JPH04318150A - Light high strengch steel sheet for molding - Google Patents

Abstract

PURPOSE:To improve moldability together with to make a steel plate light in weight and to make have high strength by specifying contents of C, Si, Mn, P and S and, moreover, specifying an adding content of Al. CONSTITUTION:A component of this steel sheet is to be 0.02-0.1% C, <=0.5% Si, 0.2-2.0% Mn, <=0.05% P, <=0.01% S, 0.5-5.0% Al and residue consisting of Fe and inevitable impurities. But a condition 4.2/Al>=(15XC+Mn+0.7)/Al>=0.7 is to be satisfied. A steel sheet having 0.5-3mm thickness and <=7.75g/cm<2> density is manufactured by controlling the condition of rolling, annealing etc.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel plate which is excellent in formability, has high strength and is lightweight, which is mainly used for automobile bodies.

0002

BACKGROUND OF THE INVENTION In recent years, it has been desired to improve the fuel efficiency of automobiles from the viewpoint of global environmental conservation, and there has been an active movement toward reducing the weight of automobile bodies in order to achieve this goal.

[0003] There are two ways to reduce the weight of a car body: optimizing the car body structure and reducing the weight of the material itself. Methods of reducing the weight of the material itself include making the steel plate for the car body thinner by increasing its strength, The use of low specific gravity metals such as aluminum, lightweight materials such as plastics, and composite materials such as FRP and laminated steel sheets are being applied.

0004

[Problem to be solved by the invention] However, the rigidity of the vehicle body, dent resistance, and strength against impact are largely controlled by the thickness of the parts rather than the strength of the material itself, and there is a limit to the thinning of steel plates. . Additionally, aluminum is inferior to steel in terms of formability, weldability, etc., and the cost of the material is also high. Plastics are not as strong as steel, so they cannot be used as structural members, and recycling is problematic. Furthermore, FRP has low reliability in terms of impact and fatigue strength, and laminated steel plates may peel during press processing.
It is difficult to apply it to members that are severely bent or drawn.

[0005] As described above, with the conventional concept of material weight reduction, it is impossible to satisfy all the characteristics required for automobile body parts, and there are also problems in terms of parts manufacturing costs. The present invention has been made in view of the above circumstances, and aims to provide a steel plate that is excellent in strength, formability, and cost, and that can achieve weight reduction without thinning.

[0006]

[Means for Solving the Problems] The present inventors have repeatedly studied ways to take advantage of the advantages of steel, which is high strength, excellent formability and cost, and to achieve a reduction in the weight of the material. We focused on reducing the weight of the steel plate itself by adding a large amount of Al to Fe.

Steel sheets to which a large amount of Al, which is a ferrite stabilizing element, is added have already been put into practical use as electromagnetic steel sheets. However, when the Al content is increased, the γ→α transformation point rises, making it difficult to raise the hot rolling finishing temperature above the transformation point, and when a large amount of Al is added, a ferrite single-phase structure results. These are preferable for electrical steel sheets aimed at coarsening the structure of the hot-rolled sheet and coarsening the structure of the final product, but are unsuitable for steel sheets that require good formability. Furthermore, in the case of electrical steel sheets, a <100>//ND texture is developed for the purpose of improving electromagnetic properties, but such a texture is unfavorable for the deep drawability of the steel sheet. Furthermore, the thickness of electromagnetic steel sheets is made thinner to reduce magnetic loss, which increases the rigidity of the car body and
It is disadvantageous in terms of dent resistance and impact strength.

[0008] As a result of repeated research by the present inventors to solve the above problems, we added a large amount of austenite stabilizing elements to balance it with Al, thereby avoiding the structure from becoming a single ferrite phase. A large amount of Al is added by making the structure of the hot rolled sheet finer by the γ→α transformation during hot rolling, and developing a <111>//ND texture that is advantageous for formability after cold rolling and annealing. without impairing formability even when
Moreover, it was discovered for the first time that weight reduction can be achieved, and that formability can be improved even in the case of hot-rolled steel sheets by using the γ→α transformation to regularize the structure, which led to the present invention. It is as follows.

