JPH0418014B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for manufacturing ultra-thin steel sheets with small in-plane anisotropy, and is particularly suitable for use as a material for composite steel sheets that require good workability. . (Prior Art) Cold-rolled steel sheets with improved workability, especially deep drawability, are mainly used as interior and exterior panels for automobiles. Therefore, material development has been carried out mainly with the use of press molding as automobile parts in mind.
Particularly in the case of steel sheets for automobiles, it is necessary to adapt them to a wide variety of designs, so there is emphasis on improving the r value to support deep drawability, or lowering yield stress and increasing work hardening rates from the perspective of shape fixability. Ru. By the way, in recent years, so-called composite steel plates in which resin is sandwiched between steel plates have come to be used in automobiles, and as a matter of course, such composite steel plates are also required to have excellent formability. (Problem to be solved by the invention) Ultra-thin steel plates with a thickness of 0.15 to 0.60 mm are often used as materials for such steel plates, but in that case, it is difficult to simply make thin steel plates using conventional steel compositions and conventional manufacturing methods. When manufactured, the following problems occurred. That is, (a) the cold rolling reduction ratio becomes excessively high, so the r value is low; on the other hand, Δr, which indicates the anisotropy of the r value, becomes high, which causes earrings after drawing; (b) the material Because the steel plate is thin, it has low elongation and is prone to cracking during forming. (c) Products that are annealed at high temperatures to improve elongation have a low YS and are prone to cracking on vertical walls due to galling. , (d) In addition, CAL is easy to break because it is thin and soft. This invention advantageously solves the above problems, and by combining the adjustment of steel plate components and appropriate control of the manufacturing process, YS is not too low, El is large, and r value is high. The purpose of this invention is to propose an advantageous method that can produce ultrathin cold-rolled steel sheets with a small Δr. (Means for Solving the Problems) As a result of extensive research to solve the above problems, the inventors have obtained the knowledge described below. In other words, (1) Reducing the amount of C is effective in improving the r value and El, and it is known that adding small amounts of Ti or Nb is usually effective in improving the material quality of such ultra-low carbon steel sheets. However, by adding a small amount of B, Δr is reduced without reducing the r value. (2) In addition, if the cold reduction rate is increased to reduce the final plate thickness, the recrystallization temperature will decrease, and if the annealing temperature is set to 800 to 900℃, which is the conventional method, Δr will increase. For Nb-B-added ultra-low carbon steel sheets, Δr becomes smaller when a very high cold rolling reduction of about 90% is used. (3) Furthermore, with this component system, plate breakage during continuous annealing of thin materials is greatly reduced. This invention is based on the above knowledge. That is, this invention has the following properties: C: 0.004wt% or less (hereinafter simply expressed as %), Si: 0.1% or less, Mn: less than 0.5%, P: 0.025% or less, S: 0.025% or less, N: 0.006% or less, Al: 0.001 to 0.100%, Ti: 0.01 to 0.10%, and contains Ti≧{(48/12)C+(48/14)N}, Nb: 0.003 to 0.03%, and B: 0.0001 to 0.0010%, the remainder being unavoidable impurities. Substantially Fe except
Hot-rolled steel plate with composition: 850-900
°C, coiling temperature: After hot rolling under the conditions of 300 to 600 °C, cold rolling, continuous annealing, and skin pass rolling are performed to produce ultra-thin steel sheets with a thickness of 0.15 to 0.60 mm. This is a method for producing ultra-thin steel sheets with small in-plane anisotropy, which comprises setting the annealing ratio to 85 to 95% and continuous annealing temperature to 650 to 750°C. This invention will be specifically explained below. First, the reason why the steel components in this invention are limited to the above range will be explained. C: Lowering the C content is effective in softening the steel and improving El and r values. C content is
If the content exceeds 0.004%, the material begins to deteriorate significantly, so the upper limit was set at 0.004%. Si, Mn: Both of these contribute effectively as deoxidizing agents, but if contained in excess, they may impair ductility, so they are limited to 0.1% or less and less than 0.5%, respectively. P, S: Both are impurity elements, and it is desirable to reduce them as much as possible, but both are acceptable if they are about 0.025% or less. Al: Al is used as a deoxidizing agent during steel manufacturing and as a solid solution N.
