JP2010275627A - High-strength steel sheet and high-strength hot-dip galvanized steel sheet having excellent workability, and method for producing them - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength steel sheet and a high-strength hot-dip galvanized steel sheet having excellent ductility, stretch flange formability and bendability. <P>SOLUTION: The steel sheet has a componential composition comprising, by mass, 0.05 to 0.3% C, 0.01 to 2.5% Si, 0.5 to 3.5% Mn, 0.003 to 0.100% P, ≤0.02% S and 0.010 to 1.5% Al, and in which the total of the contents of Si and Al is 0.5 to 3.0%, and the balance iron with inevitable impurities, and has a metallic structure comprising, by area ratio, ferrite of ≥20%, tempered martensite of 10 to 60% and martensite of 0 to 10%, and comprising, by volume ratio, retained austenite of 3 to 10%, and in which the ratio (m) between the Vickers hardness (m) of the tempered martensite and the Vickers hardness (f) of the ferrite, (m)/(f) is ≤3.0. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a high-strength steel sheet and a high-strength hot-dip galvanized steel sheet excellent in forming processability suitable for members used in industrial fields such as automobiles and electrical equipment, and methods for producing them.

In recent years, improving the fuel efficiency of automobiles has become an important issue from the viewpoint of conservation of the global environment. For this reason, a movement to reduce the thickness of the vehicle body by increasing the strength of the vehicle body material and to reduce the weight of the vehicle body is becoming active. However, increasing the strength of the steel sheet causes a decrease in ductility, that is, a decrease in forming processability, and therefore development of a material having both high strength and high processability is desired.
Furthermore, due to the recent increasing demand for improved corrosion resistance for automotive materials, development of hot-dip galvanized high-tensile steel sheets has been actively conducted.

In response to the above requirements, various high-strength steel sheets and high-strength steel structures such as ferrite, martensite duplex steel (DP steel), and TRIP steel using transformation-induced plasticity of retained austenite have been developed. Strength hot dip galvanized steel sheets have been developed.
For example, Patent Document 1 proposes an alloyed hot-dip galvanized steel sheet with excellent workability that secures retained austenite by adding a large amount of Si and achieves high ductility. Has been.
However, although these DP steels and TRIP steels are excellent in elongation characteristics, there is a problem that the hole expandability is inferior. This hole expansibility is an index indicating workability at the time of flange forming by expanding a processed hole, and is an important characteristic required for a high-strength steel sheet together with elongation characteristics.

As a method for producing a hot-dip galvanized steel sheet having excellent stretch flangeability (hole expandability), Patent Document 2 describes martensite produced by intense cooling to the Ms point or less after annealing and soaking until the hot-dip galvanizing bath. A technique for improving the hole expansibility by re-heating the tempered martensite is disclosed. However, with this technique, the hole expandability is improved by changing the martensite to tempered martensite, but the problem is that the EL is low.
Furthermore, when the tensile strength exceeds 900 MPa, the number of cases in which cracking due to bending is problematic during press molding increases, and in addition to the ductility and stretch flangeability, bendability is also required.

Japanese Patent Application Laid-Open No. 11-296991 Japanese Patent Laid-Open No. 6-93340

As described above, high-strength steel sheets and high-strength hot-dip galvanized steel sheets are required to have excellent ductility, stretch flangeability, and bendability, but conventional steel sheets and hot-dip galvanized steel sheets are all at a high level. There was nothing to combine.
Accordingly, an object of the present invention is to provide a high-strength steel sheet and a high-strength hot-dip galvanized steel sheet that are excellent in ductility, stretch flangeability, and bendability, and methods for producing them.

  In order to solve the above-described problems and obtain a high-strength steel sheet and a high-strength hot-dip galvanized steel sheet excellent in ductility, stretch flangeability, and bendability, the present inventors have earnestly studied from the viewpoint of the component composition and microstructure of the steel sheet. Repeated research. As a result, it was obtained by optimizing the content of the alloy elements and the hot rolling conditions, making the hot rolled steel sheet mainly composed of bainite and martensite, and cold rolling the hot rolled steel sheet or hot rolled steel sheet. By continuously annealing a cold-rolled steel sheet under specific conditions (in the case of a hot-dip galvanized steel sheet, hot-dip galvanizing is performed thereafter, and if necessary, a plating alloying treatment is performed), so that ferrite is 20 in area ratio. % Or more, containing 10-60% tempered martensite, containing 3-10% retained austenite by volume, and having a Vickers hardness ratio of tempered martensite and ferrite of 3.0 or less, preferably 2.0-3.0 It was found that a high-strength steel sheet and a high-strength hot-dip galvanized steel sheet having high ductility, stretch flangeability and bendability can be obtained by using a certain metal structure. In general, when retained austenite is present, ductility is improved by the TRIP effect of retained austenite. However, the martensite produced by transformation of retained austenite due to the addition of strain becomes very hard, and as a result, the hardness difference from the main phase ferrite increases and stretch flangeability and bendability may decrease. Are known. In the present invention, the coexistence of tempered martensite together with ferrite and retained austenite suppresses the deterioration of stretch flangeability and bendability as described above, and high ductility, stretch flangeability and bendability are obtained. Furthermore, it was found that the hardness ratio of tempered martensite and ferrite has a great influence on the bending characteristics, and in order to achieve high bendability, it is important to control the hardness ratio within a range of 3.0 or less. .

