KR102004077B1 - High-strength cold-rolled steel sheet, high-strength coated steel sheet, method for manufacturing high-strength cold-rolled steel sheet, and method for manufacturing high-strength coated steel sheet - Google Patents

Abstract

A high strength cold rolled steel sheet having high tensile strength (TS) of 980 MPa or more and excellent bending property, a high strength coated steel sheet, and a method of producing the same. Wherein a specific component composition and an area ratio of the ferrite phase are not less than 30% and not more than 70%, an area ratio of the martensite is not less than 30% and not more than 70%, an average grain diameter of the ferrite grains is not more than 3.5 mu m, A high strength cold rolled steel sheet having a specific steel structure such as an average aspect ratio of ferrite grains of 1.5 or less, an average aspect ratio of ferrite grains of 1.8 or less, an average grain size of martensitic grains of 3.0 mu m or less and an average aspect ratio of martensitic grains of 2.5 or less, .

Description

HIGH-STRENGTH COLD-ROLLED STEEL SHEET, HIGH-STRENGTH COATED STEEL SHEET, METHOD FOR MANUFACTURING HIGH-STRENGTH COLD-ROLLED STEEL SHEET, AND METHOD FOR MANUFACTURING 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 of producing the same, which have high tensile strength (TS) of 980 MPa or more and excellent bendability, which are useful for use in automobile skeleton members.

Recently, from the viewpoint of global environmental preservation, the automobile industry is aiming to improve fuel efficiency of automobiles in order to regulate CO ₂ emissions. In order to improve the fuel efficiency of automobiles, weight reduction of automobiles due to thinning of parts used is most effective, and therefore, the amount of high strength steel sheets used as materials for automobile parts is increasing. On the other hand, in general, the ductility and bendability of the steel sheet tend to deteriorate with the increase in the strength of the steel sheet, and such deterioration causes problems such as breakage or cracking during molding into an automobile member. Therefore, in lightening the weight of automobile parts and the like, it is required to provide a cold-rolled steel sheet having good ductility and bendability in addition to high strength.

Development of a cold rolled steel sheet having high strength and good ductility and bending property is essential, and proposals have been made so far focusing on the composition of the steel structure and the hardness of the surface layer portion.

For example, Patent Document 1 discloses a ferritic stainless steel which comprises 0.05 to 0.30% of C, 3.0% or less of Si, 0.1 to 5.0% of Mn, 0.1% or less of P, 0.02% or less of S, %, N: 0.01% or less, the balance being iron and inevitable impurities, the ferrite being a soft first phase in an area ratio of 20 to 50%, the remainder being a hard second phase, tempered martensite A technique for improving the bendability by reducing the tensile and compressive stress applied to the surface layer portion at the time of bending by making the area ratio of ferrite in the surface layer portion of the steel sheet higher than the inside by having a structure made of zite and / or tempered bainite is disclosed have.

Patent Document 2 discloses a steel sheet comprising, by mass%, C: 0.050% to 0.40%, Si: 0.50% to 3.0%, Mn: 3.0% to 8.0%, P: 0.05%, S: 0.01% A steel material having a high strength and improved ductility and impact properties is disclosed, which contains 0.001% or more and 3.0% or less of Al and 0.01% or less of Al and contains an austenite in an area ratio of 10% or more and 40% or less.

Patent Document 3 discloses a ferritic stainless steel having a composition of 0.075 to 0.300% of C, 0.30 to 2.50% of Si, 1.30 to 3.50% of Mn, 0.001 to 0.050% of P, 0.0001 to 0.0100% of S, 0.005 to 1.500% , 0.0001 to 0.0100% of N, and 0.0001 to 0.0100% of O, by controlling the hardness distribution in the thickness direction of the high strength galvanized steel sheet.

Japanese Laid-Open Patent Publication No. 2013-249502 Japanese Laid-Open Patent Publication No. 2014-25091 International Publication No. WO2013 / 018739

In the technique proposed in Patent Document 1, as shown in Fig. 1 of the embodiment of Patent Document 1, since the ferrite grain size is large and the deviation of the ferrite grain size is large, the strain may be localized in the bending process. In this case, there is a case where cracking is caused due to insufficient relaxation action of tensile stress.

The technique proposed in Patent Document 2 utilizes an austenite phase and can stably form an austenite phase from the surface layer to the surface layer of the steel sheet which is important in bending due to decarburization or a change in thermal history in the thickness direction It is very difficult to generate. Therefore, it is difficult to improve the bendability by the technique described in Patent Document 2.

The technique proposed in Patent Document 3 may also lead to cracking when the deformation is locally concentrated at the time of bending, as in Patent Document 1.

The object of the present invention is to provide a high-strength cold-rolled steel sheet, a high-strength coated steel sheet, and a method for producing the same, each of which has a high tensile strength (TS) of 980 MPa or more and excellent bendability.

