JPH0543769B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for heating a stainless alloy steel slab before hot rolling, and in particular, a method for reducing the occurrence of sagging defects caused by small cracks that occur during hot rolling of stainless alloy steel. The present invention relates to a method for heating a stainless steel alloy slab. (Prior Art) One of the main surface defects that pose a problem when hot rolling stainless steel alloys is linear sludge flaws, which are usually called slivers. This defect does not disappear even after cold rolling, and in many cases
Fine sagging defects that cannot be detected in hot-rolled coils become more noticeable during cold rolling, which has become a major problem.
In stainless steel alloys, where surface quality is important, the above-mentioned defects are fatal, resulting in lower yields and a significant increase in costs. Although the theoretical elucidation of the above defects is not necessarily sufficient, it is generally believed that they are caused by embrittlement of prior austenite grain boundaries during casting and solidification. This is because sulfur and enzymes are concentrated in the prior austenite grain boundaries, and sulfides and oxides are generated during heating, which promotes embrittlement and is thought to cause small cracks to occur before hot rolling. ing. Conventionally, as a method for preventing cracking during hot rolling, as described in Japanese Patent Application Laid-Open No. 16153/1983,
δcal (calculated value of δ ferrite amount) = 3 (Cr + Mo +
1.5Si+0.5Nb)-2.8(Ni+1/2Mn+1/2Cu)-
84(C+N)-19.8 is determined by A method of adjusting the heating temperature according to the product of the N value (temperature difference) and a method of improving hot strength by adding a special component such as Ti addition is known. However, the first method attempts to suppress the calculated value δcal of δ ferrite, which is a cause of embrittlement during hot rolling, to 4.0% or less. However, in reality, the components of stainless steel alloy are determined by considering mechanical properties and corrosion resistance, and it is difficult to satisfy the δcal value close to 0 for many steel types. Furthermore, it is difficult to achieve 100% target composition during melting of stainless alloy steel, which is a practical problem. In addition, the second method is based on ΔT and N during continuous casting.
In the operation of a heating furnace where it is necessary to heat many slabs at the same time with variable amounts and miscellaneous histories, it is practically impossible to control all the slabs. Furthermore, the addition of a third special component, e.g. Ti
Addition increases stringer flaws due to an increase in inclusions, and the addition of expensive special alloy iron significantly increases costs. (Problems to be Solved by the Invention) The present invention seeks to provide a novel method for preventing the occurrence of small cracks in prior austenite grain boundaries during hot rolling of stainless alloy steel. (Means for Solving the Problems) As a result of various experiments and studies, the present invention has developed a stainless steel alloy slab having a specific composition in an atmosphere in a slab heating furnace with a specific O ₂ concentration before hot rolling. Moreover, by heating the slab at a specific surface temperature and a specific holding time, it is possible to prevent small cracks from forming at prior austenite grain boundaries during hot rolling, thereby reducing the risk of cracking in cold rolled coils. This is based on the recognition of the fact that the occurrence of defects can be reduced. According to the invention, C in weight percent: 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: 2.0~30.0%, N: 0.001~0.060%,
Contains O: 0.015% or less, Al: 4.0% or less, Mo: 5.0% or less, Cu: 3.0% or less, Nb: 1.0%
Below, Ti: 0.05% or less, Zr: 0.10% or less, Ca:
When heating a slab of stainless steel alloy steel containing one or more of 0.06% or less, Sn: 0.10% or less, and B: 0.05% or less, with the balance consisting of Fe and unavoidable impurities before hot rolling, the slab If the O ₂ concentration in the heating furnace atmosphere is controlled to 0.5 to 5.0% by volume, and the slab surface temperature is T°C and the holding time at that temperature is H hours (H + 1.1).
It is characterized by controlling the slab heating temperature and heating time so that (T-1050)≦800. Normally, when heating a slab before hot rolling, the air-fuel ratio is controlled to reduce O ₂ in the furnace in order to minimize fuel consumption.
The concentration is controlled to be 0%. but,
Slab oxidation is minimized in a region where the O ₂ concentration in the atmosphere inside the slab heating furnace is high. Whether below or above this range, internal oxidation progresses inside the slab due to O in the steel or O penetrating from the outside, particularly at prior austenite grain boundaries, resulting in significant embrittlement. These effects become more pronounced as the heating temperature increases. The relationship between the occurrence of linear scratches on the surface of the cooling coil, the O ₂ concentration in the atmosphere inside the slab heating furnace, and the slab surface temperature is based on the results of a quantitative investigation of the austenitic stainless steel with the components shown in Table 1. An example is shown in FIG.

