CN105408511A - Ferritic stainless steel having excellent weld corrosion resistance - Google Patents

Abstract

Provided is a ferritic stainless steel having excellent weld corrosion resistance. The ferritic stainless steel is characterized by containing, in terms of mass %, 0.001-0.025% of C, 0.05-0.30% of Si, 0.35-2.0% of Mn, 0.05% or less of P, 0.01% or less of S, 0.05-0.80% of Al, 0.001-0.025% of N, 16.0-20.0% of Cr, 0.12-0.50% of Ti, 0.002-0.050% of Nb, 0.30-0.80% of Cu, 0.05% to less than 0.50% of Ni, and 0.01-0.50% of V and satisfying formula (1), with the remainder consisting of Fe and unavoidable impurities. 0.50<25*C+18*N+Ni+0.11*Mn+0.46*Cu (1) Moreover, the chemical symbols in the formula denotes the content (mass%) of the elements in question.

Description

The ferrite-group stainless steel of the excellent corrosion resistance of weld part

Technical field

Invade the ferrite-group stainless steel that the reduction of erosion resistance occurs to the carbon of weld seam and nitrogen or the nitrogen that invades from air from welding object material when the present invention relates to not easily because welding.

Background technology

Compared with austenite stainless steel, ferrite-group stainless steel can guarantee erosion resistance with few Ni amount.Ni is expensive element, and therefore, compared with austenite stainless steel, ferrite-group stainless steel can manufacture with low cost.In addition, compared with austenite stainless steel, ferrite-group stainless steel has that thermal conductivity is high, coefficient of thermal expansion is little and the excellent characteristic such as stress corrosion crack not easily occurs.Therefore, ferrite-group stainless steel is applied to the pipe device such as the material of construction such as automobile exhaust system member, roof or door and window, galley equipment or water tank, heat accumulation water pot material etc. widely in purposes.

When making these works, in most cases austenite stainless steel, particularly SUS304 (18%Cr-8%Ni) (JISG4305) etc. is combinationally used with ferrite-group stainless steel.As so stainless welding process, the general TIG of use welds.In this case, also require, to weld part, there is good erosion resistance same with mother metal portion.

For such problem, propose following method: by adding Ti or Nb larger than Cr with the avidity of C and N, C and N is fixed with the form of the carbonitride of Ti or Nb, suppresses the generation of Cr carbonitride and suppress the generation of sensitization, guaranteeing good erosion resistance.Such as, Patent Document 1 discloses the steel that improve the resistance to grain boundary corrosion resistance of ferrite-group stainless steel by compound interpolation Ti and Nb.In addition, Patent Document 2 discloses the steel that improve resistance to grain boundary corrosion by adding Nb.

But any one invention all needs the Mo of more than interpolation 0.3 quality %.Mo is the element that the erosion resistance of mother metal is improved.But Mo is strong ferrite generating elements, therefore, when being added with the Mo of 0.3 quality %, the generation of the ferritic phase of weld part is promoted, thus encourages the sensitization of weld part, therefore can not obtain the erosion resistance of sufficient weld part.In addition, the Nb of more than interpolation 0.1 quality % is all needed.But when welding the steel containing a large amount of Nb, solid solution Nb with the Form generation of thick Nb precipitate, produces the problems such as welding crack at weld part sometimes.When considering manufacturing, practicality, the erosion resistance obtaining weld part by only adding the carbonitride generting elements such as Nb, Ti cannot say it is Best strategy.

Prior art document

Patent documentation

Patent documentation 1: Japanese Unexamined Patent Publication 2007-270290 publication

Patent documentation 2: Japanese Unexamined Patent Publication 2010-202916 publication

Summary of the invention

Invent problem to be solved

Therefore, the object of the present invention is to provide without the need to excessively add Ti, Nb, the ferrite-group stainless steel of the excellent corrosion resistance of weld part.

For the method for dealing with problems

In order to solve the problem, the present inventor can suppress the technology of the reduction of the erosion resistance of weld part to conduct in-depth research since in the past.First, contriver uses the degree of 16 ~ 20 quality %Cr draw points to the sensitization that the erosion resistance for obtaining sufficient weld part allows systematically to investigate.It found that, the erosion resistance of the weld part caused by sensitization to be reduced in by the crystal boundary clad ratio of the coated ferritic phase of Cr carbonitride (about crystal boundary clad ratio, the measuring method of reference example) more than 40% time become obvious.

