JPH0920958A - Hot rolled steel sheet for press forming excellent in secondary working resistance and its production - Google Patents

Abstract

PURPOSE: To secure secondary working resistance in a steel sheet, at the time of producing a hot rolled steel sheet by using iron scrap of a low grade as an iron source, by limiting the contents of P and Sm contained in the steel sheet to a certain range. CONSTITUTION: A low alloy steel having a steel sheet compsn. contg. <=0.040% P and 0.015 to 0.040% S and also satisfying the relationship of P+Sn; <=0.060% is prepd. Preferably, this compsn. is furthermore added with 0.003 to 0.0050% B. At the time of rolling this steel, after melting, it is formed into a slab by continuous casting, which is subsequently subjected to hot rolling at a finishing temp. of the Ar3 transformation point or above by a reheating rolling method or a direct rolling method, and the obtd. steel strip is coiled. As for the temp. at the time of the coiling, the temp. range of 400 to 550 deg.C is regulated in the case the steel compsn. contains no B, and it is regulated to <=700 deg.C in the case of B-added steel to attain the coarsening of the crystalline grains and the suppression of segregation into the grain boundary of P and Sn.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled steel sheet for press forming which uses iron scrap as an iron source and is excellent in secondary workability, and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, the amount of low-grade iron scrap called discarded waste such as food cans and automobiles has increased, and in the steelmaking industry, such iron scrap is used as an iron source in consideration of environmental problems. The need to use is increasing. However, in such iron scrap, C
Many so-called trump elements such as u, Sn, Mo, Ni and Cr are contained, and these trump elements are less likely to be oxidized than Fe, so that they are hardly removed in the current ironmaking process and remain in the steel. It has been pointed out that the manufacturability, material quality, or surface quality characteristics are adversely affected (for example, JP-A-4-162943, JP-A-4-371528, and JP-A 6-1).
No. 98304).

[0003]

As described above, since the Trump element is generally not contained in steel and is not added, it is intentionally aimed at improving the corrosion resistance, strength and surface characteristics. The use of iron scrap has various restrictions, except that elements such as Cu, Cr and Ni are added as useful elements.

Further, when a large amount of Sn is contained as a playing card element or when a large amount of P and Sn are contained in a composite, in a hot rolled steel sheet produced by using that steel, after the press forming mainly by deep drawing and spreading or spreading. It is feared that if a secondary processing such as mouth drawing is performed, intergranular brittle fracture called secondary processing cracking is likely to occur.

On the other hand, in view of the environmental problems as described above, it is required to produce a steel product as a steel scrap steel-making raw material containing a large number of playing card elements, such as food cans and automobiles, in this case. It is an important issue to prevent deterioration of secondary workability.

The object of the present invention is to use a steel containing a large amount of Sn and P in the steel composition, which is produced by mixing, for example, 10% or more of a low-grade iron scrap as an iron source, and melting it. Even if it is present, it is to provide a method for producing a hot-rolled steel sheet for press forming having excellent secondary workability.

[0007]

The present invention has been made to solve the above problems, and its gist is as follows. (1) Weight%, P: 0.040% or less, Sn: 0.0
15 to 0.040% and P + Sn: 0.0
A hot-rolled steel sheet for press forming, which is made of a low alloy steel having a chemical composition satisfying a relationship of 60% or less and which has excellent secondary workability.

(2) P: 0.040% or less by weight%,
Sn: 0.015 to 0.040%, B: 0.0003 to
A hot-rolled steel sheet for press forming, which is made of a low alloy steel containing 0.0050% and has excellent secondary workability.

(3) After smelting the steel having the chemical composition according to claim 1, a slab is cast by continuous casting, and then the slab is subjected to a reheating rolling method or a direct rolling method to form an Ar ₃ transformation point. Hot rolling was carried out at the above finishing temperature, and the steel strip thus obtained was 400 to 550 ° C.
A method for producing a hot-rolled steel sheet for press forming, which is excellent in secondary workability, characterized in that it is wound within the temperature range.

(4) After smelting the steel having the chemical composition according to claim 2, a slab is cast by continuous casting, and then the slab is subjected to the reheating rolling method or the direct rolling method to change the Ar ₃ transformation point. Manufacture of hot-rolled steel sheet for press forming having excellent secondary workability, which comprises performing hot rolling at the above finishing temperature and winding the steel strip thus obtained at a temperature of 700 ° C. or less. Method.

