CN103898299B - A kind of preparation method of 2400MPa level low cost nanometer bainitic steel - Google Patents

Abstract

A preparation method for 2400MPa level low cost nanometer bainitic steel, belongs to metal material field.This steel is C-Si-Mn-Al system medium high carbon alloy, does not add the high alloying element of the prices such as Cr, Mo.Forging stock was through 900 ~ 1000 DEG C of insulations 0.5 ~ 1 hour, and rear direct input salt-bath hardening furnace insulation 3 ~ 6 hours, can the carbides-free nanometer bainite of production tensile strength 2400MPa level.This method is by falling Mn, and increase C, increase the optimizing components of Si, simplify preparation technology, without the need to high temperature diffusion annealing and course of hot rolling, forging stock directly being carried out low temperature Isothermal treatment can obtain the finished product, has significantly saved production cost simultaneously.

Description

A kind of preparation method of 2400MPa level low cost nanometer bainitic steel

Technical field

This technology relates to a kind of 2400MPa level low cost nanometer bainitic steel and preparation technology thereof, it is mainly concerned with the nanometer bainitic steel preparing a kind of superstrength at lower cost, in its ultra-fine bainite, microstructure is the thin slice bainite ferrite lath of thickness≤100nm and is distributed in the rich carbon residual austenite of film like between lath, separates out in tissue without cementite.Forging stock is directly put into stove in single phase austenite district temperature, carry out complete austenitizing, start a certain lesser temps between temperature and martensite start temperature at bainitic transformation in rear input salt-bath hardening furnace and carry out Isothermal treatment, finally steel billet water is chilled to room temperature.

Background technology

Bainitic steel is since exploitation is come out, and the comprehensive mechanical property good because of it and be widely used in structured material, therefore becomes the important channel of simultaneously improving ferrous materials intensity, plasticity and toughness to the exploitation of bainitic steel.The various New Type Bainite steel grades that domestic and international material scholar develops according to the theoretical origin different heat treatment means of bainitic transformation, comparing traditional bainitic steel has stronger intensity and better moulds (tough) property.

The thirties in 20th century Bain and Davenport Late Cambrian steel in there is bainite, people are to the understanding of Bainite Formation also gradually deeply.But due to reasons such as the complicacy of bainite transformation and the restrictions of laboratory facilities, still there is difference in current bainitic transformation.Bainite structure complex shape is various, the bainite structure that upper bainite, lower bayesian, inverse bainite and column bainite Shi Ge school are admitted jointly.

The 1950's, British Pickering and Irvine etc. have invented Mo-B series air-cooled bainitic steels, and the research of high strength and toughness bainitic steel afterwards obtains and payes attention to widely.Add Alloy Elements Mo, B makes steel obtain bainite structure in very wide continuous coo1ing velocity range, but Mo price comparison is expensive, and transition temperature is high, organize comparatively thick, product obdurability is poor.

20 century 70s, Tsing-Hua University Fang Hongsheng etc. develops a kind of Mn-B system bainitic steel that just can obtain bainite structure in as-normalized condition.In order to reduce Bs point, improving organization mechanics performance, adding Mn and replace expensive Mo.Mn and B acting in conjunction easily obtains bainite, and Bainite Phases of Some temperature is reduced, and refinement bainite size, improves toughness and intensity.

The nineties in 20th century, add alloy in the people Xiang Gang such as Wang Gui and comprise Mn, Mo etc.Tissue signature is: the E-carbide of all having separated out Dispersed precipitate through low-temperaturetempering in lath martensite and between bainite ferrite, produces second-phase dispersion strengthening effect.Meanwhile, undecomposed retained austenite has very high stability due to thermostabilization and tool stabilization, and solution strengthening in tissue like this, dispersion-strengthened, substructure strengthening and phase transformation strengthening etc. are not fully exerted, and make steel obtain superstrength.

In recent years, after high-carbon, high silicon steel are carried out reaching the Isothermal treatment of a couple of days by Caballero and Bhadeshia etc. under the cold condition of T=0.25Tm (Tm is fusing point), can obtain very tiny nano level bainite structure, its film like residual austenite being only rich carbon between the very thin bainite ferrite lath of 20 ~ 40nm and lath thereof by thickness forms.Is not carbide precipitate but retained austenitic film in the middle of bainite ferrite lath, the ultra-fine bainite of Ya Na meter of this steel and a small amount of martensite or residual austenite weave construction determine its superstrength and good toughness.The ultimate tensile strength of this nanometer bainitic steel can reach 2.5GPa, and yield strength can reach 1.7GPa, and hardness is 600 ~ 700HV, and fracture toughness property is 30 ~ 40MPam ^1/2.

