JP2006070306A - HOT WORKING METHOD FOR HIGH Ni ALLOY STEEL - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hot working method where a continuously cast slab of a high Ni alloy steel is heated as uniformly as possible, thereby the generation of thermal stress cracks is suppressed, and the generation of edge cracks can be suppressed when the heated continuously cast slab is hot-worked. <P>SOLUTION: In the method for subjecting a continuously cast slab of a high Ni alloy steel comprising, by mass, 25.0 to 85.0% N to hot working, when heating is applied prior to the working for the continuously cast slab, the difference between the maximum value and the minimum value of the temperature in the continuously cast slab is controlled to ≤200°C. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for hot working of high Ni alloy steel. The heat of high Ni alloy steel for suppressing the occurrence of cracking during heating prior to processing and during processing. It relates to the inter-machining method.

High Ni alloy steel containing 25.0 to 85.0 mass% of Ni is widely used for shadow masks for CRTs, IC lead frames, and the like as a material having a small thermal expansion in a temperature range from room temperature to 300 ° C. Conventionally, high Ni alloy steel is melted in an electric furnace, cast into a slab, and further subjected to hot rolling and cold rolling to be processed into a thin steel plate. The method of manufacturing slabs from molten steel with high Ni alloy steel melted
(A) A method for producing a slab by casting molten steel into an ingot and further rolling in pieces
(B) Broadly divided into methods for producing slabs by continuously casting molten steel.

  In recent years, from the viewpoint of improving the productivity of high Ni alloy steel, for example, in the case of high Ni alloy steel strip, a slab manufactured by continuous casting (hereinafter referred to as continuous cast slab) is arranged in series with a roughing mill and a finishing mill. A method of hot rolling in a continuous thin sheet hot rolling line and then cold rolling has been studied. However, the metal structure of the continuous cast slab of high Ni alloy steel (hereinafter referred to as the cast structure) is a so-called columnar structure in which a long and narrow structure extends from the surface layer of the continuous cast slab toward the center in the thickness direction. Because the grain boundaries are fragile, when a continuous cast slab of high-Ni alloy steel is hot-rolled, it extends both in the width direction and in the longitudinal direction, and the end in the width direction (hereinafter referred to as the edge) is locally deformed. (Referred to as part) is in a situation where cracks are likely to occur.

  Furthermore, if the temperature of the continuous cast slab after heating is not uniform, cracks (hereinafter referred to as thermal stress cracks) are likely to occur not only at the edge part, but due to thermal stress caused by the temperature difference. . When the hot rolling is performed, the influences of the rolling are manifested, and cracks are generated. In the case of thermal stress cracks, cracks are particularly likely to occur at the edge part because they are often manifested by the force applied by rolling. Hereinafter, the cracks generated at the edge portion are collectively referred to as edge cracks.

  In addition, steel materials obtained by hot-rolling continuous cast slabs of high-Ni alloy steel are collectively referred to as rolled materials in the cases of continuous cast slabs, during hot rolling, and after hot rolling. .

  When an edge crack occurs in the material to be rolled, there is a high possibility that the material to be rolled will be broken by cold rolling, which is a subsequent process. When the material to be rolled breaks, not only the productivity in cold rolling decreases, but also the product yield decreases.

  Thus, various techniques have been proposed in the past to suppress the occurrence of edge cracks in high Ni alloy steel.

  For example, JP-A-60-159157 (Patent Document 1) discloses a technique of adding B (boron) for the purpose of improving the hot workability of high Ni alloy steel. Specifically, in the melting process of high Ni alloy steel, 0.001 to 0.03% by mass of B is added, and 0.005 to 0.3% by mass of Ti is added as necessary, so that hot workability of high Ni alloy steel is improved. To improve. However, since this technique uses expensive B, it causes an increase in the melting cost of high Ni alloy steel. In addition, depending on the application of high Ni alloy steel, the addition of B or Ti may adversely affect the properties required for the product.

  In JP-A-5-65607 (Patent Document 2), when hot rolling an alloy steel containing 65 to 85% by mass of Ni, four surfaces except for both ends in the longitudinal direction are surrounded by a metal plate. A technique is disclosed in which the first hot rolling is performed, the metal plate is then removed, and the second hot rolling is performed. However, since this technique performs hot rolling twice, a decrease in productivity is unavoidable, and the rolling cost increases.

