WO2024046198A1 - Low-silicon nb-v composite microalloyed gear steel and manufacturing method therefor - Google Patents

Abstract

A low-silicon Nb-V composite microalloyed gear steel and a manufacturing method therefor, which belong to the technical field of gear steels. A gear steel comprises the following chemical components in percentages by weight: C: 0.22-0.26%, Si: ≤0.10%, 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: ≤0.010%, S: ≤0.015%, T.O: ≤10 ppm, and [N]: 60-120 ppm, with the balance being Fe and inevitable impurities. The depth of a surface oxidation layer during a gear carburizing process is effectively reduced by means of element matching and process optimization, and the obtained gear steel has a tensile strength of 1050-1200 MPa, a yield strength of 840-950 MPa, a percentage elongation after fracture of ≥30%, the percentage reduction of area of ≥40%, a room-temperature impact energy (U2) of ≥85 J, an oxidation layer depth after carburizing of ≤35 μm, and a rotating bending fatigue strength of ≥680 MPa.

Description

A kind of low silicon Nb-V composite microalloyed gear steel and its manufacturing method

This application requires the priority of the Chinese patent application submitted to the China Patent Office on August 31, 2022, with the application number 202211069305.9 and the invention title "A low-silicon Nb-V composite microalloyed gear steel and its manufacturing method". The entire contents of which are incorporated herein by reference.

Technical field

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

Background technique

Automobile gears are an important part of automobile transmission components, and carburizing technology is the main process technology for gear surface hardening treatment. During the gear carburizing process, in addition to the oxidation of the surface carburized layer, oxygen dissolves and enters the interior of the alloy, and reacts with the more active elements in the alloy to form granular oxide precipitation, which is called internal oxidation. During the long-term operation of gears, internal oxidation precipitates become fatigue sources, eventually leading to fatigue failure of parts; in addition, the alloying elements in the area after internal oxidation are reduced, resulting in uneven distribution of alloying elements and reduced hardenability, thus improving the gear Carburizing deformation and transmission noise. As commercial vehicles and new energy vehicles develop in the direction of lightweighting and high power, higher requirements have been placed on the strength and fatigue life of gear steel, and the oxidation phenomenon in the carburized layer has received more and more attention.

The literature "Research on Internal Oxidation Control Technology of Carburized Gears" studies the impact of the carburizing process on internal oxidation behavior, but does not consider how to improve the internal oxidation phenomenon during the carburizing process from the perspective of element ratio. The document "Research on Oxidation Control Technology in Carburized Gears" points out that the oxidation elements Si, Cr, and Mn are directly proportional to the depth of the internal oxide layer, while Ni and Mo have almost no impact. However, this document does not quantitatively consider each element and does not evaluate the T.O content. Make limitations.

After searching, the Chinese patent publication number is CN101319294A, and the publication date is December 10, 2008. It discloses a fine-grain carburized gear steel and its manufacturing method. The chemical composition (weight%) of the gear steel is: C 0.15 ~0.25%, Si≤0.35%, Mn 0.60～0.90%, P≤0.015%, S≤0.010%, Cr 0.80～1.20%, Mo 0.15～0.35%, Nb 0.02～0.08%, B 0.0005～0.0035%, Al 0.02～0.06%, Ti0.01～0.04%, [N]≤0.015%, [O]≤0.0015%, the rest is Fe and inevitable impurities. At the same time, it is required that Ti≥2[N], B≥([N]～Ti/3.4)/1.4+0.001. And adopt a rolling production process with a final rolling temperature lower than 900°C. This patent uses Nb-Ti-B micro-alloying based on 20CrMoH to achieve grain refinement and performance improvement. However, the patented composition uses high Si content and cannot control the oxidation phenomenon of the carburized layer.

To sum up, there is currently no disclosed gear steel that can improve the internal oxidation phenomenon during gear carburizing and is suitable for electric furnace production.

Contents of the invention

1.Problems to be solved

In view of the problem that the existing gear steel has insufficient deoxidation ability in the traditional smelting process and the depth of the internal oxide layer cannot be effectively controlled, the present invention provides a gear steel suitable for electric furnace production, which can effectively reduce gear seepage through element proportioning and rolling process optimization. Low-silicon Nb-V composite microalloyed gear steel with surface oxide layer depth in the carbon process and its manufacturing method.

2.Technical solutions

In order to solve the above problems, the technical solutions adopted by the present invention are as follows:

A low-silicon Nb-V composite microalloyed gear steel, including the following chemical components in weight percentage: C: 0.22~0.26%, Si: ≤0.10%, 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: ≤0.010%, S: ≤0.015%, T.O: ≤10ppm , [N]: 60~120ppm, the rest is Fe and inevitable impurity elements.

The microalloyed carburized gear steel has a tensile strength of 1050-1200MPa, a yield strength of 840-950MPa, an elongation after fracture ≥30%, a shrinkage of area ≥40%, room temperature impact energy (U2) ≥85J, and carburization The depth of the back oxide layer is ≤35μm, and the bending fatigue strength is ≥680MPa.

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

During the electric furnace smelting process, the strong deoxidizing ability of the electric furnace is used to control the content of Mn and Cr elements during the electric furnace smelting, thereby reducing the oxygen content in the finished steel; using the strong deoxidizing ability of the electric furnace, the addition of Cr, Mn-containing elements is added during the electric furnace smelting stage. alloy and adjust to target value;

Feeding Al wire during the LF smelting process can not only ensure the aluminum content, but also prevent excessive Al inclusions in the steel and avoid accumulation of tumors at the nozzle;

In the RH vacuum smelting process, the vacuum degree is ≥35Pa and the vacuum degassing time is ≥25min.

During the bar rolling process, the residual oxygen content of the steel billet in the heating furnace is ≤2.5%.

The carburizing temperature is 930°C for carburizing treatment. After carburizing heat treatment, oil quenching is performed at 830-880°C. After cooling to room temperature, low-temperature tempering is performed. The tempering temperature is 180-200°C.

It should be noted that among the gear steel components provided by the present invention, the functions of each component and their content are controlled as follows:

C: C is the most basic and effective strengthening element in steel. It is the most effective element affecting hardenability and has low cost. In order to ensure that gear steel has sufficient strength and sufficient hardenability, sufficient C content is required, and The appropriate amount of carbon content is beneficial to fixing the micro-alloying elements in the steel and avoiding oxidation during the carburization process. However, excessive carbon content will affect the toughness of the steel and is detrimental to the fatigue performance of the steel, so the carbon content range is determined to be 0.22 to 0.26%.

Si: Si is a strong oxidizing element that can increase the activity of C. However, Si is an element that is easily internally oxidized and the oxide formed by Si is far away from the surface and is difficult to remove during the subsequent processing of the gear. Therefore, in order to avoid internal oxidation on the material To influence fatigue performance, the Si content of the material should be reduced as low as possible, so the Si content is controlled at ≤0.10%.

Mn: Mn can expand the austenite phase area, stabilize the austenite structure, and improve the hardenability of steel, so the Mn content is ≥0.30%. However, excessive Mn will reduce the plasticity of the steel and worsen the toughness of the steel during rolling. Mn is an easily oxidized element. Higher Mn will increase the depth of the internal oxide layer, so the Mn content should be ≤0.50%. In summary, The Mn content is controlled at 0.30~0.50%.

Cr: Cr can improve the hardenability and strength of steel. Cr combines with the carbon in the steel to form fine carbides, which improves the strength and fatigue properties of the material. Therefore, the Cr content is ≥0.50%; however, Cr is an easily oxidized element, and higher Cr content will worsen the oxide layer depth, so the Cr content should be ≤0.90%. To sum up, the Cr content is controlled at 0.70~0.90%.

Mo: Mo can significantly improve the hardenability of steel and prevent temper brittleness and overheating tendencies. In addition, the reasonable combination of Mo element and Cr element in the present invention can significantly improve the hardenability and tempering resistance, and Mo can refine the grains. If the Mo content is too low, the above-mentioned effects will be limited. If the Mo content is too high, it will promote the formation of grain boundary ferrite films, which is not conducive to the thermoplasticity of the steel, increases the tendency of steel to reheat cracks, and increases the cost. The Mo element is not easily oxidized and can effectively inhibit internal oxidation during the carburizing process. Therefore, the Mo content is controlled to be 0.30 to 0.50%.

Ni: Ni can effectively improve the core toughness of steel, reduce the ductile-brittle transition temperature, improve low-temperature impact properties, and has the effect of improving the fatigue strength of steel materials. Another role of Ni in this alloy system is to increase stacking fault energy and dislocation It increases across the potential barrier and improves the torsion resistance, but the cost of Ni is higher, and too high Ni content will reduce the machinability after hot working. Therefore, the Ni content is controlled at 0.30~0.50%.

Al: Al is an effective deoxidizer and can form AlN refined grains. When the Al content is lower than 0.030%, the deoxidation effect is not obvious. When the Al content is higher than 0.040%, it is easy to form coarse inclusions and deteriorate the performance of the steel. Therefore, the timing of adding Al during the steelmaking process should be strictly controlled to ensure that the Al content should be controlled at 0.030 to 0.050%.

Nb: Nb is a very effective micro-alloying element that refines grains. Nb carbonitrides can "pin" grain boundaries, hinder the growth of austenite grains, and effectively reduce carburizing and quenching deformation. In steel The characteristic is to increase the recrystallization temperature of austenite. During the rolling process, fine niobium carbonitride precipitates due to deformation induction, thereby refining the austenite grain. The purpose of Nb is to improve the strength and toughness of steel, but too much Nb will reduce the hardenability of steel. Therefore, the Nb content is controlled between 0.030% and 0.060%.

V: On the one hand, the addition of V combines with C and N in the steel to form carbides and nitrides. The fine carbonitrides serve to pin the grain boundaries and hinder the movement of grain boundaries during the carburization process to refine the grains. On the other hand, the addition of V combines with C and N in the steel to form carbides and nitrides. On the other hand, since the precipitation temperature of the vanadium-containing precipitate phase is higher than that of the niobium-containing precipitate phase, the composite addition of Nb and V can promote the precipitation of fine Nb, which is beneficial to improving the strength and toughness of the steel, so the V content is ≥0.1%. However, higher V will cause corner cracks in the continuous casting billet and affect the product yield, so the V content is ≤0.5%. To sum up, the V content should be controlled at 0.10~0.50%.

P and S: Sulfur easily forms MnS inclusions with manganese in steel, making the steel hot and brittle. However, adding a small amount of S will significantly improve the cutting performance of gear steel without affecting product performance, and MnS also has the ability to refine The effect of grains; P is an element with a strong segregation tendency, which increases the cold brittleness of steel, reduces plasticity, and is harmful to the uniformity of product structure and performance. Control P≤0.010%, S≤0.015%.

T.O: T.O is the main source of inclusions and internal oxidation points in steel. Therefore, the control of oxygen in steel is the key to determining the performance of gear steel, so T.O≤10ppm.

[N]: [N] can form compounds with Nb and Al to refine the grains. A reasonable Al/[N] has a significant effect on grain refinement, while too high [N] will form continuous casting defects such as bubbles. . Therefore, the [N] content should be controlled between 60 and 120 ppm.

The low-silicon Nb-V composite microalloyed carburized gear steel provided by the invention should avoid internal oxidation during the carburizing process, thereby reducing the amount of carburizing deformation and improving the gear fatigue life and quality. Cr, Mn, Si and TO in steel are all easily oxidized elements, which are not conducive to the control of the oxide layer during carburization. Therefore, the contribution coefficient X to the oxide layer depth is positive, while Mo, Ni, Nb are not easily oxidized and are beneficial to improving the gear Due to the strength and toughness of steel, the contribution coefficient X to the depth of the oxide layer is negative. In order to satisfy the low carburization and oxidation phenomenon of carburized gear steel, the X value should be ≤100; but in order to ensure the good mechanical properties and production stability of the steel, the X value should be ≥50. To sum up, in order to achieve the best alloying effect, the following formula should be satisfied between each element:
X＝Cr/13+Mn/15+Si/10+10*TO-Mo*3-Ni*6-Nb*10-V*9, 50≤X≤100.

Description of drawings

The technical solutions of the present invention will be described in further detail below with reference to the accompanying drawings and examples. However, it should be understood that these drawings are only designed for explanation purposes and therefore do not limit the scope of the present invention. Furthermore, unless otherwise noted, the drawings are intended only to conceptually illustrate the structural configurations described herein and are not necessarily drawn to scale.

Figure 1 is a tissue photo of Embodiment 1 of the present invention;

Figure 2 is a tissue photo of Comparative Example 1 of the present invention;

Figure 3 is a tissue photograph of Comparative Example 2 of the present invention.

Detailed ways

The present invention will be further described below with reference to specific embodiments.

Examples 1-3 are gear steels using specific components and a specific smelting process of the present invention. Comparative Example 1 uses the components of the present invention, but does not use a specific smelting process and rolling process, which results in the T.O content not being able to be controlled to the target requirements. , Comparative Example 2 is 20CrMo produced in accordance with the requirements of GB/T 3077 standard and using conventional smelting and rolling processes. Other smelting and rolling production processes of the Examples and Comparative Examples are the same.

Table 1 Chemical composition of examples of the present invention (unit: TO, [N] is ppm, others are wt%)

Table 2 shows the mechanical properties after quenching and tempering heat treatment, the bending fatigue performance after carburization and the detection results of the oxide layer depth of the materials of the Examples and Comparative Examples. The mechanical properties heat treatment system is: 860℃×1h (oil cooling) + 200℃×2h (air cooling).

Table 2 Examples and Comparative Examples Mechanical properties and fatigue properties

As can be seen from Figures 1-2 and Tables 1 and 2, the present invention proposes a method to solve the problem of easy surface oxidation and low fatigue life during the carburization process of traditional carburized gear steel through alloy design and production process control. On the basis of 20CrMo steel, the present invention does not add Si and reduces the Mn and Cr contents, but adds appropriate amounts of C, Ni, Nb, and V elements to refine the material structure grains and improve the material mechanical properties. And by adjusting the order of alloy addition during the steelmaking process, the contact time between easily oxidized elements Cr and Mn and oxygen is reduced to achieve ultra-low oxygen content control. The ultra-low Si and ultra-low O alloy design ideas patented by this invention can also be applied to other alloy steel systems, and can effectively reduce the depth of the surface oxide layer during the gear carburizing process.

Claims (8)

1. A low-silicon Nb-V composite microalloyed gear steel, characterized by: including the following chemical components in weight percent: C: 0.22~0.26%, Si: ≤0.10%, 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: ≤0.010%, S: ≤0.015%, T.O: ≤10ppm, [N]: 60~120ppm, the rest are Fe and inevitable impurity elements.
2. A low-silicon Nb-V composite microalloyed gear steel according to claim 1, characterized in that its composition satisfies the relationship formula X=Cr/13+Mn/15+Si/10+10*T.O-Mo* 3-Ni*6-Nb*10-V*9, 50≤X≤100.
3. A low-silicon Nb-V composite microalloyed gear steel according to claim 1, characterized in that: the depth of the oxide layer after carburization is ≤35 μm, and the bending fatigue strength is ≥680MPa.
4. A low-silicon Nb-V composite microalloyed gear steel according to claim 1, characterized in that: its tensile strength is 1050-1200MPa, yield strength is 840-950MPa, elongation after fracture is ≥30%, and section shrinkage ≥40%, room temperature impact energy (U2) ≥85J.
5. A method for manufacturing low-silicon Nb-V composite microalloyed gear steel, which is characterized in that the components and proportions in any one of claims 1-4 are used for production, including the following steps:

   Step 1. Electric arc furnace smelting;

   Step 2, LF refining and RH vacuum treatment;

   Step 3: Continuous casting of round billets;

   Step 4. Rolling;

   Step five: carburizing treatment, oil quenching, cooling and low temperature tempering.
6. A method for manufacturing low-silicon Nb-V composite microalloyed gear steel according to claim 5, characterized in that: in step one, Cr and Mn-containing alloys are added during the electric furnace smelting stage and adjusted to the target value; step In the second middle, Al wire is fed during the LF smelting process. During the RH vacuum smelting process, the vacuum degree is ≥35Pa and the vacuum degassing time is ≥25min.
7. A method for manufacturing low-silicon Nb-V composite microalloyed gear steel according to claim 5, characterized in that in step three, the residual oxygen content of the steel billet in the heating furnace is ≤ 2.5%.
8. A method for manufacturing low-silicon Nb-V composite microalloyed gear steel according to claim 5, characterized in that: in step five, the carburizing temperature is 930°C for carburizing treatment, and after the carburizing heat treatment, the temperature is 830 to 830°C. Oil quenching is performed at 880°C, and then low-temperature tempering is performed after cooling to room temperature. The tempering temperature is 180~200°C.