TWI431124B - High strength steel sheet having excellent formability and method for manufacturing the same - Google Patents

Description

High-strength steel sheet excellent in workability and method of producing the same

The present invention relates to a high-strength steel sheet suitable for use as a strength member of an automobile part which is required to have excellent workability (elongation flangeability), and a method for producing the same.

In recent years, based on the perspective of global environmental protection, improving the fuel efficiency of automobiles has become an important technical issue. Therefore, it is very common to increase the strength of the material to reduce the thickness of the member, and to make the weight of the vehicle body itself very popular. As the material to be used, in particular, a high-strength steel sheet having a tensile strength of 540 MPa or more is required. However, the high strength of the steel sheet will result in a decrease in workability, so it is desirable to have a high-strength steel sheet having excellent workability, especially for a thin steel sheet (thin steel sheet), which is highly desirable. .

In response to this expectation, various technical proposals have been proposed, for example, a duplex steel plate (DP steel plate) having a two-phase structure composed of a ferrite grain iron phase and a mai field iron phase, having an iron phase containing a fat grain and a granule phase Various composite structural steel sheets such as steel sheets and steel sheets of a composite structure of a tough iron phase.

For example, the method for producing a high-strength cold-rolled steel sheet disclosed in Patent Document 1 has a composition containing C: 0.08 to 0.30%, Si: 0.1 to 2.5%, Mn: 0.5 to 2.5%, and P: 0.01 to 0.15%. The cold-rolled steel sheet is subjected to recrystallization annealing at a temperature equal to or higher than A _c1 point, and then subjected to forced air cooling from a temperature range of _{Ar 1} to 600 ° C, and then cooled at a cooling rate of 100 ° C /sec or more. Rapidly cooling, and forming a composite structure composed of a ferrite grain iron phase and a low temperature metamorphic phase, and then generating a phase hardness Hv(L) relative to a fat iron hardness Hv (at a low temperature metamorphism determined according to a specific relationship) The ratio of α), Hv(L)/Hv(α) conforms to the range of 1.5 to 3.5, and the overaging treatment is performed at a temperature in the range of 350 to 600 ° C to obtain a high-strength cold having excellent local extensibility. Rolled steel plate. The technique disclosed in Patent Document 1 is to increase the quenching start temperature to increase the volume fraction of the low temperature metamorphic phase, and then perform an aging treatment at a temperature of 350 to 600 ° C to crystallize C in the ferrite iron and let the low temperature The metamorphic phase is softened such that the ratio of Hv(L)/Hv(α) becomes smaller, thereby improving local extensibility.

Moreover, the method for producing a low-tensile ratio high-tensile hot-rolled steel sheet having excellent corrosion resistance disclosed in Patent Document 2 contains C: 0.02 to 0.25%, Si: 2.0% or less, and Mn: 1.6 to 3.5%. P: 0.03 to 0.20%, S: 0.02% or less, Cu: 0.05 to 2.0%, sol. Al: 0.005 to 0.100%, and N: 0.008% or less, the steel blank is hot rolled to form a hot rolled steel coil. After the pickling is carried out, the hot-rolled steel strip is wound on a continuous annealing line at a temperature of 720 to 950 ° C to perform annealing treatment, thereby obtaining a low-ratio ratio high-tensile hot-rolled steel sheet having excellent corrosion resistance. According to the technique disclosed in Patent Document 2, it is possible to manufacture a high-tensile hot-rolled steel sheet having a composite structure which can maintain a low drop ratio, high ductility, and good hole expandability.

Further, the "high-strength cold-rolled steel sheet excellent in the balance between the strength and the stretch flangeability" disclosed in Patent Document 3 includes C: 0.03 to 0.17%, Si: 1.0% or less, and Mn: 0.3 to 2.0%, P: 0.010% or less, S: 0.010% or less, Al: 0.005 to 0.06%, and conform to the composition of C(%)>(3/40)×Mn, and toughened iron or wave The ratio of the iron-based second phase to the fat-grained iron phase and the ratio of the (second-phase Vickers hardness) / (the fat-grained iron phase Vickers hardness) is the strength of the relationship of 1.6 A high-strength cold-rolled steel sheet excellent in balance between the stretch flange properties. In the high-strength cold-rolled steel sheet disclosed in Patent Document 3, the steel (steel blank) having the above composition is subjected to hot rolling, and then coiled at a temperature of 650 ° C or lower, followed by pickling, and then a cold roll is performed. After the rolling, the soaking treatment is performed at a temperature equal to or lower than A ₁ point to (A ₃ point + 50 ° C), and then slowly cooled at a rate of 20 ° C / sec or less in the range of 750 to 650 ° C. Until the temperature T _{1 is} reached, next, from T ₁ to 500 ° C, an annealing treatment is performed at a rate of 20 ° C /sec or more, followed by an overaging treatment at a temperature of 500 to 250 ° C. acquired.

