0Crl8Ni9 Stainless Steel Strip

The 0Crl8Ni9 stainless hot-rolled strip produced by BBN Iron and Steel (Group) Co., Ltd. was partially corroded on the surface of the coil after annealing and pickling, which seriously affected the processing and use of the product. Through the salt spray corrosion test, the influence of chemical composition, inclusion level and surface roughness on the surface corrosion resistance of 0C"8Ni9 hot-rolled sheet was studied. The research shows that w(Cr)>17.5%.w(S) ^0.005%. No surface inclusions, and low surface roughness, can effectively improve the corrosion resistance of hot-rolled stainless steel strip.
Key words : austenitic stainless steel; rust; inclusions; chemical composition; surface roughness

0Crl8Ni9 austenitic stainless steel has good comprehensive mechanical properties, excellent process performance and welding performance, so it is widely used. In the production of BBN Iron and Steel (Group) Co., Ltd. , after annealing and pickling of 0Crl8Ni9 austenitic stainless steel hot-rolled strip, the surface of the steel coil is prone to local corrosion, which affects the subsequent processing process. Therefore, this paper analyzes the problem, and studies the effects of chemical composition, inclusion level and surface roughness on the surface corrosion resistance of 0Crl8Ni9 hot-rolled sheet through salt spray accelerated corrosion test.
2 test conditions
2.1 Chemical composition of sample
0Crl8Ni9 hot-rolled plate after pickling , and its chemical composition is shown in Table 1 .
2.2 Salt spray corrosion test conditions
According to GB10125-1988 "corrosion test in artificial atmosphere
Table 1 Chemical composition of 0Crl8Ni9 hot-rolled sheet (mass percentage)%

Sample No	chemical composition
	Cr	mn	S	N
1	17. 29	1. 26	0.001	0.0387
2 , 7	1 &; 09	1. 31	0.003	0.0427
3	1 &; 36	1. 34	0.002	0.0397
4	17. 64	1. 14	0. 001	0.0320
5	17. 86	1. 25	0.007	0.0519
6	17. 82	1. 23	0. 014	0.0374
8	17. 55	1. 24	0. 006	0. 0343
9	17. 57	1. 23	0. 002	0.0285
10、ll、12	17.86	1. 28	0. 001	0.0338

——Salt Spray Test to carry out the test.
Test solution: 50g/L sodium chloride solution; test temperature: (35±; 1 )°C ; spray method: continuous; test cycle : 240h ; ° Angle.

3 Test results and analysis
3.1 Effect of chemical composition on corrosion resistance
3.1.1 Effect of Cr content on corrosion resistance
Cr element is the main alloying element of austenitic stainless steel. The corrosion resistance index of stainless steel can be judged by the following formula (4):
p-r = w(Cr) + 3. 3w(Mo) + 13w(N)
Since the sample does not contain the element Mo, the content of the element N is basically the same, so the difference in the Cr content leads to the difference in the corrosion resistance of stainless steel. The Cr content of 0Crl8Ni9 steel in GB/T3280 " Stainless Steel Cold Rolled Steel Plate and Steel Strip" 1992 edition is 17% ~ 19% ; the Cr content of 06Crl9Nil0 steel in 2007 edition is 18% ~ 20% . At present , two types of 18-8 austenitic stainless steels with different diamond content in China and European stainless steel standards are in production.
In order to study the influence of Cr element on the rust resistance of steel plate. The test adopts the same content of S and N (w(S)=0.001 % ~ 0.003%, s(N) =0.0387% ~ 0.0427 %) Under the condition of , the 1*~3« samples with Cr contents of 17.29% , 1&;09% , 1&;36% were selected for salt spray corrosion test. The test results show that when the surface corrosion rate is greater than 0.05g/(m 2 • h) , Rust defects visible to the naked eye will appear on the surface of the sample. It can be seen that the 2* and 3- samples have good rust resistance, and their Cr content is above 18% . The obvious corrosion of the wide sample is mainly due to the low Cr content, which is only 17.29% . The test results show that when the S content is 0.001% ~ 0.003%, and w (Cr) > 17.5 % , there will be no obvious visible rust on the surface of the sample.

3.1.2 Effect of S content on corrosion resistance
According to literature [2,3], the increase of sulfur content in stainless steel will reduce the corrosion resistance of the material, but there is no quantitative explanation. In this experiment , under the condition that the contents of Cr and Mn are roughly the same , 4 a ~ 6* samples were subjected to salt spray corrosion test, the results are shown in Figure 2 . It can be seen from Figure 2 that the 4# sample is not rusted , the 5 and 6* samples are all corroded, and the rust spots of the 6* sample are obviously more than that of the 5# sample.
S in stainless steel exists mainly in the form of MnS inclusions on the surface of the steel, which is a source of corrosion and affects the rust resistance of the steel.
Under the condition of a certain Mn content, with the increase of S content, the source of corrosion increases, thereby reducing the rust resistance of steel. The test results show that when w(S)<0.005 % , there will be no obvious visible rust on the surface of the sample.
3.2 Influence of inclusions on corrosion resistance
There are inclusions on the surface of some hot-rolled steel coils after pickling. The salt spray corrosion test was carried out on the 8th sample with inclusion defects and the 7* and 9# samples without defects , the results are shown in Figure 3 It can be seen from Figure 3 that the 8" sample has serious corrosion, while the 7# and 9" samples have no corrosion. Electron microscope analysis was carried out on the inclusion defects of 8# sample, and its composition is A1 2 O 3 inclusions containing Na and Ca. This type of inclusion is due to the continuous accumulation and intermittent release of A1 2 O 3 inclusions in the molten steel in the long nozzle and submerged nozzle during continuous casting, and is formed with the solidification of the molten steel on the surface of the slab. Because any surface inclusions will destroy the integrity of the passivation film on the stainless steel surface, the inclusions on the surface of the steel coil have a significant impact on the rust resistance of the steel.
3.3 Effect of surface roughness on corrosion resistance
In order to study the influence of steel coil surface roughness on its corrosion resistance, 3 samples were taken , 11* sample was the original hot-rolled pickling sample, and the surface roughness was 3.14 ; 10* sample was the hot-rolled pickling sample The sample was polished, and the surface roughness was 2.62/zm; the 12* sample was polished with coarse sandpaper, and the surface roughness was 3.86pm . Carry out corrosion comparison test on 3 samples, 12= the sample has obvious corrosion , 10«, 11« samples have no visible corrosion, but the corrosion rate of the two is different , 11* The corrosion rate of the sample is greater than 10 = sample.

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