JP3449259B2 - Method for producing hot rolled P-added steel sheet with excellent surface properties - Google Patents

Description

Description: TECHNICAL FIELD [0001] The present invention relates to a P-added hot-rolled steel sheet,
In particular, the present invention relates to a method for producing a P-added hot-rolled steel sheet having excellent surface properties containing 0.03 wt% or more of P. [0002] A hot rolled steel sheet is prepared by directly heating a slab produced by a continuous casting machine or after reheating in a heating furnace, and then using a rough rolling mill comprising a plurality of stands and a finish rolling mill. It is manufactured by rolling into a thick steel strip and winding it into a coil. One of the main factors that deteriorate the surface properties of the steel sheet produced in this way is a scale defect that occurs when the steel sheet is rolled while the scale remains. [0004] The cause of the occurrence is considered to be that descaling is not sufficiently performed before finish rolling or that a thin scale is formed that is difficult to peel between stands of the finish rolling mill. Has been done. For example, Japanese Patent Application Laid-Open No. 6-269840 discloses that a coarse bar is heated before finish rolling to increase the thickness of the scale, and then descaled with high-pressure water to completely remove the scale before finish rolling. Japanese Unexamined Patent Publication No. 57-154301 proposes a method of lowering the temperature of a rough bar before finish rolling to a certain temperature or less to suppress the formation of scale between stands. [0006] However, Japanese Patent Application Laid-Open
Even if the method described in JP-A-6-269840 or JP-A-57-154301 is applied, P is used to increase strength and corrosion resistance.
Could not prevent the scale defect of the mottled pattern generated in the hot-rolled steel sheet containing 0.03 wt% or more. The present invention has been made to solve such a problem, and an object of the present invention is to provide a method for producing a P-added hot-rolled steel sheet having excellent surface properties, which does not cause mottle pattern scale defects. [0008] The above object is achieved by setting P to 0.03 to
A slab made of P-added steel containing 0.075 wt% is roughly rolled at a temperature of 1050 ° C. or less, and the rough bar after the rough rolling is heated to raise the surface temperature by 30 ° C. or more. The problem is solved by a method for producing a P-added hot-rolled steel sheet having excellent resistance. [0009] The present inventors have investigated the cause of the occurrence of the scale of the mottled pattern of the P-added hot-rolled steel sheet, and this pattern is due to the uneven thickness of the scale. Partially exfoliated, the exfoliated scale adheres to the surface of the rolled steel material and is produced by rolling, and in areas where scale exfoliation easily occurs, scale
/ It became clear that P was concentrated at the interface of the iron and steel. Therefore, the relationship between the conditions before finish rolling, in which the surface of P tends to be concentrated, and the occurrence of scale defects in a mottled pattern was examined by using the P-added steel of the component system shown in Table 1. FIG. 1 shows the relationship between the presence or absence of a mottle pattern scale defect, the temperature of rough rolling, and the temperature rise of the rough bar surface. Here, the rise temperature of the rough bar surface is the rise of the rough bar surface temperature due to heating when the rough bar after the rough rolling is heated. It is understood that when rough rolling is performed at a temperature of 1050 ° C. or less and the surface temperature of the rough bar after the rough rolling is raised by 30 ° C. or more, mottle pattern scale defects do not occur. This is because the surface concentration of P is suppressed by lowering the temperature of the rough rolling, and the temperature of the scale generated during finish rolling is raised by increasing the surface temperature of the rough bar, and the ductility of the scale itself is increased. This is considered to be due to the fact that scale peeling during finish rolling was suppressed. Similar effects can be obtained by roughly rolling a slab of P-added steel directly from a continuous casting machine or by roughly rolling after heating in a reheating furnace. The rise in surface temperature due to the heating of the coarse bar is preferably set to 150 ° C. or less from the viewpoint of energy cost. The heating can be performed by an induction heating device using an induction coil. The finishing temperature is preferably not less than the Ar ₃ transformation point from the viewpoint of workability. [Table 1] EXAMPLE Using steel having the composition shown in Table 2, the hot rolling conditions such as heating temperature, rough rolling temperature, temperature rise of the rough bar surface, and finishing temperature were changed as shown in Table 3, Samples 1 to 16 of rolled steel sheets were prepared. Here, the temperature rise of the surface of the rough bar was performed by passing the rough bar through an induction heating device using an induction coil installed in front of a finishing mill. Also,
The descaling before the finish rolling after heating the coarse bar is 150 kgf / cm ²
Using high-pressure water. Then, the presence or absence of a mottle pattern scale defect on the steel sheet surface was visually inspected. Table 3 shows the results. It can be seen that the samples 1 to 3, 6, 8, 9, 11, 12, 14, and 15 produced by the method of the present invention do not generate a mottled pattern scale defect. On the other hand, Sample 4, whose rough rolling temperature is out of the range of the present invention,
7, 16, the temperature rise of the rough bar surface is out of the range of the present invention for samples 5, 10,
In 13, a mottled pattern scale defect occurs. [Table 2] [Table 3] Since the present invention is configured as described above, it is possible to provide a method for producing a P-added hot-rolled steel sheet which is excellent in surface properties and does not cause mottled scale defects. Further, since no scale defect of the mottled pattern occurs, a long pickling is not required, and the average surface roughness (Ra)
, A P-added pickled steel sheet having a P of 1.3 μm or less can be manufactured.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing the relationship between the presence or absence of mottle pattern scale defects, the temperature of rough rolling, and the temperature rise of the surface of a rough bar.

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Nobori Shioya 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Inside Nippon Kokan Co., Ltd. (56) References JP-A-10-219351 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) B21B 1/00-3/02

Claims (1)

(57) [Claims 1] A slab made of P-added steel containing 0.03 to 0.075 wt% of P is roughly rolled at a temperature of 1050 ° C or less, and the rough bar after the rough rolling is obtained. A method for producing a P-added hot-rolled steel sheet having excellent surface properties by heating the steel to raise its surface temperature by 30 ° C. or more, and performing finish rolling after descaling.