JPH0448865B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a stainless alloy steel, particularly a stainless alloy steel that generates fewer surface defects due to cracks during hot rolling, and a method for heating a slab thereof. (Conventional technology) One of the main surface defects that occur when hot rolling stainless steel alloys is normal line sliver (sliver).
There are linear scratches called. These defects do not disappear even after cold rolling, and in many cases, minute sagging defects that cannot be detected in hot-rolled coils become noticeable during cold rolling, posing a major problem. For stainless steel alloys, where surface quality is important, the above defects are fatal and reduce yields.
This has led to a significant increase in costs. Although the theoretical elucidation of the above-described defect occurrence is not necessarily sufficient, it is generally believed that it is caused by the embrittlement of prior austenite grain boundaries during pseudo-solidification during casting. This is because sulfur and oxygen are concentrated in the prior austenite grain boundaries, and sulfides and oxides are generated during heating, which promotes embrittlement during hot rolling, causing small cracks and sagging defects. It is estimated that it has become popular. Therefore, in order to prevent this, it was necessary to reduce the sulfur and oxygen contents in the steel, or to make it harmless. As mentioned above, in order to prevent the occurrence of surface flaws due to cracks during hot rolling, it is effective to reduce the content of sulfur and oxygen in steel as much as possible.
In reality, the cost is high and there are limits to how much it can be reduced. Conventionally, as a method for preventing cracking during hot rolling, as described in Japanese Patent Application Laid-open No. 57-16153,
δCal (calculated value of δ ferrite amount) = 3 (Cr + Mo +
1.5Si+0.5Nb)-2.8(Ni+1/2Mn+1/2Cu)-84
As described in Japanese Patent Application Laid-open No. 57-127506, the method of reducing δCal determined by (C+N)-19.8 to 4.0% or less, the heating degree ΔT of molten steel during continuous casting (difference between liquidus temperature and casting temperature) ) and the N value, a method of adjusting the slab heating temperature in a heating furnace, and a method of adding a special component such as Ti to improve hot strength are known. The first method is to suppress the calculated value δCal of δ ferrite, which is the cause of embrittlement during hot rolling, to 4.0% or less. However, in reality, the components of stainless alloy steel are determined taking into consideration mechanical properties, corrosion resistance, etc., and it is difficult to satisfy the δCal value near O for many steel types. Furthermore, it is difficult to achieve 100% target composition during melting of stainless alloy steel, which poses a practical problem. The second method is a heating furnace operation in which ΔT and N amount change during each charge or during continuous casting, and it is necessary to simultaneously heat a large number of slabs with miscellaneous covering histories, so all slabs are controlled. This is practically impossible. Furthermore, addition of a third special component, for example, addition of Ti, increases the number of defects such as stringer defects due to the increase in inclusions, and the addition of expensive special alloy components significantly increases costs. (Problems to be Solved by the Invention) The present invention aims to provide a stainless steel alloy that can render the sulfur and oxygen in the steel harmless and prevent the occurrence of surface flaws due to cracks during hot rolling as described above. It is. (Means for solving the problems) The present invention provides C: 0.001 to 0.20wt% (hereinafter simply %
), Si: 0.10-5.0%, Mn: 0.1-11.0%,
P: 0.050% or less, S: 0.020% or less, Cr: 11.0~
30.0%, Ni: 2.0~30%, N: 0.001~0.160%,
Contains O: 0.015% or less, and further contains Mo: 5.0% or less, Cu: 3.0% or less, Nb:
1.0% or less and Sn: 0.10% or less Type 1 or 2
In addition, as elements for preventing surface cracking during hot rolling, Ti: 0.05% or less, Zr: 0.10% or less, Ca:
Contains one or more of 0.006% or less and B: 0.05% or less, and contains 0.001 to 0.005% sol Al,
It is characterized by a hot-rolled stainless steel alloy with minimal surface cracks, the remainder of which is Fe and unavoidable impurities. (Function) According to the present invention, it has excellent corrosion resistance and can significantly reduce surface defects due to cracks that occur during hot rolling. Next, the reasons for limiting each component will be described. From the point of view of corrosion resistance, the lower the value of C, the better, and from the point of view of heat resistance, the higher the value, the better.
It was set at 0.001 to 0.20%. From the viewpoint of processability, it is better to have a low Si content, but if it is too low, deoxidation will be insufficient. Therefore, the lower limit
The upper limit was set at 0.1%, and since embrittlement becomes significant when it exceeds 5%, the upper limit was set at 5.0%. If Mn is too low, processability deteriorates and deoxidation becomes insufficient, so the lower limit was set at 0.1%. In addition, the more the austenite is, the more stable the austenite becomes, and the processability becomes better.
Although it is good for corrosion resistance, the effect reaches saturation, so the upper limit was set at 11.0%. As P increases, workability and corrosion resistance deteriorate, so the upper limit was set at 0.050%. S deteriorates hot workability. In particular, it segregates at austenite grain boundaries during solidification and becomes the main cause of linear heave defects that occur during hot rolling. Therefore the upper limit is
Should be 0.020%. For the same reason as S, it is better for O to be lower, and the upper limit is
It was set to 0.015%. Cr is 11.0% due to its corrosion resistance as stainless steel.
is the lower limit, and the upper limit was set at 30% in consideration of brittleness and processing difficulties. Ni has a relationship with the amount of Cr, but this relationship and
Considering the stability of the austenite phase, the content was set at 2.0% or more. The upper limit was set at 30% due to cost considerations. Although it is better to have a large amount of N in order to stabilize the austenite phase, the upper limit should be set in consideration of the limit of solid solution amount.
0.160%, and the lower limit is 0.001 due to manufacturing limitations.
%. Mo is an element that improves corrosion resistance, depending on the application.
It can be selectively added in amounts below %. Cu is effective as an element for improving corrosion resistance, but
From the viewpoint of workability and cracking during rolling, it can be added selectively with an upper limit of 3.0%. Nb is effective as an element for improving corrosion resistance, but
Due to the problem of embrittlement, it can be added selectively with an upper limit of 1.0%. Sn can be selectively added as a corrosion resistance improving element at 0.10% or less at which the effect is saturated. Ti is selectively added as an element to prevent surface cracking during hot rolling, and has the effect of preventing surface cracking during hot rolling by forming stable sulfides and at the same time making it finer.However, if added in large amounts, titanium stringer flaws may occur. Since it can also be a cause of oxidation, it should be added in small amounts selectively depending on the application, and the upper limit is set at the saturation point of 0.05%. Zr is used as an element to prevent surface cracking during hot rolling.
It has a similar effect, but considering cost and effect saturation, it is added at an upper limit of 0.10%. Ca is effective in controlling the form of inclusions as an element to prevent surface cracking during hot rolling, but since it has the effect of deteriorating corrosion resistance, it is added selectively depending on the application, with an upper limit of 0.006%. B is selectively added as an element to prevent surface cracking during hot rolling, but if it exceeds 0.05%, the intergranular corrosion resistance is significantly reduced, so B is selectively added at 0.05% or less. sol Al is 0.001-0.05% as stated below
The lower limit is set at 0.001% because its inclusion has a remarkable effect on preventing surface cracking during hot rolling.
The upper limit was set at 0.005%. Taking SUS304 as an example, Figure 1 shows the incidence rate of surface defects (sludge defects and surface defects caused by inclusions) in hot-rolled coils after sol Al and hot rolling (number of sheets with defects/total number of sheets rolled). Indicates the relationship between As is clear from Fig. 1, by containing 0.001 to 0.005% Al as sol Al, the oxygen that enters from the slab surface during heating and the oxygen that originally exists in the steel are absorbed into the fine Al ₂ crystal grains. It is made harmless and fixed by dispersing it as _O3 . The amount of sol Al is
If it exceeds 0.005%, Al in the molten steel during casting.
is combined with oxygen in molten steel or foreign oxygen as Al ₂ O ₃ and precipitates in the continuous casting tundish nozzle. This results in blockage of the nozzle and makes casting very difficult. Conventionally, Al has been used together with Si as a deoxidizing agent during melting, but since Al causes nozzle clogging during continuous casting, a small amount is added to prevent it from remaining as sol Al. In the present invention,
A small amount of sol Al that does not cause nozzle clogging
It was discovered that surface flaws can be reduced by leaving . Figure 2 shows the results of a Greeble test on the effect of sol Al addition using SUS304 as an example. The components of the sample materials are shown in Table 1. 1300 each specimen
After heating to ℃ for 20 seconds and holding at this temperature for 50 seconds,
After cooling with water at a cooling rate of -100°C/min, a Greeble test was performed. As is clear from this test, the deformability of the steel of the present invention at high temperatures is very good. This is presumed to be because internal oxidation, particularly oxidation at prior γ grain boundaries, is suppressed during high temperature heating.

