JPH07126807A - Steel sheet excellent in formability and hardenability by high energy density beam irradiation and its production - Google Patents

Abstract

PURPOSE:To produce a steel sheet excellent in formability and hardenability by high energy density beam irradiation by subjecting a steel slab contg. trace amounts of N and B in a quantitative specified relationship, coiling it round a coil, thereafter subjecting it to pickling or cold rolling and furthermore executing tempering at a specified temp. CONSTITUTION:A steel slab having a compsn. contg., by weight, 0.03 to 0.25% C, <0.6% Si, 0.1 to 2.0% Mn, <0.05% P, <0.015% S, 0.02 to 0.045 Al, <0.005% N, <0.003% O and 0.0003 to 0.0015% B and in which the quantitative relationship of N>=1.3B is satisfied between N and B is subjected to hot rolling into a hot rolled steel sheet and is coiled round a coil at <=580 deg.C to uniformly and finely distribute cementite therein. This hot rolled sheet is pickled to remove surface scales, and it is used as various surface treated steel sheet materials. Otherwise, the hot rolled steel sheet is subjected to cold rolling, is thereafter heated to the Ac1 to the Ac1+100 deg.C and is tempered. The steel sheet excellent in formability and hardenability by high energy density beam irradiation can be obtd.

Description

Detailed Description of the Invention

FIELD OF THE INVENTION The present invention relates to a steel sheet suitable for use in automobiles and the like, which is required to have curability and press formability by irradiation with a high energy density beam, that is, a hot rolled steel sheet, a surface treatment of a hot rolled steel sheet. Material, cold-rolled steel sheet, various surface-treated steel sheets of cold-rolled steel sheet, and a method for producing the same.

[0002]

2. Description of the Related Art As a strengthening method for press-formed members, a method of irradiating a press-formed material with a high energy density beam such as a laser is disclosed in JP-A-1-259911.
JP-A-4-72010 and JP-A-6-2010 disclose methods of irradiating a press-formed product with a high energy density beam.
No. 1-99629 discloses each. However, all of these are more advantageous for press forming than increasing the strength of the steel sheet itself, and are characterized in that the strain after forming is small as compared with the method of reheating and quenching the whole.

[0003]

However, the above-mentioned conventional technique is a method of simply using an existing steel sheet, and there is no mention of the possibility of improving the characteristics by combining with an appropriate steel sheet. No method is disclosed for providing a steel sheet having excellent curability and formability by beam irradiation.

[0004]

The inventors of the present invention have made extensive studies as to the relationship between the composition and structure of the steel sheet and the hardenability and formability in the case of irradiating a high energy density beam. The inventors have invented a steel sheet having excellent curability by high energy density beam irradiation and press formability after irradiation and a method for producing the same, which can maximize the effect.

The present invention has the following requirements as means for solving the above problems. (1) The invention of claim 1 has the following component composition (the composition is wt%
It is a steel sheet excellent in curability and formability by high energy density beam irradiation. (A) C: 0.03 to 0.25%, Si: 0.6% or less, Mn:
0.1% to 2%, P: 0.05% or less, S: 0.015% or less,
Al: 0.02 to 0.045%, N: 0.005% or less, O:
A steel sheet containing 0.003% or less and B: 0.0003 to 0.0015%, and (b) the N content and the B content have the following relationship. N ≧ 1.3B

(2) The invention of claim 2 is a method for producing a steel sheet having excellent curability and formability by high energy density beam irradiation, which comprises the following steps (composition is wt%). (A) C: 0.03 to 0.25%, Si: 0.6% or less, Mn:
0.1% to 2%, P: 0.05% or less, S: 0.015% or less,
Al: 0.02 to 0.045%, N: 0.005% or less, O:
0.003% or less, B: 0.0003 to 0.0015% is contained,
A slab having a composition of N ≧ 1.3B is prepared between the content and the B content, and (b) the slab is hot-rolled to obtain a hot-rolled steel sheet at 580 ° C. or lower. Winding, (c) pickling the rolled hot rolled steel sheet, or cold rolling into a cold rolled steel sheet, (d) the pickled hot rolled steel sheet, or
The cold-rolled steel sheet was heated to between Ac ₁ ~Ac ₁ +100 ℃, annealed.

