JPH0994601A - Production of hot rolled steel sheet containing copper having good surface condition - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain hot rolling technology of steel containing copper by which actual operation is easily executed and generation of a surface flaw is effectively prevented. SOLUTION: A steel slab, which has a composition consisting of, by weight, <=0.02% C, 0.10-1.50% Mn, <=0.020% S, 0.20-0.50% Cu, if necessary, <=1.2% Si, <=0.15% Ti, <=0.05% Nb, <=0.15%P, <=0.15% Cr and the balance Fe with inevitable impurities, is heated. Successively, when rolling in the slab which direction is executed at >=4% slab width, finish rolling is completed at the Ar3 point or more by means of that a rolling temp. is set to >=1170 deg.C of a rolling speed (*1) in the width direction is set to <=250mm/sec. Rolling speed (*1)=(original slab width-after rolled slab width)/width direction rolling time.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a hot-rolled copper-containing steel sheet having excellent corrosion resistance, which is mainly used for general building materials, materials for containers, materials for undercarriage parts of automobiles, and the like.

[0002]

2. Description of the Related Art Generally, it is considered effective to add Cu by 0.2% or more in order to improve the corrosion resistance at room temperature.
However, it is known that a drawback of copper-containing steel is hot brittleness, which causes cracks in the steel sheet during processing at high temperatures, which causes surface defects of the product. The causes of cracking during hot working are generally considered as follows.

That is, when the slab is heated in a heating furnace, Fe oxidizes as a scale and Cu, which is less oxidizable than Fe, forms an enriched layer at the interface between Fe and scale. When the Cu concentration in this enriched layer reaches about 8% or more, a liquid phase is generated at 1094 ° C or more. When the brittle steel plate surface in which the liquid phase is generated is subjected to hot working, cracking occurs.

On the basis of this surface crack generation mechanism, various measures have been taken to prevent cracks, that is, surface defects of products. The simplest method is to add Ni in an amount of 1/2 or more of Cu to raise the melting point of the Cu-rich layer. However, this method requires a large amount of expensive Ni, which leads to a significant cost increase. Also for Si
It is said that the Cu-enriched layer has the effect of improving the surface defects by coming to the scale side.

A method for reducing the amount of Ni added is disclosed in Japanese Patent Laid-Open No. 6-3130.
No. 2 and No. 6-297025 disclose methods of applying Ni oxide and Si oxide to the slab surface. This is Ni
The purpose is to make the effect of Si and Si only on the slab surface, but the coating method of Ni and Si oxides is unclear, and new equipment for coating must be provided.

As a method other than the addition of Ni, Japanese Patent Laid-Open No. 49-12
Japanese Patent No. 3458 discloses a method in which a heating temperature is set to 1300 ° C. or higher, rough rolling is completed at 1090 ° C. or higher, followed by cooling to 1050 ° C. or lower and then finish rolling. This aims to suppress surface defects by diffusing Cu into the scale and avoiding hot working at 1050-1100 ° C, where Cu is most brittle. However, if the heating temperature is set to 1300 ° C or higher, it is difficult to carry out in a mass production line because of damage to the furnace and a significant increase in energy cost.

Further, as a method for preventing surface defects at a heating temperature of 1300 ° C. or lower, JP-A-51-126322 discloses hot shaving the slab surface and then hot rolling, or hot shaving. After that, a method is disclosed in which the heating time and the heating temperature are strictly limited to carry out hot rolling. They are
Although the hot working is completed before the Cu-rich layer is formed, it has the drawback that new equipment must be installed for that purpose and the heating conditions are significantly restricted.

[0008]

Although the prior art is effective for preventing surface flaws in copper-containing steel, it is difficult to realize it in actual operation due to many limitations. Here, the object of the present invention is to
It is an object of the present invention to provide a method for producing a copper-containing hot-rolled steel sheet which can be easily carried out in actual operation and which can effectively prevent the occurrence of surface defects.

