JPH08291364A - Ultra low carbon steel plate with excellent lap resistance welded joint strength - Google Patents

Abstract

(57) [Summary] [Purpose] To provide an ultra-low carbon steel plate having excellent joint strength of a lap resistance welded joint, which is suitable for applications such as automobiles and electric equipment. [Composition] C: 0.005% or less, Cu: 0.01 to 0.5% by weight,
B: 0.0003 to 0.002%, or C: 0.005% or less, Si: 0.5% or less, Mn: 2% or less, Cu: 0.01 to 0.5%, P: 0.15% or less, S: 0.
02% or less, Sol.Al: 0.06% or less, N: 0.007% or less, B: 0.0003
~ 0.002%, or C: 0.005% or less, Si: 0.5% or less, Mn:
2% or less, Cu: 0.01 to 0.5%, P: 0.15% or less, S: 0.02% or less, Sol.Al: 0.06% or less, N: 0.007% or less, B: 0.0003 to 0.00
2%, Ti: 0.01 to 0.2% or Nb: 0.005 to 0.
Ultra low carbon steel sheet containing 1% of 1 or 2 types.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultra-low carbon steel sheet suitable for use in automobiles, electric appliances and the like, and having excellent deep drawability and excellent lap resistance welded joint strength.

[0002]

2. Description of the Related Art High workability is required for steel sheets used for automobiles, electric appliances and the like, and in recent years ultra-low carbon steels having good deep drawability have been used. As these deep-drawing steel sheets, an ultra-low carbon steel sheet for deep-drawing with Ti added is disclosed in JP-B-44-18066, which improves the deep drawability by adding Ti-Nb in combination and further improves the secondary work brittleness resistance by adding B. The improved ultra-low carbon steel sheet is described in JP-A-59-140333 and JP-A-61-113725.

However, these ultra-low carbon steel sheets have a large problem that the crystal grains of the lap resistance welded portion are coarsened and the weld joint strength is significantly lower than that of the low carbon steel sheet because of the low carbon content. . This has been a major factor in hindering the expansion of the range of use of ultra-low carbon steel sheets with excellent workability for structural products using lap resistance welding such as automobiles and electric equipment. Also, as an ultra-low carbon steel sheet with excellent spot weldability, B is added to make the structure of the welded part (hereinafter referred to as the nugget) fine during welding to make the nugget hard and improve joint strength and joint fatigue strength. For example, Japanese Patent Laid-Open No. 63-3
No. 17625 and Japanese Patent Laid-Open No. 63-317649.

[0004]

It is necessary to consider both the nugget strength and the heat-affected zone strength in the joint strength, but under appropriate welding conditions, the heat-affected zone strength often becomes a problem. However, the technique described above does not mention the necessity of the heat-affected zone strength, and is merely intended for hardening the nugget, which is a secondary problem from the standpoint of improving joint strength. It is not only effective, but the joint strength obtained is insufficient. The present invention has been made in view of the above circumstances, and an object thereof is to provide an ultra-low carbon steel sheet having excellent lap resistance welded joint strength.

[0005]

[Means for Solving the Problems] When considering the joint strength of a lap resistance weld, when the fracture position is not inside the nugget, that is, when fracture occurs at the heat-affected zone, the hardness of the nugget has nothing to do with the joint strength. The strength of the heat-affected zone greatly affects the joint strength. Despite the fact that the strength of the heat-affected zone affects the joint strength, no studies have been made on this point so far. Therefore, the present inventors examined the fracture phenomenon in the heat-affected zone of the welded joint, the joint strength depends on the fracture start position during the tensile test, the longer the distance from the nugget center to the fracture start position is long. It has been found that the load bearing portion becomes larger and the joint strength can be increased. Therefore, in order to improve the strength of the heat-affected zone, it was considered effective not only to increase the hardness of the heat-affected zone but also to widen the width of the heat-affected zone at the same time.

Therefore, as a result of careful investigation of the effect of additional elements on the range and hardness of the heat-affected zone in lap resistance welded joints of ultra-low carbon steel sheets, the inventors have found that the combined addition of B and Cu is extremely effective. I found that it would work. B, which is a quenching element, is suitable for increasing the hardness of the heat-affected zone, and the transformation point is lowered in order to widen the region where quenching occurs in a short-time heat cycle such as during lap resistance welding. Of the elements, Cu is the most suitable. It was found that the hardness and range of the heat-affected zone became appropriate and the excellent joint strength could be obtained by defining the addition amounts of these two elements in the appropriate ranges.

The present invention has been made on the basis of such knowledge, and its characteristic constitution is as follows.