The present invention has been made on the basis of such findings and has the following gist.
[1] C: 0.05 to 0.3% by mass, Si: 0.01 to 2.5% by mass, Mn: 0.5 to 3.5% by mass, P: 0.003 to 0.100% by mass , S: 0.02% by mass or less, Al: 0.010 to 1.5% by mass, the total content of Si and Al is 0.5 to 3.0% by mass, the balance being iron and It has a component composition consisting of inevitable impurities, including 20% or more of ferrite by area ratio, 10 to 60% of tempered martensite, 0 to 10% of martensite, and 3 to 10% of retained austenite by volume ratio. High strength with excellent workability characterized by having a metal structure in which the ratio (m) / (f) of Vickers hardness (m) of tempered martensite to Vickers hardness (f) of ferrite is 3.0 or less steel sheet.

[2] In the high-strength steel sheet of [1], Cr: 0.005 to 2.00% by mass, Mo: 0.005 to 2.00% by mass, V: 0.005 to 2.00% by mass Ni: 0.005 to 2.00% by mass, Cu: 0.005 to 2.00% by mass, containing one or more selected from high strength and excellent strength steel sheet.
[3] In the high-strength steel sheet of [1] or [2], one or more selected from Ti: 0.01-0.20% by mass and Nb: 0.01-0.20% by mass or A high-strength steel sheet excellent in workability characterized by containing two types.
[4] A high-strength steel plate excellent in workability, characterized in that the high-strength steel plate according to any one of [1] to [3] above further contains B: 0.0002 to 0.005 mass%.
[5] In the high-strength steel sheet according to any one of [1] to [4] above, it is further selected from Ca: 0.0001 to 0.005 mass% and REM: 0.0001 to 0.005 mass%. A high-strength steel sheet excellent in workability characterized by containing one or two kinds.

[6] In the high-strength steel sheet according to any one of [1] to [5] above, the ratio (m) / (f) of Vickers hardness (m) of tempered martensite to Vickers hardness (f) of ferrite is 2.0. A high-strength steel sheet excellent in workability, characterized by being -3.0.
[7] A high-strength hot-dip galvanized steel sheet excellent in workability, characterized in that the base steel sheet is made of the steel sheet according to any one of [1] to [6].
[8] Hot-rolling a steel slab having the component composition according to any one of [1] to [5] above and having a metal structure in which the total area ratio of bainite and martensite is 80% or more When performing continuous annealing on this hot-rolled steel sheet, after heating to 750-900 ° C and holding for 10 seconds or more, from 750 ° C to a temperature range of 100-350 ° C with an average cooling rate of 10 ° C / second or more A method for producing a high-strength steel sheet excellent in workability, characterized by cooling and then reheating to 350 to 600 ° C. and holding for 10 to 600 seconds, followed by cooling to room temperature.

[9] Hot-rolling a steel slab having the component composition according to any one of [1] to [5] above and having a metal structure in which the total area ratio of bainite and martensite is 80% or more In this case, the hot-rolled steel sheet was cold-rolled to obtain a cold-rolled steel sheet. When the cold-rolled steel sheet was subjected to continuous annealing, the steel sheet was heated to 750-900 ° C and held for 10 seconds or more, and then from 750 ° C to 10 ° C / Cooled to a temperature range of 100 to 350 ° C. at an average cooling rate of at least 2 seconds, then reheated to 350 to 600 ° C., held for 10 to 600 seconds, and then cooled to room temperature, excellent in workability Manufacturing method of high strength steel sheet.
[10] In the manufacturing method of [8] or [9] above, in the hot rolling step, after the rolling is finished at a finish rolling temperature not lower than the Ar ₃ transformation point, cooling is performed at an average cooling rate of 50 ° C./second or higher. The manufacturing method of the high strength steel plate excellent in workability characterized by winding at 300-550 degreeC.

[11] Hot-rolling a steel slab having the component composition according to any one of [1] to [5] above and having a metal structure in which the total area ratio of bainite and martensite is 80% or more When performing continuous annealing on this hot-rolled steel sheet, after heating to 750-900 ° C and holding for 10 seconds or more, from 750 ° C to a temperature range of 100-350 ° C with an average cooling rate of 10 ° C / second or more A method for producing a high-strength hot-dip galvanized steel sheet excellent in workability, characterized in that it is cooled, then reheated to 350-600 ° C. and held for 10-600 seconds, and then hot-dip galvanized.
[12] Hot-rolling a steel slab having the composition according to any one of [1] to [5] above and having a metal structure in which the total area ratio of bainite and martensite is 80% or more In this case, the hot-rolled steel sheet was cold-rolled to obtain a cold-rolled steel sheet. When the cold-rolled steel sheet was subjected to continuous annealing, the steel sheet was heated to 750 to 900 ° C. and held for 10 seconds or more, and then from 750 ° C. to 10 ° C. / Workability characterized in that it is cooled to a temperature range of 100 to 350 ° C. at an average cooling rate of at least 2 seconds, then reheated to 350 to 600 ° C. and held for 10 to 600 seconds, and then hot dip galvanized. For producing high-strength hot-dip galvanized steel sheets with excellent resistance.

