Background technology
In the past, produced other steel grade of high-tensile level and mostly adopted modified method, its chemical element content height, the production cycle is long, the carbon equivalent height, welding property is poor, the production cost height.
Mn content is low in the low-carbon bainite steel of early development, and Cr, Ni constituent content are higher, the cost height of steel, and to promoting that all the bainite transformation effects of tissue are undesirable, thereby the intensity rank of steel is not high, produces relatively difficulty.
" a kind of relaxation that is used for high-strength low-alloy steel production-separate out-control phase change technique " Chinese patent of University of Science ﹠ Technology, Beijing application, its application number is 01115650.3, utilize this technology, can obtain ultra-fine complex tissue, adopt oligo-elements, can obtain high strength, high tenacity and low cost, this technology is applicable to yield strength 800N/mm
2The production of level steel plate.But because its Nb, Ti constituent content are higher, weight percent is respectively 0.091,0.08, and production brings very big technical difficulty to continuous casting, is unfavorable for the production of big industrialization continuous casting, and combines the reduction that the superfluous Ti in back will inevitably cause impelling strength with N.In addition, the billet heating temperature of this technology is higher, and more than 1200 ℃, this can influence the low-temperature flexibility of product.
A patent No. of our company's application is 03110973.X, the Chinese patent of " a kind of ultra-low-carbon bainite steel and production method thereof " by name, be characterized in this steel grade with Mn element with low cost as main adding elements, add element with Cr, Ni as assisting, C content is low in the steel, below 0.05%.This technology has only adopted controlled rolling and controlled cooling+tempering process in production technique, the intensity rank of steel is relatively low, and its tensile strength is all at 900N/mm
2Below the rank, and the thickness of steel plate is generally all below 10mm.
Summary of the invention
The objective of the invention is to overcome the deficiencies in the prior art, from the angle of the market requirement, provide a kind of composition reasonable in design, technology advanced person uses oligo-elements, obtains tensile strength at 900N/mm
2The above high tenacity of rank, low carbon bainite thick steel plate cheaply.
The chemical composition content of high tensile strength low carbon bainite thick steel plate of the present invention (Wt%) is: C0.052%~0.08%, Si0.1%~0.5%, Mn1.65%~1.90%, Nb0.015%~0.060%, Ti0.005%~0.03%, B0.0005%~0.003%, Mo0.25%~0.50%, Cu0.62%~0.85%, Ni0.42%~0.80%, Al0.015%~0.05%, all the other are Fe and unavoidable impurities.
Why the present invention selects the kind of above alloying element and content mainly to be:
(1) C: carbon is very big to intensity, toughness, the influence of welding property smelting cost of steel.The intensity that carbon is lower than 0.052% steel does not reach target call; Carbon is higher than 0.08%, then generates bainite structure minimizing in the tissue, unit elongation and toughness are descended, so the present invention determines that the scope of carbon content is 0.052%~0.08%.
(2) Mn: manganese is to improve intensity and flexible effective element, bainite transformation is had bigger promoter action, and cost is very cheap, therefore in the present invention the Mn element as main adding elements.The present invention is limited to 1.65%~1.90% with manganese content.
(3) B: boron is a composition important among the present invention, and it can improve the hardening capacity of steel, and the boron that adds denier just has obvious effects, significantly postpones austenite to ferrite, pearlitic transformation, and when Nb existed simultaneously, the effect of B was more outstanding.When boron content was lower than 0.0005%, the effect that improves hardening capacity was little; When the content of boron is 0.001%, will make the tissue of steel all change bainite into; Up to 0.003% o'clock, hardening capacity reached capacity, and can be observed Fe this moment
23(CB)
6Separate out; Be higher than 0.003%, hardening capacity descends, and the toughness of steel worsens, and can form low-melting-point eutectic, concentrates on the border of crystal grain, and this will cause red brittleness, increases the thermal pressure processing difficulties.So in the present invention boron is limited between 0.0005%~0.003%.
(4) Nb: niobium is an important interpolation element of the present invention, and it is the austenitic recrystallize of delayed deformation effectively, stops austenite crystal to be grown up, and improves austenite recrystallization temperature, and crystal grain thinning improves intensity and toughness simultaneously; It and micro-boron compound action can be improved hardening capacity significantly, promote bainite transformation.This is because under the situation of only adding B, usually separates out Fe at austenite grain boundary after rolling
23(CB)
6Thereby, greatly reduce the effect of B, can not postpone the transformation of γ-α effectively, Nb easily combines with C, prevents to form Fe
23(CB)
6, along with the solubleness of Nb in steel increases, the trend that forms bainite increases.In the present invention Nb content is limited in 0.015%~0.06% scope.
(5) Ti: adding the titanium of trace, is for the nitrogen element in the fixing steel, thereby guarantees the raising hardening capacity effect of boron.The avidity of boron and oxygen, nitrogen is bigger, if the trace B and the oxygen in the steel, the nitrogen that add work, the effect of the promotion bainite transformation of boron will disappear.Therefore, gas in the steel must be reduced as far as possible, the aluminium, the titanium that add q.s carry out complete deoxidation and fixed nitrogen, just can give full play to the useful effect of boron.In the best condition, titanium, nitrogen form titanium nitride, stop steel billet the growing up of crystal grain in heating, rolling, welding process, improve the toughness of mother metal and welded heat affecting zone.Titanium is lower than at 0.005% o'clock, and nitrogen fixation effect is poor, surpasses at 0.03% o'clock, and nitrogen fixation effect reaches capacity, and superfluous titanium will make the toughness of steel worsen.When the ratio of the Ti in the steel, N atom is 1: 1, the most tiny and distribution disperse of TiN particle, the strongest to the refining effect of high temperature austenite crystal grain, not only good toughness can be obtained, and the large-line energy welding that 30KJ/cm is above can be realized.This moment, the ratio corresponding to Ti, N weight was 3.42, so in the present invention, in conjunction with the content of N in the steel, between 0.005%~0.03%, its optimization range is between 0.01%~0.019% with the titanium Composition Control.
