JP2007331014A - Hot rolled steel strip of special steel and production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hot rolled steel strip of special steel free from a scale defect pattern and having less surface roughness. <P>SOLUTION: The hot rolled steel strip of special steel is characterized in that scale is removed from the strip and the strip has an average surface roughness Ra of 1.5 to 3.5 μm. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a special steel hot-rolled steel strip and a method for producing the same.

A hot-rolled steel strip is generally a steel slab (generally called a slab) heated to 1100-1300 ° C. in a heating furnace, rolled to a thickness of 30-50 mm with a rough rolling mill, and then with a finish rolling mill. It is manufactured by rolling to a thickness of 1 to 25 mm and then cooling.
Here, if it is a so-called special steel hot-rolled steel strip containing a lot of C, Si, etc., the scale formed on the surface layer of the steel strip during hot rolling (hereinafter, there is a place abbreviated as hot-rolling) It is difficult to partly peel off and may remain as a scale defect pattern after descaling by pickling.

Such scale defect patterns of special steel hot-rolled steel strips usually remain locally or strip-like on the steel strip surface, and are also called scale patterns, red scales, and the like.
This is because when a large amount of C, Si, or the like is included, when such a special steel slab is heated, a strong scale called firelite is likely to be formed on the surface of the slab, and it is hot-rolled after extraction from the heating furnace. This is because even if descaling is performed by injecting high-pressure water in the process, it cannot be completely removed.

These strong scales can be dissolved and removed by pickling after hot rolling, but there is a step between the part where the strong scale remained before pickling and the part where it did not. As a whole, it is considered that irregularities are formed and remain as a pattern (uneven color tone).
Special steel hot-rolled steel strips, for example, are often used for stamping members for automobiles, wheels for automobiles, etc., so beautiful surface properties are often required, and there are surface defects such as scale defect patterns as described above. Steel strips are of poor quality and cannot be delivered to customers.

As a means for solving the above problems, for example, in Patent Document 1, a crack in the rolling direction is given to the scale by width reduction using a sizing press, and the supply pressure is set within a certain time after rough rolling. A method for performing descaling by injecting high-pressure water of 30 MPa or more onto a steel material during hot rolling is disclosed.
Patent Document 2 discloses that the average surface roughness Ra is 0.3 to 1.5 μm for the purpose of imparting compressive residual stress to the steel sheet surface layer portion by projecting shot grains onto the steel sheet surface. The high-strength thin steel sheet and its manufacturing method are described.

Furthermore, in Patent Document 3, the range of residual stress is optimized by projecting shot grains onto the steel sheet surface, and the average surface roughness Ra is excellent in the fretting fatigue resistance of 2.0 to 3.0 μm. A hot-rolled steel sheet and a method for producing the same are described.
Japanese Patent Laid-Open No. 2004-74178 Japanese Patent Laying-Open No. 2005-248259 JP-A-8-283896

However, the method of Patent Document 1 has a problem that the temperature of the steel material is lowered by the injection of high-pressure water, and the load on the rolling mill during subsequent finish rolling is increased. Moreover, due to variations in hot rolling conditions, it has been difficult to completely prevent the occurrence of scale defect patterns over the entire length of the hot rolled steel strip.
The methods of Patent Document 2 and Patent Document 3 are for the purpose of adjusting the residual stress inside the steel sheet, and are different in purpose from the removal of the firelight red scale which is to be solved by the present invention.

On the other hand, when a special steel hot-rolled steel strip is used as a punching member for automobiles, the plate thickness is often 6 mm or more. In these applications, the scale generated during hot rolling is generally removed to produce a product.
This is because the presence of the scale accelerates the wear of the mold used for stamping and press forming, and also causes a decarburization reaction during annealing, so that the mechanical properties of the product are reduced. It is because it falls.

Usually, the scale generated in the surface layer of the steel strip by hot rolling is removed by the pickling line, but it is preferable that the scale is also removed by the pickling line in the case of the special steel hot-rolled steel strip.
However, hot-rolled steel strips with a thickness of 6 mm or more, which are in great demand as automotive punching members as described above, often cannot pass pickling lines. In the pickling line, there are many small rolls for transport such as looper rolls, so steel strips with a large thickness are too stiff to bend along rolls with a small diameter and can be passed through. This is because there is an upper limit to the plate thickness.

If the scale cannot be removed completely by pickling, there is a method of removing the scale with a grinding brush.However, there is a problem of significant cost increase. Development of a method for efficiently removing the scale of the steel strip is desired.
As another problem, special steel hot-rolled steel strips are often used for stamping parts for automobiles, wheels for automobiles, etc., and it is mentioned above that beautiful surface properties are often required. However, in this case, it is not preferable that the surface roughness is excessively rough.

