Medium-chromium tough steel rail resistant to fracture at low temperature of 60 DEG C below zero and production method thereof

Abstract

The invention relates to a medium-Cr tough steel rail resistant to fracture at-60 DEG C and a production method thereof. The steel comprises the following chemical components in percentage by weight: 0.65%-0.70% of C, 0.70%-0.80% of Si, 1.05%-1.20% of Mn, 0.30%-0.45% of Cr, 0.17%-0.25% of Ni, 0.32%-0.40% of Cu, 55-65 ppm of N, 0.025%-0.040% of V, less than or equal to 0.015% of P, less than or equal to 0.010% of S and the balance of Fe and inevitable impurities. By optimizing alloy addition and matching with process design, the fracture toughness value at-60 DEG C reaches 32.4 MPa.m < 0.5 >-33.4 MPa.m < 0.5 >, the steel rail can keep stable obdurability under the extremely cold condition at-60 DEG C, and the low-temperature brittle fracture risk is reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of steels for rail transit, and relates to a rail material, mainly a medium-chromium low-temperature fracture-resistant and tough rail capable of resisting fracture under the condition of -60°C for railway operation lines and a production method thereof. Background Art

[0002] In recent years, with the rapid development of Chinese railways, the application environment conditions of rails that support the stable operation of trains have also changed greatly. After successively designing rails suitable for normal temperature environment conditions, 0°C environment conditions, -60°C environment conditions, and -40°C environment conditions, according to the needs of environmental service, the design of rails for extremely cold conditions exceeding 40°C has triggered the thinking of researchers, and the design of rail materials that can stably serve under extremely cold conditions has become a new topic. Generally, in extremely cold and harsh environments, the brittleness of steel materials will increase significantly, and the ductility and plasticity will decrease significantly. Rails are no exception. Under such conditions, for rails to stably serve, it is necessary to ensure that the strength does not decrease significantly, and at the same time, the low-temperature ductility and plasticity must not be too low. Therefore, the team has carried out the development work of tough rails resistant to fracture at -60°C, using medium chromium and elements to improve ductility and plasticity in combination, and through an on-line heat treatment unit for accelerated cooling to refine the microstructure, improve the strength while increasing the ductility and plasticity, forming a production technology for tough rails resistant to fracture at -60°C.

[0003] It was found in the research that the production technology of the present invention makes up for some deficiencies in the disclosed related technologies and demonstrates certain innovation. For example, CN202310394412 discloses "Manufacturing Method and Rail for Improving Low-Temperature Fracture Resistance and Contact Fatigue Resistance of Rails", which involves improving the low-temperature fracture resistance of rails, reducing the preparation cost, improving the efficiency, and taking into account both the low-temperature fracture resistance and the contact fatigue resistance. However, its process is complex, requiring wire feeding treatment and nitrogen addition treatment, and the content of Cr+Cu+Ni reaches up to 1.65%, and the content of Cu+Ni reaches 0.85%, resulting in a significant increase in cost. CN202210134161 discloses "A Production Method of Low-Temperature Resistant Rails in Alpine Regions", which involves a production method of low-temperature resistant rails in alpine regions, smelting rails with good strength and toughness ratio and excellent wear resistance, but only using carbon, silicon, and manganese as matrix elements, the tensile strength cannot meet the high-strength requirements, and there is a positive relationship between the fracture toughness and the strength, so the fracture toughness cannot meet the low-temperature requirements. CN201510750451 discloses "A Method for Improving the Low-Temperature Fracture Toughness of Rails and the Resulting Rails and Their Applications", but 1.2% silicon and 2.5% manganese will seriously damage the welding performance, increase the generation of abnormal martensite structure in the matrix, and deteriorate the fracture toughness. CN104195433B discloses "A High-Strength and Tough Pearlite Rail and Its Production Method", in which the tensile strength of the rail does not exceed 1120 MPa, which does not meet the strength requirements of rails laid on low-temperature heavy-haul or heavy-haul lines, and has insufficient wear resistance, seriously reducing the service life. CN104561816B discloses "A Rail with High Strength and Excellent Fatigue Resistance and Its Production Method", which shows that the tensile strength of the rail involved is between 1260 MPa and 1420 MPa, and the rail exhibits relatively high strength, but only emphasizes the strength and fatigue resistance, does not explain the effect of low-temperature fracture toughness, and at the same time is prone to unreasonable resource matching, increasing the design cost. CN110592496B discloses "A Pearlite Rail Steel and Its Preparation Method", and CN112501512A discloses "A Controlled Rolling and Controlled Cooling High-Strength Pearlite Rail and Its Production Method", and the rails involved exhibit relatively high strength. The silicon content of the former even reaches 1%, which will significantly reduce the toughness and plasticity of the rail, and it is very easy to have potential safety hazards under the cyclic impact of train wheels; the tensile strength of the latter reaches more than 1360 MPa, and the strength meets the requirements but the toughness does not meet the low-temperature service requirements. CN107739806A discloses "Hypereutectoid Rails with High Toughness and Ductility and Their Manufacturing Methods", and the rails involved are hypereutectoid rails, which are brittle at low temperatures. CN107475616A discloses "High-Strength and Tough Pearlite Rails and Their Manufacturing Methods", and CN107675083B discloses "Strong and Tough Pearlite Rails and Their Manufacturing Methods", and the rails involved have certain strength and hardness, but the main strengthening mechanism is the role of microalloying elements, and the combined effect of chromium and other elements is not explained.CN202211160787 discloses "a method for smelting and producing corrosion-resistant and low-temperature-resistant steel rails for plateau railways", which mainly explains the smelting method and does not describe the low-temperature fracture toughness of the steel rails.

