CN104032237B - Composite microalloyed high-strength steel forging with tensile strength of 700MPa and production method thereof - Google Patents

Composite microalloyed high-strength steel forging with tensile strength of 700MPa and production method thereof Download PDF

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CN104032237B
CN104032237B CN201410209881.8A CN201410209881A CN104032237B CN 104032237 B CN104032237 B CN 104032237B CN 201410209881 A CN201410209881 A CN 201410209881A CN 104032237 B CN104032237 B CN 104032237B
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slab
steel
forging
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rolling
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徐亮
江慧丰
方国爱
汪辉
章敏
王利
闫永超
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Hefei General Machinery Research Institute Co Ltd
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Abstract

A composite microalloyed high-strength steel forging with 700 MPa-level tensile strength and a production method thereof. The invention relates to a low-welding crack sensitivity high-strength steel forging with 700 MPa-level tensile strength and a production method thereof. The forging piece comprises the following chemical components in percentage by weight: c: 0.07-0.12%; mn: 1.2-1.8%; si: 0.2-0.4%; s is less than or equal to 0.006 percent; p is less than or equal to 0.015 percent; ni: 1.3-1.8%; cr: 0.15 to 0.3 percent; mo: 0.2-0.4%; nb: 0.02-0.1%; v is less than or equal to 0.1 percent; ti: less than or equal to 0.02 percent; cu is less than or equal to 0.25 percent; b: less than or equal to 0.001 percent, and the balance of Fe and inevitable impurities; at the same time Pcm0.26% or less. The thickness of the forged piece in the invention can reach 200mm, and the mechanical property meets the following requirements: the yield strength is more than or equal to 560MPa, the tensile strength is more than or equal to 700MPa, the elongation is more than or equal to 17 percent, and KV is2(-40℃)≥80J。

Description

A kind of tensile strength 700MPa level combined microalloying high-strength steel forging and production method thereof
Technical field
The invention belongs to high-strength low-alloy steel production field, be specifically related to a kind of tensile strength 700MPa level low-welding crack-sensitive high-strength steel forging and production method thereof.
Background technology
Along with China's metallurgical industry and " transmitting electricity from west to east " and " developing rapidly of the Important Project such as West-east Gas; hardening high-strength steel and forging are widely applied in the construction of generating steel penstock, Hydropower Unit volute, natural gas storage tank and metallurgical suite of equipment gas storage spherical tank with its excellent over-all properties; demand increases year by year, also day by day urgent to the application requiring of more high-strength hardened and tempered steel and supporting forging thereof.
Chinese Patent Application No.: 200910063768.2 disclose " a kind of low welding crack sensitivity steel with tensile strength at 700 MPa level and production method thereof ", adopt following chemical composition (wt%): C:0.08 ~ 0.12%, Mn:1.2 ~ 2.0%, Si:0.15 ~ 0.4%, S≤0.006%, P≤0.015%, Ni:0.25 ~ 0.55%, Mo:0.15 ~ 0.28%, V:0.02 ~ 0.1% and Cu:0.18 ~ 0.3%, Cr:0.15 ~ 0.3%, Ti:0.008 ~ 0.02%, two or more in B:0.0007 ~ 0.0027%, all the other are Fe and inevitable impurity.But this patent is 60mm by traditional steel plate maximum ga(u)ge that Clean Steel technique is rolled, Tempering and Quenching is produced; Adopt the steel tensile strength R that this patent is produced m>=690MPa ,-20 DEG C of KV 2>=47J.
Chinese Patent Application No.: 20131053273.5 disclose " a kind of thin strap continuous casting 700MPa level high-strength air corrosion-resistant steel manufacture method ", it comprises the steps: 1) smelting molten steel chemical component weight per-cent is C:0.03-0.1%, Si≤0.4%, Mn:0.75-2.0%, P:0.07-0.22%, S≤0.01%, N≤0.012%, Cu:0.25-0.8%, Cr:0.3-0.8%, Ni:0.12-0.4%, in addition, also comprise microalloy element Nb, V, Ti, at least one in Mo, Nb:0.01-0.1%, V:0.01-0.1%, Ti:0.01-0.1%, Mo:0.1-0.5%, all the other are Fe and inevitable impurity, 2) thin strap continuous casting, direct pouring goes out the Cast Strip that thickness is 1-5mm, 3) Cast Strip cooling, rate of cooling is greater than 20 DEG C/s, 4) the online hot rolling in Cast Strip, hot-rolled temperature 1050-1250 DEG C, draft is 20-50%, deformation rate > 20s -1, austenite online recrystallize occurs after hot rolling, and hot-rolled strip thickness is 0.5-3.0mm, 5) cool, batch, rate of cooling 10-80 DEG C/s, coiling temperature 520-670 DEG C.The steel band microstructure obtained is formed primarily of the bainite be evenly distributed and acicular ferrite.
