CN104451408A

CN104451408A - Medium-carbon ultrahigh strength bainite steel and preparation method thereof

Info

Publication number: CN104451408A
Application number: CN201410729877.4A
Authority: CN
Inventors: 余伟; 徐士新; 张昳; 陈银莉; 陈雨来
Original assignee: University of Science and Technology Beijing USTB
Current assignee: University of Science and Technology Beijing USTB
Priority date: 2014-12-04
Filing date: 2014-12-04
Publication date: 2015-03-25
Anticipated expiration: 2034-12-04
Also published as: CN104451408B

Abstract

The invention discloses medium-carbon ultrahigh strength bainite steel which consists of the following chemical components in percentage by mass: 0.28-0.5% of C, 1.0-1.7% of Si, 1.8-3.0% of Mn, 1.0-1.6% of Cr, 0-0.3% of Mo, 0-0.5% of Co, 0.6-1.3% of Al, less than or equal to 0.015% of P, less than or equal to 0.01% of S, less than or equal to 0.01% of N, and the balance of Fe and inevitable impurities. The preparation method comprises the following steps: soaking a billet obtained by smelting and casting the above components at 1150-1200 DEG C and processing the billet to required size, heating the billet by virtue of complete austenitizing, rapidly cooling to isothermal temperature, carrying out alternate isothermal treatment within intervals of 250-270 DEG C and 280-300 DEG C for 3-24hr by an alternation cycle of 0.1-1hr, and air-cooling to room temperature. The bainite steel disclosed by the invention is low in production cost, easy to weld, resistant to delayed fracture and excellent in comprehensive performance.

Description

Strong bainitic steel of carbon superelevation and preparation method thereof in one

Technical field

The invention belongs to metal material field, relate to carbon ultra-high strength bainite and preparation method thereof in one.

Background technology

The intensity of steel improves along with the increase of carbon content, but its plasticity and toughness degradation, how to realize the excellent fit ratio of intensity and plasticity and toughness, become the important topic of steel development.More research finds, under identical carbon equivalent condition, the steel with bainite structure has better anti-delayed fracture performance, fatigue strength, fracture toughness property and elongation.Therefore, under same carbon equivalent condition, bainitic steel has the application safety coefficient than martensitic stucture steel or bainite/martensite multi-phase steel longer work-ing life and Geng Gao.For the bainitic steel with lath bainitic ferrite, it is the important channel of improving its intensity that width of sheet refine to nanoscale.

The high carbon steel of disclosed a kind of Fe-C (0.6 ~ 1.1%)-Si (1.5 ~ 2.0%)-Mn (1.8 ~ 4.0%)-Cr (1.2 ~ 1.4%)-Ni (0 ~ 3%)-Mo (0.2 ~ 0.5%)-V (0.1 ~ 0.2%) alloy system of the people such as Mawella (US Patent No. 6884306B1), after the long-time homo genizing annelaing of high temperature, through austenitizing in the isothermal processes of carrying out 1 ~ 3 time-of-week a little more than Ms temperature, the carbide-free Bainite of the bainite ferrite+residual austenite body structure of nano-grade size can be obtained, such structure can reach good over-all properties, there is superstrength and higher fracture toughness property.In addition, U.K. Ministry of Defence (Chinese invention patent CN 102112644A) also utilizes the high carbon steel of Fe-C (0.6 ~ 1.1%)-Si (1.5 ~ 2.0%)-Mn (0.3 ~ 1.8%)-Cr (1.0 ~ 1.5%)-Ni (0 ~ 3%)-Mo (0.2 ~ 0.5%)-V (0.1 ~ 0.2%) alloy system to prepare by the isothermal process reaching 100h the super-high strength steel that tensile strength reaches 2048MPa, isothermal time 70h, but its room temperature impact toughness is only 4 ~ 7J.For obtaining nanometer bainite structure, all there is the shortcoming that in steel, carbon content is higher, thermal treatment isothermal time is long in above-mentioned two kinds of methods, directly causes impelling strength to reduce on the one hand, and welding property worsens, and on the other hand, its technique is unfavorable for industrial production.