[Previous Technical Literature]

[Patent Literature]

[Patent Document 1] JP-A-63-293121

[Patent Document 2] Japanese Laid-Open Patent Publication No. 05-112832

[Patent Document 3] Japanese Patent Laid-Open No. Hei 10-60593

However, the problem in the technique disclosed in Patent Document 1 is that it is necessary to use not only a continuous annealing apparatus which can perform rapid cooling (quenching treatment) after recrystallization annealing, but also to suppress overaging treatment due to high temperature. The resulting sharp drop in strength requires the addition of a large amount of alloying elements.

Further, a problem in the technique disclosed in Patent Document 2 is that although a large amount of P and Cu must be added in combination, if a large amount of Cu is contained, the inter-heat processability is lowered, and a large amount of P is contained. Will cause the embrittlement of steel. In addition, the tendency of P to segregate out in steel is strong, and this segregated P not only lowers the elongational flangeability of the steel sheet, but also causes embrittlement of the welded portion.

Further, the high-strength cold-rolled steel sheet disclosed in Patent Document 3 has excellent elongation flangeability, but has a problem that the elongation is less than 26% when the strength is 540 MPa or more. It is not possible to ensure a sufficient elongation sufficient to maintain the desired excellent processability.

An object of the present invention is to provide a high-strength steel sheet excellent in workability of a thin plate type having a thickness of about 1.0 to 1.8 mm and a method for producing the same. In addition, the term "high strength" as used herein means that the tensile strength (TS) is 540 MPa or more, more preferably 590 MPa or more, and the term "excellent workability" means elongation. The rate (El) is 30% or more (when the Japanese Industrial Standard JIS No. 5 test piece is used), and the hole expansion ratio (λ) at the time of the hole expansion test according to the specification JFST 1001-1996 established by the Japan Iron and Steel Federation is 80% or more. In the case of.

In order to achieve the above object, the inventors of the present invention have diligently studied the effects of composition and fine structure on strength and workability. As a result, it has been found that, for a hot-rolled steel sheet in which the amount of alloying elements is adjusted to an appropriate range, cold rolling is not performed, but annealing is performed by heating to an appropriate two-phase temperature range. And a suitable cooling treatment, which can be made into a ferrite-grained iron phase as the main phase, and a fine wave of iron as the main body of the second phase (metal structure), thereby ensuring a desired high The strength and the workability can be greatly improved, and a high-strength steel sheet excellent in workability having a desired elongation and a desired hole expansion ratio can be obtained.

Regarding the hot-rolled steel sheet, the detailed operation of the reason why the workability can be greatly improved by directly performing the appropriate annealing treatment is omitted, and the present inventors have not yet made clear the details. It is presumed to be the following reason.

When the hot-rolled steel sheet is not subjected to cold rolling and is heated only to the annealing treatment in the two-phase temperature range, when the annealing is performed, only the α phase→γ phase is metamorphosed, and no new recrystallization occurs. In this case, only the metamorphosis of the α phase→γ phase is preferentially generated in a place where the C concentration is high, and not only a more uniform structure but also a C having a faster diffusion rate, and the α phase and the annealing process are performed. The gamma phase will be redistributed until it becomes a balanced component. Therefore, it is possible to suppress the precipitation of the flake-shaped iron carbide on the grain boundary, and in particular, it can exert an advantageous effect for improving the elongation flange property. On the other hand, if the hot-rolled steel sheet is subjected to cold rolling and then annealed, the recrystallization phenomenon and the α phase→γ phase metamorphism will compete with each other during annealing heating, so it is easy It becomes a structure that is uneven, and it is difficult to expect that the workability can be greatly improved.

The present invention has been completed based on this novelty and further review. That is, the gist of the present invention is as follows.

(1) A high-strength steel sheet excellent in workability, comprising: C: 0.08 to 0.15%, Si: 0.5 to 1.5%, Mn: 0.5 to 1.5%, P: 0.1% by mass% Hereinafter, S: 0.01% or less, Al: 0.01 to 0.1%, and N: 0.005% or less, and the remainder is a component composed of Fe and unavoidable impurities, and a ferrite phase as a main phase, and at least a wave a metal structure composed of a second phase of iron; the iron phase of the ferrite is 75 to 90%, and the ferrite is 10 to 25%, and the wave is iron, with respect to the area ratio of the entire metal structure. The average particle diameter is 5 μm or less, and the area ratio of the aforementioned ferrite to the entire area of the second phase is 70% or more.

(2) The high-strength steel sheet having excellent workability as described in the above (1), wherein the component further contains one or more selected from the group consisting of Cr, V, and Mo, and the Cr and V are as described above. The content of Mo is, in mass%, Cr: 0.05 to 0.5%, V: 0.005 to 0.2%, and Mo: 0.005 to 0.2%.

(3) The high-strength steel sheet excellent in workability as described in the above (1) or (2), wherein the above-mentioned component further contains one or two selected from Ti and Nb, and the Ti, The content of Nb is, in mass%, Ti: 0.01 to 0.1%, and Nb: 0.01 to 0.1%.

(4) The high-strength steel sheet excellent in workability according to any one of the above (1) to (3), wherein the component further contains, in mass%, B: 0.0003 to 0.0050%.

(5) The high-strength steel sheet excellent in workability according to any one of the above (1) to (4), wherein the component further contains one or two selected from the group consisting of Ni and Cu, The content of Ni and Cu is, in mass%, Ni: 0.05 to 0.5%, and Cu: 0.05 to 0.5%.