  Japanese Patent No. 2751582 (Patent Document 3) states that when a continuous cast slab of Fe—Ni alloy is heated in a heating furnace, the surface temperature of the continuous cast slab reaches 15 ° C./min from 600 ° C. to 1000 ° C. Heating at the following heating rate, heating at a heating rate of 3 ° C / min or less from 1000 ° C to the soaking temperature, and the first pass in hot rolling at a surface temperature of 900 ° C or less and a reduction rate of 15 % Or more is disclosed. This technology suppresses the occurrence of thermal stress cracks by reducing the thermal stress due to non-uniform heating, suppresses the occurrence of ear cracks (that is, edge cracks), and also attempts to refine crystal grains by recrystallization. It is.

However, the thermal stress caused by the temperature difference of the continuous cast slab depends not only on the rate of temperature increase but also on the dimensions of the continuous cast slab. Therefore, the technique disclosed in Patent Document 3 can be applied only to extremely limited conditions. Further, the surface temperature of the continuous cast slab is not described with respect to the part for measuring the surface temperature, although it varies depending on the part to be measured (for example, upper and lower surfaces, side surfaces, etc.).
JP 60-159157 A Japanese Unexamined Patent Publication No. 5-65607 Japanese Patent No.2751582

  The present invention has been made in view of the problems of the conventional hot working technology of high Ni alloy steel as described above, and by heating the continuous casting slab of high Ni alloy steel uniformly by simple means. An object of the present invention is to provide a hot working method capable of suppressing the occurrence of edge cracks when hot-working the continuous cast slab while suppressing the occurrence of thermal stress cracks.

  The present invention is a method for hot working a continuous cast slab of high Ni alloy steel containing 25.0 to 85.0% by mass of Ni, and when the continuous cast slab is heated before the continuous cast slab is processed, This is a hot working method for high Ni alloy steel characterized in that the difference between the maximum and minimum values of the slab temperature is within 200 ° C.

  By applying the present invention, when a continuous cast slab of high Ni alloy steel is heated prior to hot working (for example, hot rolling, hot forging, etc.), It is possible to set an optimum temperature rising rate, and the continuous cast slab can be heated as uniformly as possible. As a result, it is possible to suppress the occurrence of thermal stress cracks due to heating of the continuous cast slab and to suppress the occurrence of edge cracks during hot working.

  According to the present invention, when a continuous casting slab of high Ni alloy steel is hot-worked (for example, hot rolling, hot forging, etc.), the continuous casting slab can be heated as uniformly as possible. Can suppress the occurrence of thermal stress cracking. As a result, it is possible to suppress the occurrence of edge cracks that start from thermal stress cracks. As a result, it is possible to improve the product yield by a simple means without requiring a large facility modification. Further, when hot rolling is performed as hot working, it is possible to suppress breakage of the material to be rolled in the subsequent cold rolling, so that productivity in cold rolling is also improved.

  Embodiments of the present invention will be described below.

  As described in the background section, the continuous cast slab of high Ni alloy steel is edged at the edge of the continuous cast slab (rolled material) during hot working (for example, hot rolling, hot forging, etc.). Although cracking is likely to occur, this is due to hot working because the grain structure of the high Ni alloy steel slab, which is melted and cast in an electric furnace, is weaker than that of ordinary steel such as carbon steel. This is primarily due to the fact that when a force is applied, cracks are likely to occur, particularly along the grain boundaries, at edge portions that extend both in the width direction and in the longitudinal direction and are locally deformed. This tendency is stronger in continuous cast slabs with a high cooling rate during casting.

  And, when heating the continuous cast slab of high Ni alloy steel prior to hot working, if the temperature of the continuous cast slab after heating is not uniform, thermal stress cracking is likely to occur, Due to the force applied, the influence becomes obvious, and edge cracks are more likely to occur.

  The inventors have studied the technology to suppress the occurrence of edge cracking in the hot working of continuous cast slabs of high Ni alloy steel. As a result, the continuous cast slabs are heated as uniformly as possible prior to hot working. It has been found that suppressing the occurrence of thermal stress cracking is extremely effective. The present invention has been completed based on such findings.

  The inventors have produced a continuous cast slab (150 mm thick, 1000 mm wide, 7000 mm long) produced by continuous casting after melting a high Ni alloy steel containing 36% by mass of Ni as shown in Table 1. During the process of heating, the transition of the surface temperature (upper surface, side surface) and internal temperature of the continuous cast slab was investigated. That is, after the continuous cast slab was cooled to 20 ° C., the temperature of the continuous cast slab was measured while being charged into a heating furnace and heated to 1200 ° C. The result is as shown in FIG.

