JPH0332604B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for cooling continuously cast slabs of niobium-ferrite stainless steel (hereinafter referred to as "continuous slabs"). (Prior art) Ferritic stainless steel, represented by SUS430 steel, does not contain large amounts of expensive Ni and is therefore inexpensive, so it can be used in relatively mild corrosive environments such as automobile exterior parts and architectural interior parts, and has a beautiful surface. It is widely used in applications that require. However, SUS304
Its uses have been limited because its corrosion resistance is generally inferior to that of austenitic stainless steel, which is commonly known as steel. Further, ferritic stainless steel has the disadvantage that when it is subjected to drawing or tension processing in the manufacture of parts, an uneven striped pattern called ridging or roping occurs, which significantly impairs the appearance. In response, the present applicant invented a ferritic stainless steel with a combined addition of Nb and Cu to improve rust resistance, especially with a bright sintered finish (Japanese Unexamined Patent Application Publication No. 57-1999).
140860) Furthermore, he invented a method for manufacturing ferritic stainless steel sheets with virtually no ridging by strictly controlling the N content and other components and limiting the hot rolling conditions and rough annealing conditions (Patent Application No. 140860). −
82281). The ferritic stainless steel according to these inventions can not only be used in exactly the same way as SUS304 steel, but also because it does not deteriorate the corrosion resistance of the heat-affected zone of the weld and is not susceptible to stress corrosion cracking. This significantly expands the scope of use of inexpensive ferritic stainless steel, allowing it to be used outside of the field. However, although this Nb-containing ferritic stainless steel has many advantages as described above, it has the disadvantage that slabs produced by continuous casting tend to crack during cooling. In other words, continuously cast slabs of Nb-containing ferritic stainless steel tend to crack laterally during cooling or break during reheating for hot rolling, causing accidents that force rolling to be stopped. . In addition, even if the cracking is slight and rolling is completed, significant ridge-like flaws and through holes are likely to occur, which will reduce the commercial value. On the other hand, in the past, temperatures were lowered from around 800°C using slow cooling furnaces, etc.
A method of slowly cooling to around 100°C, and a method of reheating without cooling to below 300°C since the transition temperature of the slab is around 300°C (Japanese Patent Laid-Open No. 58-39732). Furthermore, since the tensile strength of the slab becomes smaller than the thermal stress at temperatures below 150°C, a method of reheating without cooling to below 150°C has been disclosed (Japanese Patent Laid-Open Publication No. 128464/1982). All of these methods are impractical, incomplete, and not optimal. That is, the method of first slow cooling is ineffective even if the rate is reduced to about 5° C./hr, and even if it is effective to reduce the rate below that, it is disadvantageous in terms of cost. The measure of not cooling the slab below 300°C is considered to be an appropriate method to prevent cracking based on the concept described below, but the surface quality of the slab cannot be polished at temperatures above 300°C, so the emphasis is on surface quality. However, stainless steel has significant drawbacks. In addition, the measures to prevent cooling below 150℃ are as follows:
It is not a perfect method, as the thermal stress itself varies greatly depending on the trace components of the steel, the casting method, and the cooling method. Furthermore, the fact that it cannot be made into cold pieces significantly limits the degree of freedom in the manufacturing process, and can be said to be a serious drawback from a manufacturing standpoint. (Objective of the Invention) An object of the present invention is to provide a practical method for cooling a slab without the above-mentioned drawbacks. (Structure and operation of the invention) As a result of repeated investigations into the causes of cracks in continuously cast slabs, the present inventors found that plate-shaped Nb carbides were found in the grain boundaries of continuous cast slabs of Nb-containing ferritic stainless steel. It was found that coarse precipitates were formed in the shape of fern leaves, and cracks were generated starting from these areas. Furthermore, it was discovered that although intergranular cracks occur near the starting point, large overall cracks are mainly brittle fractures, and that many fine intergranular cracks exist in the slab in addition to large cracks. From these facts, slab cracking is caused by intergranular cracking of grain boundaries with coarse welded leaf-like precipitates due to thermal stress during cooling, stress concentrates at the cracked part, and in this state the temperature drops below the transition temperature. It was considered that this would lead to large brittle cracks if this occurred. Based on these considerations, the measure of not lowering the temperature below the transition temperature will prevent large cracks from occurring even if small grain boundary cracks occur (such micro cracks are crimped during rolling, so there is no problem). Therefore, it can be said that it is a very appropriate method and is in accordance with the theory. The transition temperature of a continuous slab of Nb-containing ferritic stainless steel is approximately 300°C when measured in a laboratory, so it is easy to think of measures to prevent the temperature from dropping below 300°C. On the other hand, based on these considerations, slab cracking is not a ductile fracture that occurs simply due to a difference in tensile strength. It is also well understood that cracking of slabs cannot be prevented even if measures are taken not to lower the temperature below ℃. Based on the above considerations, the present inventors investigated various ways to eliminate the starting points of cracks in Nb-containing ferritic continuous stainless steel slabs, and found that the susceptibility to starting points of cracks depends on the form of precipitates precipitated at grain boundaries. The present invention was accomplished by discovering that there is a large relationship between the following. The gist of the present invention is that Nb is 0.1% or more,
