CN104131237A - Economic type diphasic stainless steel with good toughness and weldability and manufacturing method thereof - Google Patents
Economic type diphasic stainless steel with good toughness and weldability and manufacturing method thereof Download PDFInfo
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Abstract
Disclosed are economic type diphasic stainless steel with good toughness and weldability and a manufacturing method thereof. The economic type diphasic stainless steel comprises the chemical components by the weight percentage: 0.01-0.05% of C, 0.2-1.0% of Si, 4.0-6.0% of Mn, 20.0-21.5% of Cr, 1.7-3.0% of Ni, 0.15-0.20% of N, not more than 0.5% of Mo, not more than 0.5% of W, not more than 0.5% of Cu, not more than 0.006% of B, and the balance Fe and inevitable impurities. The stainless steel mainly adopts 4.0-6.0% of Mn to replace Ni, at the same time, controls the contents of Mo and Cu in 0.5% or less, thereby significantly reducing alloy raw material costs; the content of Ni is controlled in 1.7-3.0%, the content of N is controlled in 0.15-0.2%, and the ferrite equivalent weight P[alpha] of a high-temperature heat affected zone is controlled in 20.5 or less, not only can the low temperature impact toughness of a parent metal be ensured, but also the good weldability can be obtained, and welded joints are allowed to have good low-temperature impact toughness. 304 austenitic stainless steel having the nickel content reaching up to 8% or more is replaced by the diphasic stainless steel under conditions of room temperature and low temperatures.
Description
Technical field
The present invention relates to duplex stainless steel and manufacture method thereof, especially a kind of economical diphase stainless steel and manufacture method thereof with excellent in toughness and weldability.
Background technology
Under duplex stainless steel room temperature, by ferrite and austenite two-phase, formed, and wherein every phase ratio is no less than 30%.Because the existence of two-phase structure makes duplex stainless steel have the characteristic of ferritic stainless steel and austenitic stainless steel concurrently, be that a class high strength and high anti-corrosion mate best stainless steel.With ferritic stainless steel ratio, its toughness is high, brittle transition temperature is low, intergranular corrosion resistance and weldability are significantly improved, and has retained the advantage that ferritic steel thermal conductivity is high, the coefficient of expansion is little simultaneously.Compare with austenitic stainless steel, the intensity of duplex stainless steel, particularly yield strength significantly improves, and intergranular corrosion resistance, anticorrosion stress-resistant, resistance to corrosion fatigue and wear resisting property significantly improve, and have magnetic.
The development experience of duplex stainless steel three important stages.The trade mark of developing before 1971 is first-generation duplex stainless steel, and due to the conventional levels in electric arc furnace smelting containing N amount, after its welding, performance sharply declines.The trade mark of exploitation in 1971～1989 years belongs to s-generation duplex stainless steel.By means of invention and the application of nineteen sixty-eight stainless steel precision sweetening process-argon oxygen decarburization (AOD), can make nitrogen content in duplex stainless steel significantly improve, thereby significantly improve weld seam, the toughness of heat affected zone and corrosion resistance nature.The 2205th, the representing steel grade and be widely used in a plurality of fields such as offshore oil platform, chemical industry, papermaking of s-generation dual phase steel.The trade mark occurring after nineteen ninety is third generation duplex stainless steel, and the development that enters duplex stainless steel after 2000 presents 2 kinds of trend.Improve on the one hand further steel interalloy constituent content to obtain more high strength and better corrosion resistance nature, develop on the other hand low nickel content and containing Mo or only contain the economical diphase stainless steel of a small amount of Mo, to reduce costs.
