JP2000297348A - Nigh strength cold rolled steel sheet excellent in fatigue characteristic of base material, excellent in formability after welding and hard to be softened in welding heat affected zone and high strength surface treated steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength steel sheet in which the fatigue characteristics of a base material and press formability after welding are made better, and the reduction of strength in the welding heat affected zone can be suppressed. SOLUTION: This steel sheet is the one contg., by weight, 0.01 to 0.15% C, 0.005 to 1.0% Si, 0.1 to 2.2% Mn, 0.001 to 0.05% P, 0.001 to 0.01% S, 0.0005 to 0.01% N, 0.001 to 0.10% Al, 0.001 to 0.02% Ti, 0.005 to 0.05% Nb, 0.05 to 0.5% Mo, 0.2 to 2.0% Cu, 0.05 to 2.0% Ni and Fe as the main component and also satisfying the inequality of 0.22>=C(%)+(Si/30)(%)+(Mn/20)(%)+(Mo/15)(%).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength cold-rolled steel sheet and a high-strength surface-treated steel sheet which are excellent in the fatigue properties of a base material and the formability after welding, and in which the heat affected zone of the weld heat affected zone is not easily softened. Things.

[0002]

2. Description of the Related Art Hitherto, in the production of automobile bodies or parts, members formed by press molding or the like have been integrated by spot welding, arc welding, or the like, and assembled. In recent years, in order to reduce the manufacturing cost, in order to reduce the weight of the vehicle body and improve the material yield, a method has been applied in which steel sheets having different material strengths or different thicknesses are integrated by welding and then pressed. .
In particular, the use of high-strength steel sheets has been actively promoted in order to achieve weight reduction of vehicle bodies.

[0003] However, during press forming after welding, since there is a welded portion and a heat affected zone, a problem has occurred that has not been recognized in the conventional manufacturing process in which welding is performed after press forming. That is, there is a reduction in formability due to cracks in the welded portion during pressing and softening of the material of the heat affected zone.

Up to now, the improvement of the strength of the welded portion itself has been disclosed in JP-A-3-199343 and JP-A-5-186.
Although many proposals have been made in, for example, JP-A-849, there is no need to perform molding after welding. As a method for satisfying the formability after welding, there is a proposal in Japanese Patent Application Laid-Open No. 7-26346. This technology optimizes the composition of ultra-low carbon steel to improve the formability after welding, and achieves superior formability after welding compared to conventional ultra-low carbon steel. , The following problems remained.

That is, since the above-mentioned invention is made of ultra-low carbon steel, it is a material having relatively low strength. In order to further reduce the weight of an automobile body, it is necessary to apply a high-strength material. The uncertainty of the formability after welding when a high-strength steel sheet is used, and a decrease in the strength of the weld heat-affected zone after welding, that is, softening of the weld heat-affected zone causes problems in product reliability. Remains. On the other hand, since a repeated load is applied to the automobile member as described above due to vibration during traveling, it is desired that both the base metal portion and the welded portion have excellent fatigue characteristics.

As for the fatigue properties of high-strength steel sheets, there have been many proposals mainly for hot-rolled steel sheets, and few proposals for high-strength cold-rolled steel sheets and high-strength surface-treated steel sheets. Under such circumstances, according to the proposal of JP-A-3-264646, it is possible to improve the fatigue properties by forming a specific composite structure, but the properties relating to press formability after welding are unclear, There is no steel sheet that focuses on both fatigue strength and press formability after welding.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in order to solve such problems, and has been made to improve the fatigue properties of a base material of a high strength steel sheet and the press formability after welding. The present invention relates to a high-strength cold-rolled steel sheet and a high-strength surface-treated steel sheet capable of suppressing a decrease in strength of a weld heat affected zone.

