JPH11323514A - Steel sheet with excellent surface properties - Google Patents

Abstract

(57) [Summary] (With correction) [PROBLEMS] To provide a steel sheet having excellent surface properties. SOLUTION: In weight%, C: 0.2% or less, Si: 0
To 2.0%, Mn: 0.05 to 2.0%, Al: 0.3
A hot-rolled steel sheet, a cold-rolled steel sheet, and a surface-treated steel sheet having excellent surface properties and containing no more than 2.5% and N of 0.002 to 0.01% and having no flaws due to surface cracks during continuous casting.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel sheet having excellent surface properties, and more particularly to a hot-rolled steel sheet having no flaws caused by surface cracks of a material slab by a continuous casting method, and using the hot-rolled steel sheet as a material The present invention relates to manufactured cold-rolled steel sheets and surface-treated steel sheets.

[0002]

2. Description of the Related Art When an Al-killed steel slab is manufactured by a continuous casting method, a crack-like surface defect is apt to occur on the surface of the slab. This surface defect must be removed before proceeding to the next step, and the work for removing the defect leads to an increase in cost. In addition, direct feed rolling in which hot rolling is performed directly without cooling the slab cannot be performed, which is a hindrance to energy saving.

[0003] These cracking defects are caused by the thermal stress generated in the slab in the temperature range from the low temperature γ range to the α + γ two-phase range when the slab solidifies, and the slab during straightening in this temperature range. It is caused by external stress applied to

[0004] Surface cracks occur in these temperature ranges due to embrittlement of the material, and it is known that the embrittlement is caused by γ grain boundary cracks caused by AlN precipitation. Therefore, measures have been taken to prevent the precipitation of AlN by increasing the cooling rate, and to reduce the AlN precipitates by decreasing the cooling rate. However, there are many restrictions on manufacturing equipment for adjusting the cooling rate, and at present, sufficient effects have not been obtained.

On the other hand, there is a measure by changing the chemical composition of the slab. That is, surface flaws can also be reduced by reducing Al and N and reducing the amount of AlN precipitated. However, Al is an element added for deoxidation, and it is actually difficult to reduce the amount of Al, so that deoxidation becomes insufficient and mechanical properties are greatly reduced.

Japanese Patent Application Laid-Open No. Sho 59-110762 discloses N
Is described to reduce the occurrence of surface flaws during continuous casting by reducing the content to 0.003% by weight or less. However, in order to stably reduce N to 0.003% by weight or less, there are many restrictions on manufacturing equipment, which greatly impairs economic efficiency and productivity.

[0007]

As described above, a hot-rolled steel sheet as a raw material of a hot-rolled steel sheet or a cold-rolled steel sheet or a surface-treated steel sheet excellent in surface properties by stably preventing surface cracks caused by AlN. The technology to obtain was aspired.

[0008] It is an object of the present invention to provide a steel sheet (hot-rolled steel sheet, cold-rolled steel sheet, surface-treated steel sheet) which is free from defects due to surface cracks during continuous casting and has excellent surface properties.

[0009]

FIG. 1 is a schematic diagram showing the precipitation of AlN in a normal steel slab. FIG. 1 (a) is a low temperature γ region, and FIG. 1 (b) is an α + γ region. Is the case.
As shown in FIG. 3A, in the low temperature γ region, when the slab is deformed due to the thermal stress during solidification and the external stress during correction, AlN coarsely precipitates at the grain boundaries, and at the same time, near the grain boundaries. A precipitation-free zone of AlN is formed in
1N is finely precipitated and the inside of the grains is hardened. During deformation such as straightening during continuous casting, strain concentrates on the layer along the grain boundary (no segregation zone), which is relatively lower than the inside of the grain, and fracture starts from the AlN coarsely precipitated at the grain boundary. Embrittlement occurs due to so-called grain boundary ductile fracture (γ grain boundary embrittlement).

Embrittlement in the α + γ region also occurs by a similar mechanism. In this case, as shown in FIG.
The coarse precipitation of N at the grain boundaries and the deposition of soft ferrite in the form of a film along the grain boundaries cause cracks. In other words, the strain concentrates on the film-like ferrite at the grain boundary due to the deformation, and breakage occurs at the grain boundary, resulting in embrittlement.

