CN103510014A - Economical stainless steel with resistance to urea and automobile exhaust corrosion and manufacturing method thereof - Google Patents

Abstract

The invention provides economical stainless steel with resistance to urea and automobile exhaust corrosion and a manufacturing method thereof. The economical stainless steel comprises the following components in percentage by weight: less than or equal to 0.020% of C , less than or equal to 0.60% of Si, less than or equal to 0.80% of Mn, 16.0-19.0% of Cr, 0.4-1.2% of Mo, less than or equal to 0.040% of P, less than or equal to 0.005% of S, less than or equal to 0.015% of N, 0.0010-0.0050% of Ca, less than or equal to 0.05% of Al, wherein Cr+3Mo+Nb is more than or equal to 18.5% but less than or equal to 22%, C+N is less than or equal to 0.025%, more than one of Nb and Ti are included, and the rest is Fe and unavoidable impurities. The economical stainless steel is produced by the following steps: electrosmelting, performing AOD and VOD refining, performing continuous casting to obtain a continuous cast blank, hot-rolling, pickling, cold-rolling and annealing and pickling. The economical stainless steel can be adapted to the design change of commercial vehicle emission purification systems according to the national state IV emission standards and ensures that a housing material of a urea purification spray system meets the requirements in the corrosion environments of 400-600 DEG C high temperature and urea. The economical stainless steel has the same resistance to high temperature and urea corrosion as SUS304. Besides, the material has a ferrite face-centered structure and therefore has a low thermal expansion coefficient.

Description

Economical stainless steel and the manufacture method thereof of a kind of resistance to urea and vehicle exhaust corrosion

Technical field

The present invention relates to stainless steel technical field, particularly economical stainless steel and the manufacture method thereof of a kind of resistance to urea and vehicle exhaust corrosion.

Background technology

In recent years Domestic Automotive Industry develop rapidly, comprises that the environmental pollution of motor vehicle exhaust is day by day serious.Conventional commercial car comprises that the discharge of the large-tonnage car in being heavily stuck in attracts great attention.Along with the raising of national requirements for environmental protection, commercial levels of emissions of heavily blocking automobile is brought up to state four from current state three.

Traditional state's three emission requests, do not need the obnoxious flavour of engine discharge to process general direct outer row.When Dang Cong state three brings up to state's four emission request, levels of emissions improves from technological layer to be considered, need in former blowdown system, increase a tail gas clean-up conversion system.This device use temperature is at 400～600 ℃, and by spray urea (concentration is 3%～4%), and urea can be decomposed to form NH under high temperature action ₃, as NOx reacts, form N with the toxic and harmful in engine combustion product ₂and H ₂the pollution-free product of O, reaches the object of environment protection.Hot environment also exists traditional material under the condition of resistance to urea to use austenitic stainless steel as SUS304(18Cr-8Ni).But this stainless steel is owing to containing expensive and scarce resource Ni, greatly increased the manufacturing cost of component, simultaneously, this stainless steel is austenitic stainless steel, automobile component uninterrupted operation can cause the generation of expanding with heat and contract with cold of component, add-on parts in system causes the dimensional change of Stainless Steel Shell as played the meetings such as ceramic liner of cleaning action because expanding with heat and contract with cold, ceramic liner can stressed fragmentation cause losing efficacy.Therefore need to develop a kind of manufacturing cost relatively low, its stainless steel possesses resistance to urea corrosive nature and the low thermal expansivity suitable with SUS304 stainless steel simultaneously.In addition, this Urea treatment device generally can be arranged on the outside of car body, at automobilism northern area in the winter time particularly, because adopt the motorway etc. of salting snow removing in service, also will consider the anti-chlorine ion corrosion requirement that it is outside.

The stainless composition of traditional SUS304 is: C≤0.07%, Si≤1.0%, Mn≤2.0%, Cr17.5-19.5%, Ni8.0-10.5%, P≤0.045%, S≤0.015%, N≤0.11%; Its mechanical property is: yield strength (MPa) >=230, tensile strength (MPa) >=540, unit elongation (%) >=45.

Tradition adopts the Cr-Ni stainless steel of SUS304, owing to containing expensive metallic nickel, causes the manufacturing cost of material higher.

