![<?echo $_SERVER['SERVER_NAME'];?>](/template/twentyseventeen/skin/images/header.jpg)
Abstract: This paper analyzes the influence of non-metallic inclusions, surface defects and decarburization on the quality of spring steel products, discusses the improvement of metallurgical quality of spring steel, chemical composition and process optimization, multi-alloying and rare earth treatment, and the development of new steel types. The role of other measures to improve the performance of alloy spring steels explores new ways to improve the overall performance of alloy spring steels.
Spring steel is widely used in industrial production of steel, according to chemical composition can be divided into 3 categories, namely carbon spring steel, special purpose high alloy spring steel and universal alloy spring steel. Among them, the universal alloy spring steel (hereinafter referred to as alloy spring steel) has the advantages of excellent performance, strong adaptability, low price, etc. Therefore, it is widely used and has a large amount, and is the most commonly used and most important spring material. However, due to the characteristics of the alloy elements of the alloy spring steel, many problems have been brought about for actual production and use.
1. Main problems of alloy spring steel Non-metallic inclusions, surface defects and decarburized layers in the alloy spring steel are the main factors affecting the service life of the spring. The data shows that the valve springs account for 40% of the failure caused by non-metallic inclusions under the surface layer; the surface defects and the failure caused by the decarburization layer account for 30%. A special steel mill received a total of 64 complaints from users in the past two years. The distribution of steel grades and direct complaints is shown in Table 1[1].
Table 1 Steel grades and content distribution of users' complaints Complaints from complaints Complaints Contents Number of complaints 60Si2Mn 43 Breakage and cracking 39
60Si2Cr 8 Surface Cracks 9
50CrV 4 surface decarburization 5
65Mn 7 Demolition Ear 6
55Si2MoV 1 Length and Oval 2
55SiMnVB 1 Carbon Segregation 1
Scratch and skin 2
1.1 Non-metallic inclusions Non-metallic inclusions in steel are mainly Al2O3 and TiN inclusions produced during the smelting process. Their impact on fatigue performance depends on the type, number, size, shape and distribution of inclusions on the one hand, and on the other hand, large size brittle inclusions and spherical deformations, which are restricted by the microstructure and properties of the steel substrate and weakly bonded to the matrix. Inclusions are the most dangerous. Moreover, the higher the strength level of the steel, the more harmful the harmful effects of inclusions on the fatigue limit [2].
1.2 Surface defects Surface quality problems are mainly divided into three categories: First, significant rolling defects, folding and ear defects, and partial scratches, mainly due to outdated rolling equipment, backward finishing facilities, and hole design adjustments are not in place. of. In addition, the surface of the blank is not properly ground, resulting in sharp corners and pit scratches. After rolling, it also forms folding defects. Second, it is a surface crack. It is longitudinally continuous or discontinuously distributed on the steel surface, mainly due to the residual material of the blank. The cracks and subcutaneous defects, the rolling stress and improper cooling will also produce surface cracks; Third, the surface scratches and warpage, which is related to the working conditions and improper operation, will also produce scratches in the packaging, transportation process. Their existence is necessarily the origin of failure of the material, which easily leads to the fracture of the material. However, people generally do not pay much attention to defects such as small pits, scratches, chapped skins, and pits. Some of their existence, although permitted by standards, does not become the main cause of failure, but their presence in the region is positive. It is the weak part of the material. When the overall plasticity of the material is not good, they will also become the breakthrough of cracking. Because small defects have been destroyed or sampled at the time of failure did not test the specific location, this factor is often overlooked in failure analysis. The surface quality problems accounted for 31% of the total number of 64 complaints from a special steel mill [1].
