of circle equivalent to a particle diameter exceeding 5.0 탆 is 400 pieces / mm ² or less. Inclusions having a particle size exceeding 5.0 탆 tend to become nucleation sites of ferrite phase, and the content in the area ratio of the ferrite phase is not in the desired range. Examples of inclusions exceeding 5.0 탆 include an oxide containing Al or Ti, a nitride containing Ti, and a carbonitride containing Nb.

Next, properties of the steel sheet in the high-strength plated steel sheet of the present invention will be described.

Hardness

In the high-strength plated steel sheet of the present invention, the surface hardness of the steel sheet is 95% or less when the hardness at a position of 1 / 4t (t is the thickness of the steel sheet) in the thickness direction from the surface of the steel sheet is 100%. Most of the cracks during molding occur on the surface of the steel sheet. In the present invention, the ductility of the surface layer portion of the steel sheet is improved by adjustment of the coiling temperature and decarburization of the surface layer portion of the steel sheet in the annealing step, whereby the formability can be improved. Since the hardness and ductility are in relation to each other, ductility of the surface layer of the steel sheet can be estimated according to the decrease in hardness of the surface of the steel sheet. The moldability is improved when the hardness of the surface of the steel sheet is 95% of the hardness within the steel sheet (the position of 1 / 4t (t is the thickness of the steel sheet) in the thickness direction from the surface of the steel sheet). Preferably, the surface hardness of the steel sheet is 90% or less when the hardness at a position of 1 / 4t (t is the thickness of the steel sheet) in the thickness direction from the steel sheet surface is taken as 100%. A problem in forming by bending often occurs when cracks originate from the surface to 100 mu m. Thus, the surface layer of the steel sheet is in the range from the surface of the steel sheet to a thickness of 100 mu m.

Next, the plating layer will be described. In the high-strength plated steel sheet of the present invention, the component constituting the plated layer is not particularly limited and may be a general component. For example, the plating layer may contain Fe in an amount of 5.0 to 20.0% and Al in an amount of 0.001 to 1.0% in terms of mass%, and may further contain Pb, Sb, Si, Sn, Mg, Mn, Ni, , Cu, Li, Ti, Be, Bi and REM in a total amount of 0 to 3.5%, the balance being Zn and inevitable impurities. The plating layer may be an alloyed plating layer (a plating layer containing, as a main component, an Fe-Zn alloy obtained by diffusion of Fe in steel during galvanization by alloying reaction).

Next, a method of manufacturing the high strength coated steel sheet of the present invention will be described. The method for manufacturing a high strength coated steel sheet of the present invention has a hot rolling step, a cold rolling step, a preheating heating step, an annealing step and a plating step. If necessary, the alloying process may be performed after the plating process. Hereinafter, each step will be described. In the following description, the temperature is the surface temperature unless otherwise specified. The average heating rate is ((surface temperature after heating-surface temperature before heating) / heating time) and the average cooling rate is ((surface temperature before cooling-surface temperature after cooling) / cooling time).

The hot rolling process is a process in which the steel material having the above composition is heated at a temperature of 1000 ° C or higher and 1200 ° C or lower and the average cooling rate from finish rolling temperature to 720 ° C after completion of finish rolling at a finish rolling temperature of 800 ° C or higher is set at 10 ° C / s, and winding at 580 캜 or higher and 720 캜 or lower.

The solvent method for producing the steel material is not particularly limited, and a known solvent method such as a converter, an electric furnace or the like may be employed. Further, secondary refining may be performed in a vacuum degassing furnace. After that, it is preferable to make slabs (steel materials) by a continuous casting method because of productivity and quality problems. In addition, a slab may be formed by a known casting method such as ingot casting and blooming, a thin slab playing method, or the like.

Heating temperature of steel material: 1000 ℃ or more and 1200 ℃ or less

In the present invention, it is necessary to heat the steel material before the rough rolling to make the steel structure of the steel material 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 1000 캜, the 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 1200 ° C, the production of nitride containing coarse Ti is promoted, and the number density of inclusions exceeding 5.0 μm is increased. Therefore, the heating temperature of the steel material was set to 1000 ° C or higher and 1200 ° C or lower. And preferably 1020 DEG C or more and 1150 DEG C or less. The conditions for the rough rolling of the rough rolling after the heating are not particularly limited.