[Table] Also, according to the present invention, it is preferable to set the O ₂ concentration in the atmosphere in the heating furnace to 0.5 to 5.0% when heating the slab, and the reason for this is that when the O ₂ concentration is 5.0% or more,
The oxidation of the slab becomes significant, and as a result, the surface of the slab becomes more brittle.On the other hand, if it is set to 0.5% or less, the amount of scale-off on the surface of the slab decreases, making it difficult to remove defects that originally exist on the surface of the slab. It is from. Figure 3 shows the results of an actual test.
This Figure 3 shows the relationship between the O ₂ concentration in the atmosphere in the heating furnace and the incidence of linear sagging defects in cold rolled coils (number of coils with defects/total number of coils)% for SUS304. From this, it is generally determined that when the O ₂ concentration in the heating furnace is outside of 0.5 to 5.0%, the occurrence of curling defects in cold-rolled coils increases significantly. In addition,
In the above test, the furnace slab surface temperature was
The temperature was set at 1200-1250°C and held for 3.5-4.0 hours. Moreover, the content of sol Al was 0.026% or less. The O ₂ concentration inside the furnace is controlled by detecting the exhaust gas O ₂ concentration,
It can be adjusted by controlling the fuel and air ratio of burner combustion. (Example) As an example of the present invention, Table 2 shows a comparison of SUS304 series alloy steel of the present invention manufactured by continuous casting with comparative steel. In order to offset the influence of [O], [Si] was increased or decreased in the comparison steel according to the Al level. The slab heating temperature was 1200 to 1250°C, and the O ₂ concentration in the heating furnace was controlled within the range of 0 to 8%.

