JP4710558B2 - High-tensile steel plate with excellent workability and method for producing the same - Google Patents

Description

  The present invention relates to a high-strength steel sheet excellent in workability 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. In recent years, the strength of steel sheets has been increasing further, and those exceeding 980 MPa have been desired. Further, from the viewpoint of reducing the weight of the steel sheet, further thinning is directed, and a demand for a thin object having a thickness of 2.5 mm or less is also increasing.

  Conventionally, various steel sheets of this type have been proposed. For example, Patent Document 1 proposes a steel sheet excellent in stretch flangeability, in which a bainitic ferrite structure having a high dislocation density is generated. However, this steel sheet has a drawback that it has poor elongation because it contains a bainitic ferrite structure with a high dislocation density. In addition, strong cooling on the run-out table is inevitable for the production of bainitic ferrite, and a problem arises in strip runnability on the run-out table when manufacturing a thin product. Not suitable for production.

  Patent Document 2 proposes a steel sheet excellent in stretch flangeability in which most of the structure is polygonal ferrite and precipitation strengthening and solid solution strengthening are centered on TiC. However, in order to increase the tension to 980 MPa or more with the generally well-known precipitates used in this steel sheet, a large amount of Ti is required to be added, and precipitates having a large size are likely to be formed, resulting in poor properties. There is a drawback that it tends to be stable. Moreover, since this steel positively uses Si that increases the rolling load for improving the characteristics, the rolling load increases in the production of thin objects, and it is difficult to ensure the shape of the steel sheet.

  Patent Document 3 proposes a steel sheet having an acicular ferrite structure in which fine TiC and / or NbC are precipitated and having excellent stretch flangeability. However, since this steel sheet is a structure having a high dislocation density called acicular ferrite as well as the steel sheet proposed in Patent Document 1 described above, sufficient elongation cannot be obtained. In addition, this steel, like the steel proposed in Patent Document 2, actively uses Si for increasing the rolling load to improve the characteristics, so that the rolling load increases in the production of thin objects, and the steel plate shape is secured. Is difficult.

Patent Document 4 proposes a hot-rolled steel sheet precipitation strengthened with V carbonitride. However, although this steel sheet has a ferrite main phase in its structure, there is a concern that pearlite or the like is generated at the same time, and elongation and stretch flangeability are deteriorated. Moreover, in this technique, in order to refine the ferrite, at the time of finish rolling, rolling is performed at ₃ or more Ar points in the rolling stand one stage before the end of the tandem rolling mill row, and then average cooling of 50 ° C./second or more is performed. After cooling to a temperature of “Ar ₃ points−50 ° C.” or less at a speed, it is necessary to apply a reduction of 20% or less at the final stand, which is difficult to manufacture in a normal production line.

Moreover, the technique of patent document 5 is developed as a technique of obtaining an ultra high strength steel plate. The technique of Patent Document 5 is a technique that can disperse fine carbides composed of C, Ti, and Mo in a ferrite single phase and obtain an ultra-high-strength steel sheet that is excellent in both elongation and stretch flangeability. However, in the steel sheet described in Patent Document 5, when a large amount of C or Ti is added to obtain a tensile strength of 980 MPa or more, TiC precipitated in the slab at a normal slab heating temperature (about 1250 ° C. or less). Etc. may not be completely dissolved, and higher temperatures are required to completely dissolve them, making manufacture difficult.
JP-A-6-172924 Japanese Patent Laid-Open No. 6-200351 JP-A-7-11382 JP 2004-143518 A JP 2003-89848 A

  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 an automobile member. An object of the present invention is to provide a tensile steel sheet, particularly a high-tensile steel sheet having a strength of 980 MPa or more, and a method for producing such a high-tensile steel sheet.