In the urea environment of fertilizer industry, also adopt AISI304 stainless steel (American Standard, be equivalent to SUS304 stainless steel) outside, also just like Japanese Patent Laid-Open 2007-113484(, be equivalent to SUS304,18Cr-8Ni) and Japanese Patent Laid-Open 2003-301241(duplex stainless steel 25Cr-6Ni-2Mo-2W) disclosed stainless steel.The former is mainly used in the storing unit of urea, and the latter is for the manufacturing installation of urea.

In addition, the disclosed stainless material of patent No. WO2010/010816A1 is also the alternative SUS304 stainless material of storing unit for urea, reduces costs, and maximum operation (service) temperature, at 60 ℃, is not for tail gas clean-up spray decomposing system.

Above-mentioned disclosed stainless steel does not all meet the Service Environment condition of 400～600 ℃ of use temperatures and urea spraying purification system.

Summary of the invention

The object of the present invention is to provide economical stainless steel and the manufacture method thereof of a kind of resistance to urea and vehicle exhaust corrosion, the commercial car emission abatement system design alteration that can bring for state's four emission standards, purifying carbamide sprinkling system case material need to meet high temperature and the common corrosion working conditions of urea of 400～600 ℃; Have the high temperature resistant and urea corrosive nature suitable with SUS304, meanwhile, this material has ferrite centroid structure, thereby has low thermal expansivity.

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

In order to obtain the corrosion resistance nature suitable with SUS304, steel of the present invention adopts low-carbon and nitrogen design route, as C≤0.020%, N≤0.015%, C+N≤0.025%, can improve erosion resistance by controlling impurity content, alloy Cr is controlled at 16.0～19.0%, and adds Mo element and can further improve resisting chloride ion penetration (the outside chlorion that winter, salting snow removing brought etc.) corrosive power and high temperature oxidation corrosion resistance performance.Simultaneously, by adding stabilizing element Nb, Ti, in reduction steel, total oxygen content and employing Ca processing (0.0010～0.0050%) not only can be stablized impurity element carbon and nitrogen, meanwhile, (adding calcium effect is mainly the composite oxides CaO-Al that calcium oxide and aluminum oxide form low melting point by the lower oxygen level (≤50ppm) of control smelting molten steel and Ca processing, can effectively to improve steel inclusion plasticity ₂o ₃, be equivalent to aluminate to carry out denaturing treatment, to reach, improve inclusion plasticity object), when improving erosion resistance, can also improve the purity of steel, be conducive to improve the machine-shaping property of material.In addition, in conjunction with control material structure grain fineness number (grain fineness number is controlled at 6-8 level), can fully improve the plasticity of material, make material r value surpass 1.5.By controlling Nb, Ti content and proportionlity, the carbon of appropriate Nb in not only can immobilization material, meanwhile, and the N of appropriate Ti in also can immobilization material, C, N in Nb and Ti co-controlling matrix, improve corrosion among crystalline grains and anti-urea uniform corrosion performance.When Nb or Ti stablize C and N, reduced the solid solution content in matrix, can also further improve erosion resistance and reduce wrinkling.Meanwhile, by economizing Ni, even do not add Ni, and add a small amount of Mo, adjust anti-spot corrosion equivalent value (Cr+3Mo), when can guarantee material property, reduce as far as possible material cost yet.Mentioned component design utilizes existing conventional equipment to be easy to obtain continuously cast bloom and hot-rolled sheet coil and cold product product, is conducive to manufacture and controls.

During the present invention adopts outside chromium (16.0～19.0%Cr), suitably adjust Pure Silicon Metal and manganese content, Pure Silicon Metal can play desoxydatoin in smelting, be conducive to adding and improving recovery rate of stabilizing element Ti, general silicone content is controlled and is greater than 0.20%, too high Pure Silicon Metal is unfavorable for again material formed at normal temp performance, and being controlled at 0.60% can not affect use with interior.Manganese has and the similar desoxydatoin of silicon, is too highly unfavorable for guaranteeing erosion resistance.Control certain Mn/Si ratio, relatively good between general 0.8-1.4, mainly useful for improves the pickling effect of material simultaneously, and Mn is controlled in 0.80%.In order to improve corrosion resistance nature, particularly resistance to urea corrosion and anti-chlorine ion corrosion, add Mo content and be controlled at 0.4%～1.2%, and 18.5%≤Cr+3Mo+Nb≤22%, to guarantee the equivalent of resistance to pitting attack.