  The surface temperature of the continuous cast slab was measured by attaching thermocouples to the top and side surfaces of the continuous cast slab. The internal temperature of the continuous cast slab was measured by opening a hole with a diameter of 4 mm and a depth of 100 mm in the center in the thickness direction of the continuous cast slab and embedding a thermocouple. The heating rate in the heating furnace was set so that the surface temperature of the upper surface of the continuous cast slab increased by 11 ° C. per minute (expressed as 11 ° C./min).

  As is apparent from FIG. 2, the internal temperature is always low and the surface temperature is high until the continuous cast slab charged in the heating furnace reaches 1200 ° C. The phenomenon shown in FIG. 2 can be taken into consideration when considering the temperature rising process in which heat input from the surface of the continuous cast slab in the heating furnace conducts heat toward the inside. Moreover, the surface temperature of the upper surface is slightly higher than the surface temperature of the side surface. In the heating furnace, the continuous casting slab is heated by radiation from the ceiling of the heating furnace and the furnace wall. Since the continuous cast slabs are lined up in the width direction at intervals of 50 mm, the heat input from the upper and lower surfaces of the continuous cast slab is the main, and the heat input from the side is slightly in the shadow of the adjacent continuous cast slab Because it will decrease.

  FIG. 3 shows the relationship between the thermal expansion coefficient and the temperature of the high Ni alloy steel (Ni content: 36 mass%) used for the measurement of the data in FIG. As is apparent from FIG. 3, this high Ni alloy steel has the property that the coefficient of thermal expansion increases as the temperature increases. When a temperature difference occurs in the continuous cast slab, thermal stress is generated due to the difference in thermal expansion, and the thermal stress causes cracks (ie, thermal stress cracks) along the grain boundaries of the cast structure of the continuous cast slab. To do. Even if cracks do not actually occur, cracks are likely to occur as they become apparent when cracked hot.

  When a continuous cast slab in which thermal stress cracking has occurred by heating is hot-worked, the thermal stress cracking becomes the starting point, and the cracks further expand to form edge cracks. In addition, there is a case where cracks are manifested and generated at a site where cracks are likely to occur due to hot working. Therefore, in order to suppress the occurrence of edge cracks, it is extremely effective to suppress the occurrence of thermal stress cracks due to non-uniform heating.

  However, the temperature difference generated in the continuous cast slab during heating is not necessarily constant, and depends on the dimensions of the continuous cast slab, the temperature increase rate, and the like. The transition of the temperature difference between the internal temperature and the surface temperature (upper surface) of the continuous cast slab (thickness 150 mm, width 1000 mm, length 7000 mm) shown in FIG. 2 is as shown in FIG. In addition, the transition of the temperature difference between the internal temperature and the surface temperature (upper surface) of continuous cast slabs (thickness 250mm, width 1000mm, length 7000mm) of the same material and the same thickness is the same (temperature increase rate: 11 ° C / min) The measured values are also shown in FIG.

  As is apparent from FIG. 4, even when heated under the same conditions, if the dimensions of the continuous cast slab differ, the temperature difference of the continuous cast slab also changes. That is, the temperature difference is larger when the continuous cast slab is thicker. This is because a thicker continuous cast slab requires a longer time for heat input from the surface to conduct heat to the inside.

  Next, after heating the two types of continuous cast slabs shown in FIG. 4 to 1200 ° C., hot rolling (rolling rate: 20%, rolling roll diameter: 850 mm in both upper and lower sides) occurred in the continuous cast slab. The depth of edge cracking was investigated. The relationship between the maximum temperature difference of continuous cast slab and the depth of edge cracking is shown in FIG. In addition, the maximum temperature difference of the continuous cast slab in FIG. 1 points out the maximum value of the temperature difference in FIG. That is, the maximum temperature difference is the difference between the maximum value and the minimum value of the continuous cast slab temperature that occurs between the start and end of heating. Moreover, the depth of an edge crack points out the maximum depth of the crack produced toward the inside from the surface of the continuous casting slab.

  As is apparent from FIG. 1, edge cracks are hardly observed in a continuous cast slab having a thickness of 150 mm. However, in a continuous cast slab with a thickness of 250 mm, an edge crack of about 60 mm has occurred.

  The inventors further performed the same measurement by changing various dimensions of the continuous cast slab and the heating rate of the heating furnace, and determined the maximum temperature difference of the continuous cast slab. Then, the same hot rolling was performed and the depth of the edge crack was investigated. The results are also shown in FIG.

