A method of heat treatment of a low-chromium ferritic stainless steel

Abstract

The present application relates to the field of low-chromium ferritic stainless steel heat treatment. A low-chromium ferritic stainless steel heat treatment method, a low-chromium ferritic stainless steel to be heat treated is loaded into a batch furnace, then heated to a target temperature T at a rate of 30-50 ℃ / h, and then held for a time t, and finally cooled and discharged, wherein T=A C1 +δ, A C1 is the temperature of the low-chromium ferritic stainless steel from the start of the transformation from ferrite to austenitization, in units of ℃, δ is the temperature correction amount, in units of ℃, T is in units of ℃, t=t0×h, wherein t is in units of hours, h is the thickness of the hot-rolled coil, in units of millimeters, t0 is the time modification coefficient, in units of hours / mm.

Description

Technical Field

[0001] This invention relates to the field of heat treatment of low-chromium ferritic stainless steel. Background Technology

[0002] Low-chromium ferritic stainless steel (Cr≤15%) boasts high strength, good machinability, and weldability. It also exhibits high thermal conductivity, low coefficient of thermal expansion, and excellent resistance to stress corrosion, pitting corrosion, and crevice corrosion. Coupled with its low production cost, it offers significant economic advantages and is widely used in petrochemical, transportation, construction, and home appliance industries. While in use, this material exists in a ferritic microstructure. However, in industrial production, hot-rolled coils exhibit a fibrous microstructure with high strength but poor plasticity, making them unsuitable for direct use. Heat treatment is necessary to eliminate this fibrous structure and achieve the desired ferritic microstructure and properties. Industrial heat treatment typically involves prolonged bell-type furnace annealing in the ferritic phase region. Key parameters such as heat treatment temperature and time are often determined based on experience or extensive laboratory research. It is known that the composition significantly influences the phase composition of low-chromium ferritic stainless steel. Different grades of steel require different heat treatment parameters, and even for the same grade, significant compositional fluctuations necessitate adjustments to the heat treatment parameters to achieve optimal performance. While relying on experience can quickly determine parameters, it comes at the cost of product performance to some extent, and sometimes even results in performance discrepancies (elongation <20%, or d=2a bending cracks). Although parameters obtained from laboratory research are highly accurate and well-matched to field conditions, they are labor-intensive and inefficient. Summary of the Invention

[0003] The technical problem to be solved by this invention is to overcome the problems of poor accuracy of heat treatment processes for low-chromium ferritic stainless steel based on experience and the large workload and low efficiency of determining heat treatment process parameters through laboratory research. This invention provides a heat treatment process method for low-chromium ferritic stainless steel, which can formulate a heat treatment process that meets the requirements of the site based on the actual chemical composition and size of the steel coil.

[0004] The technical solution adopted in this invention is: a heat treatment method for low-chromium ferritic stainless steel, wherein the hot-rolled coil of low-chromium ferritic stainless steel to be heat-treated is loaded into a bell-type furnace, then heated to the target temperature T at a rate of 30~50℃ / h, and held at that temperature for a time t, and finally cooled and removed from the furnace, wherein T = A C1 +δ,A C1 The temperature at which the low-chromium ferritic stainless steel begins its transformation from ferrite to austenitization is given, in °C. δ is the temperature correction factor, in °C. T is in °C. t = t0 × h, where t is in hours, h is the thickness of the hot-rolled coil in millimeters, and t0 is the time correction factor, in hours / millimeter.

[0005] When T ≥ 780°C, δ = -40°C; when 780°C > A C1 ≥ 720°C, δ = -20°C; when A C1 <720°C, 10°C ≥ δ1 ≥ -10°C.

[0006] The value range of t0 is 0.012 - 0.02 h / mm.

[0007] The chemical composition of the hot-rolled coil of low-chromium ferritic stainless steel to be heat-treated is calculated by mass percentage: 0 < C ≤ 0.08%, 0 < Si ≤ 1.0%, 0 < Mn ≤ 2.0%, 0 < P ≤ 0.045%, 0 < S ≤ 0.045%, 0 < N ≤ 0.03%, 10.5 < Cr ≤ 15%, 0 < Ni ≤ 1.0%, 0 < Ti ≤ 2.0%, 0 < Nb ≤ 1.5%, and the rest is Fe and inevitable impurities.

