Cold rolling process and quality of SUS430 stainless steel

Abstract : In response to the increasing demand for SUS430 stainless steel in the domestic market and the difficulty in its production , the stainless steel company has explored a set of production technologies. The paper introduces the problems and solutions in each process from annealing-pickling-rolling-cold-rolled coil annealing, pickling-tempering.
Keywords : stainless steel ; strip steel ; cold rolling ; process ; quality

US430 stainless steel, also known as lCrl7 stainless steel , is a ferritic stainless steel with good corrosion resistance. Its chloride corrosion resistance is better than that of general chromium-nickel austenitic stainless steel, so it is widely used in partition materials and daily decorative accessories. , car decoration, low-temperature dilute acid storage tanks, etc. can be called high-quality and low-cost stainless steel. However, due to the inherent shortcomings of ferritic stainless steel, it is difficult to produce it, so it cannot be mass-produced in my country. Baoxin Stainless Steel Co., Ltd. still has yellow spots on the surface of the strip, inconsistent smoothness, and incomplete elimination of the yield platform in the production of Baoxin Stainless Steel Co., Ltd. according to the imported production technology patent. For this reason, a set of cold-rolled SUS430 stainless steel strip production process system was explored according to its own equipment conditions , and the product quality met the market demand.

Cold rolling process of SUS430 stainless steel
For strip steel with a thickness <0.5mm , the process flow : bell furnace annealing /BAF* hot coil pickling /APH* cold rolling /CRM* cold rolled coil annealing, pickling /APC* re-cold rolling /CRM* re-cold rolling Coil annealed, pickled /APC* ( thickness <0.7mm) dry pass /SPM* wet pass /SPM* finish. The steel strip with a thickness > 0.5mm can be finished after cold-rolled coil annealing and pickling, after wet leveling.
BAF Craft System
The first phase of the project introduced three bell -type annealing furnaces from LOI Company. The structure of the heating cover is simple, and the cooling cover is changed to blowing vertically to cool the inner cover, which has a good effect.
In the process, the gas containing more than 99.9% hydrogen , the oxygen content below 10X10 -6 and the dew point below -30 °C is used as the protective gas. When the holding temperature is 840-860C , the holding time is 16 ~ 20h when the thermocouple in the protective gas is used as the control thermocouple , or the holding time is 4 ~ 8h when the thermocouple at the bottom center of the steel coil is used as the control thermocouple. Use below 300C or 400C to achieve uniform metallographic structure , optimize performance and greatly improve comprehensive production capacity. The process system emphasizes increasing the holding time to reduce the uneven oxidation caused by the large temperature difference between cold and hot spots during the heating process, thus solving the pickling effect with large differences in the strip surface length and width direction during the pickling of hot-rolled coils in the past, and even the occurrence of The oxide scale in the center of the steel coil remains and the edge of the steel coil is pickled , so that the qualified rate of the hot-rolled coil rises from about 60% to more than 95 % . The mechanical properties after annealing are shown in Table 1 .
Table 1 Mechanical properties of steel strip after BAF annealing

craft category	Mechanical properties ( min / max / average )			Stove production total time /h
	o s /MPa	%/ MPa	&/% HRB
Tradition	292/318	487/504	24.4/28.880.3/83.3	44
craft	/299	/489	/26.3 /82.5
Baoxin	272/284	467/471	27.8/30.4 80.7/82.3	37
craft	/278	/469	/29.4 /81.5

APH/C Process System of SUS430 Stainless Steel
A continuous production hot and cold belt dual-purpose annealing and pickling unit ( APH/C) was selected . The annealing furnace is provided by a Belgian company . The maximum temperature in the furnace is 1250°C , and the furnace temperature is divided into 9 zones for control. The pickling section is provided by an Austrian company, equipped with Na2SO4 electrolytic tank, HN03 or H2SO4 electrolytic tank, HN03 and HF mixed acid tank.
4.1 Tropical pickling process
Na2SO4 electrolysis ( concentration 150 ~ 220g/t, temperature 65 ~ 85C) -H2SO4 pickling ( concentration 100-200g/t, temperature 60 ~ 80C) -HNO3 + HF mixed acid ( concentration 100 ~ 200g/t , 20 ~ 25g/t , temperature 40 ~ 65C) .
42 cold strip annealing pickling process
The annealing temperature is controlled at 840 ~ 900C . Pickling process : Na 2 SO 4 electrolysis ( concentration 150 ~ 220g/t, temperature 65 ~ 85°C) fHNO3 electrochemical ( concentration 150 ~ 200g/t, temperature 50 ~ 70C) fHNO3 pickling (100 ~ 150g/t, temperature 30 ~ 50C) -Water wash.

