High-impact-toughness low-temperature B-grade steel and preparation process thereof

Abstract

The invention relates to the technical field of smelting, in particular to high-impact-toughness low-temperature B-grade steel which comprises the following components in percentage by mass: 0.17%-0.23% of C, 0.3%-0.4% of Si, 1%-1.2% of Mn, less than or equal to 0.03% of S, less than or equal to 0.03% of p, 0.3%-0.4% of Cr, 0.25%-0.3% of Ni, less than or equal to 0.04% of Cu, 0.01%-0.04% of Ti and the balance of Fe and inseparable impurities. The invention further relates to a preparation process of the high-impact-toughness low-temperature B-grade steel. The prepared low-temperature B-grade steel can keep high impact toughness at the low temperature (minus 50 DEG C), wherein the impact toughness is almost unchanged compared with the impact toughness at the normal temperature.

Description

Technical Field

[0001] The invention relates to the technical field of smelting, and in particular to a low-temperature B-grade steel with high impact toughness and a preparation process thereof. Background Art

[0002] As domestic and international railway transportation develops towards high speed and heavy load, the fatigue reliability of railway freight car bogies has attracted more and more attention, especially the product quality of the bolster and side frame castings, which are important running components of the bogies.

[0003] As the temperature decreases, the impact toughness of metal materials will decrease. When it reaches a certain temperature range, the impact toughness will suddenly drop a lot and become brittle, making the material more susceptible to brittle fracture.

[0004] The stress state of the bolster side frame during operation is very severe, and it is subjected to huge alternating loads such as tension, compression, impact, and bending. Especially when operating in extremely cold areas, low-temperature fracture of components often occurs, affecting the normal operation of railway freight cars and causing traffic accidents. Summary of the invention

[0005] The technical problem to be solved by the present invention is: how to provide a B-grade steel capable of maintaining high impact toughness at low temperature and a preparation process thereof.

[0006] The technical solution adopted by the present invention is: a low-temperature B-grade steel with high impact toughness, wherein the components of the low-temperature B-grade steel are, by mass percentage, C: 0.17%-0.23%, Si: 0.3%-0.4%, Mn: 1%-1.2%, S: ≤0.03%, P: ≤0.03%, Cr: 0.3%-0.4%, Ni: 0.25%-0.3%, Cu: ≤0.04%, Ti: 0.01%-0.04%, and the balance is Fe and impurities that cannot be separated.

[0007] A preparation process of low-temperature B-grade steel with high impact toughness, comprising the following steps Step 1: Loading charge into the eccentric bottom electric arc furnace; Step 2: Heat and melt. At the end of melting, add 1% to 2% slag-making materials and 1% to 2% crushed ore in batches. Prepare slag with a basicity of ≥ 2.5 and good fluidity in advance. Keep the slag volume at about 3%, and drain part of the slag to do a good job of early dephosphorization. Step 3: Oxidation is carried out by mineral oxygen combination method to remove gas and inclusions in molten steel. The oxidation order is phosphorus first and then carbon. When making slag, dephosphorization should be carried out first in large slag volume and then decarbonization in thin slag layer. During the oxidation process, slag should be automatically flowed and slag should be added at any time to maintain the slag volume and basicity of the oxidation process. The slag should always maintain basicity ≥ 2.5. The temperature is controlled at low first and then high. The temperature during the entire oxidation period is controlled at 1580℃~1600℃; Step 4: During the refining process, a composite deoxidizer is added in batches for multiple times for reduction to ensure that the refined slag is a well-deoxidized white slag. The total slag amount during the refining process is about 2.0% to 3.0% of the molten steel. Step 5: Feed aluminum wire at a rate of 0.5 kg / t to 1 kg / t and a wire feeding speed of 50 m / min to 80 m / min. After wire feeding, the bottom blowing time of argon shall not be less than 1 minute. Step 6: Calm the molten steel for 10 minutes, maintain the pouring temperature at 1565°C-1585°C for pouring, and cool to room temperature after pouring; Step 7, using a continuous heat treatment furnace for normalizing heat treatment, the normalizing heat treatment is divided into 9 sections, the first section is 520°C, the second section is 640°C, the third section is 730°C, the fourth section is 820°C, the fifth section is 890°C, the sixth section is 900°C, the seventh section is 900°C, the eighth section is 900°C, and the ninth section is 900°C. Sections one to five are the temperature rising sections of the normalizing heat treatment, and the time for each section is 30 minutes. Sections six to nine are the heat preservation sections of the normalizing heat treatment, and the time for each section is 30 minutes. Step 8: Enter the first cooling chamber, cool with strong cold air, cooling rate 12.5℃ / min, cooling time 30min, Then it enters the second cooling chamber and is cooled by cold air at a cooling rate of 10°C / min for 30 minutes; and then air-cooled for 20 minutes to obtain low-temperature B-grade steel with high impact toughness.

