JP4924052B2 - High yield ratio high tensile cold-rolled steel sheet and method for producing the same - Google Patents

Description

  The present invention relates to a high-yield ratio, high-tensile cold-rolled steel sheet suitable for a material for automobile members and a method for producing the same.

  From the viewpoint of improving fuel efficiency leading to environmental conservation, there is a strong demand for reducing the strength and thickness of automotive steel sheets. Many automotive members have complicated shapes obtained by press molding, and materials that have both high strength and excellent elongation and stretch flangeability, which are indexes of workability, are required. Further, in recent years, from the viewpoint of collision safety, there is a demand for a high yield ratio (yield ratio = YS / TS) type steel plate having a higher yield strength even with the same tensile strength and exceeding 980 MPa. From the viewpoint of reducing the weight of a steel plate, further thinning is directed, and there is an increasing demand for a thin material having a thickness of 1.4 mm or less.

Conventionally, as a technique for obtaining this type of steel sheet, for example, a technique related to a cold-rolled steel sheet that preferably includes C, Si, Mn, Ti, Nb, Mo, B and preferably a bainite-based structure (Patent Document 1), A technique (Patent Document 2) relating to cold-rolled steel that balances martensite strengthening and precipitation strengthening is known in which steel added with one or more of Nb, Ti, and V is water-cooled from an α + γ region in a continuous annealing furnace. . However, in the former case, the processability is still not sufficient. In the latter case, the yield ratio is as low as 80% or less.
JP 2005-105367 A Japanese Examined Patent Publication No. 2-335013

  The present invention has been made in view of such circumstances, and is suitable for an application having a complicated cross-sectional shape at the time of pressing, such as a member for automobiles. An object of the present invention is to provide a thin high yield strength high tensile cold-rolled steel sheet having a yield ratio corresponding to impact resistance of 90% or more as a structural component and a tensile strength exceeding 980 MPa, and a method for producing the same.

As a result of intensive studies to achieve the above object, the present inventors have obtained the following knowledge.
(A) When the structure has a low dislocation density and is strengthened with fine precipitates, the strength-elongation balance is improved.
(B) When the structure is substantially single phase and strengthened with fine precipitates, the strength-elongation balance is improved.
(C) When the addition balance of C, Ti, Mo, and V is appropriately controlled, carbides in which these are combined are finely precipitated.
(D) When the proportion of V in the composite precipitate is lowered, the precipitate is coarsened, so that both elongation and stretch flangeability are lowered.
(E) If there are fine precipitates in the hot rolling stage, recrystallization is suppressed by annealing after cold rolling, and both elongation and stretch flangeability are lowered.
(F) When precipitation is suppressed by cold rolling by hot rolling, recrystallization after cold rolling easily occurs, and carbides combined with Ti, Mo, and V are finely precipitated simultaneously with recrystallization or after recrystallization.

The present invention has been completed based on these findings and provides the following (1) to ( 5 ).
(1) By mass %, C: more than 0.06 to 0.24%, Si ≦ 0.3%, Mn: 0.5 to 2.0%, P ≦ 0.06%, S ≦ 0.005% , Al ≦ 0.06%, N ≦ 0.006%, Mo: 0.05-0.5%, Ti: 0.03-0.2%, V: Over 0.15-1.2% The balance is composed of Fe and inevitable impurities, and the content of C, Ti, Mo and V satisfies the following formula (I), the ferrite phase is 95% or more by area ratio, A carbide containing Ti, Mo, and V having a diameter of less than 10 nm is dispersed and precipitated, and the carbide containing Ti, Mo, and V has Ti / Mo / V expressed in atomic% as V / (Ti + Mo + V) ≧ 0. A high-yield ratio high-tensile cold-rolled steel sheet having a tensile strength of 980 MPa or more, having an average composition satisfying 3.
0.8 ≦ (C / 12) / {(Ti / 48) + (Mo / 96) + (V / 51)} ≦ 1.5 (I)
(However, C, Ti, Mo, and V represent mass% of each component)
( 2 ) The high yield ratio high-tensile cold-rolled steel sheet according to (1) above, wherein the sheet thickness is 1.4 mm or less.
( 3 ) The high yield ratio high-tensile cold-rolled steel sheet according to (1) or (2) above, which has a hot-dip galvanized film on the surface.
( 4 ) By mass%, C: more than 0.06 to 0.24%, Si ≦ 0.3%, Mn: 0.5 to 2.0%, P ≦ 0.06%, S ≦ 0.005% , Al ≦ 0.06%, N ≦ 0.006%, Mo: 0.05-0.5%, Ti: 0.03-0.2%, V: Over 0.15-1.2% In addition, the balance consists of Fe and inevitable impurities, and C, Ti, Mo, V content melts a steel slab having a component composition that satisfies the following formula (I), finish rolling finish temperature 880 ° C or more, It is characterized in that after performing hot rolling under a coiling temperature of less than 570 ° C., after scale removal, cold rolling is performed and heated to 700 to 900 ° C. at a heating rate of 10 ° C./s or more and annealed. A high yield ratio high tensile cold-rolled steel sheet having a tensile strength of 980 MPa or more.
0.8 ≦ (C / 12) / {(Ti / 48) + (Mo / 96) + (V / 51)} ≦ 1.5 (I)
(However, C, Ti, Mo, and V represent mass% of each component)
( 5 ) The method for producing a high yield ratio high tensile cold-rolled steel sheet as described in ( 4 ) above, wherein hot-dip galvanizing is performed on the surface of the steel sheet after the annealing.

