CN100462468C - A kind of ultra-fine pearlitic high-strength track steel and its preparation method - Google Patents

Abstract

This invention exposed a kind of ultra-fine pearlite high intensity railway steel. The weight proportion of alloy are C: 1.0-1.8, Al: 0.1-6.0, Cr: 0.1-2.5, Mn: 0.2-0.8, Si: 0- 1.5, Mo: 0-0.8, Nb: 0-0.1, V: 0-0.1, Ti: 0-0.1, Re: 0-O.05, remains is Fe. The preparation steps includes: smelting, casting to get bland, then deformation, air-cooled to get the material free from heating. The material can be smelted and rolled to be full-fashioned products, or reheated to austenite zone and rolled. Then it can get the mechanical characters of 900Mpa yield strength, 1300Mpa tensile strength and more than 10% Elongation. The wearability is 1.5 time of normal high-carbon steel. It has good jointing character. Hot rolling with high deformation can form good rail. It can be used to heavy load railway.

Description

A kind of ultra-fine pearlitic high-strength track steel and its preparation method

technical field

The invention relates to the preparation of rail steel, in particular to an ultra-fine pearlitic high-strength rail steel for high-speed railway tracks and heavy-duty railway tracks and a preparation method thereof.

technical background

With the rapid development of railway transportation, especially the rapid development of heavy rail lines, it is urgent to improve the strength and wear resistance of rail steel. General heavy rail steel requires tensile strength σ _b ≥880MPa, δ ₅ ≥10%, hardness (HB): 260-300. The carbon content of rail steel currently used is 0.6% to 0.8%, which belongs to eutectoid steel. In order to improve the strength and wear resistance, some patents add Nb, V, rare earth and other elements on the basis of the composition (Chinese patent application CN 96117731.4, CN99111744.1) and adopt processes such as controlled rolling to improve its mechanical properties; Tensile structure to improve strength (Chinese patent application CN99800029.9, CN94101720.6); or heat treatment after rolling to improve strength and wear resistance (Chinese patent application CN96117731.4, CN200410078390.0). The above patent applications have improved the strength and wear resistance of steel to varying degrees. However, to obtain higher strength and wear resistance, traditional ingredients and processes can no longer meet the requirements.

Carbon is the most widely used and important strengthening element in steel materials. However, the traditional concept believes that the increase of carbon content will affect the plasticity and thermal formability, and the increase of carbon content above 1% will make its plasticity decrease sharply, and the severe brittleness will also make its strength unable to increase, so it cannot be used for rails (structural materials) )Production. High-carbon hypereutectoid steel has long been excluded from practical engineering materials. However, this concept has been broken through. Recently, two patents of Nippon Steel Corporation for pearlitic steel show that the carbon content has reached 1.2% (Chinese patent application CN95191600.9) and 1.4% (Chinese patent application CN03800576. X), the former obtains high strength by further adding micro-alloying elements such as Nb, V, Mo, B, etc., to control the morphology of carbides.

High-carbon hypereutectoid steel often forms coarse network proeutectoid carbides during ingot casting, which will bring fatal brittleness to the material.

Contents of the invention

The object of the present invention is to provide a superfine pearlitic high-strength rail steel and a preparation method thereof. The prepared rail steel has high strength and high wear resistance, the material performance is significantly better than that of the current rail steel, and the process is simple and easy, the quality is stable, and it is convenient for large-scale production.

To achieve the above object, the present invention adopts the following technical solutions:

1. A superfine pearlitic high-strength rail steel, characterized in that the obtained rail steel structure is ultrafine granular carbides distributed on the superfine pearlite grains, the grain size is 2～6 μm, and the pearlite lamellar spacing is 0.1～0.2μm, the size of granular carbides is 0.3～1μm, the alloy composition weight percentage of the track steel is: C: 1.0～1.8; Al: 1.65～6.0; Cr: 0.1～2.5; Mn: 0.2～0.8; Si : 0-1.5; Mo: 0-0.8; Nb: 0.0-0.1; V: 0.0-0.1; Ti: 0.0-0.1; Re: 0.0-0.05; the balance is Fe.

The preparation process of the above-mentioned ultrafine pearlitic high-strength rail steel is carried out as follows:

1) Ferroalloy or pure metal ingredients are used to melt and cast ingots in induction furnaces or converters;

2) The ingot is subjected to hot-temperature deformation at a heating temperature of 950°C to 1150°C; and then isothermally deformed in a single austenite zone, austenite + cementite two-phase zone, or cooled from austenitizing temperature to total Continuous deformation during the analysis transition temperature;

In order to obtain good performance of heat-distorted materials, heating should be in accordance with the following specifications: preheating at 500°C to 600°C, medium temperature heating at 800°C to 900°C, uniform heating at 1050°C to 1150°C; or furnace loading below 500°C to 700°C. Heating to 1050℃～1150℃ at a lower heating rate;

The holding time for homogenization of the austenite zone is determined according to the thickness or diameter of the billet, 1 to 2.5 minutes per millimeter;

Deformation in a single austenite zone, deformation rate 0.1s ^-1 ~ 10s ^-1 ; deformation passes 1 ~ 10; total deformation 20% ~ 50%;

Continuous deformation when cooling through the austenite + carbide two-phase region, or isothermal deformation in the austenite + carbide two-phase region, deformation rate 1s ^-1 ~ 10s ^-1 ; deformation passes 1 ~ 15; total deformation 30% to 90%, air cooling after deformation.