[0008] When charging in step 1, 2%-7% of the weight of the charge is laid on the bottom of the eccentric bottom electric arc furnace to ensure the early dephosphorization and protect the bottom of the furnace, and then steel, i.e. scrap steel and / or steel scraps, is added, and at the same time, 1kg / t to 5kg / t of steel is added to the carbon electrode. When charging, from top to bottom, the steel scraps, thin and small materials, large materials, medium materials, remaining small materials, and steel scraps are charged; In step three, in order to prevent steel from running away during the oxidation process, the furnace should be tilted toward the steel outlet, but the angle should not be greater than 5°.

[0009] In step three, oxygen is blown with a water-cooled oxygen lance, the insertion angle of the oxygen lance is controlled at 20°~30°, the height from the slag surface is about 100mm, the oxygen pressure is controlled at 0.4MPa~0.8MPa, and the water-cooled oxygen lance shall not be inserted into the slag and molten steel; iron ore is added in batches, the ore adding temperature is ≥1560℃, the ore is evenly added to the center of the molten pool, the molten pool is kept evenly boiling, and large boiling is strictly prohibited. 6-10Kg is added per ton of molten steel; the decarburization rate is 0.01% / min~0.03% / min, and the decarburization amount is ≥0.30%, where the percentage is the mass percentage of carbon in the molten steel.

[0010] The beneficial effects of the present invention are as follows: the low-temperature B-grade steel prepared by the present invention can maintain high impact toughness at low temperature (minus 50 degrees Celsius) (almost unchanged compared with the impact toughness at room temperature). The Ti element in the present invention exists in the form of carbide TiC, which can prevent the growth and coarsening of grains, thereby refining the grain structure of the steel, improving the strength and toughness of the steel, and improving the plasticity and impact toughness of the steel at low temperatures. The present invention adopts a continuous heat treatment normalizing process, with a uniform temperature field and high precision, and is not prone to overburning or overheating. The insulation temperature, insulation time, and cooling process set by the present invention can make the ferrite completely austenitized, and the pearlite structure obtained after cooling is in the form of fine flakes, with a large number and a small distance between the flakes, so that the steel has higher strength, hardness and toughness. DETAILED DESCRIPTION

[0011] A low-temperature B-grade steel with high impact toughness, wherein the chemical composition of the low-temperature B-grade steel is as follows by mass percentage: C: 0.17%-0.23%, Si: 0.3%-0.4%, Mn: 1%-1.2%, S: ≤0.03%, P: ≤0.03%, Cr: 0.3%-0.4%, Ni: 0.25%-0.3%, Cu: ≤0.04%, Ti: 0.01%-0.04%, and the balance is Fe and inseparable impurities.

[0012] The preparation process of this embodiment is as follows (1) Lime weighing 2% to 7% of the weight of the charge is laid on the bottom of the eccentric bottom electric arc furnace to ensure early dephosphorization and protect the bottom of the furnace. The charge here refers to scrap steel and steel chips.

[0013] (2) Add scrap steel and steel chips, and add carbon-matching electrodes at 1kg / t to 5kg / t of steel. When loading, from top to bottom: steel chips, thin and small materials, large materials, medium materials, remaining small materials, steel chips. Carbon-matching electrodes refer to scrapped electrodes, which are used to increase carbon. Using carbon-matching electrodes for carbon increase is low cost.

[0014] Steel chips are classified into light and thin small materials, large materials, medium materials, and remaining small materials according to the size of the steel materials. The advantages of loading in this way are: more steel materials are loaded, which is conducive to melting.

[0015] (3) Heat and melt. At the end of melting, add 1% to 2% slag-making materials and 1% to 2% crushed ore in batches. Prepare slag with appropriate basicity (R ≥ 2.5) and good fluidity in advance. Keep the slag volume at about 3%, and drain some of the slag to do a good job of early dephosphorization.

[0016] Slag-making materials refer to fluorite, lime, etc. Crushed ore refers to small pieces of iron ore.

[0017] Keeping the slag volume at about 3% means 3% of the weight of the molten steel. Part of the slag is flowed away because the slag at this time contains a large amount of phosphorus.

