CN104884660A

CN104884660A - Steel for carburizing

Info

Publication number: CN104884660A
Application number: CN201380067800.9A
Authority: CN
Inventors: 小山达也; 久保田学; 吉田卓
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2012-12-28
Filing date: 2013-12-25
Publication date: 2015-09-02
Anticipated expiration: 2033-12-25
Also published as: KR20150086365A; WO2014104113A1; JP5862802B2; CN104884660B; JPWO2014104113A1; KR101745224B1

Abstract

This steel has a chemical composition which contains, in mass%, 0.16-0.30% of C, 0.01-2.0% of Si, 0.35-1.45% of Mn, 0.05-3.0% of Cr, 0.001-0.2% of Al, 0.04-5.0% of Ni, 0.015%-1.0% of Sn, 0.004-0.05% of S, 0.003-0.03% of N, 0.005% or less of O, 0.025% or less of P, 0-1.0% of Mo, 0-1.0% of Cu, 0-0.005% of B, 0-0.3% of Nb, 0-0.3% of Ti, 0-1.0% of V, 0-0.01% of Ca, 0-0.01% of Mg, 0-0.05% of Zr, 0-0.1% of Te and 0-0.005% of a rare earth element, with the balance made up of Fe and impurities. If [Si%], [Ni%], [Al%] and [Sn%] are the contents of Si, Ni, Al and Sn in terms of mass%, the relation 42 >= 21 [Si%] + 5 [Ni%] + 40 [Sn%] + 32 [Al%] >= 8.5 is satisfied.

Description

Case hardening steel

Technical field

The present invention relates to the case hardening steel not needing the adjustment carrying out car-burization condition for each converted steel parts just can improve the shock-resistant characteristic of various converted steel parts.

The Patent 2012-288131 CLAIM OF PRIORITY that the application filed an application in Japan based on December 28th, 2012, and by its content quotation in this.

Background technology

Physical structure parts are sometimes damaged because being subject to larger stress sharp.Particularly in the vehicle gear such as carburizing axle of differential gearing, transmitting gear and band gear, sometimes make tooth root damaged because of impact fracture due to the emergency start of vehicle and load when stopping in emergency.In order to prevent this phenomenon, particularly for differential gearing and transmitting gear, expect to improve its impact value (shock-resistant characteristic) further.By improving the impact value of these physical structure parts fully, the amount of physical structure parts material used can be reduced, reaching the lightweight of physical structure parts.

In the past, for above-mentioned parts, usually by by the such as C content such as JIS SCr420 and JIS SCM420 being the toughness that the case-hardened steel of about 0.2% is used as that core guaranteed by starting material.Further, for above-mentioned parts, the low-temperaturetempering of carburizing and quenching process and about 150 DEG C is implemented, the metallographic structure of parts surface is become tempered martensite that C content is about 0.8%.Thus, high reversed bending fatigue strength and the wear resistance of parts is improved.

Below the prior art for making impact value improve is described.In patent documentation 1, propose following gear steel and employ the gear of this gear steel, in this gear steel, the content of regulation Al, B and N, improves impact fatigue-resistant characteristic and face fatigue strength by solid solution B.But, the gear described in patent documentation 1 causes de-B phenomenon when carburizing, and the solid solution B of Surface of Gear disappears, and therefore the raising of its impact value is less.

In patent documentation 2, propose the gear of following excellent impact resistance, this gear is obtained by the content of regulation Mo, Si, P, Mn and Cr, the content that particularly improves Mo.But, when making Mo content increase, need the content reducing Si, Mn and Cr, the gear therefore described in patent documentation 2 causes intensity to reduce due to the reduction of hardening capacity.

In patent documentation 3, proposition has by obtaining containing appropriate Cu, having high strength and the case-hardened steel of high tenacity.But, the Cu at high temperature in steel becomes liquid layer, promotes the embrittlement of steel.Thus, there is restriction in the manufacturing condition of the case-hardened steel described in patent documentation 3.

The present inventors intently implement the investigation of the relation between Carburized Characteristic and shock-resistant characteristic.Consequently, the present inventors obtain following opinion: as described later, and the infiltration capacity making C infiltrate steel when carburizing reduces and the way that the surface C concentration of carburized material is reduced is effective for improving impact value.But, when the surface C concentration of carburized material is too low, the raising of the characteristic such as fatigue strength and wear resistance as the original object of carburizing treatment cannot be reached.Thus, in order to make converted steel parts take into account the characteristics such as shock-resistant characteristic, fatigue strength and wear resistance, need the surface C concentration of converted steel parts to control as suitable level.

The minimizing of surface C concentration realizes by making carbon potential reduce when carburizing treatment.But, be difficult to be implemented in the production process of reality employing cementing furnace.Its reason is, in the production process of reality, cementing furnace needs to the different various parts of purposes simultaneously and process in a large number continuously.The characteristic required by parts of carrying out carburizing is not limited to shock-resistant characteristic as described above.Such as, converted steel parts also require the characteristic such as wear resistance and fatigue strength.Thus, the parts of way carbon potential being reduced when carburizing treatment to the shock-resistant characteristic of major requirement are effective, but bring detrimentally affect in the parts of major requirement fatigue strength, produce the problem caused by the reduction of fatigue strength.If attempt the surface C concentration controlling converted steel parts by carrying out carburizing under the condition how not playing carburizing, now need for each parts adjustment car-burization condition.But, this causes the reduction of productivity, therefore unsatisfactory in industrial applicibility.

