JPS6117301A - Prevention of surface cracking of billet during hot rolling - Google Patents

Abstract

PURPOSE:To prevent the surface cracking of a billet during hot rolling without an increase in energy cost in direct rolling, etc. to be executed under prescribed conditions by limiting the chemical components S, N and Ti of the billet to prescribed ratios. CONSTITUTION:The chemical components of the billet are adjusted to, by wt%, to 0.003-0.02% S, 0.0015-0.009% N and 0.003-0.04% Ti and >=1.5 Ti/N. Such billet is subjected to direct rolling or hot charge rolling at 600-900 deg.C surface temp. and >=1.3 total reduction ratio.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention provides a method for immediately hot rolling a steel billet after continuous casting (hereinafter referred to as direct rolling), or for reducing the surface temperature of a steel billet after continuous casting. The present invention relates to a method for preventing surface cracking of a steel billet during hot rolling in a rolling method in which the billet is charged into a heating furnace at a temperature of 500° C. or higher and hot rolled after reheating (hereinafter referred to as hot charge rolling).

(Prior Art) Slabs cast by a continuous VT casting machine often need to be rolled before being rolled in a lower factory in order to improve quality or due to dimensional constraints at the lower factory. Conventionally, in this rolling method, the surface temperature of the continuously cast steel billet was controlled to
Rolling was performed after the temperature was lowered to around room temperature and reheated. That is, molten metal produced in a converter or electric furnace was hot rolled through the following steps.

[Continuous casting] - [Cooling (room temperature)] - [Reheating] - [Hot rolling] In this method, by cooling and reheating before rolling, the austenite grain size becomes finer and at the same time hot workability is improved. S, P, 0 (oxygen), N, AC! Since elements such as sulfides, phosphides, oxides, and nitrides are fixed in the grains, deterioration of hot workability is prevented, and surface cracking of the steel billet during rolling can be avoided.

However, this method requires a large amount of energy for reheating. Therefore, with the aim of saving energy by reducing the thermal energy consumption rate, we developed a hot charge rolling method in which hot rolling is performed by reheating the steel piece at a surface temperature of 500°C or higher without cooling it to room temperature in the cooling process. A direct rolling method has been proposed in recent years that also aims to reduce the amount of steel. However, in the direct rolling method or hot charge rolling method, the first
(a) As schematically shown in the figure, elements such as P, S, O, N, and AQ are converted into phosphides during the melting-solidification-cooling process.
Fine precipitates as sulfides, oxides, and nitrides occur almost continuously along the austenite grain boundaries. Therefore, when tensile stress due to hot working is applied, these fine precipitates become the starting point of cracks in the steel billet. Surface cracking occurs.

In order to prevent this surface cracking of the steel slab, elements such as P, S, 0, and N can be removed in advance, but this means an increase in the number of steps and is not necessarily an industrially advantageous solution. do not have.

As another countermeasure against surface cracks in direct rolling or real charge rolling, the fine precipitates are approximately 1150-9
Based on the knowledge that precipitation occurs in the temperature range of 00°C, in order to prevent this precipitation, the temperature range of 1300 to 1150
．． A method has been proposed in which rolling is performed two or more times in a temperature range of 'c' and then further rolling is carried out at a temperature lower than this temperature range (Japanese Patent Publication No. 5B-52441).

(Problems to be Solved by the Invention) However, the surface temperature of the steel billet at the start of hot rolling in direct rolling is often lower than 900°C, and the In order to achieve a surface temperature of 1150°C or higher at the start of rolling, it is necessary to charge the slab into a P1 furnace or a heat retention furnace before direct rolling, which is disadvantageous in terms of cost and equipment. . In the case of bot charge rolling, too, a large amount of energy is required to reheat to such a high temperature, and the original advantage of energy saving is lost.

Additionally, surface cracking during rolling can generally be prevented by significantly lowering the rolling reduction ratio or reduction ratio, but this is not a practical method when the required amount of processing is large.

Therefore, an object of the present invention is to provide a method for preventing surface cracking of a steel billet during direct rolling or hot charge rolling with a large processing amount without increasing energy costs.

