A cast steel having high creep resistance and a method of making the same

Abstract

The application discloses cast steel with high anti-creep performance, wherein the chemical components are as follows in percentage by weight: C: 0.3-0.4%, Si: 1.0-1.5%, Mn: ≤2.0%, Mo: ≤0.5%, Ni: 18-22%, Cr: 24-30%, V: 0.3-0.5%, Nb: 0.5-1.2%, and the rest is Fe. By adjusting the content ratio of each component, adding V and Nb with specific content, the high-temperature anti-creep performance of the material is improved, especially at a high temperature of 1000 DEG C, the material still maintains excellent anti-creep performance, the creep limit at 1000 DEG C reaches 75 MPa, the endurance strength reaches 90 MPa, and the average oxidation weight gain after oxidation for 100 h is 0.015 g / m 2 ·h.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of alloy steel preparation, in particular to a cast steel with high creep resistance and a preparation method thereof. BACKGROUND

[0002] Creep is a slow plastic deformation phenomenon of materials when they are subjected to constant external force lower than the yield strength, and is an important way of structural damage of high-temperature materials. Creep reflects the rheological properties of materials under load, i.e. the flow after loading, and also reflects the stability of materials under temperature changes. After creep occurs, the performance of materials will deteriorate with time, and when the deterioration reaches a certain degree, the materials will eventually be damaged. Many metal components work in high-temperature and high-pressure environments, and once the performance of these components decreases or the structure is damaged, it will lead to disastrous consequences and cause huge economic losses, so higher requirements are put forward for the safety and reliability of these components.

[0003] Many scholars at home and abroad have carried out a series of work on the factors of metal material creep, and the results show that temperature, stress, and alloy precipitates, pore size, composition, crystal structure, diffusion and dislocation movement are important factors affecting creep.

[0004] With the increasing demand for higher temperature-resistant gases and higher reliability under complex working conditions of engines, and the increasing demand for lightweight power systems, the exhaust pipe components made of traditional high-silicon molybdenum ductile cast iron, high-nickel austenitic cast iron, and austenitic heat-resistant cast steel cannot meet the requirements of the engine on the exhaust system. In addition, when manufacturing thin parts, the influence of material feeding performance on casting defects will be enlarged due to the restriction of narrow and thin cavities, resulting in a decrease in product forming rate. Therefore, on the one hand, the stability and reliability of materials under thermal-mechanical-chemical coupling factors are improved by improving the oxidation resistance, creep resistance, and high-temperature performance, and on the other hand, the flowability and solidifiability are improved by adjusting the composition to adapt to the casting of thin parts, so as to manufacture the exhaust system that meets the requirements of the engine.

[0005] Therefore, in view of the demand for improving the creep resistance of medium-carbon high-alloy steel, based on the existing heat-resistant cast steel material, it is necessary to develop a medium-carbon high-alloy cast steel material with more excellent high-temperature creep resistance, which can meet the requirements of longer service time and higher reliability, and meet the casting performance of thin parts, so as to expand the application range of heat-resistant cast steel in the high-temperature field. SUMMARY

[0006] The present application aims to provide a cast steel with high creep resistance.

[0007] Another object of the present application is to provide a preparation method of the cast steel with high creep resistance, which effectively ensures excellent oxidation resistance of the material and significantly improves high-temperature creep resistance of the material.

[0008] The object of the present application is achieved by the following technical solutions.

[0009] A cast steel with high creep resistance, characterized in that the chemical components in the cast steel are as follows in percentage by weight: C: 0.3-0.4%, Si: 1.0-1.5%, Mn: ≤2.0%, Mo: ≤0.5%, Ni: 18-22%, Cr: 24-30%, V: 0.3-0.5%, Nb: 0.5-1.2%, and the rest is Fe.

[0010] Preferably, the chemical components in the cast steel are as follows in percentage by weight: C: 0.3-0.35%, Si: 1.0-1.5%, Mn: ≤2.0%, Mo: ≤0.5%, Ni: 18-20%, Cr: 26-28%, V: 0.4-0.5%, Nb: 0.7-1.0%, and the rest is Fe.