[0004] In order to overcome the deficiencies of the prior art, the present invention provides a medium-chromium steel rail with high fracture toughness and resistance to -60°C low temperature, and a manufacturing method thereof. On the basis of not significantly increasing the production cost, heat-treated steel rails suitable for fracture resistance under -60°C conditions are produced. Summary of the Invention

[0005] The present invention provides a medium-Cr steel rail with high fracture toughness and resistance to -60°C, and a production method thereof. The purpose is to achieve a fracture toughness value of 32.4 - 33.4 MPa·m at -60°C through optimizing alloy addition and process design. 0.5 , so as to ensure that the steel rail has stable high fracture toughness under extremely cold conditions of -60°C and reduce the risk of low-temperature brittle fracture.

[0006] In order to achieve the above purpose, the present invention adopts the following technical solutions:

[0007] A medium-chromium steel rail with high fracture toughness and resistance to -60°C low temperature, the chemical components in the steel are calculated by weight percentage as follows: C: 0.65% - 0.70%, Si: 0.70% - 0.80%, Mn: 1.05% - 1.20%, Cr: 0.30% - 0.45%, Ni: 0.17% - 0.25%, Cu: 0.32% - 0.40%, N: 55 - 65 ppm, V: 0.025% - 0.040%, P ≤ 0.015%, S ≤ 0.010%, and the balance is Fe and unavoidable impurities.

[0008] The fracture toughness index of the steel rail is: the fracture toughness index at -60°C is 32.4 - 33.4 MPa·m 0.5 .

[0009] The room-temperature tensile strength of the steel rail is 1284 - 1311 MPa, the yield strength is 786 - 850 MPa, and the elongation after fracture is 11.0% - 14.0%.

[0010] The hardness of the tread of the steel rail is 375 - 394 HB, and the hardness of the cross-section is 36.5 - 38.5 HRC.

[0011] The metallographic structure of the rail head of the steel rail consists of sorbite, troostite and pearlite, and the pearlite lamellar spacing on the surface of the rail head is between 100 - 110 nm.

[0012] The selection of the addition amount (weight percentage) of the above elements and their function descriptions are as follows:

[0013] C is an element that increases the hardness of the matrix. Its function in the present invention is to strengthen the matrix and ensure the basic hardness of the rail. In the present invention, when the C content is less than 0.65%, the strength and hardness will decrease significantly, not meeting the wear resistance requirements of freight lines; when the C content is higher than 0.70%, the toughness and plasticity under extremely cold conditions will be significantly reduced. Therefore, the present invention selects the C content to be 0.65% - 0.70%.