[0011]

The reason why the chemical composition of the steel sheet of the present invention is limited to the above range will be described. (1) P: P is an element that deteriorates secondary workability together with Sn. If the content thereof exceeds 0.040%, intergranular brittle fracture called secondary work cracking tends to occur after press forming of a steel sheet. . The cause is that P segregates at the crystal grain boundaries at the stage of winding after hot rolling, which lowers the grain boundary strength.
Therefore, the content of P is limited to 0.040% or less. Further, when coexisting with Sn, the secondary workability is significantly deteriorated, and even if the P content is 0.040% or less, P + S
If n exceeds 0.0060%, brittle fracture tends to occur, so P + Sn is limited to 0.0060% or less.

(2) Sn: Sn is an element contained as a Trump element, which is inevitably contained if iron scrap is used as an iron source. Good secondary workability can be obtained under the conditions. However, if the Sn content is 0.015% or more, depending on the hot rolling conditions, Sn may segregate at the crystal grain boundaries to embrittle the grain boundaries and deteriorate the secondary workability. However, when the hot rolling conditions of the present invention described later are used, Sn
Does not segregate at the crystal grain boundaries and embrittle the grain boundaries. Therefore, in the present invention, the lower limit of Sn is set to 0.015%.
However, if the Sn content exceeds 0.040%, good secondary workability cannot be ensured even under the hot rolling conditions of the present invention.
Therefore, the Sn range of the present invention is 0.015 to 0.0
Limited to 40%. Further, when coexisting with P, secondary workability is significantly deteriorated, and P + Sn is 0.
If it is 0060% or more, good secondary workability cannot be obtained, so that P + Sn is 0.006 as described above.
It was limited to 0% or less.

(3) B: B segregates at the crystal grain boundaries to improve the grain boundary strength and has the effect of improving the secondary workability. As a result, the hot rolling conditions (winding temperature) can be relaxed as compared with the case where B is not added. In order to fully exhibit this effect, the B content is required to be 0.0003% or more.
However, excessive addition increases the hot deformation resistance and makes rolling difficult, so the content was limited to 0.0050% or less.

The composition of steel is adjusted by using cold iron, reduced iron or the like as an iron source together with iron scrap. In consideration of the purpose of the present invention, it is preferable to use 10% or more of the total iron source as the mixing ratio of the iron scrap. Although it depends on the composition of the iron scrap, by adding 40 to 100%, steel can be made to have a desired chemical composition.

After melting and refining the above-mentioned iron source by a converter method, an electric furnace method, or other manufacturing method, a slab is cast by continuous casting. As the slab, a thin slab having a thickness of about 50 mm may be used instead of the usual method having a thickness of about 200 mm.

Next, the reasons for limiting the method for manufacturing a steel sheet according to the present invention will be described. First, the slab manufactured by continuous casting can be applied by either a method of once cooling and then reheating it and then rolling, or a method of directly rolling without reheating.

With regard to the finishing temperature of hot rolling, at a temperature lower than the Ar ₃ transformation point, strain is applied to the ferrite grains of the steel sheet to form a mixed grain structure, which deteriorates ductility and lowers press formability. The finishing temperature must be above the Ar ₃ transformation point. The temperature is preferably 800 ° C to 900 ° C.

The winding temperature is 400 to 550 ° C. when B is not contained. If it exceeds 550 ° C., the crystal grains become coarse, P and Sn segregate at the grain boundaries, the transition temperature becomes high, and the secondary workability deteriorates. 400 ° C
If it is less than 1, there is a problem that the shape of the steel strip is deteriorated.

On the other hand, when B is contained, even if P or Sn is segregated in the crystal grain boundaries, B is segregated in the crystal grain boundaries at the same time, and the bond strength of the crystal grain boundaries is increased. Therefore, the winding temperature can be set higher than that when B is not contained. However, if it is wound at a temperature exceeding 700 ° C., the crystal grains become coarse and the elongation characteristics deteriorate significantly. Therefore, the winding temperature is 700 ° C. or lower. Suitably 550 ° C to 7
00 ° C.