Thus, even if the case hardening steel sought to carry out carburizing treatment under strong car-burization condition that can be corresponding with the carburizing of various parts, also the infiltration capacity of C can be controlled at proper level.

As the control techniques of surface C concentration, in patent documentation 4, propose the converted steel parts being suppressed overcarburizing by the relation of the content of regulation Si, Ni, Cu and Cr.But, the carburizing atmosphere becoming object be in the publication the surface C concentration of steel is set as about 1.0% carburizing atmosphere.When the surface C concentration of steel is set as such value, produce carbide on the surface of steel.In this case, reduction impact value being improved to effective surface C concentration cannot be realized.

Prior art document

Patent documentation

Patent documentation 1: Japanese Unexamined Patent Publication 2008-179848 publication

Patent documentation 2: Japanese Unexamined Patent Publication 1-108347 publication

Patent documentation 3: Japanese Patent No. 3927355 publications

Patent documentation 4: Japanese Unexamined Patent Publication 2007-291486 publication

Summary of the invention

Invent problem to be solved

As mentioned above, for the converted steel parts of physical structure, need to take into account shock-resistant characteristic and wear resistance.Can the design of change parts, to make the shock-resistant characteristic by improving converted steel parts carburized material used fully, thus while suppressing to use quantity of material, also guarantee the shock-resistant characteristic of parts.Further, the carburizing treatment that various parts different to purposes under single car-burization condition or under the car-burization condition of the least possible kind carry out in the production process of the reality of the converted steel parts of physical structure is needed.

In the public technology of patent documentation 1 ~ 4, the raising taking into account impact value and the demand avoiding productivity to reduce cannot be met fully, specifically following demand: the converted steel parts not needing just can obtain for each parts adjustment car-burization condition shock-resistant excellent.

The invention provides a kind of steel, the converted steel parts that both impact value (shock-resistant characteristic) and wear resistance are excellent can be obtained when this steel is used as the material of converted steel parts, and, when manufacturing these converted steel parts without the need to changing car-burization condition.

The means of dealing with problems

The present inventors in order to realize obtaining the steel of the converted steel parts avoiding making productivity to reduce and to have excellent impact value, to making chemical constitution and carburized material matter characteristic on a large scale and the steel systematically changed implements carburizing and shock test.Consequently, the opinion of following explanation is obtained.

Fig. 1 represents to carry out carburizing treatment to steel and the chart of relation between the surface C concentration of the carburized material obtained and impact value.The surface C concentration of steel is made to increase by carburizing treatment.Obtain following opinion: in order to make the impact value of carburized material improve, as shown in Figure 1, it is effective for controlling in suitable level by the surface C concentration after carburizing.

Further, the present inventors obtain following opinion: the surface C concentration controlling carburized material as described above can by realizing the content adjustment of the alloying element be solid-solution in steel.Specifically, when the content of each alloying element being set as respectively in specialized range and content in the steel of Si, Ni, Al and the Sn in the alloying element in steel (unit: quality %) is set as [Si%], [Ni%], [Al%], [Sn%], the present inventors have understood following content and have completed the present invention: by meeting following formula (A), the surface C concentration of carburized material becomes suitable value, and impact value is improved.

42≥21×[Si％]+5×[Ni％]+40×[Sn％]+32×[Al％]≥8.5 (A)

The present invention completes based on above neodoxy, and purport of the present invention is as described below.

(1) steel of a technical scheme of the present invention, its chemical constitution is in mass % containing C:0.16 ~ 0.30%, Si:0.01 ~ 2.0%, Mn:0.35 ~ 1.45%, Cr:0.05 ~ 3.0%, Al:0.001 ~ 0.2%, Ni:0.04 ~ 5.0%, Sn:0.015 ~ 1.0%, S:0.004 ~ 0.05%, N:0.003 ~ 0.03%, below O:0.005%, below P:0.025%, Mo:0 ~ 1.0%, Cu:0 ~ 1.0%, B:0 ~ 0.005%, Nb:0 ~ 0.3%, Ti:0 ~ 0.3%, V:0 ~ 1.0%, Ca:0 ~ 0.01%, Mg:0 ~ 0.01%, Zr:0 ~ 0.05%, Te:0 ~ 0.1% and rare earth element: 0 ~ 0.005%, and remainder is Fe and impurity, by Si, Ni, the content of Al and Sn is expressed as [Si%] in mass %, [Ni%], [Al%], time [Sn%], meet following formula (A).

42≥21×[Si％]+5×[Ni％]+40×[Sn％]+32×[Al％]≥8.5 (A)

(2) steel described in above-mentioned (1), described chemical constitution also can contain in Mo:0.05 ~ 1.0%, Cu:0.01 ~ 1.0% and B:0.0002 ~ 0.005% in mass % one or more.

(3) steel described in above-mentioned (1) or (2), described chemical constitution also can contain in Nb:0.005 ~ 0.3%, Ti:0.005 ~ 0.3% and V:0.01 ~ 1.0% in mass % one or more.