[0008]

In order to solve the above problems, the present invention provides (1) C: 0.01 to 0.15% by weight and S:
i: 0.005 to 1.0%, Mn: 0.1 to 2.2%, P: 0.00
1-0.05%, S: 0.001-0.01%, N: 0.0005-0.
01%, Al: 0.001 to 0.1%, Nb: 0.005 to 0.05%,
Mo: 0.05 to 0.5%, Cu: 0.2 to 2.0%, Ni: 0.05
-2.0%, and excellent in fatigue properties and formability after welding, characterized by satisfying the following formula (A), which is mainly composed of Fe, and hardly softens the heat affected zone by welding. High-strength cold-rolled steel sheet and high-strength surface-treated steel sheet,

0.22 ≧ C (%) + (Si / 30) (%) + (Mn / 20) (%) + (Mo / 15) (%) −−−− (A) (2) Ti : Characterized by containing 0.001 to 0.02%,
(1) A high-strength cold-rolled steel sheet and a high-strength surface-treated steel sheet which are excellent in the fatigue properties of the base material and the formability after welding and which are not easily softened in the heat affected zone of the weld, and (3) the above (1) or (2). The steel sheet according to any one of the preceding claims, wherein the dislocation density is 50 or more and 10000 or less per 1 µm ^{2 in} a planar field of view, excellent in fatigue properties of a base metal and excellent in formability after welding, and softening of a weld heat affected zone. The base material according to (1), (2) or (3), wherein the surface treatment of the high-strength cold-rolled steel sheet and the high-strength surface-treated steel sheet that is difficult to perform, and (4) the surface treatment of the high-strength surface-treated steel sheet is galvanization. And a high-strength cold-rolled steel sheet and a high-strength surface-treated steel sheet which are excellent in fatigue characteristics and formability after welding and hardly soften the heat affected zone.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors investigated a steel sheet and a welding method as a method for preventing softening of a heat affected zone of a weld while ensuring press formability of the steel sheet after welding. First, when the formability after welding was investigated, when welding a high-strength steel sheet, the strength of the base metal, the welded portion, and the weld heat-affected zone changes depending on the heat history during welding. It was found that the press-forming ability after welding is determined as a result of the strength-ductility interaction of the heat-affected zone.
And, C, Si, Mn, P, S, Al, N, Mo, N
b, Ti, Cu, Ni, among which Mo,
It has been found that when Mn, Si and C satisfy the relational expressions, the formability after welding is improved.

Further, as a result of studying a method for preventing softening of the heat affected zone, it was found that the combined addition of Nb and Mo is effective. This is because the combined addition of Nb and Mo suppresses the disappearance of dislocations in the steel sheet even when the temperature of the steel sheet rises due to welding, and the dislocations serve as precipitation nuclei, and the (Nb, Mo) C Is considered to suppress the softening of the heat-affected zone. Further, in order to more clearly exert this effect, it is desirable that the dislocation density in the steel sheet is 50 or more per 1 μm ^{2 in a} planar view.

Hereinafter, the present invention will be described in detail. First, the reasons for limiting the components of steel will be described. C is an indispensable element for maintaining the strength of the base metal itself, and at the same time, 0.01% is used for precipitating (Nb, Mo) C during welding and preventing softening of the weld heat affected zone. Need more. However, when the content is increased, the weldability is significantly hardened and the ductility is reduced at the same time as the workability of the base material is deteriorated. Therefore, the upper limit is 0.15%.

Si is used as an auxiliary element for obtaining the strength of the base material. If it is less than 0.005%, the production cost is high and it is economically disadvantageous. Therefore, the lower limit is 0.005%, and if it exceeds 1.0%, the removal of scale in the hot rolling step is costly and economical. Therefore, the upper limit is 1.0%.

Mn is an element for securing the strength of the base material, and if it is less than 0.1%, it is costly to melt and economically disadvantageous, so the lower limit is 0.1%. .
If it exceeds 2.2%, the workability of the base material will deteriorate and
Since the formability of the welded portion also deteriorates, the upper limit is 2.2%.

If P is less than 0.001%, the cost is industrially high, so the lower limit of P is 0.001%. Also, 0.
If it exceeds 05%, solidification segregation during casting remarkably lowers internal cracks and workability, and at the same time causes embrittlement of a welded portion. Therefore, the upper limit is made 0.05%.

If the content of S is less than 0.001%, the production cost is increased. Therefore, the lower limit of S is set to 0.001%. Also,
If it exceeds 0.01%, hot embrittlement occurs, so that 0.0%
The upper limit is 1%.

Al is an element necessary for deoxidizing steel.
If it is less than 01%, deoxidation will be insufficient and defects such as pinholes will occur. Therefore, the lower limit is 0.001%. If it exceeds 0.1%, inclusions such as alumina increase and the ductility of steel is impaired. The upper limit is 1%.