[0011] The preparation method of the cast steel with high creep resistance comprises alloy design, smelting, pouring, annealing treatment, solid solution treatment and aging treatment, and is characterized in that the alloy design is to adjust the chemical components and proportion of the alloy steel on the basis of Cr-Mo heat-resistant alloy steel, specifically C: 0.3-0.4%, Si: 1.0-1.5%, Mn: ≤2.0%, Mo: ≤0.5%, Ni: 18-22%, Cr: 24-30%, V: 0.3-0.5%, Nb: 0.5-1.2%, and the rest is Fe.

[0012] The element C can expand the gamma phase region and is also a constituent element of carbide. With the increase of temperature, the decomposition and segregation of carbide will reduce the creep resistance of the prepared cast steel material, so it is necessary to control the type, content, distribution and morphology of carbide. In the present application, the content of Cr is high, so C will compete with Cr to form chromium carbide, which will form a chromium-poor area near the grain boundary, reduce the chromium content near the grain boundary, and thus affect the oxidation resistance of the material. In addition, it will also increase the solidification shrinkage and affect the pouring performance, and defects such as shrinkage holes are easy to occur. However, if C is added too low, the flow performance will be poor, and the spiral line test length at 1600℃ is ≤500mm when C≤0.2%, which is not conducive to the filling of the steel liquid in the narrow cavity. Therefore, the content of C is controlled in the range of 0.25%-0.40%.

[0013] Ni is the main additive element for regulating the range of austenite region, and is the main alloying element for ensuring the gamma phase organization at room temperature. It has become a consensus in heat-resistant steel that the organization is regulated to be gamma single phase to improve the stability. However, a large amount of Ni will increase the material cost and deteriorate the casting performance. When the content of Ni exceeds 23% under the specified composition in the application, the solidification temperature range of the material increases significantly. During solidification, there is a wide paste zone between the complete solid phase zone and the complete liquid phase zone, which produces the characteristics of paste solidification. When used in thin-walled narrow cavity, the shrinkage channel is narrow, the crystals in the paste zone grow in a dendritic manner, and the volume shrinkage of the finally solidified part is blocked by the mixed dendrites, so that the solution cannot be replenished, resulting in defects such as shrinkage and porosity, and reducing the compactness of the casting. However, when the content of Ni is lower than 18%, the matching degree with Cr is insufficient, which leads to the decrease of high temperature oxidation resistance above 800℃. Therefore, the range of Ni is finally controlled in 18% to 22%.

[0014] Cr mainly functions to improve the high temperature oxidation resistance of the material, and can form a NiO·Cr2O3 composite oxide film on the surface in cooperation with Ni to prevent the diffusion of corrosive gases such as oxygen, sulfur and nitrogen into the steel. In addition, Cr is a ferrite forming element, which can regulate the size of the gamma phase region together with C and Ni. Nb and Cr have a great influence on the liquidus temperature of the alloy. When the content of Cr exceeds 30%, the range of the austenite and liquid two-phase region will increase, and the defects will increase. When the content of Cr is lower than 23%, the matching degree with C and Ni is poor. Therefore, by adjusting the matching ratio of Nb and Cr, the range of the austenite and liquid two-phase region is reduced, the solidification temperature range is reduced, the material is close to the eutectic composition point, the sequential solidification characteristics of the material are improved, and the casting defects are reduced. Therefore, considering the requirements of oxidation resistance and casting performance, the range of Cr is controlled in 23.0% to 30.0%.

[0015] V and Nb have a strengthening effect on the creep resistance of the alloy. However, when V increases to more than 1.0% or Nb increases to more than 2.0%, the hardness of the matrix will increase significantly, which will lead to the deterioration of the machining performance. Under the wear condition of the tool on the high-hardness carbide, the machining efficiency will decrease. When V decreases to less than 0.3% or Nb decreases to less than 0.5%, the creep resistance of the material at 850℃ or above will not be improved significantly. Therefore, the range of V and Nb is finally controlled in 0.3% to 0.5% and 0.5% to 1.2% respectively. In this range, the high temperature creep resistance of the material is significantly strengthened.

[0016] Si can assist Cr to form dense oxide on the surface of the material, improve the oxidation resistance, at the same time, Si can also reduce the melting point, improve the fluidity, more importantly, SiC generated by Si and C can effectively hinder the dislocation slip around it, directly improve the material creep resistance, but in the present application, when the content of Si is more than 1.5%, columnar crystal is easy to form, and the tendency of thermal cracking is increased, therefore, the content of Si is controlled to be 1.0%-1.5% in the present application.