At present, the way of bainitic steel acceleration acquisition nanometer lath structure has multiple:

Method 1: Chinese invention patent CN 102112644A with the addition of the Co element of about 1.5% and the Al element of 1% containing C (0.6 ~ 1.1%) high carbon steel in steel, shortens the isothermal processes time.

Method 2: Chinese invention patent CN 103468906A utilizes high carbon steel (Fe-0.91%C-1.65%Si-2.07%Mn-1.26%Cr-0.25%Mo-0.08%V-0.06%Nb) to carry out warm-rolling process before bainite transformation, increase dislocation desity, bainite nucleation site is increased, accelerates bainite transformation.

Method 3: the stress (tensile stress or stress) utilizing high carbon steel (Fe-0.79%C-1.56%Si-1.98Mn-1.01Cr-0.24Mo-1.51%Co-1.01%Al) to apply certain hour during bainite transformation accelerates bainite transformation, but the method for clamping of this mode to steel has strict requirement, is unfavorable for large-scale industrial production.

Method 4: by reducing austenitizing temperature and shortening austenitizing time, make austenite grain size refinement, for bainite transformation provides more nucleation site, bainitic transformation is accelerated.

Aforesaid method carbon content in chemical composition design is high, and noble alloy (Nb, V, Co, Ni) constituent content is high, and heat treatment time is long, causes complicated process of preparation, alloy and production cost high, be unfavorable for suitability for industrialized production and sizable application.

In order to obtain bainite fast, can also be played a role by alloying element:

C: reduce carbon element content and bainite transformation district can be made to move to left, shorten bainitic transformation incubation period, but the undue carbon content that reduces can cause the significantly rising of Ms point, bainite structure alligatoring, and too low carbon content can make carbon gap solution strengthening effect decline, and causes the intensity of steel significantly to reduce.Reduce the welding property that carbon element content can improve steel.

Si: non-carbide alloying element, the Si solubleness in cementite is extremely low, thus expands residual austenite stable region, makes it possible to the mixture forming bainite ferrite and residual austenite; Add the formation that appropriate Si element is conducive to carbide-free Bainite

Co: non-carbide alloying element, can increase the free energy of γ-Fe → α-Fe, and bainitic transformation district is moved to left, and shortens bainitic transformation incubation period, improves Bainite Phases of Some growth rate.But Co element price costly, is unfavorable for large-scale industrial production.

Al: the free energy increasing γ-Fe → α-Fe, also bainitic transformation district can be made to move to left, and the solubleness of Al element in carbide is lower, the precipitation of carbide can be suppressed further, the increase of Al element and then can suitably reduce Si constituent content, can make steel reduce cost of alloy further.

In order to obtain the high strength bainite steel with nanometer width of sheet, also need to add following alloying element:

Mn: manganese increases the hardening capacity of bainitic steel, improves intensity and the toughness of steel.

Cr:Cr element is mainly used for increasing the hardening capacity of bainitic steel, and Cr element can expand bainite transformation district in CCT curve, improves the stability of supercooled austenite.

Mo:Mo can suppress bainitic transformation, makes bainitic transformation start all to reduce with final temperature; Mo can reallocate between bainite ferrite and carbide, and phase transition process is slowed down.

For controlling the mechanical property of high strength bainite steel, need the residual of the elements such as control S, P:

S: sulphur easily forms FeS and MnS and is mingled with in steel, produces hot-short phenomenon, significantly reduces the toughness of steel, therefore, should reduce the sulphur content in steel as far as possible;

P: phosphorus normal segregation in steel, in crystal boundary, destroys the continuity of matrix, significantly reduces the toughness of steel, welding property is degenerated, easily produce cold short, therefore, should reduce the phosphorus content in steel as far as possible.

Summary of the invention

The object of the invention is to improve ultra-high strength bainite weldability, reduce cost of alloy and hot-work preparation cost, be beneficial to and produce and application.