The hot-rolled steel sheet thus obtained has excellent secondary workability. Therefore, a method for evaluating the secondary workability of the hot rolled steel sheet according to the present invention will be described. First, test material 11
After processing into a blank of 0mmφ, 50mmφ-10mmR
Squeeze into a cup with a cylindrical punch. Then, after cutting off the ears at a height of 40 mm, a pushing test is performed using a conical cone while maintaining various temperatures, and the presence or absence of brittle cracks is investigated. Based on this, the secondary work resistance is evaluated by the temperature at which fracture shifts from ductile cracking to brittle cracking, that is, the transition temperature. That is, it can be judged that the lower the transition temperature is, the better the secondary workability is. From a practical point of view, it is empirically known that, under the test conditions, no problem will occur as a steel sheet for press forming if the transition temperature is −70 ° C. or lower. Therefore, in the present invention, the excellent secondary workability means that the transition temperature measured by the above evaluation method is -70 ° C or lower.

As a material to be subjected to press forming such as deep drawing, low carbon steel having a C content of 0.20% or less is generally targeted. The above component limitation and the coiling temperature of the hot rolling temperature are found based on the following experiments and the like.

That is, steels T1 (B is not contained and the contents of P and Sn are smaller than those of the present invention) and T2 (B is not contained and P and Sn are contained, which have the chemical composition shown in Table 1.
Content of the present invention), T3 (including B, P, Sn
Content of the present invention) was melted in an electric furnace and then cast by continuous casting into a slab having a plate thickness of 50 mm. Next, the slab was subjected to finish rolling at 900 ° C. without reheating, and was wound at various temperatures shown in the horizontal axis of FIG. 1 to manufacture a hot rolled steel sheet. With respect to the hot rolled steel sheet having a thickness of 3.2 mm thus obtained, a test material was sampled from a steel sheet that was subjected to 1% temper rolling, and the transition temperature was evaluated by the above-described evaluation method of secondary workability. Was measured.

FIG. 1 is a graph showing the relationship between the winding temperature and the transition temperature obtained in the above test. As is clear from the figure, when T1 is 700 ° C. or lower, the transition temperature is −70 ° C. or lower at any winding temperature, but in T2, −70 ° C.
In order to obtain the following transition temperatures, it is necessary to control the winding temperature in the range of 400 to 550 ° C.

On the other hand, with regard to the steel T3 containing B, the coiling temperature shows a transition temperature of -70 ° C or lower in a wide temperature range of 700 ° C or lower. In the above, steel T1 is 700
The reason why good secondary workability is exhibited at any winding temperature as long as the temperature is not higher than 0 ° C is that the content of P and Sn is sufficiently low and the segregation amount of P and Sn at the grain boundaries is small. On the other hand, the content of P and Sn is slightly high and T2 is within the range of the present invention.
However, the transition temperature is low in the winding temperature range of 400 to 550 ° C., and the secondary workability is good, because segregation of P and Sn to the crystal grain boundaries is suppressed by winding in that temperature range. It is believed that

In the above, the winding temperature of T3 is 7
Good secondary workability can be obtained even in a normal temperature range of 00 ° C. or lower because even if P or Sn is segregated at the crystal grain boundaries, B is segregated at the crystal grain boundaries at the same time to bond the crystal grain boundaries. It is considered to increase the strength.

In the steel T2, the coiling temperature is 400 to 5
Excellent secondary workability is obtained in the temperature range of 50 ° C, whereas in the case of steel T3, the coiling temperature is 700
Excellent secondary workability was obtained at any temperature as long as it was below ℃.

Therefore, when P and Sn are slightly increased, the coiling temperature of the hot-rolled steel sheet in hot rolling is 400 to 550 ° C. for the steel containing no B, and the operating temperature for the steel containing B. It is understood that the temperature should be within a temperature range of 700 ° C. or lower, which is a normal winding temperature that is easy to perform. Note that 700
The reason why the secondary workability is deteriorated when the coiling is carried out at a temperature higher than ° C is that the crystal grains become coarse and the elongation property is remarkably lowered.