At present both at home and abroad with regard to the several similar above-mentioned patent of high-strength weather resisting steel and manufacture method application thereof, the wherein weather resisting steel of 700MPa intensity rank, mostly adopt Nb, V, Ti, Mo combined microalloying technology, improved the comprehensive mechanical property of weather resisting steel by refined crystalline strengthening and precipitation strength.The low-carbon low-alloy steel product strength utilizing above-mentioned this method for continuous casting sheet band to produce is higher, but main problem to be the unit elongation of product not high and low-temperature flexibility is poor.Because usual thin band continuous casting technique hot rolling draft is no more than 50%, by the poor effect of deformation fining austenite grains, thus the plasticity of such steel and low-temperature flexibility poor.
Summary of the invention
An object of the present invention is to provide a kind of tensile strength 700MPa level low-welding crack-sensitive high-strength steel forging, this forging not only has the outstanding mechanical properties such as high strength, high tenacity, high weldability and good plasticity, and this forging maximum ga(u)ge of still meeting the requirement of low-welding crack-sensitive reaches the 700MPa level low alloy steel forgings of 200mm.
For achieving the above object, with weight, forging is made up of following chemical composition: C:0.07 ~ 0.12%; Mn:1.2 ~ 1.8%; Si:0.2 ~ 0.4%; S≤0.006%; P≤0.015%; Ni:1.3 ~ 1.8%; Cr:0.15 ~ 0.3%; Mo:0.2 ~ 0.4%; Nb:0.02 ~ 0.1%; V≤0.1%; Ti :≤0.02%; Cu≤0.25%; B :≤0.001%, all the other are Fe and inevitable impurity; The welding crack sensibility indices P of forging simultaneously cm≤ 0.26%.
As preferred version of the present invention, the chemical component weight per-cent of forging is C:0.089%, Mn:1.335%, Si:0.25%, S:0.0038%, P:0.007%, Ni:1.31%, Cr:0.18%, Mo:0.2%, Nb:0.028%, V:0.0035%, Ti:0.013%, Cu:0.026%, B:0.0009%, and all the other are Fe and inevitable impurity.
As preferred version of the present invention, the chemical component weight per-cent of forging is C:0.11%, Mn:1.32%, Si:0.316%, S:0.0037%, P:0.006%, Ni:1.33%, Cr:0.28%, Mo:0.28%, Nb:0.031%, V:0.034%, Ti:0.016%, Cu:0.023%, B:0.0009%, and all the other are Fe and inevitable impurity.
As preferred version of the present invention, the chemical component weight per-cent of forging is C:0.086%, Mn:1.31%, Si:0.31%, S:0.0042%, P:0.007%, Ni:1.44%, Cr:0.15%, Mo:0.28%, Nb:0.035%, V:0.038%, Ti:0.011%, Cu:0.024%, B:0.0008%, and all the other are Fe and inevitable impurity.
As preferred version of the present invention, the chemical component weight per-cent of forging is C:0.08%, Mn:1.299%, Si:0.30%, S:0.0039%, P:0.007%, Ni:1.655%, Cr:0.285%, Mo:0.25%, Nb:0.036%, V:0.01%, Ti:0.012%, Cu:0.023%, B:0.0008%, and all the other are Fe and inevitable impurity.
Two of object of the present invention is to provide the production method of above-mentioned tensile strength 700MPa level low-welding crack-sensitive high-strength steel forging, comprise that desulfurizing iron is skimmed, converter smelting, the refining of LF stove, RH vacuum-treat, slab casting, heating of plate blank, rolling, cooling and tempering process;
In slab casting cycle, setting Heating temperature: 1200 ~ 1230 DEG C, time inside furnace: 220 ~ 400 minutes, to ensure the abundant austenitizing of slab;
In the operation of rolling, adopt two benches controlled rolling, finish rolling the Ith stage carries out in austenite perfect recrystallization humidity province, and 980 DEG C≤finishing temperature≤1100 DEG C, intermediate blank treats temperature; Finish rolling the IIth stage carries out in non-recrystallization zone, start rolling temperature≤900 DEG C, and finishing temperature is 840 ± 10 DEG C, non-recrystallization zone cumulative deformation >=60%;
Enter laminar cooling system after slab controlled rolling, rate of cooling is 20 ~ 40 DEG C/s, and final cooling temperature is at 450 ~ 500 DEG C;
Slab after rolling adopts quenching-and-tempering process, and quenching temperature is 910 ~ 930 DEG C, and soaking time is thickness of slab × 1.5 minute; Tempering temperature is 630 ~ 670 DEG C, soaking time: thickness of slab × 2.5 ~ thickness of slab × 3 minute; Described thickness of slab unit is mm.