As a result of intensive studies to achieve the above object, the present inventors have obtained the following knowledge.
(1) In the hot rolling stage, the base material is martensite, and by applying heat treatment such as continuous annealing, the composite fine carbide containing Ti, Mo, V precipitates while keeping the strength of the base material to be very strong. Therefore, a steel sheet having a high strength-elongation balance and stretch flangeability, which is an index of elongation characteristics with respect to strength, can be obtained.
(2) When the addition balance of C, Ti, Mo, and V is appropriately controlled, carbides in which these are combined are finely precipitated.
(3) When the proportion of V in the composite precipitate becomes low, the precipitate becomes coarse, so that both elongation and stretch flangeability are lowered.
(4) Steel added with V dissolves carbides at a lower temperature than steel added only with Ti and Mo, and fine precipitates effective for strengthening can be obtained efficiently.

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 being Fe and unavoidable impurities, C, Ti, Mo, V content have a component composition that satisfies the following formula (I), the steel sheet structure is 95% or more of martensite form an area ratio While the structure is observed, TS × El, which is the product of tensile strength (TS) and elongation (El), is a structure in which TS × El is 13000 MPa ·% or more, and is a substantially tempered martensite single-phase structure. Carbide containing Ti, Mo and V having a diameter of less than 10 nm is dispersed and precipitated, and carbide containing Ti, Mo and V Is characterized by having a composition satisfying V / (Ti + Mo + V) ≧ 0.3 in terms of Ti, Mo, V expressed in atomic%, and having excellent workability with a tensile strength of 980 MPa or more. High tensile steel plate.
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-tensile steel sheet of (1 ) above, which is a thin hot-rolled steel sheet having a thickness of 2.5 mm or less and excellent in workability having a tensile strength of 980 MPa or more.
( 3 ) A high-tensile steel sheet excellent in workability having a tensile strength of 980 MPa or more, characterized in that the high-strength steel sheet according to (1) or (2) has a hot dip galvanized coating 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% The remainder is composed of Fe and unavoidable impurities, and steel having a component composition satisfying the following formula (I) in which C, Ti, Mo, and V are contained is melted, and the finish rolling finish temperature is 880 ° C. or higher. After performing hot rolling under the conditions of a temperature of less than 400 ° C., I line annealing of 20 seconds to 24 hours at 600 to 720 ° C., and characterized by producing steel sheet according to (1) or (2) A method for producing a high-tensile steel plate having excellent workability with 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 ) A method for producing a high-tensile steel sheet having excellent workability with a tensile strength of 980 MPa or more, characterized in that hot-dip galvanizing is applied to the steel sheet after annealing in ( 4 ).

  In the present invention, the tempered martensite single-phase structure means that a small amount of other phases can be allowed in addition to the tempered martensite, and the area ratio of the tempered martensite is preferably 95% or more.

  According to the present invention, in addition to Ti and Mo, V is added in an appropriate balance, and fine carbides containing Ti, Mo and V are dispersed and precipitated, thereby obtaining a high-tensile steel plate having excellent workability. .

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

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

・ Substantially tempered martensite single phase structure:
The matrix has a tempered martensite structure. In particular, it is effective to use a tempered martensite to ensure high strength and stretch properties, and to improve the stretch flangeability, a single-phase structure should be used. This is because is effective. Although tempered martensite is inferior in elongation characteristics to ferrite structure, for example, it is easy to achieve high strength, and steel sheet excellent in TS × El balance with high elongation characteristics compared to martensite structure. Can do.

  Here, it is difficult to distinguish between tempered martensite and martensite only by observing the structure. On the other hand, as described above, as described above, in the state where martensite is not tempered, the tensile strength (TS) is high, but the elongation (El) characteristic is deteriorated, and the TS × El balance is deteriorated. Therefore, in the present invention, the structure is observed, the martensite structure is observed, and when TS × El is 13000 MPa ·% or more, the tempered martensite structure is obtained.

  That is, in the present invention, being substantially composed of tempered martensite means that a martensitic structure with an area ratio of 95% or more is observed as a steel sheet structure, and TS × El is 13000 MPa ·% or more. To do.