[0008] The coil thickness is the radial distance from the outer ring to the inner ring of the steel coil.

[0009] In the above scheme, when the heating rate is too fast (> 50°C / h), local overheating will occur on the outer ring of the steel coil, while the temperature of the core is low and the heat is insufficient; when the heating rate is too slow (< 30°C / h), although the temperature uniformity of the steel coil is good, the production efficiency is low. After research, it is found that the heating rate controlled at 30 - 50°C / h is more ideal.

[0010] During the process of low-chromium ferritic stainless steel from room temperature to high temperature, ferrite-to-austenite phase transformation will occur. Therefore, when the heat treatment temperature is too high, a considerable amount of ferrite will transform into austenite, and after cooling, the austenite will transform into hard and brittle martensite, deteriorating the performance; in addition, when the heat treatment temperature is too high, grain coarsening will also occur, which is不利 to the performance. But on the other hand, it is hoped that the heat treatment temperature is high enough to quickly eliminate the rolled fiber structure and obtain good use performance. Therefore, the general heat treatment temperature is selected in the high-temperature region of the ferrite phase region. In the present invention, the austenitization start temperature A C1 (°C) of the material is calculated by the commercial software Thermo-calc according to the actual composition of the steel coil. Based on this, and with a temperature correction of δ (°C), the heat treatment temperature T (°C) = A c1 (°C) + δ (°C) is determined. By using the above heat treatment temperature, the steel coil can quickly eliminate the rolled fiber structure, complete recovery and recrystallization, and avoid the appearance of martensite or grain coarsening.

[0011] The holding time mainly depends on the heat treatment temperature and the size of the steel coil. After adopting the above heat treatment temperature, the holding time t (h) can be set according to (0.012 - 0.02 h / mm) × coil thickness (mm), where the coil thickness is the radial distance from the outer ring to the inner ring of the steel coil. Otherwise, when the heat treatment time is too long, on the one hand, it increases energy consumption and reduces production efficiency, and on the other hand, it will also cause grain coarsening and deteriorate the properties. When the heat treatment time is too short, it will lead to insufficient annealing of the steel coil and fail to achieve the purpose of eliminating the as-rolled structure and improving the properties.

[0012] For the above heat treatment process method of low-chromium ferritic stainless steel, the chemical composition of the applicable low-chromium ferritic stainless steel is as follows by mass percentage: 0 < C ≤ 0.08%, 0 < Si ≤ 1.0%, 0 < Mn ≤ 2.0%, 0 < P ≤ 0.045%, 0 < S ≤ 0.045%, 0 < N ≤ 0.03%, 10.5 < Cr ≤ 15%, 0 < Ni ≤ 1.0%, 0 < Ti ≤ 2.0%, 0 < Nb ≤ 1.5%, and the rest is Fe and unavoidable impurities.

[0013] The beneficial effects of the present invention are: Compared with the prior art, the advantage of the present invention is that by according to the chemical composition and size of the steel coil, it can quickly formulate heat treatment process parameters that meet the on-site requirements, avoiding the problems of poor accuracy of process parameters formulated based on experience and large workload and low efficiency in determining process parameters according to laboratory research. Therefore, in actual industrial production, it has stronger applicability. Detailed implementation manners

[0014] Example 1:

[0015] According to the conventional industrial production method, low-chromium ferritic stainless steel 1 is hot-rolled, and its chemical composition is listed in Table 1, with a coil thickness of 300 mm. It is calculated by the commercial software Thermo-calc that A c1 = 818 °C. Since A c1 > 780 °C, the temperature correction δ = -40 °C is determined. Therefore, the heat treatment temperature T = A c1 + δ = 818 °C - 40 °C = 778 °C, and the heat treatment time t = 0.015 × 300 = 4.5 h.

[0016] The heat treatment process is as follows: The steel coil is loaded into a bell-type furnace, heated to 778 °C at a rate of 30 °C / h and then enters the holding stage with a holding time of 4.5 h. Finally, it is cooled to 300 °C by changing to a cooling hood and then taken out of the furnace. Through inspection, the as-rolled fiber structure is completely eliminated, the elongation in the tensile test is ≥ 23%, and the d = 2a bending is qualified, and the heat treatment effect is ideal.