[0009]

Therefore, as a result of researching a method for suppressing surface flaws by focusing on hot rolled steel sheets, the present inventors have developed a method for preventing surface flaws of copper-containing steel in hot rolled steel sheets. .
That is, the characteristics of hot-rolled steel sheet are that the roll contact surface is large in hot rolling, that the deterioration of the end surface of the product is not a problem because the product thickness is thin, and the width direction is adjusted after heating for the purpose of width adjustment or scale peeling improvement. Focusing on the fact that processing is added, by controlling the temperature during width direction processing as a method for preventing surface defects of hot rolled steel sheets, and by controlling the processing speed during width direction processing The present invention has been completed based on the knowledge that a surface flaw can be effectively prevented from occurring although deterioration is observed, and a manufacturing method that can be easily applied to a mass production line can be realized. The method for producing a copper-containing hot-rolled steel sheet having good surface properties according to the present invention is as follows.

(1) C: 0.20% or less, Mn: 0.
10 to 1.50%, S: 0.020% or less, Cu: 0.20 to 0.50%, the balance is a steel slab having a steel composition consisting of Fe and unavoidable impurities. After heating, the slab width is processed to 4% of the slab width. In the case of performing the above, the process is carried out at 1170 ° C. or higher, and finish rolling is completed at ₃ or more points of Ar.

(2) In% by weight, C: 0.20% or less, Mn: 0.
10 to 1.50%, S: 0.020% or less, Cu: 0.20 to 0.50% After heating the base steel slab having a steel composition composed of Fe and inevitable impurities, the balance is processed in the slab width direction at 4% or more of the slab width. If you do the following, in the width direction
Processing speed defined by formula (1) (* 1) Manufacturing method for copper-containing hot-rolled steel sheet with good surface properties, which is performed at 250 mm / sec or less and finish rolling is completed at Ar ₃ points or more .

Machining speed (* 1) = (original slab width-post-machining slab width) / width direction machining time (1) (3) The steel composition further contains Si: 1.2% by weight. %Less than,
The surface texture according to (1) or (2) above, which contains at least one of Ti: 0.15% or less, Nb: 0.05% or less, and / or P: 0.15% or less, Cr: 1.5% or less. For manufacturing a copper-containing hot-rolled steel sheet of good quality.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION That is, according to the present invention, Cu:
At 0.50% or less, it was found that when the widthwise processing was less than 4% of the slab width, no flaw was observed on the roll contact surface and the steel plate end surface was only slightly deteriorated. When 4% or more of the slab width is applied in the width direction, flaws may appear on the steel plate surface, but when the processing temperature is 1170 ° C or higher, or the processing speed in the slab width direction. It was found that when the value is less than 250 mm / sec, the occurrence of flaws can be suppressed. However, if Cu: 0.50% is exceeded, the processing speed in the width direction will be increased even if the processing temperature is 1170 ° C or higher.
Surface defects may occur even at 250 mm / sec or less. That is, when Cu: 0.50% or less, the processing temperature is 1170 ° C or more, or the width direction processing speed is 250 mm / sec or less, only the non-roll contact surface (free end) is flawed.

Next, the reason why the steel composition and working conditions are set as above in the manufacturing method of the present invention will be described in detail below. First, the reason for defining the chemical composition of the raw material steel slab which is the subject of the present invention will be described together with the action and effect.
In the present specification, “%” represents% by weight unless otherwise specified.

C: 0.20% or less C is an element having a great influence on mechanical properties. C is 0.
If it exceeds 20%, the workability is remarkably deteriorated, and a predetermined shape may not be obtained in cold press working. Further, in the case of a structure, softening and hardening may occur in the vicinity of the welded portion, so that sufficient strength and toughness may not be obtained. Therefore C
Content was 0.20% or less. Preferably 0.03-0.15%
It is.

Mn: 0.10 to 1.50% Mn acts as MnS to prevent hot embrittlement of S (about 900 to 1000 ° C.) and prevent surface defects from occurring. Therefore, the lower limit is 0.10
%. Mn is effective as a strengthening element, but if it is added in a large amount, elongation is reduced and workability is significantly deteriorated. Therefore, the upper limit of Mn is set to 1.50%. Preferably 0.30 to 1.30%
It is.

S: 0.020% or less S is an element that deteriorates hot workability, and it is desirable that S is as small as possible. However, if S is 0.020% or less, the deterioration is small, so the upper limit was made 0.020%. Preferably, it is not more than 0.010%.

Cu: 0.20 to 0.50% Cu is an important element for corrosion resistance. To improve corrosion resistance, 0.20% or more must be added, so the lower limit is 0.20%.
And Further, although the corrosion resistance can be increased to about 0.70%, if Cu is added in an amount of more than 0.50%, it is difficult to eliminate surface defects by the method of the present invention. Therefore, the upper limit was made 0.50%. Preferably, it is 0.30 to 0.50%.

