JP2009255107A - Method of preventing crack in rolling caused by change of set temperature of heating furnace in 18-8 stainless steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of preventing the generation of cracks by changing a hot rolling temperature by properly changing the set temperature of a heating furnace corresponding to the residence time in the heating furnace of a bloom to be heated for hot-rolling 18-8 stainless steel and suppressing the production of ferrite in the corner parts of the bloom in hot rolling. <P>SOLUTION: When the residence time in the heating furnace for heating in order to the hot-rolling of the bloom of the 18-8 stainless steel is ≥2.5 to <3.5h, the set temperature of the heating furnace is taken as 1,160-1,180°C in the preheating zone, as 1,330-1,350°C in the heating zone and as 1,330-1,340°C in the soaking zone. When the residence time is 3.5-6.0h, the preset temperature is taken as the same as the above in the preheating zone, but as 1,320-1,340°C in the heating zone and as 1,320-1,330°C in the soaking zone. By lowering the each temperature of the heating zone and the soaking zone downward by 10°C as compared with the case where the residence time is <3.5 hr, the cracks in the corner parts of the bloom in the hot rolling are prevented. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  In this application, by changing the set temperature in the heating furnace in the 18-8 stainless steel, which is an austenitic stainless steel, to lower the generation amount of ferrite in the bloom corner portion, The present invention relates to an occurrence prevention method.

  Conventionally, blooms of austenitic stainless steel by continuous casting are heated in a heating furnace for hot rolling. The set temperature of the heating furnace at the time of heating is 1160 ° C to 1180 ° C in the pretropical zone, 1330 ° C to 1350 ° C in the soaking zone, and 1330 ° C to 1340 ° C in the soaking zone. The blooming time of the bloom of No. 1 was 2.5 hours or more and 6.0 hours or less. However, if the in-furnace time from charging to extraction in the heating furnace of the bloom becomes longer than 3.5 hours, the amount of ferrite generated in the bloom corner increases, which causes cracks during hot rolling. There was a problem.

  On the other hand, under the operating conditions that stipulate the heating temperature and heating time of the conventional slab slab of austenitic stainless steel, 1% or more of δ-ferrite remains and work cracks occur during hot rolling, resulting in the occurrence of whipping. As a method of avoiding this, the heating temperature of the steel slab is maintained at 1200 ° C. to 1260 ° C., and the steel slab temperature is maintained at 1200 ° C. or higher for 40 minutes or more, thereby remaining. A method of hot rolling after eliminating δ-ferrite in the steel slab is proposed (for example, see Patent Document 1). However, this method changes the conventional technology, which has been set to a high pretropical temperature, in order to improve productivity. When heated and maintained at 1260 ° C. or higher as in the prior art, the ductility deteriorates and the heat is reduced. Since the inter-workability deteriorated, it was an improvement. By the way, the proposed prior art method is not a method of changing the heating zone or the soaking zone heat pattern according to the in-furnace time of the heating furnace.

JP 2004-137540 A

  In general, the preset temperature of the Bloom heating furnace for hot rolling of 18-8 stainless steel is 1160 ° C to 1180 ° C in the pretropical zone, 1330 ° C to 1350 ° C in the heating zone, and 1330 ° C to 1340 ° C in the soaking zone. In addition, the in-furnace time of the steel material from charging to extraction in the heating furnace was limited to 2.5 hours or more and 6.0 hours or less. However, in steel materials whose in-furnace time is 3.5 hours or more, there is a problem that the amount of ferrite generated increases at the bloom corner portion during hot rolling, and cracks occur during rolling.

  Therefore, the problem to be solved by the present invention is to appropriately change the set temperature in the heating furnace in accordance with the in-furnace time of the Bloom heating furnace heated for hot rolling of 18-8 stainless steel. It is providing the method of preventing generation | occurrence | production of a crack by changing hot rolling temperature by this and suppressing the ferrite production amount of the bloom corner part at the time of hot rolling.

  The means of the present invention for solving the above-mentioned problems is as follows. In the invention of claim 1, heating is performed according to the in-furnace time in the heating furnace for heating the 18-8 stainless steel bloom in order to hot-roll. This is a cracking prevention method during hot rolling of a bloom corner portion, characterized in that the hot rolling temperature is suppressed low by changing the set temperature of the furnace.