Granular bainite and acicular ferrite and super-refinement ferrite bainite is organized as after slab rolls.
Be organized as evenly tiny tempered sorbite after slab tempering, namely lath-shaped ferrite matrix be distributed with precipitation carbonitride.
The beneficial effect of technical scheme of the present invention is as follows:
It is the core technology means of the present invention in production high-performance welding high-strength steel that microalloying combines with controlled rolling and controlled cooling.Adopt cooling controlling and rolling controlling process, can effectively improve the obdurability of steel when the very low and alloying element content of carbon content is lower and improves the weldability of steel and various processing characteristics, combine with the microalloy element such as Nb, V, Ti, effect be better.The essence of controlled rolling obtains tiny austenite crystal, to obtain fine ferrite crystal grain and tiny other phase-change product (nodular troostite or tiny bainite etc.) after γ-α phase transformation, effectively improves the obdurability of steel.Controlling cold essence is fine austenite crystal grain and ferrite crystal grain recrystallize or grow up, to obtain small grains and respective organization after phase transformation again after preventing controlled rolling.Containing in the steel of microalloy element, controlled rolling and controlled cooling also helps and obtains tiny and microalloy carbonitride that is that be evenly distributed, improves dislocation desity, thus improves strengthening effect.
The main line of combined microalloying Steel Alloy design is that strong carbonitride-forming elements Nb, V, Ti, the Mo etc. adding appropriate proportioning in low alloy steel carry out microalloying, and combines with Controlled Rolling And Controlled Cooling means and produce tensile strength 700MPa high-performance welding high-strength steel forging.The principle that this technology utilizes deformation and phase transformation to combine, thermal distortion thinning microstructure is remained to subsequently in cooling phase-change process, for phase transformation thinning microstructure creates conditions, finally obtain the micro-alloyed steel crystal grain of refinement, thus meet the high strength of steel, high tenacity and low-welding crack-sensitive requirement.
Forged steel tool of the present invention has the following advantages:
1) composite micro-alloying low-carbon bainitic steel Composition Design, is adopted
The present invention follows low-carbon (LC), low P on Composition Design cmthe Mn-Ni-Mo-V system low-carbon bainite composition of steel of value, meets weldability requirement.Require high strength forgings not preheated weld, do not produce welding cold cracking.Reduce carbon content, good welding property can be obtained; Control carbon content 0.07 ~ 0.12%, add the elements such as Cr, Mn, Mo and reduce bainite formation temperature; Add Ni element improve steel low-temperature flexibility and improve center performance; Adopt Nb, V, Ti microalloying to carry out complex intensifying, make reduction carbon content become possibility, utilize the composite precipitation of the microalloy elements such as Nb, V, Ti to separate out simultaneously and ensure that forged steel obtains enough intensity and improves the low-temperature impact toughness of forged steel; Design without B or micro-B, avoid in steel, forming large size BN complex inclusion.The composition of low S, P, ensure that the pure of steel, thus ensure that invention steel has excellent comprehensive mechanical property.
2) two benches controlled rolling and controlled cooling rolling technology, is adopted
The present invention adopts the technique of austenite perfect recrystallization district+austenite Unhydrated cement two stage controlled rolling and controlled coolings to obtain tensile strength 700MPa high-performance welding high-strength steel forging.Two benches controlled rolling process production efficiency is high, workable.Be rolled respectively at austenite perfect recrystallization district+austenite Unhydrated cement, by the control to finishing temperature, the microalloy elements such as most Nb, V, Ti can keep admittedly melting state, until during ferrite transformation, separate out in the middle of ferrite, play precipitation strength effect; By the control to Unhydrated cement total deformation, Nb, V, Ti of part, at austenite region strain induced precipitate, play the effect of crystal grain thinning.Finally making forged steel obtain tiny ferrite bainite through the measure of two benches cooling controlling and rolling controlling process is main tissue, obtain the homogeneous microstructure of the Carbonitride Precipitation with microalloy element after tempering, crystal grain is tiny, toughness is high and the tempered sorbite of moderate strength or tempering bainite tissue, thus ensure that forged steel has good comprehensive mechanical property.