As a result of intensive studies on the structure capable of increasing the strength of the cold-rolled steel sheet while having good bendability, it has been found that locally large deformation is imparted to the region where cracks have occurred on the surface of the steel sheet of the bend-machined portion. From this knowledge, there is a possibility that the problem can be removed stably if the deformation during the bending process can be uniformly distributed over the entire bending process. As a result, it has been found that the problems of the present invention can be solved when the work hardening index is more than a predetermined value after the uniform elongation is secured to a desired level. This is because the portion to which the deformation has been introduced is processed and cured, so that the deformation is sequentially introduced to the portion where the deformation is not introduced. Next, as a result of studying a constitution for increasing the uniform elongation and the work hardening index, it has been found that by making a steel structure which tends to uniformly disperse fine martensitic phases around the fine ferrite-like ferrite grains, It is found that the softness and the work hardening of the ferrite-rich ferrite can be fully utilized. The present invention has been completed on the basis of the above knowledge, and its gist is as follows.

[1] A ferritic stainless steel comprising, by mass%, C: not less than 0.07% but not more than 0.17%, Si: not more than 0.3%, Mn: not less than 2.2% but not more than 3.0%, P: not more than 0.03%, S: not more than 0.005% : 0.0060% or less, Mo: 0.07% or more and 0.50% or less, Cr: 0.001% or more and 0.4% or less and satisfying the following expression (1), the balance being Fe and inevitable impurities, The area ratio of the martensite is not less than 30% and not more than 70%, the average grain size of the ferrite grains is not more than 3.5 mu m, the standard deviation of the grain size of the ferrite grains is not more than 1.5 mu m, The mean aspect ratio of the martensiticite is not more than 1.8, the mean particle size of the martensiticite is not more than 3.0 m, the average aspect ratio of the martensiticite is 2.5 or less, and the length of the grain boundary between the connected martensiticite satisfies the following formula (2) Of the area ratio of the martensite is 10 % Or less, and a tensile strength of 980 MPa or more.

% [Mo] + [% Cr] > = 3.15 (1)

[% C], [% Si], [% Mn], [% Mo] and [% Cr] in the formula (1) indicate the content of C, Si, Mn, Mo and Cr in mass% .

L1? 0.2? L2 (2)

In the formula (2), L1 represents the length of the grain boundary between the connected martensite trenches, and L2 represents the circumference length of the martensite trench having a large grain size in the connected martensite trench.

[2] The steel sheet according to any one of the above items [1] to [4], wherein the composition further comprises at least one of 0.001 to 0.3% V, 0.001 to 0.1% Ti, and 0.001 to 0.08% Is a high strength cold rolled steel sheet according to [1].

[3] A high-strength cold-rolled steel sheet as described in [1] or [2], wherein the high-strength cold-rolled steel sheet contains, by mass%, 20.0% or less of Fe and 0.001% or more and 1.0% or less of Al, At least one selected from the group consisting of Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi and REM in a total amount of not less than 3.5% , And the balance of Zn and inevitable impurities.

[4] The high strength plated steel sheet according to [3], wherein the plated layer of the high strength plated steel sheet is a molten plated layer or a galvannealed plated layer.

[5] A steel material having the composition described in [1] or [2], which is rolled at a temperature of not lower than 1050 캜 and not higher than 1300 캜 and at a finish rolling temperature of not lower than 800 캜, A cold rolling step of cold-rolling the hot-rolled sheet after the hot-rolling step; a step of cooling the cold-rolled sheet after the cold-rolling step at a temperature in the range of 100 DEG C to 825 DEG C or higher, , Cooling the cold-rolled sheet heated to the maximum reaching temperature to a temperature of 560 占 폚 at an average cooling rate of 12 占 폚 / s or higher, and setting the time of staying at 200 占 폚 to 520 占 폚 for 30 seconds or longer A first annealing step, and an annealing step after the first annealing step is heated to a maximum reaching temperature of 720 DEG C or more and 820 DEG C or less, and the annealed sheet heated to the maximum reaching temperature is heated to 560 DEG C Method of manufacture of a high strength cold rolled steel sheet having a second annealing step of the mean cooling rate and the cooling at 12 ℃ / s or more conditions, the residence time in the temperature range of less than 500 ℃ 200 ℃ less than 75 seconds.

[6] A high strength cold rolled steel sheet produced by the manufacturing method as described in [5], which contains Fe in an amount of not more than 20.0% and Al in an amount of not less than 0.001% and not more than 1.0%, and further contains Pb, Sb, Si, Sn , At least one selected from Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi and REM in a total amount of not less than 0% and not more than 3.5% And a plating step of forming a plating layer made of inevitable impurities.

[7] The method for producing a high strength coated steel sheet according to [6], wherein the plating layer is a hot-dip coating layer or an alloyed hot-dip plating layer.

According to the present invention, it is possible to obtain a high strength cold rolled steel sheet and a high strength coated steel sheet which are suitable for applications such as automobile structural members and have good ductility and bendability. The present invention improves the weight and reliability of automotive parts.

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

<High strength cold rolled steel plate>

Composition

The high strength cold rolled steel sheet according to the present invention is characterized by containing, by mass%, at least 0.07% C, at most 0.17% Si, at most 0.3% Si, at least 2.2% 0.08% or less, N: 0.0060% or less, Mo: 0.07% or more and 0.50% or less, and Cr: 0.001% or more and 0.4% or less. In the following description of each component, "%" means "% by mass".

