CN115522121B

CN115522121B - Low-silicon Nb-V composite microalloyed gear steel and manufacturing method thereof

Info

Publication number: CN115522121B
Application number: CN202211069305.9A
Authority: CN
Inventors: 胡芳忠; 汪开忠; 金国忠; 杨志强; 陈世杰; 杨少朋; 吴胜付; 景宏亮
Original assignee: Maanshan Iron and Steel Co Ltd
Current assignee: Maanshan Iron and Steel Co Ltd
Priority date: 2022-08-31
Filing date: 2022-08-31
Publication date: 2023-06-23
Anticipated expiration: 2042-08-31
Also published as: CN115522121A; WO2024046198A1

Abstract

The invention discloses low-silicon Nb-V composite microalloyed gear steel and a manufacturing method thereof, and belongs to the technical field of gear steel. The gear steel comprises the following chemical components in percentage by weight: c:0.22 to 0.26 percent, si: less than or equal to 0.10 percent, mn:0.30 to 0.50 percent, cr:0.70 to 0.90 percent, mo:0.30 to 0.50 percent, ni:0.30 to 0.50 percent of Al:0.030 to 0.050 percent, nb:0.030 to 0.060 percent, V:0.10 to 0.50 percent; p: less than or equal to 0.010 percent, S: less than or equal to 0.015 percent, T.O: less than or equal to 10ppm, [ N ]:60 to 120ppm, and the balance of Fe and unavoidable impurity elements. According to the gear steel obtained by optimizing the element proportion and the process, the depth of the surface oxide layer in the gear carburizing process is effectively reduced, the tensile strength of the gear steel is 1050-1200 MPa, the yield strength of the gear steel is 840-950 MPa, the elongation after fracture is more than or equal to 30%, the area shrinkage is more than or equal to 40%, the room temperature impact power (U2) is more than or equal to 85J, the depth of the oxide layer after carburization is less than or equal to 35 mu m, and the spin bending fatigue strength is more than or equal to 680MPa.

Description

Low-silicon Nb-V composite microalloyed gear steel and manufacturing method thereof

Technical Field

The invention belongs to the technical field of gear steel, and relates to low-silicon Nb-V composite microalloyed gear steel and a manufacturing method thereof, which are suitable for manufacturing high-quality steel for automobile parts.

Background

The automobile gear is an important component of an automobile transmission part, and the carburization technology is a main process technology of gear surface hardening treatment. In the gear carburization process, except that the case carburized layer is oxidized, oxygen is dissolved and enters the interior of the alloy and reacts with the more reactive elements in the alloy to form particulate oxide precipitates, known as internal oxidation. In the long-time running process of the gear, internal oxidation precipitates become fatigue sources, and finally part fatigue failure is caused; in addition, the alloy elements in the region after internal oxidation are reduced, so that the alloy elements are unevenly distributed and the hardenability is reduced, and the carburization deformation and transmission noise of the gear are improved. With the development of commercial vehicles and new energy vehicles in the direction of light weight and high power, higher requirements are put on the strength and fatigue life of gear steel, and the oxidation phenomenon in carburized layers is also more and more important.

The literature on the study of internal oxidation control technology of carburized gears researches the influence of a carburization process on internal oxidation behaviors, but how to improve the internal oxidation phenomenon in the carburization process is not considered from the aspect of element proportioning. The literature on the study of the internal oxidation control technology of carburized gears indicates that the oxidation elements Si, cr and Mn are in a proportional relation with the depth of an internal oxidation layer, and the Ni and Mo have little influence, but the literature does not quantitatively consider each element and does not limit the content of T.O.