3), rational quenching-and-tempering process is adopted
Forged steel of the present invention makes to occur a kind of granular structure in its metallographic structure through cooling controlling and rolling controlling process measure, is block ground M/A phase structure that α-Fe matrix distributes, is referred to as granular bainite.Because this ferrite adds bainite structure toughness deficiency, need to carry out tempering, bainite is decomposed, improves toughness.Tempering essence is the martensite or bainite decomposition that intensity is high by poor toughness, and to obtain homogeneous microstructure, crystal grain is tiny, toughness is high and the tempered sorbite of moderate strength or tempering bainite.Tempered sorbite adds than the ferrite of normalizing state that pearlitic structure is even, crystal grain is tiny, and the tiny and reason such as to be evenly distributed of the carbide particle of precipitation, so have the obdurability higher than normalized steel.Tempering main purpose eliminates the structural stress of steel, make tissue evenly, performance is more stable, makes the carbonitride of microalloy element separate out further simultaneously, to increase precipitation strength effect, thus improves the obdurability of steel.
The present invention adopt novel chemical composition design and the two benches controlled rolling and controlled cooling matched and reasonably Quenching plus tempering process be the prerequisite ensureing that forging has rational microtexture and every mechanical property and meets the demands.Rational thermal treatment process can ensure that forged steel has uniform microtexture, thus ensures that forging has the indexs such as good normal tensile property, low-temperature flexibility.It is as follows that forged steel of the present invention determines its best quenching-and-tempering process through serial quenching-and-tempering process performance test:
Optimal quenching temperature is 910 ~ 930 DEG C, soaking time: (thickness of slab × 1.5) minute; Optimum tempering temperature is 630 ~ 670 DEG C, soaking time: (thickness of slab × 2.5 ~ 3) minute.
Forged steel of the present invention, after two benches controlled rolling and controlled cooling and rational Tempering and Quenching, has extremely excellent comprehensive mechanical property, cold and hot working performance and welding property.Steel of the present invention can adopt the method welding such as covered arc welding, union-melt weld, shielded welding, tig welding, can be used for manufacturing 10000m 3above large natural gas spherical gasholder and the component such as pressure steel pipe of hydropower station, Hydropower Unit volute, particularly:
(1), this forging possesses low-welding crack-sensitive, and welding property is excellent, can simplify weldprocedure, reduce costs, adapt to large production requirement.
(2), this forging yield strength>=560MPa, tensile strength>=700MPa, unit elongation>=17%, KV 2(-40 DEG C)>=80J, intensity is greater than the steel grade in existing low temperature bearing device low alloy steel forging NB/T47009-2010.
(3), forging production technique is controlled, and production cost is low, can high efficiency production.
Accompanying drawing explanation
Fig. 1 is example 3 metallographic microstructure photo of the present invention.
Fig. 2 is example 6 metallographic microstructure photo of the present invention.
Fig. 3 a and Fig. 3 b example 3 transmission electron microscope of the present invention is observed pattern, the size of Second Phase Precipitation thing and distribute photo and energy spectrogram.
Fig. 4 a and Fig. 4 b example 6 transmission electron microscope of the present invention is observed pattern, the size of Second Phase Precipitation thing and distribute photo and energy spectrogram.
Fig. 3 a, 3b and Fig. 4 a, 4b all show that the microstructure of steel of the present invention meets expection, are tempered sorbite (acicular ferrite matrix is distributed with and separates out carbonitride).Ferrite grain size is on average about 8.75 μm (corresponding grain size number is 10.5 grades), ferrite matrix is uniform-distribution with spherical [Nb, Ti] (C, N) releasing characteristics of 6 ~ 25nm.
Embodiment
In order to achieve the above object, devise a kind of brand-new tensile strength 700MPa level vanadium niobium titanium-combined microalloying high-strength steel forging, forging is made up of following chemical composition: C:0.07 ~ 0.12% in the present invention; Mn:1.2 ~ 1.8%; Si:0.2 ~ 0.4%; S≤0.006%; P≤0.015%; Ni:1.3 ~ 1.8%; Cr:0.15 ~ 0.3%; Mo:0.2 ~ 0.4%; Nb:0.02 ~ 0.1%; V≤0.1%; Ti :≤0.02%; Cu≤0.25%; B :≤0.001%, all the other are Fe and inevitable impurity; The welding crack sensibility indices P of forging simultaneously cm≤ 0.26%.