In composition of the present invention, Ni, Cu, as impurity element, are mainly derived from steel scrap, control as far as possible, and common metal Ni is no more than 0.3%, can not affect use properties.Cu is no more than 0.30%, does not also affect materials'use performance.Also require metal A l to be no more than 0.05%, guarantee the least possible formation aluminate simultaneously.

The present invention can adopt a kind of or two kinds of modes in Ti, Nb to stablize C, N element, and Nb or Ti all can be for C in stabilized steel and N.But during bistable, Ti stablizes N because Ti is stronger than Ti and C keying action with N keying action; Equally, for Nb, Nb and C keying action are better than the keying action of Nb and N.Like this, the Ti in Ti, Nb bistable steel stablizes N, and Nb stablizes C, adds separately according to nitride and carbide stoichiometric ratio.By Nb and C is stable for the superfluous Ti of TiN, can also better improve the corrosion among crystalline grains of Ti or the mono-stabilized steel of Nb, improve stainless intergranular corrosion resistance corrosive nature.

The present invention adopts low-carbon and nitrogen Composition Design C≤0.020%, N≤0.015%, C+N≤0.025%, meanwhile, considers carbon and the nitrogen content of relative broad range, the independent interpolation of stabilizing element niobium should be not less than 0.20%, and guarantee to be not less than the C content of 10 times, can guarantee material solidity to corrosion, but also do not surpass 0.40%, otherwise just easily form thick Fe2Nb, and increase cost.Consider the welding effect of weldment, in material, add separately Ti, its content is guaranteeing in minimum 0.15%, also to guarantee to be not less than (C+N) content of 8 times, but also do not surpass 0.30%, otherwise the TiN forming in casting process is easily attached to continuously cast bloom surface, is also unfavorable for guaranteeing material surface quality, also can increase reconditioning quality, finally can have influence on the corrosive nature of finished product.While adopting Nb and Ti jointly to add, Nb/Ti is 0.70-1.5 for well, simultaneously, also to guarantee 0.5Nb+Ti >=0.10%+5(C+N), guarantee playing stably element stabilization, improve corrosive nature, but Nb+Ti does not surpass 0.40% yet, otherwise can increase cost and affect plasticity.This Composition Design can more easily be realized and smelting in existing production line.

Particularly, the economical stainless steel of a kind of resistance to urea of the present invention and vehicle exhaust corrosion, its composition weight percent:

C≤0.020%，

Si≤0.60%，

Mn≤0.80%，

Cr：16.0～19.0%，

Mo0.4～1.2%, and, 18.5%≤Cr+3Mo+Nb≤22%,

P≤0.040%，

S≤0.005%，

N≤0.015%，C+N≤0.025%，

Ca：0.0010～0.0050%，

Al≤0.05%, all the other are Fe and inevitable impurity element; And,

One or more in Nb and Ti are as stabilizing element, and interpolation total amount is:

Nb is monostable fixed: 0.10%≤0.5Nb≤0.20%;

Ti is monostable fixed: 0.15%≤Ti, and 8 * (C+N)≤Ti≤0.30%;

Nb, Ti bistable; 10(C+N)≤0.5Nb+Ti≤15 (C+N); Nb/Ti=0.70-1.5; Nb+Ti≤0.40%;

Impurity element Ni≤0.3%, Cu≤0.3%, O≤0.0050%, other impurity element total amount is lower than 0.05%.

In ferritic stainless steel chemical composition design of the present invention:

C and N: steel grade of the present invention belongs to the economical stainless steel of ultra-low carbon nitrogen, in order to obtain good corrosion resistance nature, therefore, in this steel, need to reduce its content as far as possible.At present smelting equipment can be controlled at total amount by carbon and nitrogen and is less than 0.025% than being easier to meet in the situation that guaranteeing throughput, reaches the requirement of carbon amount≤0.020%, nitrogen amount≤0.015% simultaneously.Reducing carbon and nitrogen total amount is mainly to reduce as far as possible stabilizing element consumption, particularly reduces the use of Ti, guarantees the surface quality of product and improves corrosion among crystalline grains.Reducing carbon and nitrogen content can also bring into play stainless steel gapless and improve forming materials performance simultaneously;

Cr: be the main alloy element that improves solidity to corrosion and intensity.Under high temperature and urea compound condition, Cr is the important element that improves stainless steel oxidation-resistance and urea corrosive nature, also improves the local corrosion abilities such as its resistance to spot corrosion in chloride soln and crevice corrosion simultaneously.Cr improves the intensity of steel, but the plasticity of too high reduction steel is unfavorable to moulding and weldability.Too lowly be unfavorable for improving its corrosive nature, range of choice 16.0～19.0%.