[0014] Si is an element that increases the strength and toughness of the matrix. Its function in the present invention is to increase the yield strength and yield ratio of the rail, improve the fatigue strength and fatigue ratio. At the same time, it prevents the nucleation and growth of ferrite and improves the ability of anti-self-tempering. When the Si content is less than 0.70%, the above effects are not obvious; when the Si content is higher than 0.80%, the strength, toughness and yield ratio of the rail decrease significantly. Therefore, the present invention selects the Si content to be 0.70% - 0.80%.

[0015] Mn is an element that increases the hardness of the matrix, impact toughness and carbide-forming element. It can increase the eutectoid transformation temperature, promote the increase of the free energy during the pearlite transformation process, increase the nucleation rate, and refine the pearlite structure. At the same time, it can improve the low-temperature toughness. In the present invention, when the manganese content is less than 1.05%, the strength and hardness of the matrix will be significantly reduced; when the manganese content is higher than 1.20%, the positive segregation degree of manganese elements will increase, and martensite abnormal structure is likely to be produced during accelerated cooling. Therefore, the present invention selects the Mn content to be 1.05% - 1.20%.

[0016] P is a harmful element in rail steel, which is likely to cause segregation and "cold brittleness". The lower the better under the premise of ensuring steelmaking conditions, steelmaking cost, etc. The present invention requires that the P content ≤ 0.015%.

[0017] S is a harmful element in rail steel, which is the main forming element of type A inclusions. At the same time, it is easy to produce "hot brittleness" during rolling. Therefore, the lower the better under the premise of ensuring no increase in unnecessary costs. The present invention requires that S ≤ 0.010%.

[0018] In the present invention, the Cr element cooperates with Ni and Cu to form a combined strengthening and toughening alloy system. Chromium can improve the low-temperature toughness of the rail. At the same time, the formation of chromium carbide can increase the hardness and improve the self-tempering stability of the rail during the on-line heat treatment process. In the present invention, when the chromium content is less than 0.30%, the improvement of low-temperature toughness is not obvious; when the chromium content is higher than 0.45%, the cooperative effect with Ni and Cu will be reduced, resulting in an increase in the low-temperature brittleness of the rail. Therefore, the present invention selects the Cr content to be 0.30% - 0.45%.

[0019] In the present invention, Ni is combined with Cr and Cu to improve the low-temperature toughness of the rail at -60°C through solid solution strengthening. At the same time, it is a component element of the Cr-Ni-Cu alloy system, which can increase the pearlite content and improve the strength and plasticity of the rail. When the nickel content is less than 0.17%, the above effects cannot be achieved; when the nickel content exceeds 0.25%, the cost increases significantly. Therefore, the Ni content is controlled at 0.17% - 0.25%.

[0020] In the present invention, Cu is combined with Cr and Ni. Its function is to increase the stability of supercooled austenite in the high-temperature transformation zone, cooperate effectively with the heat treatment process, and through the accelerated cooling process in the heat treatment process, it can refine the pearlite lamellae in the high-temperature zone, and continue to refine the pearlite lamella spacing in the transformation zone, and cooperate with Ni to enhance the low-temperature toughness of the rail. When the copper content is less than 0.32%, the function cannot be effectively exerted; when the copper content is higher than 0.40%, the cost will increase significantly. Therefore, in the present invention, the Cu content is selected to be 0.32% - 0.40%.

[0021] N and V: As interstitial atoms, they dissolve in ferrite and austenite together with carbon, hinder the movement of dislocations, and improve the strength of the rail. At the same time, they form vanadium nitride in cooperation with V, which precipitates and disperses during cooling, pins the grain boundaries, refines the grains, and improves the strength, toughness and hardness of the rail. In the present invention, to achieve the technical effects, the vanadium nitride is controlled at N: 55 - 65 ppm, V: 0.025% - 0.040%.

[0022] Based on the above composition design, to achieve the effects and purposes of the present invention and produce a tough and strong rail with medium Cr resistance to fracture at -60°C, it is necessary to cooperate with smelting process, rolling process and on-line heat treatment process that match the composition of the present invention. The whole process design is closely combined with the composition design, effectively realizing the strength and toughness of the rail under the conditions of the present invention. The designed rail is an on-line heat-treated rail.