1.3 Decarburization layer Decarburization is a common surface defect in spring steels and has a significant effect on the performance of springs. The so-called decarburization refers to the spring steel in the heating process or heat treatment, the steel surface in the furnace atmosphere under the action of all or part of the decarbonization, resulting in the carbon content of the steel surface than the internal reduction phenomenon. The decarburization of 0.1 mm on the surface of the spring steel significantly reduces its fatigue limit [3]. Moreover, as the depth of the decarburized layer on the steel surface increases, the fatigue life decreases significantly. In particular, the presence of ferrite in the decarburized layer on the steel surface can reduce the fatigue limit by 50%. Due to the decarburization, the surface hardness of the spring is reduced, and cracks are easily generated under the action of the alternating stress, so that the spring is prematurely fatigued. In addition, different coefficients of expansion during quenching in different parts of the surface layer cause stress concentration, resulting in micro cracks in the transition zone between the full decarburized layer and the partially decarburized layer. These visible or invisible microcracks become stress concentration areas. And as the origin of the continued development of the crack, it causes failure or breakage of the spring.
2. Improvement measures The quality of the alloy spring steel depends to a large extent on the smelting process, including the chemical composition of the steel, the cleanliness of the molten steel (gas, harmful elements, inclusions) and the quality of the slab (segregation, decarbonization and The surface conditions) are the key control points for smelting operations. In addition, spring steel also requires sufficient hardenability to ensure uniform microstructure and mechanical properties throughout the entire spring cross-section.
2.1 The improvement of non-metallic inclusions The main cause of fatigue cracks is the inclusion of oxides in the steel, and D-type inclusions are more harmful to fatigue life than Class B inclusions. Therefore, foreign steel mills and automobile plants put forward higher requirements for oxide inclusions in spring steels. For example, the Swedish SKF standard requires that the oxygen content in the steel is less than 15×10-6, and the D-type inclusions are lower than the B-type inclusions. . In particular, Al2O3 and TiN inclusions are the most damaging to the fatigue life of the spring. In order to produce high-quality spring steel, electric furnace-electroslag remelting or vacuum arc remelting and other special smelting methods have been used in the past.
With the development of the refining technology outside the furnace, the use of the refining process outside the furnace can significantly reduce the inclusions in the steel, such as the use of RH vacuum degassing in Japan to achieve ultra-low oxygen steel (ULO) or ultra-pure steel (UCS) production. Silica-deoxidized killed steels are refined by the use of refined synthetic slag with strictly controlled alkalinity to denature the undeformed Al2O3-rich inclusions while stirring the molten steel, allowing the inclusions to float and remove, reducing the inclusion content and making Residual inclusions are harmless, etc., thereby obtaining ultra-pure spring steel. It was confirmed that low oxygen steel (LO) with an oxygen content of less than 15×10-6 can meet the requirements for using 200MPa high stress springs. The fatigue limit of ULO+ULTiN steel produced by ultra-low oxygen plus ultra-low titanium nitride process is the same as that of vacuum arc remelted steel. Steel can be used to manufacture high-strength valve springs [4]. Xining Special Steel Plant and Jiangsu Huaiyin Steel Plant (ultra) high-power EAF initial refining + LF refining + alloy steel bloom or billet continuous casting production of spring steel, deoxidation of niobium alloy in 60 t and 80 t LF, respectively The test of inclusion and inclusion denaturation shows that the oxygen content in the steel is reduced to less than 15×10-6, the residual aluminum content is controlled at 0.020 to 0.025 %, and the proportion of Al2O3 in the oxide inclusions is less than 40 %, and the residual inclusions are small. The uniform, dispersive plastic inclusions are distributed[5]. Qingdao Iron and Steel Co., Ltd. uses the carbon smelting method, argon blowing at the bottom of the ladle, feeding, continuous casting using protective casting, reasonable control of superheat, casting speed and crystallizer parameters, and secondary cold water gas-water atomization cooling method. Produce 60Si2Mn spring steel that meets the requirements of GB1222-84, which effectively solves the quality problems in the production process and reduces the oxygen content and inclusion level in the steel [6].
