JP3496546B2 - Manufacturing method of hot rolled steel sheet with excellent scale adhesion - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a hot-rolled steel sheet having excellent scale adhesion, and particularly to a scale (black scale).
Low cost, high productivity and scale, which are used to transport automotive parts, building materials or hot-rolled steel coils with scales that are used as-is with hot-rolled steel sheets and are used for cold-rolled materials after the pickling process. The present invention relates to a method for producing a hot-rolled steel sheet having excellent adhesion. [0002] Conventionally, hot-rolled steel sheets with scales have been used for automobile parts, building materials and the like. Therefore, hot-rolled steel sheets with scales are difficult to peel off the scale during bending, hard to generate flaws during indentation, difficult to peel off the scale during coil transfer, and good appearance. From
It is required that the scale is uniform, thin and excellent in adhesion. As a technique for producing such a hot-rolled steel sheet having excellent scale adhesion, a technique for reducing the thickness of a scale is disclosed in the following publication. (1) JP-A-4-228204 and JP-A-4-228204
In the technique disclosed in the above-mentioned publication, the hot-rolled steel sheet is sealed with a roller or cooled in an inert gas or reducing gas atmosphere from the finishing mill to the winding machine. This suppresses the generation of scale, thereby reducing the thickness of the scale. Hereinafter, this method will be referred to as Conventional Technique 1
That. (2) Japanese Patent Application Laid-Open No. 61-111701 The temperature of the steel sheet on the entrance side of the finishing mill is maintained at the Ar3 point or less, cooling is performed by cooling equipment between stands of the finishing mill, and the temperature of the steel sheet on the exit side of the finishing mill Is maintained at 500 ° C. or lower, thereby suppressing the formation of scale, thereby reducing the thickness of the scale. Hereinafter, this method is referred to as Conventional Technique 2. [0004] However, the above-mentioned prior arts 1 and 2 have the following problems. [0005] In the prior art 1, safety issues are more severely required due to the necessity of enormous capital investment and the use of gas. In the prior art 2, since the deformation resistance of the steel sheet is increased, productivity is reduced due to impairment of the sheet passing property.
Standards, types and dimensions of the target materials are limited. Further, since the hot-rolled steel sheet itself is hardened, the efficiency of the operation in the post-process is significantly reduced. Accordingly, an object of the present invention is to solve the above-mentioned problems and to provide a method for producing a hot-rolled steel sheet which is low in cost, has high productivity, and has excellent scale adhesion. Means for Solving the Problems The invention according to claim 1 is:
The slab is roughly rolled, and then the following equation (1): 400-300 × logt ≦ V ≦ 1140-890 × logt (1) where, in the above equation (1), t: the thickness of the finished steel sheet (Mm), V: finish rolling under the condition expressed by the finish rolling base speed (mpm) (the speed of the steel plate when the tip of the steel sheet bites into the final stand of the finishing mill), and winding. It has features. DETAILED DESCRIPTION OF THE INVENTION The method for producing a hot-rolled steel sheet having excellent scale adhesion according to the present invention will be described in more detail. The inventors of the present application have examined in detail the effect of rolling conditions on the adhesion of scales formed on steel sheet surfaces having different thicknesses. As a result, as shown in FIG.
It has been found that a hot-rolled steel sheet having excellent scale adhesion can be obtained in a region between the two curves. The scale adhesion test was performed as follows. C: 0.0020 to 0.80%, Si:
0.01 to 0.5%, P: 0.01 to 0.10%,
S: 0.001 to 0.015%, Al: 0.01 to 0.08%, N: 0.0015 to 0.0035%, N
b: 0 to 0.04, Ti: 0 to 0.08% (all in mass%), the balance: A slab composed of iron and unavoidable impurities was prepared, and the slab was roughly rolled to obtain various finishes. Rolled sheet thickness: 2 to 10 mm, finish rolling base speed (steel plate speed when the leading end of the steel sheet bites into the final stand of the finishing mill): hot rolling is completed at 100 to 600 mpm, and the wound coil is cooled. Thereafter, the scale adhesion was evaluated by a tape peeling test for the second winding portion corresponding to the central portion in the plate width direction. Here, the tape peeling test is a method in which an adhesive tape is pressed against the entire width of a steel sheet at a constant pressure, the tape is peeled off, and the scale adhesion is evaluated based on the amount of scale adhered to the tape surface. . As is apparent from FIG. 1, when the finish rolling base speed is excessively reduced with the curve (V = 400-300 × logt) as a boundary, the steel sheet temperature decreases in a thin steel sheet. Significantly, magnetite (Fe ₃ O ₄ ) having high adhesion is not sufficiently obtained during cooling after winding, and a part of wustite (FeO) remains to lower the adhesion. On the other hand, a thick steel plate is sufficiently transformed into magnetite during cooling after being wound up due to retained heat, but on the other hand, the scale thickness is increased and the scale adhesion is reduced. The curve (V = 1140-89) in FIG.