Welding crack sensibility indices P cmdetermine according to the following formula:
P cm=C+Si/30+Ni/60+(Mn+Cr+Cu)/20+Mo/15+V/10+5B(%)。
The alloying element that the present invention adds is based on following principle:
C:0.07~0.12%。C improves the most effective element of steel strength, and along with the increase of carbon content, in steel, Fe3C amount increases, and the tensile strength of steel and yield strength improve.But, carbon content increases, the plasticity index unit elongation of steel and impact toughness decreased, particularly low-temperature flexibility significantly declines, ductile-brittle transition temperature significantly improves, welding heat affected zone can produce phenomenon of hardening, and welding cold crack sensitivity increases, and can not meet the service requirements of Natural Gas Spherical, pressure steel pipe of hydropower station and Hydropower Unit volute.In addition C still affects the principal element of Pcm, and control Pcm Zhi≤0.26% of steel, make it have low welding crack sensibility, in steel, C content controls 0.07 ~ 0.12%.
Si:0.2~0.4%。Si enters ferrite and plays solution strengthening effect, reduce yield tensile ratio, but Si can improve the ductile-brittle transition temperature of steel significantly, and also can worsen plasticity and welding property, therefore, the content of Si controls to be advisable 0.2 ~ 0.4% simultaneously.
Mn:1.20~1.80%。Mn mainly improves the hardening capacity of steel, ensures that slab has enough through hardening abilities.Meanwhile, Mn has significant solution strengthening effect, can fining austenite grains in soft steel, thus fining ferrite.When Mn content is less than 2%, do not reduce the toughness of steel.Mn deferrable F-B phase transformation, reduces Bs point, promotes to form compact grained ferrite.
Ni:1.30~1.80%。Improve the toughness of steel.Ni expands austenitic area element, can reduce A1 and A3 point, suppress thick proeutectoid ferrite, significantly improve the toughness of steel, particularly low-temperature flexibility, reduces the ductile-brittle transition temperature of steel;
Mo:0.2~0.4%。Mo can postpone pearlitic transformation effectively, does not affect bainitic transformation, promotes to form bainite.Mn, Mo, B conbined usage, is more conducive to obtaining acicular ferrite, instead of polygonal ferrite.But add the hardenability that Mo also can improve steel, thus improve welded steel cool attenuation, so the content of control Mo is 0.2 ~ 0.4%.
Nb:0.02~0.1%。The superheated susceptivity of Nb energy crystal grain thinning and reduction steel and temper brittleness, improve intensity, but plasticity and toughness decline to some extent.When adding niobium in ordinary low-alloy steel, resistant to hydrogen, nitrogen, ammonia corrosion ability under weather-resistant and high temperature can be improved.Niobium can improving welding property.In austenitic stainless steel, add niobium, intergranular corrosion phenomenon can be prevented.The most outstanding effect of Nb suppresses the recrystallize in high temperature deformation process, in forging or rolling after every a time, the carbonitride of V, Nb, Ti is because of strain induced precipitate, Nb, Ti, V carbonitride precipitates of separating out is on crystal boundary and dislocation, can pinning austenite grain boundary effectively, thus stop can the motion of recrystallize and dislocation, suppress the carrying out of Austenite Recrystallization Process, wherein Nb, Ti are stoping the effect of austenite recrystallization more remarkable.In high temperature austenitic tagma, Nb is based on the effect of dragging of admittedly molten atom; In the austenitic area of lesser temps, based on strain induced precipitate and not molten Nb (CN) particle to the pinning effect of crystal boundary.When Nb (CN) size of particles of separating out, to be greater than 200Onm inoperative to austenite recrystallization, therefore when the addition of Nb is more than 0.05%, then no longer affects by C content, reach capacity and do not have adjection to strengthening effect.Therefore the content 0.03 ~ 0.05% controlling to add Nb just can play crystal grain thinning and precipitation strength effect, is the most cost-effective means.