[0023] A production method of a medium-chromium tough and strong rail with resistance to fracture at -60°C includes smelting, rolling and on-line heat treatment. The specific method is as follows:

[0024] 1) Steel rail smelting, refining, vacuum degassing, and continuous casting processes: For the smelting of hot metal, desulfurization pretreatment is adopted. Converter or electric furnace is used for smelting, silicon-barium multi-element alloy is used for deoxidation, and (calcium carbonate + dolomite) with a total of 0.60 - 0.70 kg / t is added during the tapping process. The purpose is to promote the floating of slag, reduce the sulfide content in the steel, and at the same time ensure no loss of furnace temperature and reduce the heating cost during the refining process. LF refining is adopted to further purify the molten steel and precisely control the composition of each alloy to ensure that the alloy composition meets the design requirements. (Quicklime + fluorite) with a total of 4.0 - 4.8 kg / t is added during the tapping process, and the refining time is 15.0 - 28.0 minutes. VD or RH vacuum degassing is carried out to ensure that hydrogen and oxygen are below 1.5 ppm and 15 ppm respectively, prevent hydrogen-induced cracks in the steel rail, and control the coarse and fine series grades of non-metallic inclusions below 1.5. The continuous casting billet is used for the casting billet, and the cross-sectional size is not less than 280×380 mm. The drawing speed is controlled at 0.60 - 0.65 m / min to ensure the surface quality of the casting billet without cracks. Only through the above treatment methods can the beneficial effects of the present invention be achieved, and only by cooperating with the subsequent processes can the high-strength and high-toughness Cr-resistant -60°C fracture of the present invention be obtained.

[0025] 2) The rolling described above includes: The casting billet is descaled by high-pressure water and rolled into steel rails using a three-stand, five-stand, or seven-stand rolling mill. The initial blooming rolling temperature of the steel rail is 1060 - 1078°C, and the rolling deformation rate is 46% - 50% at the same time. The purpose is to initially break the austenite grains and prepare for the strengthening of the Cr-Ni-Cu alloy system; the pre-finishing rolling temperature is 1050 - 1080°C, and the rolling deformation rate is 48% - 54% at the same time. The purpose is to further refine the austenite grains, increase the dislocation density, and complete the element solid solution of the Cr-Ni-Cu alloy; the finishing rolling temperature is 930 - 970°C, and the rolling deformation rate is 52% - 58% at the same time. The purpose is to make vanadium combine with carbon and nitrogen elements to form V(C, N), a part of which is dissolved in austenite, hindering the movement of dislocations and improving the strength and toughness of the steel rail.

[0026] 3) The online heat treatment described above includes: after the rail rolling is completed, taking advantage of the remaining rolling temperature to enter the online heat treatment unit. The cooling medium is air. The online heat treatment unit compresses the air to complete the online accelerated cooling. The unit is divided into 3 sections, numbered from section 1 to section 3 in sequence. In order to ensure that the temperature of the rail entering the unit meets the conditions for accelerated cooling, the present invention controls the design by appropriately reducing C and increasing Si and Cr in the alloy, forming a combined strengthening alloy system of "chromium-nickel-copper" + "VN control" + segmented online heat treatment process. It gives full play to the basic strengthening effect of the basic elements of carbon, silicon, and manganese on the matrix, the supplementary effect of chromium, nickel, and copper on solid solution strengthening and fine grain strengthening, and the precipitation strengthening effect of VN and VC. Then, the online heat treatment process with stage control is used to refine the tissue units and lamellar spacing, jointly achieving the effect of strengthening the matrix and improving toughness and plasticity. The starting cooling temperature of the rail is controlled at 855°C to 870°C. Process limitations are imposed on each section of the unit: the cooling rate of the rail head tread in section 1 of the unit is 3.0 - 4.0°C / s, maintained for 30 - 40 s, preferably 35 s; the cooling rate of the rail head tread in section 2 of the unit is 1.5 - 3.0°C / s, maintained for 20 - 25 s, preferably 23 s; the cooling rate of the rail head tread in section 3 of the unit is 0.5 - 1.2°C / s, maintained for 25 - 30 s, preferably 27 s. The final cooling temperature of the rail is controlled at 530 - 540°C. In the above process, first, during the process from finish rolling to the beginning of accelerated cooling, the precipitation of vanadium carbonitride in austenite is controlled to refine and strengthen austenite grains; second, during the cooling in sections 1 and 2, the precipitation of vanadium carbonitride inside the pearlite colonies is controlled to refine the pearlite lamellae. At the same time, this precipitation can inhibit the initiation and propagation of cracks during the service of the rail and improve low-temperature toughness; third, hard carbide such as Cr7C3 and Cr 23 C6 is formed to improve the wear resistance and anti-contact fatigue performance of the rail; fourth, Ni inhibits the coarsening of brittle phases (such as cementite) and improves the fracture toughness of the rail in a low-temperature environment; fifth, the carbides of Cr (such as Cr7C3) and the ε-Cu particles of Cu act together to form multi-scale precipitation phases (nano-scale Cu particles + sub-micron-scale Cr carbides), achieving more efficient dislocation pinning and refining grains. Together, it realizes that the fracture toughness value at -60°C reaches 32.4 - 33.4 MPa·m 0.5 , meeting the requirement that the rail maintains stable strength and toughness under the low-temperature condition of -60°C in the natural environment.