Then, the present inventor is studied the method reduced by the crystal boundary clad ratio of the coated ferritic phase of Cr carbonitride.It found that, is made austenite phase stabilization be extremely effective for the erosion resistance of raising weld part by Mn and Cu added as austenite generting element.

Namely find, when making austenite phase stabilization by adding Mn and Cu, the generation of ferritic phase when welding can be suppressed, and, by with C and the N more than solid solubility limit of ferritic phase contained with the austenite being solid-solution in solid solubility limit and being greater than ferritic phase overwhelmingly mutually in state carry out stabilization, can prevent the sensitization caused because generating Cr carbonitride thus, the erosion resistance of weld part significantly improves.

Find in addition, due to C and N is solid-solubilized in austenite mutually in, therefore compare with the immobilization of C with N of Ti, Nb etc. that only utilizes in the past, even if also can sensitization be prevented under more C and N amount.Contriver systematically investigates for the composition of steel that can obtain above-mentioned effect, finds that above-mentioned effect obtains when the content of C, N, Ni, Mn and Cu by austenite phase stabilization meets following formula (1).

0.50＜25×C+18×N+Ni+0.11×Mn+0.46×Cu(1)

It should be noted that, the symbol of element in formula refers to the content (quality %) of each element.

In addition, this effect can combinationally use with the immobilization of C and N of the interpolation of Ti, Nb that utilizes as prior art, therefore, even if do not need excessively to add Ti, Nb, also can obtain the erosion resistance of weld part especially excellent compared with existing steel.

The present invention completes based on above-mentioned opinion, and its purport is as described below.

[1] a kind of ferrite-group stainless steel of excellent corrosion resistance of weld part, it is characterized in that, in mass %, containing C:0.001 ~ 0.025%, Si:0.05 ~ 0.30%, Mn:0.35 ~ 2.0%, below P:0.05%, below S:0.01%, Al:0.05 ~ 0.80%, N:0.001 ~ 0.025%, Cr:16.0 ~ 20.0%, Ti:0.12 ~ 0.50%, Nb:0.002 ~ 0.050%, Cu:0.30 ~ 0.80%, more than Ni:0.05% and be less than 0.50%, V:0.01 ~ 0.50%, and meet following formula (1), surplus is made up of Fe and inevitable impurity.

0.50＜25×C+18×N+Ni+0.11×Mn+0.46×Cu(1)

Wherein, the symbol of element in formula refers to the content (quality %) of each element.

The ferrite-group stainless steel of the excellent corrosion resistance of the weld part [2] as described in [1], it is characterized in that, in mass %, also containing be selected from Zr:0.01 ~ 0.50%, W:0.01 ~ 0.20%, REM:0.001 ~ 0.10%, Co:0.01 ~ 0.20%, B:0.0002 ~ 0.010%, Sb:0.05 ~ 0.30% more than one.

Invention effect

According to the present invention, even if the ferrite-group stainless steel also C and N invades the such welding conditions of mother metal from welding object material with excellent erosion resistance can be obtained.

Embodiment

Below the restriction reason of each constitutive requirements of the present invention is described.

1. be grouped into and metal structure about one-tenth

First, the reason that the one-tenth of regulation steel of the present invention is grouped into is described.It should be noted that, composition % all refers to quality %.

C：0.001～0.025％

C is the element inevitably contained.C amount is higher, and intensity more improves, and processibility more improves more at least.In order to obtain sufficient intensity, need containing more than 0.001%.But when content is more than 0.025%, the reduction of processibility becomes remarkable, and the locality Cr that the precipitation of easy reason Cr carbide causes lacks and the reduction (sensitization) of erosion resistance occurs.Therefore, C amount is set as the scope of 0.001 ~ 0.025%.But, although measure more low more preferred from the viewpoint of erosion resistance and processibility C, when C amount is extremely reduced, refining spended time, not preferred in manufacture.Therefore, the scope of 0.003 ~ 0.018% is preferably.Be more preferably the scope of 0.005 ~ 0.012%.

Si：0.05～0.30％

Si is to the effective element of erosion resistance improving weld part.In order to obtain this effect, need containing more than 0.05%, content more at most its effect is larger.But when Si content is more than 0.30%, plasticity, the toughness of weld part reduce, therefore not preferred.Therefore, Si amount is set as the scope of 0.05 ~ 0.30%.Be preferably the scope of 0.05 ~ 0.25%.The more preferably scope of 0.08 ~ 0.20%.