Economical diphase stainless steel refers in particular to a class Cr content below 22%, and nickel content is low, does not contain molybdenum, tungsten simultaneously or only contains a small amount of molybdenum, the duplex stainless steel of tungsten.Realize ferrite and austenite content and respectively account for approximately 50% ratio, must make the balance of austenite equivalent and ferrite equivalent.Conventional austenite equivalent calculation formula is suc as formula shown in (1):
Ni
eq＝Ni+30(C+N)+0.5Mn+0.25Cu .(1)
According to formula (1), can adopt Mn, N, Cu to replace Ni, thereby reduce costs, replace the austenitic stainless steel of traditional high nickel content.But for Ni, also have negative impact at Mn, N.Manganese can reduce the corrosion resistance nature of duplex stainless steel.Evaluate the experimental formula of the resistance to pitting attack performance of duplex stainless steel as shown in formula (2):
PREN (equivalent of resistance to spot corrosion)=Cr%+3.3Mo%+30N%-Mn% (2)
The manganese of every interpolation 1%, will make alloy PREN value reduce by 1, be equivalent to offset add 1% the Cr raising to pitting resistance.N is very strong austenite former, is conducive to improve intensity, solidity to corrosion and the weldability of duplex stainless steel.But too high N content can cause, smelt difficulty increase, hot workability declines, and easily causes separating out of nitride simultaneously.
The toughness of duplex stainless steel defines its range of application.Ni content is to determine the most important element of duplex stainless steel toughness, and the disadvantageous effect that the reduction of Ni content brings is that impelling strength reduces.In addition, N content On Impact Toughness also has remarkably influenced.Certain N content can guarantee two-phase proportion coordination, thereby improves the toughness of material, but N too high levels can cause the toughness of duplex stainless steel to decline on the contrary.
The weldability of duplex stainless steel is one of key factor determining duplex stainless steel application.The weakness zone of duplex stainless steel welded joints is generally high-temperature heat-affected zone.The process that this region experience heats up and lowers the temperature, during intensification to ferritic transformation, there is ferritic phase to austenitic transformation in austenite in temperature-fall period, after experience Thermal Cycle, conventionally can cause this region ferrite content to increase, and follow grain growth.If high-temperature heat-affected zone ferrite content is too high, can cause toughness decline, corrosion resistance nature to decline.
So far, the patent of economical diphase stainless steel mainly concentrates on and how to reduce costs, improves corrosion resistance nature or intensity, to toughness and weldability, considers few.
European patent EP 1327008 has been announced a kind of austenite-ferrite duplex stainless steel, its composition range (% by weight): 0.02～0.07%C, 3.0～8.0%Mn, 19.0～23.0%Cr, 1.1～1.7%Ni, 0.1～2.0%Si, 0.15～0.3%N, the alloying element that may comprise have and be not more than 1.0% Mo or W, be not more than 1.0% Cu, 0.003～0.005%B, surplus is Fe or inevitable impurity.Because N content is higher, this dual phase steel has higher intensity, good corrosion resistance nature and welding property.But Ni content (1.1～1.7%) lower in steel has reduced low-temperature impact toughness with higher N content.
A kind of austenite-ferrite duplex stainless steel that Chinese patent CN101215674 announces, its composition range of its composition range (% by weight): 0～0.04%C, 5.0～7.0%Mn, 18.0～20.0%Cr, 0～1.0%Si, 0.15～0.25%N.This duplex stainless steel is not containing noble metal Ni, Mo, so cost significantly declines, still the while is also lower without the dual phase steel impelling strength of Ni.
Chinese patent CN101090988 improves Ni content and Mo content, has reduced Mn content.Its Ni content is that to be increased to 0.5～2.5, Mn content be 1.5～4.5% to 1.0～3.0, Mo.The compositing range of the alloy compositions of its patent application guarantees that alloy CPT is higher than 20 ℃, and its corrosion resisting property reaches and is even better than 316L stainless steel.But the raising of Ni, Mo content will significantly improve the cost of alloy, so economy declines.
US Patent No. 4828630 has been announced a kind of economical diphase stainless steel, and its main component is: 4.0～8.0%Mn, and 17.0～21.5%Cr, 1.0～4.0%Ni, 0.05～0.15%N, is not more than 2.0% Mo, is not more than 1.5% Cu.This steel grade Mn content is high, and Ni content is low, and economy is outstanding.But due to N content and Ni content all lower, so the low-temperature flexibility of its welding joint will significantly decline.
Summary of the invention
The object of the present invention is to provide a kind of economical diphase stainless steel and manufacture method thereof with excellent in toughness and weldability, there is low-cost, good low-temperature flexibility and weldability, more than the v-notch ballistic work of mother metal-40 ℃ reaches 90J, more than the v-notch ballistic work of welded heat affecting zone-40 ℃ reaches 50J; Reduce smelting, hot-work, cold worked difficulty simultaneously, can be widely used in petrochemical industry, building, communications and transportation etc. to requiring the field that toughness and weldability are higher, under room temperature and cold condition, replace nickel content up to 304 more than 8% austenitic stainless steels.