[13] In the production method of [11] or [12] above, in the hot rolling step, after the rolling is finished at a finish rolling temperature not lower than the Ar ₃ transformation point, cooling is performed at an average cooling rate of 50 ° C./second or higher. The manufacturing method of the high intensity | strength hot-dip galvanized steel plate excellent in workability characterized by winding at 300-550 degreeC.
[14] In the manufacturing method according to any one of [11] to [13] above, a high-strength hot-dip galvanized steel sheet excellent in workability characterized by performing a plating alloying treatment after hot-dip galvanizing Production method.

The steel sheet and hot-dip galvanized steel sheet of the present invention have high strength and excellent formability (ductility, stretch flangeability, bendability). For this reason, particularly as a material for automobiles, it is possible to achieve both weight reduction of automobiles and improvement of collision safety, and greatly contribute to improvement in performance of automobile bodies.
Moreover, according to the manufacturing method of this invention, the steel plate and hot-dip galvanized steel plate which have the above outstanding performances can be manufactured stably.

First, the reason for limitation of the component composition of steel is demonstrated about the steel plate of this invention and the hot dip galvanized steel plate which has a hot dip galvanized film on the surface of this steel plate.
C: 0.05 to 0.3% by mass
C is an element that stabilizes austenite, and is an element necessary for increasing the steel sheet strength and improving the TS-EL balance by increasing the steel sheet strength in order to easily generate phases other than ferrite. If the C content is less than 0.05% by mass, it is difficult to secure a phase other than ferrite even if the production conditions are optimized, and TS × EL decreases. On the other hand, when the amount of C exceeds 0.3% by mass, the welded portion and the heat affected zone are significantly hardened, and the mechanical properties of the welded portion are deteriorated. From such a viewpoint, the C content is 0.05 to 0.3% by mass, preferably 0.08 to 0.15% by mass.

-Si: 0.01-2.5 mass%
Si is an element effective for strengthening steel. Further, it is a ferrite-forming element and has the function of promoting the formation of retained austenite because it promotes the concentration of C in austenite and suppresses the formation of carbides. If the amount of Si is less than 0.01% by mass, such an effect cannot be sufficiently obtained. On the other hand, when Si is added excessively, ductility, surface properties, and weldability deteriorate. From such a viewpoint, the Si amount is 0.01 to 2.5% by mass, preferably 0.7 to 2.0% by mass.
Mn: 0.5 to 3.5% by mass
Mn is an element effective for strengthening steel and promotes the generation of low-temperature transformation phases such as tempered martensite. Such an effect is recognized when the amount of Mn is 0.5 mass% or more. On the other hand, when Mn is added excessively exceeding 3.5 mass%, the ductile deterioration of ferrite due to excessive increase of the second phase fraction or solid solution strengthening becomes remarkable, and the formability is lowered. From such a viewpoint, the amount of Mn is 0.5 to 3.5% by mass, preferably 1.5 to 3.0% by mass.

・ P: 0.003 to 0.100 mass%
P is an element effective for strengthening steel, and this effect is obtained at 0.003% by mass or more. However, when P is added excessively exceeding 0.100 mass%, embrittlement is caused by grain boundary segregation and impact resistance is deteriorated. From such a viewpoint, the P amount is set to 0.003 to 0.100 mass%.
・ S: 0.02% by mass or less S is an inclusion such as MnS, which may cause deterioration of impact resistance and cracking along the metal flow of the weld. 0.02 mass% or less from the surface.

・ Al: 0.010 to 1.5 mass%, Si + Al: 0.5 to 3.0 mass%
Al acts as a deoxidizer and is an element effective for the cleanliness of steel, and is preferably added in the deoxidation step. If the Al content is less than 0.010% by mass, such an effect cannot be obtained sufficiently, so the lower limit is made 0.010% by mass. Al, like Si, is a ferrite-forming element and has the function of promoting the formation of retained austenite because it promotes the concentration of C in austenite and suppresses the formation of carbides. Such an effect is insufficient when the total amount of Al and Si added is less than 0.5% by mass, and sufficient ductility cannot be obtained. However, if it is added in a large amount, the risk of steel piece cracking during continuous casting increases and the productivity decreases. For this reason, the upper limit of Al amount is 1.5% by mass, and the upper limit of Si amount + Al amount is 3.0% by mass.

