JPH11350064A - High strength steel sheet excellent in shape fixability and impact resistance and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a high strength steel sheet excellent in press workability and impact energy absorbability and furthermore excellent in shape fixability at the time of press workability and to provide a method for producing it. SOLUTION: This high strength steel sheet is the one having a compsn. contg., by mass, 0.05 to 0.25% C, <=2.0% Sr, 1.0 to 4.0% Mn, <=0.100% P, <=0.030% S, 0.010 to 0.150% Al and Fe as the main component, having a steel structure of three phases of ferrite + martensite + 1 to 5% residual austenite and having <=0.50 yield ratio and >=50 N/mm<2> baking hardening quantity.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a tensile strength of 440 to 440.
The present invention relates to a high-strength steel sheet of 980 N / mm ² class, particularly to a high-strength steel sheet which has a low yield ratio, has bake hardenability (BH property), and is excellent in shape freezing property and impact resistance.

[0002]

2. Description of the Related Art In recent years, automobiles have been equipped with safety devices such as airbags from the viewpoint of protecting occupants in the event of a collision. However, a structure that can absorb the energy of the collision has been employed in the body structure. It is getting. Research and development has been actively conducted on the impact energy absorption characteristics also from the viewpoint of the material, and an approach from the structural aspect has been attempted mainly for automotive steel sheets.

On the other hand, from the viewpoint of protecting the global environment by suppressing the emission of carbon dioxide, the demand for reducing the weight of automobile bodies has been strong, and thinning of steel sheet materials by increasing the strength is still being pursued.

Under such circumstances, for example, Japanese Patent Application Laid-Open No.
6723 No. As disclosed in Japanese, about the structural member and reinforcing member of an automobile, high strength steel sheet excellent tensile strength 440~980N / mm ² class energy absorption characteristics at the time of collision have been developed.

[0005]

The technique disclosed in the above publication defines a composite structure consisting of a certain amount of martensite structure and a ferrite structure in which solid solution C is suppressed to a certain amount or less by defining the composition and structure of the steel sheet. By using a steel plate, the impact resistance is improved.

However, although the present invention is excellent in impact resistance and press formability under a high strain rate such as collision, it has a problem of shape freezing which becomes a problem when increasing the strength of a steel sheet for press forming. That is, sufficient consideration has not been given to the problem that the molded shape changes due to springback after press molding.

[0007]

In order to clarify the optimal structure of a high-strength steel sheet as a steel sheet material for a structural member of an automobile, the inventors of the present invention have developed a steel sheet having various structures as a press workability. As a result of examining the ductility and impact energy absorption characteristics in the high strain rate region, the steel structure consisting of three phases of ferrite + martensite + a small amount of retained austenite has excellent elongation characteristics and impact energy absorption characteristics. In addition, it was found that it can be applied in a wide range of strength levels.

[0008] Further, considering the shape freezing property after press working, which is one of the factors that hinder the high strength of the steel sheet material,
The yield ratio is high in the ferrite + bainite duplex steel sheet containing a relatively large amount of retained austenite, and the conventional ferrite + martensite composite structure steel sheet has a degree of shape freezing that is basically the same as that of steel sheets having other structures. The present invention achieves a lower yield ratio than before by adjusting the hardness of martensite while maintaining the same two-phase composite structure in the present invention, and achieves plastic freezing by promoting plastic deformation with low stress. Succeeded in improving the quality.

However, when the yield ratio of the steel sheet is set low, the yield stress becomes low, so that the problem of the yield stress, particularly the problem of securing the rigidity of the member, that is, sufficient work hardening does not occur in the portion with low workability. However, there is a problem that the rigidity of the portion is reduced. The inventors have found that this problem can be sufficiently compensated for by increasing the strength by heat treatment during baking coating after processing, that is, by limiting the baking hardening amount to a certain level or more.

The present invention is based on the above study and knowledge, and has completed a high-strength steel sheet which satisfies various properties required for a structural member such as press workability, impact energy absorption property and shape freezing property, and a method for producing the same. is there. That is, the high-strength steel sheet of the present invention has a mass% of C: 0.05 to 0.25%,
Si: 2.0% or less, Mn: 1.0 to 4.0%, P:
0.100% or less, S: 0.030% or less, Al:
0.010 to 0.150% and Fe as a main component, the steel structure is composed of three phases of ferrite + martensite + 1 to 5% of retained austenite, the yield ratio is 0.50 or less, and the bake hardening amount is 50N. / Mm ² or more.

