JPS62206B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for hot rolling a continuous cast slab of ferritic stainless steel containing niobium and copper (hereinafter referred to as "continuous cast slab"). (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 to maintain beautiful surfaces. Widely used for required applications. However, its applications have been limited because it generally has inferior corrosion resistance compared to austenitic stainless steels such as SUS304 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 in which the rust resistance was particularly improved by bright annealing by adding a combination of Nb and Cu (Japanese Unexamined Patent Application Publication No. 1983-1999).
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). −
No. 82281). This ferritic stainless steel has
Additive elements that have been recognized to improve corrosion resistance
By adding Ti, Cu, and Ni, further improvement in corrosion resistance can be expected. Therefore, by adding these elements according to the required corrosion resistance level, SUS304
It can exhibit corrosion resistance from the level of steel to an even higher level. 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 and Cu 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 and Cu ferrite stainless steel may crack laterally during cooling or break during reheating for hot rolling, causing accidents that force rolling to be stopped. Cheap. In addition, even if the cracking is slight and rolling is finished, significant ridge-like flaws and through holes are likely to remain, reducing the product's commercial value. On the other hand, in the past, temperatures were lowered from around 800°C using slow cooling furnaces, etc.
A method of slow cooling to around 100℃ and a method of reheating without cooling to below 300℃ since the transition temperature of the slab is around 300℃ have been proposed (Japanese Patent Laid-Open No. 1983-39732). There is. In addition, for Nb-containing steel that does not contain Cu, the tensile strength of the slab is 150℃.
A method of reheating without cooling to below 150℃ (Japanese Patent Laid-Open No. 54
128464), but this method is not effective in the case of the Nb- and Cu-containing ferritic stainless steel according to the present invention, even when the Nb-containing ferritic stainless steel is used. These methods are not effective even if the slow cooling rate is lowered to about 5°C/hr, and even if they are effective by lowering the slow cooling rate, they are not practical because they are 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 is important as it is impossible to maintain the surface of the slab at temperatures above 300°C. Stainless steel, which is made of aluminum, has significant drawbacks and is also impractical. (Object of the Invention) An object of the present invention is to provide a practical method for rolling slabs without the above-mentioned drawbacks. (Structure and operation of the invention) As a result of various investigations into the causes of cracks in continuously cast slabs, the present inventors found that plate-shaped Nb carbides were present at the grain boundaries of continuous cast slabs of Nb-containing Cu ferritic stainless steel. It was found that coarse precipitates were formed in the shape of leaves, and that cracks were generated starting from these areas. Furthermore, it was discovered that although grain boundary cracks occur near the base 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 this idea, the measure of not lowering the temperature below the transition temperature does not lead to large cracks, even if fine grain boundary cracks occur (such fine 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. Contains Nb,
The transition temperature of a Cu ferritic stainless steel continuous slab is approximately 300°C when measured in the laboratory, so 300°C
It is easy to think of measures to prevent the temperature from dropping below ℃. On the other hand, based on this consideration, slab cracking is not a ductile fracture caused by a mere difference in tensile strength, so even if the tensile strength of Nb-containing ferritic stainless steel slab becomes smaller than thermal stress at 150℃ or less, It is also well understood that cracking of slabs cannot be prevented even if measures are taken not to lower the temperature below 150℃. Based on the above idea, the present inventors investigated various methods for lowering the transition temperature of continuous cast slabs of Nb and Cu ferritic stainless steel, and as a result, they discovered that there is a relationship with the cooling rate of the slab, and the present invention has been made. accomplished. In other words, a continuously cast piece of ferritic stainless steel is
Generally, it is cut into slabs at 600 to 800℃ and then heated to 10 to
It is naturally air-cooled for 15 hours and becomes a so-called cold piece with a surface temperature of less than 100℃. Of course, it is quite possible that the cooling time until it becomes a cold slab is affected by the thickness of the slab, but there is not that much difference between slabs with a thickness of 150 to 350 mm. However, it is easy to slow down the cooling rate by immediately charging the cut slab into a heat retention furnace or the like. Figure 1 shows the components shown in Table 1 with a thickness of 215 mm.
Nb, Cu ferritic stainless steel slabs at 700℃
These are the results of measuring the transition temperature of a cold piece cooled by changing the cooling rate from 200℃ to 200℃. Since the slow cooling was carried out in a heat retention furnace, the curve almost coincides with the cooling curve obtained by extending the time axis of the natural cooling curve. While the transition temperature is 300°C when left to cool naturally, it shifts to a lower temperature side by slow cooling, and in particular, decreases to about 150°C by slow cooling for 35 hours or more. Figure 2 shows the components shown in Table 1, including Nb with a thickness of 215 mm.
Cut from Cu ferrite stainless steel slab
These are the results of measuring the transition temperature using a 50 mm x 50 mm x 100 mm sample in order to determine the temperature range that should be slowly cooled. As shown in the figure, we measured the cooling time separately from 700℃ to 400℃ and from 400℃ to 200℃.
It was found that the transition temperature can be expected to decrease by cooling from 700°C to 400°C at an average cooling rate of 18°C/hr or less, and from 400°C to 200°C at an average cooling rate of 15°C/hr or less. . If there is insufficient cooling time for either the high temperature side or the low temperature side, the effectiveness will be drastically reduced. Also
The cooling curve from 700℃ to 400℃ was good if it was close to the curve in the heat retention furnace. However, although we experimentally conducted conditions close to quenching locally (this is possible since the test material is small), there was no significant effect.
The average cooling rate from 700℃ to 400℃ should be 18℃/hr or less, and the average cooling rate from 400℃ to 200℃ should be 15℃.
By setting the average cooling rate to less than ℃/hr, the transition temperature became less than 200℃.