[0017] Further, the smelting is according to the nominal mass fraction of each element of the designed alloy composition, and pure iron, low-carbon chromium iron, nickel plate, molybdenum iron, niobium iron, vanadium iron and carbon block are weighed, and the smelting temperature is controlled at 1650-1750°C, and sufficient electromagnetic stirring and mechanical stirring are performed during the smelting process.

[0018] Further, the pouring temperature is 1550-1600°C, and the pouring rate is 2-5 kg / s.

[0019] Further, the annealing treatment is according to the heating rate of 100-120°C / h, the temperature is raised to 800-820°C, the holding time is 1-4 h, after the holding is completed, the furnace is cooled to 400°C, and then the furnace is discharged and air-cooled.

[0020] Further, the solid solution temperature of the solid solution treatment is 1200-1250°C, the time is 1-3 h, then water quenching is performed, the water quenching time is 2-5 min, and the water outlet temperature is lower than 100°C.

[0021] Further, the temperature of the aging treatment is 700-800°C, the time is 4-10 h, then the furnace is cooled to 400°C, and then the furnace is discharged and air-cooled.

[0022] The preparation method of the cast steel with high creep resistance, characterized in that the following steps are performed:

[0023] (1) alloy design: on the basis of Cr-Mo heat-resistant alloy steel, the chemical composition and ratio of the alloy steel are adjusted, specifically, C: 0.3-0.4%, Si: 1.0-1.5%, Mn: ≤2.0%, Mo ≤0.5%, Ni: 18-22%, Cr: 24-30%, V: 0.3-0.5%, Nb: 0.5-1.2%, and the rest is Fe;

[0024] (2) smelting and casting: according to the proportion of each alloy composition, pure iron, low-carbon chromium iron, nickel plate, molybdenum iron, niobium iron, vanadium iron and carbon block are weighed, and smelting is performed, the smelting temperature is controlled at 1650-1750°C, and sufficient electromagnetic stirring and mechanical stirring are performed during the smelting process;

[0025] (3) pouring: the temperature is 1550-1600°C, the pouring rate is 2-5 kg / s, and the poured steel is left to stand;

[0026] (4) annealing treatment: the blank obtained by pouring is heated to 800-820 DEG C at a heating rate of 100-120 DEG C / h, and annealing treatment is carried out, the holding time is 1-4 h, after holding ends, furnace cooling to 400 DEG C, then air cooling after furnace cooling;

[0027] (5) solid solution treatment: after annealing, solid solution treatment is carried out, the solid solution temperature is 1200-1250 DEG C, the time is 1-3 h, then quenching, the quenching time is 2-5 min, the water outlet temperature is lower than 100 DEG C;

[0028] (6) aging treatment: after solid solution treatment, aging treatment is carried out, the aging temperature is 700-800 DEG C, the time is 4-10 h, then furnace cooling to 400 DEG C, then air cooling after furnace cooling.

[0029] In addition, Cr, Mo, V, Nb and other elements form Cr 23 C6, Cr7C3, Mo2C, VC, NbC carbide, these carbides will not dissolve when heated to 900 DEG C-1000 DEG C, by adjusting the ratio of each element, adjusting the type and content of generated carbide, and under the solid solution temperature of 1200-1250 DEG C, the alloy element is fully resolubilized and uniformly diffused in the matrix, these carbides will be pinned in the grain boundary, reduce the dislocation climbing rate, prevent the creep of grain boundary at high temperature, reduce the creep rate, thereby improving the high temperature creep performance of the material. Through subsequent high temperature aging treatment, the second phase is generated in the matrix at 700-800 DEG C, forming a dispersed distribution of precipitated phase, further improving the creep resistance of the cast steel.

[0030] The present application has the following technical effects:

[0031] In the present application, by adjusting the content ratio of each component, adding a specific content of V and Nb, the high temperature creep resistance of the material is improved, especially at a high temperature of 1000 DEG C, the excellent creep resistance is still maintained, the creep limit at 1000 DEG C temperature reaches 75 MPa, the endurance strength reaches 90 MPa, and the average oxidation weight gain after 100 h of oxidation is 0.015 g / m 2 ·h. DETAILED DESCRIPTION

[0032] The present application will be described in detail below by examples, it is necessary to point out here that the following examples are only used to further illustrate the present application, and cannot be understood as limiting the protection scope of the present application, the person skilled in the art can make some non-essential improvements and adjustments to the present application according to the above content of the present application.

