With the rapid development of China's heavy equipment manufacturing industry, higher requirements are placed on the performance of heavy-duty gear steel. 17CrNiMo6 is a heavy-duty gear steel commonly used in Germany. The domestically produced 17Cr2Ni2Mo steel has a large amount of application. In order to meet the requirements of high-performance heavy-duty gear steel, the author developed V2Nb microalloying on the basis of 17Cr2Ni2Mo steel and developed high-performance 17Cr2Ni2MoVNb steel. The test results show that the V2Nb microalloying significantly improves the fatigue properties and tensile strength of the gear steel. However, the microalloying elements V and Nb may affect the high temperature plasticity of the steel, thereby having an important influence on the continuous casting, rolling, and straightening processes. Therefore, the purpose of this paper is to compare the high temperature properties of 17Cr2Ni2Mo and 17Cr2Ni2MoVNb steel, and to explore the effect of microalloying elements V, Nb and N on the high temperature properties of the test steel.
1 Test materials and methods 1.1 Test materials The chemical composition of 17Cr2Ni2MoVNb and 17Cr2Ni2Mo gear steel. The production process of test steel is: electric furnace smelting furnace refining (LF) vacuum degassing (VD), then casting into steel ingot, steel ingot heated to 1250 After °C, it is rolled into a round rod of <80mm. <80mm round bar is heated to 1200 °C and then forged into a round rod of <11mm. After forging, it is annealed at 700 °C for 3h, and finally finished into a Gleeble tensile specimen of <10mm×100mm.
1.2 Test Methods Tensile tests were carried out on a Gleeble 21500D thermal simulator. The heating and deformation system of the sample is shown in Figure 1. The temperature was raised to 1250 ° C at a rate of 20 ° C / s, and after 60 s, the temperature was lowered to 600 to 1200 ° C at a rate of 10 ° C / s. After 1 min, the film was stretched at a strain rate of 5 × 10 until fracture. During the test, the sample chamber was purged with an argon flow having a flow rate of 1 L/min. The fracture morphology was observed by Hitachi S24300 cold field emission scanning electron microscope, and the structure of the fracture site was observed with a METAVAI inverted metallographic microscope.
2 Test results The tensile strength changes with the test temperature. The tensile strength of the test steel gradually increases as the test temperature decreases. In the stretching temperature range of 600～1200°C, the tensile strength of 17Cr2Ni2MoVNb steel is higher than that of 17Cr2Ni2Mo steel. This is because V and Nb are added to 17Cr2Ni2MoVNb steel to form second phase particles MN, MC, M(C,N), effective nail The grain boundary is refined and the grain is refined.
At the same time, V exists in a solid solution state in a high temperature region (>1000 ° C), which acts as a solid solution strengthening. The preliminary test results show that the grain size of the two test steels is fine (<20μm) in the lower temperature range (<950°C), and it does not change much with the holding time; with the increase of austenitizing temperature The grain growth trend of the two test steels is obvious, and the grain coarsening of 17Cr2Ni2Mo steel is more serious than that of 17Cr2Ni2MoVNb steel.
The reduction in area is a function of the test temperature. The plasticity of 17Cr2Ni2Mo and 17Cr2Ni2MoVNb steels showed different changes with the decrease of temperature. If the reduction of the area is less than 60 as the basis for brittleness, the 17Cr2Ni2Mo steel exhibits a brittle zone in the test temperature range of 700-825 °C, which is commonly referred to as the III brittle zone, and other temperature zones exhibit good plasticity; 17Cr2Ni2MoVNb steel is tested throughout the test. It exhibits good plasticity in the temperature range and has a reduction in area of ​​75 or more. The plasticity of the two varies alternately in different temperature ranges: at 600-900 °C and 1050-1200 °C, the plasticity of 17Cr2Ni2MoVNb steel is better than that of 17Cr2Ni2Mo steel; at 925-1000 °C, the plasticity of 17Cr2Ni2Mo steel is slightly better than that of 17Cr2Ni2MoVNb steel.
