Hairline Duplex Stainless Steel Pipe 5 Inch 6 Inch SCH 40 SUS304 WT 0.4mm

Technology promotion
1 alloy elements and cooling rate

The experimental results and theoretical calculations show that the critical cooling rate of the steel is related to the manganese content. The critical cooling rate of the duplex microstructure can be estimated by the alloying elements in the root steel, which can provide a basis for the selection of appropriate cooling method in the production of heat treated duplex steel.

When the chemical composition of steel is constant, a lower cooling rate should be used as far as possible under the premise of ensuring the two-phase structure, so that the carbon in ferrite has enough time to diffuse into the austenite, so as to reduce the yield strength of dual-phase steel and improve the ductility of dual-phase steel. If the content of alloy elements in steel is 4, the critical cooling rate is too high, contains high solid solution after cooling ferrite carbon, is not conducive to obtain excellent properties of dual phase steel, then should change the chemical composition of steel, increase the content of alloy elements in steel, thus reducing the critical cooling rate, or in the production process of dual phase steel, add complementary tempering process, reduce the solid solution in the ferritic carbon, Improve the performance of duplex steel. If there are strong carbon-based forming elements in the steel, the favorable effect of these elements on the hardenability of austenite in the critical region during heating should be considered when estimating the critical cooling rate. The carbide particles of V and Ti can improve the hardenability of austenite and reduce the critical cooling rate by pinning the phase interface.

2. Two-stage cooling process

When the content of alloying element in steel is low, the microstructure of ferrite and pearlite is obtained at a slower cooling rate. When the cooling rate is faster, the solution carbon retained in ferrite is higher, which is not conducive to reducing the yield strength and improving the ductility. The performance of dual-phase steel can be improved by using two-stage cooling, that is, slowly cooling from the critical zone to a certain temperature, and then fast cooling. The carbon in ferrite can be enriched into untransformed austenite by slow cooling. Rapid cooling can avoid the isothermal decomposition of untransformed austenite and ensure the desired duplex structure and properties. For example, 0.08%C-1.4%Mn steel, from 800℃; The mechanical properties of heating to water cooling are as follows: σ0.2=365PMa, σb=700MPa, σ0.2/σb=0.52, EU =18%, et=21%. If the two-stage cooling process is used, that is, at 800℃; After heating, air cooling to 600℃; , and then cooled by water, its performance is: σ0.2=280MPa, σb=600MPa, σ0.2/σb=0.47, EU =21%, ET =29%. The yield strength decreases and ductility increases with two-stage cooling.

3. Influence of coil temperature after hot rolling of duplex steel plate

For a steel of a given composition, the hardenability of austenite when heated in the critical zone can be corrected by hot rolling of the steel plate. High temperature coiling can obviously enrich carbon, manganese and other alloying elements in the second group (pearlite or bainite). It is beneficial to improve the comprehensive properties of duplex steel in subsequent critical area treatment. Taking the test results of 0.049%C-1.99%Mn-0.028%Al-0.0019%N steel as an example, two processes are adopted: one is the common tie process, and the final rolling temperature is 900℃; → Cold oil to 600℃; Coiling → blowing air cooling to room temperature → cold rolling 70%→ continuous annealing. The results of carbon and manganese distribution analysis of the two coiling processes show that carbon and manganese can be enriched in the second phase obviously by high temperature coiling process, while manganese has no enrichment trend in ordinary rolling process.

With high temperature and volume in modified alloy content low hardenability of steel in the subsequent processing for critical area, and reduce the heat treatment of dual phase steel yield strength, improve the ductility of the technology, has been in the factory for heat treatment of dual phase steel production, the resulting heat treatment bipolar plate comprehensive performance is good, the performance of each part of the plate uniform, vertical and horizontal consistent performance. For example, for 0.09%C-0.44Si-1.54%Mn-0.023%Al steel.