Hereinafter, the present invention will be described in detail. First, the steel sheet structure of the present invention will be described. In the present invention, the structure of the steel sheet is a composite structure composed of three phases of ferrite + martensite + a small amount of retained austenite. Ferrite is necessary to improve ductility and obtain workability,
On the other hand, the transfer of the transition at a high strain rate is considered to be hindered by the hard martensite phase, and is superior in energy absorption characteristics as compared with the structure strengthened steel of a soft transformation phase such as bainite. In addition, martensite is a structure that is extremely advantageous for achieving a low yield ratio aimed at by the present invention by adjusting its hardness. On the other hand, a small amount of retained austenite has almost no effect on lowering the yield ratio, and also contributes to an improvement in ductility by a TRIP (deformation-induced plasticity) effect when subjected to deformation. The amount of retained austenite is desirably 1 to 5%. If it is less than 1%, the effect of improving ductility is too small, while 5%
Exceeding the temperature causes an increase in cost due to the addition of a large amount of component elements, and a decrease in the amount of martensite causes deterioration of impact energy absorption characteristics and a decrease in strength. In addition, a steel plate using a strengthening groove other than the structure strengthening (eg, precipitation strengthening, solid solution strengthening) as the main strengthening mechanism has poor workability typified by ductility or a strength level of at most about 490 N / mm ^2. It is not suitable as the steel sheet structure of the present invention because it is difficult to apply the steel sheet only up to this point.

The yield ratio of the steel sheet of the present invention is 0.50 or less, preferably 0.45 or less. If the yield ratio exceeds 0.50, the steel sheet having a strength level of 440 to 980 N / mm ² class changes the formed shape after press working due to springback, resulting in poor shape freezing property.

The bake hardening amount (BH amount) of the steel sheet of the present invention is 50.
N / mm ² or more, preferably 80 N / mm ² or more. 5
If the yield ratio is 0.50 or less when the yield ratio is less than 0 N / mm ² , the rigidity of the portion with a low working ratio is insufficient, and eventually the rigidity of the entire structural member cannot be secured.

Next, the reasons for limiting the steel component (unit: mass%) of the steel sheet of the present invention will be described. C: 0.05 to 0.25% C has a large effect on the strength of steel and is essential for obtaining a low-temperature transformation product such as martensite. If it is less than 0.05%, it is difficult to obtain a high strength of 440 N / mm ² or more, so the lower limit is made 0.05%. On the other hand, 0.25
%, The weldability is reduced, so the upper limit is made 0.25%.

Si: 2.0% or less Si has an action of easily increasing the strength without deteriorating the ductility, but when added in a large amount exceeding 2.0%, the chemical conversion treatment property is adversely affected. Therefore, it is limited to 2.0% or less.

Mn: 1.0 to 4.0% Mn is an element that stabilizes austenite, is indispensable for forming a small amount of retained austenite in the structure, and changes the amount of solid solution C in austenite. This has a great effect on the properties of low-temperature transformation products such as martensite formed in the cooling process, and is necessary for the formation of martensite. In order to obtain the characteristics of a high-strength steel sheet having extremely excellent workability, it is necessary to add at least 1.0%. However, if it exceeds 4.0%, not only does melting become difficult, but it also has an adverse effect on spot weldability and causes a decrease in strength, so the upper limit is 4.0%.

P: 0.100% or less P is effective for improving the corrosion resistance, but if P: more than 0.100%, the workability deteriorates. Therefore, 0.1
Stop below 00%.

S: 0.030% or less S is an impurity element and degrades stretch flangeability, so the upper limit is made 0.030%.

Al: 0.010-0.150% Al is added for deoxidation. If it is less than 0.010%, the effect is too small, while if it exceeds 0.150%, the workability deteriorates. Therefore, the lower limit is 0.010
%, And the upper limit is 0.150%.

The steel sheet of the present invention comprises the above basic components and F
e is a main component. The main component means that these actions are improved without impairing the inclusion of unavoidable impurities and the actions of the above basic components, or
The purpose is not to hinder the content of an element capable of improving the chemical properties. For example, the following Cr, B, Mo, Ti, N
One or more elements of b and Cu can be contained. That is, the following components (1) to (4) can be used. (1) The basic component further contains one or more of the following Cr and B. (2) The basic component or the component (1) further includes the following Mo
(3) The basic component, the component (1) or the component (2) further containing one or more of the following Ti and Nb (4) The basic component, the component (1), The following (2) component or the above (3) component further containing the following Cu

Cr: 2.0% or less, B: 0.0030% Cr and B have an effect of promoting the formation of martensite. However, if the content of Cr exceeds 2.0% and the content of B exceeds 0.0030%, the amount of ferrite becomes too small and workability deteriorates.

Mo: 1.0% or less Mo is effective for delayed fracture resistance, but if it exceeds 1.0%, the workability deteriorates.

Ti, Nb: 0.100% or less, respectively Ti and Nb are effective for precipitation strengthening of steel.
If it exceeds 00%, workability and shape freezing property deteriorate.