  As is clear from Fig. 1, when the maximum temperature difference of continuous cast slabs is 200 ° C or less, the depth of edge cracking was 5 mm at maximum, whereas when the maximum temperature difference exceeds 200 ° C, the depth of edge cracking Increases rapidly. Therefore, in order to suppress the occurrence of edge cracks, the maximum temperature difference of continuous cast slabs must be 200 ° C or less.

  Fig. 1 shows data obtained by unifying hot rolling at a rolling reduction of 20% and rolling roll diameter: 850 mm in both the upper and lower sides. According to the inventors' research, the rolling reduction and rolling roll are shown. Even if the diameter is changed, edge cracking can be similarly suppressed if the maximum temperature difference of the continuous slab is set to 200 ° C or less. In addition to hot rolling, other hot working techniques (for example, hot forging, etc.) have also been confirmed that edge cracking can be suppressed if the maximum temperature difference of continuous cast slabs is reduced to 200 ° C or less. It was done.

  Here, the description is based on the temperature of the continuous cast slab measured using a thermocouple. However, the measurement may be performed by attaching a thermometer such as a radiation thermometer on the heating furnace side, or heating the continuous cast slab. It is well known that given the conditions (that is, the set temperature of each zone in the heating furnace and the duration of time), the temporal transition of the temperature of the continuous cast slab can be calculated by solving the heat conduction equation by the difference method or the like. Therefore, in carrying out the present invention, the optimum temperature rise such as the set temperature of each heating zone of the heating furnace in which the maximum temperature difference of the continuous cast slab generated between the start and the end of heating is within 200 ° C in advance. It is preferable to set a pattern.

  According to the study by the inventors, the relationship shown in FIG. 1 is not limited to high Ni alloy steel having a Ni content of 36% by mass, but also holds for high Ni alloy steels containing 25.0 to 85.0% by mass of Ni. It was confirmed. These high Ni alloy steels are, for example, high Ni alloy steels (Ni content: 42% by mass) used in IC lead frames as shown in Table 2, high Ni alloy steels used in bimetals as shown in Table 3 ( Ni content: 20% by mass) and high Ni alloy steel (Ni content: 85% by mass) used for electronic materials such as magnetic shields as shown in Table 4.

  The continuous cast slab of high Ni alloy steel having the composition shown in Table 4 was heated using a heating furnace composed of the first to third heating zones and the four soaking zones. Table 5 shows the dimensions of the continuous cast slab and the heating conditions in the furnace (each set temperature and time). However, in order to make the temperature of the continuous casting slab when extracted from the heating furnace the same, all the soaking zones were set at 1200 ° C and the in-furnace time was 60 minutes. The invention examples in Table 5 are examples where the maximum temperature difference satisfies the conditions within 200 ° C., which is the range of the present invention, and the comparative example is an example where the maximum temperature difference exceeds 200 ° C. In addition, the maximum temperature difference of the continuous cast slab shown in Table 5 is a value obtained by solving the heat conduction equation by the difference method from the dimensions of the continuous cast slab and the heating conditions in the heating furnace.

  After the continuous cast slab is extracted from the heating furnace, it is hot rolled (total rolling reduction: 98%) in a continuous thin sheet hot rolling line equipped with a roughing mill and a finish rolling mill. Manufactured. Table 5 shows the thickness of the high Ni alloy steel strip.

  Next, the high Ni alloy steel strip was wound into a coil as a high Ni alloy hot rolled steel strip on the exit side of the finish rolling mill, and the presence or absence of edge cracking was visually observed over the entire length in the longitudinal direction of the high Ni alloy steel strip. . In the portion where the occurrence of edge cracks was remarkably recognized, the distance from the width edge of the high Ni alloy steel strip to the edge crack edge (that is, the depth of the edge crack) was measured. Edge cracks with a depth of less than 10 mm were evaluated as good (◯) and those with a depth of 10 mm or more were evaluated as defective (×). The results are as shown in Table 5.

  In all of the inventive examples, the evaluation of edge cracking was good (◯), but the comparative example was bad (x).

It is a graph which shows the relationship between the maximum temperature difference of a continuous casting slab, and the depth of an edge crack. It is a graph which shows transition of the temperature of a continuous casting slab. It is a graph which shows the relationship between the thermal expansion coefficient of high Ni alloy steel, and temperature. It is a graph which shows transition of the temperature difference of a continuous casting slab.

Claims (1)

A method of hot-working a continuous cast slab of high Ni alloy steel containing 25.0 to 85.0% by mass of Ni, wherein when the continuous cast slab is heated prior to processing the continuous cast slab, A hot working method for high Ni alloy steel, characterized in that the difference between the maximum and minimum values is within 200 ° C.

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