JPH0356301B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to an ultra-low carbon steel plate with good spot weldability, and in particular, the present invention relates to an ultra-low carbon steel plate with good spot weldability. This is an attempt to advantageously improve fatigue strength. (Prior Art) In recent years, the method of annealing cold-rolled steel sheets has changed from a box annealing method to a continuous annealing method in order to meet the demands for energy conservation and shortened delivery times. Further, the steel generally used for cold rolled steel sheets is low carbon Al killed steel. In order to produce cold-rolled steel sheets with good press formability using the continuous annealing method, after recrystallization annealing, overaging treatment is performed at a temperature of 300 to 500°C for about 3 to 10 minutes to improve aging resistance. Furthermore, carbonitride-forming elements such as Ti, Nb, and B have been added to improve aging resistance and drawability. On the other hand, in order to improve press formability, solid solution C,
In recent years, ultra-low carbon steels with N content reduced to several tens of ppm have been used, but even when such ultra-low carbon steels are used as raw materials, drawability, drawability,
The aging resistance is only slightly improved, and it is generally difficult to produce cold-rolled steel sheets with particularly good deep drawability or completely non-aging properties, and it is still necessary to add carbide-forming elements to improve these properties. addition is required. Furthermore, cold rolled steel sheets for automobiles are generally spot welded after press forming, and the fatigue strength of the joints is one of the important factors governing the durability of automobiles. The fatigue strength of such spot welded joints is not improved even if high-strength steel plates are used as the material, and as shown by curves A and B in Figure 3, it is higher than mild steel at high loads and low cycles, but it is higher at low loads and high cycles. In terms of cycle strength, the steel tends to be lower than that of mild steel, and is a major factor inhibiting the shift to high-strength automobiles. (Problems to be Solved by the Invention) In view of the above-mentioned circumstances, various efforts have been made to improve the fatigue strength of high-tensile steel plates. For example, in Japanese Patent Application Laid-open No. 58-3792,
Carbon equivalent is 0.06-0.60wt% (hereinafter simply expressed as %),
A method is disclosed in which a tempering current is applied when spot welding high-tensile steel plates having a tensile strength of 35 Kgf/mm ² or more. Furthermore, in JP-A No. 58-3793, when spot welding high-tensile steel plates with a C content of 0.20% or less and a tensile strength of Kgf/mm ² or more, a suitable "spatter" is required.
Disclosed is a method for welding in a current range where . However, all of the above examples relate to low carbon high tensile strength steel plates, and cannot be directly applied to ultra low carbon steel. The reality is that no technology regarding ultra-low carbon steel sheets has been disclosed to date. However, regardless of the steel type, it is possible to increase the joint strength of actual parts by increasing the number of spot welds or increasing the nugget diameter, but each method requires design changes and increases in cost, so it is difficult to do so for convenience. This was just a means to be adopted, and there was a strong demand for the development of a fundamental solution. The present invention advantageously satisfies the above-mentioned needs, and improves spot weldability, especially joint fatigue, by adjusting the component composition and continuous annealing conditions, without resorting to complicated labor or increased costs. The purpose of this study is to propose an ultra-low carbon steel plate that can advantageously improve its strength. (Means for Solving the Problems) First, the background to the elucidation of this invention will be explained. Now, the inventors have conducted a thorough reexamination of the spot weldability of mild steel plates, high-strength steel plates, and ultra-low carbon steel plates, and the fatigue strength of their joints. As a result, it was found that the mutual relationship between the hardness of the weld and the hardness of the base metal is important as a factor that affects joint fatigue strength. Figure 4 shows the hardness distribution of the spot welded joint. In mild steel plates and high-strength steel plates, the nugget and heat-affected zone (HAZ) harden, so the hardness distribution of the weld becomes as shown by curve A. Furthermore, in the case of ultra-low carbon steel sheets, the hardness distribution of the welded portion is approximately flat, as shown by curve B. Furthermore, in the conventionally known recovery annealed steel made from low carbon steel, a softened portion occurs as shown by curve C. Here, in the hardness distribution of curve A, the tensile shear fatigue strength of the joint according to JIS Z 3138 is low in the high load/low cycle region, and in curve B, it is low in the low load/low cycle region, and furthermore, in curve C, the tensile shear fatigue strength of the joint is low in the low load/low cycle region. It was found to be low both at high loads and at high loads. Based on the above results, the inventors investigated the hardness distribution that would give the best joint fatigue strength, and found that the steel type with the hardness distribution shown by curve D was extremely effective in achieving the intended purpose. We have found that this is effective and have conducted intensive research to develop such ultra-low carbon steel sheets. They developed a steel plate that exhibits fatigue strength and completed this invention. That is, in this invention, C: 0.006% or less, Mn:
Total of one or two types of Ti and/or Nd, including 0.5% or less, Al: 0.05% or less, N: 0.006% or less, and P: 0.05% or less, and excluding nitrides and sulfides: 0.001 to 0.100 % and B: 0.0001-0.005
%, the remainder is iron and unavoidable impurities, and has an unrecrystallized structure of 5 to 30% in cross-sectional structure area ratio, and has good spot weldability. This invention will be specifically explained below. First, the reason why the component composition is limited to the above range in this invention will be explained. C: 0.006% or less As the amount of C increases, the welded part becomes harder than the base metal, and as shown in curve D shown in Figure 4 above,
In other words, since it was impossible to obtain a state in which the hardness of the welded part was lower than that of the base metal, the C content was limited to a range of 0.006%. Mn: 0.5% or less Mn is an effective element for fixing S, which causes hot cracking, and must be added during the steelmaking process, but addition of more than 0.5% will harden the material.
Since it reduces ductility, the upper limit was set at 0.5%. Al: 0.05% or less Al is used as a deoxidizing agent during steel manufacturing and as a solute N.
Addition of Al is necessary because it has the effect of fixing it as AlN. However, since adding too much increases the cost of molten steel, in this invention it is set at 0.05% or less. N: 0.006% or less Similar to C, N makes crystal grains finer and reduces workability, as well as deteriorating aging resistance, so the content of N needs to be 0.006% or less. P: 0.05% or less P is an element that improves strength, but a content exceeding 0.05% hardens the material and deteriorates workability, so the upper limit was set at 0.05%. Ti and/or Nb: 0.001 to 0.100% One or two types of Ti or Nb, excluding nitrides and sulfides, 0.001 to 0.100%
The reason why it is set as % is that if it is less than 0.001%, it is difficult to improve the workability, whereas if it exceeds 0.100%,
This is because it significantly increases the recrystallization temperature, increasing not only the cost of molten steel but also the manufacturing cost. B: 0.0001-0.005% B makes the structure of the spot weld fine,
It effectively contributes to suppressing grain growth and preventing softening of the HAZ, but if it is less than 0.0001%, its effect is poor, while if it is added in a large amount exceeding 0.005%, it will cause deterioration of the material, so 0.0001 to 0.005% It is assumed that the amount is added within the following range. Now, although the appropriate range of the essential components in this invention is as described above, it is not possible to achieve the intended purpose of this invention by simply limiting the component composition to the above range. Therefore, it is important to limit the unrecrystallized structure in the steel to a predetermined range. Unrecrystallized structure The presence of an unrecrystallized structure is important for increasing the strength of the material and for making the hardness distribution of the spot weld follow the curve D shown in FIG. 4 above. If less than 5% of the unrecrystallized structure remains in cross-sectional structure area ratio, the effect is small, while if more than 30% remains, workability deteriorates significantly. It was limited to a range of 5 to 30%. Figure 1 shows 0.0015%C-0.017%Nb-0.0010%
B: Shows the results of an investigation into the relationship between the non-recrystallized structure cross section of ultra-low carbon steel having a composition of bal Fe, El, and material hardness (H _V ). As is clear from the figure, by limiting the cross-sectional area of the unrecrystallized structure to a range of 5 to 30%, a high H _V can be obtained without reducing El. In order to obtain the cross-sectional area of the unrecrystallized structure in the appropriate range as described above, it is sufficient to anneal the cold-rolled cold-rolled sheet for an appropriate time in the recrystallization temperature range. For example, a 0.0019% C- 0.024%Ti
-0.0008% B:balFe steel may be annealed for about 0.5 minutes in the temperature range of 610 to 690°C as shown in FIG. (Example) After melting steel with the composition shown in Table 1, it was hot rolled to a plate thickness of 3.2 mm, descaled, and then cold rolled to a cold rolled plate with a plate thickness of 0.7 mm, as shown in Table 2. A similar annealing process was performed. Each final product obtained is then sheared into strips,
Tensile shear fatigue test specimens were manufactured while adjusting spot welding conditions so that all nuggets had a diameter of 4.8 mm. The fatigue test was conducted with complete oscillation, and the test was stopped when the fatigue crack reached a length of about 5 mm. The test results obtained are also listed in Table 2.