Through searching, chinese patent publication No. CN101319294A, publication date is 12 months and 10 days of 2008, and discloses a fine grain carburized gear steel and a manufacturing method thereof, wherein the gear steel comprises the following chemical components in percentage by weight: 0.15 to 0.25 percent of C, less than or equal to 0.35 percent of Si, 0.60 to 0.90 percent of Mn, less than or equal to 0.015 percent of P, less than or equal to 0.010 percent of S, 0.80 to 1.20 percent of Cr, 0.15 to 0.35 percent of Mo, 0.02 to 0.08 percent of Nb, 0.0005 to 0.0035 percent of B, 0.02 to 0.06 percent of Al, 0.01 to 0.04 percent of Ti, less than or equal to 0.015 percent of [ N ], less than or equal to 0.0015 percent of [ O ], and the balance of Fe and unavoidable impurities. Meanwhile, ti is more than or equal to 2[ N ], and B is more than or equal to ([ N ]. About.Ti/3.4)/1.4+0.001. And adopts a rolling production process with the final rolling temperature lower than 900 ℃. The patent adopts Nb-Ti-B microalloying on the basis of 20CrMoH to realize grain refinement and performance improvement, but the patent component adopts high Si content and cannot control the oxidization phenomenon of a carburized layer.

In view of the above, there is no gear steel which can improve the internal oxidation phenomenon in the gear carburizing process and is suitable for electric furnace production.

Disclosure of Invention

1. Problems to be solved

Aiming at the problems that the existing gear steel is insufficient in deoxidizing capability in the traditional smelting process and the depth of an inner oxide layer cannot be effectively controlled, the invention provides the low-silicon Nb-V composite microalloyed gear steel which is suitable for electric furnace production and can effectively reduce the depth of the surface oxide layer in the gear carburizing process through element proportioning and rolling process optimization, and a manufacturing method thereof.

2. Technical proposal

In order to solve the problems, the technical scheme adopted by the invention is as follows:

the low-silicon Nb-V composite microalloyed gear steel comprises the following chemical components in percentage by weight: c:0.22 to 0.26 percent, si: less than or equal to 0.10 percent, mn:0.30 to 0.50 percent, cr:0.70 to 0.90 percent, mo:0.30 to 0.50 percent, ni:0.30 to 0.50 percent of Al:0.030 to 0.050 percent, nb:0.030 to 0.060 percent, V:0.10 to 0.50 percent; p: less than or equal to 0.010 percent, S: less than or equal to 0.015 percent, T.O: less than or equal to 10ppm, [ N ]:60 to 120ppm, and the balance of Fe and unavoidable impurity elements.

The microalloyed carburized gear steel has the tensile strength of 1050-1200 MPa, the yield strength of 840-950 MPa, the elongation after fracture of more than or equal to 30%, the area shrinkage of more than or equal to 40%, the room temperature impact power (U2) of more than or equal to 85J, the depth of an oxide layer after carburization of less than or equal to 35 mu m and the spin bending fatigue strength of more than or equal to 680MPa.

The invention provides a preparation method of low-silicon Nb-V composite microalloyed gear steel, which comprises the following steps: arc furnace smelting, LF refining, RH vacuum treatment, round billet continuous casting and rolling (finishing).

In the electric furnace smelting process, the content of Mn and Cr elements is controlled during electric furnace smelting through the strong deoxidizing capability of the electric furnace, so that the oxygen content in the finished steel is reduced; adding Cr and Mn-containing alloy in the electric furnace smelting stage by utilizing the strong deoxidizing capability of the electric furnace, and adjusting the Cr and Mn-containing alloy to a target value;

al wires are fed in the LF smelting process, so that the aluminum content can be ensured, excessive Al inclusion in steel can be prevented, and nozzle tumor accumulation is avoided;

in the RH vacuum smelting process, the vacuum degree is more than or equal to 35Pa, and the vacuum degassing time is more than or equal to 25min.

In the process of rolling the bar, the residual oxygen content of the billet in the heating furnace is less than or equal to 2.5 percent.

Carburizing at 930 deg.c, oil quenching at 830-880 deg.c, cooling to room temperature and low temperature tempering at 180-200 deg.c.

In the gear steel component provided by the invention, the actions and the contents of the components are controlled as follows:

c: c is the most effective strengthening element in steel, is the most effective element for influencing the hardenability, has lower cost, needs enough C content for ensuring the sufficient strength and the sufficient hardenability of the gear steel, and has proper carbon content which is favorable for fixing microalloy elements in the steel and avoiding oxidation in the carburization process. However, too high a carbon content affects the toughness of the steel, but is detrimental to the fatigue properties of the steel, so that the carbon content is determined to be in the range of 0.22 to 0.26%.