This nanometer bainite steel mechanical property is excellent, but obtain this tissue usually to need to add the high alloying element of a large amount of price, and before isothermal quenching, need long high temperature diffusion annealing, its large preparation process flow process is: the homogenizing annealing → stove of forging stock is chilled to room temperature → be heated to certain temperature and carries out austenitizing → Isothermal treatment for a long time → be chilled to room temperature, this technique preparation time is long, and preparation cost is too high.Therefore, the present invention is by optimizing its Composition Design, reduce its C, Mn content, add the alloying element that Si, Al etc. are comparatively cheap simultaneously, this measure not only directly decreases the cost adding alloying element, and due to its lower alloying element content, homogenizing annealing can not be carried out and directly heat-treat, thus significantly save preparation cost.So just obtain low cost, the nanometer bainitic steel of high strength.

Summary of the invention

The object of the invention is to utilize alloying constituent optimization and preparation technology to simplify combine, produce a kind of nanometer bainitic steel of low cost.

For achieving the above object, technical scheme of the present invention is:

A preparation method for 2400MPa level low cost nanometer bainitic steel, bainitic steel chemical quality per-cent is as follows:

All the other are Fe and inevitable impurity.

The present invention adopts isothermal quenching preparation technology, comprises the following steps:

1) prepare forging stock, take raw material according to above-mentioned chemical composition mass percent, carry out smelting, cast, forge;

2) forging stock is without the need to carrying out high temperature diffusion annealing directly 900 ~ 1000 DEG C of heating, takes out after soaking in 0.5 ~ 1 hour insulation;

3) forging stock of taking-up is dropped into salt-bath hardening furnace rapidly, furnace temperature 200 ~ 300 DEG C, soaking time 3 ~ 6 hours, after insulation, direct-water-quenching is cooled to room temperature.

Mn falls in utilization, increase C, increase integrative organization's refinement technology that the optimizing components of Si and low temperature isothermal quenching combine be the present invention under the prerequisite of excellent comprehensive mechanical property ensureing nanometer bainitic steel, reduce the principal character of its production cost.

The topmost Composition Design thinking of modern bainitic steel by the interpolation of alloying element, the C curve of high temperature transformation is moved to right, and is separated simultaneously, avoids ferrite and pearlite to occur in phase transformation with middle temperature transformation C curve.Alloys producing also comprises reduction martensitic transformation point (M _spoint), bainitic transformation is carried out at a lower temperature, thus refinement bainite structure.For steel grades such as high silicon bainites, for suppressing the precipitation of carbide, the alloying elements such as Si can be added in composition.Meanwhile, also strengthening effect can be played with different mechanisms to bainitic steel adding of alloying element.

C element is the most fundamental element ensureing to obtain nanometer bainite structure in the present invention, and it is as Qianghian gap solution strengthening element, and its solution strengthening is most important to raising intensity, but too high carbon content can reduce welding property and impelling strength.The solubleness of C in austenite is very large, and very little in ferrite.It can slow down the velocity of diffusion of austenite Atom, extends the incubation period before austenitic transformation, thus the velocity of transformation that slows down, increase austenitic stability, improve the hardening capacity of steel, therefore the raising of C content can make the stability of original austenite grains strengthen, and reduces Bainite Phases of Some variable Rate.On the other hand, the increase of C not only reduces B _stemperature and M _stemperature, and can B be made _stemperature and M _stemperature head between temperature expands, and this is conducive to obtaining nanometer bainite structure.

Si element is the element ensureing that in steel of the present invention, carbides-free is separated out, and it is as non-carbide forming element, when its content is lower, exists with non-metallic inclusion form, thus stops Austenite Grain Refinement.But when its too high levels, can be used as displaced type alloying element and add sosoloid, then facilitate the alligatoring of austenite crystal.Si not only has obviously suppression brittlement phase Fe ₃the effect of C type Carbide Precipitation, can also increase remained austenite content and stability thereof in tissue, a certain amount of Si can improve the toughness of nanometer bainitic steel.