(1) An ultra low carbon steel sheet having excellent lap resistance welded joint strength, including C: 0.005% or less, Cu: 0.01 to 0.5%, and B: 0.0003 to 0.002% by weight.

(2) C: 0.005% or less, Si: 0.5% by weight
Below, Mn: 2% or less, Cu: 0.01 to 0.5%, P: 0.15% or less, S:
0.02% or less, Sol.Al: 0.06% or less, N: 0.007% or less, B: 0.00
Ultra low carbon steel plate with excellent lap resistance welded joint strength including 03-0.002%.

(3) C: 0.005% or less, Si: 0.5% by weight
Below, Mn: 2% or less, Cu: 0.01 to 0.5%, P: 0.15% or less, S:
0.02% or less, Sol.Al: 0.06% or less, N: 0.007% or less, B: 0.00
Includes 03 to 0.002%, and Ti: 0.01 to 0.2% or Nb:
Ultra-low carbon steel sheet with excellent lap resistance welded joint strength containing 0.005 to 0.1% of type 1 or type 2.

[0011]

The present invention will be described below together with the reasons for its limitation.

C: In order to improve the workability, that is, the overhanging property and the deep drawing property, it is necessary to realize high elongation (EL) and r value. For this purpose, it is necessary to keep C as low as possible, and the upper limit was made 0.005%.

B is an important quenching element in an ultra-low carbon steel sheet in which the addition amount of B: C is extremely small and the quenching effect of C cannot be expected. B segregates at the grain boundaries, delaying the transformation from austenite to ferrite,
Improves hardenability. If the amount is less than 0.0003%, there is no effect, and if it exceeds 0.002%, the effect is saturated and the workability deteriorates. Therefore, the B content is 0.0003 to 0.002%.
And

Cu: The role of Cu is important in the present invention. Cu
As described later, by significantly lowering the transformation point and delaying the ferrite-pearlite transformation start time, the hardenability in the cooling process is improved and the hardness of the heat-affected zone is increased. Further, by remarkably lowering the transformation point, there is an effect of remarkably expanding the width of the heat-affected zone, which is a major feature of the present invention.

During heating for a short time of 0.1 to 0.2 seconds such as lap resistance welding, the Ac ₃ transformation point rises significantly above the equilibrium Ae ₃ transformation point, and the austenitized region is Ae ₃
It will be much narrower than expected from the transformation point. Therefore, in order to sufficiently secure the width of the heat-affected zone and increase the strength of the welded joint, it is important to suppress the increase in the Ac ₃ transformation point during rapid heating.

The inventors of the present invention have conducted extensive studies on the behavior of additional elements and transformation points during rapid heating in a non-equilibrium state,
It was newly found that Cu among the austenite stabilizing elements remarkably suppresses the increase of the transformation point in the non-equilibrium state. Figure 1 shows the chemical composition and the balance of F listed in Table 1.
e is an example of the effect of lowering the transformation point of Cu at a heating rate of 800 ° C./s by using an ultra-low carbon steel sheet consisting of unavoidable impurities, and the horizontal axis represents the Cu content in the ultra-low carbon steel sheet,
The vertical axis shows the difference between the Ac ₃ transformation point and the Ae ₃ transformation point. Cu is 0.01%
When added above, the difference between the Ac ₃ transformation point and the Ae ₃ transformation point decreases rapidly. Therefore, when Cu is added in an amount of 0.01% or more, the heat-affected zone of the welded joint is expanded, and improvement in joint strength can be expected.

[0017]

[Table 1]

Next, the extent to which this phenomenon actually contributes to the improvement of the weld joint strength of ultra low carbon steel was investigated. Table 2
~ For the Ti-B series, Nb-B series, and Ti-Nb-B series ultra-low carbon steel sheets consisting of the chemical components shown in Table 4 and the balance Fe and unavoidable impurities, the continuous casting slab was heated to a slab temperature of 1200 ° C. It is heated and hot-rolled at a finishing temperature of 920 ° C and a winding temperature of 650 ° C, then cold-rolled at a cold rolling rate of 70% and annealed at 820 ° C to obtain a sheet thickness of 0.8.
mm steel plate was manufactured.

For comparison, a low carbon steel plate having practically no problem in joint strength was manufactured under the same conditions as those described above to a steel plate having a thickness of 0.8 mm. The chemical composition of the low carbon steel sheet used is the chemical composition described in No. A-13 of Table 2 and the balance is Fe and inevitable impurities.

[0020]

[Table 2]

[0021]

[Table 3]

[0022]

[Table 4]

Spot welding was performed on each steel sheet, and the joint strength was evaluated. In addition, the presence or absence of Cu scratches during the steel sheet production was observed. The welding conditions are shown below.

