bell furnace annealing on surface quality of 430 series stainless steel

Abstract: The effect of bell furnace annealing on the structure change of oxide scale on the surface of 430 series stainless steel was studied, and the quality problems caused by this change in the subsequent cold rolling process were analyzed . For the first time , a complete theoretical explanation of the long-standing "annealing mark" defect is made , and a solution is proposed according to the results of field process tests .
Key words : stainless steel ; Chrome steel ; Annealing stamp

With the continuous fluctuation of nickel price , " substituting chrome for nickel " in the domestic stainless steel market is becoming a trend , and the demand for 430 series stainless chrome steel is growing the fastest . 430 series stainless steel is a high-grade stainless steel , which is mostly used in decoration , panel and coinage industries . It has very strict requirements on surface quality , especially the consistency of surface color , which is one of the most important acceptance indicators .
( mass fraction, % ) of 430 series stainless steel is: C<0.12, Si<1.00 , Mn<1-00, S<0.030, P<0.035, Cr
16. 00 ~ 18. 00 . The production process of 430 series stainless steel cold-rolled plate is: continuous casting f hot continuous rolling * bell furnace annealing * pickling of cold-rolled raw materials * cold rolling * pickling of cold-rolled finished products f cold-rolled finishing packaging .
the 430 series stainless steel is hot-rolled in the hot rolling workshop, it needs to be annealed in a bell furnace to fully recrystallize the material before it can be put into use . However, the steel coils after this heat treatment often have surface quality problems when they are processed in the cold rolling workshop, and the " annealing mark " defect is particularly serious . " Annealing mark " refers to a product degradation or scrapping defect caused by excessive color difference on the surface of the steel plate. Since the domestic stainless steel surface treatment technology is still in the initial stage of research, there is no theoretical explanation for this problem . Therefore, through field tests and laboratory analysis, the author believes that this defect is caused by the annealing process of the bell furnace changing the oxide scale structure on the surface of the hot-rolled coil, and took targeted improvement measures in the cold-rolling workshop, and achieved Very good practical effect .
Annealing mark " defect
Usually, the annealing of 430 series stainless steel is completed in a full hydrogen bell furnace . Figure 1 shows the commonly used annealing process .
After the steel coil is annealed at 245 °C , the surface is normal dark red when the inner cover is just removed, but after being exposed to the air for a few minutes, blue scale appears on the lateral edge of the coil. These blue scale areas are in the After the pickling of the raw materials in the cold rolling workshop, the surface is rough and pale; the area where no blue iron scale appears is smooth and dark gray. Therefore, the color difference on the lateral surface of the steel plate is extremely large, resulting in serious product degradation, which directly leads to scrapping. This is " Annealing mark " defects . The root cause of this defect is the change of scale structure on the surface of hot-rolled coils by bell furnace annealing process .

Changes in the structure of oxide scale on the surface of hot-rolled coils by bell furnace annealing process
Usually , the finishing temperature of 30 series stainless steel hot-rolled coils is higher than 800 °C , and an iron oxide layer mainly composed of FeO-Fe 3 O4-Fe 2 O 3 structure is often formed . The growth of iron oxide scale follows the parabolic law , the growth speed is fast and thick , and it is black and red . However , after entering the heat treatment in the full-hydrogen bell-type furnace , Since throughout the annealing cycle , The furnace is always full hydrogen high temperature reducing atmosphere , Therefore, the iron oxide scale on the surface of the steel coil undergoes a reduction reaction , and the thickness of the iron oxide scale layer in some areas is thinned and the structure is loose , especially the lateral edge of the steel coil and the inner and outer 3 circles that are fully in contact with hydrogen . This reduction is particularly pronounced . From the measurement results after sampling, it can be seen that when the surface of the roll is in contact with oxygen in the air , secondary oxidation occurs quickly in this part of the area .
430 series stainless steel coils are generally baked at a furnace temperature of about 245 C. Since it is an oxidation reaction that occurs in the " blue brittle " temperature range , Therefore, the generated iron oxide scale is blue ( Figure 2) . The observation result of scanning electron microscope (SEM ) confirms that the iron oxide skin layer thickness is 150 ~ 180 Hm, and its inner layer is mainly Fe 3 O 4, and the outer layer is mainly a-Fe 2 O 3, and the volume ratio of the two is about 4: 1 , during the secondary oxidation, iron oxide scale mainly composed of Fe 3 O 4 -rFe 2 O 3 is formed . At this time , there are two types of iron oxide skin layers on the surface of the steel coil . This structural difference is the root cause of the " annealing mark " defect in the cold rolling workshop .
3 The principle of the reduction of the original roughness of the steel coil surface during the pickling process
430 series stainless steel is usually carried out in the cold rolling workshop by the raw material pickling process of shot peening - H 2 SO 4 -HNO 3 + HF . When the hot-rolled coil passes through the shot peening section , Since the iron oxide scale is hard and brittle compared with the matrix , Micro-cracks will appear inside the oxide scale after being hit by steel shot . After entering the H 2 SO 4 pickling tank, the H 2 SO 4 solution enters the joint between the oxide scale and the steel matrix along the micro-cracks , The following reactions occur :
Fe+H 2 SO 4 =FeSO 4 + H 2
FeO + H 2 SO 4 = FeSO 4 + H 2 O
Fe 2 O 3 +3H 2 SO 4 =Fe 2 ( SO 4 ) 3 +3^0
Fe 3 O 4 +4H 2 SO 4 =Fe 2 ( SO 4 ) 3 +FeSO ° +4^0            
Reaction formula (1) is the main reaction , After the hydrogen gas generated by the reaction accumulates, a micro-explosion occurs , Cause the iron oxide scale to peel off from the steel matrix . Typically , 80 % The oxide scale of the steel is removed by this reaction .
H 2 SO 4 pickling to remove scale is also the process of reducing the original roughness of the steel coil surface ［ 3 ］ . This is because the chemical reaction speeds of the protrusions and recesses on the surface of the steel coil substrate are different during the pickling process . Fig. 3 is a schematic diagram of the reaction of the steel coil in the H 2 SO 4 pickling tank . Visible , During the reaction of sulfuric acid with the steel matrix , The reaction of point A in the raised area is more severe than point B in the concave area , This is because :