Si: add in steel and play deoxidation and the corrosion proof effect of improvement, the too low desoxydatoin that is unfavorable for, is generally greater than 0.2%.Be greater than 0.6%, processing and toughness are unfavorable, unfavorable to normal temperature compacted under.Consider overall performance, range of choice 0.20～0.60%.

Mn: weak austenite element, Mn can suppress the deleterious effect of sulphur in stainless steel, improves thermoplasticity.Too lowly be unfavorable for improving its thermoplasticity, be also unfavorable for improving the processability under normal temperature, be too highly unfavorable for guaranteeing its erosion resistance.Range of choice 0.20～0.80%.

Ti and Nb:Ti and Nb are mainly used in preventing that chromium concn that in steel, chromium and carbon cause in conjunction with forming chromium carbide from reducing causes erosion resistance to reduce, and particularly cause intergranular corrosion; Ti can also be combined formation TiC2S compound to prevent the caused spot corrosion of MnS by sulphur in steel.Ti and Nb can improve stainless room temperature and hot strength, improve antifatigue and cold formability and the weldability of ferritic stainless steel, but unfavorable to the brittle transition temperature of steel.The nitride (TiN) that Ti and nitrogen form can affect surface and the inner quality of steel as inclusion, and the nitride (NbN) that Nb and nitrogen form also can reduce the thermoplasticity of steel.Successful when Ti content is greater than 0.05%, too much can produce surface imperfection and generally be no more than 0.30%.Successful when Nb content is greater than 0.10%, surpassing 0.40% can too much cause forming thick Fe2Nb because of Nb, is unfavorable for forming materials performance.Consider added C, N content, select wherein a kind of or two kinds to add element, range of choice as stabilization:

Nb is monostable fixed: 0.10%≤0.5Nb≤0.20%; Ti is monostable fixed: 0.15%≤Ti and 8X(C+N)≤Ti≤0.30%;

Nb, Ti bistable: 10X(C+N)≤0.5Nb+Ti≤15 (C+N); Nb/Ti=0.70-1.5; Nb+Ti≤0.40%.

Mo:Mo mainly improves oxidation-resistance and anti-chlorine ion corrosion performance, improves corrosive nature in reductant, substitutes expensive metal Ni.Add Mo can improve material cost, by stabilizing element and other element interaction, during alloy designs, adopt: Mo0.4～1.2% and satisfied 18.5%≤Cr+3Mo≤22%, to reduce as far as possible material cost.

Cu:Cu mainly comes in stainless steel steel scrap, adds copper can improve corrosive nature, but adds copper both to increase material cost, also causes steel scrap handling cost to increase simultaneously, meeting under corrosive nature condition, does not add Cu, and remaining Cu is no more than 0.3%.

Ni: in order to reduce manufacturing cost, adopted province's nickelalloy design, coordinated 16.0～19.0% Cr simultaneously, obtained ferritic structure, thereby make its thermal expansivity low compared with SUS304.Ni belongs to control element in ferritic stainless steel, reduces as far as possible the content of Ni, and total amount is no more than 0.30%.

Al:Al is used and is conducive to reduce its inclusion as stainless steel-making reductor.But too high meeting causes Al oxide compound to form to be caused toughness drop and affects product surface quality.In actual production, be relatively difficult to control add-on, while containing higher Si in alloy, can use less Al deoxidation.The present invention adopts the Si deoxidation of certain content and adds a small amount of Al deoxidation, so Al≤0.050%.

Add Al favourable to controlling steel inclusion, therefore in deoxidation treatment process, add suitable Al.But deal with Al deoxidation products Al improperly ₂o ₃can transfer inclusion to and cause steel inclusion to affect deep drawability energy, for this reason, can eliminate Al deoxidation products Al by adding Ca ₂o ₃detrimentally affect.