(4) described any one of above-mentioned (1) ~ (3) steel, described chemical constitution also can contain Ca:0.0005 ~ 0.01%, Mg:0.0005 ~ 0.01%, Zr:0.0005 ~ 0.05%, Te:0.0005 ~ 0.1% and rare earth element in mass %: one or more in 0.0001 ~ 0.005%.

Invention effect

If use steel of the present invention to manufacture converted steel parts, then do not need the impact value in order to improve converted steel parts and adjust car-burization condition for each converted steel parts.Thus, can improve manufacture efficiency by the unification of method for carburizing, and can obtain the converted steel parts of excellent impact value, the industrial effect brought by the present invention is very big.

Accompanying drawing explanation

Fig. 1 is the chart of the relation represented between impact value and surface C concentration.

Fig. 2 is the schematic diagram in the cross section vertical with the bearing of trend of breach representing the charpy impact test sheet used in the present invention.

Fig. 3 is the schematic diagram of the measured zone of presentation surface C concentration.

Fig. 4 is the semi-log chart of the relation between presentation surface C concentration and impact value ratio.

Fig. 5 is the chart representing 21 × [Si%]+5 × [Ni%]+40 × [Sn%]+32 × [Al%] and the relation between surface C concentration and impact value ratio.

Embodiment

Below explain for implementing mode of the present invention.

First, the restriction reason of the chemical composition of the steel of present embodiment is described.Below, using " the quality % " of the unit of the content as alloying element referred to as " % ".In the following description, unless otherwise specified, the explanation about steel (case hardening steel) is also suitable for converted steel parts (carburized material).

C：0.16～0.30％

C content determines the intensity of the core of converted steel parts, and also has an impact to the degree of depth of effective hardness layer.In order to ensure required core strength, the lower value of C content is set as 0.16%.On the other hand, if C content is too much, then manufacturing reduces, and therefore the higher limit of C content is set as 0.30%.C content is preferably 0.18 ~ 0.25%.

Si：0.01～2.0％

Si is to the effective element of the deoxidation of steel, and is to the intensity needed for the converted steel parts imparting as physical structure parts and the effective element of hardening capacity.Further, by the increase of Si content, carburizing during carburizing reduces, and the impact value of converted steel parts improves.When Si content is less than 0.01%, its effect is also insufficient.In addition, if Si content is more than 2.0%, then decarburization when manufacturing becomes remarkable, the intensity of converted steel parts and effective case depth deficiency.For the above-mentioned reasons, need to make Si content in the scope of 0.01 ~ 2.0%.Si content is preferably 0.2 ~ 1.5%.

Mn：0.35～1.45％

Mn is to the effective element of the deoxidation of steel, and is to the intensity needed for steel imparting and the effective element of hardening capacity.When Mn content is less than 0.35%, martensite start temperature uprises, and causes self-tempering, and hardness is reduced.In addition, if Mn content is more than 1.45%, even then after subzero process, residual austenite is also stably present in steel, thus the intensity of steel reduces.For the above-mentioned reasons, need to make Mn content in the scope of 0.35 ~ 1.45%.Mn content is preferably 0.50 ~ 1.10%.

Cr：0.05～3.0％

Cr is to the intensity needed for steel imparting and the effective element of hardening capacity.When Cr content is less than 0.05%, its effect is insufficient.If Cr content is more than 3.0%, then its effect is saturated.For the above-mentioned reasons, need Cr content to be located in the scope of 0.05 ~ 3.0%.Cr content is preferably 0.2 ~ 1.5%.

Al：0.001～0.2％

Al is to the effective element of the deoxidation of steel, and is the element becoming nitride and separate out, play crystal grain micronized effect in steel.Further, if increase Al content, then the carburizing of steel reduces, and the impact value of converted steel parts improves thus.When Al content is less than 0.001%, its effect is insufficient.In addition, if Al content is more than 0.2%, then precipitate (Al nitride) coarsening, becomes the reason of the embrittlement of steel and converted steel parts.For the above-mentioned reasons, need Al content to be located in the scope of 0.001 ~ 0.2%.The optimum range of Al content is 0.01 ~ 0.15%.

Ni：0.04～5.0％

Ni is to the intensity needed for steel imparting and the effective element of hardening capacity.Further, by increasing Ni content, carburizing during carburizing reduces, and the impact value of converted steel parts improves thus.When Ni content is less than 0.04%, its effect is insufficient.If Ni content is more than 5.0%, even if implement subzero process to steel, residual austenite is also stably present in steel, thus the intensity of steel reduces.For the above-mentioned reasons, need Ni content to be located in the scope of 0.04 ~ 5.0%.Preferably Ni content is 1.0 ~ 2.0%.

Sn：0.015～1.0％

By the increase of Sn content, carburizing during carburizing reduces, and the impact value of converted steel parts improves thus.When Sn content is less than 0.015%, its effect is insufficient.On the other hand, if Sn content is more than 1.0%, then the high-temperature ductility of steel reduces.For the above-mentioned reasons, need the content of Sn to be located in the scope of 0.015 ~ 1.0%.The optimum range of Sn content is 0.02 ~ 0.1%.