The low temperature zone is mixed with a river-like pattern and a large plastic deformation occurs around the dimple. It is indicated that the 17Cr2Ni2MoVNb steel mainly undergoes transgranular ductile fracture and is accompanied by quasi-cleavage fracture in the low temperature zone.
There is intergranular crack on the surface of the 1150 °C fracture of 17Cr2Ni2Mo steel. This is because the impurity elements such as sulfur and oxygen are enriched in the grain boundary of 17Cr2Ni2Mo in the high temperature zone, and sulfides and oxides with lower melting points are formed, which weakens the grain boundary and is in tensile stress. Cracking along the grain boundary under the action. However, large plastic deformation occurs around the crack and in the crystal, and the fracture mode is fracture along the grain toughness. At the 900 °C fracture surface, some dimples appeared, and there was a large plastic deformation around the dimples. At the same time, intergranular cracks were observed. At this time, the fracture mode was intergranular and transgranular mixed fracture.
The 800°C fracture surface has no dimples, and there are obvious intergranular cracks. Only a small plastic deformation occurs around the crack. 3 Analysis and discussion 3.3.150~1200°C temperature range. The temperature range of the two test steels is above 82. And the plasticity of 17Cr2Ni2MoVNb steel is better than that of 17Cr2Ni2Mo steel. The two test steels exhibit good plasticity because of the dynamic recrystallization and grain boundary migration ability, and the slippage is no longer concentrated in the grain boundary during deformation, but at the interface between the inclusions of sulfides, oxides, nitrides and the like in the grains. Stress concentration occurs due to their different deformability, resulting in the formation and accumulation of micropores.
(5) The dissolution temperature of the second phase particles in the test steel was calculated according to the above formula and chemical composition as shown in Table 2. It can be seen that after the austenitization at 1250 ° C, the second phase in the test steel is theoretically completely dissolved. It is generally believed that V and Nb are harmful to the plasticity of the material, and the thermoplasticity deteriorates when the V content is high, but when the V content is low, the effect almost disappears because the compound of V is precipitated at a lower temperature.
At the same time, the N content is effectively controlled in the 17Cr2Ni2MoVNb steel, and the volume fraction of the high temperature precipitation phase Nb (C, N) is reduced. In addition, studies by WANG and Akben show that the addition of Al can slow the precipitation of Nb(C,N), which causes Nb(C,N) to precipitate at lower temperatures. Therefore, the carbon (nitrogen) of Nb in the high temperature region has little effect on the thermoplasticity of 17Cr2Ni2MoVNb steel.
The precipitation temperature of AlN in 17Cr2Ni2MoVNb steel is lower than that of 17Cr2Ni2Mo steel, which indicates that AlN is preferentially precipitated from austenite grain boundary in 17Cr2Ni2Mo steel, which affects the plasticity of 17Cr2Ni2Mo steel. At the same time, due to the enrichment of sulfur, oxygen and other impurity elements at the grain boundary of 17Cr2Ni2Mo steel to form a multi-component low melting point compound, the grain boundary melts, which is also the reason that the thermoplasticity of 17Cr2Ni2MoVNb steel in this region is slightly higher than that of 17Cr2Ni2Mo steel.
In the temperature range of 3.2925~1000°C, the plasticity of 17Cr2Ni2MoVNb steel in this temperature range decreases, because the carbon (nitrogen) and V10 of V and Nb are analyzed along the austenite grain boundary at 1000 °C, which prevents the grain boundary slip. . No V and Nb were added to the 17Cr2Ni2Mo steel, and only a part of AlN precipitated at 1000 °C. Due to the N content and the microalloying elements, the total amount of precipitation of the second phase of the two test steels at the same temperature is different.