Mn：0.35～2.0％

Mn is the element of particularly important in the present invention.Mn is as the effective element of reductor, and has the effect of austenite phase stabilization.By the Mn containing specified amount, the generation of ferritic phase when welding can be suppressed, by by with C and the N more than solid solubility limit of ferritic phase contained be solid-solution in the solid solubility limit austenite larger than ferritic phase mutually in state carry out stabilization, show the effect preventing the sensitization caused because generating Cr carbonitride, the erosion resistance of weld part improves.In order to obtain these effects, need the Mn containing more than 0.35%.But when Mn content is more than 2.0%, the excessive hardening of mother metal, ductility reduces, and produces the reduction of the toughness caused because of the hardening of weld part at weld part, therefore not preferred.Therefore, Mn amount is set as the scope of 0.35 ~ 2.0%.Be preferably the scope of 0.50 ~ 1.5%.The more preferably scope of 0.75 ~ 1.25%.

Below P:0.05%

P is the element inevitably contained in steel, and excessive containing can make weldability reduce, and easily produces grain boundary corrosion.This tendency becomes remarkable sometimes in containing more than 0.05%.Therefore, P amount is set as less than 0.05%.Be preferably less than 0.03%.

Below S:0.01%

S and P is equally also the element inevitably contained in steel, and containing more than 0.01% can make erosion resistance reduce.Therefore, S amount is set as less than 0.01%.Be preferably less than 0.008%.

Al：0.05～0.80％

Al and Si is equally also the element that the erosion resistance of weld part is improved.The avidity of Al and N is stronger than the avidity of Cr and N, therefore, when being mixed into N in weld part, making N separate out with the form of Al nitride instead of separate out with the form of Cr nitride, has the effect suppressing sensitization.In addition, Al is also element useful for the deoxidation in steel making working procedure.These effects obtain containing more than 0.05% sometimes.But during containing Al more than 0.80%, ferrite crystal grain coarsening, processibility, manufacturing reduce.Therefore, Al amount is set as the scope of 0.05 ~ 0.80%.Be preferably the scope of 0.10 ~ 0.60%.The more preferably scope of 0.15 ~ 0.50%.

N：0.001～0.025％

N and C is the element inevitably contained in steel equally.N content height then intensity improves, and processibility more improves more at least.In order to obtain sufficient intensity, more than 0.001% containing being suitable.But when content is more than 0.025%, ductility significantly reduces, and the reduction of erosion resistance can be there is because of the precipitation of growth encourage Cr nitride, therefore not preferred.Therefore, N amount is set as the scope of 0.001 ~ 0.025%.From the view point of erosion resistance, N is more low more preferred.But, need to increase refining time to reduce N amount, cause the reduction of manufacturing.Therefore, the scope of 0.003 ~ 0.025% is preferably set to.Be more preferably the scope of 0.003 ~ 0.015%.The more preferably scope of 0.003 ~ 0.010%.

Cr：16.0～20.0％

Cr is for guaranteeing the most important element of stainless erosion resistance.When Cr content is less than 16.0%, reason weld weld seam that the oxidation caused makes the Cr on top layer reduce and around, sufficient erosion resistance can not be obtained.In addition, the sensitization caused because of the N be mixed into from welding object material or air during welding is encouraged further, therefore not preferred.On the other hand, when Cr content is more than 20.0%, the deoxygenated skin after toughness reduction, annealing reduces, therefore not preferred.Therefore, Cr amount is set as the scope of 16.0% ~ 20.0%.Be preferably the scope of 16.5% ~ 19.0%.The more preferably scope of 17.0 ~ 18.5%.

Ti：0.12～0.50％

The element of the reduction of the erosion resistance that the sensitization that Ti is preferential combination with C and N and suppresses the precipitation of reason Cr carbonitride to cause causes.Its effect by more than 0.12% containing and obtain.But, when content is more than 0.50%, generates thick Ti carbonitride, cause surface imperfection, therefore not preferred.Therefore, Ti amount is set as the scope of 0.12 ~ 0.50%.Be preferably the scope of 0.15 ~ 0.40%.The more preferably scope of 0.20 ~ 0.35%.