(Means for Solving the Problems) The present inventors have determined that when performing direct rolling or bot charge rolling, the weight ratio of Ti is at least 1.5 times the N content, that is, Ti/N≧1. When using a steel type with an additive content of .5, Ti becomes a precipitation nucleus and the precipitates coarsely precipitate and become fixed within the grains, resulting in intergranular cracking as shown in Figure 1(a). The formation of fine precipitates that cause this is prevented, and as a result, the steel billet surface temperature during rolling is reduced to 900-600℃.
It has been found that even if the steel is subjected to large processing at a total reduction ratio of 1.3 or more at a relatively low temperature, cracking on the surface of the steel billet during hot rolling can be prevented, leading to improvement in the quality of rolling stock and yield.

Here, the present invention is directed to direct rolling or real charge rolling performed under the conditions of a surface temperature of 600 to 900°C and a total reduction ratio of 1.3 or more. /N≧1.5, which is a method for preventing surface cracking of a steel billet during hot rolling.

In the present invention, the "total reduction ratio" means the ratio of the thickness of a steel billet (cast slab) before rolling to the thickness of a steel billet after completion of rolling.

When converted to a rolling reduction ratio (the percentage of the difference in the thickness of a steel slab before and after rolling to the thickness before rolling), a rolling ratio of 1.3 is approximately a rolling ratio of 23.
Corresponds to %.

- Composition The steel type to which the present invention is applied is not particularly limited, but the method of the present invention is particularly applicable to steel plates for automobiles, general construction steel plates, shipbuilding steel plates, and machine structural steel plates such as aluminum killed steel, aluminum semi-killed steel, and aluminum silicon killed steel. Si used for steel plates etc.
It is effective for carbon steel containing Mn as a main component, or alloy steel for line pipes or oil country tubular goods containing Nb and ■. Specifically, for example, SS old, 5M41.5M5
0 and 5M53, and APIX60 grade steel.

In the present invention, the chemical composition of the steel is Ti/N in weight ratio.
It may be the same as the conventional method except that Ti is added so that ≧1.5. Figure 1 of the attached drawings is a schematic diagram showing the precipitation form of precipitates due to elements such as P, S, ○, N, and AQ that occur during the cooling process of '/g hot water. As explained above, the case of the conventional steel without the addition of Ti is shown, and FIG. 1(b) shows the case of the steel to which Ti is added according to the present invention so that Ti/N≧1.5. As shown in Figure 1(b), in steel to which Ti is added so that Ti/N≧1.5, the precipitates formed during cooling easily grow with Ti as the nucleus, so the precipitates become austenite. It remains within the grain, and is precipitated coarsely and discontinuously within the grain and becomes fixed. In addition, surface cracking of the steel billet during hot rolling, which is caused by sand and fine precipitates along austenite grain boundaries as shown in FIG. 1(a), is avoided. In other words, unlike the method described in Japanese Patent Publication No. 58-52441 which avoids precipitation on the high temperature side, the method of the present invention
, S, 0, N, AQ, and other elements are actively precipitated in a coarse state within the grains, thereby preventing surface cracks during hot rolling due to intergranular cracks. In order to fully exhibit such an effect of preventing surface cracking, it is necessary to add Ti at least 1.5 times as much as N by weight.

In a preferred embodiment of the present invention, the steel composition is S: 0.003 to 0.020%, N: 0
．． 0015-0.0090%, Ti: 0.003
0.040%. This is due to the following reason.

S: If it is less than 0.003%, there will be little advantage in manufacturing. On the other hand, if it exceeds 0.020%, the mechanical properties of the #7vi product will deteriorate. The reason for this is that the amount of sulfide precipitated increases.

N: If it is less than 0.0015%, like S, there will be little advantage in manufacturing, and stable manufacturing will not be possible. Also, 0.
If it exceeds 0.090%, the mechanical properties of the steel will deteriorate. The reason for this is that nitride precipitation increases.

Ti: If less than 0.003%, P, S, OlN,
The function of coarsely precipitating the precipitates of elements such as A(2) using Ti as precipitation nuclei is lost. On the other hand, if the amount exceeds 0.040%, the properties of the steel will deteriorate due to the addition of Ti.

Rolling conditions In the present invention, the rolling temperature (surface temperature of the steel billet) in direct rolling or hot charge rolling is set to 600 to 900.
The temperature range is ℃. If the temperature is less than 600°C, hot rolling will generally become difficult. The reason for setting the upper limit at 900°C is that in direct rolling, unless a heating furnace or heat retention furnace is used, the surface temperature during rolling often drops below 900°C, and even in true charge rolling, the surface temperature can exceed 900°C. This is because it is disadvantageous in terms of energy costs. Also, 900℃
At higher temperatures, the precipitates may not yet be sufficiently coarsened and may not be effective in preventing surface cracking.

