JPH0555569B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for rolling martensitic stainless steel wire rods and steel strips, such as SUS410 and SUS420J2, which can particularly prevent the occurrence of hot cracks, and which can also be used for softening treatment in the rolling line. The present invention relates to a method for rolling wire rods and steel strips. (Prior art) Stainless steels such as martensitic stainless steels, whose austenite phase is relatively unstable, may crack during hot rolling, and also crack during martensitic transformation during cooling. Occur. Usually, the rolled and wound coil is soaked at a temperature of A _c3 or higher (usually 900°C or higher), and then undergoes heat treatment such as tempering or annealing at various temperatures depending on the application. will be applied. For information on commonly used heat treatment conditions such as quenching, normalizing, and tempering, please refer to Nikkan Kogyo Shimbun Co., Ltd.
It is explained in documents such as the Stainless Steel Handbook published on August 30, 1971. Furthermore, methods for heat treatment utilizing the high temperature state after rolling have already been published in JP-A-52-65126, JP-A-53-149119, JP-A-58-
There is a disclosure in Publication No. 25419. JP-A No. 52-65126 relates to an annealing method for the purpose of reducing thermal energy of a hot coil of ferritic or martensitic stainless steel. A method of annealing at a normal annealing temperature is disclosed. JP-A No. 53-149119 discloses a method for preventing cold cracking caused by strain caused by martensitic transformation when cooling large cross-section steel slabs such as martensitic stainless steel cast slabs or slabs. It is about the method. Furthermore, JP-A No. 58-25419 relates to a method for softening a steel plate, steel pipe, or steel strip after rolling. Discloses a method of softening the rolled material by cooling it at a high speed, then reheating and holding it at a temperature close to the nose of pearlite transformation (nearly 700°C), and preventing low-temperature cracking due to martensitic transformation during cooling. are doing. As outlined above, the technical contents disclosed in the above-mentioned documents are related to high-temperature treatment of rolled materials for the purpose of energy saving, or are aimed at suppressing cold cracking caused by martensitic transformation. However, it does not aim at a method for suppressing hot cracking in rolling wire rods or steel strips, nor does it relate to energy-saving softening treatment in rolling lines. (Problems to be Solved by the Invention) The present invention was devised based on the results of detailed analysis of the structure in which cracks occur during hot rolling of martensitic stainless steel wire rods and steel strips. The object of the present invention is to provide a process-saving rolling method that suppresses the occurrence of rolling and softens the rolled wire rod and steel strip to the extent that handling in subsequent processes is not difficult (H _V ≦450). (Means for solving the problem) As mentioned earlier, martensitic stainless steel slabs develop hot rolling cracks. Hot cracks that occur during the rough rolling process of wire rod or steel strip rolling remain as flaws on the wire rod product or steel strip. As a result of detailed analysis of the structure of hot rolling cracks, the present inventors found that cracks occur at austenite grain boundaries formed during solidification, and that once the cast structure is changed to a processed structure by rolling, the austenite It was confirmed that no cracks would occur at grain boundaries. Figure 2 shows the relationship between the temperature, rolling reduction and crack occurrence in the first pass of the rough rolling process, and Figure 3 shows the relationship between the rolling schedule at the initial stage of rough rolling and the crack occurrence (in the figure, 1 step corresponds to 1 pass). However, the occurrence of cracks depends on the temperature and reduction rate during the first rolling pass, and if the reduction rate does not exceed 15% at most, cracks will not occur in subsequent rolling. I understand. In other words, the first pass of the rough rolling process is 1070
It is necessary to carry out in the temperature range from ℃ to 1250℃. At this time, when rolling at a temperature exceeding 1250°C, cracks occur due to low melting point compounds formed at grain boundaries. For this reason, the upper limit of the rolling start temperature was set at 1250°C. Next, a method of softening the rolled wire on the rolling line will be described. There are two methods for this: Figure 1 shows the process after wire rod rolling or hot steel strip rolling.
It shows the softening when held at various temperatures.
Moreover, FIG. 4 shows the softening when the material is rapidly cooled to below the M _S point after rolling and then heated to various temperatures. Fig. 1 corresponds to the decomposition of austenite into ferrite + carbide on the metal structure, and Fig. 4 corresponds to the decomposition of martensite into ferrite + carbide. When comparing Fig. 1 and Fig. 4, it is found that ferrite +
Decomposition into carbides is shorter in holding time at soaking temperature, but when softening treatment is performed on an actual rolling line, time is required to heat to soaking temperature after quenching, so The method of rapidly cooling and holding at a constant temperature after rolling produces a constant softening state (H _V ≦450 as mentioned above)
can be achieved in a shorter time. Therefore, in the present invention, it was decided to use the method shown in FIG. In the case of wire rod rolling, the temperature does not drop during the rolling process due to the heat generated during rolling, and the inlet and outlet temperatures of finish rolling are approximately the same. As mentioned earlier, rolling at high temperatures is necessary to suppress hot cracking. As a result, the rolling finish temperature inevitably becomes high. In order to set rolling conditions that both suppress the occurrence of hot cracks during rough rolling and soften the product wire during the rolling process, the present inventors set up By installing a cooling device to control the inlet temperature of the finishing rolling mill, the finishing temperature of rolling can be controlled, and then the temperature is maintained between 650 and 750°C by forced cooling.
This was made possible by maintaining the temperature at . Furthermore, in this case, if the rolling temperature in finish rolling is carried out at a temperature between A _c1 point and A _c3 point,
It was found that softening could be achieved in a shorter retention time than that shown in FIG. That is, as shown in FIG. 5, when the finishing temperature is 760°C, the desired hardness (H _V 400 or less) can be obtained in a shorter holding time than when the finishing temperature is 1000°C. (Figure 5 was obtained using the same heat schedule pattern as Figure 3.) Therefore, between rough rolling and intermediate rolling or in the middle of the intermediate rolling process, the rolled material is cooled by a cooling device.
If finish rolling is performed after cooling to a temperature between A _c1 point and A _c3 point, the finish rolling end temperature will be approximately in the above temperature range, and the above effects can be obtained. (Function) Next, the steps of the present invention will be explained in more detail. As mentioned earlier, in order to prevent hot cracking of martensitic stainless steel, it is necessary to
The slab, which has been soaked at ℃, is lightly rolled down in the first pass of the rough rolling process. At this time, the rolling reduction rate of the first pass is 15% or less,
Preferably it is 10% or less. Next, between the rough rolling mill and the intermediate rolling mill or between the intermediate rolling mill row, the temperature of the rolled material is cooled to a temperature between the A _c1 point and the A _c3 point using a cooling device. Next, at the exit of the finishing rolling mill, it is cooled to 700°C by a cooling device and wound into a coil. The coil is placed in a soaking furnace (usually
After being heat-retained for a short time (usually 3 to 5 minutes) at a temperature of 700°C, it is air-cooled to produce wire rod products or steel strip products. Through the above steps, a softened product without hot cracking can be obtained. (Example) Table 1 shows examples. No.1, No.2, No.4,
No. 7 is an example of the present invention, but the rolling temperature and reduction rate of the first pass of rough rolling are within the range of the present invention, and it is cooled between intermediate rolling mills and at the exit of the finishing mill, and then heated to 700°C for 30 minutes. By soaking for 5 minutes, the hardness decreased to a predetermined value (H _V <450). On the other hand, No. 3 had a high first pass reduction rate, which caused hot cracking, and No. 5 had insufficient cooling.
High hardness due to short retention time at 700℃.
No. 6 cracked similarly to No. 3, and at 700℃
The hardness is high because there is no uniform heating. No.8 is
Similar to No. 3, cracks occurred because the first pass reduction rate was high.