(1) C: 0.02 to 0.1 wt%, Si
: 0.5 wt% or less, Mn: 0.2 to 2.0 wt%, P
: 0.05wt% or less, S: 0.01wt% or less, Al
:0.5-5.0wt%, however, 4.2/Al≧(1
5×C+Mn+0.7)/Al≧0.7, the remainder consists of Fe and inevitable impurities, and the plate thickness is 0.5 mm.
A lightweight high-strength steel plate for forming, characterized in that it has a thickness of 3 mm or less and a density of 7.75 g/cm3 or less. (2) C: 0.02-0.1wt%, Si: 0.5wt
% or less, Mn: 0.2-2.0wt%, P: 0.05w
t% or less, S: 0.01wt% or less, Al: 0.5-5
．． 0wt%, Ni: 0.05-1.0wt%, Cu: 0
．． 05 to 0.5 wt%, provided that 4.2/Al≧(15
×C+Mn+Ni+2×Cu+0.7)/Al≧0.7
with the balance consisting of Fe and unavoidable impurities, the plate thickness is 0.5 mm or more and 3 mm or less, and 7.75 g/cm3
A lightweight high-strength steel plate for forming, characterized by having the following density: (3) Nb: 0.05wt% or less, Ti: 0.05wt
% or less, V: 0.05wt% or less, Zr: 0.05wt
% or less, B: 1 or 2 of 0.005 wt% or less
The lightweight, high-strength steel sheet for forming according to (1) or (2) above, characterized in that it contains at least 0.05 wt% of at least one type of carbon dioxide in total.

0010

[Operation] Regarding the present invention as described above, first, the reasons for limiting the alloying elements in the present invention are as follows.

[0011] Al is the most important element in the present invention, and the greater the amount added, the lower the density of the steel, and the more weight reduction of the material can be promoted. Therefore, the lower limit of Al is set at 0.5 because the density reduction is small if too small a quantity is added.
%. However, when added in excess of 5.0%, excessive austenite stabilizing elements must be added to avoid the steel becoming a single ferrite phase, thereby significantly reducing formability. Moreover, since the steel becomes extremely brittle and cracks are likely to occur during cold rolling, it is difficult to apply it to cold rolled steel sheets. Therefore, the upper limit of Al is 5.0%.

C is an austenite stabilizing element,
By adding an appropriate amount of C, the steel undergoes γ→α transformation during hot rolling, which makes it possible to make the structure regular and fine-grained. As a result, the formability of the hot-rolled sheet is improved, and at the same time, nucleation of <111>//ND texture from grain boundaries is promoted during annealing after cold rolling, and the formability of the cold-rolled annealed sheet is improved. . C is 0.
If it is less than 0.02%, these effects will not appear. On the other hand, if it is added in too large a quantity, the texture will deteriorate, resulting in poor formability and aging resistance. Therefore, the upper limit is 0.1%
shall be.

Mn is an austenite stabilizing element and is an important element that prevents steel from becoming a single ferrite phase when a large amount of Al is added. Similar to C, addition of 0.2% or more is essential to exhibit its effect. However, if it is added in excess, the moldability will decrease, so 2.
Limited to 0% or less.

In the present invention, C and Mn are used as the base elements for stabilizing austenite, but Ni and Cu are also effective, and it is even more effective to add a combination of these elements.

Ni is also an austenite stabilizing element,
In order to exhibit its effect, it is necessary to add 0.05% or more. However, since it is expensive, adding it in excess will increase the cost, so the upper limit is set at 1.0%.

[0016] Cu also has the effect of stabilizing austenite, and this effect does not appear unless it is added in an amount of 0.05% or more. However, if it is added in an amount exceeding 0.5%, formability deteriorates due to precipitation of the ε phase. Therefore, the upper limit is set at 0.5%.