2.2 Surface defects and decarburization layer improvement Part surface strength is an important factor affecting fatigue strength. Surface heat treatment and surface cold plastic deformation processing are very effective for improving fatigue strength, such as surface quenching, carburizing, carbonitriding, nitriding, shot peening and rolling. Increasing the surface strength of the part can reduce the effective tensile stress and local non-uniform deformation of the part surface and reduce the formation of fatigue cracks.
Yamata et al. [7] studied the effect of different surface preparation and surface treatment methods on the fatigue properties of spring steel SUP10. Table 2 shows the measured surface roughness, surface residual stress and fatigue limit.
Table 2 SUP10 steel fatigue specimen surface roughness, residual stress and fatigue limit specimen surface roughness Rrmax / μm surface residual stress σw/MPa fatigue limit σw/MPa fatigue limit increase Δσw/%
HT 4.7 -5 420 0
HT+BF 2.3 -324 470 11.9
HT+SP 5.7 -470 620 47.6
HT+RG 4.3 -156 630 50.0
HT+FG 1.1 -139 675 60.7
HT+RG+SP 3.8 -628 675 60.7
HT+FG+SP 1.8 -623 700 67.7
FG+HT 2.2 -03 490 16.7
FG+HT+SP 4.1 -591 640 52.4
Note: 1 negative sign indicates compressive stress; 2HT indicates heat treatment, BF indicates barrel grinding, SP indicates shot peening, RG indicates rough grinding, and FG indicates fine grinding.
It can be seen that: (1) The decarburized layer produced by heat treatment can significantly increase the fatigue limit; (2) The surface decarburized layer produced after heat treatment is not removed. Direct shot peening is better than removing decarburization and then shot peening. The rate is large, the former is 30%-50%, and the latter is only 3%-6%.
In order to reduce the influence of surface decarburization, the stripping of light on the hot rolled spring bar surface and the avoidance of surface decarburization should eliminate or reduce the carbonization degree gradient existing between the two. Heating with a protective atmosphere is an effective measure to avoid or reduce surface decarburization. Shortening the heating time reduces the depth of decarburization and rapid induction heating should be used. Because different alloying elements have different effects on the activity and diffusion of carbon, spring steels with different compositions under different conditions will show different decarburization behaviors. For example, Si can increase the elastic limit, strength, tempering stability, and elastic decompression resistance, but it must also pay attention to the fact that Si increases the activity of carbon in austenite and the serious decarburization of the surface caused by chemical gradients. The low-carbon spring steel 28MnSiB produced by Shigang Company reduced the content of carbon-silicon in the steel and effectively reduced the tendency of surface decarburization. The inspection results showed that the actual carbon content was 0.10%-0.16%, with an average of 0.12%, reaching the standard. The carbon content required is less than 0.23% [8].
3 The new production technology of spring steel makes the comprehensive performance and service life of the alloy spring steel significantly improved. It is not enough to improve the smelting process alone. We must continue to seek for beneficial alloy elements, develop new production processes and new steel grades. Meet the ever-increasing demands.
3.1 Spring Steel Multi-alloyed Standard Alloys Spring steels used alloy elements are not extensive enough. The alloy series is relatively simple and fails to fully exploit the effects of multi-alloying. The latest trend in the alloying of spring steel is to fully utilize the alloying effect of alloying elements and to expand the use of alloying elements, especially the use of previously unused trace alloying elements, using multiple (even seven or more) alloy series. To improve the hardenability of spring steel, reduce the tendency of decarburization, and improve the overall performance of spring steel.
For steels that are sensitive to decarburization, adding a small amount of chromium, vanadium, niobium, and molybdenum to the steel can improve the decarburization sensitivity of the steel and reduce decarburization of the spring steel. Adding trace amounts of boron, vanadium, molybdenum, niobium, and chromium can increase Spring steel anti-reduced.