  In the invention of claim 2, the method of changing the set temperature of the heating furnace in accordance with the in-furnace time in the heating furnace is the in-furnace from charging to extraction of the 18-8 series stainless steel bloom to the extraction furnace. When the time is 2.5 hours or more and less than 3.5 hours, the set temperature of the heating furnace is 1160 ° C to 1180 ° C in the pretropical zone, the heating zone is 1330 ° C to 1350 ° C, and the soaking zone is 1330 ° C to 1340 ° C. However, when the in-furnace time is 3.5 hours or more to 6.0 hours or less, the point where the pre-tropical zone is 1160 ° C to 1180 ° C is not changed, but the heating zone is 1320 ° C to 1340 ° C, The temperature of the heating zone and the soaking zone is lowered by 10 ° C. each by 10 ° C. compared to the case where the temperature in the furnace is less than 3.5 hours. To prevent cracks during hot rolling A.

  By using the above-described means of the present invention, in 18-8 stainless steel, by suppressing the amount of ferrite produced in the hot-rolled bloom corner portion, generation of cracks during hot rolling can be prevented. The quality of the rolled material can be improved. Especially when the in-furnace time in the heating furnace is 3.5 hours or more and 6.0 hours or less, the heating zone of the heating furnace and the temperature of the soaking zone are set to 3.5 respectively. Reduced by 10 ° C as compared to the case of less than the time, suppressing the generation of ferrite in the hot-rolled bloom corner, thereby preventing the occurrence of cracks during hot rolling and the quality of the rolled material Therefore, the present invention is a method that exhibits an excellent effect that has never been achieved.

  The best mode for carrying out the present invention will be described below with reference to the table. Prior to this, in order to hot-roll a bloom made of JIS standard SUS304, which is 18-8 stainless steel, from the setting temperature of the heating furnace for heating to the hot rolling temperature and from the charging to the extraction of the bloom Experiments were conducted to investigate the occurrence of flaws at the corners of the hot-rolled bloom, depending on the in-furnace time.

  First, the in-furnace time of the bloom in the conventional heating furnace is 2.5 hours to less than 3.0 hours, 3.0 hours to less than 3.5 hours, 3.5 hours to 6.0 hours. The temperature at the corner of the bloom during rolling is 1120 ° C. for 2.5 hours to less than 3.0 hours, and 1120 ° C. for 3.0 hours to less than 3.5 hours. The bloom was heated to 1135 ° C. for 3.5 hours to 6.0 hours. Furthermore, the surface center temperature during rolling is 1115 ° C. for 2.5 hours to less than 3.0 hours, 1117 ° C. for 3.0 hours to less than 3.5 hours, and 3.5 hours to 6. Below 0 hours, the bloom was heated to 1125 ° C. Next, these blooms were hot-rolled, and the number of wrinkles generated per one hot-rolled bloom (the number when N = 10) was investigated. As a result, when the in-furnace time of the bloom was 2.5 hours or more and less than 3.0 hours, the number of soot was 2.4. When the in-furnace time of the bloom was 3.0 hours or more and less than 3.5 hours, the number of soot was 2.5. When the bloom time in the bloom was 3.5 hours to 6.0 hours, the number of soot was 5.4. In addition, when this in-furnace time is 3.5 hours or more, it is thought that the amount of ferrite generation increased and the cracks increased in the bloom corner portion. The evaluation based on the number of the generated wrinkles is shown in Table 1, with less than 3 wrinkles being ◯, 3 to 5 wrinkles being Δ, and five or more wrinkles being ×.

  Next, a rolling test was performed on the bloom that had been heated by lowering the set temperature of the heating furnace at a blooming time of 3.5 hours or longer. That is, the temperature of the heating zone and the soaking zone of the heating furnace was lowered by 10 ° C., the bloom corner temperature during rolling was 1120 ° C., and the temperature at the center of the surface during rolling was 1114 ° C. In this case, the number of ridges (number when N = 10) generated per bloom by rolling was 2.5, and the evaluation was ◯.

