JPS6059967B2 - Method for preventing surface defects in low Mn-low Al slabs - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for preventing the occurrence of surface defects in low Mn-low slabs.

Low Mn-low aluminum killed steel has recently been widely used as a general-purpose continuous casting steel for thin plates because it has excellent cold rollability and is inexpensive in terms of cost.

However, if this slab is heated to a high temperature during rolling, sludge-like surface defects may occur on the surface of the hot-rolled sheet, which poses a problem in terms of surface quality. Regarding this point, the cause has not been clear in the past, and therefore countermeasures have generally only been taken in the direction of lowering the heating temperature.In order to prevent the occurrence of surface defects, the heating temperature has been quantitatively controlled by considering the components. Nothing was done. This invention was made in view of the above-mentioned circumstances, and its purpose is to prevent the occurrence of surface defects in rolled steel sheets by heating the slab within a quantitative heating temperature range that takes into account the ingredients. The present invention aims to provide a method for preventing the occurrence of surface defects in low Mn-low Al slabs.

The characteristics of the low Mn-low N slab heating method of this invention are as follows:
n: 0.03-0.25%, 501A10.005-0
．． When heating a slab having 0.020%, the maximum heating temperature in the heating furnace T° C. is within the range of 1000 T<, 562 [Mn] + 1165, where Mn content % is [Mn]. .

An embodiment of the method of this invention will be described below.

First, the materials of the low Mn-low Al slab that is the target of the method of this invention are: C: 0.06% or less, Si: 0.03% or less, SolA1: 0.005 to 0.020%, N: 0. 0
025% or less, except for Mn, the remainder consists of iron and unavoidable impurities.
It is a so-called low Mn-low N type killed steel with Mn/5>7). c decreases the value to increase the strength of the steel,
Since it also significantly reduces cold workability, the upper limit is set to 0.0.
It was set at 6%. (・Si is a necessary component for steelmaking, but if it is included in a large amount, it will reduce cold workability, so the upper limit should be set to 0.
．． 03%. The upper limit of SOlAl was set at 0.020%, but this means that if the content is higher than 1, it becomes expensive.
To prevent the cost aspect of eliminating the characteristics of inexpensive low A1 steel, and to prevent the steel from hardening due to solid solution hardening of 2A1 and the occurrence of abnormally coarse grains in hot rolled sheets that tend to occur when the heating temperature is lowered. This is due to material issues such as.

On the other hand, the lower limit was set at 0.005% in order to prevent deterioration of the cleanliness, surface texture, and strain aging properties of the steel. N is the upper limit of 0.00
The reason for this is 25% because of cold rolling properties, and if it exceeds this value, the wear of the work rolls during cold rolling becomes severe.

Regarding Mn, lowering Mn improves mechanical properties and improves mechanical properties, as shown in (1) softening and improvement of ductility in hot-rolled sheets, and (2) improvement in 〒 value (including reduction in in-plane anisotropy) in cold-rolled sheets. Since it has the same effect as improving cold rolling properties,
The upper limit was set at 0.25%.

The lower limit was set at 0.03% because it is industrially extremely difficult to lower the content below this. Also S
From the viewpoint of preventing hot embrittlement, the range was set to satisfy Mn/S≧7.

Next, regarding the heating temperature, when a low Mn-low A1 slab is heated to a high temperature, sagging surface defects occur on the rolled steel sheet, which is thought to be due to selective oxidation of the surface layer of the slab in the heating furnace. Therefore, the present inventors conducted a high temperature oxidation experiment simulating heating furnace conditions to investigate the oxidation status of the surface layer of the slab.

The results are shown in FIG. This means that for Mn: 0.13% and 0.3% slabs, the heating temperature is 1150-13%.
When heated at 4 levels in the range of 00°C for 3 minutes each, a certain unit length of the surface layer of the slab (here, 507T
The number of selectively oxidized portions generated on the line r1n is measured using a cross-sectional microscope. This result shows that when the Mn content is low and the heating temperature is high, the number of selectively oxidized parts is large, and many of them are deep. It became clear that there is a relationship between heating temperature and heating temperature. Therefore, the relationship between the surface quality of the rolled steel sheet, the Mn content of the slab, and the maximum heating temperature was investigated, and the results shown in FIG. 2 were obtained.

From this result, it was found that, as in the case of the oxidation experiment described above, the surface defects of the rolled steel sheet tend to occur more easily as the content (1) is lower and the maximum heating temperature is higher. In addition, the lower SOlAl was, the more likely it was to occur. However, as a result of the investigation, there was no particular correlation between the quality of the surface texture of the steel sheet and Mn/S, so it is difficult to think that hot embrittlement is caused by the generally considered low Mn/S. In connection with the oxidation experiment described above, it is thought that surface defects during rolling are caused by selective oxidation of the surface layer of the slab. As a result of organizing Figure 2, the maximum allowable heating temperature depends on the components, especially the Mn content, so low Mn and low A
In order to obtain a steel plate with excellent surface quality, the maximum slab heating temperature T°C should be maintained within the range corresponding to the Mn content, where the Mn content% is [Mn]. Something good has become clear.

Note that the lower limit of temperature was set at 1000'C from the viewpoint of melting behavior of AlN and rolling workability. Next, low Mn samples A1 to 0.16% and high Mn samples Xl and X2 of 0.3% were hot-rolled to a thickness of 2.6 mm and then pickled to improve the surface texture. Table 1 shows the results of the quality inspection.

Except for sample A1, which was heated at a high temperature, all the steel plates heated by the method of the present invention had good surface properties.

The tensile properties of the cold-rolled sheets of these samples also showed good results, as shown in Table 2.

This is a 70%
Cold pressed plate thickness 0.8w! A cold-rolled plate with a temperature of n is annealed at 680°C.

The method for preventing the occurrence of surface defects in a low Mn-low N slab according to the present invention is as described above. can be prevented.

[Brief explanation of the drawing]

Figure 1 is an explanatory diagram showing the relationship between the Mn content and heating temperature of the slab and the number of selectively oxidized parts generated on the surface layer of the slab. Figure 2 is an explanatory diagram showing the relationship between the Mn content and heating temperature of the slab and the number of selectively oxidized parts generated on the surface layer of the slab. Figure 2 is the relationship between the Mn content and heating temperature of the slab. It is an explanatory view showing the relationship with the maximum heating temperature in the furnace.

Claims (1)

[Claims] 1 Mn: 0.03 to 0.25%, SolAl 0.00
When heating a slab having 5 to 0.020%, the maximum heating temperature in the heating furnace T° C. is within the range of 1000≦T≦562 [Mn] + 1165 when the Mn content % is [Mn]. A method for preventing the occurrence of surface defects in a low Mn-low Al slab.