C: 0.07% or more and 0.17% or less

C is an element that hardens martensite and contributes to the substantial increase in strength of the steel sheet. In order to obtain the tensile strength required in the present invention: 980 MPa or more, the C content needs to be 0.07% or more. On the other hand, if the C content exceeds 0.17%, the area ratio of the martensite excessively increases, and the work hardening of the ferrite phase can not be utilized, and the ductility and bendability are deteriorated. Therefore, the C content is 0.07% or more and 0.17% or less. The preferred C content for the lower limit is 0.08% or more, and the preferable C content for the upper limit is 0.15% or less.

Si: less than 0.3%

Si is an element that makes it easy to generate a ferrite phase. If the Si content is excessive, coarse ferrite grains remain during the annealing, and a fine and regular ferrite phase is not obtained, and the bendability is lowered. Therefore, in the present invention, the Si content needs to be less than 0.3%. The Si content is preferably 0.25% or less. The lower limit is not specifically defined, but Si of 0.01% may inevitably be incorporated into the steel.

Mn: not less than 2.2% and not more than 3.0%

Mn is an effective element for removing coarse ferrite grains contained in the steel structure during annealing and increasing microstructure to produce fine ferrite grains in the retention process. On the other hand, if it is contained excessively, ferrite phase formation is inhibited during the cooling and holding process, and ductility and bendability are deteriorated. From the above viewpoint, the Mn content is 2.2% or more and 3.0% or less. A preferable Mn content is 2.3% or more for the lower limit, and a preferable Mn content is 2.8% or less for the upper limit.

P: not more than 0.03%

P is segregated at the grain boundary, which causes grain boundary cracking at the time of bending, and therefore it is desirable to reduce the P content as much as possible. In the present invention, the P content can be allowed up to 0.03% or less. The preferable P content is 0.02% or less. The P content is preferably reduced as much as possible, but 0.001% of the P content may be inevitably incorporated.

S: not more than 0.005%

S exists as an inclusion such as MnS in the steel. This inclusion becomes an inclined extension to the wedge-like shape by rolling, resulting in a remarkable adverse effect on the bendability. 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. Although it is preferable to reduce the S content as much as possible, 0.0001% in the production may be inevitably incorporated.

Al: 0.08% or less

When Al is added as a deoxidizer at the stage of steelmaking, the Al content is preferably 0.02% or more. On the other hand, if the Al content exceeds 0.08%, the adverse effect on the bendability becomes present due to the influence of inclusions such as alumina. Therefore, the Al content should be 0.08% or less. Preferably 0.07% or less.

N: 0.0060% or less

N has negative effects on endurance. If the N content exceeds 0.0060%, the influence of deterioration over time with respect to ductility and bendability can not be ignored. Therefore, the upper limit of the N content is set to 0.0060%. The preferable N content is 0.0050% or less.

Mo: 0.07% or more and 0.50% or less

Like Mn, Mo is an effective element for increasing the quenching property to produce a fine and uniform ferrite phase. To obtain this effect, the Mo content needs to be at least 0.07%. On the other hand, if the Mo content exceeds 0.50%, the area ratio of the martensite deviates from the range required by the present invention, so that ductility and bendability are lowered. From the above, the Mo content is 0.07% or more and 0.50% or less. The preferable Mo content for the lower limit is 0.07% or more, and the preferable Mo content for the upper limit is 0.30% or less.

Cr: not less than 0.001% and not more than 0.4%

Cr, like Mn and Mo, is an element that has an effect of increasing quenching. In order to obtain this effect, the Cr content needs to be at least 0.001%. On the other hand, when the Cr content exceeds 0.4%, the surface properties of the steel sheet are adversely affected, and the chemical conversion treatment and plating quality are deteriorated. Therefore, the Cr content is 0.001% or more and 0.4% or less. A preferable Cr content for the lower limit is 0.005% or more, and a preferable Cr content for the upper limit is 0.3% or less.

In the composition of the present invention, the C content, the Si content, the Mn content, the Mo content, and the Cr content satisfy the following expression (1).

% [Mo] + [% Cr] &gt; = 3.15 (1)

[% C], [% Si], [% Mn], [% Mo] and [% Cr] in the formula (1) indicate the content of C, Si, Mn, Mo and Cr in mass% .

In the present invention, in order to obtain a fine and regular ferrite phase, it is necessary to remove coarse ferrite lips during annealing to secure quench for suppressing ferrite transformation in cooling after annealing. From the above viewpoint, the coefficient of each element of the formula (1) was obtained from the result of examining the effect of each element. When the left side of the formula (1) is less than 3.15, fine ferrite phases can not be obtained due to the coarse ferrite grains remaining or the ferrite grains growth at high temperature due to lack of quenching. On the other hand, if the left side of equation (1) exceeds 4.30, it may be difficult to obtain a ferrite phase. Therefore, the left side of equation (1) is preferably 4.30 or less.

These are essential components of the composition of the high-strength cold-rolled steel sheet of the present invention. The component composition may contain, as an optional component, at least one member selected from V, Ti and Nb as an optional component.

V: at least 0.001% and not more than 0.3%, Ti: at least 0.001% and not more than 0.1%, Nb: at least 0.001% and not more than 0.08%

The above elements are mainly dispersed finely as precipitates, and thus contribute to the enhancement of the strength of the steel sheet. On the other hand, when these elements are contained excessively, they remain as coarse carbides without being dissolved at the time of heating the slab. Coarse carbides adversely affect bendability. From the above viewpoint, the V content is 0.001% or more and 0.3% or less, the Ti content is 0.001% or more and 0.1% or less, and the Nb content is 0.001% or more and 0.08% or less.