(Welding condition) Welding machine: Single-phase AC spot welding machine Plate thickness of test material: 0.8 mm Electrode: JIS CR type, tip angle: 120 °, tip diameter: 4.5
mm, Material: Chromium copper (Class 2) Pressure: 1852N Squeeze: 36 cycles / 60Hz Energization time: 10 cycles / 60Hz Hold time: 60 cycles / 60Hz Welding current: Nugget diameter of 5mm (no dust was generated ) Joint strength test method: JIS Z3136 tensile shear strength (TS
S), JIS Z3137 cross tensile strength (CTS)

Generally, the tensile shear strength and the cross tensile strength are affected by the plate thickness of the test material, the tensile strength (TS) of the test material, and the nugget diameter. There is no problem because the plate thickness and the nugget diameter are constant, but the TS differs among the test materials. Regarding TS, the effect of TS was standardized and evaluated by taking the ratio of joint strength and TS.
The joint strength was evaluated as ◯ when the joint strength was equal to or larger than the joint strength of the low carbon steel plate (No. A-13) which had no problem in practical use, and as x when the joint strength was small. Regarding the presence or absence of Cu scratches, the case where Cu scratches did not occur at the time of production was marked with ◯, and the case where Cu scratches occurred was marked with x. The results are shown in Tables 5 to 7, respectively.

[0026]

[Table 5]

[0027]

[Table 6]

[0028]

[Table 7]

When the amount of Cu is less than 0.01%, the ratio of the joint strength to the tensile strength is lower than that in the case of the standard low carbon steel sheet with the chemical composition No. A-13. Further, when the Cu amount exceeds 0.5%, Cu scratches are generated during the steel sheet manufacturing.

Although the detailed mechanism has not been clarified, the addition amount of Cu is specified as follows based on such experiments. If it is less than 0.01%, there is no effect of suppressing the increase of the Ac ₃ transformation point and there is no effect of improving the joint strength, so 0.01% is made the lower limit. On the other hand, if it exceeds 0.5%, Cu scratches will occur during manufacturing, so the upper limit is 0.5%.

In the present invention, in addition to the components of the above composition, Si, Mn, P, S, Sol.
Can include N.

Si: An element effective as a deoxidizing agent, but excessive addition raises YP, hinders moldability and deteriorates the surface properties due to generation of scale, so the upper limit is 0.5%.
And

Mn: added as an element for improving hardenability, but excessive addition reduces EL and hinders deep drawability, so the upper limit was made 2%. Further, the amount of Mn is preferably 0.01% or more in order to prevent hot embrittlement due to S 2.

P: Excessive addition raises the yield strength and is not preferable from the viewpoint of moldability and shape fixability, so its upper limit is set to 0.
15%.

S: S is an element that deteriorates the workability, so it is desirable that it be as low as possible, but considering steelmaking costs, the upper limit was made 0.02%.

Sol.Al:Al is a deoxidizer and solid solution N is AlN.
It is an important element that plays the role of fixing as. However, excessive addition of Al causes a large amount of precipitates in the steel sheet to deteriorate the workability, so the upper limit was made 0.06%. In order to sufficiently exert the effects of deoxidizing and fixing AlN 3, the amount of Sol.Al is preferably 0.01% or more.

N: When N is excessively present in the steel sheet, the crystal grains become finer, the workability is lowered and the aging resistance is lowered, so the upper limit was made 0.007%.

In the present invention, in order to improve the processability of the components having the above-mentioned composition, the following range of components is further added.
It may contain one or two selected from Ti or Nb.

Ti: Ti has the function of capturing solid solution C and N in the form of carbides and nitrides and improving the workability of steel sheets. How much is added
It is 0.01 to 0.2%. If it is less than 0.01%, there is no effect,
On the other hand, if it exceeds 0.2%, the effect will be saturated, leading to an increase in cost.

Like Nb: Ti, Nb has the function of capturing solid solution C and N in the form of carbides and nitrides and improving the workability of steel sheets. The added amount is 0.005 to 0.1%. If it is less than 0.005%, the effect is ineffective, and if it exceeds 0.10%, the effect is saturated and the cost increases.

Various elements are added to the ultra-low carbon steel sheet of the present invention according to the purpose or various elements are mixed in during the manufacturing process, but for Ca, Zr and Sn, the effect of Cu is reduced. Since it works, it is usually less than 1%, preferably 0.0
5% or less.

Cr, Ni, and Co may be added in order to improve the corrosion resistance, but since they act to reduce the effect of Cu, they are made 5% or less, preferably 1% or less.