India's Ved.Parkash research shows that adding chromium to the silicon-manganese spring steel can increase the hardenability and yield strength of the spring steel (add 0.5% Cr to increase 15%), and the decarburization layer is significantly reduced. When smelting 60Si2Mn spring steel, the Nanchang Steel Works increased the residual chromium content in the steel to 0.35% to 0.85%, and found that the mechanical properties of the flat steel produced had reached the level of 55SiMnVB. The trace amount of boron can prolong the incubation period of the phase change of the spring steel, reduce the critical quenching rate, thereby improving the hardenability of the steel, and its optimum content is 0.0005% to 0.003%. According to relevant literature reports: steel contains 0.0015 %-0.003% B, can replace 1.0%-1.25% Ni, 0.1%-0.25% Mo, 0.30%-0.35% Cr, 0.2%-0.7% Mn, 0.1% V, 1.6 % Si, 0.001 % B can be equivalent to 1.33% Ni + 0.31% Cr + 0.04% Mo [9]. Vanadium and niobium can refine grains and improve the quality of steel; molybdenum and nickel can improve the plasticity and surface finish of steel [4].
3.2 Rare earth treatment of spring steel Because the main inclusions of electric furnace steel with lower content* are Al2O3 and aluminosilicates, adding rare earth to the steel can reduce the number of inclusions in the steel and reduce the formation of fatigue cracks. The rare earth can also act as a micro-alloy on the steel, thereby increasing the fatigue life of the spring steel.
3.3 Development of New Steel Types Various countries have conducted extensive research on the development of steel types for spring steel. Overseas studies have focused on improving the design stress of spring steel, and correspondingly proposed some steel types with higher stress. Such as: Si-Mn alloy spring steel has a higher anti-reducing, Germany revised in 1988 when the spring steel standard increased a Si-Cr alloy steel 54SiCr6 steel; Another example is Japan's SUP12 is also Si-Cr alloy steel . The United States has developed chromium-free spring steel because of insufficient chromium resources. British Tinsley Bridge Company developed the low-alloy spring steel 0.4C-Mn(Cr)-B, which is hereinafter referred to as "test steel", using "optimum metallurgy" technology. Compared with the conventional spring steel 525H60 leaf spring, the test steel did perform. Excellent surface core fatigue crack propagation ability, and the elastic strength and tensile strength are higher than conventional spring steel [10]. U.S. Patent No. 5,009,843 also describes a low-carbon spring steel having excellent fatigue resistance and resistance to erosion: C 0.35% to 0.55%, Si 1.80% to 3.00%, Mn 0.50% to 1.50%, and Ni 0.50% to 3 0%, Cr 0.10%-1.50%, Al 0.01%-0.05%, N 0.010%-0.025% [4].
The development of spring steel in China should proceed from the following aspects: 1 Based on 60Si2Mn steel, study the optimal composition and application of 60Si2MnB steel, 60Si2Mn silicon steel and 60Si2MnCr steel; 2 Introduction of foreign advanced steel grades; 3 due to low carbon horses The quaternary spring steel is easy to smelt, and the cost is low, the decarburization tendency is small, and the thermal processing performance is good. Research and development and application should be made. In recent years, China's low-carbon martensitic spring steel is based on 28MnSiB steel. By increasing the content of silicon and carbon in steel, 33MnSiB steel, 33MnSiB steel, and 35MnSiB steel have been developed to increase the strength and resistance to springback of spring steel.
4. Conclusion 1) Optimizing the smelting process and denaturing Al2O3 and other harmful inclusions can greatly reduce the content of harmful inclusions. The surface strength of parts can be increased by surface quenching, carburizing, carbonitriding, nitriding and shot blasting.
2) Avoiding surface decarburization It is very effective to eliminate or reduce the carbonization degree gradient existing between the two, and to adopt protective atmosphere heating and shorten the heating time.
3) to expand the use of alloying elements, in particular the use of trace alloy elements and rare earth processing, make full use of alloying elements alloying effect, can improve the comprehensive performance and service life of alloy spring steel.