The components other than the above essential components and optional components are Fe and inevitable impurities. Inevitable impurities include components added inevitably in addition to the components that are inevitably incorporated during production, insofar as the effects of the present invention are not impaired for imparting desired characteristics. When the content of the arbitrary component is less than the lower limit value, it is assumed that the optional component is contained as an inevitable impurity.

River structure

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 is 30% or more and 70% or less, the area ratio of the martensite is 30% or more and 70% or less and the average grain size of the ferrite grains (Mean grain size of the martensite) of 3.0 탆 or less, and the average grain size of the ferrite grains is 3.0 탆 or less, and the mean deviation of the mean grain size of the ferrite grains is 1.5 탆 or less, Of the total martensite is 2.5 or less and the sum of the area ratios of the martensitic phases satisfying the formula (2) is the area ratio between the connected martensitic phases (the total area ratio of the total martensitic phases ).

Ferritic phase: 30% or more and 70% or less, martensitic phase: 30% or more and 70% or less

In the present invention, the work hardening of the ferrite phase is realized by the composite structure of the ferrite phase and the martensite phase. In this composite structure, there is a tendency that the martensitic phase is present around the ferrite-like ferrite grains. By becoming a steel structure with this tendency, the ferrite phase can be processed and hardened. If the area ratio of any of the ferrite phase and the martensite phase is excessively large, a desired structure can not be obtained. From this viewpoint, the area ratio of the ferrite phase is 30% or more and 70% or less, and the area ratio of the martensite is 30% or more and 70% or less. A preferable area ratio of the ferrite phase to the lower limit is 35% or more, and a preferable area ratio of the ferrite phase to the upper limit is 65% or less. The area ratio of the martensite phase to the lower limit is preferably 35% or more, and the area ratio of the martensite phase to the upper limit is preferably 65% or less.

Ferrite average particle diameter of 3.5 mu m or less, standard deviation of ferrite particle diameter of 1.5 mu m or less, average aspect ratio of ferrite lips: 1.8 or less

If the ferrite lips are in coarse or coarse state, all of the ferrite lips are not uniformly worked and cured. In addition, even when the ferrite lips are in an expanded form, the work hardening behavior is adversely affected. Therefore, in order to obtain a steel structure having a high work hardening index, it is necessary to make ferrite lips which are fine and have no deviation in their diameters. Therefore, the average grain size of the ferrite grains is 3.5 mu m or less, and the standard deviation of the ferrite grain size and the average aspect ratio of the ferrite grains are 1.5 mu m or less and 1.8 or less, respectively. Preferably, the average grain size of the ferrite grains is not more than 3.0 mu m, and the standard deviation of the ferrite grain size and the average aspect ratio of the ferrite grains are not more than 1.0 mu m and not more than 1.5, respectively. The term "coarse grains" as defined in the present invention refers to a group of grains having an average aspect ratio of more than 1.8 and a deviation in ferrite grain size (a standard deviation of the ferrite grain size is more than 1.5 μm). The sizing means that the average aspect ratio is 1.8 or less and the standard deviation is 1.5 占 퐉 or less.

Wherein the average particle size of the martensite is 3.0 m or less, the average aspect ratio of the martensiticite is 2.5 or less, and the length of the grain boundaries between the connected martensitic phases satisfies the following formula (2): martensite Percentage of area ratio: Not more than 10%

As described above, a high work hardening of the ferrite phase can be obtained by bringing many ferrite grains into contact with the martensitic phase. Therefore, when the existing martensitic tripping is coarse, a desired structure is not obtained because the distribution of the martensitic trip is localized. In addition, even if the martensitic phases are dispersed in a fine martensitic phase, if the martensitic phases are connected to each other, the work hardening is reduced. For this reason, the average particle size of the martensiticite is limited to 3.0 mu m or less for the fine martensite. In order to specify that the martensitic phases are hardly connected to each other, the total area of the martensitic phase of the total area ratio of the martensitic phases satisfying the following formula (2) Rate is 10% or less and the average aspect ratio of the martensiticide is 2.5 or less. Preferably, the average particle size is 2.5 占 퐉 or less, the ratio is 5% or less, and the average aspect ratio is 2.0 or less.

L1? 0.2? L2 (2)

In the formula (2), L1 represents the length of the grain boundary between the connected martensite trenches, and L2 represents the circumference length of the martensite trench having a large grain size in the connected martensite trench.

Here, the ratio of the area ratio, the average grain size, the standard deviation, the average aspect ratio of each phase, and the area ratio of the total area ratio of the martensiticite satisfying the above formula (2) to the area ratio of the martensitic phase is derived by the following method.

First, 20 samples are taken from the widthwise center position at intervals of 50 m in the longitudinal direction of the coil from cold-rolled steel sheets having a length of about 1000 m. Next, the observation field of view of each sample is set to 10, and the steel structure is observed in each field of view, and an average value of the area ratio of the ferrite phase in 10 fields of view and the area ratio of the matting phase in 10 fields of view is calculated . Thereafter, an average value of the area ratio of the ferrite phase and an average value of the martensite in the 20 samples are calculated, and the average value is defined as the area ratio of the ferrite phase and the area ratio of the martensite phase.