It is useful as an element to be fixed as AlN, and it is necessary to add at least 0.001%. However, adding too much increases the cost of molten steel, so the upper limit was set at 0.100%. N: Like 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. Ti: Not only is Ti useful for fixing solid solution components such as C, N, and S, but it is also effective in improving material quality by forming precipitates with these elements. However, if the content is less than 0.01%, the effect of adding it will be poor, and if it is added more than 0.10%, no increase in the effect can be expected, so the content was limited to a range of 0.01 to 0.10%. In addition, in this invention, Ti is Ti≧{(48/12)C+ so that C and N can be completely fixed with this Ti.
(48/14)N}. Nb: By adding Nb in combination with Ti, it effectively contributes to increasing YP while ensuring high El and high r value. The effect appears above 0.003%, but 0.030
Nb content was limited to a range of 0.003 to 0.030% because exceeding % would lead to an excessive increase in YP and decrease in El. B: Addition of a small amount of B effectively contributes to reducing Δr, that is, improving the anisotropy of r value, without causing a decrease in r value, but adding too much B causes deterioration of the material. Therefore, B was added in a range of 0.0001 to 0.001% so that the above-mentioned effects are effectively exhibited. Next, the reasons for limiting the manufacturing process conditions according to the present invention will be explained. Hot rolling finishing temperature: If the hot rolling finishing temperature is less than 850°C, the r value will deteriorate due to residual strain, while if it exceeds 900°C, the r value will also deteriorate due to coarsening of the crystal grains. The hot rolling finishing temperature was limited to a range of 850 to 900°C. Coiling temperature: In order to avoid excessive softening and increase in hot-roll scale due to high CT, it is necessary to keep the temperature below 600°C, but the temperature should be set in consideration of the time and capacity required for cooling and the shape of the coil to be wound. Lower limit is 300℃
And so. Cold rolling reduction ratio: 85 to 95% Figure 1 shows the results of an investigation into the influence of the cold rolling reduction ratio on Δr. As a sample material, 0.001
%C-0.049%Ti-0.005%Nb-0.0009%B-
BalFe (○ mark in the figure), 0.002%C-0.015%Ti-
3: balFe (same □ mark), 0.005%C-balFe (same △ mark)
Each component was used and prepared under the following conditions. Hot rolling finishing temperature: 870℃ Coiling temperature: 550℃ Cold rolling reduction: 70-98% Continuous annealing temperature: 700℃, 60s Skin pass rolling: 0.5% As is clear from the figure, conventional ultra-low carbon steel
In Ti-added ultra-low carbon steel, Δr increases as the rolling reduction increases;
Nb-B-added ultra-low carbon steel exhibits a small Δr value as the rolling reduction increases, particularly in the range of rolling reduction of 85% or more. However, an ultra-high reduction rate exceeding 95% places a heavy burden on the cold rolling equipment, so the upper limit was set at 95%. Continuous annealing temperature: 650-750℃ Figure 2 shows 0.001%C-0.049%Ti-0.005%Nb
A steel slab with a composition of -0.0009%B-BalFe,
Hot rolling at a hot finishing temperature of 880°C and a winding temperature of 500°C, followed by rolling at rolling reductions of 75% and 90%, followed by continuous annealing at a temperature of 600 to 820°C, followed by skin pass rolling. The results of an investigation into the effect of continuous annealing temperature on the material properties of the ultra-thin steel sheets obtained by this process are shown below. As is clear from the figure, the relationship between the material and the continuous annealing temperature shifts to a lower annealing temperature at a high rolling reduction of 90% compared to the normal cold rolling reduction (75%), and at 650℃ With the above, excellent strength and ductility can be obtained.
However, when Δr exceeds 750°C, it becomes 0.6 or more, leading to deterioration of in-plane anisotropy. Based on the above results, the continuous annealing temperature range was limited to 650 to 750°C. Note that skin pass rolling, which is the final stage of the manufacturing process, may be carried out using normal methods and conditions, that is, approximately % of the plate thickness or less. Through the component adjustment and manufacturing process described above, an ultra-thin steel sheet for composite steel sheets with excellent material quality can be obtained. It should be noted that this type of soft ultra-thin steel plate has conventionally had a problem of plate breakage during continuous annealing, particularly breakage due to weld netting after seam welding on the entry side. However, another major advantage of the invention component steel is that there is very little netting in the weld. Table 1 shows the results of an investigation into the occurrence of netting when cold-rolled steel plates with a thickness of 0.2 mm were subjected to narrow seam welding in a continuous annealing furnace and then subjected to an experimentally higher tension than normal. show. As is clear from the table, the Ti-Nb-B additive steel has extremely low occurrence of necking, which is thought to be because the weldable range is wide and the softening of the welded part is small.