S：0.004～0.05％

S forms MnS in steel, makes the machinability of steel improve thus.When S content is less than 0.004%, its effect is insufficient.On the other hand, if S content is more than 0.05%, then its effect is saturated, instead produce grain boundary segregation and cause embrittlement of grain boundaries.For the above-mentioned reasons, need the content of S to be located in the scope of 0.004 ~ 0.05%.The optimum range of S content is 0.01 ~ 0.04%.

N：0.003～0.03％

N combines with Al, Ti, Nb and V etc. and generates nitride or carbonitride in steel.These nitride and carbonitride have the effect of the coarsening suppressing crystal grain.When N content is less than 0.003%, its effect is insufficient.If N content is more than 0.03%, then its effect is saturated.For the above-mentioned reasons, need N content to be located in the scope of 0.003 ~ 0.03%.The optimum range of N content is 0.005 ~ 0.008%.

Below O:0.005%

O forms oxide compound in steel.Sometimes this oxide compound makes grain boundary segregation and causes embrittlement of grain boundaries.In addition, O is the element being easy to form harder oxide based inclusion in steel and causing brittle rupture.O content is needed to be limited in less than 0.005%.The optimum range of O content is less than 0.0025%.O content is more few more preferred, and therefore the lower value of O content is 0%.

Below P:0.025%

P segregation when carburizing, in austenite grain boundary, causes intercrystalline failure thus.Namely, P makes the impact value of converted steel parts reduce.Thus, need P content to be limited in less than 0.025%.The optimum range of P content is less than 0.01%.P content is more few more preferred, and therefore the lower value of P content is 0%.But, when the removing of P has been carried out more than required degree, manufacturing cost has increased.Thus, the substantial lower value of P content is about 0.004% usually.

The steel of present embodiment, also can containing the one in Mo, Cu and B or two or more in order to improve impact value.But, be not required containing these elements.

Mo：0～1.0％

Mo, for suppressing P at grain boundary segregation, is therefore the effective element of impact value to improving steel.If Mo content is more than 1.0%, then its effect is saturated, therefore needs the upper limit of Mo content to be set as 1.0%.The lower value of Mo content is 0%, but when obtaining above-mentioned effect when making steel contain Mo, preferred Mo content is more than 0.05%.The scope preferably of Mo content is 0.05 ~ 0.25%.

Cu：0～1.0％

Cu is the effective element of hardening capacity to improving steel, and is make by improving hardening capacity the element that the impact value of steel improves.If Cu content is more than 1.0%, then high-temperature ductility reduces, and therefore needs the upper limit of Cu content to be set as 1.0%.The lower value of Cu content is 0%, but when obtaining above-mentioned effect when making steel contain Cu, preferred Cu content is more than 0.01%.The scope preferably of Cu content is 0.01 ~ 0.2%.

B：0～0.005％

B has the effect of the grain boundary segregation suppressing P.In addition, B also has the raising effect of grain-boundary strength and intragranular intensity and the raising effect of hardening capacity, and these effects make the impact value of steel improve.If B content is more than 0.005%, then its effect is saturated, therefore needs the higher limit of B content to be set as 0.005%.The lower value of B content is 0%, but when obtaining above-mentioned effect when making steel contain B, preferred B content is more than 0.0002%.The scope preferably of B content is 0.0005 ~ 0.003%.

The steel of present embodiment, also can containing the one in Nb, Ti and V or two or more in scope shown below in order to also can prevent the reduction of impact value when having carried out long-time carburizing.But, be not required containing these elements.

Nb：0～0.3％

Nb generates Nb carbonitride in steel.When applying carburizing temperature and being the so-called high temperature carburizing of more than 980 DEG C and when applying carburizing time and being the so-called long-time carburizing of more than 10 hours; all by making the Nb carbonitride of the amount be suitable for be present in steel; austenite grain grain refined can be made, the reduction of impact value can be prevented.If Nb content is more than 0.3%, then machinability is deteriorated, and the upper limit of Nb content is set as 0.3%.The lower value of Nb content is 0%, but when obtaining above-mentioned effect when making steel contain Nb, preferred Nb content is more than 0.005%.The scope preferably of Nb content is 0.02 ~ 0.05%.

Ti：0～0.3％

Ti generates fine TiC and/or TiCS in steel.When applying carburizing temperature and being the so-called high temperature carburizing of more than 980 DEG C and when applying carburizing time and being the so-called long-time carburizing of more than 10 hours; all by making TiC and TiCS of the amount be suitable for be present in steel; austenite grain grain refined can be made, the reduction of the impact value of steel can be prevented.If Ti content is more than 0.3%, then the precipitate of TiN main body becomes many, and the fatigue characteristic of steel reduce.For the above-mentioned reasons, need the higher limit of Ti content to be set as 0.3%.The lower value of Ti content is 0%, but when obtaining above-mentioned effect when making steel contain Ti, preferred Ti content is more than 0.005%.The scope preferably of Ti content is 0.02 ~ 0.2%.

V：0～1.0％

V generates V carbonitride in steel.When applying carburizing temperature and being the so-called high temperature carburizing of more than 980 DEG C and when to apply carburizing time be the so-called long-time carburizing of more than 10 hours; all by making the V carbonitride of the amount be suitable for be present in steel; austenite grain grain refined can be made, the reduction of the impact value of steel can be prevented.If V content is more than 1.0%, then the machinability of steel is made to be deteriorated.For the above-mentioned reasons, need the higher limit of V content to be set as 1.0%.The lower value of V content is 0%, but when obtaining above-mentioned effect when making steel contain V, preferred V content is more than 0.01%.The scope preferably of V content is 0.03 ~ 0.1%.