The high-strength steel sheet and high-strength hot-dip galvanized steel sheet according to the present invention have the above-described component composition as a basic component, and the balance is composed of iron and unavoidable impurities, but appropriately contain the components described below according to desired characteristics. be able to.
Cr: 0.005-2.00 mass%, Mo: 0.005-2.00 mass%, V: 0.005-2.00 mass%, Ni: 0.005-2.00 mass%, Cu : One or more selected from 0.005 to 2.00% by mass Cr, Mo, V, Ni, Cu suppresses the formation of pearlite during cooling from the annealing temperature, and generates a low-temperature transformation phase. Promotes and works effectively in strengthening steel. Such an effect is obtained when each content of Cr, Mo, V, Ni, and Cu is 0.005 mass% or more. However, when each content of Cr, Mo, V, Ni, and Cu exceeds 2.00% by mass, the effect is saturated, resulting in a cost increase. For this reason, content of Cr, Mo, V, Ni, and Cu shall be 0.005-2.00 mass%, respectively.

-Ti: 0.01-0.20% by mass, Nb: 1 or 2 types selected from 0.01-0.20% by mass Ti, Nb forms carbonitrides and precipitates steel by precipitation strengthening Has the effect of increasing strength. Such an effect is recognized when each content of Ti and Nb is 0.01% by mass or more. On the other hand, when Ti and Nb each contain more than 0.20 mass%, it will become excessively strong and ductility will fall. For this reason, content of Ti and Nb shall be 0.01-0.20 mass%, respectively.

B: 0.0002 to 0.005 mass%
B has the effect of suppressing the formation of ferrite from the austenite grain boundaries and increasing the strength. Such an effect is obtained when the B content is 0.0002 mass% or more. However, if the amount of B exceeds 0.005% by mass, the effect is saturated and causes an increase in cost. For this reason, the amount of B shall be 0.0002-0.005 mass%.
-Ca: 0.0001 to 0.005 mass%, REM: One or two types selected from 0.0001 to 0.005 mass% Ca and REM are both workable by controlling the form of sulfide. It has the effect of improving, and such an effect is obtained when the content of Ca and REM is 0.0001% by mass or more. However, excessive addition can adversely affect cleanliness. For this reason, content of Ca and REM shall be 0.0001-0.005 mass%, respectively.

Next, the reasons for limiting the metal structures of the high-strength steel sheet and high-strength hot-dip galvanized steel sheet according to the present invention will be described.
-Ferrite area ratio: 20% or more If the ferrite area ratio is less than 20%, the balance between TS and EL decreases. Therefore, the area ratio of ferrite is set to 20% or more.
-Area ratio of tempered martensite: 10-60%
Tempered martensite is a composite structure of ferrite and cementite having a high dislocation density obtained by heating martensite to a temperature equal to or lower than the Ac ₁ transformation point, and effectively works to strengthen steel. Tempered martensite is a metal phase that has less adverse effects on hole expandability than retained austenite and martensite and is effective in ensuring strength without a significant decrease in hole expandability. Furthermore, when tempered martensite coexists with retained austenite and martensite, a decrease in stretch flangeability due to retained austenite and martensite is also suppressed. If the area ratio of tempered martensite is less than 10%, such an effect cannot be sufficiently obtained, while if it exceeds 60%, the balance between TS and EL is lowered. Therefore, the area ratio of tempered martensite is 10 to 60%.

-Martensite area ratio: 0-10%
The martensite phase works effectively to increase the strength of the steel, but if the area ratio exceeds 10% and exists excessively, the stretch flangeability is significantly lowered. For this reason, the area ratio of martensite is 10% or less. Even if the martensite is not included at all and the area ratio is 0%, the effect of the present invention is not affected and there is no problem.
-Volume ratio of retained austenite: 3 to 10%
Residual austenite not only contributes to the strengthening of the steel, but also works effectively to improve the balance between steel TS and EL. Such an effect is obtained when the volume ratio is 3% or more. On the other hand, the retained austenite phase is transformed into martensite by processing, and stretch flangeability is lowered. When the volume fraction of retained austenite exceeds 10%, stretch flangeability and bendability are significantly lowered. For this reason, the volume ratio of retained austenite is 3 to 10%.

The high-strength steel sheet and the high-strength hot-dip galvanized steel sheet of the present invention may contain one or two of pearlite and bainite as metal phases other than the above-described ferrite, tempered martensite, martensite, and retained austenite. There is no problem as long as the above phase structure is satisfied. However, from the viewpoint of securing ductility and stretch flangeability, the area ratio of pearlite is preferably 3% or less.
-Ratio (m) / (f) of Vickers hardness (m) of tempered martensite and Vickers hardness (f) of ferrite: 3.0 or less, preferably 2.0 to 3.0
When the ratio (m) / (f) of Vickers hardness (m) of tempered martensite to Vickers hardness (f) of ferrite exceeds 3.0, the bendability decreases, so the hardness ratio (m) / (f) is 3 0.0 or less. On the other hand, when the hardness ratio (m) / (f) is less than 2.0, it tends to be difficult to ensure the strength. From the viewpoint of balance between TS and EL and bendability, the hardness ratio (m) / (f) is 2. It is preferable to set it as 0-3.0.

