JP2008291289A - Method for preventing crack flaw at rolling time of 18-8 based stainless steel containing calcium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for preventing cracking in the corner part of a bloom generated in the hot-rolling of the bloom thereafter even if the staying time in a heating furnace for heating the bloom exceeds 3.5 hr, when the bloom of a 18-8 based stainless steel containing Ca is hot-rolled. <P>SOLUTION: When the staying time for heating the bloom in the heating furnace exceeds 3.5 hr, in order to hot-roll the bloom of the 18-8 based stainless steel containing Ca, the preventive method of the generation of the crack flaw shown in the attached Figure 1 at the hot-rolling of the bloom is performed as the followings: in the heating furnace composed of a pre-heating zone, a heating zone and a soaking zone, the setting temperatures of the heating zone and the soaking zone are respectively made to lower by 10°C than the setting temperatures, when the furnace-staying time is <3.5 hr, to control to lower by 10°C hot-rolling temperature of the bloom, in order to perform hot-rolling so that the generating quantity of ferrite at the corner part of the bloom is restrained. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention is a method for preventing the occurrence of cracking flaws during subsequent hot rolling by changing the set temperature of a heating furnace that is heated to hot-roll the bloom of 18-8 stainless steel containing Ca. About.

  Conventionally, the preset temperature of a heating furnace that heats a bloom of 18-8 stainless steel containing Ca, particularly 30-50 ppm of Ca, is 1160-1180 ° C. The heating zone is set to 1330 to 1350 ° C., the soaking zone is set to 1330 to 1340 ° C., and the blooming time of the bloom from the charging of the bloom into the heating furnace to the extraction at the set temperature is 2.5 to 6. Heating is performed for a range of 0 hours. However, when the in-furnace time of the bloom from charging the bloom into the heating furnace until extraction is longer than 3.5 hours, cracks occurred at the corner of the bloom during subsequent hot rolling.

  In conventional hot rolling of steel, it is known that if the amount of reduction is too small, side cracks may occur due to insufficient training (see, for example, Non-Patent Document 1). However, this does not relate to Ca-containing 18-8 stainless steel, and therefore, no countermeasure has been shown.

Steel Handbook 4th Edition, p. See 2247

  The problem to be solved by the present invention is that, when hot-rolling a bloom of 18-8 stainless steel containing Ca, even if the in-furnace time of the heating furnace for heating the bloom exceeds 3.5 hours, The object is to provide a method for preventing cracks at the corners of the bloom that occurs in subsequent hot rolling of the bloom.

  When hot rolling a bloom of 18-8 stainless steel containing Ca, the bloom generated in the subsequent hot rolling when the in-furnace time of the heating furnace for heating the bloom exceeds 3.5 hours The inventor has intensively studied the cause of cracks in the corners of the steel and the countermeasures. As a result, the inventors have found that the ferrite produced in the corners of the bloom is the cause, and the heating by the heating furnace for a long time exceeding 3.5 hours. It has been found that by lowering the heating temperature than before, the formation of ferrite at the corners of the bloom can be suppressed and cracking can be prevented by lowering the heating temperature.

  Therefore, the means of the present invention for solving the above-described problems is that, in the invention of claim 1, the bloom is heated in a heating furnace in order to hot-roll the bloom of 18-8 stainless steel containing Ca. If the in-furnace time for the heating is longer than 3.5 hours, the set temperature in the heating zone and soaking zone in the pre-tropical zone, heating zone, and soaking zone is less than 3.5 hours. During the hot rolling of the bloom, the hot rolling temperature of the bloom is controlled to be lower by 10 ° C. by lowering the temperature by 10 ° C. to suppress the amount of ferrite generated at the corner of the bloom. This is a method for preventing the occurrence of cracks.

  The invention according to claim 2 is characterized in that the 18-8 stainless steel containing Ca is 18-8 stainless steel containing 30 to 50 ppm of Ca. This is a method for preventing the occurrence of cracks at the time.

  In the heating in a heating furnace for hot rolling of a bloom made of 18-8 stainless steel containing Ca, particularly 18-8 stainless steel containing 30 to 50 ppm of Ca, the in-furnace time of the heating furnace is By reducing the heating zone of the furnace and the temperature of the soaking zone by 10 ° C. below those in the conventional method when the time is 3.5 hours or more, the ferrite in the corner portion due to subsequent hot rolling of this bloom It is possible to reduce cracking flaws by reducing the amount of formation of.