Note that during hot rolling, the surface temperature of the steel billet generally changes in the direction of decreasing, so the above temperature range is 90°C at the start of rolling.
It means that the rolling finishing temperature is 600°C or higher.

As for the reduction conditions, the total reduction ratio was set to 1.3 or more because, as can be seen from the graph in Figure 2 showing the results of the examples, surface cracking does not occur in machining where the total reduction ratio is less than 1.2. That's because there isn't.

The amount of reduction per pass may be within a normal range, but if the amount of reduction is excessive, surface cracks are likely to occur. From this point of view, it is preferable that the rolling reduction amount per pass be in the range of 5 to 40 mm.

An example of the rolling pass schedule during hot rolling is shown below: 270 + U steel billet - 4 times of 20 dragons/pass reduction - 4 times of 115N/pass reduction → 10 dragons/pass reduction 2 times - 51 strokes/pass of 2 steel slabs of 100 pieces of the same thickness, total reduction ratio = 2.7 (total reduction ratio = 63%).

Next, the present invention will be specifically explained with reference to Examples.

A molten steel having the chemical composition shown in Table 1, in which the amount of Ti added was varied within the range where the weight ratio of Ti/N was θ to 10, was continuously cast to 270 mm (thickness) x 1350 mm (width). Direct rolling was started at a surface temperature of 750°C, and the total reduction ratio was 1.2.1 according to the rolling pass schedule shown in Table 2.
．． 3 and 2.7 rollings were performed. The occurrence of surface cracks on each of the obtained steel slabs was visually inspected after scarfing, and the surface crack occurrence rate (number of steel slabs with surface cracks/number of rolled steel slabs) was determined. The results are shown graphically in FIG.

As can be seen from FIG. 2, surface cracking does not occur in direct rolling with a total reduction ratio of 1.2 even without the addition of T. However, when the total reduction ratio becomes 1.3 or more, the incidence of surface cracking increases as the amount of processing increases in the case of no Ti addition.

Although surface cracking is suppressed by adding T1, the effect is not sufficient when the Ti/N ratio is less than 1.5. On the other hand, when the Ti/N ratio is 1.5 or more, surface cracking is completely prevented even at a high total reduction ratio of 2.7. However, if the total reduction ratio is greater than 4.0, even if Ti is added, surface splitting will occur at about 1, which is economically ineffective, so the total reduction ratio should be 4.0 or less. It is preferable to do so.

(Effects of the Invention) As is clear from the above description and examples, according to the present invention, direct rolling and body 7 shipping are possible.

It is possible to prevent surface cracking of steel slabs during large processing, which has been a problem in the rolling method, without increasing energy costs, and has extremely high practical value.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing the precipitation form of precipitates due to P, S, O, N, AQ, etc. that occur during cooling, and +a+ is T
In the case of no addition of i, (bl is Ti/N≧1.
Fig. 2 is a graph showing changes in surface crack incidence due to hot rolling at various total reduction ratios with respect to the Ti/N weight ratio in steel. Applicant Sumitomo Metal Industries Co., Ltd. Agent Patent Attorney Akira Hirose −(tth” 'IL
) Figure 2 Tesho'ε Neiff Official Book July 72, 1985 1, Indication of the incident 1982 Patent H No. 135228 2, Name of the invention Method for preventing surface cracking of steel billet during hot rolling 3, Relationship with the case of the person acting as the city attorney Patent applicant address: 5-15 Kitahama, Higashi-ku, Osaka Name (211) Sumitomo Metal Industries, Ltd. 4, Representative Column 6 of the detailed explanation of the invention in the official specification, Contents of the amendment Add the following statement in a paragraph on the last line of page 4 of the specification. ``In addition, here, [surface crack 1] of the steel billet due to hot working
refers to a hair crack with a length of ~5 mm that occurs in an area of 10 mm 1 yen directly below the surface of a steel piece,
In rare cases, it may be seen on the surface of a steel piece. j or more

Claims (2)

[Claims] (1) In direct rolling or hot charge rolling performed at a surface temperature of 600 to 900°C and a total reduction ratio of 1.3 or more, the chemical composition of the steel billet is determined by the weight ratio of titanium and nitrogen: Ti/N≧1. .5 A method for preventing surface cracking of a steel billet during hot rolling. (2) The chemical components of the steel slab are as follows in weight ratio: S: 0.003-0.020%, N: 0.0015-0.0090%, Ti: 0.003-0.040%. The method of claim 1, further characterized.