  That is, special steel hot-rolled steel strips are often plated or polished to make the final product. If the surface roughness is excessively rough, the plating will be uneven in applications where the surface is plated. In addition, in the case of use for polishing the surface, there is a problem that the number of polishing passes increases and the productivity decreases. For this reason, the surface roughness after scale removal is preferably as small as possible, and development of a method capable of efficiently obtaining such a steel strip is desired.

  This invention solves the above subjects, and provides the special steel hot-rolled steel strip excellent in surface quality, and its manufacturing method.

As a result of intensive studies, the inventors have been able to eliminate scale defect patterns by shot blasting using fine shot grains instead of removing the scale by conventional pickling, and the surface of the pickled steel strip It was found that a steel strip close to roughness could be obtained, and the present invention was completed.
That is, the present invention is as follows.

1. A special steel hot-rolled steel strip having an average surface roughness Ra of 1.5 μm to 3.5 μm with scale removed.
2. A special steel hot-rolled steel strip having an average surface roughness Ra of 1.5 μm to 3.5 μm and a plate thickness of 6 mm to 18 mm with scale removed.
3. 1. produced by projecting shot grains having an average particle size of 0.15 mm or more and 0.40 mm or less. Or 2. The special steel hot-rolled steel strip described in 1.

4). A method of producing a special steel hot-rolled steel strip from which scale has been removed, wherein shot grains having an average particle size of 0.15 mm to 0.40 mm are projected.
5). A method for producing a special steel hot-rolled steel strip characterized by projecting shot grains having an average particle size of 0.15 mm or more and 0.40 mm or less onto a steel strip having a thickness of 6 mm or more and 18 mm or less.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to provide a hot-rolled steel strip with a small surface roughness without a scale defect pattern.

  The experiment that led to the completion of the present invention will be described below. In the experiment, a sample having a width of 150 mm and a length of 300 mm cut from a special steel hot-rolled steel strip having a thickness of 4 mm was used. The chemical composition of the hot-rolled steel strip used in the experiment is as shown in Table 1.

Shot blasting was performed under conditions of a projection speed of 70 m / sec and a projection density of 100 kg / m ² . The projection speed was calculated from the wheel diameter and the rotation speed in accordance with JIS B 6614-1989. The projection density is the weight of shot grains projected on the steel strip per unit area. The projection distance is a vertical distance between the centrifugal projection device and the sample, and is 350 mm. The shot grains include steel grids (square bars) with an average particle diameter of 0.60 mm, and steel shots (spherical) with an average particle diameter of 0.40 mm, 0.30 mm, 0.25 mm, 0.15 mm, and 0.10 mm. A total of 6 types were used. The average particle size is determined according to JIS Z 8801 using a standard sieve with a nominal opening of 710, 500, 425, 355, 300, 250, 212, 180, 150, 100, 75, 53 μm. And determined by weight average. For comparison, pickling with hydrochloric acid was performed. The pickling was performed by immersing in hydrochloric acid having a temperature of 60 ° C. and a concentration of 15 mass% for 60 seconds.

Scale defect patterns were evaluated for the steel strips obtained under each condition. The scale defect pattern was judged visually, and the case where the scale defect pattern was observed was evaluated as x, and the case where it was not observed was evaluated as ◯. Further, the weight of the sample before and after pickling or shot blasting was measured and the difference was taken to determine the descaling amount.
Further, in accordance with JIS B 0601-2001 and JIS B 0651-2001, the stylus type surface roughness measuring instrument is moved to the sample surface in the length direction, and the reference length 1r (λc) for the roughness curve Ra (arithmetic mean roughness) was measured with 0.8 mm for the waviness curve, 8 mm for the reference length 1 w (λf) for the waviness curve, and 40 mm for the evaluation length 1p (λn) for the cross-sectional curve.

Moreover, the surface polishing property of the obtained hot-rolled steel strip was evaluated. The evaluation is that if the surface of a hot-rolled steel strip is polished with a # 400 grinding brush, how many passes of polishing are required in order to completely remove the crater-like shot marks formed during shot blasting and to obtain a flat surface. I went to see if it was necessary.
Table 2 shows the results obtained.

No. which performed normal pickling. 1 or No. 1 in which shot blasting was performed using shot grains having an average grain size of 0.10 mm. In No. 7, a scale defect pattern was observed, but when shot blasting was performed using shot grains having an average particle size of 0.15 to 0.60 mm, no scale defect pattern was observed.
The reason why the scale defect pattern is no longer observed will be described below.