Si: si is a strong oxidizing element, and can improve the activity of C, but Si is an element easy to oxidize internally, and oxides formed by Si are far away from the surface and are difficult to remove in the subsequent processing process of the gear, so in order to avoid the influence of internal oxidation on the fatigue performance of the material, the Si content of the material is reduced as low as possible, and the Si content is controlled to be less than or equal to 0.10 percent.

Mn: mn can enlarge an austenite phase region, stabilize an austenite structure and improve the hardenability of steel, so that the Mn content is more than or equal to 0.30 percent. However, excessive Mn reduces the plasticity of the steel, deteriorates the toughness of the steel in the rolling process, is an easily oxidized element, and higher Mn increases the depth of an inner oxide layer, so that the Mn content is less than or equal to 0.50%, and in sum, the Mn content is controlled to be 0.30-0.50%.

Cr: cr can improve the hardenability and strength of steel, and Cr and carbon in the steel are combined to form tiny carbide, so that the strength and fatigue performance of the material are improved, and the Cr content is more than or equal to 0.50%; however, cr is an easily oxidizable element, and a higher Cr content deteriorates the oxide layer depth, so that the Cr content should be less than or equal to 0.90%. In summary, the Cr content is controlled to be 0.70-0.90%.

Mo: mo can obviously improve the hardenability of steel and prevent tempering brittleness and overheating tendency. In addition, the reasonable matching of Mo element and Cr element in the invention can obviously improve hardenability and tempering resistance, and Mo can refine grains. However, if the Mo content is too low, the effect is limited, and if the Mo content is too high, the formation of a grain boundary ferrite film is promoted, which is unfavorable for the thermoplasticity of steel, increases the reheat cracking tendency of steel, and has high cost. Mo element is not easy to oxidize, and can effectively inhibit internal oxidation in the carburizing process. Therefore, the Mo content is controlled to be 0.30-0.50%.

Ni: ni can effectively improve the core toughness of steel, reduce ductile-brittle transition temperature, improve low-temperature impact performance, has the effect of improving the fatigue strength of steel materials, and the other effect of Ni in the alloy system is to improve the stacking fault energy, improve dislocation crossing potential barrier and improve torsion resistance, while Ni has higher cost, and the machinability after hot working can be reduced due to the fact that the Ni content is too high. Therefore, the Ni content is controlled to be 0.30-0.50%.

Al: al is an effective deoxidizer and forms AlN refined grains, and when the Al content is less than 0.030%, the deoxidizing effect is not obvious, and when the Al content is more than 0.040%, coarse inclusions are easily formed, so that the performance of the steel is deteriorated. Therefore, the adding time of Al in the steelmaking process should be strictly controlled, and the content of Al should be controlled to be 0.030-0.050%.

Nb: nb is a very effective microalloying element for refining grains, and carbon nitride of Nb can be used for pinning grain boundaries to prevent austenite grains from growing, so that carburizing and quenching deformation is effectively reduced, and the characteristic in steel is that the recrystallization temperature of austenite is increased. In the rolling process, fine niobium carbonitride is separated out due to deformation induction, so that austenite grains are thinned, the toughness of the steel is improved, and the hardenability of the steel is reduced due to excessive Nb. Therefore, the Nb content is controlled to be 0.030 to 0.060%.

V: on the one hand, the addition of V is combined with C, N in steel to form carbide and nitride, fine carbonitride plays a role of pinning grain boundary to prevent the grain boundary from moving in the carburization process so as to refine grains, and on the other hand, the precipitation temperature of a vanadium-containing precipitation phase is higher than that of a niobium-containing precipitation phase, and the Nb and V composite addition can promote the precipitation of fine Nb, so that the strength and toughness of the steel are improved, and the V content is more than or equal to 0.1%. However, the higher V can lead to angle cracks of the continuous casting blank, and influence the yield of products, so that the V content is less than or equal to 0.5 percent. In summary, the V content should be controlled to be 0.10-0.50%.