The ultra-low carbon steel sheet of the present invention is a hot-rolled steel sheet descaled by pickling or the like, a descaled steel sheet is heat treated, a heat-treated descaled steel sheet, or the like It includes surface-treated steel sheets that have been galvanized or tin-plated.

The ultra low carbon steel sheet of the present invention includes a cold rolled steel sheet which is hot rolled, pickled, cold rolled and then recrystallized and annealed.

The ultra-low carbon steel sheet of the present invention includes a surface-treated steel sheet, and may be a hot-rolled steel sheet or a cold-rolled steel sheet subjected to surface treatment.

The ultra low carbon steel sheet of the present invention having the above chemical composition can be manufactured by a conventional method. That is, a hot rolled steel sheet can be manufactured by melting, casting, and hot rolling steel. The steel may be melted in either a converter or an electric furnace. Outside furnace refining can also be applied as needed. The casting may be either ordinary ingot or continuous casting. The hot rolling may be performed after reheating the cold slab or after lightly reheating (including heat retention and retention) the hot slab after casting. Moreover, the case where hot rough rolling is not performed may be performed. Although not particularly limited, the slab heating temperature of hot rolling is 1050 ° C to 1250 ° C from the viewpoint of surface state change and rolling load due to scale generation, and the finishing temperature is Ar ₃ transformation point or more from the viewpoint of workability, The winding temperature is preferably 700 ° C or lower in terms of suppressing the generation of scale, and Ti, Nb
In the case of containing Ti, N is preferably 400 ° C. or higher for precipitation of carbonitrides and coarsening of Nb.

If necessary, the hot-rolled steel sheet may be subjected to heat treatment, descaling such as pickling, heat treatment of the descaled steel sheet, and after heat treatment, to descaling such as pickling. Alternatively, these steel plates can be further surface-treated.

After hot rolling, pickling and cold rolling, recrystallization annealing can be performed to produce a cold rolled steel sheet. In this case, although not particularly limited, workability,
Particularly, from the viewpoint of deep drawability, the rolling rate of cold rolling is preferably 30 to 90%. The annealing temperature is preferably 650 ° C. or higher for softening and 900 ° C. or lower for suppressing coarse grains. The annealing method may be continuous annealing or box annealing. The continuous annealing may be continuous annealing in a continuous hot dip galvanizing line. Regarding temper rolling, it does not affect hardenability, so
There are no restrictions on the conditions for temper rolling.

Further, the effect of the present invention is not impaired even when surface treatment such as electroplating, organic composite coating or chemical conversion treatment is carried out individually or in combination through temper rolling after annealing.

The present invention can be widely applied to lap resistance welding, but is particularly effective in resistance spot welding and projection welding.

[0051]

EXAMPLES Examples of the present invention will be described below. (Example 1) A continuously cast slab containing the chemical components shown in Table 8 and the balance Fe and unavoidable impurities was added to the slab heating temperature.
Heated to 1150 ℃, finishing temperature 970 ℃, winding temperature 630 ℃
Cold rolling at 75% cold rolling, 780 ° C
Was annealed to manufacture a steel plate with a thickness of 1 mm.

For comparison, a low carbon steel plate having practically no problem in joint strength was manufactured under the same conditions as those described above, with a plate thickness of 1.0 mm. The chemical composition of the low carbon steel sheet used is the chemical composition described in No. D-1 of Table 8 and the balance is Fe and inevitable impurities.

Spot welding was performed on each steel sheet, and the joint strength was evaluated. In addition, the presence or absence of Cu scratches during the steel sheet production was observed. The welding conditions are shown below.

[0054]

[Table 8]

(Welding conditions) Welding machine: Single-phase AC spot welding machine Plate thickness of test material: 1 mm Electrode: JIS CR type, tip angle: 120 °, tip diameter: 5 mm
φ, Material: Chromium copper (Class 2) Pressurization: 2315N Squeeze: 36 cycles / 60Hz Energization time: 12 cycles / 60Hz Hold time: 60 cycles / 60Hz Welding current: Nugget diameter 5mm current (no dust generation ) Joint strength test method: JIS Z3136 tensile shear strength (TS
S), JIS Z3137 cross tensile strength (CTS) evaluation standard: Generally, the tensile shear strength and cross tensile strength are affected by the thickness of the specimen, TS of the specimen, and nugget diameter. . There is no problem because the plate thickness and nugget diameter are constant, but TS
Varies among the test materials. Regarding TS, the effect of TS was standardized and evaluated by taking the ratio of joint strength and TS. The joint strength is evaluated using a low carbon steel plate (No.
-1) the joint strength equal to or larger than the joint strength was marked with ◯, and the smaller one was marked with x. Regarding the presence or absence of Cu scratches, the case where Cu scratches did not occur at the time of production was marked with ◯, and the case with Cu scratches was marked with x.

