JPH0873995A - Hot rolled steel plate free from crack and surface flaw and its production - Google Patents

Abstract

PURPOSE: To prevent the occurrence of cracks in a cast slab at the time of continuously casting a steel containing tramp elements in a steel composition, to improve the hot workability of the slab, and to produce a hot rolled steel plate free from surface flaw and excellent in quality. CONSTITUTION: This hot rolled steel plate has a composition consisting of 0.01-0.15% C, <=0.3% Si, 0.1-1.0% Mn, <=0.03% P, <=0.025% S, 0.01-0.07% sol.Al, <=0.012% N, 0.04-0.4% Cu, 0.004-0.04% Sn, 0.01-0.1% Ni, 0.01-0.1% Cr, 0.01-0.1% Mo, and Fe, satisfying C+(Cu+5Sn)/3<=0.25% in the stage of a molten steel, and also satisfying Cu+10Sn<=0.4% in the stage of a slab. A steel having this composition is refined and cast into a slab by continuous casting. The cast slab is heated to >=1100 deg.C, and both edges of the heated slab, in a plate-width direction, are heated by means of edge heaters. Then, the slab is hot-rolled at a finishing temp. not lower than the Ar3 transformation point and coiled at 500-700 deg.C. By this method, the hot rolled steel plate free from crack and surface flaw can be produced.

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 using iron scrap as an iron source and free from cracks and surface flaws, and a method for producing the hot rolled steel sheet.

[0002]

2. Description of the Related Art In recent years, the amount of iron scrap generated from discarded food cans and automobiles has increased, and the use of such iron scrap as an iron source has begun to be considered in the steel industry from the viewpoint of environmental problems. .

However, in the iron scrap, Cu,
Sn, As, Ni, Cr and the like are contained as playing card elements, and these elements are less likely to be oxidized than Fe, so that they are hardly removed in the current ironmaking process and remain in steel. As a result, hot ductility deteriorates, cracks are likely to occur during continuous casting of slabs, hot workability of slabs deteriorates, and surface defects tend to occur. Effect of alloying elements on "378
Page Sebundou Shinkosha issue).

In order to reduce such surface defects generated during hot working, Ni is positively added to steel containing a playing card element such as Cu and Sn to remove Cu and Sn in the steel.
It has been proposed to prevent the generation of surface flaws by fixing as a Cu-Sn-Ni alloy. However,
This method has a problem that the manufacturing cost increases because Ni is expensive.

In order to solve the above problems, Japanese Patent Laid-Open No. 5-2224
No. 32 discloses that high scrap steel is continuously tin-cast with Sn / Cu ≤ 1/20, and the cast slab is
A method of hot rolling at a surface temperature of 50 ° C. or lower (hereinafter referred to as prior art 1) has been proposed, and is disclosed in Japanese Patent Laid-Open No. 5-220505.
In the publication, a method of heating a slab containing Cu and Sn at a temperature of 950 ° C. or lower, heating the heated slab to a rolling temperature in a non-oxidizing atmosphere, and then hot rolling (hereinafter referred to as prior art 2 It is proposed).

[0006]

However, the prior art 1 has a problem that it has a large facility restriction and is inferior in practicability since it requires a detinning facility. In addition, the prior art 2 is
There is a problem in operability and there is a problem that the cost becomes high.

Therefore, an object of the present invention is to solve the above-mentioned problems and to prevent cracks occurring in a cast slab during continuous casting of steel containing Trump elements such as Cu and Sn using iron scrap as an iron source. And to improve the hot workability of the slab and to obtain a hot-rolled steel sheet without surface flaws.

[0008]

The hot-rolled steel sheet of the present invention comprises:
Substantially the following chemical composition, produced by using iron scrap as an iron source, carbon (C): 0.01 to 0.15 wt.% Silicon (Si): 0.3 wt.% Or less, manganese (Mn): 0.1 to 1.0 wt.%, phosphorus (P): 0.03 wt.% or less, sulfur (S): 0.025 wt.% or less, soluble aluminum (sol.Al): 0.01 to 0.07 wt.% nitrogen (N): 0.012 wt.% or less , Copper (Cu): 0.04 to 0.4 wt.%, Tin (Sn): 0.004 to 0.04 wt.%, Nickel (Ni): 0.01 to 0.1 wt.%, Chromium (Cr): 0.01 to 0.1 wt.%, Molybdenum (Mo): 0.01-0.1 wt.% And iron (Fe). At the molten steel stage, the following formula (1), C + (Cu + 5Sn) / 3 ≤ 0.25 wt.% ─────── (1 ) Is satisfied, and the following formula (2) Cu + 10Sn ≤ 0.4 wt.% ───────────── (2) is satisfied at the stage of the slab cast from the molten steel. It has characteristics.