N is involved in the precipitation of (Nb, Mo) C,
Since this precipitate is slightly contained, the content is made 0.0005% or more. Also, if it is contained over 0.01%,
In the hot rolling process, NbN precipitates and the amount of Nb effective for preventing the softening of the heat affected zone during welding is reduced, so that the amount of NbN is reduced.
01% is the upper limit.

Nb, together with Mo, has an effect of preventing softening of the heat affected zone by welding, and is an essential element in the present invention.
If it is less than 0.005%, the corrosion resistance and the effect of preventing softening of the weld heat affected zone are lost, so the lower limit is made 0.005%.
If the content exceeds 0.05%, the workability of the base material deteriorates.
The upper limit is 0.05%.

Mo is an element effective in preventing softening of the heat affected zone by adding Nb in a complex manner, and is an essential element in the present invention. If it is less than 0.05%, the effect of preventing softening of the weld heat affected zone is lost, so the lower limit is 0.05%, and if it exceeds 0.5%, the effect is saturated and the number of inclusions causing scratches increases. , 0.5% as the upper limit.

Cu is an element effective for improving the fatigue strength, and if it is less than 0.2%, an effect effective for improving the fatigue characteristics cannot be obtained. Therefore, the lower limit is set to 0.2%. On the other hand, if it exceeds 2.0%, the effect of improving the fatigue characteristics is saturated, and the cost is increased. Therefore, the upper limit is set to 2.0%.

Ni is necessary for suppressing surface defects (Cu scab) generated during hot rolling of Cu-added steel, and keeps the surface quality of the steel sheet at high quality and prevents hot brittleness. Therefore, 0.05% or more of Ni is added. The amount added is 2.
Even if it is added in excess of 0%, the effect of improving the surface quality is saturated and the cost is increased, so the upper limit is 2.0%. In addition, since the effect of Ni addition is exhibited according to the addition amount of Cu, the ratio of Ni addition amount is desirably set to 0.25 to 0.60 according to Ni / Cu.

Ti improves the formability after welding by fixing C, N, and S. In order to exhibit this effect, 0.001% or more must be added. However, if it is added excessively, the workability of the base material is deteriorated by a large amount of carbonitride precipitated, so the upper limit is made 0.02%.

Further, in the present invention, it is important that the amounts of C, Si, Mn, and Mo satisfy the following formula (A) among the above various components.

0.22 ≧ C (%) + (Si / 30) (%) + (Mn / 20) (%) + (Mo / 15) (%) −−−− (A) Conducted an overhang test after butt welding of the same material on high-strength cold-rolled steel sheets having various chemical components, and investigated the relationship between the value on the right side of the above formula (A) and the overhang height. The result is shown in FIG. The horizontal axis is (A)
The value calculated from the right side of the equation, the vertical axis is a value (formability index) that is standardized by dividing the overhang height of the steel sheet by welding by the overhang amount of the steel sheet before welding, the larger the formability index is, The formability after welding is excellent. From FIG. 1, when the formula (A) is satisfied, that is, C, Si, Mn, Mo
It can be seen that when the addition amount of the compound is in accordance with the present invention, the moldability index is large and the moldability is excellent.

This is because C, Si, Mn, and Mo increase the strength of the weld zone and the heat-affected zone during welding, and excessive addition lowers the ductility of the weld zone and the heat-affected zone. It is considered that the moldability afterwards is deteriorated.

The data shown in FIG. 1 are the results of various welding methods (TIG welding, plasma welding, laser welding, seam welding (mash seam) welding).
In the present invention, even if the welding method is different, the formability after welding is almost the same as long as the chemical component satisfies the formula (A).

Further, Cr inevitably present in the steel sheet,
Subcomponents such as B, V, Ca, and Mg do not impair the properties of the steel of the present invention at all, but if present in large amounts, increase the recrystallization temperature and decrease the rollability, making production difficult. There is fear. Therefore, these subcomponents contain 0.1% or less of Cr, 0.01% or less of Mg and Ca, and 0.1% or less of B.
005% or less, and V is preferably limited to 0.01% or less. The method for producing the high-strength cold-rolled steel sheet and the high-strength surface-treated steel sheet of the present invention may be appropriately selected according to the application and required characteristics.