P and S: sulfur is easy to form MnS inclusion with manganese in steel, so that the steel is hot and brittle, but a small amount of S is added, the cutting performance of gear steel can be obviously improved while the product performance is not influenced, and MnS has the effect of grain refinement; p is an element with strong segregation tendency, increases the cold brittleness of steel, reduces plasticity, and is harmful to uniformity of product structure and performance. Controlling P to be less than or equal to 0.010 percent and S to be less than or equal to 0.015 percent.

T.O: T.O is the main source of inclusions and internal oxidation points in steel, so the control of oxygen in steel is the key to determine the performance of gear steel, and T.O is less than or equal to 10ppm.

[ N ]: the N can form a compound with Nb, al and the like to refine grains, and reasonable Al/[ N ] has obvious effect on grain refinement, and excessively high N can form continuous casting defects such as bubbles and the like. Therefore, the [ N ] content should be controlled to 60 to 120ppm.

The low-silicon Nb-V composite microalloyed carburized gear steel provided by the invention has the advantages that the internal oxidation phenomenon in the carburization process is avoided, so that the carburization deformation is reduced, and the fatigue life and quality of the gear are improved. Cr, mn, si and T.O in the steel are all easily oxidized elements, which is unfavorable for the control of an oxide layer in the carburization process, so that the contribution coefficient X to the depth of the oxide layer is positive, while Mo, ni and Nb are not easily oxidized, and the toughness of the gear steel is favorable to be improved, so that the contribution coefficient X to the depth of the oxide layer is negative. In order to meet the low carburization and oxidation phenomenon of carburized gear steel, the X value is less than or equal to 100; however, in order to ensure good mechanical properties and production stability of the steel, the X value should be more than or equal to 50. In summary, in order to achieve the best alloying effect, the following formula should be satisfied between the elements: x=cr/13+mn/15+si/10+10×t.o-Mo 3-Ni 6-Nb 10-V9, 50.ltoreq.x.ltoreq.100.

Drawings

The technical solution of the present invention will be described in further detail below with reference to the accompanying drawings and examples, but it should be understood that these drawings are designed for the purpose of illustration only and thus are not limiting the scope of the present invention. Moreover, unless specifically indicated otherwise, the drawings are intended to conceptually illustrate the structural configurations described herein and are not necessarily drawn to scale.

FIG. 1 is a photograph of an organization according to example 1 of the present invention;

FIG. 2 is a photograph of an organization of comparative example 1 of the present invention;

FIG. 3 is a photograph showing the structure of comparative example 2 of the present invention.

Detailed Description

The invention is further described below in connection with specific embodiments.

Examples 1-3 are gear steels using specific components and specific smelting processes according to the present invention, comparative example 1 is gear steels using components according to the present invention, but no specific smelting process and no specific rolling process are used, which results in a t.o content that cannot be controlled to the target requirement, and comparative example 2 is 20CrMo produced according to the GB/T3077 standard and using conventional smelting and rolling processes. The examples are the same as the comparative examples in other smelting and rolling processes.

TABLE 1 chemical components (unit: T.O, [ N ] ppm, others wt%)

Examples	C	Si	Mn	P	S	Cr	Mo	Ni	Al	Nb	V	[N]	T.O	X
															Example 1	0.24	0.05	0.32	0.005	0.005	0.72	0.31	0.36	0.032	0.027	0.18	96	6	55.1
Example 2	0.23	0.04	0.38	0.007	0.007	0.81	0.35	0.41	0.036	0.032	0.46	95	7	62.1
															Example 3	0.25	0.02	0.47	0.007	0.008	0.87	0.44	0.48	0.042	0.043	0.32	98	6	52.6
Comparative example 1	0.23	0.09	0.45	0.008	0.009	0.82	0.36	0.36	0.037	0.039	0.26	99	13	124.1
															Comparative example 2	0.21	0.23	0.84	0.009	0.007	1.23	0.21	0.32	0.024	0.032	0.25	96	14	135.1
Comparative example 3	0.21	0.25	0.80	0.010	0.010	1.20	0.20	/	0.020	/	/	80	18	179.6

Table 2 shows the mechanical properties, the spin-bend fatigue properties after carburization and the oxide layer depth detection results of the example and comparative example materials after quenching and tempering heat treatment. The mechanical property heat treatment system is as follows: 860 ℃ x 1× (oil cooling) +200 ℃ x 2× (air cooling).

