CN106636590B - A kind of medium carbon steel thermo-mechanical processi method of alternative modifier treatment - Google Patents

Abstract

A medium-carbon steel thermomechanical treatment method that can replace quenching and tempering treatment, which mainly includes: heating medium-carbon steel round bars to the α+γ two-phase region for a period of time, and rolling them out of the furnace for 9 passes, accumulating The amount of deformation is 70%. After every 3 passes of rolling, return to the furnace for 5 minutes for heating, and after the 9th pass of rolling, air cool to room temperature. The microstructure of the rolled steel bar is composed of fibrous ferrite matrix and granular cementite and forms a strong <101> silk texture. The comprehensive mechanical properties of the medium carbon steel bars prepared by the present invention can not only reach the performance level of the medium carbon steel treated by the traditional quenching and tempering process, but also the process of the present invention is simple, the production efficiency is high, and it avoids the impact of the quenching and tempering cooling medium on the environment. It does not involve workpiece deformation, internal micro-crack initiation and cracking caused by quenching stress. The process performance and mechanical properties of the product are not affected by hardenability and temper brittleness. For large cross-section workpieces, there is no need to use alloy steel, which greatly reduces the material cost.

Description

A thermomechanical treatment method for medium carbon steel that can replace quenching and tempering treatment

technical field

The invention relates to a method for processing iron and steel materials.

Background technique

The traditional heat treatment of medium carbon steel is quenching and tempering treatment, that is, high temperature tempering after quenching to obtain tempered sorbite structure, which is composed of ferrite matrix and spherical cementite particles evenly distributed, so as to ensure good strength , plasticity and toughness, that is, to obtain good comprehensive mechanical properties. This traditional process is mainly used in the manufacture of mechanical parts such as shafts, pins, gears and connecting rods. The quenching and tempering process involves quenching and high temperature tempering, which has the following disadvantages: (1) Quenching and high temperature tempering require long-term heating and heat preservation, consume a lot of energy, and the production efficiency is low, and the cooling medium used for quenching and tempering is harmful to the environment Certain pollution. (2) In order to ensure a uniform tempered sorbite structure for workpieces with larger cross-sections, alloy steels with high hardenability and higher alloying elements must be used to meet the requirements for the depth of the hardened layer of workpieces with large cross-sections. The material cost is high; for some alloy steels, there is still high temperature tempering brittleness sensitivity, and the more expensive alloying element Mo has to be added. (3) The quenching process is accompanied by the volume expansion effect caused by rapid martensitic transformation, which causes a large quenching stress, and there is a tendency of workpiece deformation, internal micro-crack initiation and cracking.

Contents of the invention

The object of the present invention is to provide a medium carbon steel thermomechanical treatment method that can not only ensure the internal metallographic phase of the medium carbon steel, but also save energy and have high efficiency, which can replace quenching and tempering treatment. The technical route of the method of the present invention is: heating the medium-carbon quenched and tempered steel to the α+γ two-phase region, carrying out multi-pass rolling deformation after heat preservation, and then air cooling to room temperature to obtain ferrite+ with fibrous characteristics Spherical cementite structure.

The technical scheme that the present invention adopts is specifically as follows:

(1) Heat the medium-carbon steel bar in a heating furnace to the temperature of the α+γ two-phase region, the temperature range is 720-820°C, and keep it for 40 minutes to obtain a ferrite+austenite dual-phase structure;

(2) Take out the steel bar and immediately carry out 9-pass rolling, the total cumulative deformation is controlled at not less than 70%, and after every 3 passes during the rolling process, put it back into the heating furnace for reheating for 5 minutes, and then proceed to the next step. A 3-pass rolling, air cooling to room temperature after the 9th rolling. Because of the dynamic recrystallization of ferrite at the initial stage of rolling, the austenite is elongated into a fibrous shape, and the grain boundaries and dislocations increase. The rolling process is accompanied by the cooling of the bar. When the temperature drops to A _r1 , the austenite undergoes dynamic pearlite transformation, and at the same time, the lamellar carbides will be broken under the action of rolling stress and strain. In order to prevent the rapid increase of rolling resistance caused by excessive temperature drop in the rolling process, the rolling bar is put back into the rolling heating furnace for 5 minutes after every 3 passes. At this time, due to the promotion of carbon atom diffusion by grain boundaries and dislocations As a result, the cementite undergoes rapid spheroidization, and at the same time, a part of the austenite transformation process also occurs, so as to ensure that the subsequent rolling process can be completed without a significant increase in the rolling resistance. After the 9th rolling pass, it is air-cooled to room temperature, and a fibrous structure composed of ferrite and granular carbide is obtained, and has a strong <101> silk texture, which makes a large amount of {001} of ferrite cleaved The surface is parallel to the rolling direction, which is not only conducive to the crack deflection of the impact fracture perpendicular to the rolling direction to improve the impact toughness, but also to improve the strength and plasticity.