The steel adjusted to the above components is formed into a steel sheet according to the following method, for example. First, steel is melted in a converter and formed into a slab by a continuous casting method. Leave this slab in a high temperature state or after cooling it to room temperature, insert it into a heating furnace,
Heating in a temperature range of 1000 to 1250 ° C.,
Finish rolling is performed in the temperature range of 00 to 950 ° C.
It is wound at a temperature of 00 ° C. or less to form a hot-rolled steel sheet. Next, after pickling and cold rolling, annealing is performed to obtain a cold rolled steel sheet. In the case of a high-strength surface-treated steel sheet, plating is further performed. Annealing is
700 ° C. or higher and lower than 900 ° C. is preferable. If the temperature is lower than 700 ° C., sufficient recrystallization is not performed, and it is difficult to stably obtain the workability of the base material itself. Therefore, the annealing temperature is 700 ° C.
Is the lower limit. On the other hand, if the temperature exceeds 900 ° C., the crystal grains of the base material become coarse and the surface may be roughened during pressing.
This is the upper limit.

For example, most high-strength surface-treated steel sheets used for automobiles, home appliances, and building materials are hot-dip galvanized steel sheets.
Annealing and plating are performed simultaneously in the same equipment (or the same equipment row). The plating amount is 3 mg / m ^{2 to} 800 g /
subjected to the m ² on the surface of the steel sheet. If it is less than 3 mg / m ² , the anticorrosion effect is lost and the purpose of plating cannot be achieved. On the other hand, if it exceeds 800 g / m ² , defects such as blowholes are liable to occur remarkably during welding, so the plating amount is set within the above range.

The effect of the present invention is not impaired when annealing and plating are performed simultaneously, as in hot-dip galvanizing, or when electroplating and an organic composite film are applied after annealing.

Further, when the dislocation density of the obtained high-strength cold-rolled steel sheet and high-strength surface-treated steel sheet is 50 or more per 1 μm ^{2 in a} planar view, there is an effect of suppressing softening of the weld heat affected zone. Although the number of dislocation densities varies depending on the location and orientation, the average value of 10 visual fields of a transmission electron microscope is taken. If the value is 50 or more / 1 μm ² or more, (Mb,
Mo) C precipitates in a short time, and suppresses the softening of the heat affected zone more effectively. In addition, the dislocation density is 10,000
If it exceeds μm ² , press formability may be degraded and cracks may occur, so the upper limit was set to 10000 pieces / 1 μm ² . In addition, in a normal annealing material, the dislocation density is 5 to 20
Since this is 1 μm ² , in order to obtain this effect, a plastic strain of 1.0% or more and less than 10.0% in elongation may be applied. As a method for applying strain, skin pass rolling or a method of cutting out a steel sheet and then applying a tensile strain is used. Thus, a high-strength steel sheet, such as a high-strength cold-rolled steel sheet or a high-strength surface-treated steel sheet, which has excellent formability after welding and hardly softens the heat affected zone of the weld.

[0031]

EXAMPLES (Example 1) Steel having the chemical components shown in Table 1 was melted in a converter, turned into a slab by continuous casting, and then subjected to hot rolling and cold rolling (sheet thickness: 1.4 mm). Thereafter, some of them were subjected to hot-dip galvanizing (45 g / m ² ). Plastic strain was applied by skin pass rolling. The dislocation density in the steel sheet was measured. As for the dislocation density, the number of dislocations per 1 μm ² of the square visual field was measured for 10 visual fields using a transmission electron microscope, and the average value was defined as the dislocation density. Table 1 shows the results of these measurements.

These high-strength steel sheets were subjected to a tensile test in a rolling direction and a swing plane bending fatigue test according to JIS No. 5. Fatigue properties, the stress at 10 ⁷ cycles and fatigue strength (.sigma.w), tensile measured tensile strength divided by the (TS) in test (.sigma.w / TS) was evaluated as a fatigue limit ratio. The results are shown in Table 2.

Next, these high-strength steel sheets were subjected to butt welding of high-strength steel sheets of the same steel type and evaluated. The welding was performed by laser welding (laser output: 2 k
W, welding speed: 2 m / min, shielding gas: Ar (20 L
/ min))

For evaluation after welding, the formability and the state of softening of the heat affected zone were examined. The moldability is determined by the Erichsen test (JIS
Z 2247, B method), and the critical overhang height of the weld was divided by the critical overhang height of the base material to obtain a formability index. The softening condition of the heat affected zone was measured by a Vickers hardness tester (Hv0.1) at intervals of 0.3 mm at a position 1/2 of the plate thickness in the section including the welded zone, and the base metal hardness and the softened The difference in hardness was measured, and the softening property of the heat affected zone was evaluated. The results are shown in Table 2. In the case of the steel of the present invention, it can be seen that the fatigue properties of the base metal, the formability after welding, and the softening properties of the heat affected zone of welding are all superior to the comparative steel.