【table】

[Table] Figure 1 shows the O ₂ concentration in the atmosphere in the heating furnace and the linear sagging flaws on the surface of the cold-rolled coil when the slab surface temperature is 1200 to 1250℃ and the holding time at that temperature is 1.5 to 3.5 hours. The relationship with the initial birth rate is shown. As is clear from FIG. 1, when the O ₂ concentration in the atmosphere inside the heating furnace is in the range of 0.5 to 5.0% by volume, the linear sagging defects are minimized. Even if the _O2 concentration is less than 0.5%
Even if it exceeds 5.0%, the area where linear bald spots occur will increase. That is, when it exceeds 5%, oxidation becomes significant;
This is because if it is less than 0.5%, the amount of scale-off on the slab surface layer will be reduced, and defects will likely remain on the slab surface. In addition, Figure 2 shows the heating temperature and the holding time at that temperature when using the austenitic stainless steel in Table 1 and controlling the O ₂ concentration in the atmosphere in the heating furnace to 0.5 to 5.0% by volume. It shows the occurrence rate of linear scratches on the surface of rolled coils (number of coils with scratches/total number of coils), and in the figure, the symbols ●, ×, △, and ○ indicate the occurrence rate of linear scratches of 31% or more, 16~30%, 5~
15% and less than 5%, respectively, and curve A shows the line of (H+1.1)·(T-1050)=800. This line A indicates the limit line for the occurrence of linear sagging defects when the O ₂ concentration in the atmosphere in the heating furnace is 0.5 to 5.0% by volume. As can be seen from these survey results, in order to prevent linear sagging defects that occur during hot rolling, the O ₂ concentration in the atmosphere inside the heating furnace must be set to 0.5 to 5.0% by weight, and the slab surface temperature must be If T°C is the holding time at that temperature and H hours, then T and H are (H+
1.1) It is necessary to adjust it so that it is in the range of (T-1050)≦800. Next, the reasons for limiting each component will be described. For C, the lower the better from the point of view of corrosion resistance, and the higher it is from the point of view of heat resistance, but considering practical use,
It was set at 0.001 to 0.20%. A lower Si content is better from the viewpoint of processability, but if it is too low, deoxidation will be insufficient. Therefore, the lower limit is set to 0.1%
Remove 5% or more of the material that causes significant embrittlement, and set the upper limit.
It 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%. Also,
The higher the amount, the more stable the austenite becomes, and the better the workability and corrosion resistance, but the effect reaches saturation, so it is 11.0%.
was set as the upper limit. As P increases, workability and corrosion resistance deteriorate, so the upper limit was set at 0.050%. S deteriorates hot workability. In particular, during solidification, it segregates at austenite grain boundaries and becomes the main cause of linear heave defects that occur during hot rolling. Therefore, the upper limit should be 0.020%. For O, for the same reason as S, lower is better, so the upper limit was set at 0.015%. Cr is selected from the viewpoint of corrosion resistance as a stainless steel.
The lower limit is 11.0%, and the upper limit was set at 30% in consideration of increased brittleness and difficulty in processing. Ni has a relationship with the amount of Cr, but considering this relationship and the stability of the austenite phase, it was set to 2.0% or more. The upper limit was set at 30% due to cost considerations. In the sense that N stabilizes the austenite phase,
The more the better, but the upper limit should be set considering the limit of the amount of solid solution.
0.060%, and the lower limit is 0.001 with the lower manufacturing limit.
%. Mo is effective in improving corrosion resistance, depending on the application, 5.0%
You can be selective. Cu is also effective for corrosion resistance, but the upper limit was set at 3.0% due to problems with workability and cracking during rolling. Nb forms stable carbides and is therefore effective in corrosion resistance, but due to the problem of embrittlement, the upper limit was set at 1.0%. Ti has the effect of preventing surface cracking during hot rolling by forming stable sulfides and at the same time making it fine. However, if added in large amounts, it can cause titanium stringer flaws, so it may be added in small amounts depending on the application. However, the upper limit is set at the saturation point of 0.05%. Zr also has the same effect as Ti, but the upper limit was set at 0.10% in consideration of cost and effect saturation. Ca has a similar effect and is effective in controlling the form of inclusions, but since it degrades corrosion resistance, it is added selectively depending on the application, with an upper limit of 0.06%. In order to improve corrosion resistance, Sn is selectively added up to 0.10%, which saturates the effect, depending on the application. B is added to prevent cracking during hot heating, but if it exceeds 0.05%, the intergranular corrosion resistance will be significantly reduced, so it is selectively added at 0.05% or less. Al is effective for deoxidation, but is not necessary depending on the amount of Si, Mn, etc. Furthermore, depending on the application, a large amount may be added to achieve precipitation hardening, and the upper limit was set at 4.0%, at which the effect is saturated. (Example) Stainless alloy steel having the composition range of the present invention shown in Table 2 was melted in a converter and made into a slab by continuous casting. Table 2 shows the results of controlling the heating furnace using the method of the present invention. Comparative methods are also shown. As is clear from this table, if the method of the present invention is used, linear scratches that occur during hot rolling can be significantly reduced, but even if stainless steel is within the composition range of the present invention, If the _O2 concentration in the internal atmosphere is outside the range of 0.5-5.0% by volume, or the slab surface temperature Tc and holding time H
If it deviates from the relationship of (H+1.1)×(T-1.050)≦800, many linear flaws have occurred during hot rolling.