50 to 100 ferrite grains were randomly selected in observation at 1 field in each sample, and the average grain size of these ferrite grain sizes (diameter when the ferrite grains were circularly approximated), the average value of the aspect ratios of the ferrite grains (Average aspect ratio) and standard deviation of the ferrite grain size are derived. Subsequently, an average value in one sample (10 fields of view) was calculated and the same was also conducted in the other 19 samples to calculate an average value in all of the samples (20), and the average value of all the samples was regarded as a ferrite average particle size, The standard deviation, and the average aspect ratio of the ferrite lips. The average particle size and the average aspect ratio of the martensitic trip are calculated in the same manner as the average particle size of the ferrite grains and the average aspect ratio of the ferrite grains. The ratio of the total area ratio of the martensitic grains satisfying the above formula (2) to the total area ratio of the martensite between the connected martensitic trips is determined by the following method. Here, the connected martensititan manganese triplet indicates that the grain boundaries are connected to each other. In short, in the connected martensitic trip, the grain boundary is a part of the circumference of one of the martensite, and a part of the circumference of the other martensite. The length of this grain boundary corresponds to &quot; the length of the grain boundary between connected martensite trenches &quot;. When the circumferential length of the one side of the mattenzaitrip is compared with the circumferential length of the other side of the mattenzaitrip, the longer side is referred to as L2. It should be noted that in the observation at one field of view in each sample, the mattenza triplets are connected to each other, and the perimeter length of each mattenza triplet is measured for all of the connected samples, and the larger one is designated as L2, Is measured. Then, the total area ratio of the mattenzaitrip satisfying the formula (2) is calculated, and from the area ratio and the area ratio of the martensite, the total area ratio of the mattenzaitrip satisfying the above formula (2) The ratio of the area ratio of the martensite is calculated. The average value is calculated in the same manner in the entire field of view, and this average value is set as a ratio of the total area ratio of the matantiastrip satisfying the above formula (2) to the area ratio of the martensitic phase. If the connected martensitic trip satisfies the formula (2), the sum of the area ratios of the two connected martensitic trips is the area ratio of the mattenzaitrip satisfying the formula (2). In addition, if three martensite trips are connected and listed, for example, if the martensite trip is connected to the right side martensite trip and also to the left side martensite trip, (2) is satisfied in the Zai Trip and the central Martensi trip, and it is confirmed whether or not the formula (2) is satisfied in the left side martensite trip and the central martensite trip, If satisfied, the central martensite trip is a martensite trip on a martensite that satisfies (2).

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

Next, a method of manufacturing the steel sheet of the present invention will be described.

The method for manufacturing a high strength cold rolled steel sheet of the present invention has a hot rolling step, a cold rolling step, a first annealing step and a second annealing step. Each step will be described below.

Hot rolling process

The hot rolling step is a step of heating the steel material having the above composition composition at a temperature of 1050 占 폚 or higher to 1300 占 폚 or lower and winding at 500 占 폚 or higher and 700 占 폚 or lower after completion of finish rolling at a finish rolling temperature of 800 占 폚 or higher.

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

The heating temperature of the steel material should be 1050 ℃ or more and 1300 ℃ or less. The steel material thus obtained is subjected to rough rolling and finish rolling. In the present invention, it is necessary to heat the steel material prior to rough rolling to obtain a substantially homogeneous austenite phase. If the heating temperature is lower than 1050 占 폚, hot rolling can not be completed at a finish rolling temperature of 800 占 폚 or higher. On the other hand, if the heating temperature exceeds 1300 占 폚, the scale is dried to deteriorate the surface properties of the hot-rolled steel sheet, and the yield is reduced by the scale loss. Therefore, the heating temperature of the steel material is set to 1050 to 1300 占 폚. And preferably 1100 DEG C or more and 1270 DEG C or less. However, when the steel material is subjected to hot rolling and the steel material after casting is in a temperature range of 1050 ° C to 1300 ° C, or when the carbon material of the steel material is dissolved, the steel material is not directly heated You can. The rough rolling conditions are not particularly limited.

The finishing rolling temperature should be 800 ℃ or higher. When the finish rolling temperature is lower than 800 캜, the ferrite transformation starts during the finish rolling to form a structure in which the ferrite lips are stretched, and the ferrite lips partially grow, resulting in poor plate thickness precision during cold rolling, This has an adverse effect on the bendability. Therefore, the finishing rolling temperature should be 800 ° C or higher. Preferably 820 DEG C or more. The upper limit of the finishing rolling temperature is not particularly limited, but is usually 1000 占 폚 or lower in the present invention.

The coiling temperature is set to 500 캜 to 700 캜. If the coiling temperature is lower than 500 캜, the martensitic phase is excessively generated, and the deformation resistance during cold rolling is increased, adversely affecting the plate thickness accuracy. If the plate thickness accuracy is lowered, the deformation becomes localized at the time of bending and becomes a factor of crack generation. On the other hand, when the coiling temperature exceeds 700 ° C, an internal oxide layer is formed on the surface layer of the steel sheet, thereby lowering the bendability. Therefore, the range of the coiling temperature is set to 500 ° C or more and 700 ° C or less. The preferred coiling temperature is 520 DEG C or higher and 670 DEG C or lower.