As described above, since an increase in the amount of Al promotes grain boundary embrittlement, it has been conventionally operated to minimize the content of Al which is a deoxidizing element.

However, the present inventors have significantly reduced the Al content (usually 0.1% by weight or less; the chemical composition weight% is simply referred to as% in the following description) of the slab required for deoxidation. It has been found from the following tests that surface cracks can be prevented by setting the Al concentration to be higher than 0.3%.

In the test of the present inventors, slabs having a cross section of 240 mm in thickness and 1200 mm in width and having various Al contents were manufactured, and the degree of occurrence of surface cracks was observed. The casting conditions were as follows: a curved continuous casting machine with a radius of 15 m and a casting speed of 80.
The slab temperature at the straightening point was 850 ° C. at 0 mm / min. Table 1 shows the chemical composition of the slab used for the test and the observation results of surface cracks.

[0014]

[Table 1]

[0015] As shown in Table 1, it was judged that the slab having a high Al content had no surface cracks, and that the maintenance step before proceeding to the next step could be omitted. The mechanism by which the surface cracks during continuous casting are suppressed by greatly increasing the Al content is still largely unknown, but is presumed as follows.

FIG. 2 shows a cast slab for a steel sheet according to the present invention.
FIG. 3A is a schematic diagram showing the precipitation state of 1N, and FIG.
FIG. 3B shows the case of the α + γ region. It is considered that the mechanism of suppressing the embrittlement in the low-temperature γ region is because the coarsening of AlN in the grains is promoted, as shown in FIG. As a result, intragranular hardening is suppressed, the difference in strength from the non-segregated zone near the grain boundary is eliminated, and the concentration of strain in the non-segregated zone is suppressed, so that embrittlement can be prevented.

On the other hand, the mechanism that suppresses embrittlement in the α + γ region is because, as shown in FIG. 3B, the formation of ferrite at the grain boundaries is promoted by Al and the formation of film-like ferrite is suppressed. it is conceivable that. That is, it is considered that the concentration of strain is suppressed and the embrittlement is prevented by increasing the amount of ferrite.

Based on the above findings, the gist of the completed present invention is as follows: "Chemical composition is% by weight, C: 0.2% or less;
i: 0 to 2.0%, Mn: 0.05 to 2.0%, Al:
0.3 to 2.5%, and N: 0.002 to 0.01%.
Here, the steel sheet of the present invention refers to a hot-rolled steel sheet, a cold-rolled steel sheet, and a surface-treated steel sheet.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The components of the present invention and the operation thereof will be described in detail below. The following chemical components% are% by weight
Means

(1) The chemical composition C of the slab is 0.20% or less. C is a preferable component for increasing the strength of the steel sheet. However, if it exceeds 0.20%, workability is reduced and weldability is deteriorated. The lower limit is not particularly defined, but is 0.002 in consideration of the application and manufacturing cost.
% Is desirable.

The content of Si is set to 0 to 2.0%. Si is a preferred component that solid-solution strengthens steel and improves strength without impairing ductility. However, even if the content exceeds 2.0%, the effect of the above-mentioned action is saturated and the weldability is deteriorated. Si may be 0%. That is, Si need not be positively added unless it is included as an unavoidable impurity. Considering the effect or adverse effect of Si content and the production cost, the content of Si is preferably 0.004% or more.

Mn is set to 0.05 to 2.0%. Mn has the effect of increasing the strength of the steel sheet, fixing S in the steel as MnS, and suppressing cracks generated during continuous casting or hot rolling. When the content of Mn is less than 0.05%, the above-mentioned effects cannot be obtained. When the content exceeds 2.0%, not only the effect is saturated, but also the workability is reduced.

Al is 0.3 to 2.5% as a solid solution. Al is an important element in the present invention. Al is
It has the effect of promoting the coarsening of AlN in the grains, increasing the amount of ferrite generated at the grain boundaries, suppressing embrittlement due to grain boundary ductile fracture, and preventing surface cracking during continuous casting. If the Al content is less than 0.3%, the effect of suppressing cracking cannot be obtained. On the other hand, if the content exceeds 2.5%, the effect is saturated. The preferred range is 0.5-2.0%.