Finishing rolling temperature: 800 ℃ or more

When the finishing rolling temperature is lower than 800 캜, ferrite transformation is initiated during finishing rolling to form a structure in which ferrite grains are stretched and a duplex grain microstructure in which ferrite grains are partially grown. Therefore, the plate thickness accuracy at the time of cold rolling is adversely affected, and the cementite is not finely dispersed in the steel structure before annealing. Therefore, the finishing rolling temperature should be 800 ° C or higher. Preferably 820 DEG C or more. Further, if the finish rolling temperature is excessively high, it is preferable that the finish rolling temperature is 940 占 폚 or less because the surface properties are deteriorated by biting scale.

When the average cooling rate from the finish rolling temperature to 720 占 폚 is 10 占 폚 / s or more

After finish rolling, it is cooled to just above the coiling temperature by forced cooling. There is a limitation in the length of the forced cooling zone (run-out table), and when the temperature is cooled to less than 10 ° C / s, the desired coiling temperature is not attained. Therefore, the average cooling rate is set to 10 ° C / s or more. The upper limit is not particularly limited, but is substantially 200 ° C / s. When the coiling temperature is less than 720 占 폚, the average cooling rate from 720 占 폚 to the cooling stopping temperature may be 10 占 폚 / sec or less and less than 10 占 폚 / sec.

Coiling temperature: 580 캜 or more and 720 캜 or less

And the decarburization from the scale is promoted by winding, thereby reducing the C concentration in the surface layer portion of the steel sheet. If the coiling temperature is lower than 580 占 폚, decarburization does not proceed and the hardness of the surface layer portion of the steel sheet does not decrease. On the other hand, if the coiling temperature exceeds 720 占 폚, an internal oxide layer is generated in the surface layer of the steel sheet, which causes cracking during molding. Thus, the coiling temperature was set to 580 ° C or higher and 720 ° C or lower. And preferably 600 ° C or more and 690 ° C or less.

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

The pre-heating step to be carried out subsequently is a method in which the cold-rolled sheet is heated up to the Ac ₃ point or higher, which is the maximum reaching temperature, after the cold rolling step, and the cold-rolled sheet after the heating is cooled to 580 캜 at a cooling rate of 5 캜 / The time during which the cold-rolled sheet stays at Ac ₃ point or more in the heating and cooling thereof is 60 seconds or less, the dew point at the Ac ₃ point or more is -45 ° C or more and -20 ° C or less, And the time at which the cold rolled sheet stays at 530 [deg.] C or lower is 20 seconds or longer.

Heating temperature (maximum reaching temperature): Ac ₃ points or more

It is necessary to further carry out decarburization in the continuous annealing line to reduce the hardness of the surface layer portion of the steel sheet. Therefore, it is necessary to pass the continuous annealing line before the continuous hot-dip plating line (the processing carried out in the continuous annealing line is the pre-processing heating and the processing carried out in the furnace in the continuous hot-dip coating line is the annealing processing in the present invention). The Ac ₃ point is the temperature at which it becomes the austenite single phase inversion from the bimetallic of ferrite and austenite. In the treatment in the continuous annealing line, it is necessary to disperse the cementite finely by promoting bainite transformation. However, unless the formation of the ferrite phase is sufficiently suppressed, the solubility of the element (C (carbon)) is small in the ferrite phase, so that the cementite is not precipitated in the ferrite phase and the dispersed form of the cementite becomes uneven, It does not become a finely dispersed form. Therefore, it is necessary to sufficiently remove the ferrite phase in this process. Thus, the heating temperature was set to Ac ₃ point or more. The upper limit of the heating temperature is not particularly limited, but a temperature higher than 1000 占 폚 is preferably 1000 占 폚 or lower because the load caused by heat in the annealing furnace is large to shorten the service life.

Cooling speed from maximum reaching temperature to 580 ℃: 5 ℃ / s or more

When the cooling rate to 580 占 폚 is slow, the ferrite transformation starts in the cooling process, and the ferrite lattice growth proceeds. When the ferrite grains grow, the arrangement of the finely dispersed cementite is changed, and a fine martensite structure is not obtained. Thus, excessive ferrite grain growth needs to be suppressed. For this, it is necessary to perform forced cooling at an average cooling rate of 5 占 폚 / s or more from the cooling start temperature (maximum attained temperature) to 580 占 폚. Preferably 7 DEG C / s or more. The cooling stop temperature of this cooling is set to 100 占 폚 or less in order to control the average heating rate from 100 占 폚 in the next annealing step.