[Table] Based on the above findings, continuous cast pieces of ferritic stainless steel containing Nb and Cu were heated at 18℃/
We have invented a rolling method that does not cause cracking of slabs, which involves hot rolling by cooling from 400°C to 200°C at an average cooling rate of 15°C/hr or less, and reheating without cooling to less than 200°C. As a result, it has become possible to perform surface treatment of slabs at 200 to 300°C. Next, the reasons for limiting the starting steel components will be briefly explained. Since C and N become carbonitrides of Nb and cause surface flaws and deteriorate corrosion resistance, they were set to 0.05% or less and 0.04% or less, respectively. Cr was set in the range of 10 to 30% as the amount of Cr normally used in ferritic stainless steel. Si and Mn can be added from the viewpoint of strength, but if they are added in large amounts, hot workability deteriorates, so the upper limit for both was set at 1%. Nb is 0.2% or more and 8% of the sum of C% and N% in order to fix C and N and improve corrosion resistance.
It was more than doubled. However, since the effect does not change even if 1.0% or more is added, the upper limit was set at 1.0%. Cu improves corrosion resistance when combined with Nb, but when added in large amounts, hot workability deteriorates, so it was set at 0.2 to 1.0%. Additionally, Ti, which can be added as necessary,
If Ni and Mo are added in large amounts, they are not only disadvantageous in terms of cost but also deteriorate hot workability, so the upper limits were set to 1% or less, 1% or less, and 3% or less, respectively. Next, the present invention will be further explained with reference to Examples. Nb-containing, with the components shown in Table 2 as representative components,
Cu ferritic stainless steel continuous cast slabs were cooled using various cooling methods, surface treated between 200 and 300°C, and then hot rolled. the result,
Table 3 shows the presence or absence of cracks in slabs ( )
The number of rollings is shown in the figure. While no cracking occurred in the method of the present invention, cracking occurred in all of the comparative methods except for the method (No. 5) in which the temperature was allowed to cool naturally and the temperature was not lowered below 300°C. Further, although no cracking occurred in method No. 5, the surface could not be cleaned at all, and a great deal of work had to be done to remove surface flaws after hot rolling.

【table】

[Table] (Effects of the Invention) As described above, according to the present invention, the remarkable effect of hot rolling continuous cast slabs of niobium and copper-containing ferritic stainless steel without causing any cracks can be achieved, which is useful for industrial use. There is an extremely large amount of benefits to be had.

[Brief explanation of the drawing]

Figure 1 shows the relationship between the transition temperature and the time required for cooling a 215 mm thick Nb- and Cu-containing ferritic stainless steel slab from 700℃ to 200℃.
The figure also shows the relationship between the cooling rate and transition temperature of ferritic stainless steel containing Nb and Cu.

Claims (1)

[Claims] 1 C0.05% or less, N0.04% or less, Cr10-30%,
Si1% or less, Mn1% or less, Nb0.2-1.0%, and C
After casting a continuously cast slab of Nb and Cu ferritic stainless steel containing 0.2 to 2% of Cu, the rest consisting of Fe and unavoidable impurities.
Average cooling rate 18℃/hr or less from 700℃ to 400℃, average cooling rate 15℃/hr from 400℃ to 200℃
A method for rolling a continuous cast piece of ferritic stainless steel containing niobium and copper, characterized by cooling it at a temperature below, heating it again without cooling it below 200°C, and hot rolling it. 2 C0.05% or less, N0.04% or less, Cr10-30%,
Si1% or less, Mn1% or less, Nb0.2-1.0%, and C
% and N%, Cu0.2 to 2%, and one or more of Ti1% or less, Ni1% or less, Mo3% or less, and the rest consists of Fe and unavoidable impurities. After casting, continuously cast slabs of Nb and Cu ferrite stainless steel are cooled from 700°C to 400°C at an average cooling rate of 18°C/hr or less, and from 400°C to 200°C at an average cooling rate of 15°C/hr or less. ,and
A method for rolling continuous cast pieces of ferritic stainless steel containing niobium and copper, characterized by hot rolling by heating again without cooling to below 200°C.