  According to the present invention, both elongation and stretch flangeability, which are indexes of workability, are excellent, the yield ratio corresponding to the collision resistance as a vehicle body structural component is 90% or more, and the high yield of a thin object having a tensile strength of 980 MPa or more. A specific high-tensile cold-rolled steel sheet and a method for producing the same can be obtained. Such a cold-rolled steel sheet is suitable for an application having a complicated cross-sectional shape during pressing, such as a member for an automobile.

  Hereinafter, the present invention will be specifically described by dividing it into a metal structure, a chemical component, and a production method.

[Metal structure]
The high-tensile steel sheet according to the present invention has a substantially ferrite single-phase structure, and carbides including Ti, Mo, and V are precipitated.

• The ferrite phase is 95% or more in area ratio. The matrix is essentially a single ferrite phase. The reason why the matrix has a ferrite single phase structure is that ferrite with a low dislocation density is effective for improving the elongation, and a single phase structure is effective for improving the stretch flangeability. This is because the effect is remarkable in a single-phase structure. However, the matrix does not necessarily have to be a ferrite single phase structure completely, and may be a ferrite having an area ratio of 95% or more. Here, the ferrite means ferrite that has been annealed after cold rolling and has been recrystallized, and does not include processed ferrite that has not been recrystallized.

-Carbide containing Ti, Mo, V Since carbide containing Ti, Mo, V becomes fine, it is effective for strengthening steel. Conventionally, TiC was mainly used as a precipitate. However, Ti has a strong tendency to form precipitates, so when it does not contain Mo and V, it is easy to coarsen and the effect on strengthening is reduced. In order to obtain the above, a precipitate is required until the workability is deteriorated. On the other hand, the composite carbide containing Ti, Mo, and V can finely precipitate and strengthen steel without degrading workability. This is presumably because the Mo and V precipitate formation tendency is weaker than that of Ti, so that it can exist stably and finely, and can be effectively strengthened with a small addition amount that does not deteriorate the workability. The effect of refinement of precipitates can be obtained only by adding Mo to Ti. However, when Ti is added in a large amount, the slab heating temperature before hot rolling must be increased to an area where special equipment of 1300 ° C. or higher is necessary. . The combination of V and C has a very low melting temperature, and can be dissolved at a normal heating temperature even if added in a large amount to increase the strength.

  In order for the carbides to exist stably and finely, the composition of the carbides influences, and Ti, Mo, V in which the carbide composition is expressed in atomic% satisfies the V ratio; V / (Ti + Mo + V) ≧ 0.3 If it becomes like this, the effect which suppresses the coarsening of a precipitate will become high, and a desired fine precipitate can be obtained. Therefore, it is required that carbides containing Ti, Mo, and V are dispersed and precipitated in a range where Ti, Mo, and V expressed in atomic% satisfy V / (Ti + Mo + V) ≧ 0.3.

  Further, by setting the average particle size of the composite carbide to less than 10 nm, the strain around the precipitate becomes more effective as the resistance of dislocation movement, and the steel can be strengthened efficiently. For this reason, it is required that the carbide containing Ti, Mo, V having an average particle size of less than 10 nm is dispersed and precipitated. More preferably, the average particle size is 5 nm or less.