For achieving the above object, technical scheme of the present invention is:
Present invention focuses on, for obtaining lower cost, use the Mn of 4-6% to replace part Ni, Mo control of element is below 0.5%; For the low-temperature impact toughness of assurance mother metal at-40 ℃, improve Ni content and be controlled at 1.7～3.0%, N content is controlled to 0.15-0.2%.For further guaranteeing the low-temperature impact toughness of welding high-temperature heat-affected zone, need to control the ferrite content of weld seam high-temperature heat-affected zone.By a large amount of theoretical and experimental studies, the present invention controls this region ferrite content by controlling high-temperature heat-affected zone ferrite equivalent P α.
P
α＝2Cr+Mo+2Si-90C-75N-3Ni-0.75Mn-Cu (3)
Up to now, also do not have patent to be described high-temperature heat-affected zone ferrite equivalent.In conjunction with the control to alloying elements such as Ni, N, the present invention is by P
αbe controlled at below 20.5, thus can be so that more than the ballistic work of high-temperature heat-affected zone reaches 50J.
Particularly, the present invention has the economical diphase stainless steel of excellent in toughness and weldability, and its chemical component weight per-cent is: C0.01～0.05%, Si0.2～1.0%, Mn4.0～6.0%, Cr20.0～21.5%, Ni1.7～3.0%, N0.15～0.20%, Mo≤0.5%, W≤0.5%, Cu≤0.5%, B≤0.006%, all the other are Fe and inevitable impurity; P
α(2Cr+Mo+2Si-90C-75N-3Ni-0.75Mn-Cu) <20.5.
In Composition Design of the present invention:
Carbon carbon is strong austenite former, promotes austenitic formation, and stable austenite tissue.According to austenite equivalent calculation formula (1), Ni
eq=Ni+30 (C+N)+0.5Mn+0.25Cu, the austenitic formation effect of visible carbon can reach 30 times of Ni, so carbon can partly replace Ni.According to formula (3), C can significantly reduce the ferrite content of high-temperature heat-affected zone.But on the other hand,, in welding process, carbon electrode is easily combined with chromium and is formed carbide, causes on the contrary intergranular corrosion, and significantly reduces impelling strength.Too low carbon content, by the difficulty and the cost that increase in preparation process, reduces austenitic stability simultaneously, destroys austenite and ferritic balancing each other.Therefore, in steel of the present invention, designing carbon content is 0.01～0.05%.
Silicon silicon is the element conventionally containing in Manufacture Of Iron And Steel By Melting.In duplex stainless steel, silicon is that ferrite forms and stable element.Silicon for deoxidation, contains more than 0.2% silicon in therefore general duplex stainless steel in fusion process.When but silicone content is too high, can accelerate separating out of weld seam Low Temperature Thermal zone of influence intermetallic phase, unfavorable to material corrosion resistance and toughness.Therefore, in steel of the present invention, designing silicone content is 0.2～1.0%.
Manganese manganese is a kind of austenitic formation and stable element, from formula (1), utilizes manganese to replace to a certain extent nickel; In addition the interpolation of manganese can significantly improve the solubleness of nitrogen.Therefore Mn is that economical diphase stainless steel reduces costs very important alloy element.According to formula (3), manganese has certain effect to reducing high-temperature heat-affected zone ferrite content, but not remarkable.But manganese is all negative on the impact of stainless erosion resistance substantially.According to formula (2), the manganese of every interpolation 1%, will make alloy PREN value reduce by 1, so in steel of the present invention, priority control Mn content is 4.0%～6.0%.
Chromium chromium is the most important element that steel obtains corrosion resistance nature, is a kind of ferrite former.For duplex stainless steel, when chromium content is lower, solidity to corrosion will decline, and likely occur martensitic phase simultaneously, all unfavorable to mechanics and corrosion resistance nature; And when chromium too high levels, can increase the tendency of separating out of weld heat-affected zone intermetallic phase, carbide and nitride, and need the Ni element of corresponding high-content to match, thus increase cost.In addition, according to formula (3), during chromium too high levels, can cause the ferrite content of high-temperature heat-affected zone to improve, reduce low-temperature impact toughness.Therefore can cause Cr content in steel of the present invention to be controlled at 20.0～21.5%.