4) The development of new technologies and new steel grades can improve the hardenability of spring steels, reduce decarburization tendencies, and improve the overall performance of spring steels.
Spring steel is widely used in industrial production of steel, according to chemical composition can be divided into 3 categories, namely carbon spring steel, special purpose high alloy spring steel and universal alloy spring steel. Among them, the universal alloy spring steel (hereinafter referred to as alloy spring steel) has the advantages of excellent performance, strong adaptability, low price, etc. Therefore, it is widely used and has a large amount, and is the most commonly used and most important spring material. However, due to the characteristics of the alloy elements of the alloy spring steel, many problems have been brought about for actual production and use.
1. Main problems of alloy spring steel Non-metallic inclusions, surface defects and decarburized layers in the alloy spring steel are the main factors affecting the service life of the spring. The data shows that the valve springs account for 40% of the failure caused by non-metallic inclusions under the surface layer; the surface defects and the failure caused by the decarburization layer account for 30%. A special steel mill received a total of 64 complaints from users in the past two years. The distribution of steel grades and direct complaints is shown in Table 1[1].
Table 1 Steel grades and content distribution of users' complaints Complaints from complaints Complaints Contents Number of complaints 60Si2Mn 43 Breakage and cracking 39
60Si2Cr 8 Surface Cracks 9
50CrV 4 surface decarburization 5
65Mn 7 Demolition Ear 6
55Si2MoV 1 Length and Oval 2
55SiMnVB 1 Carbon Segregation 1
Scratch and skin 2
1.1 Non-metallic inclusions Non-metallic inclusions in steel are mainly Al2O3 and TiN inclusions produced during the smelting process. Their impact on fatigue performance depends on the type, number, size, shape and distribution of inclusions on the one hand, and on the other hand, large size brittle inclusions and spherical deformations, which are restricted by the microstructure and properties of the steel substrate and weakly bonded to the matrix. Inclusions are the most dangerous. Moreover, the higher the strength level of the steel, the more harmful the harmful effects of inclusions on the fatigue limit [2].
1.2 Surface defects Surface quality problems are mainly divided into three categories: First, significant rolling defects, folding and ear defects, and partial scratches, mainly due to outdated rolling equipment, backward finishing facilities, and hole design adjustments are not in place. of. In addition, the surface of the blank is not properly ground, resulting in sharp corners and pit scratches. After rolling, it also forms folding defects. Second, it is a surface crack. It is longitudinally continuous or discontinuously distributed on the steel surface, mainly due to the residual material of the blank. The cracks and subcutaneous defects, the rolling stress and improper cooling will also produce surface cracks; Third, the surface scratches and warpage, which is related to the working conditions and improper operation, will also produce scratches in the packaging, transportation process. Their existence is necessarily the origin of failure of the material, which easily leads to the fracture of the material. However, people generally do not pay much attention to defects such as small pits, scratches, chapped skins, and pits. Some of their existence, although permitted by standards, does not become the main cause of failure, but their presence in the region is positive. It is the weak part of the material. When the overall plasticity of the material is not good, they will also become the breakthrough of cracking. Because small defects have been destroyed or sampled at the time of failure did not test the specific location, this factor is often overlooked in failure analysis. The surface quality problems accounted for 31% of the total number of 64 complaints from a special steel mill [1].
1.3 Decarburization layer Decarburization is a common surface defect in spring steels and has a significant effect on the performance of springs. The so-called decarburization refers to the spring steel in the heating process or heat treatment, the steel surface in the furnace atmosphere under the action of all or part of the decarbonization, resulting in the carbon content of the steel surface than the internal reduction phenomenon. The decarburization of 0.1 mm on the surface of the spring steel significantly reduces its fatigue limit [3]. Moreover, as the depth of the decarburized layer on the steel surface increases, the fatigue life decreases significantly. In particular, the presence of ferrite in the decarburized layer on the steel surface can reduce the fatigue limit by 50%. Due to the decarburization, the surface hardness of the spring is reduced, and cracks are easily generated under the action of the alternating stress, so that the spring is prematurely fatigued. In addition, different coefficients of expansion during quenching in different parts of the surface layer cause stress concentration, resulting in micro cracks in the transition zone between the full decarburized layer and the partially decarburized layer. These visible or invisible microcracks become stress concentration areas. And as the origin of the continued development of the crack, it causes failure or breakage of the spring.