-Carbides containing Ti, Mo, V:
Since carbides containing Ti, Mo, and V become fine, they are effective for strengthening steel. Conventionally, it has been mainstream to use TiC containing no Mo or V as a precipitate. However, since Ti has a strong tendency to form precipitates, when it does not contain Mo and V, it is easy to coarsen, and the effect on strengthening is reduced. Therefore, precipitation until the workability is deteriorated to obtain the required strengthening amount. Things are needed. On the other hand, since the composite carbide containing Ti, Mo, and V precipitates finely, the steel can be strengthened without degrading workability. This is because Mo and V precipitate formation tendency is weaker than Ti, so it can exist stably and finely without becoming coarse precipitates that do not contribute to strengthening, and it is effective in a small amount that does not reduce workability. This is thought to be because it can be strengthened.

  The effect of refinement of precipitates can be obtained only by adding Mo to Ti. However, when Ti is added at a level at which a tensile strength of 980 MPa or more can be obtained only by carbide containing Ti and Mo, as described above, general hot rolling is performed. In some cases, a high temperature that exceeds the previous heating temperature is required, and in order to increase the temperature, for example, special equipment is required, resulting in an increase in cost. On the other hand, the combination of V and C has a very low melting temperature, and can be easily dissolved at a normal heating temperature even if added in a large amount in order to obtain a high strength of 980 MPa or more.

  Conventionally, when a large amount of V is added in addition to Ti and Mo, the elongation tends to decrease. Therefore, the addition of V has been suppressed to a relatively low range. However, as a result of detailed examinations by the present inventors, the amount of precipitation of V increases as the amount of addition of V is increased, and the added V is sufficiently precipitated as carbides. It has been found that by precipitating, high strength can be achieved while securing sufficient elongation.

  In order for the carbide to exist stably and finely, the composition of the carbide influences so that Ti, Mo, V expressed in atomic% satisfy V / (Ti + Mo + V) ≧ 0.3. If it becomes, the effect which suppresses the coarsening of a precipitate will become high, and a desired fine precipitate can be obtained. Therefore, in the present invention, 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 distortion around the precipitate becomes more effective as the resistance of dislocation movement, and the steel can be strengthened efficiently. For this reason, in this invention, it is required that the carbide | carbonized_material containing Ti, Mo, and V with an average particle diameter of less than 10 nm has precipitated. More preferably, the average particle size is 5 nm or less. Furthermore, Ti, Mo, and V are included by adjusting 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. Since carbides easily precipitate and formation of fine precipitates of less than 10 nm is facilitated, V and V so that the atomic ratio between C and (Ti + Mo + V) in the steel is in the range of 0.8 to 1.5. It is preferable to add Ti and Mo. A more preferable range of the atomic ratio is 0.8 to 1.3. When the atomic ratio 0.8 to 1.5 of C and (Ti + Mo + V) is converted into mass%, the following formula (I) is satisfied.
0.8 ≦ (C / 12) / {(Ti / 48) + (Mo / 96) + (V / 51)} ≦ 1.5 (I)
However, in said Formula (I), C, Ti, Mo, and V represent the mass% of each component.

[Chemical composition]
In the present invention, as long as the metal structure is satisfied, desired elongation and stretch flangeability and tensile strength of 980 MPa or more can be obtained, and the chemical composition is not particularly limited, but by mass%, C: more than 0.06 to 0.00. 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%, with the balance being Fe and inevitable impurities, C, Ti, Mo It is preferable that the V content has a component composition satisfying the following formula (I).
0.8 ≦ (C / 12) / {(Ti / 48) + (Mo / 96) + (V / 51)} ≦ 1.5 (I)
However, in said Formula (I), C, Ti, Mo, and V represent the mass% of each component.
Hereinafter, each of these components will be described.

C: Over 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%. . More preferably, it is 0.07% or more, and in particular, in order to obtain a tensile strength of 1100 MPa or more, 0.1% or more is desirable.

Si: 0.3% or less 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 precipitates at grain boundaries. It becomes easy and stretch flangeability falls. In the present invention, the rolling load of austenite is reduced by reducing Si that has been positively used for high-tensile steel sheets in the past, and the manufacture of thin objects is facilitated. If Si is added in excess of 0.3%, rolling of a material of 2.5 mm or less becomes unstable, and the plate shape is also deteriorated. 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 to 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 and stretch flangeability deteriorates. For this reason, the Mn content is preferably 0.5 to 2.0%.