[0028]

Next, examples of the present invention will be described in comparison with comparative examples. On average Cu: 0.4%, Sn: 0.0
Low-grade iron scrap containing 4% was used as an iron source together with cold iron and reduced iron. Here, the iron scrap was made to obtain a steel having a desired chemical composition with a mixing ratio of 10% or more of the total iron source. Various steels shown in Table 2 (invention example No. 1 having a chemical composition within the scope of the present invention)
~ 8 and Comparative Example No. Comparative Examples Nos. 1 to 4 and Comparative Example Nos. 5-6) were melted in an electric furnace, and then a slab having a thickness of 50 mm was cast by a continuous casting method. Next, the hot slab thus obtained was directly hot-rolled without cooling. In this direct hot rolling, as shown in Table 3, the invention sample No. For the slabs 1 to 8, the manufacturing conditions were within the scope of the present invention, and the comparative example No. Comparative Examples Nos. 1 to 4 were manufactured under manufacturing conditions outside the scope of the invention. 5-6
For the slab of, we will do under the manufacturing conditions within the scope of the invention,
A hot rolled steel strip having a plate thickness of 3.2 mm was prepared. Then, the hot-rolled steel strip thus obtained was subjected to temper rolling of 1% to produce a hot-rolled steel sheet.

A test material was sampled from the hot-rolled steel sheet manufactured as described above, and a tensile test piece (JIS No. 5 test piece) was used to evaluate the strength and elongation measurement test and the secondary work resistance under the above-mentioned conditions. . Moreover, the surface quality test of the hot rolled steel sheet was performed. The results are shown in Table 3.

The evaluation of the secondary workability was conducted at a transition temperature of -70 ° C.
The following are considered good (marked with ∘) and those exceeding -70 ° C are regarded as bad (marked with x).
Regarding the surface quality test, the presence or absence of surface defects on the hot-rolled steel sheet was inspected.

The following matters were clarified from Tables 2 and 3. Comparative Example No. In Nos. 1 to 4, the winding temperature of the hot-rolled steel strip was a high temperature outside the range of the present invention, so that any steel sheet had a transition temperature higher than -70 ° C and had a secondary work resistance. It was inferior.

Comparative Example No. 5 is Sn, and Comparative Example N
o. No. 6 had too much P + Sn outside the scope of the invention,
The winding temperature was within the range of the invention, but the secondary workability was poor.

Inventive Example No. Comparative Examples Nos. 7 and 8 are steels to which B is added and which are B-free steels. Despite being wound at a high temperature as compared with 1-3,
The secondary workability is good, and the effect of adding B is clear.

Inventive Example No. 1 whose chemical composition and manufacturing conditions of steel are within the scope of the present invention. In each of 1 to 8, hot-rolled steel sheets excellent in secondary workability were obtained. As described above, hot-rolled steel sheets having even one chemical composition and manufacturing condition of steel outside the scope of the present invention deteriorated in secondary workability, whereas all the conditions were within the scope of the present invention. The hot-rolled steel sheet was excellent in secondary workability.

[0035]

[Table 1]

[0036]

[Table 2]

[0037]

[Table 3]

[0038]

The effect of the present invention is that when the hot rolled steel sheet is manufactured using iron scrap containing a large number of playing cards as a raw material for steelmaking, the present invention limits the chemical composition and manufacturing conditions as described above. The hot-rolled steel sheet for press forming has excellent characteristics that can be used without any other restrictions in the secondary workability and can be used without problems even when subjected to press forming such as deep drawing. And a method for producing the same, which brings about an industrially beneficial effect.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a coil winding temperature and a transition temperature.

Claims (4)

[Claims] 1. Weight%, P: 0.040% or less, S
n: 0.015 to 0.040%, and P + S
n: A hot-rolled steel sheet for press forming, which is made of a low alloy steel having a chemical composition satisfying a relationship of 0.060% or less and which is excellent in secondary workability. 2. In weight%, P: 0.040% or less, S
n: 0.015 to 0.040%, B: 0.0003 to
A hot-rolled steel sheet for press forming, which is made of a low alloy steel containing 0.0050% and has excellent secondary workability. 3. A slab is cast by continuous casting after the steel having the chemical composition according to claim 1 is melted, and then this slab is subjected to a reheating rolling method or a direct rolling method to obtain an Ar ₃ transformation point or higher. Hot rolling at the finishing temperature of
A method for producing a hot-rolled steel sheet for press forming having excellent secondary workability, comprising winding the steel strip thus obtained within a temperature range of 400 to 550 ° C. 4. A steel having the chemical composition according to claim 2 is melted, a slab is cast by continuous casting, and the slab is then reheat-rolled or directly-rolled to obtain an Ar ₃ transformation point or higher. Hot rolling at the finishing temperature of
A method for producing a hot-rolled steel sheet for press forming having excellent secondary workability, which comprises winding the steel strip thus obtained at a temperature of 700 ° C. or lower.