  By changing the set temperature in the heating furnace according to the in-furnace time of the heating furnace of the bloom made of 18-8 series stainless steel containing Ca: 30-50 ppm, and controlling the rolling temperature of the hot rolling, the corner of the bloom The amount of ferrite produced in the steel was suppressed to prevent rolling flaws. For that purpose, the furnace time of the bloom heating furnace, the temperature of the corner portion during rolling of the bloom, the temperature of the center portion of the surface during rolling, the number of ridges per bloom (N-10) and the evaluation of the folds, Table 1 shows the test results of the conventional method at the in-furnace time and the heating furnace set temperature.

  Furthermore, the temperature was lowered in the heating zone and in the soaking zone, the temperature of the corner part during rolling of the bloom, the temperature of the center part of the surface during rolling, the number of cracks per bloom (N-10) and the evaluation of the soot Table 2 shows the test results of the present invention in the in-furnace time of 3.5 hours or more and the heating furnace set temperature. These cracks are shown as cracks 1 in FIGS. 1 and 2, respectively.

  In Table 1 and Table 2 above, 疵 of o evaluation indicates less than 3 疵, Δ indicates 3 to 5 疵, and x indicates 6 or more 疵. Furthermore, in Table 2, when the temperature lowered in the heating zone and the soaking zone is 20 ° C. or higher compared to the conventional temperature in Table 1, the hot workability deteriorates due to the temperature drop of the bloom, and cracking occurs. Presumed to have occurred. That is, as shown in Table 1, in the soot evaluation of the conventional method, the in-furnace time is from 2.5 hours to less than 3.0 hours and from 3.0 hours to less than 3.5 hours. It was x from more than 5 hours to 6.0 hours. On the other hand, as shown in Table 2, in the soot evaluation of the method of the present invention, even when the in-furnace time is 3.5 hours or more to 6.0 hours, the temperature of the heating furnace and the soaking furnace is set to 10 When it was lowered by 0 ° C., it was ◯, and when it was lowered by 20 ° C. or 30 ° C. than those of conventional ones, it was x.

  Based on the test results shown in Table 1 and Table 2 above, the heating set temperature of the heating furnace when hot rolling a bloom made of 18-8 stainless steel containing Ca: 30 to 50 ppm is based on the in-furnace time of the bloom. Management was carried out as shown in Table 3 below, and the occurrence of rolling defects due to hot rolling was suppressed. In Table 3, for comparison, the in-furnace time according to the conventional method, and the temperatures of the pretropical zone, heating zone, and soaking zone are shown in the old, and those in the present invention are shown in the new.

  With regard to the bloom operation status, Bloom, which was frequently subjected to hot rolling due to hot rolling this time, had no record of abnormal operation during bloom rolling. However, looking at the record of the inspection results of the rolled material, it was found that the area ratio of the surface defect Lo in the 0-2.5 mm section below the surface by the probe rotating ultrasonic flaw detector R-UST was high. In addition, even in blooms with long in-furnace time in the furnace due to past troubles, etc., there is a record that corner cracks occurred in the hot rolled material, and a temporary outflow of hot rolled material with wrinkles occurred. Measures were taken to confirm the quality status.

  As countermeasures against the occurrence of corner cracks in hot rolled material, the first zone of the pre-tropical zone of the heating furnace, the second zone and the third zone of the heating zone, and the fourth zone of the soaking zone in order to prevent overheating The set temperature in each zone consisting of was changed. Table 4 shows each heating time and each set temperature of the heating furnace in each zone with the conventional method as the current state and a revised invention method.

  Furthermore, as a countermeasure against the outflow of the hot rolled material with corner cracks in the hot rolled material to the next process, the hot rolled material with a Lo area ratio of R-UST of 2.5% or more A visual inspection is conducted for confirmation of 3 lots. If there is any remaining flaws, a re-inspection is performed. Depending on the situation of the remaining flaws, a part of the flaws is removed by adding a peeling process to the next process. did.

It is a figure of the macro photograph which shows the crack of the corner surface of a hot rolling material. It is a figure of the macro photograph which shows the crack of the surface of a rod-shaped rolling material.

Explanation of symbols

  1 Crack

Claims (2)

  In order to hot-roll blooms of 18-8 stainless steel containing Ca, when the in-furnace time for heating the blooms in the heating furnace becomes longer than 3.5 hours, The hot rolling temperature of the bloom is controlled to be 10 ° C. lower by setting the heating zone and the setting temperature in the soaking zone in the soaking zone to be 10 ° C. lower than the setting temperature when the in-furnace time is less than 3.5 hours. Rolling and suppressing the amount of ferrite produced at the corners of the bloom, and a method for preventing the occurrence of cracks during hot rolling of the bloom.   The method for preventing the occurrence of cracks in hot rolling of bloom according to claim 1, wherein the 18-8 stainless steel containing Ca is 18-8 stainless steel containing 30 to 50 ppm of Ca. .