[0018] (4) The main purpose of oxidation is to dephosphorize and remove gas and inclusions in the molten steel. The oxidation order is phosphorus first and then carbon. When making slag, the large amount of slag should be dephosphorized first and then the thin slag layer should be decarbonized. During the oxidation process, the slag should be automatically flowed and slag should be added at any time to maintain the slag volume and basicity of the oxidation process. The slag should always maintain high basicity (R ≥ 2.5). The temperature should be controlled from low to high. The temperature during the oxidation period should be controlled at 1580℃ ~ 1600℃. In order to prevent steel from running during the oxidation process, the furnace should be tilted towards the outlet, but the angle should not be greater than 5°.

[0019] (5) Perform oxygen blowing operation: Use a water-cooled oxygen lance to blow oxygen. The insertion angle of the oxygen lance is controlled at 20° to 30°, and the height from the slag surface is about 100 mm. The oxygen pressure is controlled at (0.4 to 0.8) MPa. The water-cooled oxygen lance must not be inserted into the slag or molten steel.

[0020] (6) Ore is added in batches, and the ore adding temperature is ≥ 1560℃. Ore is added evenly to the center of the molten pool to keep the molten pool boiling evenly. Large boiling is strictly prohibited. Ore refers to iron ore. 6-10Kg of iron ore is added to each ton of molten steel.

[0021] (7) Decarburization rate (0.01 ~ 0.03)% / min, decarburization amount ≥ 0.30%. During the oxidation period of the steelmaking process, decarburization is carried out by oxidation to remove gases and inclusions in the molten steel. The percentage here is the mass percentage of carbon in the molten steel.

[0022] (8) During the refining process, composite deoxidizer is added in batches for reduction to ensure that the refined slag is well deoxidized white slag. The total slag volume during the refining process is about 2.0% to 3.0% of the molten steel.

[0023] Composite deoxidizer is a highly efficient steelmaking auxiliary material, containing elements with strong oxygen affinity such as silicon and calcium. The main principle is that the calcium in the composite deoxidizer can convert the oxides in the molten steel into low-melting-point, easily floating deoxidized products. The color of well-deoxidized slag is white.

[0024] (9) Feed aluminum wire at a rate of 0.5 kg / t to 1 kg / t and a wire feeding speed of (50 to 80) m / min. After wire feeding, the bottom blowing time of argon should be no less than 1 minute.

[0025] (10) The molten steel is cooled for 10 minutes, the pouring temperature is not higher than 1585℃, and after pouring, it is cooled to room temperature to obtain Grade B steel. The pouring temperature is not higher than 1585℃, and the pouring temperature is generally controlled at 1565℃-1585℃.

[0026] (11) The grade B steel poured in step 10 is subjected to normalizing heat treatment in a continuous heat treatment furnace. The temperature setting parameters of each heating section are: 520°C for the first section, 640°C for the second section, 730°C for the third section, 820°C for the fourth section, and 890°C for the fifth section; the temperature setting parameters of the insulation section are: 900°C for the sixth section, 900°C for the seventh section, 900°C for the eighth section, and 900°C for the ninth section. The insulation time is 2 hours. Sections 1 to 5 are the heating sections of normalizing heat treatment, and the time of each section is 30 minutes. Sections 6 to 9 are the insulation sections of normalizing heat treatment, and the time of each section is 30 minutes.

[0027] (12) Cooling the Grade B steel after the insulation treatment in step 11, the cooling process is as follows: First, it enters the first cooling chamber and is cooled by strong cold air at a cooling rate of 12.5℃ / min and a cooling time of 30min; Then it enters the second cooling chamber and is cooled by cold air at a cooling rate of 10°C / min for 30 minutes. After exiting the second cooling chamber, it is air-cooled for another 20 minutes to obtain low-temperature B-grade steel with high impact toughness.

[0028] When Ti exists in ferrite as a solid solution, its strengthening effect is higher than that of elements such as aluminum, manganese, nickel, and molybdenum. The present invention adds Ti element to B-grade steel to form carbide TiC with carbon in the steel, thereby preventing the growth and coarsening of grains, thereby refining the grain structure of the steel, improving the strength and toughness of the steel, and improving the plasticity and impact toughness of the steel, especially the low-temperature impact toughness.

[0029] The present invention adopts a continuous heat treatment normalizing process, the temperature field is uniform, the precision is high, and it is not easy to overburn or overheat. The holding temperature, holding time, and cooling process set by the present invention can make the ferrite completely austenitized, and the pearlite structure obtained after cooling is in the form of fine flakes, with a large number and a small distance between the flakes, so that the steel has higher strength, hardness, and toughness.