P: 0.06% or less P is effective for solid solution strengthening, but if added over 0.06%, segregation occurs and the stretch flangeability is deteriorated, so 0.06% or less is preferable.

S: 0.005% or less S is preferably as small as possible, and if it exceeds 0.005%, the stretch flangeability deteriorates, so 0.005% or less is preferable.

Al: 0.06% or less Al is added as a deoxidizer. However, if the Al content in the steel exceeds 0.06%, elongation and stretch flangeability deteriorate, so 0.06% or less is preferable. There is no particular lower limit, but in order to sufficiently obtain the effect as a deoxidizer, the Al content is preferably 0.01% or more.

N: 0.006% or less N is preferably as small as possible. If 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 ferrite transformation and pearlite transformation during cooling after hot rolling, and a martensitic structure can be easily obtained. Further, by forming fine precipitates with Ti and V, the steel can be strengthened while ensuring excellent elongation and stretch flangeability. However, even if added over 0.5%, the effect is saturated and only the cost is increased, 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. If it exceeds 0.2%, the stretch flangeability deteriorates, and the slab heating temperature before hot rolling must be 1300 ° C or higher. Since the carbide does not dissolve, it cannot be effectively precipitated as a fine precipitate even if it is added more than this. 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, in order for the carbide to exist stably and finely, the composition of the carbide influences, and the composition of the carbide is Ti / Mo / V expressed in atomic% as V / (Ti + Mo + V) ≧ 0. When 3 is satisfied, the effect of suppressing the coarsening of the precipitate becomes high, and a desired fine precipitate can be obtained. In addition, C is included in a large amount exceeding 0.06%, and by adding a large amount of V, the precipitation efficiency of V is increased, and a precipitate satisfying V / (Ti + Mo + V) ≧ 0.3 can be obtained. Turned out to be.

  Specifically, by containing C in a large amount exceeding 0.06% and adding V exceeding 0.15% to obtain a very good precipitation efficiency, the composition of the carbide is atomic%. Ti, Mo, and V represented by V / (Ti + Mo + V) ≧ 0.3, forming fine composite carbide with Ti and Mo, and ensuring excellent elongation and stretch flangeability It turns out that it can be strengthened. However, when the content of V exceeds 1.2%, center segregation appears strongly, leading to a decrease in elongation and toughness. Therefore, the V content is preferably more than 0.15 to 1.2%. More preferably, it is 0.2 to 1.2%. Even when 1.2% of V is contained, if the slab heating temperature is a normal temperature of about 1200 ° C., the carbide is completely dissolved.

0.8 ≦ (C / 12) / {(Ti / 48) + (Mo / 96) + (V / 51)} ≦ 1.5
(However, C, Ti, Mo, and V in the formula represent mass% of each component)
In the present invention, the balance of addition of C, Ti, Mo and V is very important. With the C, Ti, Mo, and V contents in the predetermined ranges, (C / 12) / {(Ti / 48) + (Mo / 96) + (V / 51)} is set to 0.8 to 1. By setting it to 5, a large amount of carbide having a composition satisfying V / (Ti + Mo + V) ≧ 0.3 for Ti, Mo, and V can be finely dispersed, that is, with an average particle diameter of less than 10 nm, and can be easily dispersed and precipitated. When (C / 12) / {(Ti / 48) + (Mo / 96) + (V / 51)} is less than 0.8, the precipitate becomes coarse and a strength of 980 MPa or more cannot be obtained stably. On the other hand, when (C / 12) / {(Ti / 48) + (Mo / 96) + (V / 51)} is more than 1.5, C becomes excessive and pearlite is generated, so that the formability is lowered. .

[Production method]
In the present invention, steel having the above component composition is melted and hot rolled at a finish rolling finish temperature of 880 ° C. or more and a coiling temperature of less than 400 ° C., then at 600 to 720 ° C. for 20 seconds to 24 hours. Annealing is performed. In general, steel having the above composition is cast into a slab according to a conventional method after melting, and is subjected to hot rolling as it is or after being cooled and reheated. Even in the case where the slab is reheated, in the present invention, the heating temperature of the slab may be about 1200 to 1250 ° C., which is the range of ordinary methods.