Ac Dew point in the temperature range of ₃ points or higher: -45 ° C or higher -20 ° C or lower

When the dew point is lower than -45 캜 at a temperature range of Ac ₃ point or more in heating and cooling, the decarburization does not progress, so that the hardness of the surface layer of the steel sheet does not decrease. On the other hand, if the above-mentioned dew point exceeds -20 占 폚, an inner oxide layer or iron oxide is generated in the surface layer of the steel sheet, so that the moldability and the surface property are impaired. Therefore, the dew point was set at -45 캜 or higher and -20 캜 or lower. Preferably -45 캜 or higher and -25 캜 or lower. The dew point in the temperature range lower than the Ac ₃ point is not particularly limited and may be suitably determined.

Ac Retention time in temperature range of ₃ points or more: 60 seconds or less

If the time during which the cold-rolled sheet is in the state of Ac ₃ point or more in the heating and cooling is long, the austenite grains are coarsened, and a steel structure in which martensite is finely dispersed can not be obtained. In the present invention, up to 60 seconds can be allowed.

Retention time in the temperature range from 440 ° C to 530 ° C: 20 seconds or more

It is possible to obtain fine martensite at the time of passing through the continuous plating line by forming fine cementite in the steel sheet in accordance with progress of bainite transformation at a temperature range of 440 DEG C or higher and 530 DEG C or lower. In the present invention, the temperature range in which the bainite transformation progresses most is a temperature range of 440 ° C to 530 ° C. Preferably from 460 DEG C to 520 DEG C. In order to proceed the bainite transformation, it is necessary to set the residence time at the above-mentioned temperature range for cooling to 20 seconds or longer, so that the residence time lower limit is 20 seconds. Preferably 30 seconds or more. The upper limit of the residence time is not particularly limited, but is preferably 900 seconds or less because it is due to facility constraints.

Further, in order to further lower the hardness of the surface layer portion of the steel sheet, the steel sheet may be passed through the continuous annealing line for carrying out the above-mentioned preheating and heating step two or more times. However, from the viewpoint of coarsening of the austenitic grains, it is preferable to set the number of times of passage to four times or less.

The subsequent annealing step is a step of heating the cold-rolled sheet after the pretreatment heating step in a condition that the average heating rate from 100 deg. C to (Ac ₃ point -10) deg. C or higher to the maximum attained temperature is 3.0 deg. C / s or higher, the heated cold-rolled sheet was cooled by an average cooling rate of 10 ℃ / s or more to 560 ℃ condition of the heating and cooling in its (Ac ₃ point 10) the time at which cold-rolled plate is less than 60 seconds residence at least ℃ , And the time during which the cold-rolled sheet stays at 440 DEG C or higher and 530 DEG C or lower in the cooling is 20 seconds or longer and 180 seconds or shorter.

Average heating rate from 100 ° C to the maximum attained temperature: 3.0 ° C / s or more

100 ° C is the temperature at which C begins to diffuse, and the finely dispersed cementite is coarsened under heating conditions in which an average heating rate of 100 ° C or more at which C or Fe is diffused is less than 3.0 ° C / s. The cementite becomes a martensite-generating site, but when the cementite is coarsened, the fine martensite can not be obtained. In order to obtain finer martensite, it is necessary to suppress the coarsening of the austenite under annealing. If the average heating rate is lower than 3.0 DEG C / s, the austenite is coarsened and the average diameter of the desired martensite phase is not obtained. As described above, the average heating rate from 100 deg. C to the maximum attained temperature was 3.0 deg. C / s or more. The preferred heating rate is 4.0 [deg.] C / s or more. Here, the maximum reaching temperature is (Ac ₃ point -10) ° C or more. The area ratio of the ferrite phase does not become 20% or less unless heating is carried out to at least (Ac ₃ point -10) ° C. The preferred maximum attained temperature is Ac ₃ point or higher.

Average cooling rate from maximum attained temperature to 560 캜: 10 캜 / s or more

In the cooling after the heating, if the cooling rate to 560 deg. C is slow, the ferrite transformation is started in the cooling process, and an excessive ferrite phase is generated. In order to avoid this, it is necessary to set the average cooling rate to 560 占 폚 at 10 占 폚 / s or more. The cooling stop temperature in this cooling is not particularly limited, but usually the cooling stop temperature is 460 to 540 占 폚. The cooling rate to the cooling stop temperature after the temperature reaches 560 占 폚 is not particularly limited, and may be 10 占 폚 / s or less and 10 占 폚 / s or less.

(Ac ₃ point -10) Dwell time in temperature range above ℃: 60 seconds or less

If the time during which the cold-rolled sheet is retained in the temperature range of (Ac ₃ point -10) ° C or more in the heating and cooling is set to exceed 60 seconds, the austenite under annealing is coarsened and fine martensite is not obtained. In view of the above, it is preferable that the time of staying at a temperature range of (Ac ₃ point -10) DEG C or higher is 60 seconds or less and 50 seconds or less.