Si, Ti, Nb, P, Cr: These may be added if necessary. The addition of Si, Ti, Nb, P and Cr is a case where the following effects are expected.

Si: 1.2% or less Si is an element suitable for improving the strength of a steel sheet with little deterioration in formability. For this reason, adding Si is an effective means when using copper-containing hot-rolled steel sheets for undercarriage parts of automobiles. However, when added in excess of 1.2%, Fe ₂ S
A large amount of iO ₄ is generated, causing abnormal oxidation and increasing scale loss. Therefore, the upper limit was 1.2%. It is preferably 0.50 to 1.10%.

Ti: 0.15% or less, Nb: 0.05% or less Ti and Nb are elements effective in increasing strength by precipitating as TiC and NbC, respectively. However, even if Ti is added in excess of 0.15%, its effect will be small and only increase in cost, so the upper limit was made 0.15%. For the same reason, the upper limit of Nb was set to 0.05%. Preferably, Ti: 0.02 to 0.12%, Nb:
It is 0.01 to 0.035%.

P: 0.15% or less P is an element effective for corrosion resistance. Therefore, if it is necessary to further improve the corrosion resistance, it may be added. However, if P is added in excess of 0.15%, not only the corrosion resistance is not improved but also the toughness of the steel is deteriorated, so the upper limit is 0.15%.
%. Preferably, it is 0.05 to 0.12%.

Cr: 1.5% or less Cr is also an element effective in corrosion resistance, and particularly effective in seawater corrosion resistance. Therefore, if seawater corrosion resistance is required, it must be added. However, the effect is saturated even if it exceeds 1.5%, so the upper limit was made 1.5%. Preferably 1.0-1.5
%.

Next, the reasons for limiting the manufacturing conditions will be described. The hot rolling is performed after each step of extracting the slab from the heating furnace and then processing the slab in the width direction → descaling → rough rolling → finish rolling → cooling → winding.

Slab width direction working amount: 4% or more of the slab width If the working amount in the slab width direction is less than 4% of the reduction amount of the slab width, the slab has surface flaws even if it is processed in the width direction. Hard to do. Therefore, if the amount of processing in the slab width direction is less than 4% of the slab width, it is outside the scope of the present invention. Therefore, the processing amount in the slab width direction is set to 4% or more in terms of the reduction amount of the slab width. It is preferably at least 6%.

Slab width direction processing temperature: 1170 ° C. or higher It is important to prevent the surface flaws by setting the slab width direction processing temperature to 1170 ° C. or higher. The cause of surface defects of the copper-containing hot-rolled steel sheet is largely due to the processing in the width direction. That is, in the width direction processing, the surface of the steel sheet becomes a free end, which may cause flaws on the surface side during product processing. Surface flaws of the copper-containing hot-rolled steel sheet can be prevented by performing the widthwise working at a temperature other than the brittleness temperature of Cu. Therefore, the processing temperature of the slab width direction processing was set to 1170 ° C or higher. Preferably 1200
~ 1250 ℃.

Processing speed of slab width direction processing: 250 mm / sec
From the other aspect, the present invention aims to prevent surface flaws during processing by limiting the processing speed in the width direction.

The processing speed of the slab width direction processing is 250 mm / sec.
The following is important for preventing surface defects. The cause of surface defects of the copper-containing hot-rolled steel sheet is largely due to the processing in the width direction. That is, in the width direction processing, the surface of the steel sheet becomes a free end, which may cause flaws on the surface side during product processing. As a result of studying the surface flaws on the steel sheet and the machining speed in the width direction, it was found that the probability of surface flaws increasing as the machining speed increased. Therefore, by specifying the upper limit of the processing speed in the width direction, the embrittlement of the surface due to Cu can be prevented. Therefore, the upper limit of the processing speed for slab width direction processing was set to 250 mm / sec or less. Preferably 200m
It is less than m / sec. Here, the slab width direction processing speed is
As shown in the above equation (1), it is defined as (original slab width-post-machining slab width) / width direction machining time.

Finishing rolling temperature: Ar ₃ points or more Ar ₃ points or more is the reason for completing the finishing rolling. Not only the flatness and dimensional accuracy of the steel sheet are greatly deteriorated due to changes in rolling load when transformation occurs during rolling, but also The reason is that it becomes difficult for the threading itself.