Finish finish rolling temperature: 880 ° C. or higher Finish finish rolling temperature is important for securing elongation and stretch flangeability and reducing rolling load. If the temperature is lower than 880 ° C., the amount of accumulated strain increases due to the progress of rolling due to non-recrystallization, and it becomes difficult to hot-roll thin materials due to a significant increase in rolling load.

-Winding temperature: less than 400 ° C. When the winding temperature is 400 ° C. or higher, bainite and ferrite are produced and the martensite base material of the present application cannot be obtained. In addition, since the water used at the time of cooling after hot rolling will not evaporate and water may accumulate in a coil and a steel plate may corrode if a coiling temperature becomes too low, it is preferable to set it as 100 degreeC or more.

-Annealing temperature: 600-720 ° C
The purpose of annealing in the present invention is to temper martensite, which is a base material, to recover ductility while ensuring strength, and at the same time, to precipitate composite carbide containing Ti, Mo and V. When the annealing temperature is less than 600 ° C., neither tempering of martensite nor precipitation of composite carbides is sufficient. On the other hand, when it exceeds 720 ° C., a γ phase is generated and transformed into soft ferrite during cooling. 720 ° C.

Annealing time: 20 seconds to 24 hours The annealing time is 20 seconds to 24 hours. If the annealing time is less than 20 seconds, the tempering of martensite and the precipitation of composite carbides are not sufficient. On the other hand, if the annealing time exceeds 24 hours, the tempering progresses and the precipitate becomes coarser, so the annealing time is 20 seconds to 24 hours.

  In addition, annealing may be performed by any of the usual annealing methods such as a continuous annealing line, a continuous melting galvanizing line, and box annealing, and any of these can obtain a desired effect.

  The high-tensile steel plate of the present invention includes a hot-dip galvanized steel sheet formed on the surface thereof. For example, annealing after hot rolling can be performed in a continuous melting galvanizing line, and hot dip galvanizing or further alloying can be performed continuously after annealing. Since the high-tensile steel sheet of the present invention has good workability, good workability can be maintained even when a hot dip galvanized film is formed. Here, the hot dip galvanizing is hot dip plating mainly composed of zinc and zinc, and includes those containing alloy elements such as Al and Cr in addition to zinc. In addition, the plating may be performed as it is or after the plating. In addition, the high-tensile steel plate of the present invention can be made into a thin material having a thickness of 2.5 mm or less, which has been strongly demanded in the past without problems such as securing the shape of the steel plate.

Example 1
After melting a slab having the composition shown in Table 1 and hot rolling to a plate thickness of 1.8 mm at a slab heating temperature (SRT) of 1250 ° C., a hot rolling finishing temperature (FDT) of 900 ° C., and a winding temperature (CT) of 200 ° C. , Continuous annealing was performed at 700 ° C. for 120 seconds.

  After pickling the obtained steel plate, the thin film produced from the steel plate was observed with a transmission electron microscope (TEM), and the size of the precipitate was measured. The composition of Ti, Mo, and V in the precipitate is determined by analysis with an energy dispersive X-ray spectrometer (EDX) equipped in the TEM, and the V ratio (atomic ratio) of the precipitate = V / (Ti + Mo + V) (formula Among them, Ti, Mo, and V are atomic%). The structure was observed with a scanning electron microscope (SEM). As for the microstructure after annealing, as described above, the martensitic structure is observed in an area ratio of 95% or more, and the case where TS × El is 13000 MPa ·% or more is tempered martensite, and less than 13000 MPa ·%. The case was simply martensite.