In the method for producing a hot-rolled steel sheet according to the present invention, steel having the above chemical composition is melted in a converter or an electric furnace, cast into a slab by continuous casting, and then,
The slab is heated to a temperature of 1,100 ° C or higher, then both edges of the heated slab in the plate width direction are heated by edge heaters to make the temperature distribution in the plate width direction uniform, and then the plate width. The slab with a uniform temperature distribution in the direction is hot-rolled at a finishing temperature above the Ar ₃ transformation point, and the resulting steel strip is wound at a temperature of 500 to 700 ℃. .

[0010]

The reason why the chemical composition of the hot-rolled steel sheet of the present invention is limited to the above range will be described below. (1) C: C is a basic element that enhances the strength of steel.
If the C content is less than 0.01 wt.%, Secondary working embrittlement tends to occur, while if the C content exceeds 0.15 wt.%, The strength becomes too high and the workability deteriorates. Therefore, the C content is 0.01
It should be limited to the range of ~ 0.15 wt.%.

It is generally known that when a molten steel containing a large amount of C is continuously cast to produce a slab, cracks occur in the curved portion of the slab to which low strain tensile stress is applied. The crack generated in such a slab has a strong relationship with the hot ductility of steel. Cu and Sn are known as elements that reduce the hot ductility in the Trump element contained in the low-grade scrap.

According to the research conducted by the present inventors, it has been clarified that when Cu and Sn are contained in the steel, C in the steel is also involved in the reduction of hot ductility. FIG. 1 is a graph showing the relationship between the amount of C in steel and the “(Cu + 5Sn) / 3” value with respect to the reduction value. From Fig. 1, it is said that there is no risk of cracks in the slab during continuous casting, and the aperture value is 60.
It can be seen that the region of% or more needs to satisfy the following formula (1), C + (Cu + 5Sn) /3≦0.25 wt.% ────────── (1), in the molten steel stage. From the above points,
In the present invention, it is necessary that the C content is within the range of 0.01 to 0.15 wt.% And that the above formula (1) is satisfied at the molten steel stage.

(2) Si: Si is an element that enhances the strength of steel. However, if the Si content exceeds 0.3 wt.%, Surface defects called red scale are likely to occur. Therefore, the Si content should be limited to 0.3 wt.% Or less.

(3) Mn: Mn is also an element that enhances the strength of steel. If the Mn content is less than 0.1 wt.%, Flaws are likely to occur on the surface of the steel sheet. On the other hand, when the Mn content exceeds 1.0 wt.%, The strength becomes too high and the workability deteriorates. Therefore, the Mn content should be limited to the range of 0.1-1.0 wt.%.

(4) The content of S: S is preferably as small as possible in order to improve workability such as bending workability and stretch flangeability of the steel sheet, and therefore should be limited to 0.025 wt.% Or less. This value is also important from the viewpoint of ensuring weldability.

(5) P: P tends to segregate at the grain boundaries,
It is an element that causes intergranular brittle fracture called secondary work cracking after press forming of a steel sheet. Therefore, it is desirable that the content is small, and it should be limited to 0.03 wt.% Or less.

(6) N: N is an element that reduces the aging property, so it is desirable that its content be small. Therefore, its content should be limited to 0.012 wt.% Or less.

(7) sol.Al:Al is an element effective for deoxidizing steel. However, the content of sol.Al is 0.01w
If it is less than t.%, its effect is insufficient, while if the content of sol.Al exceeds 0.07 wt.%, the amount of silicate-based inclusions increases and the workability deteriorates, and the cost is high. Become. Therefore, the content of sol.Al should be limited to the range of 0.01 to 0.07 wt.%.