V:0.01~0.1%。V is strong carbonitride-forming elements.Because the carbonitride dissolves degree of V is larger, V, Ti easily form nano level M (CN) type carbonitride, when hot rolling, V (CN) is at austenite grain boundary strain induced precipitate, stop Austenite Grain Growth, tissue after refinement phase transformation, and the V separated out on austenite grain boundary (CN) particle also can provide the nucleation site of more horn of plenty for ferrite, obtains more tiny ferrite crystal grain at austenite to during ferritic transformation; V (CN) separates out in austenite inside, can induce the forming core of two kinds of Intragranular Acicular Ferrite, namely wait axle ferrite and acicular ferrite, thus these fining ferrite can significantly improve normal temperature and the hot strength of steel.But when V and Cr, Mo exist simultaneously, then can form complicated carbonitride and reduce plasticity and the toughness of welding joint in drawing process.The welding joint of the ben Cr of being, Mo, V steel thick-walled vessel is higher to the susceptibility of crackle when postwelding carries out hidden lino removal (SR process), therefore be no matter ensure plasticity and toughness or crack in process of avoiding eliminating stress, all must the content (being limited in 0.01 ~ 0.1%) of strict control V.
Cu:0.10~0.25%。Cu mainly plays precipitation strength effect in steel, is conducive in addition obtaining good low-temperature flexibility, improves the antifatigue extended capability of steel.But when Cu too high levels, during Steel Rolling, very easily produce check crack.Therefore Cu content controls 0.10 ~ 0.25%.
Cr:0.15~0.30%。Cr reduces austenite γ phase block elements, forms γ phase and encloses, infinitely dissolve each other in α iron in steel.Increase the hardening capacity of steel and have secondary hardening effect, improving carbon steel wear resistance.Its content lower limit is 0.15%.When Cr-Cu-Ni compound is added, add Cr and can improve welded steel cool attenuation.The content of Cr controls 0.15 ~ 0.30% in the present invention.
Ti:0.01 ~ 0.02%, Ti is a kind of strong carbide and nitride forming element, and it can improve room temperature strength and the hot strength of steel significantly, because Ti can play crystal grain thinning, therefore also can improve the toughness of steel.The effect of Ti is the N dissolved in fixing steel, otherwise N will be combined with B and causes B to lose efficacy.Research shows, wants to be completely fixed N, and the concentration of Ti should be approximately 3.4 times of N concentration.But excessive Ti can cause toughness to decline.Another effect of Ti is to oxide morphology control action kou.Because Ti and S has stronger avidity, increase and can form (MnTi) S or Ti4C2S2 of not easily viscous deformation containing Ti amount, can improving SNR ununiformity, improve impelling strength.In addition, appropriate Ti can improve the toughness of weld metal, but excessive Ti can be formed in steel be again mingled with make it reduce.Consider from the toughness improving weld metal in low-alloy high-strength steel, add be no more than 0.02% Ti comparatively suitable, Second Phase Particles TiN, Ti (CN) of utilizing Ti to be formed etc. stop the grain growth of coarse grain zone, welded heat affecting zone, ensure that welding joint has good low-temperature flexibility.
B:0.0005~0.001%。B is the element strongly improving hardening capacity, B adds, effectively can suppress forming core and the generation of proeutectoid ferrite, due to the dgnamic segregation of B on austenite grain boundary, strongly inhibited γ-α phase transformation, impels austenite to form the martensite of tiny low-carbon (LC) when quenching, thus improve yield strength and the tensile strength of steel, in addition, the interaction of B and N, can significantly improve the low-temperature flexibility of test steel.Because Ti has been completely fixed the N in steel, therefore, all B added adding before the rolling, hankers obtaining dissolving.Along with the increase of B content, intensity is improved, and the bainite mark in result tissue increases, and the tensile strength index when B content is more than 0.002% tends towards stability.But, along with B content more than 0.001% time, low-temperature flexibility sharply worsens.It is because along with the increase of B content, the B segregation caused in γ grain boundary and γ crystal grain causes that the reason producing this result is pushed off.In addition, in steel B too high levels can make to eliminate stress process (SR process) afterwards " B phase " in netted precipitation, produce SR crackle.B easily forms carbide and nitride, and is easily collected at original austenite crystal prevention, impels dislocation desity in the neighbourhood to increase, and as the trap of hydrogen in partial area, thus can impel and grain boundary separation occurs herein.So B content is selected 0.0005 ~ 0.001%.