Moreover, a JIS No. 5 tensile test piece and a hole expansion test piece were collected from the obtained steel plate. Tensile test specimens were taken from the vertical direction of rolling, and for the hole expansion test, a specimen having a hole with a clearance (one side) punched out at 12.5% of the plate thickness was prepared at the center of a 130 mm square steel plate. The hole diameter d at the time when the crack penetrated the steel plate was measured by pushing up with a 60 ° conical punch from the opposite direction of the burr side of the punched hole, and the hole expansion ratio λ was calculated from the following equation.
λ (%) = {(d−10) / 10} × 100
Table 2 shows the A value, the structure after hot rolling, the structure after annealing, the average particle size of precipitates, the composition of precipitates (V ratio), the yield strength (YS), the tensile strength (TS), the elongation (El), and the TS × El and hole expansion rate (λ) are shown. In Table 2, the A value indicates the value of (C / 12) / {(Ti / 48) + (Mo / 96) + (V / 51)} in the above formula (I).

  As shown in Table 2, steels A to C, which are within the scope of the present invention, have excellent workability, specifically, TS × El ≧ 13000 MPa ·%, excellent elongation characteristics, and an index of stretch flangeability. It was confirmed that a high-tensile steel plate of 980 MPa or more with a certain hole expansion ratio (λ) of λ ≧ 50% can be obtained. In addition, it is confirmed by structure observation that the steel sheet has a martensite single-phase structure, and since the value of TS × El is 13000 MPa ·% or more, the martensite generated after annealing is tempered martensite. It was confirmed. On the other hand, the steel D as a comparative example has a value of TS × El exceeding 13000 MPa ·%, which is tempered martensite, but because no fine precipitates are formed, the tensile strength does not exceed 980 MPa, The workability was also inferior to the steel of the present invention.

(Example 2)
Using a slab having the composition of Steel B shown in Table 1, annealing was performed by hot rolling to a plate thickness of 1.8 mm under the conditions shown in Table 3 (steel plates No. 1 to 11). In addition, No. 6 was annealed in a hot dip galvanizing line, and subsequently alloyed hot dip galvanizing (galvanizing bath temperature: 480 ° C., alloying temperature: 530 ° C.) was performed in the line.
The obtained steel plate was evaluated in the same manner as in Example 1. The results are shown in Table 4.

  As shown in Table 4, Example No. of the present invention produced under conditions satisfying the present invention. It was confirmed that Nos. 1 to 6 were excellent in workability and high-tensile steel sheets of 980 MPa or more were obtained. On the other hand, No. which is a comparative example. 7 to 11 were inferior in workability and strength.

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 of Fe and unavoidable impurities, C, Ti, Mo, V content have a component composition that satisfies the following formula (I), the steel sheet structure is less than 95% of martensite-like structure with an area ratio observed In addition, TS × El, which is the product of tensile strength (TS) and elongation (El), is a substantially tempered martensite single-phase structure having a structure of 13000 MPa ·% or more, and an average particle size of less than 10 nm The carbide containing Ti, Mo and V is dispersed and precipitated, and the carbide containing Ti, Mo and V % Represented Ti, Mo, V in is, you characterized by having a composition satisfying V / (Ti + Mo + V ) ≧ 0.3, high tensile strength and excellent more workability 980MPa Tensile steel plate.
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-strength steel sheet excellent in workability having a tensile strength of 980 MPa or more according to claim 1, which is a thin hot-rolled steel sheet having a thickness of 2.5 mm or less.   The high-tensile steel sheet excellent in workability having a tensile strength of 980 MPa or more according to claim 1 or 2, wherein the surface has a hot-dip galvanized coating film. 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 composed of Fe and inevitable impurities and having a C, Ti, Mo, and V content satisfying the following formula (I) is melted, and finish rolling finish temperature is 880 ° C. or higher, winding temperature is 400 ° C. After performing hot rolling at less than condition, line annealing of 20 seconds to 24 hours at six hundred to seven hundred and twenty ° C., characterized by producing the steel sheet according to claim 1 or claim 2, tensile A method for producing a high-tensile steel sheet having excellent workability with a 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 method for producing a high-tensile steel sheet excellent in workability having a tensile strength of 980 MPa or more according to claim 4 , wherein hot-dip galvanizing is performed on the annealed steel sheet.