The steel of present embodiment, also can containing the one in Ca, Mg, Zr, Te and rare earth element or two or more in scope shown below in order to improve machinability.But, be not required containing these elements.

Ca：0～0.01％

Ca makes the fusing point of oxide compound reduce, and occurs softening, therefore improve machinability when machining because temperature rises.But, if Ca content is more than 0.01%, then CaS generates in a large number, and machinability reduces.For the above-mentioned reasons, desirably the higher limit of Ca content is set as 0.01%.The lower value of Ca content is 0%, but when obtaining above-mentioned effect when making steel contain Ca, preferred Ca content is more than 0.0005%.The scope preferably of Ca content is 0.0005 ~ 0.0015%.

Mg：0～0.01％

Mg is deoxidant element, in steel, generate oxide compound.Further, the Mg system oxide compound that Mg is formed is easy to become the crystallization of MnS and/or the core of precipitation.In addition, the sulfide of Mg becomes the complex sulfide of Mn and Mg, thus makes MnS balling.So, Mg is dispersion to control MnS, improves the effective element of machinability.But, if Mg content is more than 0.01%, then MgS generates in a large number, and the machinability of steel reduces, therefore desirably the higher limit of Mg content is set as 0.01%.The lower value of Mg content is 0%, but when obtaining above-mentioned effect when making steel contain Mg, preferred Mg content is more than 0.0005%.The scope preferably of Mg content is 0.0005 ~ 0.0015%.

Zr：0～0.05％

Zr is deoxidant element, generates oxide compound.Further, the Zr system oxide compound that Zr is formed is easy to become the crystallization of MnS and/or the core of precipitation.So, Zr is dispersion to control MnS, improves the effective element of machinability.But, if Zr amount is more than 0.05%, then its effect is saturated, therefore desirably the higher limit of Zr content is set as 0.05%.The lower value of Zr content is 0%, but when obtaining above-mentioned effect when making steel contain Zr, preferred Zr content is more than 0.0005%.In addition, in order to promote the balling of MnS, particularly preferably the lower value of Zr content is set as more than 0.003%.

Te：0～0.1％

Te promotes the balling of MnS, therefore improves the machinability of steel.If Te content is more than 0.1%, then its effect is saturated, and therefore the preferred higher limit by Te content is set as 0.1%.The lower value of Te content is 0%, but when obtaining above-mentioned effect when making steel contain Te, preferred Te content is more than 0.0005%.The scope preferably of Te content is 0.0005 ~ 0.0015%.

Rare earth element: 0 ~ 0.005%

Rare earth element generates sulfide in steel, and this sulfide becomes the precipitation core of MnS, and therefore rare earth element is the element of the generation promoting MnS, improves the machinability of steel.But, if the total content of rare earth element is more than 0.005%, then sulfide is greatly thicker, and the fatigue strength of steel is reduced, and therefore needs the higher limit of the total content of rare earth element to be set as 0.005%.The lower value of the total content of rare earth element is 0%, but when obtaining above-mentioned effect containing rare earth element, the total content of preferred rare earth element is more than 0.0001%.The scope preferably of the total content of rare earth element is 0.001 ~ 0.003%.

The steel of present embodiment contains above-mentioned alloying constituent, and remainder contains Fe and impurity.As long as the element beyond above-mentioned alloying constituent does not bring the level of impact on the characteristic of steel from its mixed volume of this situation that starting material and manufacturing installation are mixed into steel as impurity, then allow.

Above the content of each alloying constituent contained by the steel of present embodiment is discussed.But control separately by means of only to the content of each alloying constituent, cannot realize following steel, how this steel all can obtain the converted steel parts with sufficient impact value for the shape obtaining no matter converted steel parts under single car-burization condition.The present inventors have found following opinion: need the content controlling alloying constituent based on formula (1).

42≥21×[Si％]+5×[Ni％]+40×[Sn％]+32×[Al％]≥8.5 (1)

In formula (1), [Si%], [Ni%], [Sn%] and [Al%] refer to, represent the content of Si, Ni, Sn and Al with quality %.Below the basis of the derivation of formula (1) is described.

First, below illustrate that the present inventors are in order to evaluate the shock-resistance of carburized material and the content of research of carrying out.

First, by containing C:0.20 quality %, Si:0.24 quality %, Mn:0.79 quality %, P:0.014 quality %, S:0.015 quality %, Cr:1.21 quality %, Al:0.031 quality %, Ni:0.05 quality %, Sn:0 quality %, N:0.005 quality % and O:0.001 quality % and the case hardening steel that remainder is Fe and impurity is defined as benchmark steel.Secondly, by shown in Fig. 2, physical dimension is 10mm × 10mm × 55mm and to have radius-of-curvature be 10mm and the degree of depth is the charpy impact test sheet 3 that the charpy impact test sheet of the breach (recess) of the arc-shaped of 2mm is defined as in present embodiment.For the charpy impact test sheet 3 formed for material with benchmark steel, first treatment temp be 930 DEG C, the treatment time is 5 hours and the car-burization condition (hereinafter sometimes referred to benchmark car-burization condition) that carbon potential is 0.8 carries out gas cementation, then carry out tempering temperature and be 150 DEG C and tempering time is the tempering of 90 minutes, the Xia Shi shock absorption 25 DEG C time of thus obtained carburized material can be defined as standardized impact value.