Quality problems and countermeasures of SUS430 stainless steel
On the basis of learning and mastering the original imported annealing and pickling technology, the above-mentioned process system has been formulated, but there are still quality problems such as hot coil scale residue, over-pickling and cold-rolled coil surface pickling yellow spots, and surface whitening.
(1) Causes and countermeasures of hot coil quality problems
Generally, the scale content on the surface of hot coil produced by hot rolling mill is about 80 ~ 100g/m 2 . SUS430 hot coil is annealed for a long time in the full hydrogen bell furnace, so the thickness of the oxide layer is basically unchanged. However , the chromium in the strip matrix diffuses to the strip surface and combines with iron oxide , which makes the oxide composition more complex , especially the increase of iron chromium oxide FeCr 2 O4 , which increases the difficulty of pickling.
SUS430 hot coils mainly needs to solve the oxide scale residue caused by insufficient pickling and the yellowing of the surface caused by over-pickling. Stainless steel, the dissolution rate in mixed acid is even higher than that of oxides , but its scale is faster than 300 Series stainless steel thick. Therefore , the scope of pickling for 430 hot coils is relatively narrow , and it is easy to produce scale residues or surface yellowing , and even the two phenomena exist on the surface of the same coil.
Due to the influence of BAF annealing on the scale structure of 430 hot coils, it is of great significance to control the hot coil annealing process for pickling. In addition , the most important thing about the pickling process is effective shot blasting and mixed acid pickling. Baoxin Company has a shot blasting machine, which can achieve uniform and good shot blasting effect by adjusting the shot blasting angle of the shot blasting wheel. In order to control the over-pickling problem , the pickling temperature of the mixed acid section is mainly controlled , especially the pickling temperature of the last mixed acid tank is generally controlled at about 40-50 °C , so as to avoid yellowing of the strip surface.
(2) Causes and countermeasures of cold coil quality problems
According to practice, there are three reasons for surface yellow spots, surface dirt, and surface whitening after pickling of cold coils: ① The gas generated during electrolytic pickling is concentrated on the lower surface of the strip and cannot escape in time, forming large bubbles that will The strip is isolated from the pickling solution ; ②The neutral salt and the sludge in the electrochemical section are deposited on the bottom of the strip driven by the gas ; ③ Improper use of the scrubbing machine. In view of the existing problems, according to the general market requirements for electro-chemical pickling surface with bright effect for SUS430 cold coils , a complete set of technologies for producing SUS430 cold coil electrochemical products is summarized .
The following methods are adopted to solve the pickling macula on the surface of strip steel : ① Strictly control before production and maintain the cleanliness of Na2SO4 solution during production , and the angle of the solution outlet should be reasonable. The pH value is between 2 and 6, and the temperature is between 65 and 85°C . ② Keep all acid tanks clean. ③It is necessary to prevent the strip steel from drying before entering the scrubbing , and keep the scrubbing machine running normally and the bristles reasonable. ④ Cr 6 + within the concentration range of 4 ~ 10g/t .
In order to solve the whitening of the strip steel surface , Na2SO4 electrolysis + HNO3 pickling is adopted , and the method of high current and short pickling time is adopted :

1. Increase the NaSO4 electrolysis current and control it at 3000-4500A .
2. Shorten the residence time of the strip in the pickling section, and keep the strip speed above 25m/min . ③ The HNO3 pickling section is produced with the maximum current allowed by the equipment, and the temperature is controlled below 50C .