Research shows, when adding separately B, and meeting austenite grain boundary Precipitation Fe after rolling usually 23(CB) 6, thus significantly reduce the strengthening effect of B, cause the transformation of γ → α not to be effectively suppressed, therefore add Nb in steel to stop Fe 23(CB) 6formation because Nb is more easily combined with C, along with the increase of Nb content of dissolving, the tendency forming bainite also considerably increases.The appropriate dissolving of Nb can stable austenite show and add with B compound the effect promoting bainite transformation.
The combined action mechanism of Nb and B can describe like this: first, Nb can hinder the recrystallize of distortion γ effectively, so forming new crystal boundary by prevention due to recrystallize makes γ crystal boundary stablize, this just makes B have time enough to be diffused near γ crystal boundary, thus adds the hardening capacity of γ.Secondly, Nb can reduce C diffusivity and activity in γ, and therefore, the Nb dissolved in γ can protect B, and is unlikely to the C compound forming B, as Fe 23(CB) 6.3rd is dissolve Nb itself in γ to have considerable influence for suppression γ → α transformation.Obviously also need more research to illustrate its exact mechanism, because the solubility limit of Nb in γ is 0.03%, therefore in typical low-carbon bainite steel, Nb content is usual not higher than 0.04%.
P cm≤ 0.26%, P sR< 0%; Wherein, P sRfor reheat cracking susceptibility index, P sR=Cr+Cu+2Mo+10V+7Nb+5Ti-2 (%).Control alloying element overall content, to ensure that steel plate has excellent welding property and eliminates reheat cracking susceptibility.
P≤0.015%,S≤0.006%。The impurity elements such as P, S, As, Sb, Sn are the toughness affecting steel, particularly the major cause of low-temperature flexibility and weldability.Not only strict control P, S content, also reduces the impurity elements such as As, Sb, Sn as much as possible.
The production method of described tensile strength 700MPa level low-welding crack-sensitive low alloy steel forging, adopt that desulfurizing iron is skimmed, converter smelting, the refining of LF stove, RH vacuum-treat, slab casting, heating of plate blank, rolling, cooling and tempering process; Wherein:
In slab casting cycle, setting Heating temperature: 1200 ~ 1230 DEG C, time inside furnace: 220 ~ 400 minutes, to ensure the abundant austenitizing of slab;
In the operation of rolling, adopt two benches controlled rolling, finish rolling the Ith stage carries out in austenite perfect recrystallization humidity province, finishing temperature >=920 DEG C, and intermediate blank treats temperature, and object is by the recrystallization zone abundant refine austenite tissue of recrystallize repeatedly.Finish rolling the IIth stage carries out in non-recrystallization zone, start rolling temperature≤900 DEG C, finishing temperature 840 ± 10 DEG C, non-recrystallization zone cumulative deformation >=60%; Object is by the distortion in non-recrystallization zone, and number of nuclei during phase transformation is increased, the brilliant rate of refinement;
Enter laminar cooling system after slab controlled rolling, rate of cooling is 20 ~ 40 DEG C/s, and final cooling temperature is at 450 ~ 500 DEG C;
Slab after rolling adopts quenching-and-tempering process, and optimal quenching temperature is 910 ~ 930 DEG C, soaking time: (thickness of slab × 1.5) minute; Optimum tempering temperature is 630 ~ 670 DEG C, soaking time: (thickness of slab × 2.5 ~ 3) minute, and the unit of described thickness of slab is millimeter.
For the technical characterstic of this programme can be clearly demonstrated, carry out 9 embodiments by steel chemical composition of the present invention and manufacturing technique requirent, and by reference to the accompanying drawings, the technical scheme in the present invention has been set forth.
The chemical composition of 9 embodiment steel is in table 1, and rolling and thermal treatment process are in table 2, and mechanical property is in table 3 and table 4.
The chemical composition of table 1 embodiment steel
The rolling of table 2 embodiment steel and quenching-and-tempering process
The mechanical property of table 3 embodiment steel
The low temperature impact properties of table 4 embodiment steel
Can be learnt by above-described embodiment: the present invention is by conservative control carbon content on Composition Design, Nb, V, Ti etc. is adopted to carry out combined microalloying process, and improve the purity of forged steel, adopt the two benches controlled rolling and controlled cooling of coupling and rational quenching-and-tempering process, thus ensure that forging of the present invention has high strength, high tenacity and low-welding crack-sensitive, also ensure that forging of the present invention has excellent over-all properties.