[0007]

In the present invention, by adding an appropriate amount of B, the hardenability of the steel sheet by high energy density beam irradiation is enhanced, and at the same time, B is fixed by nitrogen in the base metal from the viewpoint of ensuring formability, It is characterized in that only the heat-affected zone is cured. Generally, for quenching-tempering type low alloy steel and the like, a method is known in which B is added to enhance the quench hardening of the base material. In such a case, if B is combined with nitrogen, the hardenability does not improve.
An important technique is to fix nitrogen with 1, etc., heat it just above the A ₃ transformation point to bring B into a free state, and segregate it to austenite grain boundaries during subsequent cooling.

On the other hand, in the present invention, conversely, B is fixed as BN to suppress the extreme hardening of the base material and to secure the formability, which is the basic technique. However, since the high energy density beam irradiation part in the present invention is heated up to around the melting point which is much higher than the A ₃ transformation point, BN is also easily decomposed and quench hardening occurs in the beam irradiation part and its heat affected part. To do.

This point is different from conventional quenching-tempering steels in terms of compositional design. Therefore, Ti was not added, the upper limit of the Al content was regulated, and the lower limit of nitrogen was regulated according to the B content. It is the essence of the present invention. Also, regarding the manufacturing conditions, low temperature winding after hot rolling was performed with the aim of improving not only formability but also curability by high energy density beam irradiation,
It is characterized by refining carbides and reducing band structure by annealing in the two-phase region. The operation will be described in detail below.

C is an element effective for enhancing the curability by high energy density beam irradiation, and 0.03% or more is necessary. However, if added excessively, the base material is hardened and the formability is lowered, so the upper limit is made 0.25%.

Si is an element that contributes to an increase in strength in spite of its small adverse effect on formability, but addition of a large amount causes remarkable deterioration of formability, so the upper limit is made 0.6%.

Mn is an element effective for enhancing the curability by high energy density beam irradiation, and 0.1% or more is necessary. However, if a large amount is added, the base material becomes hard and the formability is lowered, so the upper limit is made 2%.

Like P, P is an element that contributes to an increase in strength in spite of having a small adverse effect on formability, but addition of a large amount causes embrittlement due to segregation, so the upper limit is made 0.05%.

S is present as a sulfide-based inclusion and reduces the ductility, so it is desirable that the content be small, and the upper limit is 0.0.
15%.

Since Al is deoxidized in molten steel, 0.02% or more must be added. However, excessive addition causes a decrease in formability due to an increase in oxide, and at the same time forms a nitride and indirectly makes B free, which causes a decrease in formability of the base material. Is 0.045%.

If the amount of N is too large, the formability is lowered due to the increase of nitrides, so the upper limit is made 0.005%. However, if the amount is too small, B cannot be fixed as BN, leading to hardening of the base material, that is, deterioration of moldability, and curing by the irradiation beam. Therefore, 1.3 times or more addition to B is essential. .

B has the function of enhancing the curability of high energy density beam irradiation, and it is necessary to add 0.0003% or more. However, excessive addition not only saturates the effect on curing, but also hardens the base material and lowers the formability, so the upper limit is set to either 0.0015% or N / 1.3, whichever is lower.

O forms an oxide, resulting in deterioration of moldability,
In the high energy density beam irradiation part and its heat affected part, it acts as a nucleation site of ferrite and causes deterioration of hardenability. Therefore, the lower the content, the better, and the upper limit is made 0.0030%.

Other component elements may be contained as long as the above component composition is substantially contained, unless the essence of the steel of the present invention is changed. By the above component control, it is possible to secure the curability and formability by irradiation with a high energy density beam. However, by optimizing the manufacturing conditions described below, both characteristics can be further improved.

First, a steel slab having the above component composition is hot-rolled, and then a hot-rolled steel sheet is wound at 580 ° C. or lower to uniformly disperse cementite. At a coiling temperature exceeding 580 ° C, cementite becomes coarse, so the upper limit is made 580 ° C. This steel sheet can be used as it is as a hot rolled steel sheet, a pickled or shot-finished steel sheet of a pickled hot rolled steel sheet, and an original sheet of various surface-treated steel sheets after descaling the hot rolled steel sheet.