【table】

[Table] Figure 4 shows the percentage of occurrence of linear scratches (number of coils with scratches/total number of coils) in cold-rolled coils produced by heating in a heating furnace and hot rolling cooling. show. As is clear from FIG. 4, the steel of the present invention is
Scratches can be significantly reduced. In Figure 4, when sol Al is higher than 0.005%, nozzle clogging occurs easily during continuous casting, and Al ₂ O ₃
This is not preferable as it increases the number of inclusions. (Effects of the Invention) According to the steel of the present invention, it has become possible to suppress the increase in cost due to the addition of a process for polishing away flaws found in conventional hot rolled coils using a grinder. Furthermore, the reduction in yield due to the occurrence of defects in cold-rolled coils was significantly reduced.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between the amount of sol Al and the surface defect occurrence rate, Figure 2 is a graph showing the relationship between the diameter reduction rate of the specimen in the Greeble test and the test temperature, and Figure 3 is the graph inside the heating furnace. A graph showing the influence of O ₂ concentration in the atmosphere on linear sagging defects in cooling coils. FIG. 4 is a graph showing the incidence of linear sagging defects due to hot rolling of cooling coils of the present invention steel and conventional steel. .

Claims (1)

[Claims] 1C: 0.001 to 0.20wt% (hereinafter simply expressed as %),
Si: 0.10-5.0%, Mn: 0.1-11.0%, P: 0.050%
Below, S: 0.020% or less, Cr: 11.0-30.0%,
Ni: 2.0~30%, N: 0.001~0.160%, O: 0.015
% or less, and further contains Mo: 5.0% or less, Cu:
3.0% or less, one or more types of Nb: 1.0% or less and Sn: 0.10% or less, Ti: 0.05% or less,
Zr: 0.10% or less, Ca: 0.006% or less, and B:
A stainless steel alloy steel containing 0.05% or less of one or more sol Al, and 0.001 to 0.005% of sol Al, with the balance being Fe and unavoidable impurities, and exhibits few surface cracks when hot rolled. 2 C: 0.001-0.20%, Si: 0.10-5.0%, Mn:
0.1-11.0%, P: 0.050% or less, S: 0.020% or less, Cr: 11.0-30.0%, Ni: 2.0-30%, N:
Contains 0.001 to 0.160%, O: 0.015% or less, Mo: 5.0% or less, Cu: 3.0% or less, Nb: 1.0%
One or more of the following and Sn: 0.10% or less, Ti: 0.05% or less, Zr: 0.10% or less, Ca:
Contains one or more types of 0.006% or less and B: 0.05% or less, and sol Al from 0.001 to
When heating a slab of stainless alloy steel containing 0.005% O 2 and the remainder consisting of Fe and unavoidable impurities, the O ₂ concentration in the atmosphere inside the slab heating furnace is set to 0.5%.
A method for heating slabs of stainless alloy steel with less surface cracking during hot rolling, characterized by reducing the temperature to ~5.0%.