Continuously cast Nb-containing ferritic stainless steel slab containing 10% or more of Cr can be continuously cast without cooling to below 300℃.
Once at a temperature of 800℃ or higher and 1300℃ or lower for 1 hour or more10
Heating time or less, then average cooling rate 40℃/hr
A method for cooling a continuous cast piece of niobium ferrite-containing stainless steel, characterized by cooling it to 300°C at the following cooling rate to obtain a cold piece. The following is an explanation based on the survey results. When the cracked slab was examined in detail, it was found that the grain boundaries where the cracks occurred mainly consisted of fern leaf-like precipitates, and no cracks occurred at the grain boundaries where there were many spherical or needle-shaped precipitates. Based on this, we investigated various methods including rolling and other methods to change the state of the fern leaf-like precipitates.
The results showed that this was possible by heating for more than 1 hour. At 800°C or higher, the form of a considerable portion of the precipitate changes, but some portions remain in the form of leaves, so a temperature of 900°C or higher is desirable because it can change the overall form of the precipitate. However, simply changing the form of the precipitate in this way was insufficient. In other words, it is assumed that some carbon is still dissolved in the steel at temperatures above 800°C, and during the cooling process, carbon forms in the form of weeping lobes or forms that have the same degree of cracking susceptibility as weeping lobes. This seems to be because it precipitates out as a precipitate. Therefore, as a result of examining the cooling conditions, it was found that if gradual cooling is performed at an average cooling rate of 40°C/hr or less from the heating temperature to 300°C, the shape of the grain boundary precipitates will change from spherical to acicular, and The amount of precipitation within was also increased. As a result, cracks no longer occur even when the slab is cooled to room temperature. Next, reasons for limiting the constituent elements of the present invention will be shown. If the amount of Nb is less than 0.1%, leaf-like precipitates will not form at the grain boundaries of the slab, and the frequency of cracking of the slab will be low, so it is set at 0.1% or more. If the Cr content is less than 10%, the basic corrosion resistance of stainless steel cannot be maintained, so the content was limited to 10% or more. The handling temperature of the slab before heating was limited to reheating without cooling to below 300°C, as cooling to below 300°C would cause it to become brittle and cause cracks to become apparent. If the heat treatment temperature is less than 800℃, it will take a long time to change the form of the leaf-like precipitates, and if the temperature exceeds 1300℃, the grains will become coarser and the quality after cold rolling will deteriorate. The temperature was 800℃ or higher and 1300℃ or lower. If the heating time is less than 1 hour, the effect will be small, and if it exceeds 10 hours, the quality of the final product will deteriorate due to coarsening of crystal grains, and it will be disadvantageous in terms of cost. The cooling rate was set to 40°C/hr or less, since the effect of changing the form of the leaf-like precipitates in the weeping becomes small if the cooling rate exceeds 40°C/hr. (Example) Next, an example will be shown. 16%Cr−0.4Nb, 19%Cr−0.4%Nb and 19%
Continuously cast slabs of three types of Nb-containing ferritic stainless steels with Cr-0.6%Nb as the representative components were cooled to cold slabs using various cooling methods, and then heated again and hot rolled. As a result, Table 1 shows whether or not cracks occurred in the slabs. In Table 1, No.1~
The heat-treated slab in No. 6 is naturally air-cooled during casting to reduce the surface temperature.
It was placed in a heating furnace that had been preheated to a predetermined temperature between 350 and 700°C and reheated. After heating, it was controlled and cooled to 250°C at a predetermined average cooling rate, and then air-cooled to room temperature by natural air cooling to obtain a cold piece. The unheated material No. 7 was air-cooled to room temperature as it was. The ratio between the number of cracked slabs of all steel types and the total number of rolled slabs is shown in parentheses in the column of crack incidence in Table 1 as a fraction. In the method of the present invention, no cracking occurred even when the pieces were made into cold pieces.

【table】

[Table] (Effects of the invention) As detailed above, the present invention makes it possible to make Nb-containing ferritic stainless steel, which is prone to cracking, into cold pieces in the same way as through-threaded stainless steel. It can be manufactured without taking away the degree of freedom, and its industrial benefits are enormous.

Claims (1)

[Claims] 1 A continuously cast slab of Nb-containing ferritic stainless steel containing 0.1% or more Nb and 10% or more Cr is heated once at a temperature of 800°C or higher and 1300°C or lower for 1 hour or more and 10 hours or less, without cooling it to 300°C or lower. After that, the temperature is reduced to 300℃ at an average cooling rate of 40℃/hr or less.
1. A method for cooling a continuously cast piece of niobium ferrite-containing stainless steel, the method comprising cooling it to a cold piece.