  Furthermore, the heating zone of the heating furnace and the soaking zone temperature were lowered by 20 ° C., the bloom corner temperature during rolling was set to 1102 ° C., and the temperature at the center of the surface during rolling was set to 1098 ° C. In this case, the number of ridges (number when N = 10) generated per bloom by rolling was 5.5, and the evaluation was x.

  Further, the temperature of the heating zone and the soaking zone of the heating furnace was lowered by 30 ° C., the bloom corner temperature during rolling was set to 1099 ° C., and the temperature at the center of the surface during rolling was set to 1089 ° C. In this case, the number of ridges (number when N = 10) generated per bloom by rolling was 7.0, and the evaluation was x.

  In these tests, when the heating zone of the heating furnace and the temperature of the soaking zone are lowered to 20 ° C or higher, the amount of ferrite produced increases at the bloom corner when the above furnace time is 3.5 hours or more. Unlike the case in which cracks increase, the temperature drop of the rolled bloom becomes remarkable, so that hot workability deteriorates, and it is speculated that cracks are generated particularly by rolling. These test results are shown in Table 2.

  Based on the above test results, the present invention changes the heating temperature and the time in the zone of the pre-tropics, heating zone, and soaking zone in the Bloom heating furnace, and further generates ferrite in the bloom corner portion. This is a method for preventing the occurrence of cracks during hot rolling by suppressing the amount. For that purpose, it is a method of changing the temperature of the heating furnace according to the in-furnace time of the bloom in the heating furnace to be charged in order to heat and soothe the bloom made of 18-8 stainless steel.

  In the best mode of the method of the present invention, in Table 3, when the material furnace time limit is 2.5 hr ≦ in-furnace <3.5 hr, the pre-tropical zone, the heating zone and The heating temperature in the soaking zone is exactly the same as the heating temperature in the pretropical zone, heating zone, and soaking zone in the old case where the conventional in-furnace time limit is 2.5 hr ≦ in-furnace ≦ 6.0 hr. The temperatures were 1160 ° C to 1180 ° C in the pretropical zone, 1330 ° C to 1350 ° C in the heating zone, and 1330 ° C to 1340 ° C in the soaking zone.

  However, in Table 3, in the case of the new 2 in which the in-furnace time limit is 3.5 hr ≦ in-furnace ≦ 6.0 hr, the temperature of the pretropical zone is 1160 ° C. to 1180 ° C. which is the same as the new 1 and old temperatures. However, the temperature of the heating zone is 1320 ° C to 1340 ° C, the temperature of the soaking zone is 1320 ° C to 1330 ° C, and the temperature of the heating zone and soaking zone is 10 ° C compared to the new 1 and the old temperature, respectively. Shifted gradually and controlled low. As a result, in the new 1 and new 2 methods, it was possible to suppress the occurrence of cracks in the bloom corner portion to less than three.

Claims (2)

  It is characterized by suppressing the hot rolling temperature low by changing the set temperature of the heating furnace corresponding to the in-furnace time in the heating furnace heated to hot-roll the bloom of 18-8 stainless steel. How to prevent cracks during hot rolling at the bloom corner.   The method of changing the set temperature of the heating furnace corresponding to the in-furnace time in the heating furnace is that the in-furnace time from charging to extraction of the 18-8 stainless steel bloom into the heating furnace is 2.5 hours or more. In the case of less than 3.5 hours, the set temperature of the heating furnace is 1160 ° C to 1180 ° C in the pretropical zone, the heating zone is 1330 ° C to 1350 ° C, the soaking zone is 1330 ° C to 1340 ° C, but the in-furnace time is 3 In the case of 5 hours or more and 6.0 hours or less, the temperature of the pretropical zone is not changed, but the heating zone is set to 1320 ° C to 1340 ° C, and the soaking zone is set to 1320 ° C to 1330 ° C. The temperature of the heating zone and the soaking zone is lowered by 10 ° C respectively in comparison with the case where the in-furnace time is less than 3.5 hours, and at the time of hot rolling of the bloom corner portion according to claim 1, How to prevent cracks.