2. Improvement measures The quality of the alloy spring steel depends to a large extent on the smelting process, including the chemical composition of the steel, the cleanliness of the molten steel (gas, harmful elements, inclusions) and the quality of the slab (segregation, decarbonization and The surface conditions) are the key control points for smelting operations. In addition, spring steel also requires sufficient hardenability to ensure uniform microstructure and mechanical properties throughout the entire spring cross-section.
2.1 The improvement of non-metallic inclusions The main cause of fatigue cracks is the inclusion of oxides in the steel, and D-type inclusions are more harmful to fatigue life than Class B inclusions. Therefore, foreign steel mills and automobile plants put forward higher requirements for oxide inclusions in spring steels. For example, the Swedish SKF standard requires that the oxygen content in the steel is less than 15×10-6, and the D-type inclusions are lower than the B-type inclusions. . In particular, Al2O3 and TiN inclusions are the most damaging to the fatigue life of the spring. In order to produce high-quality spring steel, electric furnace-electroslag remelting or vacuum arc remelting and other special smelting methods have been used in the past.
With the development of the refining technology outside the furnace, the use of the refining process outside the furnace can significantly reduce the inclusions in the steel, such as the use of RH vacuum degassing in Japan to achieve ultra-low oxygen steel (ULO) or ultra-pure steel (UCS) production. Silica-deoxidized killed steels are refined by the use of refined synthetic slag with strictly controlled alkalinity to denature the undeformed Al2O3-rich inclusions while stirring the molten steel, allowing the inclusions to float and remove, reducing the inclusion content and making Residual inclusions are harmless, etc., thereby obtaining ultra-pure spring steel. It was confirmed that low oxygen steel (LO) with an oxygen content of less than 15×10-6 can meet the requirements for using 200MPa high stress springs. The fatigue limit of ULO+ULTiN steel produced by ultra-low oxygen plus ultra-low titanium nitride process is the same as that of vacuum arc remelted steel. Steel can be used to manufacture high-strength valve springs [4]. Xining Special Steel Plant and Jiangsu Huaiyin Steel Plant (ultra) high-power EAF initial refining + LF refining + alloy steel bloom or billet continuous casting production of spring steel, deoxidation of niobium alloy in 60 t and 80 t LF, respectively The test of inclusion and inclusion denaturation shows that the oxygen content in the steel is reduced to less than 15×10-6, the residual aluminum content is controlled at 0.020 to 0.025 %, and the proportion of Al2O3 in the oxide inclusions is less than 40 %, and the residual inclusions are small. The uniform, dispersive plastic inclusions are distributed[5]. Qingdao Iron and Steel Co., Ltd. uses the carbon smelting method, argon blowing at the bottom of the ladle, feeding, continuous casting using protective casting, reasonable control of superheat, casting speed and crystallizer parameters, and secondary cold water gas-water atomization cooling method. Produce 60Si2Mn spring steel that meets the requirements of GB1222-84, which effectively solves the quality problems in the production process and reduces the oxygen content and inclusion level in the steel [6].
2.2 Surface defects and decarburization layer improvement Part surface strength is an important factor affecting fatigue strength. Surface heat treatment and surface cold plastic deformation processing are very effective for improving fatigue strength, such as surface quenching, carburizing, carbonitriding, nitriding, shot peening and rolling. Increasing the surface strength of the part can reduce the effective tensile stress and local non-uniform deformation of the part surface and reduce the formation of fatigue cracks.