1. A is thin and easy to crack, so it reacts preferentially with acid ;
2. A has a large contact area with the acid liquid, and the effective reaction area is large ;
3. The reaction equilibrium in the region of point A is easily affected by the liquid flow in the H2SO4 tank , The local concentration balance is easily destroyed so that the reaction continues to the right , Therefore, the dissolution process of the matrix in the convex area of point A lasts for a long time ; and because the area of point B is in the concave , Liquids flow slower , slower .

This shows that the main reason for the decrease of the original roughness of the hot-rolled coil surface is the difference in the dissolution process between the convex part and the concave part of the steel matrix .
Causes, hazards and solutions of " annealing marks " defects
41 Causes of Defects
The 430 series stainless steel hot-rolled coils annealed in the bell furnace are pickled before the raw materials are pickled in the cold rolling workshop . There are two kinds of iron oxide skin layers with different structures on the surface , that is, the primary oxidation zone distributed in the middle of the steel coil surface is black and red, and the secondary oxidation zone distributed in the lateral edge of the steel coil surface is blue ( Figure 4 ) . These two oxidation zones with different structures are under the same pickling process conditions , showing different scale removal processes , This directly leads to the generation of " annealing mark " defects .
The H 2 SO 4 liquid enters the joint between the iron oxide scale and the steel matrix along the micro-cracks . During the process of peeling off the iron oxide scale [ Formula ( 1 )] , because the secondary oxidation zone is connected with the steel matrix is Fe 3 O 4 , and the Fe 3 The bonding strength of O 4 to the matrix is stronger than that of FeO in the primary oxidation zone
100 times higher , Therefore, the peeling process of the oxide scale in the secondary oxidation zone takes a relatively long time , which directly results in a short contact time between the subsequent H 2 SO 4 liquid and the substrate in the secondary oxidation zone . Therefore, the degree of decrease in the surface roughness of the steel coil is far less than that of the primary oxidation zone [ 1 ] .
Due to the difference in the decrease speed of the surface roughness of the primary oxidation zone and the secondary oxidation zone , the final surface roughness is different , and the different surface roughness directly leads to the difference in the surface color of the steel coil , resulting in the " annealing mark " defect .
difference in the degree of surface roughness reduction cannot be resolved by prolonging the pickling time , because during the process of prolonging the H2SO4 pickling time , Although the secondary oxidation zone has sufficient time to reduce the surface roughness , in the primary oxidation zone , when the reaction time with H 2 SO 4 solution exceeds 10% of the normal time , the electric charge of H 2 SO 4 solution on the surface of the steel substrate Chemical corrosion will suddenly increase ［ 3 ］ It was observed on site that the surface of the primary oxidation zone was destroyed at this time , and the surface roughness of this zone decreased and then increased sharply. After the pickling of raw materials , the surface roughness of the secondary oxidation zone was lower than that of the primary oxidation zone . The color difference is still large , and another " annealing mark " defect with the opposite color distribution appears .
4.2 Hazards and Solutions of Defects
Due to the heredity of surface defects of stainless steel, the " annealing marks " defects that appear after pickling of raw materials in the cold rolling workshop often appear on the surface of the final cold-rolled product , resulting in product degradation or even scrapping due to color differences . 15%, resulting in huge economic losses .
Although the root cause of this defect is caused by the bell furnace annealing process in the hot rolling workshop , the bell furnace process must take into account factors such as structure , performance , carburization and yield , and cannot be easily changed , so it can only be used in cold rolling. The workshop solves the " annealing mark " defect . In the process of H 2 SO 4 pickling to remove scale and reduce surface roughness in the cold rolling workshop , the " annealing mark " defect can be alleviated by adjusting the on -site process and shortening the pickling time of raw materials . Moderately shortening the pickling time of raw materials has little effect on the secondary oxidation zone , but significantly reduces the level of surface roughness in the primary oxidation zone , and significantly improves the consistency of surface color after pickling of raw materials ( Figure 5 ) . Although the average value of surface roughness has also increased , as long as the cold rolling deformation is not less than 60% , there is basically no effect on the surface quality of the final cold rolled sheet .
It should be pointed out that in addition to moderately shortening the pickling time of raw materials, field tests have also proved that reducing the concentration of H 2 SO 4 and HF solution and reducing the reaction temperature of H 2 SO 4 solution are also effective in solving the " annealing mark " defect. In actual production , these four methods are often used in combination to achieve the best results .

1. the 430 series stainless steel is annealed in the bell furnace, the structure change of the surface oxide scale is the root cause of the " annealing mark " defect in the pickling process of raw materials in the cold rolling workshop .
2. Appropriately shorten the pickling time of raw materials and reduce H 2 SO . The defect of " annealing mark " can be basically solved by adjusting the concentration of HF solution, reducing the reaction temperature of H 2 SO 4 solution, and adopting a reasonable amount of cold rolling deformation .

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