The winding temperature has no influence on the surface of the copper-containing hot-rolled steel sheet, but is 70% to prevent winding defects.
It is desirable that the temperature is 0 ° C. or less. Next, the function and effect of the present invention will be described more specifically based on Examples.

[0031]

【Example】

(Example 1) In this example, C = 0.10%, Mn = 0.50%, S = 0.
Raw steel slab with 3 kinds of steel composition of Cu = 0.2, 0.5, 0.8% added to 015% basic steel composition 240t ×
1200w × 6000〜7000L (mm) is produced and heated in a heating furnace.
After extraction, slab width direction processing → descaling → rough rolling →
Hot rolling was performed in the order of finish rolling, cooling, and winding.

Processing in the slab width direction was performed by a sizing press at about 12.5% (150 mm) of the slab width. The slab heating temperature was set to about 1220 ° C., and then the slab was cooled to a predetermined width-direction processing temperature on a hot run table. After rolling in the width direction, rough rolling and tandem finish rolling were performed to a plate thickness of 2.6 mm.

FIG. 1 shows the processing temperature and the occurrence rate of surface defects at this time.
It is a graph showing the relationship with (%). As shown in FIG. 1, it can be seen that the surface defects are most generated when the processing temperature in the width direction is about 1100 ° C. Further, when Cu: 0.50% or less, the surface flaw generation is suppressed when the width-direction processing temperature is 1170 ° C or higher. Conversely Cu
= 0.8%, surface defects may occur at a processing temperature in the width direction of 1170 ° C or higher, albeit slightly.

(Example 2) Steel containing the components shown in Table 1 (Ar ₃ ≈ 800)
(° C) was melted by a conventional method, and then hot rolling was performed while changing the width direction working temperature and the working amount. The size of the manufactured slab is 24
0t × 1200w × 6000 to 7000L (mm), after extraction as in the case of Example 1, a width direction rolling reduction of 2 by a sizing press.
% (24 mm) to 20% (240 mm) slab width direction processing was performed, and then rough rolling, finish rolling, and coil winding were performed.

The results are shown in Table 2. The presence or absence of surface flaws is the result of visual inspection in the next step, the pickling line. From this result, it can be seen that the method of the present invention does not cause flaws on the surface of the copper-containing hot-rolled steel sheet.

The mechanical properties are the results of a tensile test in the rolling direction using a JIS No. 5 test piece of a sample taken from the position of 1 / 4w of the hot rolled coil in the pickling line. It can be seen that the steel obtained by the method of the present invention has good mechanical properties, whereas the alloys containing C and Mn added in excess have markedly deteriorated elongation. Furthermore, Ti is 0.15% or more, Nb is 0.05%.
It can be seen that the effect on strength is small even if added in a large amount.

Further, in order to evaluate the corrosion resistance of the sample collected on the pickling line, an accelerated bakuro test was carried out.
In addition to the usual bakuro test, the accelerated bakuro test is performed once
Characterized by spraying 5% saline solution weekly.

From these test results, it can be seen that the corrosion resistance is significantly deteriorated when the added amount of Cu is less than the range specified in the present invention. It is also found that the effects of P and Cr, which are effective in improving the corrosion resistance, are saturated even if added in a large amount beyond the range of the present invention. As for the scale loss, it can be seen that the scale loss is large when a large amount of Si is added.

[0039]

[Table 1]

[0040]

[Table 2]

(Example 3) In this example, as in the case of Example 1, the basic steel composition of C = 0.10%, Mn = 0.50% and S = 0.015% was added to Cu = 0.2, 0.5 and 0.8%, respectively. Material steel slab with three kinds of added steel composition 240t × 1200w × 6000〜70
00L (mm) was prepared and the relationship between the width direction processing speed and the surface defect occurrence rate when rolling was examined.

Processing in the slab width direction was carried out by a sizing press at a reduction rate of the slab width of about 20% (240 mm). Further, the slab heating temperature was all set to about 1200 ° C. After rolling in the width direction, rough rolling and tandem finish rolling were performed to a plate thickness of 2.6 mm.