Nb：0.002～0.050％

The element of the reduction of the erosion resistance that the sensitization that Nb is preferential combination with C and N and suppresses the precipitation of reason Cr carbonitride to cause causes.In addition, Nb also have make weld part the miniaturization of crystallization particle diameter, improve the toughness of weld part and the effect of bendability.These effects obtain when containing more than 0.002%.On the other hand; Nb is also the element that recrystallization temperature is raised; when content is more than 0.050%; annealing temperature high temperature needed for recrystallize; therefore, in the annealing operation using high speed cold-reduced sheet annealed wire, annealing becomes insufficient; because non-recrystallization crystal grain and recrystal grain mixing exist and the reduction of processibility occurs, therefore not preferred.Therefore, Nb amount is set as the scope of 0.002 ~ 0.050%.Be preferably the scope of 0.010 ~ 0.045%.The more preferably scope of 0.015 ~ 0.040%.

Cu：0.30～0.80％

Cu is the element that erosion resistance is improved, and is the effective especially element of erosion resistance for mother metal when improving in the aqueous solution or be attached with slightly acidic water droplet and weld part.In addition, Cu and Ni is strong austenite generting element equally, has the effect of the generation suppressing the ferritic phase of weld part, the sensitization suppressing the precipitation because of Cr carbonitride to cause.These effects obtain when containing more than 0.30%.On the other hand, during containing Cu more than 0.80%, hot workability reduces, therefore not preferred.Therefore, Cu amount is set as the scope of 0.30 ~ 0.80%.Be preferably the scope of 0.30 ~ 0.60%.The more preferably scope of 0.35 ~ 0.50%.

More than Ni:0.05% and be less than 0.50%

Ni is the element that stainless erosion resistance is improved, and is the element that the carrying out suppressing corrosion in the corrosive environment of active dissolution occurs not forming passivation tunicle.In addition, Ni is strong austenite generting element, has to suppress that the ferrite of weld part generates, the effect of the sensitization that suppresses the precipitation because of Cr carbonitride to cause.These effects by more than 0.05% containing and obtain.But during Ni containing more than 0.50%, processibility reduces, in addition, stress corrosion opening cracking maleate sensitivity grow.In addition, Ni is expensive element, therefore causes the increase of manufacturing cost, therefore not preferred.Therefore, Ni amount is set as less than 0.05% and is less than the scope of 0.50%.Be preferably the scope of 0.10 ~ 0.30%.The more preferably scope of 0.15 ~ 0.25%.

V：0.01～0.50％

V is the element that erosion resistance, processibility are improved, and has when to be mixed into N in weld part by being combined the effect of the generation suppressing Cr nitride, the sensitization reducing weld part with N.This effect by more than 0.01% containing and obtain.But when content is more than 0.50%, processibility reduces, therefore not preferred.Therefore, V amount is set as the scope of 0.01 ~ 0.50%.Be preferably the scope of 0.05 ~ 0.30%.The more preferably scope of 0.08 ~ 0.20%.

0.50＜25×C+18×N+Ni+0.11×Mn+0.46×Cu(1)

It should be noted that, the symbol of element in formula refers to the content (quality %) of each element.

In order to make C and N of weld part be solid-solubilized in austenite mutually in and immobilization, need to generate austenite phase in cools tissue after welding.In order to generate austenite phase in process of cooling after welding, the above-mentioned formula of demand fulfillment (1).When the right of formula (1) is less than 0.50, the stabilization of austenite phase becomes insufficient, can not generate for the austenite phase by solid solution C provided by the invention and solid solution N immobilized degree effectively at weld part.Therefore, the right of formula (1) is set greater than 0.5.Be preferably more than 0.60.Be more preferably more than 0.70.

Be more than basic chemical composition of the present invention, surplus is made up of Fe and inevitable impurity.It should be noted that, as inevitable impurity, can below Ca:0.0020% be allowed.

In addition, on the basis of above-mentioned basal component, for suppressing the sensitization of weld seam and improving the object of erosion resistance etc., also following element can be contained.

Zr：0.01～0.50％

Zr has and is combined with C and N and suppresses the effect of sensitization.This effect by more than 0.01% containing and obtain.But when content is more than 0.50%, processibility reduces.In addition, Zr is expensive element, and therefore, excessive interpolation can cause the increase of manufacturing cost, therefore not preferred.Therefore, when containing Zr, be preferably set to the scope of 0.01 ~ 0.50%.Be more preferably the scope of 0.10 ~ 0.35%.