Nitrogen nitrogen element is a kind of important element that forms austenite phase.In austenite equivalent calculation formula (1), the austenitic formation ability of nitrogen is 30 times of nickel, therefore with N, replaces Ni significantly to reduce costs.Nitrogen can improve the corrosion resistance nature of austenite phase, especially resistance to pitting attack performance and slit and corrosion resistant performance, and in the calculation formula (2) of PREN value, its equivalent of resistance to spot corrosion is 30 times of chromium.According to calculation formula (3), N can significantly improve ferrite in Thermal Cycle process and, to austenitic transformation, reduce high-temperature heat-affected zone ferrite content, thereby improves the low-temperature flexibility of weld seam high-temperature heat-affected zone.Therefore,, for improving the weldability of duplex stainless steel, conventionally add higher nitrogen content.When but nitrogen content is too high, can reduce the toughness of mother metal; Increase the risk that heat affected zone nitride forms simultaneously, reduce on the contrary the toughness of welded heat affecting zone.Therefore, nitrogen in steel content of the present invention is controlled at 0.15～0.20%.
Molybdenum molybdenum is very beneficial for improving the corrosion resistance nature of steel.According to RREN value calculation formula (2), its equivalent of resistance to spot corrosion is 3.3 times of chromium.Its mechanism is to stablize passive film and promote the enrichment of chromium element in passive film, and the Main Function that adds molybdenum is to improve erosion resistance.According to formula (3), molybdenum content improves can improve high-temperature heat-affected zone ferrite content, reduces corrosion resistance nature and toughness.In addition, molybdenum too high levels, by causing the acceleration of phase between the friable metal of welded heat affecting zone to be separated out, increases cost of alloy simultaneously, so the molybdenum content in steel of the present invention is controlled at below 0.5%.
Copper copper is also one of optional element in the present invention.Copper is a kind of austenite former, and adding of copper can improve the erosion resistance of dual phase steel in reductive acid, is conducive to improve slit and corrosion resistant performance simultaneously.According to formula (3), it is not remarkable to reducing the contribution of high temperature heat affecting ferrite content to improve Cu content, but is unfavorable for hot workability increasing cost during copper too high levels simultaneously.Therefore nitrogen in steel content of the present invention is controlled at below 0.5%.
Tungsten tungsten is one of optional element in the present invention.The effect of tungsten in dual phase steel is similar to molybdenum, can improve the corrosion resistance nature of steel.Tungsten can also reduce the activity at austenite/ferritic phase interface, suppresses the formation of intermetallic phase.When but W content is too high, promote on the contrary intermetallic phase to generate.Therefore in steel of the present invention, W content is controlled at below 0.5%.
The interpolation of boron trace B can thinning microstructure, improves grain-boundary strength, improves thermoplasticity, but excessive interpolation boron will cause materials hot working to worsen on the contrary, therefore the present invention, through exploring, find in this alloy system, so in steel of the present invention, boron content is controlled at below 0.006%.
The manufacture method of duplex stainless steel of the present invention, comprises the following steps:
1) smelt, cast
By following composition weight percent, smelt: C0.01～0.05%, Si0.2～1.0%, Mn4.0～<6.0%, Cr20.0～21.5%, Ni1.7～3.0%, N0.15～0.20%, Mo≤0.5%, W≤0.5%, Cu≤0.5%, B≤0.01%, all the other are Fe and inevitable impurity; Smelting process selects vacuum induction to smelt, and electric furnace-argon oxygen decarburization AOD or electric furnace-argon oxygen decarburization AOD-external refining LF stove are smelted; Molten steel is carried out to die casting or continuous casting, and during die casting, controlling superheating temperature is 20～50 ℃; Or employing continuous cast method, avoid the effusion of nitrogen, during continuous casting, controlling superheating temperature is 20～50 ℃, slab pulling rate is 0.8～2m/min;
2) die casting base or continuous casting steel billet are put into process furnace and be heated to 1100～1250 ℃ and insulation, on forging line or hot rolling unit, be machined to desired thickness, then anneal or annealing and pickling; For guaranteeing final low-temperature impact toughness, its hot rolling compression ratio is greater than 10, and annealing temperature is controlled at 1030～1150 ℃, annealing soaking time t >=1h, and the unit of t is minute; H is thickness, and unit is mm.