Residence time at a temperature range of 440 ° C to 530 ° C: 20 seconds or more and 180 seconds or less

In order to promote bainite transformation and obtain a bainite structure containing fine martensite, it is necessary to keep the cold-rolled sheet for 20 seconds or more at a temperature range of 440 DEG C to 530 DEG C in cooling. On the other hand, if the retention time exceeds 180 seconds, an excessively large bainite phase is generated, and a large number of bainite phases not in contact with the martensite phase are generated. The preferred residence time is not less than 25 seconds but not more than 150 seconds.

The subsequent plating step is a step of performing plating after the annealing step to form a plating layer on the annealing plate. For example, in the case of performing the hot-dip coating commonly used for steel sheets for automobiles as a plating process, the annealing is carried out in a continuous hot-dip coating line, followed by cooling after annealing and dipping in a hot- . Further, after the plating step, an alloying step for performing the alloying treatment of the plating layer may be set as necessary.

Example 1

A steel material having a composition shown in Table 1 having a thickness of 250 mm was subjected to a hot rolling process under the hot rolling conditions shown in Table 2 to obtain a hot rolled plate and subjected to a cold rolling process under cold rolling conditions shown in Table 2 to obtain a cold rolled plate , A preheating heat treatment under the conditions shown in Table 2 was conducted in a continuous annealing line and annealing under the conditions shown in Table 2 was carried out in a continuous hot-dip plating line. Thereafter, plating treatment was carried out, and further alloying treatment was carried out if necessary. Here, the temperature of the plating bath (plating composition: Zn-0.13 mass% Al) to be dipped in the continuous hot-dip coating line is 460 ° C and the plating amount is one surface of GI material (hot-dip galvanized steel sheet) and GA material (galvannealed steel sheet) To 45 g / m &lt; 2 &gt; and the amount of Fe contained in the plating layer is in the range of 6 to 14 mass%. The Ac ₃ point was measured using a thermal expansion measuring apparatus. The Ac ₃ point was measured at an average heating rate of 5 ° C / s.

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

(i) Image of tissue observation

The area ratio of each phase was evaluated by the following method. From the steel sheet, the cross section parallel to the rolling direction (the cross section which is vertical when the steel sheet is placed and parallel to the rolling direction) is cut out to be the observation surface, and the central portion of the sheet thickness is corroded with 1% It was enlarged 2000 times with a scanning electron microscope, and 1 / 4t of sheet thickness was photographed at 10 fields. 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 martensitic phase is a structure in which carbides are not observed in the particles and observed with a white contrast. These were subjected to image analysis to separate the bainite phase, bainite phase and martensite phase, and the area ratio to the observation field was obtained. The average diameter of the martensite image The area occupied by each particle on the martensite was determined by the image analysis, and the equivalent diameter equivalent to the area was obtained. As for the portion where the martensitic phase is connected at 0.5 m or less in length, the martensite connected to the portion is regarded as two, and the respective equivalent diameter is obtained. The maximum interval length between martensite was obtained as the maximum length in the 10 field of view. The distance means the distance between the outer periphery of the martensite and the outer periphery of the neighboring martensite.

(ii) tensile test

A tensile test specimen of JIS No. 5 was prepared from the obtained steel sheet in the direction perpendicular to the rolling direction (in a direction perpendicular to the rolling direction) and subjected to a tensile test according to JIS Z 2241 (2011) five times to obtain an average yield strength (YS) Strength (TS), and total elongation (El). The crosshead speed of the tensile test was 10 mm / min. In Table 3, the mechanical properties of the steel sheet required for the steel of the present invention were set as tensile strength: 780 MPa or more and work hardening index (n value): 0.16 or more. Here, the work hardening index is a value determined according to a method defined in JIS Z 2253 (1996), and is obtained in a true strain range of 0.02 to 0.05. This is because this region is the region with the highest sensitivity to the crack generation phenomenon caused by the effect of work hardening in the press working.

(iii) Hardness test

The hardness of the steel sheet surface and inside of the steel sheet was determined by Vickers hardness test. The hardness of the surface of the steel sheet was measured from the surface of the steel sheet where the plating layer was removed by pickling at a test load of 0.2 kgf in total of 20 points to obtain an average value. The hardness inside the steel plate was measured by measuring a total of 5 points of 1 / 4t of the thickness of the section parallel to the rolling direction with a test load of 1 kgf, and the average value was obtained. If the average value of the hardness of the surface of the steel sheet is not more than 95% of the average value of the hardness inside the steel sheet (not more than 0.95 in the table), the steel sheet properties required in the present invention are obtained.