Claims

1. A low temperature B grade steel with high impact toughness, characterized in that: The low temperature B grade steel has the following components by mass percentage: C: 0.17%~0.23%, Si: 0.3%~0.4%, Mn: 1%~1.2%, S: ≤0.03%, P: ≤0.03%, Cr: 0.3%~0.4%, Ni: 0.25%~0.3%, Cu: ≤0.04%, Ti: 0.01%~0.04%, and the balance is Fe and inseparable impurities.

2. A process for preparing the low temperature B grade steel with high impact toughness as claimed in claim 1, characterized in that: The following steps are included Step 1: Loading charge into the eccentric bottom electric arc furnace; Step 2: Heat and melt. At the end of melting, add 1% to 2% slag-making materials and 1% to 2% crushed ore in batches. Prepare slag with a basicity of ≥ 2.5 and good fluidity in advance. Keep the slag volume at about 3%, and drain part of the slag to do a good job of early dephosphorization. Step 3: Oxidation is carried out by mineral oxygen combination method to remove gas and inclusions in molten steel. The oxidation order is phosphorus first and then carbon. When making slag, dephosphorization should be carried out first in large slag volume and then decarbonization in thin slag layer. During the oxidation process, slag should be automatically flowed and slag should be added at any time to maintain the slag volume and basicity of the oxidation process. The slag should always maintain basicity ≥ 2.

5. The temperature is controlled at low first and then high. The temperature during the entire oxidation period is controlled at 1580℃~1600℃; Step 4: During the refining process, a composite deoxidizer is added in batches for multiple times for reduction to ensure that the refined slag is a well-deoxidized white slag. The total slag amount during the refining process is about 2.0% to 3.0% of the molten steel. Step 5: Feed aluminum wire at a rate of 0.5 kg / t to 1 kg / t and a wire feeding speed of 50 m / min to 80 m / min. After wire feeding, the bottom blowing time of argon shall not be less than 1 minute. Step 6: Calm the molten steel for 10 minutes, maintain the pouring temperature at 1565°C-1585°C for pouring, and cool to room temperature after pouring; Step 7, using a continuous heat treatment furnace for normalizing heat treatment, the normalizing heat treatment is divided into 9 sections, the first section is 520°C, the second section is 640°C, the third section is 730°C, the fourth section is 820°C, the fifth section is 890°C, the sixth section is 900°C, the seventh section is 900°C, the eighth section is 900°C, and the ninth section is 900°C. Sections one to five are the temperature rising sections of the normalizing heat treatment, and the time for each section is 30 minutes. Sections six to nine are the heat preservation sections of the normalizing heat treatment, and the time for each section is 30 minutes. Step 8: Enter the first cooling chamber, cool with strong cold air, cooling rate 12.5℃ / min, cooling time 30min, Then it enters the second cooling chamber and is cooled by cold air at a cooling rate of 10°C / min for 30 minutes; and then air-cooled for 20 minutes to obtain low-temperature B-grade steel with high impact toughness.

3. The process for preparing a low-temperature B-grade steel with high impact toughness according to claim 2, characterized in that: When charging in step 1, 2%-7% of the weight of the charge is laid on the bottom of the eccentric bottom electric arc furnace to ensure early dephosphorization and protect the bottom of the furnace, and then steel, i.e. scrap steel and / or steel chips, is added. At the same time, carbon electrodes are added at a rate of 1kg / t to 5kg / t of steel. When charging, charge from top to bottom according to the order of light and small steel chips, large materials, medium materials, remaining small materials, and steel chips.

4. The process for preparing a low-temperature grade B steel with high impact toughness according to claim 2, characterized in that: In step three, in order to prevent steel from running away during the oxidation process, the furnace should be tilted toward the steel outlet, but the angle should not be greater than 5°.

5. The process for preparing a low-temperature B-grade steel with high impact toughness according to claim 2, characterized in that: In step three, oxygen is blown with a water-cooled oxygen lance, the insertion angle of the oxygen lance is controlled at 20°~30°, the height from the slag surface is about 100mm, the oxygen pressure is controlled at 0.4MPa~0.8MPa, and the water-cooled oxygen lance shall not be inserted into the slag and molten steel; iron ore is added in batches, the ore adding temperature is ≥1560℃, the ore is evenly added to the center of the molten pool, the molten pool is kept boiling evenly, and large boiling is strictly prohibited. 6-10Kg is added per ton of molten steel; the decarburization rate is 0.01% / min~0.03% / min, and the decarburization amount is ≥0.30%, where the percentage is the mass percentage of carbon in the molten steel.