If the finish rolling base speed is excessively increased with the boundary of 0 × logt), the exit temperature and the winding temperature of the finishing mill increase, and especially, the thickness of the scale increases near the coil edge, and the scale adhesion increases. Inferior. In the above curves and (t),
The finish thickness (mm) of the steel sheet is shown, and (V) shows the finish rolling base speed (mpm). For the reasons described above, in the present invention, in order to produce a hot-rolled steel sheet having excellent scale adhesion, the relationship between the finished sheet thickness of the steel sheet and the finish rolling base speed is limited as in the following equation (1). . 400-300 × logt ≦ V ≦ 1140-890 × logt (1) The finish rolling base speed (V) is the average threading speed on the run-out table which has the largest influence on the scale generation amount. It is proportional to the speed and used because of the ease of control in production. The steel sheet manufactured according to the present invention can be used also as a cold rolled material. In this case, the pickling step is performed before the cold rolling. However, according to the present invention, the scale of the steel sheet can be reduced in thickness, which is preferable from the viewpoint of pickling properties. In manufacturing a steel sheet according to the present invention, a method of rolling after heating a slab, a method of performing a short-time heat treatment after continuous casting, or a method of immediately rolling without the heating step are used. Either method may be adopted. However, in order to impart excellent scale adhesion, after rough rolling, using a high-pressure water jet immediately before the finish rolling mill,
It is preferable to perform descaling and completely remove the primary scale (scale generated by heating) from the steel sheet surface. In addition, you may heat with a bar heater during hot rolling. In order to suppress the generation of secondary scale (scale generated during rolling), descaling is preferably performed between finishing stands. The exit temperature of the finish rolling mill of the steel sheet is preferably set as low as possible within the range in which the steel sheet can be passed, and at a temperature of three or more Ar points in order to ensure the uniformity of the material of the steel sheet. Therefore, in order to secure an appropriate finish rolling mill exit side temperature, descaling that also serves as cooling between finishing stands is preferably performed by adjusting water pressure and water amount according to a finished plate thickness and a finish rolling speed. . Furthermore, even if an inert gas or a reducing gas is used during hot rolling, there is no possibility that the above-described characteristics are deteriorated. Next, the present invention will be further described with reference to examples. Steel having the chemical composition shown in Table 1 was continuously cast to prepare a slab, and the slab was roughly rolled. After descaling using high-pressure water, various bases were continuously produced by a finishing mill. It was rolled at various speeds to various thicknesses and wound up into coils. Next, samples A1 to A4, B1 to B4, and C1 were obtained from the coils thus obtained.
From C1, D1 to D4 and E1 to E4, respectively. The sampling position of the sample was set at 2 in the slitter line or skin pass line, corresponding to the center in the plate width direction.
It was a wound part (at a position 10 m from the coil bottom part).