Cu: 1.0% or less Cu is effective for improving corrosion resistance, but if Cu: more than 1.0%, workability is deteriorated. In addition, when adding Cu, it is preferable to add 1.0% or less of Ni in order to improve the surface properties.

Next, the manufacturing method will be described. The steel sheet of the present invention is produced by melting a steel having the above-mentioned composition in a converter or an electric furnace according to a conventional method, and then, according to the following conditions, by hot rolling,
Alternatively, it can be further manufactured by cold rolling.

In the case of a hot-rolled steel sheet, the slab heating temperature, the finishing temperature, and the winding temperature may be in accordance with ordinary methods.
In the cooling process until winding, the hot-rolled steel sheet is cooled from the two-phase region of ferrite + austenite to a temperature below the Ms point and most of the austenite is transformed into martensite so that 1-5% of austenite remains. After
After maintaining the temperature in a temperature range of 100 to 200 ° C. for 10 seconds or more and 10 minutes or less, it is cooled. Further, in the case of a cold-rolled steel sheet, after performing recrystallization annealing after cold rolling, the above-described temperature holding treatment may be performed on the steel sheet after annealing in a continuous annealing furnace. To leave some of austenite together with martensite,
It is convenient to adjust the components. That is, the amount of retained austenite increases as the contents of C, Mn, and B that increase the stability of austenite increase, and the desired three-phase structure can be obtained by adjusting the contents of these elements. In addition, the faster the cooling rate, the more austenite tends to remain.

The cooling from the ferrite + austenite two phase region to a temperature below the Ms point may be generally carried out by water quenching. It is not necessary to quench rapidly by putting, and it is sufficient to cool to a temperature below the Ms point at a cooling rate of 20 ° C / sec or more, preferably 100 ° C / sec or more.
However, in this case, an increase in the component cost is inevitable, so that it is advantageous to make the steel component as low as possible and to perform water quenching in terms of cost and production efficiency.

After completion of the martensitic transformation, 100
1 to 200 ° C., preferably 100 to 150 ° C.
By holding for 0 second or more, the amount of solid solution C in the steel and the martensite hardness are adjusted while the structure is made into three phases of ferrite + martensite + a small amount of retained austenite,
Thereby, excellent workability is obtained, and a predetermined yield ratio and bake hardening amount are obtained. That is, when the heat treatment is not performed, or when the holding time is less than 100 ° C., or when the holding time is less than 10 sec even when the holding temperature is 100 to 200 ° C., the precipitation of carbide hardly occurs, and the workability of the steel sheet is significantly deteriorated. . On the other hand, if the temperature is maintained at a temperature higher than 200 ° C. or at a temperature of 100 to 200 ° C. for 10 minutes or more, the retained austenite is decomposed to deteriorate the workability. Further, excessive precipitation of carbides occurs, so that the amount of solid solution C in the steel becomes too small, and it becomes difficult to secure a necessary amount of bake hardening. Also, martensite is excessively softened, and the yield ratio increases. Further, when the strength is high, the delayed fracture resistance is adversely affected. 1
Holding in a temperature range of 00 to 200 ° C. is performed, for example, by cooling to a temperature lower than 100 ° C. and completing martensitic transformation, as in the case of water quenching, and reheating to the temperature range. Although it is necessary to raise the temperature, when the martensitic transformation is completed at a temperature higher than 100 ° C., the temperature holding treatment may be performed in the subsequent cooling process without reheating.

In the case of a cold-rolled steel sheet, temper rolling and the like may be performed as necessary after annealing. However, if excessive strain is added, the yield ratio will increase. Care must be taken not to exceed.

The method for producing a steel sheet according to the present invention can be applied not only to the production of a hot-rolled steel sheet and a cold-rolled steel sheet, but also to the production of a galvanized steel sheet or an alloyed galvanized steel sheet. In the case of a hot-dip galvanized steel sheet, the above-mentioned temperature holding treatment may be performed on the hot-dip galvanized steel sheet in a cooling process after immersion in a zinc bath, and in the case of an alloyed hot-dip galvanized steel sheet, The same temperature holding treatment may be performed on the steel sheet plated in the cooling process.

[0031]

EXAMPLES Steel having the chemical components shown in Table 1 was melted to form slabs. The slab was hot-rolled in a conventional manner, and then further cold-rolled to obtain a cold-rolled steel sheet having a thickness of 1.2 mm, and continuously annealed in a continuous annealing line under the conditions shown in Table 2 to obtain various 590s.
An N / mm ² grade steel sheet was obtained. The microstructure of the obtained steel sheet was observed under a microscope, and the amount of retained austenite was determined by X-ray measurement. Further, test specimens were taken along the rolling direction, and mechanical properties were examined by a tensile test. These results are also shown in Table 2.