[0033] Example 1

[0034] A method for preparing cast steel with high creep resistance, which is carried out in the following steps:

[0035] (1) Alloy design: on the basis of Cr-Mo heat-resistant alloy steel, the chemical composition and proportion of the alloy steel are adjusted, specifically C: 0.35%, Si: 1.2%, Mn: 1.5%, Mo: 0.2%, Ni: 20%, Cr: 24%, V: 0.45%, Nb: 0.8%, and the rest is Fe;

[0036] (2) Melting and casting: according to the designed proportion of each alloy component, pure iron, low-carbon chromium iron, nickel plate, molybdenum iron, niobium iron, vanadium iron and carbon block are weighed and melted, and the melting temperature is controlled at 1700℃, and sufficient electromagnetic stirring and mechanical stirring are carried out during the melting process;

[0037] (3) Pouring: the temperature is 1580℃, the pouring rate is 4kg / s, and the pouring is placed after pouring;

[0038] (4) Annealing treatment: the blank obtained by pouring is annealed according to the heating rate of 110℃ / h and heated to 810℃, the holding time is 3h, and after the holding is finished, the furnace is cooled to 400℃ and then taken out and air cooled;

[0039] (5) Solid solution treatment: after annealing, solid solution treatment is carried out, the solid solution temperature is 1220℃, the time is 2h, then water quenching is carried out, the water quenching time is 4min, and the water outlet temperature is lower than 100℃;

[0040] (6) Aging treatment: after solid solution treatment, aging treatment is carried out, the aging temperature is 750℃, the time is 6h, then the furnace is cooled to 400℃ and then taken out and air cooled.

[0041] Creep limit and endurance strength are important indicators for evaluating the anti-creep performance of materials.

[0042] The creep limit refers to the tensile force index under which the material does not produce brittle deformation under high temperature and long time load.

[0043] The endurance strength refers to the maximum stress value that causes the material to break at a certain temperature and specified duration.

[0044] On the basis of Example 1, by changing the proportioning relationship of V and Nb, the corresponding cast steel material is prepared, and the corresponding performance data of the material at 800℃ is tested, and the results are shown in Table 1, wherein the creep limit and the endurance strength are tested according to GB / T2039-2012 Metal Material Uniaxial Tensile Creep Test Method, and the oxidation resistance is tested according to GB / T 13303-91 Steel Oxidation Resistance Test Method.

[0045] Table 1:

[0046]

[0047] Other conditions being the same, the influence of the change of V and Nb on the creep resistance of the material is as shown in the above table, when the addition amount of V and Nb is between 0.3-0.5% and 0.5-1.2% respectively, the material has excellent creep resistance, and the oxidation resistance at 800℃ is also relatively excellent, when the addition amount of V and Nb is between 0.4-0.5% and 0.7-1.0%, the material has better creep resistance and oxidation resistance.

[0048] Comparative Example 1

[0049] Different from Example 1, the solid solution temperature is 1180℃, and the holding time is 2h, and the rest of the steps and parameters are consistent with Example 1.

[0050] Example 2

[0051] A preparation method of a cast steel with high creep resistance is carried out in the following steps:

[0052] (1) Alloy design: on the basis of Cr-Mo heat-resistant alloy steel, the chemical composition and ratio of the alloy steel are adjusted, specifically C: 0.3%, Si: 1.0%, Mn: ≤2.0%, Mo ≤0.5%, Ni: 18%, Cr: 24%, V: 0.3%, Nb: 0.5%, and the rest is Fe;

[0053] (2) Melting and casting: according to the designed proportion of each alloy component, pure iron, low-carbon chromium iron, nickel plate, molybdenum iron, niobium iron, vanadium iron and carbon block are weighed and melted, the melting temperature is controlled at 1650℃, and sufficient electromagnetic stirring and mechanical stirring are carried out during the melting process;

[0054] (3) Pouring: the temperature is 1550℃, the pouring rate is 5kg / s, and the pouring is placed after pouring;