It is considered that the scale defect pattern is generated by a mechanism schematically shown in FIG.
That is, in the special steel containing a large amount of C and Si, a scale that is partially eroded is formed as shown in FIG. In the process of being extracted from the heating furnace and hot-rolled, when descaling with high-pressure water, as shown in FIG. 1 (2), the portion of the scale with deep roots is difficult to remove and remains. Thereafter, as shown in FIG. 1 (3), a scale is generated and stretched thinly from finish rolling to winding.

By pickling the hot-rolled steel strip after finish rolling, the remaining scale with deep roots can be removed, but as shown in FIG. Will see uneven color tone, scale defect pattern, and red scale pattern.
Here, in the present invention, as shown in FIG. 1 (4b), by projecting shot grains having an average particle diameter of 0.15 mm or more, the scale can be completely removed, and the steel grains after the shot grains are projected The present inventors have found that the entire surface can be made uneven and the occurrence of uneven color tone can be prevented.

In any case, the descaling amount is 24 to 25 g / m ² . Separately, since the scale thickness of the measured hot-rolled steel strip was 4 μm, if the scale average density is considered to be 6000 kg / m ³ , the product of both, 24 g / m ² corresponds to the scale weight. In the case of the condition, it can be said that the scale is completely removed.
In addition, when shot grains having a mean particle diameter larger than 0.40 mm, which is a comparative example, are used, the surface roughness is remarkably increased as compared with the conventional example, whereas the present invention example,. When shot grains of 15 mm or more and 0.40 mm or less, particularly shot grains of 0.30 mm or less are used, the shot grains are not projected, and only the pickling is performed. I understand.

Furthermore, no. In No. 5, as in the case of pickling, it can be confirmed that the shot marks can be sufficiently removed by one-pass polishing, and it can be seen that the film has good polishing properties.
As described above, according to the present invention, a steel plate having no scale defect pattern and a small surface roughness can be obtained.

In the present invention, the reason why the average surface roughness Ra of the steel strip is limited to 1.5 μm or more and 3.5 μm or less will be described.
When the average surface roughness Ra is less than 1.5 μm, the scale defect pattern does not become inconspicuous because the unevenness is not less than or equal to the step forming the color tone unevenness in FIG. 1 (4a). On the other hand, as described above, the average surface roughness Ra is preferably as small as possible from the viewpoints of surface polishing properties, plating properties, etc., and it is not preferable to exceed 3.5 μm. Therefore, in the present invention, the average surface roughness Ra of the steel sheet is specified to be 1.5 μm or more and 3.5 μm or less. Preferably, they are 1.5 micrometers or more and 2.0 micrometers or less.

Next, the reason why the average grain size of the shot grains is defined in the present invention will be described below.
The reason why the average grain size of the shot grains is specified to be 0.15 mm or more is that if the average grain diameter of the shot grains is smaller than this, the weight of the shot grains will be reduced, so that the particles when projected and collided with the steel strip This is because the collision energy per unit is reduced and the scale removal capability is reduced. In this case, productivity is remarkably lowered from the viewpoint of substitution for pickling. In addition, shot particles with an average particle size of less than 0.15 mm are difficult to separate from dust such as scale powder and shot particle wear powder generated during shot blasting, and are therefore virtually impossible to collect and reuse. It is not suitable for use because it becomes possible and disadvantageous in terms of cost.

  The reason why the average grain size of the shot grains is specified to be 0.40 mm or less is that if the average grain diameter of the shot grains is larger than this, the average surface roughness Ra of the steel strip after removing the scale becomes larger than 3.5 μm. It is. In order to obtain a preferable average surface roughness Ra of 1.5 μm or more and 2.0 μm or less, it is preferable to use shot grains having an average particle diameter of 0.30 mm or less. As the shot grains, it is preferable to use steel spherical ones (steel shots).

  In the present invention, the special steel hot-rolled steel strip refers to C of 0.1 mass% to 1.0 mass%, Si of 0.1 mass% to less than 1.2 mass%, and Mn of 0.1 mass% to 2.0 mass%. Contains below, with the balance being Fe and other inevitable impurities. Inevitable impurities include P: 0.050 mass% or less, S: 0.01 mass% or less, N: 0.01 mass% or less, O: 0.040 mass% or less, Al: 0.0020 mass% or less.

  Also, depending on the application and purpose, Ni is 0.1 mass% to 3.0 mass%, Cr is 0.1 mass% to 2.0 mass%, Mo is 0.1 mass% to 0.3 mass%, Al is 0 More than 0020 mass% and 0.1 mass% or less, Ti is 0.005 mass% or more and 0.1 mass% or less, Nb is 0.005 mass% or more and 0.1 mass% or less, V is 0.005 mass% or more and 0.1 mass% or less, Zr Is 0.005 mass% to 0.1 mass%, Cu is 0.05 mass% to 1.0 mass%, W is 0.05 mass% to 1.0 mass%, and Ca is 0.0005 mass% to 0.0050 mass%. B is 0.0003 mass% or more and 0.0030 mass% or less, and Mg is 0.0005 mas. % Or more 0.0050 mass% or less, the REM than 0.0005 mass% 0.0050 mass% or less, it may be allowed to contain.