(6) The high-strength steel sheet excellent in workability according to any one of the above-mentioned (1) to (5), wherein the component further contains one selected from the group consisting of Ca and rare earth metal (REM). Or two, and the content of the above-mentioned Ca and rare earth metal (REM) is 0.001 to 0.005% by mass and rare earth metal (REM): 0.001 to 0.005% by mass%.

(7) A method for producing a high-strength steel sheet excellent in workability, comprising: C: 0.08 to 0.15%, Si: 0.5 to 1.5%, and Mn: 0.5 to 1.5% by mass%; P: 0.1% or less, S: 0.01% or less, Al: 0.01 to 0.1%, N: 0.005% or less, and the remainder is a steel material composed of Fe and unavoidable impurities, and the following hot rolling process and continuous In the annealing step, the hot rolling step is performed by hot rolling to form a hot-rolled steel sheet; and the continuous annealing step is performed after pickling the hot-rolled steel sheet, and then the hot-rolled steel sheet is subjected to a continuous annealing line on A _C1 An annealing treatment for 5 to 400 seconds in the first temperature range of the metamorphic point ~A _C3 metamorphic point, and after the annealing treatment, performing: from the first temperature range up to 700 ° C, at 5 ° C / sec or more The average cooling rate is cooled, and the residence time in the second temperature range from 700 ° C to 400 ° C is selected to be a cooling treatment of 30 to 400 seconds.

(8) The method for producing a high-strength steel sheet having excellent workability according to the above (7), wherein the hot rolling step is performed after the steel material is heated to a temperature in the range of 1,100 to 1,280 ° C. The hot-rolled steel sheet was prepared by hot rolling at a temperature of 870 to 950 ° C. After the hot rolling was completed, the hot-rolled steel sheet was taken up, and the coiling temperature at this time was set to 350 to 720 °C.

(9) The method for producing a high-strength steel sheet having excellent workability as described in the above (7) or (8), wherein, in the second temperature range, a cooling time in a temperature range of 700 to 550 ° C is Selected for more than 10 seconds.

(10) The method for producing a high-strength steel sheet excellent in workability according to any one of the above (7), wherein the component further comprises: selected from the group consisting of Cr, V, and Mo One or two or more kinds, and the content of the above Cr, V, and Mo, in terms of mass%, Cr: 0.05 to 0.5%, V: 0.005 to 0.2%, and Mo: 0.005 to 0.2%.

(11) The method for producing a high-strength steel sheet having excellent workability according to any one of the above (7), wherein the component further contains one selected from Ti or Nb or Two kinds, and the content of the above Ti and Nb is, in mass%, Ti: 0.01 to 0.1%, and Nb: 0.01 to 0.1%.

(12) The method for producing a high-strength steel sheet having excellent workability according to any one of the items (7), wherein the component further contains, in mass%, B: 0.0003 to 0.0050. %.

(13) The workability as described in any one of the above (7) to (12) A method for producing a high-strength steel sheet, wherein the component contains one or two selected from the group consisting of Ni and Cu, and the content of the Ni and Cu is 0.05% to 0.5% by mass%, Cu: 0.05 to 0.5%.

(14) The method for producing a high-strength steel sheet having excellent workability according to any one of the above (7), wherein the component further contains: from Ca, a rare earth metal (REM) One or two of the selected ones, and the content of the above-mentioned Ca and rare earth metal (REM), in terms of mass%, Ca: 0.001 to 0.005%, and rare earth metal (REM): 0.001 to 0.005%.

According to the present invention, it is possible to easily and inexpensively manufacture a high strength having a tensile strength (TS) of 540 MPa or more, and an elongation (E) of 30% or more and elongation flangeability ( λ) is a high-strength steel sheet having an elongation-flange property of 80% or more and excellent in workability, and can achieve a special effect that contributes to the industry. Moreover, in the present invention, the step of cold rolling can be omitted, so that it is also advantageous in terms of reducing the manufacturing cost and improving the productivity. Further, in particular, when the steel sheet of the present invention is applied to, for example, an automobile body part, it contributes greatly to the weight reduction of the automobile body.

First, the reason for limiting the components of the steel sheet of the present invention will be explained. In the following description, the mass % is simply expressed in % unless otherwise specified.

C: 0.08~0.15%

The C system is helpful for increasing the strength of the steel sheet, and is an element which contributes to the formation of a composite structure composed of a ferrite grain iron phase and a second phase other than the ferrite grain iron phase. In the present invention, It is necessary to ensure a high tensile strength of 540 MPa or more, and it is necessary to contain 0.08% or more. On the other hand, if the content is more than 0.15%, the weldability of the spot welding is lowered, and the workability such as ductility is also lowered. Therefore, the content of C is limited to the range of 0.08 to 0.15%. It is preferably 0.10~0.15%.