W：0.01～0.20％

W and Mo has the effect that erosion resistance is improved equally.This effect by more than 0.01% containing and obtain.But when content is more than 0.20%, intensity raises, and the reduction of the manufacturing causing the increase etc. because of rolling loads to cause is therefore not preferred.Therefore, when containing W, be preferably set to the scope of 0.01 ~ 0.20%.Be more preferably the scope of 0.05 ~ 0.15%.

REM：0.001～0.10％

REM has the effect that oxidation-resistance is improved, and the formation formation by inhibited oxidation skin being suppressed to the scarce Cr region of the below of adjacent welding tempering color is effective.In order to obtain this effect, need containing more than 0.001%.But, when content is more than 0.10%, the manufacturings such as pickling can be made to reduce.In addition, REM and Zr is expensive element equally, therefore, excessively containing the increase that can cause manufacturing cost, therefore not preferred.Therefore, when containing REM, be preferably set to the scope of 0.001 ~ 0.10%.Be more preferably the scope of 0.010 ~ 0.08%.

Co：0.01～0.20％

Co is the element that toughness is improved.This effect by more than 0.01% containing and obtain.On the other hand, when content is more than 0.20%, manufacturing can be made to reduce.Therefore, when containing Co, be preferably set to the scope of 0.01 ~ 0.20%.Be more preferably the scope of 0.05 ~ 0.15%.

B：0.0002～0.010％

B is the element improving secondary processing brittleness, its effect by more than 0.0002% containing and obtain.But, when content is more than 0.010%, the reduction of the ductility caused because of excessive solution strengthening can be brought out.Therefore, when containing B, be preferably set to the scope of 0.0002 ~ 0.010%.Be more preferably the scope of 0.0010 ~ 0.0075%.

Sb：0.05～0.30％

Sb and Al has the effect catching the N be mixed into from air when the gas shield of TIG welding is insufficient equally, when being applied to the structure having and be difficult to the complicated shape carrying out sufficient gas shield, is effective especially element.Its effect by more than 0.05% containing and obtain.But when content is more than 0.30%, generate non-metallic inclusion in steel making working procedure, surface texture is deteriorated.In addition, the degraded toughness of hot-rolled sheet is made.Therefore, when containing Sb, be preferably set to the scope of 0.05 ~ 0.30%.Be more preferably the scope of 0.05 ~ 0.15%.

2. about manufacturing condition

Then, the preferable production process of steel of the present invention is described.Utilize the known methods such as converter, electric furnace, vacuum melting stove to carry out melting on the molten steel that mentioned component forms, make the former material of steel (steel billet) by continuous metal cast process or ingot casting-cogging method.Carry out hot rolling after this steel billet is heated 1 ~ 24 hour at 1100 ~ 1250 DEG C or do not carry out heating and directly carry out hot rolling under the state of casting, thus making hot-rolled sheet.

Usually, hot-rolled sheet is implemented at 800 ~ 1100 DEG C the hot-rolled sheet annealing of 1 ~ 10 minute.It should be noted that, according to purposes, also can omit hot-rolled sheet annealing.Then, after hot-rolled sheet pickling, make cold-reduced sheet by cold rolling, then, implement recrystallization annealing, pickling, thus make product.

From the view point of extensibility, bendability, press formability and shape, the cold rolling preferred draft with more than 50% carries out.About the recrystallization annealing of cold-reduced sheet, generally speaking, when surface finishing, the No.2B finished goods of JISG0203, from the viewpoint of obtain good mechanical property viewpoint and from pickling, preferably carry out at 800 ~ 950 DEG C.In addition, for the component at position requiring gloss further, in precision work, carry out BA annealing (clean annealing) is effective.It should be noted that, in order to improve surface texture further afterwards and after processing cold rolling, grinding etc. can be implemented.

Embodiment 1

Below, based on embodiment, the present invention is described in more detail.

Stainless steel melting in 50kg small size vacuum smelting furnace of the chemical constitution shown in table 1 will be had.After these blooms are heated 1 hour at 1150 DEG C, implement hot rolling and make the thick hot-rolled sheet of 3.5mm.Then, after these hot-rolled sheets being implemented to the hot-rolled sheet annealing of 1 minute at 950 DEG C, effects on surface carries out shot peening, then, flood in temperature 80 DEG C, 20 quality % sulphuric acid solns after 120 seconds, flood 60 seconds in the mixing acid of the temperature 55 DEG C be made up of 15 quality % nitric acid and 3 quality % hydrofluoric acid, carry out pickling thus, carry out deoxygenated skin.