[Table] (Effects of the Invention) As described above, according to the present invention, excellent wire rod products and steel strip products that are free of defects caused by hot cracking and that are softened so that they are not difficult to handle in subsequent processes can be produced. Since it can be obtained with fewer steps, its industrial effects are enormous.

[Brief explanation of drawings]

Fig. 1a shows the softening curve of the present invention, Fig. 1b shows the heat schedule of the invention, and Fig. 2 shows the softening curve of the present invention.
A diagram showing the relationship between pass rolling reduction, rolling temperature, and crack occurrence. Figure 3 is a diagram showing the relationship between rolling conditions and crack occurrence in the initial stage of rough rolling. Figure 4 a shows softening when cooled to room temperature after rolling. FIG. 5 is a diagram showing the influence of the rolling end temperature on the softening curve in the same heat schedule pattern as FIG. 1b.

Claims (1)

[Claims] 1 In the rolling process of martensitic stainless steel wire rod and steel strip, the first pass of the rough rolling process is
Reduction rate of 15% in the temperature range of 1070℃ to 1250℃
Then, between the rough rolling process and the intermediate rolling process, or in the middle of the intermediate rolling process, the rolled material is cooled to a temperature between A _c1 point and A _c3 point, and then rolled, and then the material is rolled in the finishing mill. 650℃ or more at the outlet
A method for producing martensitic stainless steel wire and steel strip, which comprises cooling to 750°C, winding into a coil, and then retaining heat in a temperature range of 650°C to 750°C on a rolling line.