Si: 0.5~1.5%

The Si system can be solid-melted in steel to contribute to the strengthening of the ferrite iron, and is an element which contributes to the improvement of ductility. In order to secure a high tensile strength of 540 MPa or more, it is necessary to contain 0.5% or more. On the other hand, when the content is excessively contained in an amount of 1.5%, the occurrence of red scale or the like is promoted, and not only the surface properties of the steel sheet but also the chemical conversion treatability are lowered. In addition, excessive inclusion of Si causes an increase in resistance during resistance welding, thereby impeding resistance weldability. Therefore, Si is limited to the range of 0.5 to 1.5%. It is best 0.7 to 1.2%.

Mn: 0.5~1.5%

The Mn system contributes to an increase in the strength of the steel sheet and is an effective element for forming a composite structure, and in order to obtain such an effect, it is necessary to contain 0.5% or more. On the other hand, if the content exceeds 1.5%, the granule iron phase is easily formed during the cooling during annealing, which results in a decrease in workability, particularly elongation flangeability. Therefore, Mn is limited to the range of 0.5 to 1.5%. It is best 0.7 to 1.5%.

P: 0.1% or less

P is an effective element that can be solidified in steel and contributes to the strength of the steel sheet. However, it tends to segregate at the grain boundary, which causes a decrease in the bonding strength of the grain boundary, resulting in a decrease in workability and Concentration on the surface of the steel sheet reduces chemical conversion treatability, corrosion resistance, and the like. This adverse effect caused by P tends to be significant when its content exceeds 0.1%. Therefore, P is limited to 0.1% or less. In addition, in order to avoid such adverse effects caused by P, the content of P is 0.1% or less, and it is preferable to reduce as much as possible. However, if the excessive reduction causes the manufacturing cost to rise, it is selected to be 0.001% or more. It is appropriate.

S: 0.01% or less

In steel, S mainly forms sulfides (intermediates) such as MnS, which reduces the workability of the steel sheet, particularly local elongation. In addition, the presence of sulfide (intermediate) will reduce weldability. This adverse effect caused by S tends to be remarkable when its content exceeds 0.01%. Therefore, S is limited to 0.01% or less. In addition, in order to avoid such adverse effects caused by S, the content of S is 0.01% or less, and it is preferable to reduce it as much as possible. However, if the excessive reduction causes the manufacturing cost to rise, it is selected to be more than 0.0001%. It is appropriate.

Al: 0.01~0.1%

The Al system can function as a deoxidizing agent, and is an element necessary for improving the cleanliness of the steel sheet, and can also effectively act to improve the yield of the carbide forming element. In order to obtain this effect, the content must be 0.01% or more. If the content is less than 0.01%, the Si system will be The removal of the starting point for delayed damage is insufficient and increases the risk of delayed damage. On the other hand, even if the content has exceeded 0.1%, the above effects have become saturated, and it is not expected to obtain an effect in accordance with the content thereof, which is not only uneconomical, but also has reduced workability and surface defects. The tendency is huge. Therefore, Al is limited to the range of 0.01 to 0.1%. It is preferably 0.01 to 0.05%.

N: 0.005% or less

N is considered to be an essential harmful element in the present invention and should be minimized, but its content can be tolerated to 0.005%. Therefore, N is limited to 0.005% or less. Further, if the content of N is excessively lowered, the production cost is increased, so that it is preferably selected to be 0.0001% or more.

Although the above-mentioned components are basic components, in addition to these basic components, they may be additionally added as necessary: for example, from Cr: 0.05 to 0.5%, V: 0.005 to 0.2%, and Mo: 0.005 to 0.2%. 1 or 2 or more; and/or 1 or 2 selected from Ti: 0.01 to 0.1%, Nb: 0.01 to 0.1%; and/or from B: 0.0003 to 0.0050%, and/or Ni: 0.05 to 0.5%, Cu: 0.05 to 0.5% selected one or two; and/or selected from Ca: 0.001 to 0.005%, rare earth metal (REM): 0.001 to 0.005% 1 or 2 types.

One or more selected from the group consisting of Cr: 0.05 to 0.5%, V: 0.005 to 0.2%, and Mo: 0.005 to 0.2%

Any element of Cr, V, or Mo is an element which increases the strength of the steel sheet and contributes to the formation of a composite structure, and may be selectively contained as necessary. One or two or more. In order to obtain such an effect, the content of each of the three is preferably 0.05% or more, V: 0.005% or more, and Mo: 0.005% or more. On the other hand, if the contents of the three are excessively contained in Cr: 0.5%, V: 0.2%, and Mo: 0.2%, it is difficult to generate a desired amount of Boron iron in the cooling treatment after the annealing treatment. Therefore, it is impossible to ensure the desired composite structure, the elongation flangeability is lowered, and the workability is also lowered. Therefore, if it is desired to contain, the content of each of the three is limited to a range of Cr: 0.05 to 0.5%, V: 0.005 to 0.2%, and Mo: 0.005 to 0.2%.

One or two selected from Ti: 0.01 to 0.1% and Nb: 0.01 to 0.1%

Both Ti and Nb are elements which can increase the strength of the steel sheet by crystallization strengthening, and may be selectively contained in one or two types as necessary. In order to obtain such an effect, it is preferable that the content of both is Ti: 0.01% or more and Nb: 0.01% or more, but if the contents of both are excessively contained such as Ti: 0.1% and Nb: 0.1%, It will reduce the workability and shape freeze. Therefore, if it is to be contained, it is preferable to limit the content of both to a range of Ti: 0.01 to 0.1% and Nb: 0.01 to 0.1%.

