CN105492645A

CN105492645A - Hot-rolled steel sheet having excellent cold workability and excellent surface properties and hardness after working

Info

Publication number: CN105492645A
Application number: CN201480048219.7A
Authority: CN
Inventors: 梶原桂; 高知琢哉
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 2013-09-04
Filing date: 2014-09-02
Publication date: 2016-04-13
Also published as: US20160201172A1; MX2016002690A; JP6058508B2; WO2015033933A1; JP2015048527A; DE112014004028T5

Abstract

A hot-rolled steel sheet which has a thickness of 3-20 mm and contains specific amounts of C, Si, Mn, P, S, Al and N, with the balance made up of iron and unavoidable impurities. This hot-rolled steel sheet contains a specific amount of solid-solved N, and the contents of C and N satisfy the relation 10C + N <= 3.0. This hot-rolled steel sheet contains bainitic ferrite and pearlite, respectively in an area ratio of 5% or more and in an area ratio of less than 20% relative to the whole structure, with the balance made up of polygonal ferrite. The average crystal grain size of the bainitic ferrite is 3-50 [mu]m.

Description

The hot-rolled steel sheet of cold-workability and the surface property after processing and hardness excellence

Technical field

The present invention relates to both work in-processes and demonstrate good cold-workability, demonstrate again set surface property (also claiming " surface quality ") and the hot-rolled steel sheet of the rear hardness of processing after processing.

Background technology

In recent years, from the view point of environment protection, for the purpose of the fuel efficiency improving automobile, for the various parts of automobile, such as, for the lightweight of the steel of drive disk assembly and the shells etc. such as gear, namely the requirement of high strength grows to even greater heights.In order to comply with such lightweight, the requirement of high strength, as steel generally used, use the steel (forge hot material) bar steel being carried out to forge hot.In addition, in order to cut down the CO in parts manufacturing process ₂quantity discharged, so far, for the cold forging of the parts such as the gear processed by forge hot requirement also growth.

So, cold working (cold forging) and hot-work and temperature are processed and are compared, and have productivity high and all good advantage of the yield rate of dimensional precision and steel.But, become problem by during such cold working manufacture component, in order to guarantee more than the preset value of hope by the intensity of cold worked parts, the steel that working strength, i.e. resistance to deformation are high must be needed., the resistance to deformation of the steel of use is higher, more can cause the lost of life of cold working metal die.

In addition, in the field of drive disk assembly, from the forged article () such as forge hot, cold forging of bar steel, also the research manufactured with the parts that the lightweight of parts, cost degradation are target exploitation steel plate is being carried out, but exist in the parts after the cold working of steel plate, the problem being called as the surface imperfection of tension strain trace (StretcherStrainMark, hereinafter referred to as " SS trace ") is easily produced on surface.

Therefore, in the past steel cold forging being become both after shaped, also sometimes implemented by carrying out the thermal treatments such as Q-tempering, thus manufactured the method guaranteeing the high-strength parts of set intensity (hardness).But part dimension must change in the thermal treatment after cold forging, therefore need to be revised by mechanical workouts such as two-dimensional cuttings, expect to omit thermal treatment and the such solution of processing thereafter.

In order to solve above-mentioned problem; such as; disclose and utilize solid solution C to suppress the carrying out of room temperature ageing in soft steel, guarantee the set age hardening amount based on strain aging, the cold forging wire rod of excellent in distortion aging property, bar steel (with reference to patent documentation 1) can be obtained thus.

But this technology is into utilizing solid solution C to measure the technology of controlled strain timeliness, be difficult to obtain the steel of the required surface quality after having sufficient cold-workability, processing concurrently and hardness, intensity.

Therefore, the applicant is conceived to solid solution C contained in steel and solid solution N to the difference of the impact that resistance to deformation and static strain timeliness cause, carry out various research, found that the amount by suitably controlling these solid solution elements, both work in-processes can be obtained and play good cold-workability, after cold working (cold forge), demonstrate again the machine structural steel product of set hardness (intensity), carry out patent application (with reference to patent documentation 2).

These steel achieve the taking into account of high rigidity (high strength) after cold-workability and processing, but with the wire rod described in above-mentioned patent documentation 1, bar steel is same is forge hot material, has the problem that manufacturing cost is high.Therefore, in order to make manufacturing cost low cost further, for alternative existing forge hot material, making automotive part with hot-rolled steel sheet by cold working and being also studied.

Such as, a kind of hot-rolled steel sheet (with reference to patent documentation 3) that can obtain the nitriding treatment of high surface hardness and sufficient hardening depth after nitriding treatment is proposed.

But this technology Problems existing is, also needs to carry out nitriding treatment, can not realize sufficient cost degradation after cold working.

In addition, propose a kind of hot-rolled steel sheet, it consists of containing below C:0.10%, Si: lower than 0.01%, below Mn:1.5% and below Al:0.20%, and in (Ti+Nb)/2: contain in the scope of 0.05 ~ 0.50%, S, N and O adds up to less than 0.0100% and containing below S:0.005%, below N:0.005%, below O:0.004%, and make microtexture be the ferrite single phase structure of the essence of more than 95%, the dimensional precision of the precision stamping machined surface of this hot-rolled steel sheet is excellent, and the surface hardness of the stamping surface after processing is high, in addition resistance to red rust defective also excellent (with reference to patent documentation 4).