The superfine pearlite high-strength rail steel prepared by the present invention shows through metallographic examination that the obtained high-carbon steel structure is ultrafine granular carbides distributed on ultrafine pearlite grains, the grain size is 2-6 μm, and the pearlite The distance between sheets is 0.1-0.2 μm, and the size of the granular carbide is 0.3-1 μm.

The inventor's test results show that the above-mentioned ultra-fine pearlitic high-strength steel has good structural stability at higher temperatures (750°C-1100°C), and the grain growth is very slow; as the temperature increases, the deformation resistance decreases significantly , the plasticity increases, and it can be hot deformed at a higher strain rate.

The present invention adopts the process route of smelting, ingot casting and thermal deformation, which is basically consistent with the production process route of traditional iron and steel enterprises. The material obtained after thermal deformation (forging, rolling) is:

1. The deformation amount of one pass is 10% to 20%, the total deformation amount is 90%, and no cracks are found after thermal deformation;

2. Air-cooled after thermal deformation, the material structure is a structure of spherical carbide (0.3 μm ~ 1 μm) dispersedly distributed on the matrix of ultrafine grain pearlite (2 μm ~ 6 μm), and the structure is uniform;

3. The material obtained after thermal deformation can obtain the following room temperature properties without other heat treatment:

Yield strength: above 930MPa;

Tensile strength: above 1300MPa;

Elongation: more than 10%;

Hardness: HB370~410 (HRC40~44).

Description of drawings

Figure 1 is the microstructure of ultra-fine pearlitic high-strength steel (C: 1.31, Cr: 1.49, Al: 1.65, Mn: 0.43, Nb: 0.025, V: 0.036, Ti: 0.050, excess Fe) after hot rolling. Heating in sections to 1050°C-1150°C, continuous rolling during cooling, 10%-20% deformation per pass, 12 passes.

Figure 2 is a hot-rolled ultrafine pearlitic high-strength steel (C: 1.58, Cr: 1.54, Al: 1.66, Mn: 0.44, Si: 0.50, Re(Ce): 0.02, Ti: 0.06, Mo: 0.3, and Fe). later organization. Heating in sections to 1050°C-1150°C, continuous rolling during cooling, 10%-20% deformation per pass, 12 passes.

In order to further understand the present invention, the present invention will be further described in detail below in conjunction with the specific examples given by the inventor.

Detailed ways

Ultra-fine pearlitic high-strength track steel, the alloy composition weight percentage is: C: 1.0-1.8; Al: 0.1-6.0; Cr: 0.1-2.5; Mn: 0.2-0.8; Si: 0-1.5; Mo: 0-0.8 ; Nb: 0.0-0.1; V: 0.0-0.1; Ti: 0.0-0.1; Re: 0.0-0.05; the balance is Fe.

The performance of the above-mentioned ultra-fine pearlitic high-strength rail steel with this structure feature is characterized by two aspects. One is that the tensile strength above 1300MPa can be obtained without quenching and tempering treatment, the yield strength above 900MPa, and the yield strength above 10% elongation; second, high wear resistance can be obtained without surface quenching.

Make the proeutectoid carbides broken and spheroidized, the crystal grains are ultra-fine, and the structure is ultra-fine granular carbides distributed on the ultra-fine pearlite grains, the grain size is 2-6 μm, and the pearlite lamellar spacing is 0.1-0.2 μm , The size of granular carbides is 0.3-1 μm.

The above-mentioned ultra-fine pearlitic high-strength steel and steel ingot are subjected to hot-temperature deformation processing, including rolling, forging, and extrusion. Taking rolling as an example, the following steps are specifically included:

(1) The steel ingot is put into the furnace and heated according to the preset heating specification. After heating to the final temperature, heat preservation, the heat preservation time is determined according to the size of the steel ingot, that is, 1 to 2.5 minutes per millimeter.

Sectional heating: heating in a section heating furnace, 500°C to 600°C for 1 hour, 800°C to 900°C for 1 hour, 1050°C to 1150°C for heat preservation. Or install the furnace below 500-700°C, heat to 1050-1150°C at a lower heating rate, and keep warm.

(2) Rolling the above steel ingot

The initial rolling temperature is not higher than 1100°C, and the final rolling temperature is not lower than 650°C;

Rolling deformation rate: 1s ^-1 ~ 10s ^-1 ;

Deformation per pass: 10% to 20%;

Total deformation: 50% to 90%;

Cool in air after rolling.

Below are several embodiments provided by the inventor.

1. Alloy smelting: use a 50kg induction furnace to melt and cast a 40kg steel ingot.

2. Blank forging: Heating in stages, first preheating at 500°C-600°C for 1 hour; raising the furnace temperature to 800-900°C for 1 hour; then increasing the furnace temperature to 1100-1150°C for 1 hour. Take out the steel ingot, continuously forge it into a 50cm×50cm section billet on the free forging hammer, and air cool.