FIG. 2 shows the processing temperature and the occurrence rate of surface defects at this time.
It is a graph showing the relationship with (%). As shown in FIG. 2, it can be seen that the surface flaw occurrence rate increases as the width direction processing speed increases. Furthermore, when Cu: 0.50% or less, surface flaws are suppressed when the width direction processing speed is 250 mm / sec or less.
On the other hand, when Cu = 0.8%, surface defects may occur considerably even when the processing speed in the width direction is 250 mm / sec or less.

(Embodiment 4) Steel containing the components shown in Table 3 (Ar ₃ = about 80)
(0 ° C.) was melted by a conventional method, and then hot rolling was performed while changing the width direction working temperature and the working amount. The dimensions of the manufactured slab are
240t x 1200w x 6000 to 7000L (mm), after extraction as in the case of Example 3, slab width processing is performed with a sizing press in the width direction rolling reduction of 2% (24mm) to 20% (240mm) and processing speed. The rolling was performed at 150 to 350 mm / sec, and then rough rolling, finish rolling with ₃ or more points of Ar, and coil winding were performed.

The results are shown in Table 4. The presence or absence of surface flaws is the result of visual inspection in the next step, the pickling line. From this result, it can be seen that the surface of the hot-rolled steel sheet obtained by the method of the present invention has no flaws. The mechanical properties are the result of a tensile test in the rolling direction using a JIS No. 5 test piece of a sample taken from the position of 1 / 4w of the hot rolled coil in the pickling line.

It can be seen that the steel obtained by the method of the present invention has good mechanical properties, whereas the alloys containing C and Mn added in excess have markedly deteriorated elongation. In addition, Ti,
It can be seen that the effect on strength is small even if Nb is added in a large amount of 1.0% or more.

Further, in order to evaluate the corrosion resistance of the sample collected on the pickling line, an accelerated bakuro test was carried out.
In addition to the usual bakuro test, the accelerated bakuro test is performed once
Characterized by spraying 5% saline solution weekly.

From these test results, it is understood that the corrosion resistance is significantly deteriorated when the added amount of Cu is less than the range specified in the present invention. Further, it is understood that the effects of P and Cr, which are effective in improving the corrosion resistance, are saturated even if they are added within the range of the present invention.
As for the scale loss, it can be seen that the scale loss is large when a large amount of Si is added.

[0049]

[Table 3]

[0050]

[Table 4]

[0051]

According to the present invention, a steel-containing hot-rolled steel sheet having a good surface can be manufactured without greatly limiting expensive Ni and heating conditions. When the Cu content is 0.5% or less, the width direction processing is performed at 1170 ° C or higher, or the slab width direction processing speed is set to 250 mm / sec or less to substantially complete the generation of surface defects due to Cu embrittlement. Can be prevented.

[Brief description of drawings]

FIG. 1 is a graph showing a relationship between a width direction working temperature of steel and a surface flaw occurrence rate when the Cu content is changed.

FIG. 2 is a graph showing the relationship between the width direction working speed of steel and the surface flaw occurrence rate when the Cu content is changed.

Claims (3)

[Claims] 1. A raw material steel having a steel composition comprising C: 0.20% or less, Mn: 0.10 to 1.50%, S: 0.020% or less, Cu: 0.20 to 0.50%, and the balance being Fe and unavoidable impurities. After heating the slab, if processing in the slab width direction is performed at 4% or more of the slab width, the processing is performed at 1170 ° C or higher, and finish rolling is completed at Ar ₃ points or more. Good surface quality. For producing a copper-containing hot rolled steel sheet. 2. A raw steel slab having a steel composition comprising C: 0.20% or less, Mn: 0.10 to 1.50%, S: 0.020% or less, Cu: 0.20 to 0.50% with the balance being Fe and inevitable impurities. After heating the slab, if processing in the slab width direction is performed at 4% or more of the slab width, the processing is performed at a processing speed in the width direction (* 1) of 250 mm / sec or less, and finish rolling is completed with Ar ₃ points or more. A method for producing a copper-containing hot-rolled steel sheet having good surface properties. Machining speed (* 1) = (original slab width-post-machining slab width) / width direction machining time 3. The steel composition further comprises, by weight, at least one of Si: 1.2% or less, Ti: 0.15% or less, Nb: 0.05% or less, and / or P: 0.15% or less, Cr: 1.5. % Or less at least one or more kinds, The manufacturing method of the copper-containing hot-rolled steel sheet with favorable surface property of Claim 1 or 2.