The sample thus collected was observed with an optical microscope to measure the thickness of the scale, the scale composition was examined by X-ray diffraction, and the scale adhesion was evaluated by a tape peel test. The results are shown in Table 2 and FIG. [Table 1] [Table 2] As is clear from Table 2, comparative sample A1
Because the relationship between the finish plate thickness and the finish rolling base speed is below the curve, magnetite with excellent adhesion is not sufficiently generated, and some wustite remains,
Scale adhesion was poor. In Comparative Sample A4, since the relationship between the finished plate thickness and the finished rolling base speed was out of the curve, the magnetite excellent in adhesion was sufficiently generated due to the retained heat. The thickness became thick and the scale adhesion was poor. In contrast, the samples A2 and A3 of the present invention
Are excellent in adhesion since the relationship between the finish plate thickness and the finish rolling base speed satisfies the formula (1) of the present invention, that is, is between the curves. Magnetite was sufficiently generated, and the scale adhesion was excellent. In each of Comparative Samples B1 and B3, the relationship between the finished plate thickness and the finished rolling base speed was below the curve, so that magnetite having excellent adhesion was not sufficiently produced, and some of the wustite was not produced. As a result, the scale adhesion was poor. On the other hand, Samples B2 and B4 of the present invention
Are excellent in adhesion since the relationship between the finish plate thickness and the finish rolling base speed satisfies the formula (1) of the present invention, that is, is between the curves. Magnetite was sufficiently generated, and the scale adhesion was excellent. In the comparative sample C4, the relationship between the finish plate thickness and the finish rolling base speed is out of the curve, so that magnetite excellent in adhesion is sufficiently generated due to retained heat. The scale thickness was increased and the scale adhesion was poor. On the other hand, in the samples C1, C2 and C3 of the present invention, the relationship between the finish plate thickness and the finish rolling base speed satisfies the formula (1) of the present invention, that is, the curve And the curve, the magnetite with excellent adhesion was sufficiently generated, and the scale adhesion was excellent. In the comparative sample D1, the relationship between the finished plate thickness and the finish rolling base speed was below the curve and deviated from the curve, so that magnetite having excellent adhesion was not sufficiently produced, and some wustite remained. Scale adhesion was poor.
In Comparative Sample D4, since the relationship between the finish plate thickness and the finish rolling base speed was out of the curve, the magnetite with excellent adhesion was sufficiently generated due to the retained heat. The thickness became thick and the scale adhesion was poor. In contrast, the samples D2 and D3 of the present invention
Are excellent in adhesion since the relationship between the finish plate thickness and the finish rolling base speed satisfies the formula (1) of the present invention, that is, is between the curves. Magnetite was sufficiently generated, and the scale adhesion was excellent. In the comparative sample E3, the relationship between the finished plate thickness and the finished rolling base speed was below the curve and deviated from the curve, so that magnetite excellent in adhesion was not sufficiently generated, and some wustite remained. Scale adhesion was poor. On the other hand, in the samples E1, E2 and E4 of the present invention, the relationship between the finished plate thickness and the finish rolling base speed satisfies the formula (1) of the present invention. And the curve, the magnetite having excellent adhesion was sufficiently generated, and the scale adhesion was excellent. As described above, according to the present invention, the finish rolling is performed by the following equation: 400-300 × logt ≦ V ≦ 1140-890 × l
ogt, where t: Finished steel plate thickness (mm), V: Finished rolling base speed (mpm) (steel plate speed when the tip of the steel sheet bites into the final stand of the finishing mill) By this, the thickness of the generated scale can be reduced, and as a result, the school adhesion is excellent, so that the peeling of the scale at the time of bending, the generation of flaws due to indentation can be prevented, and thus the surface properties are excellent. Production of hot-rolled steel sheets becomes possible. In addition, in the case of a material for cold rolling, pickling properties are also improved, so by optimizing the finish rolling speed for each sheet thickness, scale peeling,
Since the occurrence of flaws due to the indentation can be prevented, useful effects such as improvement in yield and production of a hot-rolled steel sheet excellent in scale adhesion at low cost can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing the influence of a finished plate thickness and a finished rolling base speed on scale adhesion by a tape peeling test. FIG. 2 is a graph showing test results of an example.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor, Noboru Shioya 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (72) Katsumi Nakajima 1-1-2, Marunouchi, Chiyoda-ku, Tokyo Japan (56) References JP-A-10-180338 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B21B 1/00-1/46 B21B 45/00

Claims (1)

(57) [Claims] [Claim 1] A slab is roughly rolled, and then the following (1)
Formula, 400-300 × logt ≦ V ≦ 1140-890 × logt (1) In the above formula (1), t: Finished plate thickness of steel sheet (mm), V: Finished rolling base speed (mpm) Hot-rolled steel sheet with excellent scale adhesion, characterized in that it is finish-rolled and rolled up under the condition expressed by (the steel sheet speed when the tip of the steel sheet bites into the final stand of the finishing mill). Manufacturing method.