While Mn element and Si act synergistically and can ensure that steel grade of the present invention obtains high strength, high rigidity, still there is higher toughness.Mn belongs to displaced type alloying element, and it has the effect of solution strengthening in steel.As expansion austenite phase field element, add the A that Mn can make steel _r1, A _r3, B _sand M _stemperature declines, and is conducive to nanometer bainite structure and is formed.During the phase boundary place enrichment of appropriate Mn below middle temperature, having moved effect of dragging to phase boundary, significantly reduced the phase driving force of bainite simultaneously, bainite transformation temperature is reduced, is one of principal element ensureing the tiny bainite ferrite size of acquisition.Too high Mn can cause Bainite Phases of Some variable Rate slack-off, extends the bainitic transformation deadline.

It is act synergistically to ensure that steel grade obtains better toughness with Si that Al element acts in the present invention.It is similar that Al acts on Si in bainitic steel, can suppress the precipitation of cementite.And can consoluet element as one, Al can not send segregation in the tissue, also can not produce oxide on surface.Meanwhile, Al can suppress martensitic transformation, increases stabilization of austenite.Bainite transformation is accelerated in another effect of Al, this is because it makes phase driving force increase by the impact on phase driving force magnetic and nonmagnetic portion.Add phase driving force, Enhancing Nucleation Density is increased, reduce the retained austenite island to detrimental on the one hand, more nucleation site can make the refinement of bainite ferrite lath on the other hand.

Traditional nanometer bainite composition of steel, often high C height Mn height Si, then add the alloying elements such as a certain amount of Cr and Mo.Cr can improve hardening capacity, but is easy to form carbide; Mo can postpone the precipitation of perlitic transformation and carbide, also can fining austenite grains.The present invention, by reducing Mn content, has abandoned expensive Cr and Mo, then has added a certain amount of Si and Al, significantly shortened the bainitic transformation deadline, makes forging organize more evenly and not carbide-containing simultaneously, directly can carry out low-temperature heat treatment without the need to annealing.

Beneficial effect of the present invention:

1) excellent comprehensive mechanical property is possessed.The tensile strength of the nanometer bainitic steel that the present invention relates to is up to 2.5GPa, and hardness can reach 750HV, and toughness is 30 ~ 40MPam ^1/2.

2) reduce costs, raise the efficiency.Compared with the nanometer bainitic steel of routine, due to the optimization of alloying element, shorten technical process, significantly saved raw materials cost and time cost, mechanical property indices is still excellent simultaneously.

3) application prospect is extensive.The nanostructure bainitic steel that the present invention prepares, its intensity superelevation, unit elongation are good, and can be applicable to automobile steel field, also can be used for the military domain such as armor plating and bulletproof halmet, prospect is considerable.

Accompanying drawing illustrates:

Fig. 1 is thermal treatment process schematic diagram.

Embodiment

Chemical composition given by table 1, adopts electromagnetic induction furnace vacuum melting, becomes forging stock, to carry out subsequent technique to the blank forging of casting.

Table 1 is the mass percent of each composition

Table 1

Forging stock is all heated to 950 DEG C of austenitizings 30 minutes, then is taken out by sample and directly drop into salt-bath hardening furnace and carry out Isothermal treatment, holding temperature 200 ~ 300 DEG C, soaking time is 6 hours, finally by its water-cooled to room temperature.

The present invention have employed different holding temperatures to different sample, No. 1 200 DEG C, 300 DEG C insulations, No. 2 230 DEG C, 300 DEG C insulations, No. 3 230 DEG C, 300 DEG C insulations, soaking time is identical.Performance corresponding to different technical parameters is as shown in table 2:

Table 2 is mechanical property corresponding to main technologic parameters

Table 2

Claims (1)

1. a preparation method for low cost nanometer bainitic steel, is characterized in that chemical quality per-cent is as follows:

All the other are Fe and inevitable impurity;

Comprise following preparation process:

1) prepare forging stock, take raw material according to above-mentioned chemical composition mass percent, carry out smelting, cast, forge;

2) forging stock is without the need to carrying out high temperature diffusion annealing directly 900 ~ 1000 DEG C of heating, takes out after soaking in 0.5 ~ 1 hour insulation;

3) forging stock of taking-up is dropped into salt-bath hardening furnace rapidly, furnace temperature 200 ~ 300 DEG C, soaking time 3 ~ 6 hours, after insulation, direct-water-quenching is cooled to room temperature.