[0017] Example 2:

[0018] Low-chromium ferritic stainless steel 2 was hot-rolled using conventional industrial production methods. Its chemical composition is listed in Table 1, and the coil thickness was 500 mm. A was calculated using the commercial software Thermo-calc. c1 =751℃, since 780>A c1 >750℃, the temperature correction δ is determined to be -20℃, therefore the heat treatment temperature T=A c1 +δ=751℃-20℃=731℃. Heat treatment time t=0.012×500=6h.

[0019] The heat treatment process is as follows: the steel coil is loaded into a bell-type furnace and heated to 778℃ at a rate of 40℃ / h. After entering the heat preservation stage, the heat preservation time is 6h. Finally, the cooling hood is replaced and the temperature is cooled to 280℃ before the coil is taken out of the furnace. After inspection, the rolled fiber structure is completely eliminated, the tensile test elongation is ≥25%, and the d=2a bending is qualified. The heat treatment effect is ideal.

[0020] Example 3:

[0021] Low-chromium ferritic stainless steel 3 was hot-rolled using conventional industrial production methods. Its chemical composition is listed in Table 1, and the coil thickness was 800 mm. A was calculated using the commercial software Thermo-calc. c1 =670℃, due to A c1 <720℃, the temperature correction δ is determined to be ±10℃, therefore the heat treatment temperature T=A c1 +δ=670℃±10℃=660~680℃, select 670℃. Heat treatment time t=0.018×800=14.4h.

[0022] The heat treatment process is as follows: the steel coil is loaded into a bell-type furnace and heated to 670℃ at a rate of 50℃ / h. After that, it enters the heat preservation stage and the heat preservation time is 14.4h. Finally, the cooling hood is replaced and the temperature is cooled to 300℃ before the coil is taken out of the furnace. After inspection, the rolled fiber structure is completely eliminated, the tensile test elongation is ≥27%, the d=2a bending is qualified, and the heat treatment effect is ideal.

[0023] Table 1. Chemical composition of steels in Examples 1-3 (units): w.t. %

[0024]

[0025] Comparative example:

[0026] For steel coils of the same composition and size as in Example 3, it was difficult to develop a bell-type furnace heat treatment process based on experience. Therefore, a laboratory research method was adopted, and a total of 20 heat treatment experiments with different heating temperatures and different holding times were carried out. Finally, the heat treatment temperature was determined to be 670℃ and the holding time was 13h~15h, which is basically similar to that of Example 3. However, from sample preparation, heat treatment, inspection to the final determination of heat treatment process parameters, it took 10 days.

[0027] The above embodiments are merely preferred embodiments of the present invention and should not be construed as limiting the scope of protection of the present invention. Any non-substantial changes and substitutions made by those skilled in the art based on the present invention shall fall within the scope of protection claimed by the present invention.

Claims

1. A heat treatment method for low-chromium ferritic stainless steel, characterized in that: Hot-rolled coils of low-chromium ferritic stainless steel to be heat-treated are loaded into a bell-type furnace, then heated to the target temperature T at a rate of 30~50℃ / h, and held at that temperature for a time t. Finally, the coils are cooled and removed from the furnace, where T = A. C1 +δ,A C1 The temperature at which the transformation from ferrite to austenitization begins in low-chromium ferritic stainless steel is expressed in °C. δ is the temperature correction factor, also in °C. T is in °C. t = t0 × h, where t is in hours, h is the thickness of the hot-rolled coil in millimeters, and t0 is the time correction factor in hours / mm. When A C1 When ≥780℃, δ=-40℃; 780℃>A C1 When the temperature is ≥720℃, δ = -20℃; A C1 When <720℃, 10℃≥δ1≥=-10℃.

2. The heat treatment method for low-chromium ferritic stainless steel according to claim 1, characterized in that: The value range of t0 is 0.012 to 0.02 hours / mm.

3. The heat treatment method for low-chromium ferritic stainless steel according to claim 1, characterized in that: The chemical composition of the hot-rolled coil of low-chromium ferritic stainless steel to be heat-treated is by mass percentage: 0 < C ≤ 0.08%, 0 < Si ≤ 1.0%, 0 < Mn ≤ 2.0%, 0 < P ≤ 0.045%, 0 < S ≤ 0.045%, 0 < N ≤ 0.03%, 10.5 < Cr ≤ 15%, 0 < Ni ≤ 1.0%, 0 < Ti ≤ 2.0%, 0 < Nb ≤ 1.5%, and the rest is Fe and unavoidable impurities.

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