(8) Cu: Cu is an element contained as a trump element, but even if iron scrap is used as an iron source, the maximum Cu content is 0.04 wt. The lower limit of is 0.04 wt.%. Further, as described with respect to C, in order to prevent cracks occurring in the slab during continuous casting, the Cu content in the molten steel stage is the same as in the above (1).
It is necessary to satisfy the formula, that is, C + (Cu + 5Sn) /3≦0.25 wt.%.

If the Cu content is high, the hot-rolled steel sheet contains Cu.
Surface defects called defects occur and the surface quality of the product deteriorates. That is, continuously cast Cu-containing slab, after heating in an oxidizing atmosphere, during hot rolling, Cu is not oxidized during hot rolling, so a Cu-rich phase is formed immediately below the scale. It Since the melting point of the Cu-rich phase is relatively low, the Cu-rich phase becomes a melt at a general slab heating temperature of 1100 ° C or higher, and as a result of this melt penetrating into the austenite grain boundary, during hot rolling. Surface cracks occur and become surface defects. In order to prevent the occurrence of such surface defects, it is necessary to set the upper limit of the Cu content in steel to 0.4 wt.%.

Further, as in the present invention, in the case of steel in which Cu and Sn are added in combination, Sn is concentrated in the Cu-enriched phase and the melting point of this phase is lowered, so that the steel sheet containing only Cu is added. Compared with, surface defects are more likely to occur and the surface quality deteriorates.
FIG. 2 is a graph showing the relationship between the Cu and Sn contents and the state of occurrence of surface defects. From FIG. 2, it can be seen that the region where the surface flaw is not likely to occur in the hot-rolled steel sheet needs to satisfy the following expression (2) at the slab stage. Cu + 10Sn ≦ 0.4 wt.% ──────── (2)

From the above points, in the present invention in which the slab containing Cu and Sn is reheated and then hot-rolled,
In order to improve the surface quality of hot rolled steel sheet, the Cu content is
It is necessary to be within the range of 0.04 to 0.4 wt.% And to satisfy the above formula (2) at the slab stage.

(9) Sn: Sn is an element contained as a trump element, but even if iron scrap is not used as an iron source, the maximum Sn content is 0.004 wt.%. The lower limit of is 0.004wt.%. on the other hand,
If the Sn content exceeds 0.04 wt.%, The hot ductility deteriorates, vertical cracks occur during slab production by continuous casting, and the surface quality of the steel sheet deteriorates. Therefore, the Sn content is 0.004 to 0.
It should be limited to the range of 04 wt.%. Furthermore, as described above, the Sn content in the molten steel stage is set to the above formula (1), that is, C +, in order to prevent cracks occurring in the slab during continuous casting.
It is necessary to satisfy (Cu + 5Sn) /3≦0.25 wt.%, And in order to improve the surface quality of the steel sheet, the Sn content in the slab stage is expressed by the formula (2), Cu + 10Sn ≦ 0. .
It is necessary to satisfy 4 wt.%.

(10) Ni: Ni is an element contained as a Trump element, but even if iron scrap is not used as an iron source, the maximum Ni content is 0.01 wt.%. The lower limit of is 0.01 wt.%. On the other hand, Ni
Is an element effective in preventing the generation of Cu defects and improving hardenability. However, when the Ni content exceeds 0.1 wt.%,
Strength increases and workability deteriorates. Therefore, the Ni content is 0.
It should be limited to the range of 01-0.1wt.%.

(11) Cr, Mo: Cr and Mo are also elements contained as Trump elements, but in all cases, even if iron scrap is not used as an iron source, a maximum of 0.01
Since Cr and Mo are contained in wt.%, the lower limits of the Cr and Mo contents are 0.01 wt.%, respectively. On the other hand, Cr
If the content of Mo and Mo exceeds 0.1 wt.%, The workability deteriorates due to the increased strength. Therefore, the contents of Cr and Mo should be limited to the range of 0.01 to 0.1 wt.%.