B: 0.0003~0.0050%

The B-series is segregated in the Worthfield iron grain boundary, and has an action of suppressing the formation of ferrite iron from the grain boundary and suppressing the growth of the ferrite iron, and may be contained as necessary. In order to obtain such an effect, the content is preferably 0.0003% or more, but if the content exceeds 0.0050%, the workability is lowered. Therefore, if you want to contain it, limit B to 0.0003~0.0050%. The range is appropriate. Further, in order to obtain the above-described effect by B, it is necessary to suppress the formation of BN, so it is preferable to contain it together with Ti.

One or two selected from Ni: 0.05 to 0.5% and Cu: 0.05 to 0.5%

Both Ni and Cu have an effect of increasing the strength of the steel sheet, and have an action of promoting internal oxidation to enhance the adhesion of the plating layer, and may be selectively contained as necessary. In order to obtain such an effect, the content of both of them is preferably 0.05% or more and Cu: 0.05% or more, respectively, but if the contents of the two are excessively contained in Ni: 0.5% and Cu: 0.5%, In the cooling treatment after the annealing treatment, it is difficult to form a desired amount of the ferrite, and thus the desired composite structure cannot be secured, the elongation flangeability is lowered, and the workability is also lowered. Therefore, if it is intended to be contained, it is preferably limited to a range of Ni: 0.05 to 0.5% and Cu: 0.05 to 0.5%.

One or two selected from Ca: 0.001 to 0.005% and rare earth metal (REM): 0.001 to 0.005%

Both Ca and rare earth metals (REM) are elements that help control the form of sulfides, and can spheroidize the shape of sulfides, and have an adverse effect on the processability of sulfides, especially on the elongation flangeability. The role. In order to obtain such an effect, the contents of the two are preferably Ca: 0.001% or more and rare earth metal (REM): 0.001% or more, respectively, but the contents of the two are respectively Ca: 0.005%, rare earth metal (REM). If 0.005% is excessively contained, it will cause an increase in the intervening substance, resulting in many surface defects as well as internal defects. Therefore, if you want to contain it, limit it to Ca: 0.001~0.005%, rare earth metal (REM): 0.001~ A range of 0.005% is preferred.

The rest except the above components are composed of Fe and unavoidable impurities.

The steel sheet of the present invention has the above-described composition and has a metal structure composed of a main phase, that is, a ferrite-grained iron phase, and a second phase containing at least ferrite.

In the steel sheet of the present invention, the area ratio of the main phase, that is, the ferrite iron phase, is selected to be 75 to 90% with respect to the area ratio of the entire structure. If the area ratio of the ferrite-grained iron phase is less than 75%, the desired elongation and the desired hole expansion ratio cannot be ensured, and the workability is lowered. On the other hand, if the area ratio of the ferrite grain iron phase exceeds 90%, the area ratio of the second phase is lowered, so that the desired high strength cannot be ensured. Therefore, the area ratio of the main phase, that is, the ferrite iron phase, is limited to the range of 75 to 90%. In addition, the area ratio of the better ferrite iron phase is 80 to 90%.

Further, the steel sheet of the present invention contains at least Bola iron in the second phase. The area ratio of the Borne iron is 10 to 25% based on the area ratio of the overall organization. If the area ratio of the ferritic iron is less than 10%, the desired hole expansion ratio cannot be ensured, the elongation flangeability is lowered, and the workability is also lowered. On the other hand, if the area ratio of the Borne iron is too much and exceeds 25%, the interface between the ferrite grain iron phase and the Borne iron will increase, and voids are easily generated during processing, and the elongation flangeability is lowered, and the workability is also improved. Will decrease.

Further, the Borite iron is selected as fine particles having an average particle diameter of 5 μm or less. If the average particle diameter of the ferrite is coarse and exceeds 5 μm, the stress concentrates on the ferrite particles (interface) when the steel sheet is processed, and fine voids are generated, so that the elongation flangeability is lowered. Processability will also decrease. base For this reason, the average particle diameter of the Borne iron is limited to 5 μm or less. More preferably, it is 4.0 μm or less.

The second phase in the structure of the steel sheet of the present invention contains at least Boron iron, and the area ratio of the total area of the ferrite to the second phase is 70% or more, that is, the second phase is mainly composed of Borne iron. Phase. If the area ratio of the total area of the Borne iron to the second phase is less than 70%, the hard granulated iron phase, the tough iron phase or the residual γ phase will become too much, and the workability is easily lowered. Therefore, the area ratio of the total area of the Borne iron to the second phase is limited to 70% or more. Better is 75~100%.

In the second phase, in addition to the Borne iron, it may contain: toughened iron, 麻田散铁, residual Worthite iron (residual γ phase), etc., but especially the toughened iron and the maitian iron are hard. The phase, while the residual γ phase undergoes metamorphism during processing, and metamorphoses into granulated iron, which respectively deteriorates the workability. Therefore, it is preferable that these toughened iron, the granulated iron and the residual Worth iron are reduced as much as possible, and the total area ratio of the whole structure is preferably 5% or less. More preferably, the total is 3% or less.