The physical metallurgical principle of the method of the present invention is: the initial pass rolling deformation of the medium carbon steel in the two-phase region makes the ferrite undergo dynamic recrystallization and refinement, and the austenite deformation makes the grains elongate along the rolling direction Fibrosis occurs, resulting in an increase in the number of austenite grain boundaries and dislocation density. During the rolling process, as the billet temperature decreases, the austenite transforms into a pearlite-type structure. Due to the high grain boundary number and dislocation density in the deformed austenite, the cementite sheets in the pearlite are refined, and the distance is reduced. After a short period of returning to the furnace for heating and rolling deformation, the cementite will be fragmented and the cementite will be spheroidized. At the same time, these cementites will also inhibit the grain growth of ferrite. Then, when cooled to room temperature, fibrous fine ferrite and spherical cementite structures can be obtained.

Compared with traditional tempering treatment, the present invention has the following advantages:

1. By adopting this method, the same microstructure as its quenching and tempering treatment can be obtained in medium carbon steel, that is, ferrite and spherical cementite microstructure, and its tensile strength, yield strength, elongation and impact toughness are all higher than GB/ T 699-1999 Specifications for quenching and tempering treatment of medium carbon steel.

2. The preparation process is simple and the production efficiency is high.

3. It does not involve the quenching and tempering heat treatment process, avoiding the pollution of the cooling medium to the environment, and does not involve the deformation of the workpiece caused by the quenching stress, the initiation and cracking of internal micro-cracks, and the process performance and mechanical properties of the product are not affected by hardenability and recovery. Fire brittleness and other effects.

4. For workpieces with large cross-sections, there is no need to select alloy steel, and the medium-carbon steel treated by the present invention can fully meet the requirements, greatly reducing the cost of materials.

Table 1 has listed the comparison of the mechanical properties of medium carbon steel treated with the method of the present invention and GB/T 699-1999 quenching and tempering:

Table 1

Description of drawings

Figure 1 is a scanning electron micrograph of the rod microstructure prepared in Example 1 of the present invention.

Fig. 2 is a scanning electron micrograph of the rod microstructure prepared in Example 2 of the present invention.

Fig. 3 is a scanning electron micrograph of the rod microstructure prepared in Example 3 of the present invention.

Fig. 4 is a scanning electron micrograph of the rod microstructure prepared in Example 4 of the present invention.

Fig. 5 is a scanning electron micrograph of the rod microstructure prepared in Example 5 of the present invention.

Fig. 6 is a scanning electron micrograph of the rod microstructure prepared in Example 6 of the present invention.

Fig. 7 is a scanning electron micrograph of the rod microstructure prepared in Example 7 of the present invention.

Detailed ways

The process of the present invention will be further described below according to specific embodiments and accompanying drawings.

Example 1

A commercial 45 steel round bar with a diameter of 30mm was heated to 720°C, held for 40 minutes, and rolled out of the furnace for 9 passes (square-diamond-square) with a cumulative deformation of 70% to obtain a bar with a square cross-section. Among them, the 9th pass is the shaping process, and after the 8th pass rolling, the steel bar is turned 90° and then rolled into the 8th hole. After the 3rd and 6th rolling passes, the steel rods were put back into the furnace at 720°C for 5 minutes to keep warm. Air cooling directly after the 9th pass rolling.

Embodiment 2～6

According to the scheme of Example 1, the difference from Example 1 is that the heating temperature is changed from 720°C to 740, 760°C, 780°C, 800°C and 820°C, and the steel Correspondingly, the temperature of rod reheating also changed from 720°C to 740°C, 760°C, 780°C, 800°C and 820°C.

Example 7

According to the scheme of embodiment 3, the difference from embodiment 3 is that the steel used is changed to 35 steel.

Table 2 lists the mechanical properties of the bars prepared in Examples 1-7.

Table 2

Claims (2)

1. a kind of medium carbon steel thermo-mechanical processi method of alternative modifier treatment, it is characterised in that：

(1) medium carbon steel bar is heated to α+γ two-phase section temperature, 720~820 DEG C of temperature range, insulation in heating furnace 40min, it is set to obtain ferrite+austenitic duplex tissue；

(2) take out rod iron and carry out 9 passage groove rollings immediately, total accumulative deformation amount controlling was rolled not less than 70% Heating furnace is put back in journey after every 3 passage and reheats 5min, then carries out next 3 passes, air cooling arrives after the 9th passes Room temperature.

2. the medium carbon steel thermo-mechanical processi method of alternative modifier treatment according to claim 1, it is characterised in that：Gained To the bacillar structure that is made up of ferrite and granular carbide of medium carbon steel, and with stronger<101>Fiber texture degree.