To Patents analysis, can find out, the basic ideas of existing economical diphase stainless steel Composition Design are main added elements for take Cr, Mn, Ni, N etc., replace Ni, thereby significantly reduce costs by Mn and N, replace expensive austenitic stainless steel.
Chinese patent CN101215674 and European patent EP 1327008 are that typical high Mn, high N are for the economical diphase stainless steel design of Ni, for replacing now widely used 304 austenitic stainless steels.But because Ni content is too low, N content is higher, its low-temperature impact toughness affects adversely.
US Patent No. 4828630 is when design is compared with high Mn content, and Ni content is improved, but N content significantly declines, but this performance after to welding is brought disadvantageous effect.
Chinese patent CN101090988 suitably improves Ni content, reduces N content, and impelling strength and solidity to corrosion are all improved, but because Ni, Mo content significantly improve, its cost significantly improves, and its purpose of design is mainly to replace 316L austenitic stainless steel.For when maintenance reduces costs, obtain good impelling strength, the impelling strength after especially welding, the present invention, adding certain Mn content to reduce costs, suitably improves Ni content, and designs the N content of moderate 0.15-0.2%.
The most important thing is, for guaranteeing the low-temperature impact toughness of welding heat affected zone afterwards, by high-temperature heat-affected zone ferrite equivalent P α (2Cr+Mo+2Si-90C-75N-3Ni-0.75Mn-Cu) <20.5.
The present invention compared with prior art, has the following advantages and positively effect:
(1) use the Mn of 4.0-6.0% to replace Ni, Mo, Cu content are controlled at below 0.5% simultaneously, significantly fall low-alloyed material cost.
(2) Ni content is controlled to 1.7-3.0%, N content is controlled at 0.15-0.2%, can guarantee the low-temperature impact toughness of mother metal, makes-40 ℃ of ballistic works be greater than 90J.
(3) for to guarantee that welded heat affecting zone obtains good low-temperature impact toughness, guaranteeing that Ni is at 1.7-3.0%, N content, under the prerequisite of 0.15-0.2%, is controlled high-temperature heat-affected zone ferrite equivalent P
α(2Cr+Mo+2Si-90C-75N-3Ni-0.75Mn-Cu) <20.5, is greater than 50J thereby make to weld high-temperature heat-affected zone at-40 ℃ of ballistic works.
(4) this duplex stainless steel can utilize existing stainless steel to produce line batch production, concrete preparation method is casting after vacuum induction furnace, the smelting of electric furnace-AOD stove or electric furnace-AOD-LF stove are smelted, when die casting, controlling superheating temperature is 20～50 ℃ of left and right, and coordinate cooling fast, or adopt cooling rate continuous cast method faster, avoid the effusion of nitrogen, during continuous casting, controlling superheating temperature is 20～50 ℃, and slab pulling rate is 0.8～2m/min.Hot rolling compression ratio is greater than 10, and the annealing temperature after hot rolling is controlled between 1030～1150 degree, annealing soaking time t >=1h, and the unit of t is minute; H is thickness, and unit is mm.
Accompanying drawing explanation
Fig. 1 is embodiments of the invention 1 alloy microstructures.
Fig. 2 is that embodiments of the invention 2 alloys weld rear high-temperature heat-affected zone tissue.
Embodiment
Below in conjunction with embodiment and accompanying drawing, the present invention will be further described.