Next, the manufacturing conditions of the hot-rolled steel sheet of the present invention will be described.
The reason for limiting the above range will be described. The melting and refining of steel may be carried out by either a converter method or an electric furnace method, and low-grade iron scrap as an iron source is melted using 10 wt.% Or more of pig iron. Casting of a slab is performed by a continuous casting method in terms of its quality, yield, production efficiency, etc., and a thick slab or a thin slab having a normal wall thickness is cast.

In order to prevent cracks occurring in the cast slab during continuous casting, a vacuum degassing device is used before casting to satisfy the above formula (1), C + (Cu + 5Sn) /3≦0.25 wt.%. So that the C content in the molten steel is adjusted.

The heating temperature of the slab may be 1,100 ° C. or higher, which is a conventional method. The finishing temperature during hot rolling should be limited to the _{Ar 3} transformation point or higher. If the finishing temperature is _lower than the _{Ar 3} transformation point, strain is applied to the ferrite grains of the steel sheet to form a mixed grain structure and the ductility deteriorates. The coiling temperature during hot rolling may be within the range of 500 to 700 ° C. which is a conventional method.

According to the present invention, after the continuously cast slab is heated to the above temperature and before hot rolling, both ends in the plate width direction in which the temperature is lower than the central part in the plate width direction of the slab Is heated using an edge heater. In this way, by heating the both ends of the slab in the plate width direction, Cu and Sn in the steel prevent edge cracks that occur during hot rolling, and also make the temperature distribution in the plate width direction of the slab uniform. It

[0030]

EXAMPLES Next, the present invention will be described by way of examples in comparison with comparative examples. As an iron source, Cu: 0.4w on average
t.%, Sn: 0.04wt.%, Cr: 0.1wt.% and Mo: 0.1wt.%
Low-grade iron scrap containing 10 wt.% Of pig iron
Steel Nos. 1 to 5 of the present invention having the chemical composition within the scope of the present invention shown in Table 1 used above, and at least 1
Comparative Steel Nos. 1-6, in which two elements have chemical composition outside the scope of the present invention, were melted by an electric furnace and continuously cast into a slab.

[0031]

[Table 1]

Next, continuously cast steel Nos. 1 to 5 of the present invention
Slabs and slabs of comparative steel Nos. 1 to 6 are shown in Table 2 for heating temperature (ST), finish rolling temperature (FT) and winding temperature (C).
T) hot-rolled to a plate thickness of 3.2 mm, then 1
% Hot rolling of the present invention (hereinafter referred to as the present invention specimen) Nos. 1 to 5 and comparative hot rolled steel sheet specimens (hereinafter referred to as the comparative specimen). No.1
~ 6 were prepared. In addition, regarding the present invention sample Nos. 1 to 5 and the comparative sample No. 1, after heating the continuously cast slab and before hot rolling, both end portions in the plate width direction of the slab are edge-heated. Was heated using.

[0033]

[Table 2]

With respect to each of the sample of the present invention and the sample for comparison produced as described above, the presence or absence of cracks generated on the surface of the continuously cast slab was visually inspected and evaluated by the following. ◯: No cracks, X: Cracks.

Further, in order to evaluate the hot ductility of each of the above-mentioned test pieces during slab, a round bar-shaped test piece having a diameter of 8 mm and a parallel portion length of 16 mm was cut out from each test piece slab, and the cut-out test piece was Hold in a vacuum at a temperature of 1250 ° C for 5 minutes, then rapidly cool to a temperature of 700 to 1000 ° C, and at that temperature perform a tensile test until it breaks at a low strain rate (strain rate 10 ^-3 / sec), The drawing ratio (reduction ratio of the cross section) of the fracture surface was obtained.

A JIS No. 5 test piece was sampled from each of the sample of the present invention and the sample for comparison, and its mechanical properties, that is, the yield strength, tensile strength and total elongation were examined. Further, the surface quality of each of the sample of the present invention and the sample for comparison, that is, the presence or absence of surface defects, was visually examined and evaluated by the following. ◯: No surface flaw occurred, ×: Surface flaw occurred.

The slab manufacturability of each specimen, that is, the presence or absence of slab cracking and the hot ductility, that is, the drawing ratio, and the material characteristics of each specimen, that is, the yield strength, the tensile strength, and the total strength The elongation and the surface quality are also shown in Table 2.