3. Hot rolling: the above-mentioned billet is heated to 1100 ℃ ~ 1150 ℃ in stages in the walking heating furnace, and kept at this temperature for 50 minutes; The rolling temperature is 1100°C, the final rolling temperature is 850°C, the cross-section reduction is 10% to 20% in one pass, and the rolling is carried out continuously for 12 passes until the bar is rolled to a diameter of 20mm.

4. The mechanical properties of the material at room temperature after rolling and spheroidizing in Example 1 (Table 1), and the mechanical properties of the material at room temperature after rolling and spheroidizing in Example 2 (Table 2). The room temperature mechanical properties of the material in Example 3 are equivalent to those in Example 2. The room temperature mechanical properties of the materials of Examples 4 and 5 are equivalent to those of Example 1.

Table 1 Mechanical properties of steel (where C: 1.31; Cr: 1.49, Al: 1.65, Mn: 0.43, Nb: 0.025; V: 0.036, Ti: 0.050, the rest is Fe)

processing status Yield strength MPa Tensile strength MPa Elongation% HB Hot Rolled Air Cooled 934 1327 12 370

Table 2 Mechanical properties of steel (where C: 1.58, Cr: 1.54, Al: 1.66, Mn: 0.44, Si: 0.50, Mo: 0.3, Ti: 0.06, Re(Ce): 0.02, remaining Fe)

processing state Yield strength MPa Tensile strength MPa Elongation% HB Hot Rolled Air Cooled 985 1472 11 400

Multiple experiments by the applicant have proved that the present invention adopts hypereutectoid steel components, and the carbon content can be as high as 1.8%. Through rolling and forging, the proeutectoid carbides are spheroidized and the austenite grains are ultrafine to obtain super The structure of ultra-fine spherical carbide dispersed on the fine-grained matrix can obtain high strength, high toughness and good plasticity. Due to its high carbon content, there are 10% to 30% carbides in the final structure, which will undoubtedly significantly improve its wear resistance.

High-carbon hypereutectoid steel often forms coarse network proeutectoid carbides during ingot casting, which will bring fatal brittleness to the material. In order to solve this problem, the steel can be heated to the complete austenite zone to homogenize, so that carbides and other elements can be fully dissolved, and then through forging and rolling during the cooling process, the proeutectoid carbides are broken and precipitated in spherical form. Through heat treatment, the structure can be further improved and excellent properties can be obtained.

It is required that the limit of impurity content in steel reaches the standard of ordinary carbon steel. Adding Al can increase the eutectoid transformation temperature of low-alloy high-carbon steel, increase the carbon concentration at the eutectoid transformation point, and reduce the number of pro-eutectoid carbides; it can enhance the high-temperature deformation ability and deform at a higher stress edge rate. Adding Cr can avoid graphitization and inhibit the growth of carbide particles during thermal processing. The purpose of adding Mn is to eliminate the harmful effects of impurity elements such as S and P in steel. Adding Nb, V, Ti, and Re can improve the structure of steel and improve the comprehensive mechanical properties.

Claims (2)

1. ultra-fine pearlite high-strength rail steel, it is characterized in that, this track structure of steel that obtains is a distributed superfine granular carbide on the ultra-fine pearlite crystal grain, grain-size is 2～6 μ m, perlite sheet spacing is 0.1～0.2 μ m, granular carbide is of a size of 0.3～1 μ m, and the alloying constituent weight percent of this track steel is: C:1.0～1.8; Al:1.65～6.0; Cr:0.1～2.5; Mn:0.2～0.8; Si:0～1.5; Mo:0～0.8; Nb:0.0～0.1; V:0.0～0.1; Ti:0.0～0.1; Re:0.0～0.05; Surplus is Fe.

2. the preparation technology of described this ultra-fine pearlite high-strength rail steel of claim 1 is characterized in that, carries out as follows:

1) with iron alloy or pure metal batching, with induction furnace or converter melting, ingot casting, or the continuous casting and rolling once-forming;

2) ingot casting is carried out hot warm deformation, 950 ℃～1150 ℃ of Heating temperatures; Then in single austenitic area, austenite+warm deformations such as cementite two-phase region, or be cooled to continuous modification in the eutectoid transformation temperature course from austenitizing temperature;

Hot warm deformation is by the heating of following standard: 500 ℃～600 ℃ preheatings, temperature heating in 800 ℃～900 ℃, 1050 ℃～1150 ℃ homogenizing heating; Or 500 ℃～shove charge below 700 ℃, be heated to 1050 ℃～1150 ℃;

Austenitic area homogenizing soaking time is fixed according to sotck thinkness or diameter, every millimeter 1～2.5 minute;

At the distortion of single austenitic area, rate of deformation 0.1s ^-1～10s ^-1Rolling pass 1～10; Total deformation 20%～50%;

At cooling continuous modification during by austenite+carbide two-phase region, or at austenite+warm deformations such as carbide two-phase region, rate of deformation 1s ^-1～10s ^-1Rolling pass 1～15; Total deformation 30%～90%, distortion back air cooling gets final product.

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