Table 3 shows the results obtained by the above.

It can be seen that all of the inventive examples have obtained a steel sheet having a tensile strength (TS) of 780 MPa or more and a high work hardening index. On the other hand, a comparative example deviating from the scope of the present invention, particularly a steel sheet in which a desired ferrite area ratio is not obtained, has a low tensile strength. When the area ratio and shape of the martensite were not desired, the work hardening index was low. In addition, in some cases where the coiling temperature or the range defined by the present invention in the continuous annealing line is not satisfied, the hardness of the surface of the steel sheet is substantially the same as the inside of the steel sheet.

Claims (7)

A high strength coated steel sheet having a steel sheet and a plating layer formed on the steel sheet,
Wherein the composition of the steel sheet comprises, by mass%, C: 0.06 to 0.18%, Si: 0.50%, Mn: 1.9 to 3.2%, P: 0.03% % Of N, 0.006% or less of N, 0.0002% or more and 0.0030% or less of B, 0.007% or more and 0.030% or less of Nb and Ti with the balance being Fe and inevitable impurities Lt; / RTI &
The steel structure of the steel sheet has a ferritic phase in an area ratio of 20% or less (including 0%), a bainite phase in an area ratio of 35% or more and 90% or less, a martensite phase in an area ratio of 10% or less contained, and also contains less than 400 / ㎜ ² inclusions which circle equivalent diameter exceeds 5.0 ㎛ in number density,
Wherein the average particle diameter of the granular martensite constituting the martensite phase is 4.0 m or less and the maximum length between martensite is 5.0 m or less,
Wherein the surface hardness of the steel sheet is 95% or less when the hardness at a position of 1 / 4t (t is the thickness of the steel sheet) in the thickness direction from the surface of the steel sheet is 100%.
[% N] -14 [% Ti] / 48? 0 (1)
[% N] in the formula (1) means the N content, and [% Ti] means the Ti content. The method according to claim 1,
0.001 to 0.3% of Mo, 0.001 to 0.3% of Mo, and 0.001 to 0.3% of Mo, respectively, in terms of% by mass of Cr, 0.001 to 0.9% Or more and 0.2% or less, and Hf: 0.001% or more and 0.3% or less, based on the total weight of the steel sheet. 3. The method according to claim 1 or 2,
Wherein said composition further comprises at least one of REM, Mg and Ca in an amount of not less than 0.0002% and not more than 0.01% in total, in mass%. 4. The method according to any one of claims 1 to 3,
Wherein the plating layer is any one of a molten plated layer and a galvannealed plated layer. A steel material having a composition according to any one of claims 1 to 3 is heated at a temperature of 1000 ° C or higher and 1200 ° C or lower and subjected to finish rolling at a finish rolling temperature of 800 ° C or higher and finish rolling to a finish rolling temperature of 720 ° C A hot rolling step of cooling at an average cooling rate of 10 DEG C / s or higher and winding at 580 DEG C or higher and 720 DEG C or lower;
A cold rolling step of cold-rolling the hot-rolled sheet after the hot-rolling step,
After the cold rolling step, the cold-rolled sheet is heated to a temperature of Ac ₃ or higher, which is the maximum reaching temperature, and the cold-rolled sheet after the heating is cooled under the condition of a cooling rate of 5 ° C / s or higher to 580 ° C, The time at which the cold rolled sheet stays at Ac ₃ point or more is 60 seconds or less, the dew point at Ac ₃ point or more is -45 ° C or more and -20 ° C or less, the cold rolled sheet remains at 440 ° C to 530 ° C A heating process step of setting the time for the heating process to 20 seconds or longer,
The cold-rolled sheet after the pretreatment heating step was heated under the conditions that the average heating rate from 100 ° C to (Ac ₃ point -10) ° C or higher was 3.0 ° C / s or higher, and the cold- , And the time during which the cold-rolled sheet stays at (Ac ₃ point -10) 占 폚 or higher in the heating and cooling thereof is set to 60 seconds or less, and 440 Lt; RTI ID = 0.0 &gt; 530 C &lt; / RTI &gt; to at least 20 seconds and less than or equal to 180 seconds,
And a plating step of performing plating after the annealing step to form a plating layer on the annealing plate. 6. The method of claim 5,
Wherein 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, Li, Ti, Be, Bi, and REM in a total amount of 0 to 3.5%, and the balance of Zn and inevitable impurities. The method according to claim 5 or 6,
Wherein the plating treatment is any one of a hot-dip galvanizing treatment and an alloying hot-dip galvanizing treatment.