As is clear from Tables 1 and 2, in the comparative test sample No. 1 in which the finish rolling temperature (FT) was out of the range of the present invention and was low, the microstructure was ferrite mixed grains, Growth has dropped significantly.

Even if the respective contents of Cu and Sn are within the scope of the present invention, "C + (Cu + 5Sn) /
Comparative specimen No. 3 having many “3” values exceeding the range of the present invention
In Nos. 6 to 6, cracking occurred during slab production by continuous casting. Further, even if the respective contents of Cu and Sn are within the range of the present invention, in the comparative specimens Nos. 1 and 3 in which the “Cu + 10Sn” value in the slab stage exceeds the range of the present invention, the surface A flaw was generated and the surface quality was deteriorated.

In the comparative specimen No. 1 having a large Mn content exceeding the range of the present invention and the comparative specimen Nos. 4 to 6 having a large C content exceeding the scope of the present invention, the tensile strength was increased. The strength was over 50 Kgf / mm ² , and the workability was deteriorated due to the low elongation. In addition, in the comparative specimen Nos. 2 to 6,
Before the slab was hot-rolled, the edge heaters did not heat both ends of the plate width direction, so the total elongation of both ends of the steel plate was significantly reduced compared to the center part, The uniformity was poor.

On the other hand, the present invention sample N in which the chemical composition and manufacturing conditions of steel are within the scope of the present invention
In o.1 to 5, cracks did not occur during the production of the slab by continuous casting, and the material properties such as mechanical properties and surface quality were all excellent.

The use ratio of the iron scrap in the above-mentioned embodiment is 10 wt.% Or more with respect to the pig iron, but it goes without saying that the iron scrap may be used in any ratio as the iron source.

[0043]

As described above, according to the present invention,
During continuous casting of steel containing Trump elements such as Cu and Sn using iron scrap as the iron source, cracks that occur in the cast slab are prevented, and the hot workability of the slab is improved, and the surface Industrially useful effects can be obtained in which a hot-rolled steel sheet with excellent quality and no flaws can be obtained.

[Brief description of drawings]

[Fig. 1] C amount in steel and "(Cu + 5Sn) /
It is a graph which shows the relationship with a "3" value.

FIG. 2 shows the amount of Sn and the amount of Cu in steel regarding surface defects,
It is a graph which shows the relationship with a "Cu + 10Sn" value.

Claims (2)

[Claims] 1. Substantially the following chemical composition, produced by using iron scrap as an iron source, carbon (C): 0.01 to 0.15 wt.% Silicon (Si): 0.3 wt.% Or less, manganese (Mn ): 0.1 to 1.0 wt.%, Phosphorus (P): 0.03 wt.% Below, sulfur (S): 0.025 wt.% Or less, soluble aluminum (sol.Al): 0.01 to 0.07 wt.% Nitrogen (N): 0.012 wt.% Or less, Copper (Cu): 0.04 to 0.4 wt.%, Tin (Sn): 0.004 to 0.04 wt.%, Nickel (Ni): 0.01 to 0.1 wt.%, Chromium (Cr): 0.01 to 0.1 wt.%, molybdenum (Mo): 0.01 to 0.1 wt.%, and iron (Fe). At the stage of molten steel, the following formula (1), C + (Cu + 5Sn) /3≦0.25 wt.% ──── ─── (1) is satisfied, and the following formula (2) Cu + 10Sn ≦ 0.4 wt.% ──────────── (2) is satisfied at the stage of the slab cast from the molten steel. A hot-rolled steel sheet having no cracks and surface defects. 2. A steel having the chemical composition according to claim 1 is melted in a converter or an electric furnace, cast into a slab by continuous casting, and then the slab is heated to 1,100.
After heating to a temperature of ℃ or more, then the both ends of the heated slab in the plate width direction are heated by edge heaters to uniformize the temperature distribution in the plate width direction of the slab, and then to the temperature distribution in the plate width direction. The slab homogenized is hot-rolled at a finishing temperature equal to or higher than the Ar ₃ transformation point, and the obtained steel strip is
A method for producing a hot-rolled steel sheet free from cracks and surface flaws, which comprises winding at a temperature of 500 to 700 ° C.