[0027] To achieve the technical effects of the present invention, the present invention integrates production technologies such as alloy composition design, steelmaking, rolling, and online heat treatment. In particular, it gives full play to the role of the "C-Si-Mn" based alloy system + "chromium-nickel-copper" in the rolling and heat treatment processes, and cooperates with the production process to give full play to the advantages of each process, reflecting the systematicness and innovation from design to production. According to the element characteristics, the rolling process and segmented cooling process are designed, enabling the technical indicators of the rail to meet the design expectations and realizing mass production and application.

[0028] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0029] 1) Fracture toughness index at -60°C: 32.4 - 33.4 MPa·m 0.5 , far exceeding 26 MPa·m of the National Science and Technology Support Program 0.5 .

[0030] Conditions for fatigue pre-crack: Span S = 4W, sine wave, f = 31 Hz, r = 0.1, ΔK control: Pre-crack Initial Kmax = 23 MPa·m 0.5 , Pre-crack Final Kmax = 16.5 MPa·m 0.5 , test temperature: -60°C. During testing, span S = 4W, displacement control, loading rate V = 1.0 mm / min.

[0031] 2) Tensile properties and hardness: Tensile strength (R m ) at room temperature is 1284 - 1311 MPa, yield strength (Rp 0.2 ) is 786 - 850 MPa, elongation after fracture is 11.0% - 14.0%, tread hardness is 375 - 394 HB, cross-sectional hardness is 36.5 - 38.5 HRC, hardness transition is uniform, without abnormal steep high points.

[0032] 3) Microstructure composition: The microstructure of the rail head consists of sorbite, troostite and pearlite. The lamellar spacing on the surface of the rail head is between 100 - 110 nm, the microstructure transition is uniform, and no other microstructures are contained. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is the microstructure scanning morphology diagram of the rail in Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0034] In order to make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the following further illustrates the specific implementation manners of the present invention in combination with embodiments. The following embodiments are used to specifically illustrate the content of the present invention. These embodiments are only general descriptions of the content of the present invention and do not limit the content of the present invention.

[0035] The present invention forms a combined strengthening alloy system of "chromium-nickel-copper" + "VN control" and a segmented on-line heat treatment process through appropriate control design of reducing C and increasing Si and Cr alloy. It gives play to the basic strengthening effect of carbon, silicon and manganese on the matrix, the supplementary effect of chromium, nickel and copper on solid solution strengthening and fine grain strengthening, and the precipitation strengthening effect of VN and VC. Then, the on-line heat treatment process with stage control is used to refine the tissue units and lamellar spacing, jointly achieving the effect of strengthening the matrix and improving toughness and plasticity.

[0036] The production processes, tensile properties, hardness, fracture toughness at -60°C, etc. corresponding to the chemical components of the embodiments of the present invention are specifically shown in Tables 1 to 4. The scanning morphology of the rail microstructure of Example 1 is shown in Figure 1 .

[0037] Table 1 Chemical Components of the Embodiments

[0038]

[0039]

[0040] Table 2 Key Parameters for the Control of the Steelmaking Process of the Embodiments

[0041]

[0042] Table 3 Key Parameters for the Control of the Rolling and Online Heat Treatment Processes of the Embodiments

[0043]

[0044]

[0045] The segmented cooling time of the embodiment adopts the optimized time.