Claims (7)

1. a production method for tensile strength 700MPa level vanadium niobium titanium-combined microalloying high-strength steel forging, it is characterized in that: with weight, forging is made up of following chemical composition: C:0.07 ~ 0.12%; Mn:1.2 ~ 1.8%; Si:0.2 ~ 0.4%; S≤0.006%; P≤0.015%; Ni:1.3 ~ 1.8%; Cr:0.15 ~ 0.3%; Mo:0.2 ~ 0.4%; Nb:0.02 ~ 0.1%; V≤0.1%; Ti :≤0.02%; Cu≤0.25%; B :≤0.001%, all the other are Fe and inevitable impurity; The welding crack sensibility indices P of forging simultaneously cm≤ 0.26%;
Present method comprises that desulfurizing iron is skimmed, converter smelting, the refining of LF stove, RH vacuum-treat, slab casting, heating of plate blank, rolling, cooling and tempering process;
In slab casting cycle, setting Heating temperature: 1200 ~ 1230 DEG C, time inside furnace: 220 ~ 400 minutes, to ensure the abundant austenitizing of slab;
In the operation of rolling, adopt two benches controlled rolling, finish rolling the Ith stage carries out in austenite perfect recrystallization humidity province, and 980 DEG C≤finishing temperature≤1100 DEG C, intermediate blank treats temperature; Finish rolling the IIth stage carries out in non-recrystallization zone, start rolling temperature≤900 DEG C, and finishing temperature is 840 ± 10 DEG C, non-recrystallization zone cumulative deformation >=60%;
Enter laminar cooling system after slab controlled rolling, rate of cooling is 20 ~ 40 DEG C/s, and final cooling temperature is at 450 ~ 500 DEG C;
Slab after rolling adopts quenching-and-tempering process, and quenching temperature is 910 ~ 930 DEG C, and soaking time is thickness of slab × 1.5 minute; Tempering temperature is 630 ~ 670 DEG C, soaking time: thickness of slab × 2.5 ~ thickness of slab × 3 minute; Described thickness of slab unit is mm.
2. the production method of tensile strength 700MPa level vanadium niobium titanium-combined microalloying high-strength steel forging according to claim 1, it is characterized in that the chemical component weight per-cent of forging is C:0.089%, Mn:1.335%, Si:0.25%, S:0.0038%, P:0.007%, Ni:1.31%, Cr:0.18%, Mo:0.2%, Nb:0.028%, V:0.0035%, Ti:0.013%, Cu:0.026%, B:0.0009%, all the other are Fe and inevitable impurity.
3. the production method of tensile strength 700MPa level vanadium niobium titanium-combined microalloying high-strength steel forging according to claim 1, it is characterized in that the chemical component weight per-cent of forging is C:0.11%, Mn:1.32%, Si:0.316%, S:0.0037%, P:0.006%, Ni:1.33%, Cr:0.28%, Mo:0.28%, Nb:0.031%, V:0.034%, Ti:0.016%, Cu:0.023%, B:0.0009%, all the other are Fe and inevitable impurity.
4. the production method of tensile strength 700MPa level vanadium niobium titanium-combined microalloying high-strength steel forging according to claim 1, it is characterized in that the chemical component weight per-cent of forging is C:0.086%, Mn:1.31%, Si:0.31%, S:0.0042%, P:0.007%, Ni:1.44%, Cr:0.15%, Mo:0.28%, Nb:0.035%, V:0.038%, Ti:0.011%, Cu:0.024%, B:0.0008%, all the other are Fe and inevitable impurity.
5. the production method of tensile strength 700MPa level vanadium niobium titanium-combined microalloying high-strength steel forging according to claim 1, it is characterized in that the chemical component weight per-cent of forging is C:0.08%, Mn:1.299%, Si:0.30%, S:0.0039%, P:0.007%, Ni:1.655%, Cr:0.285%, Mo:0.25%, Nb:0.036%, V:0.01%, Ti:0.012%, Cu:0.023%, B:0.0008%, all the other are Fe and inevitable impurity.
6. the production method of tensile strength 700MPa level vanadium niobium titanium-combined microalloying high-strength steel forging according to claim 1, is characterized in that: be organized as granular bainite and acicular ferrite and super-refinement ferrite bainite after slab rolls.
7. the production method of tensile strength 700MPa level vanadium niobium titanium-combined microalloying high-strength steel forging according to claim 1, it is characterized in that: after slab tempering, be organized as evenly tiny tempered sorbite, namely lath-shaped ferrite matrix is distributed with precipitation carbonitride.
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