The balance of Al, C, Mn, Ni, and Cu described above is very important in the present invention. This is because if the austenite stabilizing elements C, Mn, Ni, and Cu are insufficient relative to the amount of Al added, the steel becomes a ferrite single-phase structure, resulting in poor formability. on the other hand,
Excessive austenite stabilizing elements also reduce formability. To avoid this, 4.2/
Al≧(15×C+Mn+Ni+2×Cu+0.7)/
Al, C, Mn, Ni, C that satisfies Al≧0.7
u Need to maintain balance. FIG. 1 shows a graph showing the relationship between this formula and E1. In this figure, the hatched range corresponds to the range that satisfies the above formula and C: 0
．． 02-0.1 wt%, Mn: 0.2-2.0 wt%,
Ni: 0.05-1.0 wt%, Cu: 0.05-1.
By setting the content to 0 wt% and Al: 0.5 to 5.0 wt%, preferable moldability can be obtained. In addition, when Ni and Cu are not added, 4.2/Al≧(15×C+Mn+0
．． 7) C and Mn may be added so as to satisfy /Al≧0.7.

[0018] Si is a ferrite stabilizing element,
Since it impairs the effect of the austenite stabilizing element, it is better to have less, and since it also makes the steel brittle, the upper limit should be set at 0.5.
%. It is more preferable to set it to 0.1% or less.

P is also a ferrite stabilizing element, but its action is greater than that of Si, so it is necessary to reduce the amount added, and the upper limit is 0.05%. Preferably 0.0
It is preferable to keep it at 1% or less.

[0020] S significantly impairs the ductility of steel and also reduces spot weldability, so it is limited to 0.01% or less.

In the present invention, by adding Nb, Ti, V, and Zr, fine carbonitrides are precipitated during slab heating or hot rolling, and this effect makes it possible to further refine the grain structure of the hot rolled sheet. However, if added in excess of 0.05%, the amount of C that contributes to austenitization of steel will be insufficient. Further, B also has the effect of refining the structure, but the effect is saturated even if it is added in an amount exceeding 0.005%. Therefore, Nb: 0.05% or less, Ti: 0.05% or less, V: 0.05% or less, Zr: 0.05% or less, B: 0
．． 0.05% or less, and the upper limit of the total amount added is 0.05%.
%.

[0022] Steel having the above composition is generally made into a steel plate by only hot rolling or by subsequent cold rolling and annealing. In this case, hot rolling is carried out at a heating temperature of 1200°C or less, a reduction rate of 50% or more, and a finishing temperature of γ→
A more preferable TS-El balance can be obtained by carrying out the winding at a temperature higher than the α transformation point and a winding temperature of 550 to 750°C. Further, it is preferable that the rolling reduction during cold rolling is 50% or more. Annealing may be performed by either continuous annealing or batch annealing, but by setting the annealing temperature to 800 ° C. or higher for continuous annealing and 700 ° C. or higher for batch annealing, <111>
//The ND texture develops more easily, and the formability of the steel sheet can be further improved.

In the present invention, the thickness of the steel plate is 0.5 mm or more and 3 mm or less. By setting the plate thickness to 0.5 mm or more, we can ensure appropriate rigidity, dent resistance, impact resistance, etc. when used as a car body, and the upper limit is 3 m.
This is to achieve moldability as a structural member and to reduce weight by setting it to m.

Further, by setting the density to 7.75 g/cm 3 or less, it is possible to obtain a density reduction rate of 1% or more, and it is possible to reduce the weight while ensuring the strength of the steel plate.

[0025]

[Example 1] To explain a specific example according to the present invention, the chemical composition of typical steel according to the present invention example and comparative example specifically adopted by the present inventors is as shown in Table 1 below. and C2, C4, D2, D3,
D4, D8, E2, E3, E4, E5, F2
, F3, G2, and G3 are examples of the present invention, and the others are comparative examples.

[0026]

[Table 1]

[0027] Steel having the composition as shown in Table 1 was melted and cast, and the heating temperature was 1200°C, the reduction rate was 87%, and the finishing temperature was 9.
00 to 950°C and a coiling temperature of 640 to 700°C, followed by cold rolling at a reduction rate of 80% to obtain a plate with a thickness of 0.
．． It was made into a 7 mm cold rolled plate. In addition, the cold-rolled sheets thus obtained were subjected to continuous annealing at 850°C and pressure adjustment of 1.0% to produce products.The mechanical properties and density of the obtained products were investigated, and the results are as follows. The results were as shown in Table 2.