And then, be cold-rolled to thickness of slab 0.8mm, in weak reducing atmosphere (hydrogen: 5 volume %, nitrogen: 95 volume %, dew point :-40 DEG C), carry out the recrystallization annealing of 1 minute in 900 DEG C, obtain cold rolled annealed plate.This cold rolled annealed plate is carried out electrolytic pickling in temperature 50 C, the mix acid liquor that is made up of 15 quality % nitric acid and 0.5 quality % hydrochloric acid, carries out the process of deoxygenated skin thus, obtain cold-rolling pickling annealed sheet.

It should be noted that, table 1-1, table 1-2 are a series of tables of continuous print.

The thickness of slab of the cold-reduced sheet of use and commercially available austenite stainless steel SUS304 (C:0.07 quality %, N:0.05 quality %, Cr:18.2 quality %, Ni:8.2 quality %) is the cold-reduced sheet of 0.8mm, carry out docking TIG and weld (buttTIGwelding) (cold-reduced sheet of the present invention: mother metal, welding object material: SUS304).Welding current is set as 90A, welding speed is set as 60cm/ minute, for shielding gas, uses with 15L/ minute the Ar gas comprising the nitrogen of 8 volume %, the oxygen of 2 volume %.The width of the weld seam of the face side obtained is about 3mm.

Cut the test film comprising weld seam of making, carry out following test.

1. the pitting potential test of mother metal and weld part

Cut the square test film of 20mm from the test film after the test film after cold rolled annealed and welding, make the square mensuration face of residual 10mm and carried out coated test film with sealing material.For the test film after welding, cut test film in the mode comprising weld seam, keep the state of the tempering color (oxide film thereon) remained because of welding generation.For these test films, in the 3.5 quality %NaCl solution of 30 DEG C, measure the pitting potential of mother metal and weld part.During mensuration, do not carry out the grinding of test film, Passivation Treatment, but measuring method is in addition according to JISG0577 (2005).

The pitting potential of the pitting potential by mother metal: more than 150mV, weld part: more than 0mV is set as qualified.

2. neutral salt spray round-robin test

Cut the square test film of the 100mm that comprises weld seam from the test film after welding, makings #600 sand paper effects on surface carries out by the test film of end face portion sealing after grinding precision work, for the neutral salt spray round-robin test specified in JISH8502.In neutral salt spray round-robin test, 5 quality %NaCl solution sprays (35 DEG C, 2 hours) → drying (60 DEG C, 4 hours, relative humidity 20 ~ 30%) → wetting (moistness) (40 DEG C, 2 hours, relative humidity more than 95%) is a circulation.It is qualified that the situation that this test enforcement 15 circulated does not occur to corrode afterwards from mother metal or weld part is set as.

3. measured by the crystal boundary clad ratio of the coated ferritic phase of Cr carbonitride

The right angle orientation of the weld seam of test film after welding cuts metal structure observation test film, after mirror ultrafinish, by utilizing the etching of picrate aqueous acid, metal structure and precipitate are manifested, carry out using scanning electron microscope and the structure observation of energy dispersion type X-ray optical spectroscopy and the identification of phases of precipitate, measure the crystal boundary clad ratio coated by Cr carbonitride of the ferritic phase of welding seam part.

About crystal boundary clad ratio, utilize the crystal boundary length of the crystal grain in the macrograph captured by image analysis apparatus measurement, and utilize same image analysis apparatus to measure the diameter in the direction parallel with the crystal boundary of the Cr carbonitride of separating out on crystal boundary, calculated by the formula of crystal boundary clad ratio (%)=(total of the diameter in the direction parallel with the crystal boundary of the Cr carbonitride on crystal boundary) ÷ (total of the crystal boundary length of crystal grain) × 100.

It is qualified that the situation being less than 40% by the crystal boundary clad ratio by the coated ferritic phase of Cr carbonitride is set as.

4. mechanical characteristics evaluation

From made cold rolled annealed plate, cut JIS13B tension test sheet abreast with rolling direction, carry out tension test according to JISZ2241, measure elongation at break.

By elongation at break be more than 25% be set as qualified.

5. surface quality evaluation

Detect by an unaided eye cold rolled annealed after, surface of steel plate after deoxygenated skin, be confirmed whether the surface imperfection such as the bad and line defect of deoxygenated skin.