[0046] Table 4 Mechanical Properties, Hardness, Fracture Toughness at -60°C of the Embodiments

[0047]

[0048]

Claims

1. A medium-chromium rail resistant to low-temperature fracture with high strength and toughness at -60°C, characterized in that: The chemical components in the steel are by weight percentage: C: 0.65% - 0.70%, Si: 0.70% - 0.80%, Mn: 1.05% - 1.20%, Cr: 0.30% - 0.45%, Ni: 0.17% - 0.25%, Cu: 0.32% - 0.40%, N: 55 - 65 ppm, V: 0.025% - 0.040%, P ≤ 0.015%, S ≤ 0.010%, and the balance is Fe and inevitable impurities.

2. The medium-chromium wear-resistant and -60°C low-temperature fracture-resistant and tough steel rail according to claim 1, wherein: The rail fracture toughness index is: the fracture toughness index at -60°C is 32.4 - 33.4 MPa·m 0.5 .

3. A medium-chromium wear-resistant and -60°C low-temperature fracture-resistant and tough steel rail according to claim 1, characterized in that: The tensile strength of the rail at room temperature is 1284 - 1311 MPa, the yield strength is 786 - 850 MPa, and the elongation after fracture is 11.0% - 14.0%.

4. A medium-chromium corrosion-resistant and -60°C low-temperature fracture-resistant and tough steel rail according to claim 1, characterized in that: The hardness of the rail tread is 375 - 394 HB, and the hardness of the cross-section is 36.5 - 38.5 HRC.

5. The medium-chromium resistant -60°C low-temperature fracture resistant and tough steel rail according to claim 1, characterized in that: The metallographic structure of the rail head consists of sorbite, troostite, and pearlite, and the interlamellar spacing of the pearlite on the rail head surface is between 100 - 110 nm.

6. A production method of the medium-chromium wear-resistant and -60°C low-temperature fracture tough rail as described in any one of claims 1 - 5, including smelting, rolling, and online heat treatment, characterized in that: The rolling includes: the initial blooming rolling temperature of the rail is 1060 - 1078°C, and the rolling reduction rate is 46% - 50% at the same time; the pre-finishing rolling temperature is 1050 - 1080°C, and the rolling reduction rate is 48% - 54% at the same time; the finishing rolling temperature is 930 - 970°C, and the rolling reduction rate is 52% - 58% at the same time. The online heat treatment includes: after the rail rolling is completed, it enters the online heat treatment unit, and the online cooling is completed by using compressed air. The starting cooling temperature is controlled at 855°C - 870°C, the final cooling temperature of the rail is controlled at 530 - 540°C, and finally the rail is air-cooled to room temperature.

7. The production method of a medium-chromium wear-resistant and -60°C low-temperature fracture-resistant and tough steel rail according to claim 6, characterized in that: In the smelting process, the hot metal for smelting adopts desulfurization pretreatment, and a total of 0.60 - 0.70 kg / t of calcium carbonate + dolomite is added during the tapping process.

8. The production method of a medium-chromium wear-resistant and -60°C low-temperature fracture-resistant and tough steel rail according to claim 6, characterized in that: In the smelting process, LF refining is adopted, a total of 4.0 - 4.8 kg / t of quicklime + fluorite is added during the tapping process, the refining time is 15.0 - 28.0 minutes, and VD or RH vacuum degassing is adopted.

9. The production method of a medium-chromium wear-resistant and -60°C low-temperature fracture-resistant and tough steel rail according to claim 6, characterized in that: In the smelting process, continuous casting billets are used for the cast billets, and the drawing speed is controlled at 0.60 - 0.65 m / min.

10. The production method of a medium-chromium corrosion-resistant and -60°C low-temperature fracture-resistant and tough steel rail according to claim 6, characterized in that: In the online heat treatment process, the unit is divided into 3 sections, numbered 1 section to 3 section in sequence. The cooling rate of the rail head tread in the 1 section unit is 3.0 - 4.0°C / s and is maintained for 30 - 40 s; the cooling rate of the rail head tread in the 2 section unit is 1.5 - 3.0°C / s and is maintained for 20 - 25 s; the cooling rate of the rail head tread in the 3 section unit is 0.5 - 1.2°C / s and is maintained for 25 - 30 s.

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