[0028]

[Table 2]

That is, as shown in the results in Table 2, sample No. 1 according to the present invention. 4, 6, 9, 10, 11, 15, 18,
Nos. 19, 20, 21, 24, 25, 28, and 29 have a density reduction rate of more than 1% and 2% or more, and the TS is also good at 40 Kgf/mm2 or more. Further, an El of 30% or more and an r value of 1.0 or more were obtained.

On the other hand, sample No. 1, which is a comparative example, 1
In Sample No. 2, the weight reduction effect did not appear because the amount of Al added was insufficient, while in Sample No. 2 with excess Al. 31, 3
In No. 2, cracks occur during cold rolling, so it is not practical. In addition, sample No. lacking C, Mn, Ni, and Cu
．． 3, 8, 17, 23, 27, and C, Mn, Ni,
Sample No. with excess Cu. 7, 16, 22, 26,
It is clear that No. 30 is not suitable for press working because El is significantly reduced.

[0031]

[Example 2] Steel E2 in Table 1 above was hot rolled, cold rolled and annealed under the conditions shown in Table 3 below to give a plate thickness of 1.
．． A 0 mm cold rolled steel plate was manufactured. Table 4 shows the mechanical properties after applying 1.0% pressure adjustment.

[0032]

[Table 3]

[0033]

[Table 4]

From Table 4, the heating temperature for hot rolling is 1200°C.
Below, the reduction rate is 50% or more, the finishing temperature is γ→α transformation point or more,
If the coiling temperature is 550 to 750°C, the rolling reduction during cold rolling is 50% or more, and the annealing temperature is 800°C or more for continuous annealing or 700°C or more for batch annealing, the forming of the steel sheet can be made. It turns out that it is possible to make more factories.

[0035] Using the steel shown in Table 1 above, heating temperature 11
Hot rolling was performed at 60°C, rolling reduction rate of 90%, finishing temperature of 890-930°C, and coiling temperature of 620-710°C, resulting in a plate thickness of 1.6
A hot-rolled steel plate of mm was manufactured. Table 5 shows the results of investigating the mechanical properties of these steel plates.

[0036]

[Table 5]

From Table 5, it is clear that the hot rolled steel sheets according to the present invention have a large density reduction rate and are excellent in formability.

[0038]

[Effects of the Invention] According to the present invention as explained above, a low-density and high-strength steel plate can be manufactured, so weight reduction can be achieved without reducing the plate thickness, and the strength of the molded product can be maintained. Since it is possible to do this, it has great industrial utility value, and is particularly useful for reducing the weight of automobile bodies, so it is an invention with great industrial effects.

[Brief explanation of the drawing]

[Figure 1] Balance of Al, C, Mn, Ni, Cu and El
This is a chart showing the relationship between

Claims (3)

[Claims] [Claim 1] C: 0.02 to 0.1 wt%, Si:
0.5 wt% or less, Mn: 0.2 to 2.0 wt%, P:
0.05wt% or less, S: 0.01wt% or less, Al:
0.5 to 5.0 wt%, provided that 4.2/Al≧(15
×C+Mn+0.7)/Al≧0.7, the remainder consists of Fe and unavoidable impurities, the thickness is 0.5 mm or more and 3 mm or less, and the density is 7.75 g/cm3 or less. A lightweight high-strength steel plate for forming. [Claim 2] C: 0.02 to 0.1 wt%, Si:
0.5 wt% or less, Mn: 0.2 to 2.0 wt%, P:
0.05wt% or less, S: 0.01wt% or less, Al:
0.5-5.0wt%, Ni: 0.05-1.0wt%
, Cu: 0.05 to 0.5 wt%, however, 4.2/A
l≧(15×C+Mn+Ni+2×Cu+0.7)/A
l≧0.7, the remainder consists of Fe and unavoidable impurities, the plate thickness is 0.5 mm or more and 3 mm or less, and 7.75
A lightweight high-strength steel plate for forming, characterized by having a density of g/cm3 or less. [Claim 3] Nb: 0.05wt% or less, Ti: 0
．． 05wt% or less, V: 0.05wt% or less, Zr: 0
．． 05 wt% or less, and B: 0.005 wt% or less in a total amount of 0.05 wt% or less. Strength steel plate.