To remain there is no oxide skin and surface imperfection, the situation that obtains good appearance are set as qualified benchmark.

Test-results is shown in Table 2.

Steel No.1 ~ 20 meeting important document of the present invention are all that the pitting potential of mother metal is more than 150mV, the pitting potential of weld seam is more than 0mV, and, all produce corrosion less than because of neutral salt spray round-robin test, even if when implementing to have also been obtained sufficient erosion resistance when welding with austenite stainless steel.In addition, the crystal boundary clad ratio of the ferritic phase coated by Cr carbonitride after the welding of steel No.1 ~ 20 is less than 40%, obtains the anti-sensitization effect of regulation.In addition, the elongation at break based on tension test is more than 25%, obtains good processing characteristics, and does not also observe surface imperfection.

On the other hand, exceed for the steel No.21 of scope of the present invention for Cr amount, the toughness of the steel plate after hot rolling significantly reduces, the manufacturing process after can not implementing, and can not evaluate characteristic.

Cr is measured for the steel No.22 lower than scope of the present invention, can not get sufficient pitting potential, in addition, in neutral salt spray round-robin test, from mother metal and weld part, produce corrosion, the erosion resistance specified can not be obtained.

Mn is measured for the steel No.23 lower than scope of the present invention, although obtain sufficient erosion resistance for mother metal, sufficient erosion resistance can not be obtained for weld part.

On the other hand, Mn amount or Al amount are exceeded for the steel No.24 and 25 of scope of the present invention, although obtain mother metal and the corrosion resistance at welded part of regulation, because of steel plate hardening, ductility is reduced, the mechanical characteristics specified can not be obtained.

Ti amount being exceeded for the steel No.26 of scope of the present invention, although obtain erosion resistance and the mechanical characteristics of regulation, creating surface imperfection because generating thick Ti system inclusion in a large number, the surface texture specified can not be obtained.

Though scope of the present invention is met for the content of each element the content of austenite stabilizer element lower than the scope of formula (1) steel No.27 ~ 30 for, obtain mother metal erosion resistance and the mechanical characteristics of regulation.But, do not obtain the pitting potential of welding seam part and the erosion resistance of weld part of regulation.Investigate the fractography of the weld part of No.27 ~ 30, result all confirms, and has separated out the Cr carbonitride counting more than 50% such huge amount with crystal boundary clad ratio at ferritic phase grain boundaries.Think for steel No.27 ~ 30, austenite stabilizer element is not enough, therefore, ferritic phase is generated in cooling after welding, can not suppress the generation of Cr carbonitride on crystal boundary, there is significant sensitization in result, can not obtain the erosion resistance of the weld part specified.

From above result, according to the present invention, the ferrite-group stainless steel without the need to excessive interpolation Ti, Nb with excellent erosion resistance, mechanical characteristics and surface texture can be obtained.

Utilizability in industry

The ferrite-group stainless steel obtained in the present invention is applicable to being applied to and is undertaken in the purposes of the making of structure by welding, in automobile exhaust system material, door and window or the structure material such as ventilation opening, pipeline etc. such as such as sound damper.

Claims (2)

1. a ferrite-group stainless steel, it is characterized in that, in mass %, containing C:0.001 ~ 0.025%, Si:0.05 ~ 0.30%, Mn:0.35 ~ 2.0%, below P:0.05%, below S:0.01%, Al:0.05 ~ 0.80%, N:0.001 ~ 0.025%, Cr:16.0 ~ 20.0%, Ti:0.12 ~ 0.50%, Nb:0.002 ~ 0.050%, Cu:0.30 ~ 0.80%, more than Ni:0.05% and be less than 0.50%, V:0.01 ~ 0.50%, and meet following formula (1), surplus is made up of Fe and inevitable impurity

0.50＜25×C+18×N+Ni+0.11×Mn+0.46×Cu(1)

Wherein, the symbol of element in formula refers to the content (quality %) of each element.

2. ferrite-group stainless steel as claimed in claim 1, it is characterized in that, in mass %, also containing be selected from Zr:0.01 ~ 0.50%, W:0.01 ~ 0.20%, REM:0.001 ~ 0.10%, Co:0.01 ~ 0.20%, B:0.0002 ~ 0.010%, Sb:0.05 ~ 0.30% more than one.