JPS6158526B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a heat treatment method for a hot-rolled martensitic stainless steel strip, and its purpose is to omit possible softening annealing such as cold rolling. Conventionally, to obtain a hot-rolled martensitic stainless steel strip, hot rolling is completed at a temperature of more than 900°C but less than 950°C, and after being coiled into a coil, it is allowed to cool to room temperature, but at around 350°C The austenite phase transforms into the martensite phase. The martensitic phase has hardness
It is hard (Hv > 600) and has low ductility and toughness, making it impossible to develop steel strips. For this reason, generally, annealing (usually referred to as bell annealing) is performed while the roll is in the form. This bell annealing is performed at 850 to 890 degrees Celsius while keeping the rolled form from room temperature.
After holding the steel strip at a high temperature for a long time (5 to 15 hours), it is slowly cooled to the transformation completion temperature.However, in the case of normal steel strip, it is rolled into a roll of 6 to 12 tons, and because the mass is large, it is necessary to reach the target temperature all the way to the center. Not only does it take a long time to raise the temperature and a large amount of thermal energy is required,
Due to the high temperature heating, the inner cylinder of the furnace wears out and deforms, requiring periodic replacement, which is costly.Furthermore, the annealing rate is set at 10 to 30℃/Hr, so the bell annealing described above requires a total of 60~ It requires a long time of up to 100 hours. As mentioned above, bell annealing involves a lot of energy and process loss, and although it occupies a large proportion of the price structure of relatively low-priced martensitic stainless steel, bell annealing does not cause softening. Since the purpose was to complete the transformation from austenite phase to ferrite phase + carbide, there was a strong impression that improvement was impossible, and no countermeasures have been taken to date. According to experiments conducted by the present inventors, it has been found that bell annealing can be omitted if the steel has a recrystallized structure immediately after hot rolling and the slow cooling start temperature is 780° C. or higher in subsequent slow cooling. This point will be discussed in detail below.As is clear from the relationship between the slow cooling start temperature and the hardness shown in Figure 1, the slow cooling start temperature has a large effect on the degree of softening, and The higher the cooling start temperature, the softer it becomes at any cooling rate. On the other hand, the hardness of martensitic stainless steel that can be cold rolled is
Hv200 or less, and as is clear from the drawing, if the cooling rate is 30℃/Hr, this value will be satisfied with a slow cooling start temperature of 780℃ or higher, but if slow cooling starts at 780℃ or lower, Possible hardness is not achieved. Furthermore, as a result of investigating the relationship between hot working temperature and recrystallization, it was confirmed that a recrystallized structure can be obtained by performing hot working at 950° to 1280°C. Therefore, the hot rolled material can be cold rolled by ensuring the final hot rolling temperature is 950°C or higher to obtain a recrystallized structure, followed by prompt handling, and then placing it in a slow cooling furnace and slowly cooling it from a temperature of 780°C or higher as described above. A steel strip can be obtained, and bell annealing can be completely omitted. As mentioned above, the end temperature of hot rolling is 950°C or higher because a recrystallized structure can be obtained, but if it exceeds 1280°C, scale will occur due to severe oxidation and hot rolling will not be possible. Therefore, the upper limit is set to 1280℃. However, in reality, it is desirable to set the temperature to 1,100°C or less, because if the temperature is higher than this, the recrystallized austenite grains may become coarse, so the hot rolling end temperature should be 950° to 1,280°C, preferably 1,280°C.
950° C. to 1100° C. Characteristics of the present invention will be explained below based on Examples. Table 1 below shows the materials used for cutlery materials, etc.
An example will be described in which the method of the present invention is applied to SUS420J2 steel. The specimen material is a continuously cast slab melted in an AOD furnace.

[Table] In this example, during hot rolling, with a rolling ratio of 25 to 30%, the final temperature was aimed at 950℃ or higher, and the actual temperature was 970℃.
Controlled rolling is carried out at ℃ and immediately transferred to a slow cooling furnace after coiling.
During this time, there is a temperature drop of 120-140℃, but the temperature is 800-820℃.
Start slow cooling from ℃ to 650℃ which is the completion temperature of transformation.
After slow cooling at a cooling rate of 10 to 30°C/Hr, the mixture was left to cool. The mechanical properties of the obtained steel plate are shown in Table 2 below in comparison with conventional materials.

[Table] Here, the conventional method involves bell annealing after air cooling to room temperature after normal hot rolling.

[Brief explanation of the drawing]

FIG. 1 is a chart showing the influence of slow cooling start temperature on hardness.

Claims (1)

[Claims] 1. Perform controlled hot rolling to obtain a martensitic stainless steel strip so that the final temperature of hot rolling is 950° to 1280°C, and then slowly cool from a temperature of 780°C or higher to the completion temperature of austenite → ferrite + carbide transformation. A heat treatment method for hot-rolled martensitic stainless steel strip characterized by: