Application Status and Prospects of Non-quenched and Tempered Steel on Construction Machinery

Most of the components in construction machinery are made of steel materials, the working environment of construction machinery is bad, and modern construction machinery is developing in the direction of high power and low self-weight. Limited by the space structure of the car body, it is impossible to achieve high dynamic performance by expanding the structure and increasing the number of engine cylinders. Therefore, the solution can only be to continuously improve the material strength and machining accuracy, thereby improving the running performance of the components and reducing Power loss, improve output power.

Non-quenched and tempered steel is a steel that has developed rapidly in recent years. It is made by adding trace (≤0.2%) alloying elements (mainly Ti, V, Nb, etc.) to medium carbon steel to make one or several The mechanical properties of the steel have significant changes. For example, when 0.10% V is added to steel, the strength increment of 250 MPa or more can be generated, and in special cases, it can be increased by 300 MPa. This kind of steel is called microalloyed non-tempered medium carbon steel in Europe and America, and Japan is called non-tempered structural steel or non-tempered steel.

1. Non-tempered steel development application background

Since the 1970s, Germany's THYSSEN has developed the first non-tempered steel 49MnVS3 to replace the quenched and tempered CK45 steel. Since the manufacture of automotive crankshafts, non-tempered steel has been widely used in the automotive field, Japan's 55% shaft parts, 75 % of forged structural parts have been made of non-tempered steel. Among them, the hot forging and tempering parts used by Mitsubishi Motors' steering system and transmission system are almost non-tempered. Toyota, Sumitomo and other companies have 90% of crankshafts and 75 The % connecting rod is made of non-tempered steel, and the Volkswagen connecting rod made of 27MnSiVS6 non-tempered steel is used by the German public to produce 2.5 million pieces per year. Table 1 lists examples of domestic and foreign manufacturers applying non-tempered steel to automobiles.

Table 1 Examples of domestic and foreign manufacturers applying non-tempered steel to automobiles

2. Characteristics of non-tempered steel

(1) Within a certain range, the strength of steel increases with the increase of carbon content, and the increase of carbon content increases the strength and reduces the toughness of steel. Dispersion strengthening and solid solution strengthening contribute more to the strength of non-quenched and tempered steel than quenched and tempered steel. In non-tempered steel, the carbon content can be reduced and the same strength as quenched and tempered steel is still obtained. Therefore, non-tempered steel has better toughness at the same strength.

(2) Non-quenched and tempered steel has good cutting performance. Under the same hardness, non-quenched and tempered steel with ferritic-pearlite structure has better cutting performance than quenched and tempered structural steel with sorbite structure.

(3) Non-tempered steel has good surface strengthening properties In order to improve the wear resistance and fatigue strength of the surface of engineering machinery parts, surface strengthening treatment is required. Compared with quenched and tempered steel of the same strength grade, under the same nitriding and soft nitriding process conditions, the non-quenched and tempered steel layer can obtain higher hardness and deeper seepage, and the core part after nitriding treatment Hardness is not reduced.

(4) Non-tempered steel can save the heat treatment cost of 6% of the total cost of quenched and tempered steel production. Germans using 49MnVS3 steel instead of quenched and tempered steel to manufacture connecting rods can save 38% of the total cost; Japan Aichi analysis, non-tempered steel due to the omission of quenching and tempering treatment process, can reduce the cost of hot forging products by 18%.

(5) The labor cost and production management cost are saved because the quenching and tempering process is omitted. In addition, for existing heat treatment workshops, maintenance costs for some heat treatment equipment can be eliminated.

(6) Non-tempered steel eliminates the quenching and tempering process, thus avoiding the transfer of the workpiece and eliminating the deformation caused by the heat treatment, eliminating the straightening process and greatly improving the production efficiency.

(7) Eliminate the waste water, waste gas and waste oil generated during the quenching and tempering process, greatly improving the environment of the production site, and is known as “green steel”.

3. Application status of non-quenched and tempered steel on construction machinery

There are many types of construction machinery, and the products are mainly 18 categories of lifting machinery, excavating machinery, reinforced concrete machinery, earthmoving machinery and road machinery. In Europe, the United States and Japan, the application of non-tempered steel to the above-mentioned machinery is extensive, and in the domestic due to the backwardness of smelting technology and low cost of quenching and tempering, the use of non-tempered steel in the field of construction machinery lacks power, so it is still in its infancy .

The successful application of non-quenched and tempered steel in the automotive field is a good inspiration. There are many similarities between the automotive industry and the construction machinery field. For example, forklifts and automobiles have little difference in structure and power, so non-tempered steel can be used. Apply to the parts of the forklift.

Hydraulic cylinders are strength members of main engines such as excavators and forklifts. The hydraulic cylinder consists essentially of the cylinder and cylinder head, the piston and piston rod, the sealing device, the damping device and the exhaust device. Table 2 is a summary table for applying non-quenched and tempered steel to the hydraulic cylinder parts. The mechanical properties of the materials on the hydraulic cylinders. The parts made of non-quenched and tempered steel were sampled and measured, and the results showed that the non-tempered steel used can meet the technical requirements.

Table 2 Mechanical performance requirements of hydraulic cylinder parts and replacement of corresponding parts with non-tempered steel

Excavators, crawler cranes and bulldozers, which account for a large share of construction machinery, are crawler-type walking construction machinery, and the four-wheeled belt (roller wheel, drive wheel, guide wheel, roller and walking track) are the supporting components. Modern engineering machinery is getting more and more powerful, the weight of the machine itself is increasing, and the flexibility of work is getting higher and higher. Therefore, the roller, the drive wheel and the track are the key to the technology.

Japan first applied non-tempered steel to the axle of the excavator, replacing the carbon structural steels S43C, S45C and SCM435. The excavator roller is mainly composed of the wheel body and the shaft. It bears the weight of the whole body and needs to bear more impact during work. All of these require high strength and toughness of the wheel body and shaft. In addition to the weight of the fuselage, the roller axle has the function of advancing the track along the wheel. Therefore, it has a higher yield strength to make the shaft less bendable, that is, a larger yield ratio is used for the roller axle. The key factor. An excavator manufacturer in Japan replaced the S45C used to manufacture the roller axle with non-tempered steel. The process comparison is shown in Table 3.

Table 3 Comparison of process routes between quenched and tempered steel for heavy duty axles

It can be seen from Table 3 that the use of non-tempered steel can eliminate the roughing and quenching and tempering process, thus greatly reducing the processing and logistics costs, making the product more competitive.

In Europe and the United States, the technology of applying non-tempered steel to the axles of the supporting axles is also very mature. For example, Caterpillar, the leader of construction machinery, not only produces non-tempered steel roller axles locally, but also with China's steel companies. Co-developed non-tempered steel to assemble the roller that its company produces in China, thereby reducing logistics costs.

The jaw crusher is an indispensable equipment for material crushing in mines, quarries, etc., and it has to withstand considerable impact during work. The eccentric shaft is an important part of the jaw crusher. In order to ensure the reliability of the jaw crusher, the eccentric shaft is required to have good comprehensive mechanical properties. The traditional manufacturing material is 45 steel, the process is complicated, the heat treatment defect is easy to occur in the quenching and tempering process, and the energy consumption is large. Shanghai Jianlu Luqiao Machinery Equipment Co., Ltd. has achieved good application results by using 38MnVS6 non-tempered steel instead of 45 steel. Non-quenched and tempered steel is also used in the production of fasteners, but only in a few foreign companies.

In summary, non-tempered steel is mostly applied to shaft products, but it is rarely applied to the wheel and structural parts of construction machinery. Non-tempered steels are now of many types and their performance is relatively stable. The application of non-tempered steel in construction machinery should be accelerated.

4. Prospects for the application of non-quenched and tempered steel in construction machinery

Non-tempered steel has excellent properties, good processability and weldability, which make it suitable for use in construction machinery.

There are many types of chassis parts for steel used in construction machinery, such as excavators, bulldozers, and crawler cranes. The chassis member can be summarized as "four wheels and one belt", that is, the guide wheel, the drive wheel, the roller, the roller and the track assembly. The main function of the guide wheel is to guide the smooth running of the body and adjust the tension of the track, especially when the machine drives over the rock. The wheel body is mostly a casting part, and the guiding wheel axle is a shaped step shaft, and the step is locally heated and then forged, as shown in FIG. The manufacturing process of the shaft is complicated, and the hot working process includes processes such as quenching and tempering, forging and induction hardening. Because of its complicated process, it requires a large amount of management expenses, labor costs, electricity charges, etc. in the production process, which is easy to produce unqualified parts, resulting in high prices. If the non-tempered steel for forging is used instead of the original material to be tempered, the cost can be greatly reduced and the production efficiency can be improved.

Figure 2 Metallographic picture in different processing states

Our company has now begun to try to use forged non-tempered steel. The use of non-quenched and tempered steel to produce forged products should solve the problem of coarse grain after forging. Figure 2 shows the metallographic structure before and after forging the axle with non-tempered steel. It can be seen that the grain is abnormally coarse after forging. Table 4 shows the mechanical properties before and after forging. It can be seen that the impact properties, elongation and reduction of area are greatly reduced.

In order to solve the problem that the mechanical properties of the non-quenched and tempered steel decreased after forging, the forged shaft was treated by a more economical and convenient method. The mechanical properties after treatment are shown in Table 4. It can be seen from Table 4 that the mechanical properties of the treated shaft are comparable to those before forging and far superior to the mechanical properties after forging. At present, our company is working hard to promote the application of this process on the guiding axle.

Table 4 Comparison of mechanical properties under different treatment conditions

The application of non-quenched and tempered steel on the guide axle can not only reduce the production cost of the enterprise, increase the profit rate, but also reduce the pollution of the environment during production because it reduces the production process. In short, if the non-tempered steel meets the technical requirements, it can replace the materials that need to be tempered to meet the technical requirements. Table 5 summarizes the engineering machinery components that can be replaced with non-tempered steel.

Table 5 Summary of construction machinery parts that can be made from non-quenched and tempered steel

It can be seen that the application prospects of non-quenched and tempered steel on construction machinery are very broad, but limited to the low level of smelting in domestic steel mills and the high price of non-quenched and tempered steel, so that non-tempered steel is not widely used in construction machinery. Therefore, by adjusting the chemical composition of non-quenched and tempered steel, improving the refining technology, improving the segregation control technology of the slab composition, and developing the multiphase structure in the steel, the development cost is lowered, and the non-tempered steel with higher performance is developed for use in construction machinery. .

About the author: Zhang Kun, Ma Qiang, Shantui Construction Machinery Co., Ltd.