【table】

【table】

[Table] (Effects of the Invention) According to the method of the present invention, linear scratches due to hot rolling were significantly reduced, and as a result, the yield of cooling coils was significantly improved. Moreover, it is possible to suppress the increase in cost due to the addition of a process for removing flaws on the surface of the hot-rolled coil using a grinder after hot-rolling.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between the incidence of linear sagging defects and the O ₂ concentration in the atmosphere in the hot-rolling furnace in a cold-rolled coil of austenitic stainless steel having a specific composition range according to the present invention. 1 is a graph showing the degree of influence of the pre-hot rolling heating surface temperature and holding time of a slab of austenitic stainless steel having a specific composition range of the present invention on the occurrence of linear sludge defects on the surface of a cold rolled coil.

Claims (1)

[Claims] 1 C: 0.001-0.20%, Si: in weight percent
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:
Contains 2.0 to 30.0%, N: 0.001 to 0.060%, O: 0.015% or less, Al: 4.0% or less, and further Mo: 5.0%.
Below, Cu: 3.0% or less, Nb: 1.0% or less, Ti0.05
% or less, Zr: 0.10% or less, Ca: 0.06% or less, Sn:
When heating a stainless steel slab containing one or more of B: 0.10% or less, B: 0.05% or less, and the remainder consisting of Fe and unavoidable impurities before hot rolling, O ₂ in the atmosphere in the slab heating furnace The concentration is controlled to 0.5 to 5.0% by volume, and the slab surface temperature is T℃ and the holding time at that temperature is H.
A method for heating a stainless steel alloy slab, characterized by controlling the slab heating temperature and heating time so that (H+1.1)·(T-1050)≦800 when expressed as time.