  As a preferred method for producing the steel strip of the present invention, the components are adjusted by melting a blast furnace slag in a converter, degassing with VOD, etc., and then making a steel ingot by a method such as continuous casting, 1000 to 1350 ° C. The method of projecting shot grains having an average grain size of 0.15 mm or more and 0.40 mm or less, etc., is exemplified as a steel strip that has been heated to a desired thickness after being hot-rolled. It is not necessarily limited to the method described above.

The reason why the present invention is suitable for use in a special steel hot-rolled steel strip having a thickness of 6 mm to 18 mm will be described below.
When the plate thickness exceeds 6 mm, it is impossible to pass through a general pickling line. Therefore, the method of removing scale by shot blasting as in the present invention is particularly useful. Since no defect pattern is produced and the subsequent polishing load can be reduced, it can be said to be an optimum method. Moreover, although it is not necessary to prescribe | regulate especially the upper limit of preferable plate | board thickness using this invention, since the plate | board thickness does not exceed 18mm from the dimension of the product which exists as a special steel hot-rolled steel strip, fortune-telling, It is sufficient that the present invention is applied to a plate thickness of 18 mm or less.

  The above explanation is not intended to prevent the present invention from being applied to steel strips other than special steel or steel strips with a thickness of less than 6 mm, and there is no problem in applying the present invention to them.

  A high-carbon steel hot-rolled steel strip having a thickness of 10 mm and a width of 1200 mm was used with the chemical components shown in Table 3.

  In the shot blast line shown in FIG. 2, the shot blasting treatment was performed while passing through the hot-rolled steel strip at a penetration rate of 10 m / min. The shot blast line is No. 1 shot blasting device 11 and No. The two-shot blasting device 21, the dispensing machine 31, and the winder 41 are configured with No. 1 on the upstream side from the passing direction of the steel strip. 1 shot blasting machine, downstream side is No. A two-shot blasting device is used. Each shot blasting device has one centrifugal projection device 12, 13, 14, 15 (22, 23, 24, 22) at each position on the OP side upper surface, DR side upper surface, OP side lower surface, DR side lower surface. 25), and each shot blasting device has a recovery device 16 (26) and a circulation device 17 (27). In this example, a steel shot having an average particle size of 0.25 mm was used with a projection weight of each centrifugal projection device of 1000 kg / min, a projection speed of 70 m / sec, a projection distance of 1200 mm.

  About the obtained hot-rolled steel strip, the presence of the hot-rolled scale residue and the scale defect pattern was evaluated over the entire length of the front and back surfaces in the longitudinal direction. As a result, the remaining hot rolled scale and the scale defect pattern were not observed at all. Further, samples were collected from the central part in the longitudinal direction and the central part in the width direction, and the surface roughness Ra was measured by the same method as described above. As a result, it was confirmed that the surface roughness Ra was as small as 1.6 μm.

  Note that the shot blast line shown in FIG. 2 shows a general line configuration, and the object to which the present invention can be applied is not limited to this. A mechanical scale breaker for introduction may be provided.

It is a figure which shows the generation | occurrence | production cause of a scale defect pattern, and the effect of this invention compared with a prior art example. It is a figure which shows the shot blast line of a hot-rolled steel strip.

Explanation of symbols

11 No. 1 shot blasting device 21 2-shot blasting device 31 Dispensing machine 41 Winding machine 12, 13, 14, 15, 22, 23, 24, 25 Centrifugal projector 16, 26 Recovery device 17, 27 Circulating device

Claims (5)

  A special steel hot-rolled steel strip having an average surface roughness Ra of 1.5 μm to 3.5 μm with scale removed.   A special steel hot-rolled steel strip having an average surface roughness Ra of 1.5 μm to 3.5 μm and a plate thickness of 6 mm to 18 mm with scale removed.   The special steel hot-rolled steel strip according to claim 1 or 2, produced by projecting shot grains having an average particle size of 0.15 mm or more and 0.40 mm or less.   A method of producing a special steel hot-rolled steel strip from which scale has been removed, wherein shot grains having an average particle size of 0.15 mm to 0.40 mm are projected.   A method for producing a special steel hot-rolled steel strip characterized by projecting shot grains having an average particle size of 0.15 mm or more and 0.40 mm or less onto a steel strip having a thickness of 6 mm or more and 18 mm or less.

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