Then, the hot rolled steel sheet is pickled or cold-rolled and then heated to Ac _{1 to} Ac ₁ + 100 ° C. and annealed to reduce the band structure. Ac
_If it is less than ₁ , recrystallization occurs but it does not transform, so
The band structure will remain. On the other hand, Ac ₁ +10
If the temperature exceeds 0 ° C, the transformation grain growth and the coarsening of the carbide become remarkable, so that the heating temperature range of the annealing is Ac ₁ to Ac ₁ +100.
℃.

By making the carbides finer and reducing the band structure, the ductility is improved and the formability is improved. Regarding the curability by irradiation with a high energy density beam, although the maximum hardness itself does not change due to such structure control, the rate of increase in strength as a whole increases as the width of the hardened portion expands. This steel sheet can be applied to various surface-treated steel sheets including hot-rolling, pickling, and hot dip galvanizing after annealing, and also to cold-rolled steel sheets and various surface-treated steel sheets such as cold-rolled steel sheets.

The steel sheet having the above-mentioned composition is usually melted in a converter or an electric furnace, cast, and then hot-rolled or pickled and then cold-rolled into a steel sheet having a desired thickness. Although it is not particularly limited, when hot rolling is performed with a heating temperature of 1150 ° C. or higher and a finishing temperature of Ar ₃ or higher, and when cold rolling is performed, a reduction ratio of 50% or more is ensured. The effect of the invention is maximized. Even if rough rolling is not performed, the effects of the present invention are not impaired at all.

Annealing after pickling or cold rolling may be carried out by either a continuous annealing line or a continuous hot-dip galvanizing line equipped with continuous annealing. Further, in the case of continuous hot dip galvanizing, alloying plating treatment may be performed. Further, the effect of the present invention is not impaired even when a surface treatment such as electroplating, an organic composite film, or a chemical conversion treatment is performed alone or in combination through temper rolling after annealing.

The high energy density beam in the present invention refers to a laser beam, an electron beam, a plasma arc, or the like. Especially, in the irradiation with a laser beam, the output is 2 to 5 kw and the welding speed is 1 to 15 m / mi.
n, focus position: -2 to +2 mm, shield gas flow rate: 1
A typical condition is 0 to 30 l / min. In the case where the irradiation unit by the irradiation are melted and resolidified, the effect of the present invention is maximized, Ac ₃ point +
Melting is not always an essential condition if heating at 200 ° C. or higher. Further, there is no particular limitation on the pattern of the beam irradiation, such as a linear shape, a dot shape, or a grid shape.

[0026]

EXAMPLES Examples of the present invention will be described below. First, the composition of the present invention steel and the comparative steel, the laser irradiation hardenability, and the formability are shown in Table 1, respectively. Each steel in Table 1 is cast after melting and heating temperature is 1
Hot rolling is performed under the conditions of 220 ° C and finishing temperature of 870 ° C.
After winding at 560 ° C. to form a steel plate having a thickness of 2.8 mm, scale removal was performed by pickling.

Steels 2 and 25 are hot rolled steel sheets. The cold rolled steel sheet was cold rolled into a steel sheet having a thickness of 0.8 mm.
For such pickled steel sheets or cold rolled steel sheets, steel 3,
6, 8, 19, 29 are continuously annealed into cold rolled steel sheets,
For the rest of the steel, continuous hot dip galvanizing (500 ° C
Alloying treatment, weight per unit side 60g / m ² )
Temper rolling with an elongation of 1.2% was performed.

The annealing temperature was 750 to 760 ° C. These steel sheets were irradiated with CO ₂ laser to obtain test steels. The curability is the change in breaking strength before and after irradiation of a JIS No. 5 tensile test piece with three 5 mm pitches in the center parallel to the tensile direction as shown in Fig. 3. Strength increase rate (%) ((strength after laser irradiation-laser Pre-irradiation intensity) x 100 /
It was evaluated by the intensity before laser irradiation). The formability was evaluated by a plane overhang test (LDHo) using a plane strain overhang test using only one test piece irradiated at right angles to the maximum principal strain direction as shown in FIG.