Further, by impact value than being defined as following value: according to the car-burization condition (, benchmark car-burization condition) applied when obtaining standardized impact value, carburizing and tempering are carried out to charpy impact test sheet 3 and obtain must 25 DEG C of carburized material time the Xia Shi shock absorption value that can obtain divided by standardized impact value.

Said reference steel is the steel with the chemical composition being equivalent to SCr420 being often used as gear steel, identical with the steel of comparative example 26 described later.The gas cementation of carrying out under said reference car-burization condition is the common carburizing treatment of carrying out in order to manufacturing machine structure parts.

The side view (shape in the cross section vertical with the bearing of trend of breach) of above-mentioned charpy impact test sheet 3 represents in fig. 2.The radius-of-curvature of breach 2 is 10mm.The shape of charpy impact test sheet 3 is different from the shape (shape specified in such as JIS-Z2242 " the charpy impact test method of metallic substance ") of common charpy impact test sheet.The shape that the shape of the breach 2 of charpy impact test sheet 3 is intended to the tooth root portion of simulating gear decides.By carrying out charpy impact test to the test film with this breach, the shock-resistant characteristic in the tooth root portion of gear can be estimated.The test film with this breach is such as widely used as the test film shape measured the shock-resistant characteristic of the steel of carburizing as described in Japanese Unexamined Patent Publication 2013-40376 publication.Except the shape of charpy impact test sheet 3, carry out the measurement of Xia Shi shock absorption energy according to JIS-Z2242 " the charpy impact test method of metallic substance ".The enforcement temperature of charpy impact test is 25 DEG C.Charpy impact test sheet 3 is made into by mechanical workout.

Then, the present inventors forge the various steel (wherein not considering the regulation relevant to formula 1) containing alloying element within the scope of mentioned component, mechanical workout and carburizing, obtain the impact value ratio of the carburized material obtained from these steel.Further, the surface C concentration of the present inventors to each carburized material is measured.

The measuring method of surface C concentration is below described.First, cut off charpy impact test sheet 3 along the direction vertical with the recess face (forming face jaggy) of charpy impact test sheet 3 and breach 2 of having carried out gas cementation process under benchmark car-burization condition, cut surface is ground.The sketch chart of cut surface has been shown in Fig. 3.Then, in the region (surface C concentration measuring area 1) of the lower surface 5 ~ 50 μm of the short transverse distance breach 2 towards charpy impact test sheet 3 with 5 μm of interval measurements C concentration.The measurement of C concentration has been carried out by EPMA.The size (beam diameter of EPMA) of measurement point is φ 5 μm.Thus, the mean value of obtained 10 take off data is set as surface C concentration.The unit of surface C concentration is quality %.

Like this, to the surface C concentration after carburizing when making the chemical composition of steel and carburized material matter characteristic on a large scale and systematically change measure as a result, the present inventors have found following opinion: surface C concentration correspondingly changes along with the change of the addition of alloying element.It is believed that this phenomenon is that chemical interaction between the C that utilizes alloying element and infiltrated steel surface by carburizing is produced.The impact of Si, Ni, Al and Sn particularly effects on surface C concentration is comparatively strong, and along with the increase of the content of these elements, surface C concentration reduces.According to above-mentioned opinion, in the chemical constitution of the steel of present embodiment, the relational of content of Si, Ni, Al and Sn is specified by formula 1, thus can control the carburizing of steel.

Based on following reason, impact value ratio needs to be more than 1.2.As mentioned above, can the design of change parts, to make by making the shock-resistant characteristic of converted steel parts carburized material used improve fully, thus also guarantee shock-resistant destructiveness while suppression uses quantity of material.In the technical field of physical structure parts, in order to implement such design alteration, need, relative to said reference impact value (having carried out the impact value of the SCr420 of carburizing under common car-burization condition), impact value is improved 20%.

As mentioned above, between impact value ratio and surface C concentration, there is correlationship.Fig. 4 is the semi-log chart of the correlationship represented between the surface C concentration of carburized material and impact value ratio.In the diagram, the impact value ratio of the data point under dotted line is less than 1.2.As shown in Figure 4, known: in order to obtain the carburized material of the impact value ratio with more than 1.2, need the surface C concentration of the carburized material having carried out gas cementation in benchmark car-burization condition to control below 0.75%.

At this, carry out multiple regression analysis using the respective content of Si, Ni, Al and Sn as factor pair surface C concentration.Consequently, when having carried out gas cementation with benchmark car-burization condition, as obtaining the critical condition that surface C concentration is the carburized material of 0.75 quality %, following formula (1 ') and formula (2 ') are obtained.