N is set to 0.002 to 0.01%. N is an element that is finely precipitated as AlN in the grains and causes surface cracks. However, when the Al content is increased, AlN can be coarsened and cracks can be suppressed. When the content of N is less than 0.002%, not only the coarsening of AlN is suppressed but also the low N
The necessity of the treatment of the conversion necessitates the economy. On the other hand, 0.0
If it exceeds 1%, AlN becomes too coarse and the content of AlN itself increases, so that P, which causes surface cracking, is an inevitable impurity. In order to ensure the above, the content of P is preferably set to 0.05% or less. In order to uniformly disperse ferrite, the content is more preferably 0.01% or less.

S, like P, is an inevitable sulfuric acid impurity, but forms oxide inclusions and lowers workability. Therefore, the S content is preferably set to 0.05% or less. In order to obtain particularly excellent workability, the content is more preferably 0.003% or less.

The steel sheet of the present invention is basically composed of Fe and the above-mentioned composition. However, when the strength and formability of the steel sheet are required, Nb, V, Cr, Mo, Zr or a rare earth element may be used. One or more of the above may be added as appropriate.

Among the steel sheets of the present invention, the hot-rolled steel sheet is prepared by heating a slab obtained by a known continuous casting, performing a rough rolling of a known hot rolling, performing a finish rolling, cooling, and winding it. Manufactured by the method. Using the hot-rolled steel sheet as a raw material, the cold-rolled steel sheet or surface-treated steel sheet of the present invention is manufactured by a known method.

When manufacturing the above hot rolled steel sheet, if a thin slab can be manufactured at the time of continuous casting, it may be manufactured by omitting hot rolling rough rolling.

[0029]

EXAMPLES Table 2 shows the chemical compositions of the samples used for the test. Steels A to K are steels having the chemical composition defined in the present invention (Examples of the present invention), whereas steels L to O are steels for comparison with the present invention (Comparative Examples).

[0030]

[Table 2]

The cross section of the slab has a thickness of 240 mm and a width of 120.
0 mm. The casting conditions were a bending type continuous casting machine with a radius of 15 m, a casting speed of 800 mm / min, and a slab temperature at the correction point of 850 ° C.

These slabs were heated to 1220 ° C. in a heating furnace in a hot-rolling step without roughening, rough-rolled, and subjected to finish rolling and winding under the conditions shown in Table 3 to obtain a sheet thickness of 2 mm. .
A 3 mm steel plate was obtained.

The surfaces of these steel sheets were visually observed, and the surface properties were evaluated according to the following criteria.

：: Good surface properties (frequency of occurrence of scabs is 0.1 / m ²⁾
Δ: Slightly reduced surface properties (as above, exceeding 0.1 / m ² .
5 months / m ² or less) ×: surface texture deterioration (ibid exceeds 0.5 months / m ²⁾ was also evaluated the mechanical properties were taken No.5 tensile test piece prescribed in JISZ2201.

[0035]

[Table 3]

As shown in Table 3, steel plates A to K having a chemical composition within the range specified by the present invention (test numbers 1 to 11)
No surface defects caused by surface cracks during continuous casting typified by hedging occurred. On the other hand, trial No. 12 using steels L to O whose Al or N content deviates from the regulation of the present invention.
In Nos. To 15, the surface properties were deteriorated.

[0037]

The hot-rolled steel sheet according to the present invention has no surface flaws due to surface cracks during the production of slabs and has excellent surface properties. The cold-rolled steel sheet or surface-treated steel sheet of the present invention using this hot-rolled steel sheet as a raw material also has excellent surface properties. In addition, since slab care before hot rolling is not required, the economic effect is high in terms of working cost and energy cost.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a precipitation state of AlN in a normal steel slab, wherein FIG. 1 (a) shows a low temperature γ region and FIG. 1 (b) shows an α + γ region.

FIG. 2 is a schematic view showing the precipitation of AlN in a slab for a steel sheet of the present invention, wherein FIG. 2 (a) is a low-temperature γ region, and FIG.
Is the case of the α + γ region.

Claims (1)

[Claims] 1. A chemical composition in weight%, C: 0.2% or less, Si: 0 to 2.0%, Mn: 0.05 to 2.0%,
Al: 0.3 to 2.5%, N: 0.002 to 0.01%
A steel sheet having excellent surface properties, characterized by containing.