(6) Si: silicon is the bioelement of deoxidation in steel making, also has certain solution strengthening effect, when being lower than 0.1%, is difficult to obtain sufficient deoxidation effect; Surpass at 0.5% o'clock, the degree of cleaning of steel descend, and toughness reduces, and weldability is poor.So in the present invention silicon is limited in 0.1%~0.5% the scope.
(7) Al: aluminium is deoxidant element, can be used as the AlN forming element, crystal grain thinning effectively, and when it contained quantity not sufficient 0.01%, effect was less; Surpass at 0.07% o'clock, desoxydatoin reaches capacity; High more then harmful to mother metal and welding heat influence area toughness.So, aluminium content should be limited in 0.010%~0.07% the scope.The present invention is limited to 0.015%~0.050% with aluminium content.
(8) Mo: molybdenum helps the refinement of austenite crystal when rolling and the generation of fine bainite, improves the intensity of steel, surpasses at 0.5% o'clock but add, and cost improves, and weldability obviously reduces.Thereby, in the present invention, be limited between 0.25%~0.50% according to the difference of armor plate strength and thickness content with molybdenum.
(9) Cu: copper does not only have detrimentally affect to welded heat affecting zone hardening and toughness, the intensity of mother metal is improved, and low-temperature flexibility is improved greatly, also can improve solidity to corrosion.In Ultra-low carbon Bei Shi steel, add copper, can utilize the comprehensive action of Cu-B, further improve the hardening capacity of steel, promote the formation of bainite.Under appropriate condition, Cu can precipitate to separate out and generate ε-Cu, further makes matrix strengthening; But easily crack when steel billet heating or hot rolling when Cu content is high, worsen the surface of steel plate performance, must add an amount of Ni to stop the generation of this crackle.In the present invention, different according to armor plate strength and thickness are controlled at copper content between 0.62%~0.85%.
(10) Ni: nickel does not have detrimentally affect to welded heat affecting zone hardening and toughness, the intensity of mother metal is improved, and low-temperature flexibility is improved greatly.It is a noble element, causes the cost of steel to rise significantly, and is less economical.The purpose of adding the Ni element in the present invention mainly is to stop to contain the tendency that the high steel billet of Cu amount cracks when heating or hot rolling.So different according to armor plate strength and thickness are controlled at Ni content between 0.42%~0.80% in the present invention.
(11) upper limit of the impurity element in the steel is controlled at P≤0.02%, and S≤0.01% is to improve the toughness of steel.Preferably N≤0.006% to avoid forming the nitride of boron, lost efficacy the hardening capacity of boron, and the pure more effect of steel is better.
The production method of low carbon bainite thick steel plate of the present invention is as follows:
Smelting technology: adopt converter smelting, by top blast or top bottom blowing, dark decarburization; Adopt RH or VD vacuum-treat and LF stove to handle, to reduce the content of obnoxious flavour such as O, H, N and S; Add alloying element, carry out microalloying; In conjunction with S content in the steel, by feeding Si-Ca line 2~7m/ ton steel, carry out Ca and handle, with the control oxide morphology, improve ductility and toughness, reduce the horizontal and vertical poor performance of steel plate; Continuous casting adopts induction stirring.
Rolling technology: adopt controlled rolling and controlled cooling technology (TMCP)+relaxation-separate out-control phase change technique (RPC)+tempering (T).Heating temperature is 1050 ℃~1190 ℃ before rolling, and to guarantee obtaining tiny austenite crystal, can have the Nb of a great deal of to dissolve in austenite, helps rolling the formation of bainite in the postcooling process; Adopt the controlled rolling of two stages, recrystallization zone rolling temperature 〉=1000 ℃, non-recrystallization district rolling temperature be 950 ℃~(Ar3+0 ℃~80 ℃), the accumulation of distortion amount is greater than 50%, roll back relaxation time 10~120s, with the after-acceleration cooling, best speed of cooling scope is at 10~40 ℃/S, quicken the refrigerative final cooling temperature and be controlled at following 0~150 ℃ of Bs point, best final cooling temperature is 500 ℃~300 ℃; Air cooling afterwards; Last temper of carrying out different time again generates ε-Cu so that the Cu precipitation is separated out, and further improves product strength and toughness, and tempering temperature is controlled at 480 ℃~680 ℃.
The present invention with Mn element with low cost as main adding elements, with elements such as Cu, Mo, Ni, Nb, B to the abundant combined utilization of the effect of bainite transformation.This steel grade carbon content is low, and low-temperature flexibility is good, and tough crisp transition temperature is respectively below-40 ℃, welding property is good, does not need preheating before the welding, does not need thermal treatment after the welding, can realize the large-line energy welding that 30KJ/cm is above, and have good cold roll forming performance; Nb, Ti constituent content are low, are suitable for the production of big industrialization continuous casting; Higher Cu content helps improving The comprehensive performance.The present invention adopts TMCP+RPC+T technology can obtain tensile strength 900N/mm
2High strength, high tenacity and low-cost steel grade that rank is above.The present invention adopts lower billet heating temperature, and both save energy has improved the low-temperature flexibility of steel again.The steel plate thickness that goes out with this explained hereafter can reach 12mm~60mm, field such as product can be widely used in engineering machinery, excavates machinery, heavy-duty car and maritime facilities.