Next, preferable production conditions for the high-strength steel sheet and the high-strength hot-dip galvanized steel sheet according to the present invention will be described.
When manufacturing the steel plate of this invention, the steel adjusted to the component composition as mentioned above is melted in a converter or the like, and is made into a steel slab by a continuous casting method or the like. The steel slab is hot-rolled to obtain a hot-rolled steel sheet, or further cold-rolled to obtain a cold-rolled steel sheet, and the hot-rolled steel sheet or the cold-rolled steel sheet is subjected to continuous annealing. Moreover, when manufacturing a hot dip galvanized steel sheet, after this continuous annealing, hot dip galvanization is given, and also the galvanization alloying process is given as needed. In the manufacture of such a hot dip galvanized steel sheet, the continuous annealing may be performed in a continuous hot dip galvanizing line.

-Hot rolling conditions A steel slab is hot-rolled to obtain a hot rolled steel sheet having a metal structure with a total area ratio of bainite and martensite of 80% or more. When annealing a hot-rolled steel sheet (or a cold-rolled steel sheet obtained by cold-rolling a hot-rolled steel sheet), austenite is generated by heating to the Ac ₁ transformation point or higher. In particular, it is preferentially generated from the position of bainite or martensite in the hot-rolled sheet structure, but if the total area ratio of bainite and martensite is less than 80% in the structure of the hot-rolled steel sheet, austenite is generated during annealing. Becomes non-uniform, and austenite is preferentially generated from the position where alloy elements such as Mn are segregated during casting. Austenite generated during annealing partially becomes tempered martensite by subsequent cooling and reheating treatment, but austenite generated nonuniformly as described above is unlikely to undergo martensitic transformation during cooling or tempering by reheating. For example, it is difficult to obtain the final structure in the present invention for reasons such as being difficult to occur. For this reason, the sum total of the area ratio of the bainite and martensite of a hot-rolled steel sheet shall be 80% or more.

In order to obtain the metal structure of the hot-rolled steel sheet as described above, in the hot rolling, after finishing the rolling at a finish rolling temperature not lower than the Ar ₃ transformation point, the steel sheet is cooled at an average cooling rate of 50 ° C./second or higher. It is preferable to wind up at ˜550 ° C.
When the finish rolling finish temperature is less than Ar ₃ points or the average cooling rate after rolling is less than 50 ° C./second, ferrite is excessively generated during rolling or cooling, and the hot rolled sheet structure has an area ratio of bainite and martensite. It is difficult to make the total of 80% or more. On the other hand, if the winding temperature exceeds 550 ° C., ferrite and pearlite are generated after winding, and it becomes difficult for the hot rolled sheet structure to have a total area ratio of bainite and martensite of 80% or more. Moreover, if coiling temperature is less than 300 degreeC, the shape of a hot-rolled steel plate will deteriorate, the intensity | strength of a hot-rolled steel plate will rise excessively, and cold rolling will become difficult.

-Continuous annealing conditions When continuously annealing a hot-rolled steel sheet or a cold-rolled steel sheet obtained by cold rolling this hot-rolled steel sheet, after heating to 750-900 ° C and holding for 10 seconds or more, from 750 ° C to 10 ° C Cool to a temperature range of 100 to 350 ° C. at an average cooling rate of at least ° C./second, then reheat to 350 to 600 ° C. and hold for 10 to 600 seconds.
If the heating temperature for continuous annealing is less than 750 ° C. or the holding time is less than 10 seconds, austenite is not sufficiently generated during annealing, and a sufficient amount of low-temperature transformation phase cannot be secured after annealing cooling. On the other hand, if the heating temperature exceeds 900 ° C., it is difficult to secure 20% or more of ferrite in the final structure. The upper limit of the holding time is not particularly specified, but holding for 600 seconds or more saturates the effect and leads to an increase in cost. Therefore, the holding time is preferably less than 600 seconds.

  When the average cooling rate from 750 ° C. is less than 10 ° C./second, pearlite is produced, and ductility and hole expansibility are lowered. The upper limit of the average cooling rate is not particularly defined. However, if the cooling rate is too high, the shape of the steel sheet deteriorates and it becomes difficult to control the temperature at which the cooling reaches, and therefore it is preferably 200 ° C./second or less. The cooling ultimate temperature is one of the most important conditions in the present invention. When the cooling is stopped, a part of austenite is transformed into martensite, and the rest becomes untransformed austenite. After reheating from there (further, after plating or plating / alloying treatment if necessary), by cooling to room temperature, martensite becomes tempered martensite, and untransformed austenite becomes residual austenite or martensite. Become. The lower the temperature reached from the annealing, the lower the amount of martensite generated during cooling and the lower the amount of untransformed austenite. Therefore, the final martensite and residual austenite and tempered martensite are controlled by controlling the cooling temperature. Is determined (area ratio or volume ratio). When the cooling attainment temperature exceeds 350 ° C., the martensite transformation at the time of cooling stop is insufficient and the amount of untransformed austenite is increased, the final martensite or residual austenite is excessively generated, and the hole expandability is lowered. On the other hand, when the temperature reached by cooling is less than 100 ° C., austenite is almost transformed into martensite during cooling, and the amount of untransformed austenite is reduced, and 3% or more of retained austenite cannot be obtained. For this reason, cooling ultimate temperature shall be the range of 100-350 degreeC.