Yamata et al. [7] studied the effect of different surface preparation and surface treatment methods on the fatigue properties of spring steel SUP10. Table 2 shows the measured surface roughness, surface residual stress and fatigue limit.
Table 2 SUP10 steel fatigue specimen surface roughness, residual stress and fatigue limit specimen surface roughness Rrmax / μm surface residual stress σw/MPa fatigue limit σw/MPa fatigue limit increase Δσw/%
HT 4.7 -5 420 0
HT+BF 2.3 -324 470 11.9
HT+SP 5.7 -470 620 47.6
HT+RG 4.3 -156 630 50.0
HT+FG 1.1 -139 675 60.7
HT+RG+SP 3.8 -628 675 60.7
HT+FG+SP 1.8 -623 700 67.7
FG+HT 2.2 -03 490 16.7
FG+HT+SP 4.1 -591 640 52.4
Note: 1 negative sign indicates compressive stress; 2HT indicates heat treatment, BF indicates barrel grinding, SP indicates shot peening, RG indicates rough grinding, and FG indicates fine grinding.
It can be seen that: (1) The decarburized layer produced by heat treatment can significantly increase the fatigue limit; (2) The surface decarburized layer produced after heat treatment is not removed. Direct shot peening is better than removing decarburization and then shot peening. The rate is large, the former is 30%-50%, and the latter is only 3%-6%.
In order to reduce the influence of surface decarburization, the stripping of light on the hot rolled spring bar surface and the avoidance of surface decarburization should eliminate or reduce the carbonization degree gradient existing between the two. Heating with a protective atmosphere is an effective measure to avoid or reduce surface decarburization. Shortening the heating time reduces the depth of decarburization and rapid induction heating should be used. Because different alloying elements have different effects on the activity and diffusion of carbon, spring steels with different compositions under different conditions will show different decarburization behaviors. For example, Si can increase the elastic limit, strength, tempering stability, and elastic decompression resistance, but it must also pay attention to the fact that Si increases the activity of carbon in austenite and the serious decarburization of the surface caused by chemical gradients. The low-carbon spring steel 28MnSiB produced by Shigang Company reduced the content of carbon-silicon in the steel and effectively reduced the tendency of surface decarburization. The inspection results showed that the actual carbon content was 0.10%-0.16%, with an average of 0.12%, reaching the standard. The carbon content required is less than 0.23% [8].
3 The new production technology of spring steel makes the comprehensive performance and service life of the alloy spring steel significantly improved. It is not enough to improve the smelting process alone. We must continue to seek for beneficial alloy elements, develop new production processes and new steel grades. Meet the ever-increasing demands.
3.1 Spring Steel Multi-alloyed Standard Alloys Spring steels used alloy elements are not extensive enough. The alloy series is relatively simple and fails to fully exploit the effects of multi-alloying. The latest trend in the alloying of spring steel is to fully utilize the alloying effect of alloying elements and to expand the use of alloying elements, especially the use of previously unused trace alloying elements, using multiple (even seven or more) alloy series. To improve the hardenability of spring steel, reduce the tendency of decarburization, and improve the overall performance of spring steel.
For steels that are sensitive to decarburization, adding a small amount of chromium, vanadium, niobium, and molybdenum to the steel can improve the decarburization sensitivity of the steel and reduce decarburization of the spring steel. Adding trace amounts of boron, vanadium, molybdenum, niobium, and chromium can increase Spring steel anti-reduced.