Example 2 Using a part of the steel slab shown in Table 1, high strength cold rolled steel sheets and high strength surface treated steel sheets having different final sheet thicknesses were manufactured. The production process was almost the same as the production conditions shown in Table 1, and the thickness was changed by changing the rolling reduction of hot rolling.

These steel sheets were combined and subjected to butt welding by various welding methods (laser welding, mash seam welding, plasma welding), and the formability and softening of the heat affected zone were investigated. The combinations of steel types, welding methods, formability and the results of the softening investigation of the heat affected zone are summarized in Tables 3 and 4.
The method for investigating the formability and the state of softening of the weld heat affected zone is the same as in Example 1. The welding conditions are laser welding:
Welding speed: 2m / min, shielding gas: Ar (20L / mi)
n), plasma welding: welding speed: 0.7 m / min, shielding gas: Ar (6 L / min), mash seam welding: welding speed: 4 m / min, pressing force: 10 kN, lap allowance: 2 m
m, and the heat input in each welding method was appropriately changed as the maximum heat input that would not cause burn-out or welding at the welded portion under the conditions of the combination of the steel sheets.

From the results of Tables 3 and 4, it can be seen that, under the conditions of the plate combination between the steels of the present invention, the formability after welding and the softening characteristics of the heat affected zone of the weld were compared with the conditions of the plate combination between the comparative steels. It turns out that both are excellent. In addition, when the steel of the present invention and the comparative steel are combined, the weld heat affected zone is softened, but it can be seen that the formability after welding is superior to the case where the comparative steels are combined.

[0038]

[Table 1]

[0039]

[Table 2]

[0040]

[Table 3]

[0041]

[Table 4] (continuation of Table 3)

[0042]

According to the present invention, it is possible to provide a high-strength cold-rolled steel sheet and a high-strength surface-treated steel sheet in which the fatigue properties of the base material, the formability after welding, and the heat-affected zone of the weld are not easily softened. The effect can be expected.

[Brief description of the drawings]

FIG. 1 Right side of equation (A) (C (%) + (Si / 30) (%) + (Mn / 20) (%) + (M
FIG. 3 is a diagram showing the effect of o / 15) (%)) on the formability index.

Claims (4)

[Claims] C: 0.01 to 0.15%, Si: 0.005 to 1.0%, Mn: 0.1 to 2.2%, P: 0.001 to 0.05%, S: 0.001 to 0.01%, N: 0.0005 to 0.01% by weight% , Al: 0.001 to 0.1%, Nb: 0.005 to 0.05%, Mo: 0.05 to 0.5%, Cu: 0.2 to 2.0%, Ni: 0.05 to 2.0%, and Fe as a main component and the following formula (A) A high-strength cold-rolled steel sheet and a high-strength surface-treated steel sheet, which have excellent fatigue properties and formability after welding and are less likely to soften a heat-affected zone by welding, characterized by satisfying the following conditions. 0.22 ≧ C (%) + (Si / 30) (%) + (Mn / 20) (%) + (Mo / 15) (%) −−−− (A) 2. The high-strength cold steel according to claim 1, wherein Ti is contained in an amount of 0.001 to 0.02%, and is excellent in fatigue characteristics and formability after welding, and hardly softens the heat affected zone of the weld. Rolled steel sheets and high-strength surface-treated steel sheets. 3. The fatigue properties of a base material and forming after welding according to claim 1, wherein the dislocation density is 50 or more and 10000 or less per 1 μm ^{2 in} a planar field of view. High-strength cold-rolled steel sheet and high-strength surface-treated steel sheet that have excellent heat resistance and are less likely to soften the heat affected zone. 4. The base material according to claim 1, wherein the surface treatment of the high-strength surface-treated steel sheet is galvanization, wherein the base material is excellent in fatigue characteristics and formability after welding, and is affected by welding heat. High-strength cold-rolled steel sheet and high-strength surface-treated steel sheet that are difficult to soften.