Ca: by Ca, process, can make Ca deoxidation form CaO and with molten steel in Al ₂o ₃, MgO, SiO ₂deng inclusion, form compound type inclusions CaO-SiO ₂-MgO-Al ₂o ₃, this compound type inclusions has certain high-temp plastic, is conducive to high temperature rolling distortion broken, improves the processability of finished-product material.Appropriate Ca is also conducive to promote desulfurization, reduces S content, avoids forming MnS because S reacts with Mn, affects pitting corrosion.Ca is too low can not be played a role, and too high meeting exists with inclusion form, worsens material property, and suitable addition is Ca:0.0010～0.0050%.

O:O is one of impurity element in steel, mainly with the oxide inclusion form forming, exists, and higher O shows that inclusion is higher.Reduce O content in steel and just can guarantee that inclusion content of steel is lower, is conducive to improve plasticity and pitting corrosion.In steel, can guarantee that material has higher impact briquetting performance and corrosion resistance during O≤0.0050%.

P and S: in ferritic stainless steel, p and s can have a strong impact on stainless solidity to corrosion and processing characteristics, must strictly control, and general hope is controlled at P≤0.035%, S≤0.003%.

In order to ensure the corrosion resistance of material, main alloy element Cr composition, between 16.0～19.0%, adopts Mo and stabilizing element as Ti simultaneously, Nb, a kind of element of V etc. or compound interpolation.Add Al favourable to controlling steel inclusion, therefore in deoxidation treatment process, add suitable Al.But deal with Al deoxidation products Al improperly ₂o ₃can transfer inclusion to and cause steel inclusion to affect deep drawability energy, for this reason, can eliminate Al deoxidation products Al by adding Ca ₂o ₃detrimentally affect.

The economical stainless manufacture method of a kind of resistance to urea of the present invention and vehicle exhaust corrosion, it comprises the steps:

1) smelt, cast

Will be according to mentioned component design by Electric furnace steel making, AOD decarburization, the steel-making of VOD deoxidation three-step approach and feed Si-Ca-Ba silk and titanium silk is processed after VOD finishes, through continuous casting, obtain continuously cast bloom again, casting process pulling rate is 1000～1600 amperes, 0.9～1.1m/min, induction stirring electric current, make proportion of equiaxed grain in continuously cast bloom be not less than 60%, continuously cast bloom is with warm figuring of surface, reconditioning starting temperature is not less than 380 ℃, reconditioning finishing temperature is not less than 180 ℃, after reconditioning, be with temperature to send process furnace to be heated to 1100～1200 ℃, and be incubated 180～360min, carry out hot rolling;

2) hot rolling,

First carry out 5～7 passage roughing, 1050～820 ℃ of temperature ranges, and remove surface scale, roughed bloom is through 5～7 passage finish rolling, and 980～820 ℃ of temperature ranges are cooling and batch acquisition hot-rolled sheet coil;

3) cold rolling

After the annealed pickling of hot-rolled sheet coil, carry out cold rolling processing, cold rolling draft 60～80%;

4) annealing,

Cold rolling after annealing, 920～1050 ℃ of annealing temperatures; Pickling, by controlling 25～50 meters/min of annealing speed, make fully recrystallize of cold-rolling stainless steel, and grain size number reaches 6-8 level, and guarantee surface roughness Ra 0.10～0.50 μ m, so not only can guarantee that deep drawability can but also be conducive to improve general corrosion performance, obtain the cold-rolling stainless steel product that over-all properties meets the demands.

The present invention obtains the cold rolled stainless steel sheet that thickness is 0.5～2.5mm.

Beneficial effect of the present invention:

Adopt above-mentioned design stainless steel its at 400～600 ℃ and the corrosion resistance nature that sprays in the environment of urea, reached the stainless level of SUS304, its pitting resistance is also better than SUS304 stainless steel, composite score, corrosion resistance nature reaches and is even better than traditional SUS304 stainless steel.

Embodiment

Below in conjunction with embodiment, the present invention will be further described.

Table 1 is depicted as the chemical composition of experiment steel grade, and wherein 1-7 is steel grade of the present invention, and 8-10 is contrast steel grade.