India's Ved.Parkash research shows that adding chromium to the silicon-manganese spring steel can increase the hardenability and yield strength of the spring steel (add 0.5% Cr to increase 15%), and the decarburization layer is significantly reduced. When smelting 60Si2Mn spring steel, the Nanchang Steel Works increased the residual chromium content in the steel to 0.35% to 0.85%, and found that the mechanical properties of the flat steel produced had reached the level of 55SiMnVB. The trace amount of boron can prolong the incubation period of the phase change of the spring steel, reduce the critical quenching rate, thereby improving the hardenability of the steel, and its optimum content is 0.0005% to 0.003%. According to relevant literature reports: steel contains 0.0015 %-0.003% B, can replace 1.0%-1.25% Ni, 0.1%-0.25% Mo, 0.30%-0.35% Cr, 0.2%-0.7% Mn, 0.1% V, 1.6 % Si, 0.001 % B can be equivalent to 1.33% Ni + 0.31% Cr + 0.04% Mo [9]. Vanadium and niobium can refine grains and improve the quality of steel; molybdenum and nickel can improve the plasticity and surface finish of steel [4].
3.2 Rare earth treatment of spring steel Because the main inclusions of electric furnace steel with lower content* are Al2O3 and aluminosilicates, adding rare earth to the steel can reduce the number of inclusions in the steel and reduce the formation of fatigue cracks. The rare earth can also act as a micro-alloy on the steel, thereby increasing the fatigue life of the spring steel.
3.3 Development of New Steel Types Various countries have conducted extensive research on the development of steel types for spring steel. Overseas studies have focused on improving the design stress of spring steel, and correspondingly proposed some steel types with higher stress. Such as: Si-Mn alloy spring steel has a higher anti-reducing, Germany revised in 1988 when the spring steel standard increased a Si-Cr alloy steel 54SiCr6 steel; Another example is Japan's SUP12 is also Si-Cr alloy steel . The United States has developed chromium-free spring steel because of insufficient chromium resources. British Tinsley Bridge Company developed the low-alloy spring steel 0.4C-Mn(Cr)-B, which is hereinafter referred to as "test steel", using "optimum metallurgy" technology. Compared with the conventional spring steel 525H60 leaf spring, the test steel did perform. Excellent surface core fatigue crack propagation ability, and the elastic strength and tensile strength are higher than conventional spring steel [10]. U.S. Patent No. 5,009,843 also describes a low-carbon spring steel having excellent fatigue resistance and resistance to erosion: C 0.35% to 0.55%, Si 1.80% to 3.00%, Mn 0.50% to 1.50%, and Ni 0.50% to 3 0%, Cr 0.10%-1.50%, Al 0.01%-0.05%, N 0.010%-0.025% [4].
The development of spring steel in China should proceed from the following aspects: 1 Based on 60Si2Mn steel, study the optimal composition and application of 60Si2MnB steel, 60Si2Mn silicon steel and 60Si2MnCr steel; 2 Introduction of foreign advanced steel grades; 3 due to low carbon horses The quaternary spring steel is easy to smelt, and the cost is low, the decarburization tendency is small, and the thermal processing performance is good. Research and development and application should be made. In recent years, China's low-carbon martensitic spring steel is based on 28MnSiB steel. By increasing the content of silicon and carbon in steel, 33MnSiB steel, 33MnSiB steel, and 35MnSiB steel have been developed to increase the strength and resistance to springback of spring steel.
4. Conclusion 1) Optimizing the smelting process and denaturing Al2O3 and other harmful inclusions can greatly reduce the content of harmful inclusions. The surface strength of parts can be increased by surface quenching, carburizing, carbonitriding, nitriding and shot blasting.
2) Avoiding surface decarburization It is very effective to eliminate or reduce the carbonization degree gradient existing between the two, and to adopt protective atmosphere heating and shorten the heating time.
3) to expand the use of alloying elements, in particular the use of trace alloy elements and rare earth processing, make full use of alloying elements alloying effect, can improve the comprehensive performance and service life of alloy spring steel.
4) The development of new technologies and new steel grades can improve the hardenability of spring steels, reduce decarburization tendencies, and improve the overall performance of spring steels.
Embedded Electric Fireplace,Practical Electric Fireplace,Ornamental Electric Fireplace,Electric Fireplace Without Heating
JINAN LODOR CNC EQUIPMENT CO.,LTD , https://www.zgfireplace.com