It is example that the embodiment of the present invention be take the Production Flow Chart that electric furnace-AOD smelts: add electric furnace to melt ferrochrome, ferronickel and steel scrap etc., after melting clearly, pour molten steel into AOD stove, in AOD stove, take off the blowing of C, de-S and increasing N, control N, when smelting composition reaches requirement, pour molten steel into tundish, and cast on vertical-bending caster; The superheating temperature of continuous casting is 20～50 ℃, and slab pulling rate is 0.8～2m/min; Continuous casting steel billet is put into process furnace and be heated to 1100～1250 ℃, on hot rolling unit, be rolled down to 15mm, then carry out continuous annealing pickling, obtain the tissue of ferritic-austenitic double structure; Annealing temperature is controlled at 1030～1150 ℃, and annealing time is 20min.
Table 1 is depicted as the chemical composition of embodiment of the present invention steel grade, and table 1 has provided the chemical composition of object 304 austenitic stainless steels that typically economical diphase stainless steel US4828630 and steel plan of the present invention replace at present as a comparison case simultaneously.Welding process is: argon tungsten-arc welding (TIG), fill out 2209 welding wires.Table 1 has provided mother metal and welded heat affecting zone at the ballistic work (bulk sample, V-type) of-40 ℃, all higher than US4828630.
The metallographic structure of the alloy of embodiment 1 as shown in Figure 1.To after sample polishing polishing, carry out electrolytic corrosion, etching reagent is 40gKOH+100mlH
2o, corrosion current 0.3～0.8A/cm
2, finally under metaloscope, by Quantitative metallography, austenite ratio is analyzed.In figure, black is organized as ferrite, and white is organized as austenite, and austenite accounts for 53% mutually.The metallographic structure of embodiment 2 welded heat affecting zones as shown in Figure 2.
Table 1 unit: weight percent
Table 2
From table 1, table 2, steel of the present invention, owing to using Mn and N to replace Ni content, does not contain simultaneously or only containing alloying elements such as a small amount of Mo, Cu, compares with 304 austenitic stainless steels, and its cost of alloy significantly reduces.Compare with duplex stainless steels such as US4828630, its Ni content is higher, so mother metal low-temperature flexibility is higher, more than reaching 90J; The N that keeps 0.15-0.2%, combines with higher Ni content, and by heat affected zone ferrite equivalent P
αbe controlled at below 20.5, make the low-temperature impact toughness of welded heat affecting zone good, more than reaching 50J.
It is worthy of note, the embodiment providing can not be interpreted as limiting the scope of the invention, and some nonessential improvement that the person skilled in the art in this field makes the present invention according to the content of the invention described above and adjustment must belong to protection domain of the present invention.
Claims (2)
1. the economical diphase stainless steel with excellent in toughness and weldability, its chemical component weight per-cent is: C0.01～0.05%, Si0.2～1.0%, Mn4.0～6.0%, Cr20.0～21.5%, Ni1.7～3.0%, N0.15～0.20%, Mo≤0.5%, W≤0.5%, Cu≤0.5%, B≤0.006%, all the other are Fe and inevitable impurity; P
α(2Cr+Mo+2Si-90C-75N-3Ni-0.75Mn-Cu) <20.5.
2. the manufacture method with the economical diphase stainless steel of excellent in toughness and weldability, comprises the following steps:
1) smelt, cast
By following composition weight percent, smelt: C0.01～0.05%, Si0.2～1.0%, Mn4.0～<6.0%, Cr20.0～21.5%, Ni1.7～3.0%, N0.15～0.20%, Mo≤0.5%, W≤0.5%, Cu≤0.5%, B≤0.01%, all the other are Fe and inevitable impurity; Smelting process selects vacuum induction to smelt, and electric furnace-argon oxygen decarburization AOD or electric furnace-argon oxygen decarburization AOD-external refining LF stove are smelted; Molten steel is carried out to die casting or continuous casting, and during die casting, controlling superheating temperature is 20～50 ℃; Or employing continuous cast method, avoid the effusion of nitrogen, during continuous casting, controlling superheating temperature is 20～50 ℃, slab pulling rate is 0.8～2m/min;
2) die casting base or continuous casting steel billet are put into process furnace and be heated to 1100～1250 ℃ and insulation, on forging line or hot rolling unit, be machined to desired thickness, then anneal or annealing and pickling; For guaranteeing final low-temperature impact toughness, its hot rolling compression ratio is greater than 10, and annealing temperature is controlled at 1030～1150 ℃, annealing soaking time t >=1h, and the unit of t is minute; H is thickness, and unit is mm.
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