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

The steel material with the above ingredients is selected as the starting material. The method for producing the steel material is not particularly limited. However, from the viewpoint of productivity, the molten steel of the above-mentioned components is melted by a general melting method such as a converter or an electric furnace, and continuous casting is used. It is preferable to produce a steel material such as a steel embryo by a commonly used casting method such as a method. Further, an agglomerate-block rolling method, a thin steel blank casting method, or the like can also be applied.

The steel material having the above composition is subjected to a hot rolling process to form a hot rolled steel sheet. In the hot rolling step, the steel material is heated to a temperature in the range of 1,100 to 1,280 ° C, and hot-rolled at a temperature of 870 to 950 ° C at the end of hot rolling to form a hot-rolled steel sheet. After the completion of the rolling, the hot-rolled steel sheet is preferably wound up at a coiling temperature of 350 to 720 °C.

If the heating temperature of the steel material is less than 1100 ° C, sometimes the deformation resistance becomes too high, the rolling load becomes too large, and it is difficult to perform hot rolling. On the other hand, if it exceeds 1280 ° C, the crystal grains are too coarse, and even if hot rolling is performed, the desired fine steel sheet structure cannot be secured. Therefore, the heating temperature at which hot rolling is performed is preferably selected in the range of 1100 to 1280 °C. Even better is less than 1280 ° C.

In addition, if the end temperature of the hot rolling is less than 870 ° C, ferrite iron (α phase) and Vostian iron (γ phase) are formed during the rolling, and it is easy to form a band structure in the steel sheet. . Such a band-like structure remains even after annealing, and sometimes the characteristics of the steel sheet are made to have an omnidirectional property or a decrease in workability with respect to the obtained steel sheet. On the other hand, if the hot-rolling end temperature exceeds 950 ° C, the hot-rolled steel sheet structure becomes coarse, and sometimes the desired structure cannot be obtained even after annealing. Therefore, it is preferable that the hot rolling end temperature is selected to be 870 to 950 °C.

In addition, if the coiling temperature after the hot rolling is less than 350 ° C, it will form a toughened ferro-fermented ferrite, toughened iron, and granulated iron, which can easily become a hard and non-granular hot-rolled structure. Even in the subsequent annealing treatment, the hot-rolled structure is inherited, and it is easy to become a non-granular structure, and sometimes the desired workability cannot be ensured. On the other hand, if it is at such a high temperature condition exceeding 720 ° C, it is difficult to ensure uniform mechanical properties in the entire length direction of the steel sheet and the entire width direction. Therefore, the coiling temperature is preferably selected to be in the range of 350 to 720 °C. More preferably, it is 500 to 680 °C.

For the hot-rolled steel sheet obtained by the hot-rolling step, in order to remove the scale formed on the surface of the steel sheet, the conventional method is followed, and after the pickling is performed, the cold-rolled steel sheet is not subjected to cold rolling. Instead, a continuous annealing process is performed directly on the continuous annealing line, and an annealing process and subsequent cooling treatment are performed.

The annealing treatment is performed for 5 to 400 seconds in the first temperature range from the A _c1 metamorphic point to the A _c3 metamorphic point.

If the temperature in the first temperature range of the annealing treatment (heating temperature) does not reach the A _c1 metamorphic point, or the holding time (annealing time) in the first temperature range is less than 5 seconds, the carbide in the hot rolled steel sheet has not yet been Fully melted or the metamorphosis of the α phase→γ phase has not occurred or is insufficient, so the subsequent cooling treatment does not ensure the desired composite structure, so it cannot be obtained: having ductility that can meet the desired elongation and hole expansion ratio, Elongated flanged steel plate. On the other hand, if the heating temperature of the annealing treatment is too high and exceeds the A _c3 metamorphic point, the coarsening of the Worthite iron particles becomes remarkable, and even if the subsequent cooling treatment is performed, the generated structure is coarsened, sometimes Will reduce the processability. Further, if the holding time (annealing time) in the first temperature range exceeds 400 seconds, the processing time is too long, and the energy consumed is too much, resulting in an increase in manufacturing cost. For this reason, the annealing treatment is limited to a treatment that is maintained for 5 to 400 seconds in the first temperature range from the A _c1 metamorphic point to the A _c3 metamorphic point.

Further, the A _c1 metamorphic point of each steel sheet is a value calculated by the following formula (1), and the A _c3 metamorphic point is a value calculated by the following formula (2). In addition, if there is an element that is not included in an element in the formula, the element is considered to be zero.

A _c1 metamorphic point (°C)=723+29.1Si-10.7Mn-16.9Ni+16.9Cr+6.38W+290As ...(1)

A _c3 metamorphic point (°C)=910-203√C+44.7Si-30Mn+700P+400Al-15.2Ni-11Cr-20Cu+31.5Mo+104V+400Ti+13.1W+120As (2)

Here, the content (% by mass) of each element of C, Si, Mn, Ni, Cr, W, As, C, P, Al, Cu, Mo, V, Ti.