[0032]

[Table 1]

[0033]

[Table 2]

In order to examine the shape freezing property, a steel strip having a width of 40 mm was sampled from the obtained steel sheet in the rolling direction, and as shown in FIG. 1, a hat-shaped drawing bending test member (dimension unit: mm)
Was press-molded, and after release, the radius of curvature ρ of the warpage generated in the vertical wall portion was measured.

Further, in order to examine the dynamic energy absorption characteristics, an impact crush test member (dimension unit: mm) shown in FIG. 2 was manufactured, and the dynamic (impact) absorbed energy was measured. The test member is manufactured by bending a main body 21 having a hat-shaped cross section and then attaching a flat plate 22 of the same material to the opening.
In the flange portion, spot welding is performed at a pitch of 50 mm, side edges are TIG welded, and end plates 23, 23 are TIG welded to both ends in the axial direction. Using this test member, a 200 kg drop weight was dropped in the member axial direction so that the speed at the time of collision was 50 km / hr, and the amount of deformation was 15
The absorbed energy up to 0 mm was measured as the dynamic absorbed energy. On the other hand, in order to examine the static absorbed energy, the test member was compressed at a speed of 1.0 mm / sec by a tensile tester, and the absorbed energy up to 150 mm was determined as the static absorbed energy as in the above case.

Further, a test piece was sampled from the steel sheet along the rolling direction, a tensile strain of 2% was applied to the test piece, and then a baking treatment (processing condition: holding at 170 ° C. × 20 min) was performed to obtain a bake hardening amount. (BH amount) was examined. In addition, in order to examine whether or not there is a problem in the rigidity of the structure when lightly worked, a 2% tensile strain was applied to the test piece, and after the baking treatment (treatment condition: holding at 170 ° C. × 20 min). The yield stress itself was measured and evaluated based on this value. These test results are shown in Table 3.

[0037]

[Table 3]

From Tables 2 and 3, it can be seen that the microstructures, characteristic values, and components are within the scope of the present invention (Sample Nos. 1, 2, 2).
4,8) have a high elongation (El) characteristic of 30% or more and a 0.1% elongation.
A low yield ratio (YR) of less than 50 is realized, and both the shape freezing property and the impact energy absorption property are superior to those of the comparative examples that do not satisfy any of the conditions of the present invention. I understand. Further, the baking hardening amount is sufficiently high, and a sufficient yield strength is obtained by the baking treatment after light working, and it can be seen that there is no problem in terms of rigidity as a steel sheet for a structural member.

[0039]

According to the high-strength steel sheet of the present invention, a three-phase structure of a predetermined component, ferrite and martensite having a small amount of retained austenite, and a yield ratio of 0.5 are provided.
0 or less, the tensile strength was 44
While realizing high strength 0~980N / mm ² grade, good press formability and excellent ductility, excellent shape freezing in excellent impact energy absorption properties and press during processing under a high strain rate at the time of collision It can have sex. Further, since the bake hardening amount is specified to be 50 N / mm ² or more, the rigidity of the processed portion can be ensured even in the press working in a low strain rate region. Also, according to the production method of the present invention, regardless of the type of steel sheet, shape freezing and impact resistance, and further, has excellent rigidity in lightly processed parts,
A high-strength steel sheet having a tensile strength of 440 to 980 N / mm ² can be easily manufactured.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing a test procedure of shape freezing property.

FIG. 2 is a perspective view of an impact crush test member used in the example.

Claims (4)

[Claims] 1. C .: 0.05 to 0.25 in mass%
%, Si: 2.0% or less, Mn: 1.0 to 4.0%, P
: 0.100% or less, S: 0.030% or less, A
l: 0.010 to 0.150% and Fe as a main component, steel structure consisting of three phases of ferrite + martensite + 1 to 5% retained austenite, and a yield ratio of 0.50
A high-strength steel sheet excellent in shape freezing property and impact resistance having a baking hardening amount of 50 N / mm ² or more. 2. The component according to claim 1, further comprising C
The high-strength steel sheet having excellent shape freezing properties and impact resistance according to claim 1, wherein the high-strength steel sheet contains at least one element among elements of r: 2.0% or less and B: 0.0030% or less. 3. The high-strength steel sheet according to claim 1 or 2, further comprising Mo: 1.0% or less in addition to the components according to claim 1 or 2. 4. A steel sheet having the component according to any one of claims 1 to 3 is cooled from a two-phase region of ferrite + austenite to a temperature not higher than the Ms point to reduce the austenite to 1 or less.
After transforming most of the austenite to martensitic so that about 5% remains, 1% at 100-200 ° C.
A method for producing a high-strength steel sheet having excellent shape freezing properties and impact resistance after cooling for 0 sec or more and 10 min or less.