It is preferable to adjust the contents of C, Ti, Mo, and V so that the atomic ratio between C and (Ti, Mo, V) in the steel is 0.8 to 1.5. Thereby, the carbide | carbonized_material containing Ti, Mo, and V becomes easy to precipitate, and formation of the fine precipitate below 10 nm becomes easy. A more preferable range of the atomic ratio is 0.8 to 1.3. In addition, when the atomic ratio 0.8 to 1.5 of C and (Ti + Mo + V) is converted to mass%, the following equation (1) is satisfied.
0.8 ≦ (C / 12) / {(Ti / 48) + (Mo / 96) + (V / 51)} ≦ 1.5 (1)
(However, in the above formula (1), C, Ti, Mo, V represent mass% of each component)

[Chemical composition]
In the present invention, as long as the above metal structure is satisfied, desired elongation and stretch flangeability and strength of 980 MPa or more can be obtained, and the chemical composition is not particularly limited, but C: more than 0.06 to 0.24% by mass% , Si ≦ 0.3%, Mn: 0.5 to 2.0%, P ≦ 0.06%, S ≦ 0.005%, Al ≦ 0.06%, N ≦ 0.006%, Mo: 0 0.05 to 0.5%, Ti: 0.03 to 0.2%, V: more than 0.15 to 1.2%, and the balance is preferably Fe and inevitable impurities. Hereinafter, these components will be described. Unless otherwise specified,% represents mass%.

・ C: more than 0.06 to 0.24%
C forms carbides and is effective for strengthening steel. However, if it is 0.06% or less, the steel is not sufficiently strengthened, and if it exceeds 0.24%, spot welding becomes difficult, so the C content is preferably more than 0.06 to 0.24%.

・ Si ≦ 0.3%
Si is an effective element for solid solution strengthening, but if added over 0.3%, C precipitation from ferrite is promoted, and coarse iron carbide tends to precipitate at the grain boundaries, resulting in reduced stretch flangeability. To do. Moreover, in the present invention, the rolling load of austenite is reduced by reducing Si that has been actively used in the past, and the production of thin materials becomes easy. However, when it exceeds 0.3%, hot rolling is performed. Since the rolling load at the time becomes large, the rolling becomes unstable and the plate shape also deteriorates. For these reasons, the Si content is preferably 0.3% or less. More preferably, it is 0.15% or less, and desirably 0.05% or less.

・ Mn: 0.5-2.0%
Mn is preferably 0.5% or more from the viewpoint of strengthening the steel by solid solution strengthening, but if added over 2.0%, segregation occurs, a hard phase is formed, and stretch flangeability deteriorates. For this reason, the Mn content is preferably 0.5 to 2.0%.

・ P ≦ 0.06%
P is effective for solid solution strengthening, but if added over 0.06%, it segregates and lowers the stretch flangeability. Therefore, it is preferably made 0.06% or less.

・ S ≦ 0.005%
The smaller the amount of S, the better, and when it exceeds 0.005%, the stretch flangeability deteriorates, so 0.005% or less is preferable.

・ Al ≦ 0.06%
Al is added as a deoxidizer. However, if it exceeds 0.06%, elongation and stretch flangeability deteriorate, so 0.06% or less is preferable.

・ N ≦ 0.006%
N is preferably as small as possible, and when it exceeds 0.006%, coarse nitrides increase and stretch flangeability deteriorates, so 0.006% or less is preferable.

Mo: 0.05-0.5%
Mo is an important element in the present invention. Adding 0.05% or more suppresses pearlite transformation, forms fine precipitates with Ti and V, and ensures excellent elongation and stretch flangeability. Steel can be strengthened. However, if added over 0.5%, a hard phase is formed and stretch flangeability is lowered, so the Mo content is preferably 0.05 to 0.5%.

Ti: 0.03-0.2%
Ti is an important element in the present invention. By forming composite carbide with Mo and V, steel can be strengthened while ensuring excellent elongation and stretch flangeability. However, if it is less than 0.03%, the effect of strengthening the steel is insufficient, and if it exceeds 0.2%, the stretch flangeability deteriorates and the slab heating temperature before hot rolling must be as high as 1300 ° C or higher. Since the carbide does not dissolve, even if it is added more than this, it cannot be effectively precipitated as a fine precipitate. Therefore, the Ti content is preferably 0.03 to 0.2%.