Further, the cooling treatment after the annealing treatment is performed at a cooling rate of an average of 5 ° C /sec or more from the first temperature range of the first temperature range of 700 ° C and a second temperature range of 700 ° C to 400 ° C. The residence time in the interior is selected to be 30 to 400 seconds.

If the average cooling rate from the first temperature range up to 700 ° C is less than 5 ° C / sec, the amount of ferrite iron is too much, and the desired composite structure cannot be obtained, and the workability is lowered, sometimes it is impossible. Ensure the desired tensile strength (540 MPa or more). Therefore, the cooling rate from the first temperature range up to 700 ° C is limited to an average of 5 ° C / sec or more. More preferably, it is 20 ° C / sec or less; preferably 5 to 15 ° C / sec.

Further, the residence time in the second temperature range of 700 ° C to 400 ° C is an important factor for the formation of the ferrite contained in the second phase. The term "residence time" as used herein means a period of residence in the second temperature range, including: maintaining in a specific temperature condition within the second temperature range, and in the second temperature range The case where cooling is performed at a specific cooling rate, and cooling is performed in such a manner that the two cases are mixed. If the residence time in the second temperature range is less than 30 seconds, the wave-forming iron metamorphosis is not yet formed, or the amount of generated b-iron is insufficient, and the desired composite structure cannot be secured. On the other hand, if the residence time in the second temperature range is too long and exceeds 400 seconds, the productivity is lowered. Therefore, the residence time in the second temperature range is limited to the range of 30 to 400 seconds. More preferably, it is less than 150 seconds. Further, the cooling time in the temperature range of 700 to 550 ° C in the second temperature range is selected to be 10 seconds or longer, that is, the cooling rate in the temperature range of 700 to 550 ° C is an average of 15 ° C / sec or less. It is a good practice to ensure the expected amount of iron. When the cooling time in the temperature range of 700 to 550 ° C is less than 10 seconds, the amount of generated ferrite is insufficient, and the desired composite structure cannot be obtained, and sometimes the desired workability cannot be ensured.

The present invention will be further specifically described below based on the examples. Further, the present invention is not limited to those disclosed in the embodiments.

[Examples]

The molten steel of the composition shown in Table 1 is first melted, and then the steel material is produced by a commonly used method. With respect to these steel materials, hot rolling was performed under the conditions of the heating temperature and the hot rolling end temperature shown in Table 2 to prepare a hot rolled steel sheet having a thickness of 1.6 mm, and after the hot rolling was completed, The coiling temperature shown in Table 2 was taken up into a steel strip roll. Then pickling is carried out. In addition, for a part of the hot-rolled steel sheet (thickness: 3.2 mm), after pickling, cold rolling was performed at a rolling ratio of 50% to form a cold-rolled steel sheet having a thickness of 1.6 mm as a comparative example. .

The hot-rolled steel sheet or the cold-rolled steel sheet produced is subjected to a continuous annealing step to form an annealed steel sheet under the conditions shown in Table 2, and the continuous annealing step includes heating to a temperature within the first temperature range. And maintaining the temperature annealing treatment; and cooling from the temperature in the first temperature range up to 700 ° C at the average cooling rate shown in Table 2, and 700 to 550 ° C in the second temperature range The cooling was performed at the cooling rate (cooling time) shown in Table 2, and the residence time in the second temperature range of 700 to 400 ° C was selected from the cooling treatment shown in Table 2. Further, the transformation points of the respective steel sheets shown in Table 2 are values calculated by the above formulas (1) and (2).

Test pieces were taken from the obtained annealed steel sheets, and were subjected to a structure observation, a tensile test, and a hole expansion test. The test method is as follows.

(1) Organizational observation

A test piece for observation of the structure was taken from the obtained annealed steel sheet, and a cross section (L section) parallel to the rolling direction was polished, and it was etched with a nitric acid solution using a scanning electron microscope (magnification: 3000 times). Three fields of view were observed for the metal structure, and photographing was performed, and the type of the tissue and the area ratio of each phase structure with respect to the whole tissue were measured, and the area ratio of the total area of the second phase with respect to the entire tissue was calculated. Further, the average crystal grain size of the ferrite contained in the second phase was also calculated. Further, the average crystal grain size of the ferrite is obtained by first measuring the area of each of the iron particles, calculating the diameter corresponding to the circle from the area, and arithmetically averaging the diameters of the corresponding circles of the obtained particles. And as the average crystal grain size of the Borne iron particles. The number of particles of the ferrite measured was 20 or more. In addition, the area ratio of the second phase of the Borne iron relative to the total area is also calculated.

(2) Tensile test

The JIS No. 5 test piece was obtained from the obtained annealed steel sheet (that is, the direction of the stretching direction and the direction perpendicular to the rolling direction), and the tensile test was carried out in accordance with the regulation of JIS Z 2241. The tensile properties (peak point (YP), tensile strength (TS), elongation (El)) were determined.

(3) Hole expansion test

A 100 mm square hole-expanding test piece was taken from the obtained annealed steel sheet. Then, the hole expansion test was carried out in accordance with the specification of the specification JFST 1001-1996 established by the Japan Iron and Steel Federation to obtain the hole expansion ratio λ (%).

The results obtained are shown in Table 3.