【table】

[Table] As is clear from the results in Table 2, only steels A1, B1, and C1, which have the steel composition of this invention and whose unrecrystallized ratio satisfies the appropriate range of this invention, have a joint fatigue strength of 10 ⁴ 400Kgf/spot or more in cycle,
In 10 ⁷ cycles, it showed a high value of 90Kgf/spot or more across the entire low cycle range to high cycle range. In contrast, good joint fatigue strength could not be obtained with any of the other steels. (Effects of the Invention) Thus, according to the present invention, strength is ensured by partially leaving an unrecrystallized structure in the steel plate, and in spot welding, nuggets are not hardened and HAZ hardening is also suppressed. This makes it possible to significantly improve the fatigue strength of spot welded joints from low cycle to high cycle range, even though it is an inexpensive high-strength steel plate.
It has been successfully applied to mechanical parts where the fatigue strength of spot welded joints is a problem.

[Brief explanation of drawings]

Figure 1 is a graph showing the relationship between the area ratio of unrecrystallized structures in steel, El, and material hardness, and Figure 2 is
A graph showing the relationship between continuous annealing temperature and unrecrystallized structure area ratio, Figure 3 is a tensile shear fatigue diagram of a spot welded joint, and Figure 4 is a diagram showing the hardness distribution of a spot welded joint. .

Claims (1)

[Claims] 1 C: 0.006wt% or less, Mn: 0.5wt% or less, Al: 0.05wt% or less, N: 0.006wt% or less, and P: 0.05wt% or less, and nitride, Ti and/or sulfides not included
Contains a total of one or two types of Nb: 0.001 to 0.100wt% and B: 0.0001 to 0.005wt%, the remainder consists of iron and unavoidable impurities, and the cross-sectional structure area ratio is 5 to 30% unregenerated. An ultra-low carbon steel sheet with good spot weldability characterized by having a crystalline structure.