Table 2 mechanical properties and fatigue properties of examples and comparative examples

As can be seen from fig. 1-2, table 1 and table 2, the present invention provides a method for solving the problem of easy surface oxidation and lower fatigue life in the carburization process of the conventional carburized gear steel by alloy design and production process control. On the basis of 20CrMo steel, si is not added, the contents of Mn and Cr are reduced, C, ni, nb, V elements are added in proper amounts, the material structure grains are refined, and the mechanical properties of the material are improved. And the contact time of the easily oxidized elements Cr and Mn and oxygen is reduced by adjusting the alloy addition sequence in the steelmaking process, so that the ultra-low oxygen content is controlled. The design thought of the alloy with ultra-low Si and ultra-low O of the invention is also applicable to other alloy steel systems, and can effectively reduce the depth of the surface oxide layer in the gear carburizing process.

Claims

1. A low-silicon Nb-V composite microalloyed gear steel is characterized in that: comprises the following chemical components in percentage by weight: c: 0.22-0.26%, si: less than or equal to 0.10 percent, mn: 0.30-0.50%, cr: 0.70-0.90%, mo: 0.30-0.50%, ni: 0.30-0.50%, al: 0.030-0.050%, nb: 0.030-0.060%, V: 0.10-0.50%; p: less than or equal to 0.010 percent, S: less than or equal to 0.015 percent, T.O: less than or equal to 10ppm, [ N ]: 60-120 ppm, and the balance of Fe and unavoidable impurity elements;

the components meet the relation of X=Cr/13+Mn/15+Si/10+10. O-Mo 3-Ni 6-Nb 10-V9, and X is more than or equal to 50 and less than or equal to 100.

2. The low silicon Nb-V composite micro-alloyed gear steel according to claim 1, wherein: the depth of an oxide layer after carburization is less than or equal to 35 mu m, and the spin bending fatigue strength is more than or equal to 680MPa.

3. The low silicon Nb-V composite micro-alloyed gear steel according to claim 1, wherein: the tensile strength is 1050-1200 MPa, the yield strength is 840-950 MPa, the elongation after fracture is more than or equal to 30%, the area shrinkage is more than or equal to 40%, and the room temperature impact power U2 is more than or equal to 85J.

4. A manufacturing method of low-silicon Nb-V composite microalloyed gear steel is characterized by comprising the following steps of: the production using the components and proportions of any one of claims 1 to 3, comprising the steps of:

smelting in an arc furnace;

step two, LF refining and RH vacuum treatment;

step three, round billet continuous casting;

step four, rolling;

and fifthly, carburizing treatment, oil quenching, cooling and low-temperature tempering.

5. The method for manufacturing the low-silicon Nb-V composite microalloyed gear steel in accordance with claim 4, which is characterized in that: adding Cr and Mn alloy in an electric furnace smelting stage, and adjusting to a target value; in the second step, al wires are fed in the LF smelting process, and in the RH vacuum smelting process, the vacuum degree is more than or equal to 35Pa, and the vacuum degassing time is more than or equal to 25min.

6. The method for manufacturing the low-silicon Nb-V composite microalloyed gear steel in accordance with claim 4, which is characterized in that: in the third step, the residual oxygen content of the steel billet in the heating furnace is less than or equal to 2.5 percent.

7. The method for manufacturing the low-silicon Nb-V composite microalloyed gear steel in accordance with claim 4, which is characterized in that: and fifthly, performing carburization treatment at 930 ℃, performing oil quenching treatment at 830-880 ℃ after carburization heat treatment, cooling to room temperature, and performing low-temperature tempering at 180-200 ℃.