[Table] (Example) Molten steel having various compositions shown in Table 2 was continuously cast, heated to 1150℃, and finished at a temperature of 860℃.
Hot rolling was carried out at ~900°C and a winding temperature of about 550°C, followed by cold rolling and continuous annealing at the rolling reduction and temperature shown in Table 3, respectively, followed by 0.4% skin pass rolling. Table 3 shows the results of examining the thickness and material of the obtained thin plate.

【table】

【table】

[Table] In all cases (Nos. 1 to 3) that satisfy the composition and manufacturing process conditions of this invention, YS19Kg
An excellent material with f/mm ² , TS30Kgf/mm ² , El 45%, and r2.0 was obtained, and Δr0.4, showing almost no internal anisotropy. On the other hand, those whose component composition is suitable but whose manufacturing process conditions are outside the appropriate range (Nos. 4 to 6) are YS
was low, and Δr exceeded 0.8. In addition, for those having the composition of the comparative steel, regardless of whether the manufacturing process conditions are within the appropriate range of this invention or not,
The Δr value exceeds 0.7, and characteristics as good as expected were not obtained in this invention. As described above, only steel that satisfies both the composition and manufacturing conditions specified in the present invention is an ultra-thin steel sheet that exhibits excellent properties. When these steel plates were laminated together to form a damping steel plate and a cylindrical deep drawing test was conducted using an Erichsen testing machine, all of them exhibited excellent critical drawing ratios of 2.0 or higher, and almost no earring occurred. . (Effects of the Invention) Thus, according to the present invention, it is possible to obtain an ultra-thin steel plate that has excellent formability, low in-plane anisotropy, and does not break during continuous annealing. Composite steel plates, which are used as materials for shaken steel plates and lightweight steel plates and are manufactured by laminating them with plastics, exhibit excellent formability and have a great effect on the automobile industry and other industries. Furthermore, the steel sheet of this invention has excellent formability not only when used as a composite steel sheet material but also when used alone, so it can be used in many ways as an original sheet for surface-treated steel sheets or as a bare sheet. It is also possible.

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between cold rolling reduction and Δr, and FIG. 2 is a graph showing the relationship between continuous annealing temperature and various mechanical properties.

Claims (1)

[Claims] 1 C: 0.004wt% or less, Si: 0.1wt% or less, Mn: less than 0.5wt%, P: 0.025wt% or less, S: 0.025wt% or less, N: 0.006wt% or less, Al : 0.001 to 0.100wt%, Ti: 0.01 to 0.10wt%, and includes Ti≧{(48/12)C+(48/14)N}, Nb: 0.003 to 0.03wt%, and B: 0.0001 to 0.0010wt%. , the remainder is essentially Fe, excluding unavoidable impurities.
Hot-rolled steel plate with composition: 850-900
℃, coiling temperature: After hot rolling under the conditions of 300 to 600℃, cold rolling, continuous annealing, and skin pass rolling are performed to produce ultra-thin steel sheets with a thickness of 0.15 to 0.60 mm. A method for producing an ultra-thin steel sheet with small in-plane anisotropy, characterized in that the annealing rate is 85 to 95% and the continuous annealing temperature is 650 to 750°C.