・ V: Over 0.15 to 1.2%
V is an important element in the present invention. As described above, the composition of the carbide has an influence on the fact that the carbide can exist stably and finely. Specifically, when the average composition of carbides satisfies V / (Ti + Mo + V) ≧ 0.3 with Ti, Mo, and V expressed in atomic%, the effect of suppressing the coarsening of precipitates becomes high, By forming fine composite carbide, steel can be strengthened while ensuring excellent elongation and stretch flangeability. In this regard, as a result of detailed investigations by the present inventors, it is possible to add a large amount of C exceeding 0.006%, and by adding a large amount of V exceeding 0.15%, the precipitation rate of V, That is, it is understood that the ratio of V that contributes to the actual formation of precipitates in the added V increases, and precipitates satisfying the condition of V / (Ti + Mo + V) ≧ 0.3 can be obtained. It was. Therefore, the V content is set to exceed 0.15%. However, if added over 1.2%, a martensite phase is formed after annealing, or center segregation appears strongly, leading to a decrease in elongation or toughness, so the V content exceeds 0.15 to 1.2%. Is preferred. Even when 1.2% of V is added, if the slab heating temperature is a normal temperature of about 1200 ° C., the carbide is completely dissolved.

[Production method]
・ Finish rolling finish temperature: 880 ° C. or higher If the finish rolling finish temperature is less than 880 ° C., the amount of accumulated strain increases due to the progress of rolling by non-recrystallization, and the rolling load increases significantly, and in the hot rolling stage Is promoted to lower the cold-rollability (cold-rollability), and recrystallization during annealing is suppressed to reduce the elongation of the cold-rolled steel sheet.

-Winding temperature: less than 570 degreeC If a fine precipitate produces | generates in a hot-rolled sheet, it will become difficult to recrystallize by annealing after cold rolling. Therefore, it is necessary to prevent carbides including Ti, Mo, and V from being precipitated during winding, and the winding temperature is set to less than 570 ° C. Desirably, it is less than 550 degreeC. When the coiling temperature is less than 400 ° C., a hard phase such as bainite or martensite is generated and the cold rolling property is lowered. The hot-rolled steel sheet produced as described above is subjected to cold rolling after scale removal by pickling or the like according to a conventional method. As described above, although there is an increasing demand for high yield ratio type high strength steel sheets with a thickness of 1.4 mm or less, the hot-rolled steel sheets obtained by the above-described method are used for the carbide in the hot rolling stage. Precipitation is suppressed and cold rolling to a thickness of 1.4 mm or less is easy. In the present invention, the rolling reduction of the cold rolling is not particularly limited, but is preferably about 40 to 80% which is a range of a conventional method. The cold-rolled cold-rolled sheet is annealed under the following conditions.

-Heating to 700-900 ° C at a heating rate of 10 ° C / s or more and annealing The heating rate during annealing was set to 10 ° C / s or more because it is necessary to recrystallize before the carbide is precipitated. is there. If it is less than 10 ° C./s, the carbide precipitates before recrystallization, so recrystallization is suppressed and elongation is lowered. Desirably, it is 30 ° C./s or more. Also, the annealing temperature is set to 700 to 900 ° C., if it is less than 700 ° C., a large amount of carbides are precipitated before recrystallization and recrystallization is suppressed and elongation is lowered, and if it exceeds 900 ° C., the austenite single phase is heated. This is because the carbide is dissolved and the strength is lowered.

・ Plating The high-tensile cold-rolled steel sheet of the present invention can be a hot-dip galvanized cold-rolled steel sheet that is a cold-rolled steel sheet that has a hot-dip galvanized film on the surface and a hot-dip galvanized film on the surface. It is. The hot-dip galvanized cold-rolled steel sheet also includes an galvannealed cold-rolled steel sheet that has been subjected to an alloying reaction after hot-dip galvanizing. When manufacturing a hot-dip galvanized cold-rolled steel sheet, it is possible to perform hot-dip galvanizing in a continuous hot-dip galvanizing line continuously after annealing, for example. Here, the hot dip galvanizing is hot dip plating whose plating film is substantially composed of Zn or hot dip plating mainly composed of Zn, and contains alloy elements such as Al, Cr, Mn in addition to Zn. Also good.