Each of the examples of the present invention has a high tensile strength (T) of 540 MPa or more, a high ductility of elongation (El) of 30% or more, and a hole expansion ratio (λ) of 80% or more. A high-strength steel sheet excellent in workability and excellent in stretch flangeability. On the other hand, in the steel sheet of the comparative example outside the scope of the present invention, the desired high strength cannot be obtained, that is, the desired elongation cannot be obtained, or the desired hole expansion ratio (λ) cannot be obtained, thereby reducing the steel sheet. Its processability.

Claims (7)

A high-strength steel sheet excellent in workability, comprising: C: 0.08 to 0.15%, Si: 0.5 to 1.5%, Mn: 0.5 to 1.5%, P: 0.1% or less, and S by mass% : 0.01% or less, Al: 0.01 to 0.1%, N: 0.005% or less, the remainder is composed of Fe and unavoidable impurities, and the ferrite phase as the main phase, and at least containing Boron The metal structure composed of the second phase; the ferrite phase accounts for 75-90% of the total grain metal relative to the entire metal structure, and the Boron iron accounts for 10-25%, and the average particle size of the Boron iron It is 5 μm or less, and the area ratio of the above-mentioned wave iron to the entire area of the second phase is 70% or more. The high-strength steel sheet excellent in workability as described in the first aspect of the invention, wherein the component contains one or more of the following groups (A) to (E), (A) ) one or two or more selected from the group consisting of Cr, V, and Mo, and the contents of Cr, V, and Mo, in terms of mass%, Cr: 0.05 to 0.5%, V: 0.005 to 0.2%, Mo: 0.005 ~0.2%, (B) one or two selected from Ti and Nb, and the content of Ti and Nb, in terms of mass%, Ti: 0.01 to 0.1%, Nb: 0.01 to 0.1%, (C In terms of mass%, B: 0.0003 to 0.0050%, (D) one or two selected from Ni and Cu, and the content of Ni and Cu described above, in mass%, Ni: 0.05 to 0.5%, Cu: 0.05~ 0.5%, (E) one or two selected from Ca, rare earth metal (REM), and the content of the above Ca, rare earth metal (REM), in mass%, Ca: 0.001 to 0.005%, rare earth metal (REM): 0.001~0.005%. A method for producing a high-strength steel sheet excellent in workability, comprising: C: 0.08 to 0.15%, Si: 0.5 to 1.5%, Mn: 0.5 to 1.5%, P: 0.1 in mass% % or less, S: 0.01% or less, Al: 0.01 to 0.1%, N: 0.005% or less, and the remaining part is a steel material of a component composed of Fe and unavoidable impurities, and the following hot rolling step and continuous annealing step are performed. The hot rolling step is performed by hot rolling to produce a hot rolled steel sheet. The continuous annealing step is performed after pickling the hot rolled steel sheet, and the hot rolled steel sheet is subjected to a continuous annealing line on the continuous line of A _C1 . An annealing treatment is maintained for 5 to 400 seconds in the first temperature range of the A _C3 transformation point, and after the annealing treatment, an average cooling rate of 5 ° C /sec or more is performed from the first temperature range up to 700 ° C Cooling is performed, and the residence time in the second temperature range from 700 ° C to 400 ° C is selected as a cooling treatment of 30 to 400 seconds. The method for producing a high-strength steel sheet having excellent workability according to the third aspect of the invention, wherein the component contains one or more of the following groups (A) to (E), (A) one or two or more selected from the group consisting of Cr, V, and Mo, and the contents of Cr, V, and Mo, in terms of mass%, Cr: 0.05~ 0.5%, V: 0.005 to 0.2%, Mo: 0.005 to 0.2%, (B) one or two selected from Ti and Nb, and the content of the above Ti and Nb, in mass%, Ti: 0.01 ~0.1%, Nb: 0.01~0.1%, (C) in mass%, B: 0.0003~0.0050%, (D) one or two selected from Ni and Cu, and the content of Ni and Cu described above In terms of mass%, Ni: 0.05 to 0.5%, Cu: 0.05 to 0.5%, (E) one or two selected from Ca, rare earth metal (REM), and the above Ca, rare earth metal (REM) The content is, in terms of mass%, Ca: 0.001 to 0.005%, and rare earth metal (REM): 0.001 to 0.005%. The method for producing a high-strength steel sheet having excellent workability as described in the third or fourth aspect of the invention, wherein the hot rolling step is performed by heating the steel material to a temperature in a range of 1100 to 1280 ° C. At the end of hot rolling, the temperature is set to 870 to 950 ° C for hot rolling to prepare a hot-rolled steel sheet. After the hot rolling is completed, the hot-rolled steel sheet is wound up, and the coiling temperature at this time is set to 350~ 720 ° C. A method for producing a high-strength steel sheet excellent in workability according to the third or fourth aspect of the invention, wherein a cooling time in a temperature range of 700 to 550 ° C is selected in the second temperature range. It is more than 10 seconds. The method for producing a high-strength steel sheet excellent in workability according to the fifth aspect of the invention, wherein in the second temperature range, the cooling time in a temperature range of 700 to 550 ° C is selected to be 10 seconds or longer. .