Cold rolling process

The cold rolling step is a step of cold rolling a hot rolled sheet after the hot rolling step. The conditions for the cold rolling are not particularly limited, and it is preferable that the reduction rate is 30 to 80%.

Annealing process

In the manufacturing method of the present invention, the first annealing step performed after the cold rolling step and the second annealing step performed after the first annealing step are performed twice. First, the reason why annealing is required twice in the present invention will be described.

In the first annealing step (first annealing step), it is necessary to completely recrystallize from the structure expanded by cold rolling by heating to destroy the coarse ferrite grains and to generate the bainite single phase by staying after heating. Since bainite is a structure containing cementite, which becomes a nucleation site for ferrite formation during the second annealing, the ferrite grains after the second annealing become finer and more sophisticated due to the increase in water density of cementite . In order to form a bainite phase in which many cementites are dispersed, it is necessary to generate as fine austenite phase as possible during the first heating. Hereinafter, the first annealing process and the second annealing process will be described.

In the first annealing step, the cold-rolled sheet after the cold-rolling step is heated at 100 ° C to the maximum attained temperature of 825 ° C or higher at an average heating rate of 1.5 ° C / s or higher, Cooling is carried out under the condition that the average cooling rate is 12 占 폚 / s or more, and the time of staying at a temperature range of 200 占 폚 to 520 占 폚 is 30 seconds or more.

If the average heating rate from 100 deg. C to 825 deg. C or higher to the maximum attained temperature is lower than 1.5 deg. C / s, the potential to become the nucleation site of the austenite phase is restored and the austenite phase becomes coarse. If the maximum reaching temperature is lower than 825 캜, the desired austenite is not obtained and the bainite in which the fine cementite is dispersed is insufficient, and a fine ferrite phase can not be obtained at the second annealing. In view of the above, the heating conditions of the cold-rolled sheet after the cold-rolling step were set at an average heating rate from 100 占 폚 to the maximum attained temperature of 1.5 占 폚 / sec or more and a maximum attainable temperature of 825 占 폚 or more. Preferably, the heating conditions of the cold-rolled sheet after the cold-rolling are an average heating rate from 100 ° C to the maximum attained temperature of 2.1 ° C / s or more and a maximum attainable temperature of 830 ° C or more. In addition, when the average heating rate is extremely high, it is preferably 100 DEG C / s or less because it causes reverse transformation without recrystallization and the microstructure is likely to be uneven. The upper limit of the maximum attained temperature is not particularly limited, but is generally 870 占 폚 or lower in the present invention.

The reason why the cold-rolled sheet heated to the maximum reaching temperature is cooled under the condition that the average cooling rate up to 560 占 폚 is 12 占 폚 / s or more and the residence time in the temperature range of 200 占 폚 to 520 占 폚 is 30 seconds or more . In order to obtain a desired bainite structure, it is necessary to prevent ferrite transformation occurring at a high temperature. Therefore, the average cooling rate from the maximum attained temperature to 560 占 폚 needs to be 12 占 폚 / s or more. And preferably not less than 15 [deg.] C / s. Thereafter, for the purpose of promoting the bainite transformation, it is necessary to set the time of staying at a temperature range of 200 DEG C to 520 DEG C to 30 seconds or more. Preferably, the residence time in the temperature range of 300 DEG C to 500 DEG C is 30 seconds or more. The upper limit of the average cooling rate is not particularly limited. However, it is preferably 50 DEG C / s or less because the flaking of the steel sheet in the annealing line is suppressed and stabilized. The upper limit of the residence time is not particularly limited, but is preferably 300 seconds or less because the productivity is lowered.

Next, the second annealing process will be described. In the second annealing step, the annealing plate after the first annealing step is heated to a maximum attained temperature of 720 ° C. or higher and 820 ° C. or lower, and the heated cold rolled sheet up to the maximum attainable temperature is cooled at an average cooling rate of 12 ° C./s Or more, and the time of staying in the temperature range of 200 DEG C to 500 DEG C is set to 75 seconds or less.

It is necessary to make a structure containing fine ferrite and austenite at the time of heating in the second annealing process (second annealing process). When the maximum attained temperature is lower than 720 占 폚, austenite is not produced. When heated to a temperature exceeding 820 占 폚, the austenite is coarsened and the desired structure is not obtained. From the above, the maximum reaching temperature in the heating of the annealing plate after the first annealing step is 720 DEG C or higher and 820 DEG C or lower. The preferred maximum attained temperature is 720 DEG C to 810 DEG C.