  After cooling to a temperature range of 100 to 350 ° C., reheating to 350 to 600 ° C. and holding for 10 to 600 seconds, the martensite generated during the cooling is tempered to become tempered martensite. Expandability is improved, and untransformed austenite that has not been transformed into martensite at the time of cooling is stabilized. Finally, 3% or more of retained austenite is obtained, and ductility is improved. Although the details of the mechanism of stabilization of untransformed austenite by reheating and holding are unknown, it is considered that the concentration of C in the untransformed austenite proceeds and the austenite is stabilized. If the reheating temperature is less than 350 ° C., the tempering of martensite and the stabilization of austenite are insufficient, and the hole expandability and ductility are lowered. On the other hand, when the reheating temperature exceeds 600 ° C., untransformed austenite at the time of cooling stop is transformed into pearlite, and finally 3% or more of retained austenite cannot be obtained. If the holding time is less than 10 seconds, the stabilization of austenite becomes insufficient, and if it exceeds 600 seconds, untransformed austenite at the time of cooling stop is transformed into bainite, and finally 3% or more of retained austenite cannot be obtained. Therefore, the reheating temperature is in the range of 350 to 600 ° C., and the holding time in that temperature range is 10 to 600 seconds.

-Hot dip galvanizing treatment conditions The conditions of the hot dip galvanizing treatment are not particularly limited, but 0.12 to 0.22 mass% at the time of hot dip galvanized steel sheet (GI) production, and at the time of galvannealed steel sheet (GA) production. It is preferable that the amount of dissolved Al is 0.08 to 0.18% by mass, and is preferably performed in a plating bath having a bath temperature of 440 to 500 ° C. After the steel sheet has entered the plating bath, the amount of plating adhered by gas wiping or the like Adjust. At the time of manufacture of the galvannealed steel sheet, after adjusting the coating amount, the alloying treatment is performed by heating to 450 to 600 ° C. and holding for 1 to 30 seconds.
In addition, you may add temper rolling to the steel plate (including alloyed hot-dip galvanized steel plate) after the hot dip galvanizing treatment for adjustment of shape correction, surface roughness, and the like. Moreover, you may give processes, such as resin or oil-fat coating and various paintings.
Moreover, in the manufacture of hot dip galvanized steel sheets (including alloyed hot dip galvanized steel sheets), it is preferable to perform the above-described continuous annealing of hot-rolled steel sheets or cold-rolled steel sheets in a continuous hot-dip galvanizing line.

Other manufacturing conditions are not particularly limited, but preferred examples are shown below.
Casting conditions The steel slab to be used is preferably produced by a continuous casting method in order to prevent macro segregation of components, but may be produced by an ingot-making method or a thin slab casting method. In addition, after manufacturing the steel slab, it was cooled to room temperature and then heated again, and then it was charged into the heating furnace as it was without cooling to room temperature, or a little warming was performed. Energy saving processes such as direct feed rolling and direct rolling, which are rolled immediately afterwards, can be applied without any problem.

-Hot rolling conditions The slab heating temperature is preferably 1100 ° C or higher. Low-temperature heating is preferable in terms of energy, but if the heating temperature is less than 1100 ° C., carbides cannot be sufficiently dissolved, and problems such as increased risk of trouble during hot rolling due to an increase in rolling load occur. Cheap. Note that the slab heating temperature is desirably 1300 ° C. or less because of an increase in scale loss accompanying an increase in oxidized weight.
Moreover, in order to prevent the trouble at the time of hot rolling at the time of making slab heating temperature low, you may utilize what is called a sheet bar heater which heats a sheet bar.

In the hot rolling step, part or all of the finish rolling may be lubricated rolling in order to reduce the rolling load during hot rolling. Performing lubrication rolling is also effective from the viewpoint of uniform steel plate shape and uniform material. In addition, it is preferable to make the friction coefficient in the case of lubrication rolling into the range of 0.25-0.10. Moreover, it is preferable to set it as the continuous rolling process which joins the sheet | seat bars which precede and follow, and finish-rolls continuously. The application of the continuous rolling process is also desirable from the viewpoint of the operational stability of hot rolling.
When performing cold rolling on a hot-rolled steel sheet, preferably the oxidized scale on the surface of the hot-rolled steel sheet is removed by pickling, and then subjected to cold rolling to obtain a cold-rolled steel sheet having a predetermined thickness. Pickling conditions and cold rolling conditions are not particularly limited, and may be according to ordinary methods. The rolling reduction of cold rolling is preferably 40% or more.