  Steel having the chemical components shown in Table 1 was heated to 1250 ° C. and hot-rolled to a sheet thickness of 4 mm at various finishing temperatures (FT) and winding temperatures (CT). The obtained steel plate was pickled and then cold rolled at a cold rolling rate of 75% to a plate thickness of 1 mm. Subsequently, continuous annealing was performed at various heating rates and heating temperatures. Table 2 shows the hot rolling and calcination conditions. The thin film produced from the obtained steel sheet was observed with a transmission electron microscope (TEM) for 10 fields of view (magnification: × 1,000,000 by photo enlargement), and the size of each precipitated carbide was analyzed by image analysis using circle approximation. The average particle size of the carbide was obtained by arithmetic and averaging. Moreover, the element which comprises each carbide | carbonized_material was analyzed using the energy dispersive X ray spectrometer (EDX). Further, the concentration of each element (Ti, Mo, V) in each carbide was calculated from the peak value, and the average composition of the Ti, Mo, V content in the carbide was obtained by arithmetic averaging to obtain the V ratio. Further, JIS No. 5 tensile test piece and hole expansion test piece were collected from the obtained steel plate. Tensile specimens were taken from the direction perpendicular to rolling. The hole expansion test was conducted in accordance with Japan Iron and Steel Federation standard JFST1001, and the hole expansion ratio (λ) was obtained. The structure was observed with a scanning electron microscope (SEM) after nital etching. The area ratio of the tissue was determined by image analysis of the SEM image obtained above. In Table 2, No. Nos. 12 and 13 were subjected to hot dip galvanization using zinc as a plating bath, and then subjected to alloying treatment to obtain a galvannealed cold-rolled steel sheet.

  As is apparent from the results shown in Table 2, the examples of the present invention satisfying the scope of the present invention have a tensile strength exceeding 980 MPa, excellent elongation and stretch flangeability, which are processability indexes, and The yield ratio corresponding to the collision resistance was 90% or more.

Claims (5)

In mass%, C: more than 0.06 to 0.24%, Si ≦ 0.3%, Mn: 0.5 to 2.0%, P ≦ 0.06%, S ≦ 0.005%, Al ≦ 0.06%, N ≦ 0.006%, Mo: 0.05 to 0.5%, Ti: 0.03 to 0.2%, V: more than 0.15 to 1.2%, the balance being Fe and inevitable impurities, C, Ti, Mo, V content has a component composition that satisfies the following formula (I), ferrite phase is 95% or more by area ratio, average particle size less than 10nm The carbide containing Ti, Mo, and V is dispersed and precipitated, and the carbide containing Ti, Mo, and V satisfies Ti / Mo / V expressed by atomic% satisfying V / (Ti + Mo + V) ≧ 0.3. A high-yield ratio high-tensile cold-rolled steel sheet having an average composition and a tensile strength of 980 MPa or more.
0.8 ≦ (C / 12) / {(Ti / 48) + (Mo / 96) + (V / 51)} ≦ 1.5 (I)
(However, C, Ti, Mo, and V represent mass% of each component) The high yield ratio high-tensile cold-rolled steel sheet according to claim 1 , wherein the thickness is 1.4 mm or less. High yield ratio high-strength cold-rolled steel sheet according to claim 1 or claim 2, characterized in that it has a hot-dip galvanized coating on the surface. In mass%, C: more than 0.06 to 0.24%, Si ≦ 0.3%, Mn: 0.5 to 2.0%, P ≦ 0.06%, S ≦ 0.005%, Al ≦ 0.06%, N ≦ 0.006%, Mo: 0.05 to 0.5%, Ti: 0.03 to 0.2%, V: more than 0.15 to 1.2%, the balance being A steel slab consisting of Fe and inevitable impurities and having a C, Ti, Mo, V content satisfying the following formula (I) is melted, and finish rolling finish temperature is 880 ° C or higher, winding temperature Tensile strength, characterized in that after hot rolling under conditions of less than 570 ° C., after scale removal, cold rolling is performed, and annealing is performed by heating to 700 to 900 ° C. at a heating rate of 10 ° C./s or more. Is a method for producing a high yield cold rolled steel sheet having a high yield ratio of 980 MPa or more.
0.8 ≦ (C / 12) / {(Ti / 48) + (Mo / 96) + (V / 51)} ≦ 1.5 (I)
(However, C, Ti, Mo, and V represent mass% of each component) The manufacturing method of the high yield ratio high tension cold-rolled steel sheet according to claim 4 , wherein hot-dip galvanizing is performed on the surface of the steel sheet after annealing.