21×[Si％]+5×[Ni％]+40×[Sn％]+32×[Al％]＝α (1’)

α＝8.5 (2’)

Fig. 5 is the chart representing the relation between α value and surface C concentration and the relation between α value and impact value ratio.In Figure 5, the α value of the data point kept left than left dotted line is less than 8.5, and the α value of the data point of keeping right than right dotted line is more than 42.From this chart, if α value is more than 8.5, then, when having carried out gas cementation with benchmark car-burization condition, surface C concentration becomes below 0.75 quality %.Along with α value increases, the surface C concentration of carburized material reduces, and therefore corresponding therewith, the impact value of carburized material increases thereupon.Preferred α value is more than 12.

On the other hand, in the significantly reduced situation of carburizing, because surface hardness reduces, wear resistance is reduced significantly, the intensity as converted steel parts becomes and insufficient.When having carried out gas cementation with benchmark car-burization condition, desirably the surface hardness of carburized material is higher than HV550.In order to reach this surface hardness, the surface C concentration of having carried out the carburized material of gas cementation with benchmark car-burization condition is needed to be more than 0.4 quality %.Further, known: in order to make the surface C concentration of having carried out the carburized material of gas cementation with benchmark car-burization condition be more than 0.4 quality %, to need α value to be less than 42.In addition, preferred surface C concentration of having carried out the carburized material of gas cementation with benchmark car-burization condition is more than 0.55 quality % further, and in order to reach this surface C concentration, preferred α value is less than 25.

For obtaining the method for carburizing preferred gas carburizing (also can be the either type in modification stove mode and dripping type) of converted steel parts from the steel of present embodiment.In addition, nitriding can also be implemented except implementing carburizing.

In addition, also sometimes the effective case depth of the metewand as converted steel parts is studied, but the such characteristic required by parts of the vehicle gears such as differential gearing is more associated with surface hardness consumingly compared to effective case depth.Thus, adopt and the surface C concentration of converted steel parts is controlled, for suitable level, the steel that can make the optimized present embodiment of surface hardness thus, can obtain effect favourable in industry is practical.

The steel of present embodiment also can be such as first be made into pole steel by hot rolling, then, is applied in forging, machining and be made for the shape of gear etc., then implements carburizing and quenching and make converted steel parts.

Embodiment

Then, embodiments of the invention are described, but the condition in embodiment is that the present invention is not limited to this condition example in order to confirm exploitativeness of the present invention and effect and the condition example adopted.Only otherwise depart from purport of the present invention and reach object of the present invention, the present invention can adopt various condition.

After the sectional dimension that the various steel billet forgings with the chemical composition shown in table 1-1 and table 1-2 are stretched as length direction is the square rod shape of vertical 50mm and horizontal 50mm (50mm × 50mm); after implementing equal thermal treatment and normalizing (normalizing), the sectional dimension being divided into 4 length directions is further the square rod shape of vertical 25mm and horizontal 25mm.Acquire the physical dimension shown in Fig. 2 from obtained each rod along its central axis and be 10mm × 10mm × 55mm and to have radius-of-curvature be 10mm and the degree of depth is the charpy impact test sheet of the breach (recess) of the arc-shaped of 2mm.This test film shape is identical with above-mentioned charpy impact test sheet 3.Then, carburizing treatment is implemented to this charpy impact test sheet.In embodiment beyond comparative example 29 and comparative example, with treatment temp be 930 DEG C, the treatment time is 5 hours and the car-burization condition that carbon potential is 0.8 has carried out gas cementation.These treatment condition are identical with said reference car-burization condition.In comparative example 29, with treatment temp be 930 DEG C, the treatment time is 5 hours and the car-burization condition that carbon potential is 0.6 has carried out gas cementation.Be 150 DEG C and tempering time implements tempering under being the condition of 90 minutes in tempering temperature.

After tempering, the surface C concentration of each sample is measured.The measuring method of surface C concentration is as described below.First, the direction vertical along the recess face (forming face jaggy) and breach with charpy impact test sheet cuts off charpy impact test sheet, grinds cut surface.Then, in the region (surface C concentration measuring area 1) of the lower surface 5 ~ 50 μm of the short transverse distance breach 2 towards charpy impact test sheet with 5 μm of interval measurements C concentration.The measurement of C concentration has been carried out by EPMA.The size (beam diameter of EPMA) of measurement point is set as φ 5 μm.Thus, using the mean value of 10 take off data that obtains as surface C concentration.The unit of surface C concentration is quality %.

In addition, implement charpy impact test after tempering, Xia Shi shock absorption can (impact value) be measured.Except the shape of the recess of charpy impact test sheet, implement charpy impact test according to the method for JIS-Z2242 defined with test temperature 25 DEG C.

Further, by the impact value of each sample to be calculated the impact value ratio of each sample divided by the impact value of comparative example 26.In addition, the steel of comparative example 26 is said reference steel.

In addition, in order to evaluate the wear resistance of each sample, wear test being carried out to each sample and the abrasion degree of depth is measured.The wear test sheet of following shape is acquired: to have diameter be 26mm and length is the cylindrical portion of 28mm and the cylindric grip with the diameter that this cylindrical portion has a same central axis to be 24mm and length be 51mm from the square rod of the 50mm × 50mm after the normalizing be made into aforesaid method along its central axis.Grip is configured in the two ends of the length direction of cylindrical portion.Further, with the condition identical with aforesaid charpy impact test sheet, carburizing treatment is implemented to this wear test sheet.The abrasion degree of depth refers to, roller is pressed on the cylindrical portion of wear test sheet and makes this roller have rotated 1,000,000 times afterwards in the degree of depth of the abrasion of wear test sheet generation.Wear test condition is as described below.The sample that the abrasion degree of depth is less than 30 μm is judged as has sufficient wear resistance.