Steel having the composition shown in Table 1 and the balance being Fe and unavoidable impurities was melted in a converter and made into a slab by a continuous casting method. The obtained slab was hot-rolled to a thickness of 3.0 mm under the conditions shown in Tables 2 and 3. After pickling this hot-rolled steel sheet, it is cold-rolled to a sheet thickness of 1.4 mm to obtain a cold-rolled steel sheet, which is continuously annealed under the conditions shown in Tables 2 and 3 in a continuous hot-dip galvanizing line (CGL), Hot dip galvanizing was performed. In some examples, the steel sheet hot-rolled to a thickness of 2.3 mm was pickled, and the hot-rolled steel sheet was continuously annealed under the conditions shown in Table 3 in a continuous hot-dip galvanizing line. Plating was applied. The hot dip galvanizing temperature was 460 ° C., and after the plating, alloying treatment was performed at 520 ° C., and cooling was performed at an average cooling rate of 10 ° C./second. Moreover, in some Examples, the hot dip galvanized steel plate which does not perform an alloying process was manufactured. The plating adhesion amount was 35 to 45 g / m ² per side. Moreover, about some Examples (No. 36-42), the cold-rolled steel plate and hot-rolled steel plate mentioned above were continuously annealed by the continuous annealing line (CAL), and it was set as the product steel plate without performing hot dip galvanization.

About the obtained steel plate (non-plated steel plate) and hot-dip galvanized steel plate, the cross-sectional microstructure, tensile properties, stretch flangeability (hole expandability), and bendability were measured and evaluated. The results are shown in Tables 4 and 5.
The cross-sectional microstructure of the steel sheet is revealed with a 3% nital solution (3% nitric acid + ethanol), and the position of the thickness in the depth direction 1/4 is observed with a scanning electron microscope. Image analysis processing (this image analysis processing can use commercially available image processing software) was performed, and the area fraction of the ferrite phase was quantified. The martensite area ratio and tempered martensite area ratio were quantified with image processing software by taking SEM photographs at an appropriate magnification of 1000 to 3000 times depending on the fineness of the structure.

The volume ratio of retained austenite was determined by polishing the steel plate to a ¼ surface in the plate thickness direction and diffracting X-ray intensity of the ¼ surface thickness. MoKα rays are used as incident X-rays, and the peaks of {111}, {200}, {220}, {311} planes of retained austenite and {110}, {200}, {211} planes of ferrite phases are used. Intensity ratios were determined for all combinations of integrated intensities, and the average value of these ratios was taken as the volume fraction of retained austenite.
The hardness of ferrite and tempered martensite was measured using a micro Vickers hardness meter at a load of 1 g and a load time of 15 s. The vicinity of the center of each phase was measured for 10 particles, and the average value was taken as the hardness of the phase.
Also, the cross-sectional microstructure of the hot-rolled steel sheet was photographed by revealing the structure with a 3% nital solution (3% nitric acid + ethanol), observing the depth direction thickness 1/4 position with a scanning electron microscope. Image analysis processing was performed using the structure photograph, the area ratio of the bainite phase and the martensite phase was quantified, and the sum of them was obtained.

Tensile properties are obtained by performing a tensile test in accordance with JIS-Z2241, using a JIS No. 5 test piece sampled so that the tensile direction is perpendicular to the rolling direction of the steel sheet, and TS (tensile strength) and EL (elongation). The strength-elongation balance value represented by the product of strength and elongation (TS × EL) was determined.
Stretch flangeability was evaluated by a hole expansion rate λ by conducting a hole expansion test in accordance with Japan Iron and Steel Federation standard JFST1001.
The bendability is obtained by collecting a strip-shaped test piece having a width of 30 mm and a length of 120 mm in a direction perpendicular to the rolling direction, and smoothing the end so that the surface roughness Ry is 1.6 to 6.3 s. A bending test was performed at a bending angle of 90 ° by the push bending method, and the minimum bending radius at which no crack or necking occurred was evaluated as the critical bending radius R.

  According to Tables 4 and 5, in all of the examples of the present invention, TS × EL: 21000 MPa ·% or more, hole expansion ratio λ: 70% or more, ratio of critical bending radius R to sheet thickness t R / t: 1 or less As a result, excellent strength / ductility balance, stretch flangeability and bendability are obtained. In contrast, the comparative example is inferior in one or more of TS × EL, hole expansion ratio λ, ratio R / t, and has excellent strength / ductility balance, stretch flangeability and bendability as in the present invention example. Combined performance cannot be obtained.