Table 2 is mechanical property and the pitting potential comparison of steel of the present invention and comparative example steel.Wherein, mechanical property detects yield strength, tensile strength, unit elongation, hardness.Determination of Corrosion Property pitting potential and uniform corrosion performance.

Pitting potential measurement utilizes potentiodynamic technique to measure the pitting potential of stainless steel in neutral 3.5% sodium chloride solution.In experiment, get 3 mean value calculation net results.

The uniform corrosion of sample adopts U.S. AK company phlegma to consist of: 100ppm Cl ^-, 100ppmNO ₃ ^-, 5000ppmSO ₄ ^2-, positively charged ion is NH ₄ ⁺, with dilute sulphuric acid, pH being adjusted to 3,5 or 10, the content that makes urea is 2.4wt%, and envrionment temperature adopts 450 ℃ and 560 ℃, and sample first heats 2 hours, then in test temperature is the corrosive fluid of 80 ℃, corrodes 30 minutes, and this is a corrosive cycle.Repeat 30 cycles, measure sample weightless, relatively the corrosion resistance nature of different-alloy.

From the results shown in Table 2, the economical stainless steel of province of the present invention nickel has with the suitable mechanical properties such as intensity and the corrosion resistance nature of contrast steel grade, can substitute SUS304 stainless steel for urea and the high-temperature corrosion environment of commercial car exhaust gas purification system.

The mechanical property of table 2 steel of the present invention and comparative example steel and corrosive nature comparison

Claims (2)

1. the economical stainless steel that resistance to urea and vehicle exhaust corrode, its composition weight percent:

C≤0.020%，

Si≤0.60%，

Mn≤0.80%，

Cr：16.0～19.0%，

Mo0.4～1.2%, and, 18.5%≤Cr+3Mo+Nb≤22%,

P≤0.040%，

S≤0.005%，

N≤0.015%，C+N≤0.025%，

Ca：0.0010～0.0050%，

Al≤0.05%, all the other are Fe and inevitable impurity element; And,

One or more in Nb and Ti are as stabilizing element, and interpolation total amount is:

Nb is monostable fixed: 0.10%≤0.5Nb≤0.20%;

Ti is monostable fixed: 0.15%≤Ti, and 8 * (C+N)≤Ti≤0.30%;

Nb, Ti bistable; 10(C+N)≤0.5Nb+Ti≤15 (C+N); Nb/Ti=0.70-1.5; Nb+Ti≤0.40%;

Impurity element Ni≤0.3%, Cu≤0.3%, O≤0.0050%, other impurity element total amount is lower than 0.05%.

2. the economical stainless manufacture method that resistance to urea as claimed in claim 1 and vehicle exhaust corrode, it comprises the steps:

1) smelt, cast

To by Electric furnace steel making, AOD decarburization, VOD deoxidation three-step approach, make steel according to mentioned component design, and after VOD finishes, feed Si-Ca-Ba silk and titanium silk and process, through continuous casting, obtain continuously cast bloom again, casting process pulling rate is 1000～1600 amperes, 0.9～1.1m/min, induction stirring electric current, make proportion of equiaxed grain in continuously cast bloom be not less than 60%, continuously cast bloom is with warm figuring of surface, reconditioning starting temperature is not less than 380 ℃, reconditioning finishing temperature is not less than 180 ℃, after reconditioning, be with temperature to send process furnace to be heated to 1100～1200 ℃, and be incubated 180～360min and carry out hot rolling;

2) first hot rolling carries out 5～7 passage roughing, 1050～820 ℃ of temperature ranges, and remove surface scale, and roughed bloom is through 5～7 passage finish rolling, and 980～820 ℃ of temperature ranges are cooling and batch acquisition hot-rolled sheet coil;

3) cold rolling

After the annealed pickling of hot-rolled sheet coil, carry out cold rolling processing, cold rolling draft 60～80%;

4) annealing

Cold rolling after annealing, 920～1050 ℃ of annealing temperatures; Pickling, by controlling 25～50 meters/min of annealing speed, make fully recrystallize of cold-rolling stainless steel, and grain size number reaches 6-8 level, and guarantee surface roughness Ra 0.10～0.50 μ m, so not only can guarantee that deep drawability can but also be conducive to improve general corrosion performance, obtain the cold-rolling stainless steel product that over-all properties meets the demands.