The results are also shown in Table 8. As shown in Table 8, the steel sheets containing Cu and B in the range of the present invention have excellent joint strength and C
u There are no scratches. On the other hand, a steel plate containing Cu and B outside the scope of the present invention has poor joint strength or Cu scratches.

Example 2 A continuously cast slab consisting of the chemical components shown in Table 9 and the balance being Fe and unavoidable impurities was heated to a slab heating temperature of 1120 ° C., and finished at a finishing temperature of 980 ° C. and a winding temperature of 600 ° C. It was rolled, then cold-rolled at a cold rolling rate of 85% and annealed at 810 ° C to manufacture a steel sheet having a thickness of 1 mm.

For comparison, a low carbon steel plate having practically no problem in joint strength was manufactured under the same conditions as those described above, and a steel plate having a plate thickness of 1.0 mm was manufactured. The chemical composition of the low carbon steel sheet used is that shown in No. E-1 of Table 9 and the balance is Fe and inevitable impurities.

[0059]

[Table 9]

Each steel plate was pot-welded under the same conditions as in Example 1 and the joint strength was evaluated.
The joint strength is evaluated as ○ if it is equal to or greater than the joint strength of the low carbon steel plate (No.E-1), and as × if it is smaller.
And Further, as in Example 1, Cu
The presence or absence of scratches was observed and evaluated. The results are shown in Table 10.

[0061]

[Table 10]

As shown in Table 10, the steel sheets in the composition range of the present invention have excellent joint strength and no Cu scratches. On the other hand, the steel sheets out of the composition range of the present invention have poor joint strength or have Cu scratches.

Example 3 A continuously cast slab consisting of the chemical components shown in Table 11 and the balance being Fe and unavoidable impurities was heated to a slab heating temperature of 1180 ° C., a finishing temperature of 950, and a winding temperature of 6
Hot rolling at 20 ℃, then cold rolling at 80% cold rolling
Annealing was performed at 0 ° C to manufacture a steel plate having a thickness of 1 mm.

For comparison, a low carbon steel sheet having practically no problem in joint strength was manufactured under the same conditions as described above, and a steel plate having a thickness of 1.0 mm was manufactured. The chemical composition of the low carbon steel sheet used is that shown in No. F-56 of Table 11 and the balance is Fe and inevitable impurities.

[0065]

[Table 11]

Each steel sheet was pot-welded under the same conditions as in Example 1 and the joint strength was evaluated.
The joint strength is evaluated as ○ if it is equal to or larger than the joint strength of the low carbon steel plate (No. F-56), and as × if it is smaller.
And Further, as in Example 1, Cu
The presence or absence of scratches was observed and evaluated. Table 12 shows the results.

[0067]

[Table 12]

As shown in Table 12, Ti-added steel, Nb-added steel,
In both the Ti- and Nb-added steels, the steel plates in the composition range of the present invention have excellent joint strength and no Cu scratches occur. On the other hand, the steel sheets out of the composition range of the present invention have poor joint strength or have Cu scratches.

[0069]

The ultra-low carbon steel sheet of the present invention does not cause Cu scratches during production, and when lap resistance welding is performed, the joint strength can be made stable and strong. It is possible to remarkably expand the range of application of the ultra-low carbon steel sheet for the application of lap resistance welding.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of a transformation point lowering effect of Cu at a heating rate of 800 ° C./s.

Claims (3)

[Claims] 1. By weight%, C: 0.005% or less, Cu: 0.01-0.
Ultra low carbon steel sheet with excellent lap resistance welded joint strength, characterized by containing 5% and B: 0.0003 to 0.002%. 2. In% by weight, C: 0.005% or less, Si: 0.5% or less, Mn: 2% or less, Cu: 0.01 to 0.5%, P: 0.15% or less, S: 0.
02% or less, Sol.Al: 0.06% or less, N: 0.007% or less, B: 0.0003
Ultra low carbon steel sheet with excellent lap resistance welded joint strength characterized by containing ~ 0.002%. 3. By weight%, C: 0.005% or less, Si: 0.5% or less, Mn: 2% or less, Cu: 0.01 to 0.5%, P: 0.15% or less, S: 0.
02% or less, Sol.Al: 0.06% or less, N: 0.007% or less, B: 0.0003
~ 0.002%, Ti: 0.01 ~ 0.2% or Nb: 0.0
An ultra-low carbon steel plate with excellent lap resistance welded joint strength, characterized by containing one or two of 05 to 0.1%.