Then, the cooled cold-rolled sheet up to the maximum attained temperature is cooled under the condition that the average cooling rate up to 560 占 폚 is 12 占 폚 / s or more and the time for staying in the temperature range of 200 占 폚 to 500 占 폚 is made 75 seconds or less Will be described. As in the first annealing step, when the ferrite transformation starts at a high temperature, the ferrite grains grow into a coarse and coarse grain structure. Therefore, the average cooling rate from the maximum attained temperature to 560 占 폚 is set to 12 占 폚 / s or more. The preferred average cooling rate is at least 15 ° C / s. After cooling, to transform the austenite phase to martensite, it is necessary to suppress bainite transformation after cooling. From this point of view, the time of staying in the temperature range of 200 DEG C to 500 DEG C is set to 75 seconds or less. The upper limit of the average cooling rate is not particularly limited. However, it is preferably 50 DEG C / s or less because the flaking of the steel sheet in the annealing line is suppressed and stabilized. The upper limit of the residence time is not particularly limited, but is preferably 300 seconds or less because the productivity is lowered.

After staying in the temperature range of 200 ° C or higher and 500 ° C or lower for 75 seconds or less, martensitic transformation is preferably carried out by water cooling, and cooling may be performed at a cooling rate of 30 ° C / s or higher.

&Lt; High Strength Plated Steel Sheet and Manufacturing Method Thereof &

In the high strength coated steel sheet of the present invention, a plating layer is formed on the high strength cold rolled steel sheet. Sb, Si, Sn, Mg, Mn, Ni, Fe, and Ni in an amount of not more than 20% by mass and not more than 0.001% At least one selected from the group consisting of Cr, Co, Ca, Cu, Li, Ti, Be, Bi and REM in a total amount of not less than 0% and not more than 3.5%, and the balance of Zn and inevitable impurities . The plating layer may be alloyed. When the plating layer is a hot-dip coating layer, the Fe content is less than 5.0%, and in the case of the alloyed hot-dip coating layer, the Fe content is not less than 5.0% and not more than 20.0%.

The production method of the high strength coated steel sheet can be carried out by a general method. After the plating step for forming the plating layer, the alloying treatment to be carried out if necessary may be carried out by a general method.

Example

Cold rolled steel sheets having a thickness of 250 mm and having the composition shown in Table 1 were subjected to cold rolling at a cold rolling rate of 30% or more and 80% or less by using hot rolled steel sheets under the hot rolling conditions shown in Table 2. Subsequently, the annealing process (the first annealing process and the second annealing process) was carried out in the continuous annealing line under the conditions shown in Table 2 to form a cold-rolled steel sheet (CR material) or a hot-dip galvanized Steel plates ("GI material" and "GA material") were produced. Here, the temperature is based on the surface temperature of the steel sheet measured with a multiple reflection thermometer multiple reflection thermometer. The temperature of the plating bath (plating composition: Zn-0.13 mass% Al) immersed in the continuous annealing hot dip galvanizing line is 460 ° C. and the plating amount is 45 to 65 g / m 2 per one side of the GI material and the GA material , The amount of Fe contained in the plating layer was 6 to 14 mass%, and the amount of Al was in the range of 0.001 to 1.0 mass%.

A test piece was taken from the cold-rolled steel sheet or the coated steel sheet obtained by the above, and the structure was observed and evaluated for its performance in the following manner.

(I) Image of tissue observation

The area ratio of each phase was evaluated by the following method. Twenty samples were collected from cold-rolled steel sheets and coated steel sheets of approximately 1000 m in intervals of 50 m with respect to the coil length direction. The cross section parallel to the rolling direction was cut out to be the observation plane, and the central portion of the plate was corroded with 1% or more of deviation, and enlarged to 2000 times by a scanning electron microscope to obtain a plate thickness of 1/4 t 1 / 4t (t is plate thickness)) was taken at 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 martensite phase is a structure in which carbides are not observed in the particles and observed with a white contrast rather than a ferrite phase. With respect to the observation field area, the area occupied by the obtained image was determined as the area ratio of each image, and was obtained by image analysis. In this image analysis, the average particle diameter, the standard deviation of the ferrite lips, the average aspect ratio of the ferrite lips and the martensite trip were determined from the circle equivalent diameters of the ferrite lips and the martensite trips. The aspect ratio was determined by dividing the length of the crystal grains in the rolling direction by the length of the crystal grains in the plate thickness direction. In this image analysis, the connected martensite trips are checked, and the perimeter length of each mattenzaitrip is determined for the martensitic trip, the length of the longer side is taken as L2 and the length L1 of the grain boundaries is The total area ratio of connecting martensite trips, which was more than 20% of the circumference length, was calculated and allocated from the total area ratio of martensite occupying the field of view. In Table 3, this area ratio is defined as "connected martensitic area ratio ".

(Ii) Tensile test

JIS No. 5 tensile test specimens were prepared from the obtained cold-rolled steel sheet and coated steel sheet in the direction perpendicular to the rolling direction and subjected to a tensile test according to JIS Z 2241 (2011) five times to measure tensile strength, yield strength, elongation, The uniform elongation and the work hardening index (n value) were obtained. The crosshead speed of the tensile test was 10 mm / min. The work hardening index is a value determined according to the method defined in JIS Z 2253 (1996), and the true strain range is obtained from 0.02 to 0.05.

(Iii) Bending test

The steel sheet pieces were cut at intervals of 100 m with respect to the longitudinal direction of the cold-rolled steel sheet and the coated steel sheet and the No. 3 test piece described in JIS Z 2248 was taken so that the direction perpendicular to the rolling direction was the longitudinal direction of the test piece. Test. The limiting bending radius (R / t) was obtained by dividing the radius (R (mm)) of the tip of the pressing metal when cracks were found in the bending ridgeline by the plate thickness (t (mm)). For all the samples, if R / t is 3.0 or less, the range obtained in the present invention is evaluated as "? &Quot; in Table 3.