Claims (14)

  C: 0.05-0.3 mass%, Si: 0.01-2.5 mass%, Mn: 0.5-3.5 mass%, P: 0.003-0.100 mass%, S: 0.02% by mass or less, Al: 0.010 to 1.5% by mass, the total content of Si and Al is 0.5 to 3.0% by mass, the balance being iron and inevitable impurities The composition comprises: 20% or more ferrite by area ratio, 10-60% tempered martensite, 0-10% martensite, 3-10% residual austenite by volume, and tempered martensite. A high-strength steel sheet excellent in workability, characterized by having a metal structure in which a ratio (m) / (f) of Vickers hardness (m) of a site to Vickers hardness (f) of a ferrite is 3.0 or less.   Furthermore, Cr: 0.005-2.00 mass%, Mo: 0.005-2.00 mass%, V: 0.005-2.00 mass%, Ni: 0.005-2.00 mass%, The high-strength steel sheet with excellent workability according to claim 1, comprising one or more selected from Cu: 0.005 to 2.00% by mass.   Furthermore, 1 or 2 types chosen from Ti: 0.01-0.20 mass% and Nb: 0.01-0.20 mass% are contained, The 1 or 2 characterized by the above-mentioned. High-strength steel sheet with excellent workability.   Furthermore, B: 0.0002-0.005 mass% is contained, The high-strength steel plate excellent in workability in any one of Claims 1-3 characterized by the above-mentioned.   Furthermore, 1 type or 2 types chosen from Ca: 0.0001-0.005 mass% and REM: 0.0001-0.005 mass% are contained, Any of Claims 1-4 characterized by the above-mentioned. High-strength steel sheet with excellent workability as described in Crab.   6. The ratio (m) / (f) of Vickers hardness (m) of tempered martensite and Vickers hardness (f) of ferrite is 2.0 to 3.0. High-strength steel plate with excellent workability as described.   A high-strength hot-dip galvanized steel sheet excellent in workability, wherein the base steel sheet is made of the steel sheet according to any one of claims 1 to 6.   A steel slab having the composition according to any one of claims 1 to 5 is hot-rolled to form a hot-rolled steel sheet having a metal structure with a total area ratio of bainite and martensite of 80% or more. When performing continuous annealing on the steel sheet, after heating to 750 to 900 ° C. and holding for 10 seconds or more, the steel sheet is cooled from 750 ° C. to a temperature range of 100 to 350 ° C. at an average cooling rate of 10 ° C./second or more, and then 350 to A method for producing a high-strength steel sheet excellent in workability, characterized by reheating to 600 ° C. and holding for 10 to 600 seconds and then cooling to room temperature.   A steel slab having the composition according to any one of claims 1 to 5 is hot-rolled to form a hot-rolled steel sheet having a metal structure with a total area ratio of bainite and martensite of 80% or more. The steel sheet is cold-rolled to form a cold-rolled steel sheet. When the cold-rolled steel sheet is subjected to continuous annealing, it is heated to 750 to 900 ° C. and held for 10 seconds or more, and then an average cooling rate of 750 ° C. to 10 ° C./second or more. After cooling to a temperature range of 100 to 350 ° C., then reheating to 350 to 600 ° C. and holding for 10 to 600 seconds, followed by cooling to room temperature, a method for producing a high-strength steel sheet having excellent workability . The hot rolling process, after completion of the rolling at Ar ₃ transformation point or more of the finish rolling temperature, cooling at an average cooling rate of more than 50 ° C. / sec, claim, characterized in that wound at 300 to 550 ° C. 8 Or the manufacturing method of the high strength steel plate excellent in workability of 9.   A steel slab having the composition according to any one of claims 1 to 5 is hot-rolled to form a hot-rolled steel sheet having a metal structure with a total area ratio of bainite and martensite of 80% or more. When performing continuous annealing on the steel sheet, after heating to 750 to 900 ° C. and holding for 10 seconds or more, the steel sheet is cooled from 750 ° C. to a temperature range of 100 to 350 ° C. at an average cooling rate of 10 ° C./second or more, and then 350 to A method for producing a high-strength hot-dip galvanized steel sheet excellent in workability, characterized in that it is reheated to 600 ° C. and held for 10 to 600 seconds, and then hot-dip galvanized.   A steel slab having the composition according to any one of claims 1 to 5 is hot-rolled to form a hot-rolled steel sheet having a metal structure with a total area ratio of bainite and martensite of 80% or more. The steel sheet is cold-rolled to form a cold-rolled steel sheet. When the cold-rolled steel sheet is subjected to continuous annealing, it is heated to 750 to 900 ° C. and held for 10 seconds or more, and then an average cooling rate of 750 ° C. to 10 ° C./second or more. The steel is cooled to a temperature range of 100 to 350 ° C., then reheated to 350 to 600 ° C., held for 10 to 600 seconds, and then subjected to hot dip galvanization. Manufacturing method of galvanized steel sheet. The hot rolling step, after finishing rolling at a finish rolling temperature not lower than the Ar ₃ transformation point, is cooled at an average cooling rate of 50 ° C / second or higher, and is wound at 300 to 550 ° C. Or the manufacturing method of the high intensity | strength hot-dip galvanized steel plate excellent in the workability of 12.   The method for producing a high-strength hot-dip galvanized steel sheet excellent in workability according to any one of claims 11 to 13, wherein a plating alloying treatment is performed after hot-dip galvanizing.