The precipitation of some second phase particles in the test steel at different temperatures is calculated by the second phase solid solubility product formula. See Table 3. It can be seen that as the temperature decreases, the precipitation of the second phase in the two test steels gradually increases, and The total amount of precipitation of the second phase particles in the 17Cr2Ni2MoVNb steel in the temperature range of 925 to 1000 ° C is higher than the precipitation amount of AlN in the 17Cr 2 Ni 2 Mo steel. Therefore, the second phase particles in 17Cr2Ni2MoVNb steel have stronger inhibition effect on grain boundary slip than AlN in 17Cr2Ni2Mo steel, which makes the plasticity of 17Cr2Ni2MoVNb steel lower than that of 17Cr2Ni2Mo steel.
The temperature of the temperature range from 3.3600 to 900 °C drops below 900 °C, and the plasticity of 17Cr2Ni2Mo steel changes greatly. At 800 °C, the plasticity drops to the low valley (the reduction of area is only 55). At the same time, the plasticity of 17Cr2Ni2MoVNb steel also decreases. The lowest rate is 75). In addition to the precipitation of a large amount of AlN second phase, the precipitation of ferrite along the grain boundary is the main reason for the plasticity difference between the two test steels. Because the strength of ferrite is only 1/4 of austenite, under stress, the deformation will mainly concentrate on the ferrite film with lower strength at the grain boundary, and the stress can exceed the grain boundary α phase. At the time of the strength, micropores are generated in the α phase, the micropores are polymerized, grown, and finally developed into cracks, resulting in a different decrease in the plasticity of the two test steels. The ferrite dissolution temperature of the two test steels was calculated according to the formula (6): AcCr: 17Cr2Ni2MoVNb steel was 835 ° C; 17Cr 2 Ni 2 Mo steel was 820 ° C.
Ac3=910-203w(C)-15.2w(Ni) 44.7w(Si) 104w(V) 31.5w(Mo)(6) It can be inferred that the Ar3 temperature of 17Cr2Ni2MoVNb steel is higher than the Ar3 temperature of 17Cr2Ni2Mo steel, so 17Cr2Ni2MoVNb steel precipitates ferrite along the original austenite grain boundary before 17Cr2Ni2Mo steel. When the ferrite film of 17Cr2Ni2MoVNb steel grows up and precipitates in the crystal, the ferrite film in 17Cr2Ni2Mo steel is analyzed along the grain boundary. As a result, the plasticity of 17Cr2Ni2Mo steel is lower than that of 17Cr2Ni2MoVNb steel. In addition, the plasticity of 17Cr2Ni2Mo steel decreases sharply in the range of 800-850 °C, which may be related to the segregation of impurity elements such as Cu and P at the grain boundary.
Nachtrab and Chou found that segregation of impurity elements (Cu, Sn, and Sb) at the grain boundaries during the deformation process can lead to intergranular fracture. At the same time, a large amount of segregation of P at the grain boundary increases the sensitivity of the steel to cracks.
In summary, by reducing the N content and the content of impurities such as Cu and P, and the thermoplasticity of the 17Cr2Ni2MoVNb steel after the V, Nb microalloying treatment is improved, the continuous casting and the hot working process can be ensured.
4 Conclusions (1) Due to the addition of V and Nb microalloying elements to produce fine grain strengthening and solid solution strengthening, the tensile strength of 17Cr2Ni2MoVNb steel is slightly higher than that of 17Cr2Ni2Mo.
(2) 17Cr2Ni2MoVNb steel maintains good plasticity at 600-1200 °C, the surface shrinkage rate is above 75, the fracture mode is transgranular ductile fracture, and the valley region appears at 700-900 °C; while the plasticity change of 17Cr2Ni2Mo steel is more severe, 950~ The plasticity is good at 1200 °C, and the surface shrinkage rate is maintained above 80. The sample exhibits ductile transgranular fracture, but below 900 °C, the plasticity deteriorates, and the brittle zone appears at 700-825 °C. The surface shrinkage rate drops to 55. It is a brittle fracture along the crystal.
(3) The high temperature ductility of the gear steel can be improved by lowering the N content and the content of impurity elements such as Cu and P, and adding appropriate amounts of microalloying elements (V, Nb).
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