Table 3 shows the results obtained by the above.

In all of the examples of the present invention, it is found that good bendability is obtained at a tensile strength of 980 MPa or more. The comparative example deviating from the scope of the present invention can not achieve good bending property and high strength. Particularly, as is clear from Comparative Example No. 8 and the like, which are within the scope of the present invention, and Comparative Example No. 4, in which only the second annealing process is within the scope of the present invention, only two annealing processes are all under predetermined conditions As a result, the n value is as high as 0.14 or more, and the strength ductility balance is also good at 18420 ㎫ ·%.

Claims (7)

At least 0.07% and not more than 0.17%, Si: less than 0.3%, Mn: at least 2.2% and not more than 3.0%, P: at most 0.03%, S: at most 0.005%, Al: at least 0.02% : 0.0060% or less, Mo: 0.07% or more and 0.50% or less, Cr: 0.001% or more and 0.4% or less and satisfying the following expression (1), the balance being Fe and inevitable impurities,
The area ratio of the martensite is not less than 30% and not more than 70%, the average grain size of the ferrite grains is not more than 3.5 mu m, the standard deviation of the grain size of the ferrite grains is not more than 1.5 mu m, The average aspect ratio of the lips is 1.8 or less, the average particle diameter of the martensitic phase is 3.0 m or less, the average aspect ratio of the martensitic phases is 2.5 or less, the length of the grain boundaries between the connected martensitic phases satisfies the following formula (2) Wherein the total area ratio of the ziotrips is not more than 10% of the area ratio of the martensite,
A tensile strength of 980 MPa or more,
A high strength cold rolled steel sheet having a limiting bending radius (R / t) of 3.0 or less as measured by the following measurement method:
% [Mo] + [% Cr] &gt; = 3.15 (1)
[% C], [% Si], [% Mn], [% Mo] and [% Cr] in the formula (1) represent contents of C, Si, Mn, Mo and Cr in mass% ,
L1? 0.2? L2 (2)
In the formula (2), L1 represents the length of the grain boundary between the connected martensite trenches, and L2 represents the circumference length of the martensite trench having a large grain size in the connected martensite trench.
(How to measure)
The steel sheet pieces were cut at intervals of 100 m with respect to the longitudinal direction of the steel sheet and a No. 3 test piece described in JIS Z 2248 was taken so that the direction perpendicular to the rolling direction was the longitudinal direction of the test piece and the bending test was carried out by the V- . The limiting bending radius (R / t) is obtained by dividing the radius (R (mm)) of the tip of the pressing metal when cracks are confirmed in the bending ridgeline by the plate thickness (t (mm)). The method according to claim 1,
Wherein said composition further comprises at least one member selected from the group consisting of 0.001% to 0.3% of V, 0.001% to 0.1% of Ti, and 0.001% to 0.08% of Nb, Steel plate. A high-strength cold-rolled steel sheet according to any one of claims 1 to 3,
Wherein said high-strength cold-rolled steel sheet contains, on a mass% basis, Fe: not more than 20.0% and Al: not less than 0.001% and not more than 1.0%, and further contains Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, A high strength layer having a composition layer containing at least one selected from the group consisting of Ca, Cu, Li, Ti, Be, Bi and REM in a total amount of not less than 0% and not more than 3.5%, the balance being Zn and inevitable impurities Plated steel plate. The method of claim 3,
Wherein the plating layer is a hot-dip coating layer or an alloyed hot-dip coating layer. A hot rolling step in which a steel material having the composition described in claim 1 or 2 is rolled at a temperature of 1050 占 폚 to 1,300 占 폚 and at a finish rolling temperature of 800 占 폚 or more and after finishing rolling at 500 占 폚 to 700 占 폚 and,
A cold rolling step of cold-rolling the hot-rolled sheet after the hot-rolling step,
The cold-rolled sheet after the cold-rolling step is heated at a temperature of 100 ° C to an ultimate temperature of 825 ° C or higher at an average heating rate of 1.5 ° C / s or higher, and the cold- A first annealing step of cooling at a temperature of 12 占 폚 / s or higher and a time of 30 seconds or more staying in a temperature range of 200 占 폚 to 520 占 폚;
The annealing plate after the first annealing step is heated to a maximum reaching temperature of 720 DEG C or more and 820 DEG C or less and the annealing plate heated to the maximum reaching temperature is cooled under the condition that an average cooling rate up to 560 DEG C is 12 DEG C / And a second annealing step of keeping the temperature at 200 deg. C or higher and 500 deg. C or lower within 75 seconds or less. Sb, Si, Sn, Mg, Fe, and Fe in an amount of not more than 20.0% and not more than 1.0% of Al in terms of% by mass, At least one selected from Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi and REM in a total amount of not less than 0% and not more than 3.5%, the balance being Zn and inevitable impurities And a plating step of forming a plating layer on the plating layer. The method according to claim 6,
Wherein the plating layer is a hot-dip coating layer or an alloyed hot-dip coating layer.