Middle carbon ultra-high strength bainite of the present invention, its chemical composition is C:0.28 ~ 0.5% by mass percentage, Si:1.0 ~ 1.7%, Mn:1.8 ~ 3%, Cr:1.0 ~ 1.6%, Mo:0 ~ 0.3%, Co:0 ~ 0.5%, Al:0.6 ~ 1.3%, P :≤0.015%, S :≤0.01%, N :≤0.01%, all the other are Fe and inevitable impurity.

Concrete steps of the present invention are as follows:

1) smelt according to above-mentioned chemical composition, be then cast into blank;

2) after 1150 ~ 1200 DEG C of insulations, desired size steel are thermally processed into;

3) by steel after complete austenitizing heating and thermal insulation, be quickly cooled to isothermal treatment temperatures, replace isothermal processes at 250 ~ 270 DEG C and 280 ~ 300 DEG C of temperature ranges, alternate cycle 0.1 ~ 1 hour, the then air cooling that processes 3-24 hour is to room temperature.

Beneficial effect of the present invention:

1) super-high strength steel that Mawella, U.K. Ministry of Defence etc. are described is compared, while guarantee superstrength, by a relatively large margin improve unit elongation and reduce carbon content, the mass percent reduction of carbon is about 0.2% ~ 0.7%, weldability and toughness are significantly improved, and heat treatment time also significantly shortens;

2) have excellent comprehensive mechanical property, tensile strength reaches 1500 ~ 1900MPa, unit elongation 12% ~ 15%, U breach room temperature impact merit 53 ~ 90J;

3) considerably reduce noble alloy element in steel, its cost is only 1/90 of maraging steel;

4) middle carbon bainite super-high strength steel of the present invention, can be used as high-strength mechanical component, as products such as lightweight automotive sheet, armor plate, bulletproof helmet and Micro Alloying axial workpieces.

Embodiment

The present invention adopts compared steel 1 (patent CN 102112644A high carbon steel), compared steel 2 (CN 103451549A high carbon steel), as a comparison steel, and its chemical composition is as shown in table 1.

The chemical composition (massfraction, %) of table 1 steel

Steel billet is processed into desired size steel by steel billet after 1150 ~ 1200 DEG C of insulations, and be heated to 850 ~ 1000 DEG C of austenitizings, and replace isothermal processes at 250 ~ 270 DEG C and 280 ~ 300 DEG C of temperature ranges respectively, alternate cycle 0.1 ~ 1 hour, then air cooling is to room temperature.

Mechanical property corresponding to heterogeneity is as shown in table 2.

Table 2 mechanical property

Claims

1. carbon ultra-high strength bainite in a kind, it is characterized in that the mass percent of each chemical composition is C:0.28 ~ 0.5%, Si:1.0 ~ 1.7%, Mn:1.8 ~ 3%, Cr:1.0 ~ 1.6%, Mo:0 ~ 0.3%, Co:0 ~ 0.5%, Al:0.6 ~ 1.3%, P :≤0.015%, S :≤0.01%, N :≤0.01%, all the other are Fe and inevitable impurity.

2. the preparation method of middle carbon ultra-high strength bainite as claimed in claim 1, is characterized in that preparation process is as follows: carry out smelting, casting according to chemical composition described in claim 1; Gained steel billet is after 1150 ~ 1200 DEG C of soaking, be machined to desired size steel, again after complete austenitizing heating and thermal insulation, be quickly cooled to isothermal treatment temperatures, isothermal processes is replaced at 250 ~ 270 DEG C and 280 ~ 300 DEG C of temperature ranges, alternate cycle 0.1 ~ 1 hour, the then air cooling that processes 3-24 hour is to room temperature; Structure of steel is the ferrite bainite of nanoscale width, tensile strength 1500 ~ 1900MPa, elongation 12% ~ 15%, and U-shaped breach room temperature impact merit is 53 ~ 90J.