[0055] (4) Annealing treatment: the blank obtained by pouring is annealed according to the heating rate of 120℃ / h, and the heating is carried out to 820℃, the holding time is 1h, and after the holding is completed, the furnace is cooled to 400℃ and then taken out and air cooled;

[0056] (5) Solid solution treatment: after the annealing is completed, the solid solution treatment is carried out, the solid solution temperature is 1200℃, the time is 3h, and then the water quenching is carried out, the water quenching time is 5min, and the water temperature is lower than 100℃;

[0057] (6) Aging treatment: after the solid solution treatment is completed, the aging treatment is carried out, the aging temperature is 800℃, the time is 4h, and then the furnace is cooled to 400℃ and then taken out and air cooled.

[0058] Example 3

[0059] A method for preparing cast steel with high creep resistance, which is carried out in the following steps:

[0060] (1) Alloy design: on the basis of Cr-Mo heat-resistant alloy steel, the chemical composition and proportion of the alloy steel are adjusted, specifically C: 0.4%, Si: 1.5%, Mn: ≤2.0%, Mo ≤0.5%, Ni: 22%, Cr: 30%, V: 0.5%, Nb: 1.2%, and the rest is Fe;

[0061] (2) Melting and casting: according to the designed proportion of each alloy component, pure iron, low-carbon chromium iron, nickel plate, molybdenum iron, niobium iron, vanadium iron and carbon block are weighed and melted, and the melting temperature is controlled at 1750℃, and sufficient electromagnetic stirring and mechanical stirring are carried out during the melting process;

[0062] (3) Pouring: the temperature is 1600℃, the pouring rate is 2kg / s, and the pouring is placed after pouring;

[0063] (4) Annealing treatment: the blank obtained by pouring is annealed according to the heating rate of 100℃ / h, and the temperature is raised to 800℃, the holding time is 4h, and after the holding is finished, the furnace is cooled to 400℃ and then taken out and air cooled;

[0064] (5) Solid solution treatment: after annealing, solid solution treatment is carried out, the solid solution temperature is 1250℃, the time is 1h, then water quenching is carried out, the water quenching time is 2min, and the water outlet temperature is lower than 100℃;

[0065] (6) Aging treatment: after solid solution treatment, aging treatment is carried out, the aging temperature is 700℃, the time is 10h, then the furnace is cooled to 400℃ and then taken out and air cooled.

[0066] The creep resistance limit of the samples of each scheme under the given pressure, temperature and time conditions is shown in Table 2.

[0067] Table 2:

[0068]

[0069] Under lower solid solution temperature, the dissolution and diffusion of alloy elements are affected, which leads to the decrease of creep resistance. However, by using higher solid solution temperature, the alloy elements are fully dissolved and uniformly diffuse in the matrix, which pins at the grain boundary, reduces the dislocation climbing rate, prevents the grain boundary from creeping at high temperature, reduces the creep rate, and thus improves the high temperature creep resistance and oxidation resistance of the material.

Claims

1. A method for preparing cast steel with high creep resistance, comprising alloy design, melting, pouring, annealing treatment, solid solution treatment and aging treatment, characterized in that: The alloy design is based on Cr-Mo heat-resistant alloy steel, adjusting the chemical composition and proportion of the alloy steel, specifically C: 0.3~0.4%, Si: 1.0~1.5%, Mn: ≤2.0%, Mo ≤0.5%, Ni: 18~22%, Cr: 24~30%, V: 0.3~0.5%, Nb: 0.5~1.2%, and the rest is Fe; the solid solution treatment solid solution temperature is 1200~1250℃, the time is 1~3h, then quenching, quenching time is 2~5min, and the water temperature is lower than 100℃; the aging treatment temperature is 700~800℃, the time is 4~10h, then furnace cooling to 400℃ and air cooling after discharging.

2. A method for producing cast steel having high creep resistance according to claim 1, characterized in that: The pouring temperature is 1550~1600℃, and the pouring rate is 2~5kg / s.

3. A method for producing a cast steel having high creep resistance according to claim 1 or 2, characterized in that: The annealing treatment is according to the heating rate of 100~120℃ / h, heating to 800~820℃, the holding time is 1~4h, and after the holding ends, furnace cooling to 400℃ and air cooling after discharging.

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