The laser irradiation conditions and the plane strain overhanging conditions are as follows. Laser irradiation conditions Laser output: 3 Kw Irradiation speed: 3 m / min Focal length of condenser lens: 254 mm Focal position: -0.4 mm Shield gas type: Argon Shield gas flow rate: 20 l / min

Plane strain overhang condition Punch: φ100 mm-R50 mm ball head Die: φ106 mm-Shoulder R50 mm, with triangular beads (bead position: φ133 mm) Wrinkle holding force: 60 tonf (constant) Lubrication: Polyethylene film + high viscosity press oil

As shown in FIGS. 1 and 2, the invention steels are more significantly hardened by laser irradiation than the comparative steels, and have a good balance between the rate of increase in strength and the plane strain limit overhang height. For example, among comparative steels, steel 16 and B having a low C content
Steels with low or no additions 23, 25-28
Both have a low rate of increase in strength. Also, steel with high Al content 1
7, 18 and the steels 19 and 20 with a low N content, the steels 20 and 21 with Ti added, and the steel 24 with a high B content have a slightly low strength increase rate because their base materials are hardened. In addition, the plane strain bulging formability is also poor. Steels 29 and 30 having a high O content are also inferior in hardenability and formability.

In Table 2, steel 6,1 which is an invention steel as a component
4 shows the change in strength due to irradiation and the plane strain limit overhang height when the winding temperature and the annealing temperature were changed. If the coiling temperature or annealing temperature is not appropriate, the formability is lowered. To achieve both curability and formability by irradiation with a high energy density beam, the composition of components and the coiling temperature and annealing temperature should be optimized. Turns out to be important.

[0033]

[Table 1]

[0034]

[Table 2]

[0035]

[Table 3]

[0036]

The steel sheet according to the present invention has excellent hardenability and formability by irradiation with a high energy density beam. That is,
High energy density beam irradiation increases the strength of the steel sheet,
Moreover, the moldability is also improved. Therefore, for example, hot-rolled steel sheet for automobiles, surface-treated material of hot-rolled steel sheet, cold-rolled steel sheet,
It is a steel sheet excellent as various surface-treated steel sheets for cold rolled steel sheets.
In addition, the method of the present invention can provide a steel sheet having the above performance.

[0037]

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between an intensity increase rate by laser irradiation and an intensity before laser irradiation.

FIG. 2 is a diagram showing a relationship between a strength increase rate and a plane strain limit overhang height.

FIG. 3 is a view showing a tensile test piece irradiated with laser.

FIG. 4 is a view showing a plane strain bulge test piece irradiated with laser.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shinichiro Nakamura 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Corporation (72) Inventor Aoshi Tsuyama Marunouchi 1-2-2 Marunouchi, Chiyoda-ku, Tokyo In-house (72) Inventor Tomoyoshi Okita 1-2-2 Marunouchi, Chiyoda-ku, Tokyo Nihon Steel Tube Co., Ltd. (72) Inventor Yukio Maho 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Steel Tube Co., Ltd. (72) Inventor Kai Tsu, 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Nihon Steel Tube Co., Ltd. ( 72) Inventor Hiroyuki Tsunoda 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Steel Tube Co., Ltd.

Claims (2)

[Claims] 1. The following component composition (composition is wt%)
A steel sheet excellent in curability and formability by irradiation with a high energy density beam, characterized by containing. (A) C: 0.03 to 0.25%, Si: 0.6% or less, Mn: 0.1% to 2%, P: 0.05% or less, S: 0.015% or less, Al: 0.02 to 0.045%, N: 0.005% or less, O : 0.003% or less, B: 0.0003 to 0.0015%, wherein (b) the N content and the B content have the following relationship. N ≧ 1.3B 2. A method of manufacturing a steel sheet having excellent curability and formability by high energy density beam irradiation, which comprises the following steps (composition is wt%): (A) C: 0.03 to 0.25%, Si: 0.6% or less, Mn: 0.1% to 2%, P: 0.05% or less, S: 0.015% or less, Al: 0.02 to 0.045%, N: 0.005% or less, O : 0.003% or less, B: 0.0003 to 0.0015%, and prepare a slab having a composition such that N ≧ 1.3B is satisfied between the N content and the B content, (b) the slab Hot-rolling the obtained hot-rolled steel sheet,
(C) The rolled hot-rolled steel sheet is pickled or cold-rolled into a cold-rolled steel sheet, and (d) the pickled hot-rolled steel sheet or the cold-rolled steel sheet. the steel sheet was heated to between Ac ₁ ~Ac ₁ +100 ℃, annealed. [0001]