The material of roller: bearing steel (SUJ2)

The hardness of roller: HV700 ~ 800

Roller diameter: 130mm

Roller width: 18mm

Roll shape: the convex surface being formed on the outer periphery R=150mm

Roller contact force: hertz stress 1500MPa (face pressure)

Slip rate :-100%

The surface C concentration of each sample, impact value ratio and the abrasion degree of depth have been shown in table 2.Comparative example 26 has the chemical constitution of the SCr420 of the JIS-G4053 defined being equivalent to be typically used as gear steel, 21 × [Si%]+5 × [Ni%]+40 × [Sn%]+32 × [Al%] is 6.3, when having carried out gas cementation with benchmark car-burization condition, impact value has been 10J/cm ².Can it is clear that: the impact value ratio of example 1 ~ 25 is more than 1.3, has excellent impact strength.Such as, in example 1 21 × [Si%]+5 × [Ni%]+40 × [Sn%]+32 × [Al%] is 41.1, and surface C concentration is 0.46%, therefore obtains good impact value.

On the other hand, comparative example 26 ~ 35 does not have gratifying characteristic.

Comparative example 26 and 28 is not containing Sn, and therefore carburizing has exceedingly been carried out, and compared with embodiment, only has lower impact value.In addition, the Sn content of comparative example 31 is lower than specialized range of the present invention, and therefore same with comparative example 26 and 28, carburizing has exceedingly been carried out, and compared with embodiment, only has lower impact value.

About comparative example 27, the content of each alloying element is in specialized range of the present invention, but 21 × [Si%]+5 × [Ni%]+40 × [Sn%]+32 × [Al%] is beyond specialized range of the present invention.Thus, the wear resistance of comparative example 27 is low.

About comparative example 30, the content of each alloying element is in specialized range of the present invention, but 21 × [Si%]+5 × [Ni%]+40 × [Sn%]+32 × [Al%] is lower than specialized range of the present invention, and therefore carburizing has exceedingly been carried out.Thus, comparative example 30, compared with embodiment, only has lower impact value.

Exceeded specialized range of the present invention about comparative example 32, Sn content, therefore high-temperature ductility reduces.Thus, on the surface of the carburized material obtained in comparative example 32, crackle is multiple.

Exceed specialized range of the present invention about comparative example 33, Ni content, therefore intensity reduces.Thus, the wear resistance of comparative example 33 is low.

Exceed specialized range of the present invention about comparative example 34, Al content, therefore create embrittlement.Thus, the impact value of comparative example 34 is than low.

Reference example 29 is steel identical with comparative example 26, but car-burization condition is different, set lower by carbon potential (carburizing treatment of 0.6), therefore known: surface C concentration is lower, obtains good impact value.But, carbon potential is set lower meeting causes productivity to reduce in the production of reality, therefore not good.

Table 1-1

Mark "-" in ※ table represents that the content of element is 0 or is considered the amount of impurity.

Table 1-2

Table 2

The car-burization condition of ※ comparative example 29 is different from other examples

Nomenclature

1 surface C concentration measuring area

2 breach (recess)

3 charpy impact test sheets (carburized material)

Claims

1. a steel, is characterized in that, its chemical constitution contains in mass %:

C：0.16～0.30％、

Si：0.01～2.0％、

Mn：0.35～1.45％、

Cr：0.05～3.0％、

Al：0.001～0.2％、

Ni：0.04～5.0％、

Sn：0.015～1.0％、

S：0.004～0.05％、

N：0.003～0.03％、

Below O:0.005%,

Below P:0.025%,

Mo：0～1.0％、

Cu：0～1.0％、

B：0～0.005％、

Nb：0～0.3％、

Ti：0～0.3％、

V：0～1.0％、

Ca：0～0.01％、

Mg：0～0.01％、

Zr：0～0.05％、

Te:0 ~ 0.1% and

Rare earth element: 0 ~ 0.005%,

Further, remainder is Fe and impurity,

When the content of Si, Ni, Al and Sn being expressed as in mass % [Si%], [Ni%], [Al%], [Sn%], meet following formula (1),

42≥21×[Si％]+5×[Ni％]+40×[Sn％]+32×[Al％]≥8.5 (1)。

2. steel according to claim 1, is characterized in that, described chemical constitution is in mass % containing one or more in Mo:0.05 ~ 1.0%, Cu:0.01 ~ 1.0% and B:0.0002 ~ 0.005%.

3. steel according to claim 1 and 2, is characterized in that, described chemical constitution is in mass % containing one or more in Nb:0.005 ~ 0.3%, Ti:0.005 ~ 0.3% and V:0.01 ~ 1.0%.

4. the steel according to any one of claims 1 to 3, it is characterized in that, described chemical constitution is in mass % containing Ca:0.0005 ~ 0.01%, Mg:0.0005 ~ 0.01%, Zr:0.0005 ~ 0.05%, Te:0.0005 ~ 0.1% and rare earth element: one or more in 0.0001 ~ 0.005%.