 CRM ( Twelve High Cold Rolling Mill ) Craft system
The rolling mill is a 12-high cold rolling mill specially developed in Japan for the production of special steel , with a maximum rolling speed of 800m/min . When this rolling mill was cold rolling SUS430 and 300 series stainless steel , it was found that the former was prone to scratches and broken strips due to its soft material, which made it difficult to control the product quality and affected the output. However, according to the SUS304 process , the corrosion resistance of stainless steel is lower than ordinary. When the diameter of the work roll is small, the roughness is small , and the tension is large, slipping is easy to occur in the first two passes of rolling; The oil film between them is easy to break, resulting in damage to the surface finish of the strip and damage to the surface of the roll. To this end, the following measures have been taken and achieved good results.

1. Appropriately increase the roughness of the work rolls , And distinguish the roughness of the rolls used in the first two passes from the roughness of the work rolls in the remaining passes.
2. Check frequently to keep the roll surface in good condition.
3. Use work rolls with larger diameters as much as possible.
4. the 1st pass , In particular, it is necessary to master the tension difference between the front and back.
5. The reduction rate in the first two passes is controlled at 18% to 24% .
6. Using pure mineral oil as the base oil , adding 10 % fat additive rolling oil , Enhance oil film strength.

SUS430 stainless steel SPM process
The market's demand for product surface processing grades is mostly No - 2B grade. The single-frame two-roller reversible skin pass machine and wet skin pass method made in Japan are selected , and the working roll size is ① 860mm X 1450mm . The leveling speed is 0 ~ 150/300m/min , and the maximum elongation rate is 2%
In the early stage of production , when the strip thickness is greater than 0.7mm , 10/8 rollers ( roller combinations representing different convexity values of the upper and lower rollers ) are used , Wet smoothing 1 pass , But the shape of the board is mostly medium wave ; Such as the shape of a slab , The elongation rate should be more than 2% . The hardness index is greatly increased, so that the steel coil head and tail section yield and extend for a long distance. When the thickness is less than 0.7mm , one dry leveling and one wet leveling are required , Dry leveling uses 13/13 rollers , At this time, the shape of the plate is difficult to control. When the thickness is < 0.5mm , 2 to 3 cramp-like creases are often formed in the center of the strip , and the shape of the strip is still unsatisfactory after wet leveling by 5/4 rollers. Not only the yield and elongation of the steel coil head and tail section are more unqualified , Moreover, the full-length yield and elongation of the steel coil do not match.
For this reason , Formulate corresponding process systems according to their own equipment conditions ; According to the chemical composition of raw materials from different manufacturers , adopt different process systems ; Due to the location and climate of different manufacturers , the roll profile of the SPM work roll will change . When the elongation is too large, it is necessary to consider whether the roll profile is too large ; In order to solve the insufficient extension of the strip head and tail section , Set the preload when the head is flat. For steel strips with a thickness of < 0.7mm , when the middle pass is leveled , reversible rolling is adopted for about 100m from the head to the tail. After improvement, the unqualified rate has been completely eliminated from the original 24 % to August 2000 . At this point, the SUS430 No-2B product with high-quality electrochemical surface that is satisfactory to users can be produced completely . Its mechanical properties are shown in Table 2 . For the surface roughness of No-2B products , when the thickness of the steel strip is 0.3 ~ 3.0mm , the general R a a 0 · 12? x m , when the internal control level reaches B level , R a is 0.07 .

Table 2 Mechanical properties of SUS430 NO.2B products

Thickness /mm	finishing	o s /MPa	%/MPa		HRB
0.3 ~ 0.8	2B	324	493	28	156
0.9 ~ 3.0	2B	317	481	29	155

SUS430 stainless steel cold rolling conclusion :
SUS430 is a high-quality product that is difficult to produce in cold rolling production. Through nearly two years of exploration, a set of experience has been summed up, and the product quality can fully meet customer requirements. With the customer's understanding of the performance and use of SUS430 stainless steel , The current products are in short supply. At present, only 1000 ~ 1500t products can be provided to the market every month . If the second phase of the project is put into production, the output can be doubled.

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