But in this hot-rolled steel sheet, N is limited in extremely low content as harmful element, with the hot-rolled steel sheet of the present application utilizing N energetically, completely different inwardly in technology.

Prior art document

Patent documentation

Patent documentation 1: the flat 10-306345 publication of Japanese Laid-Open

Patent documentation 2: Japanese Laid-Open 2009-228125 publication

Patent documentation 3: Japanese Laid-Open 2007-162138 publication

Patent documentation 4: Japanese Laid-Open 2004-137607 publication

Summary of the invention

The problem that invention will solve

The present invention is conceived to above-mentioned situation and is formed, and its object is to, provides a kind of both work in-processes to demonstrate good cold-workability, demonstrate again set surface property and the hot-rolled steel sheet of hardness after processing.

For solving the means of problem

First invention, is the hot-rolled steel sheet of cold-workability and the surface property after processing and hardness excellence, it is characterized in that,

Thickness of slab is 3 ~ 20mm,

Become to be grouped in mass % (following, relate to chemical composition all with) be,

Below C:0.3% (not containing 0%),

Below Si:0.5% (not containing 0%),

Mn：0.2～1％、

Below P:0.05% (not containing 0%),

Below S:0.05% (not containing 0%),

Al：0.01～0.1％、

N：0.008～0.025％，

Surplus is made up of iron and inevitable impurity,

More than solid solution N:0.007%,

And the content of C and N meets the relation of 10C+N≤3.0,

Organize in the area occupation ratio relative to whole tissue,

Bainite ferrite: more than 5%,

Perlite: lower than 20%,

Surplus: polygonal ferrite,

The average crystal grain diameter of described bainite ferrite is the scope of 3 ~ 50 μm.

Second invention is in the hot-rolled steel sheet described in the first invention, becomes to be grouped at least one also comprised in following (a) ~ (e),

At least one in (a) below Cr:2% (not containing 0%) and below Mo:2% (not containing 0%)

B () is selected from least one in below Ti:0.2% (not containing 0%), below Nb:0.2% (not containing 0%), below V:0.2% (not containing 0%)

(c) below B:0.005% (not containing 0%)

D () is selected from least one in below Cu:5% (not containing 0%), below Ni:5% (not containing 0%), below Co:5% (not containing 0%)

E () is selected from least one in below Ca:0.05% (not containing 0%), below REM:0.05% (not containing 0%), below Mg:0.02% (not containing 0%), below Li:0.02% (not containing 0%), below Pb:0.5% (not containing 0%), below Bi:0.5% (not containing 0%).

Invention effect

According to the present invention, in the tissue of bainite ferrite+polygonal ferrite main body with set median size, by guaranteeing that solid solution N measures, and make the content of C and the content of N meet set relation, the resistance to deformation that can obtain in cold working is reduced, the life-span of metal die is extended, and steel plate is difficult to generation crackle, and the parts obtained after processing can guarantee the hot-rolled steel sheet of set surface property and the rear hardness of processing.

Embodiment

Below, hot-rolled steel sheet of the present invention (hereinafter also referred to " steel plate of the present invention ", or is only called " steel plate ".) illustrate in greater detail.Steel plate of the present invention and the forge hot material described in above-mentioned patent documentation 2 guarantee N solid solution capacity and make C content and N content meet set relation in common, but, higher scope is reached in permission C content, make tissue become bainite ferrite-polygonal ferrite-perlite heterogeneous structure, and make the aspect of ferrite grain miniaturization different.

(thickness of slab of steel plate of the present invention: 3 ~ 20mm)

First, steel plate of the present invention with the steel plate of thickness of slab 3 ~ 20mm for object.When thickness of slab is lower than 3mm, the rigidity as structure can not be guaranteed.On the other hand, if thickness of slab is higher than 20mm, be then difficult to reach tissue morphology given to this invention, can not get the effect expected.Preferred thickness of slab is 4 ~ 19mm.

Next, be grouped into for the one-tenth forming steel plate of the present invention and be described.Below, the unit of chemical composition is all quality %.

(one-tenth of steel plate of the present invention is grouped into)

Below < C:0.3% (not containing 0%) >

C is the element made a big impact to the formation of the tissue of steel plate, although tissue is bainite ferrite-polygonal ferrite-perlite heterogeneous structure, but in order to become perlite the least possible bainite ferrite-polygonal ferrite subject organization, it is the element needing limited content.If make C contain superfluously, then the perlite in steel plate tissue divides rate to rise, because pearlitic work hardening causes resistance to deformation likely excessive.Therefore, the C content in steel plate is less than 0.3%, is preferably limited to less than 0.25%, more preferably below 0.2%, particularly preferably in less than 0.15%.But if the content of C is very few, then the deoxidation of steel in melting becomes difficulty, and is difficult to the intensity after meeting cold working, hardness, is therefore preferably more than 0.0005%, is more preferably more than 0.0008%, is particularly preferably more than 0.001%.

Below < Si:0.5% (not containing 0%) >

Si makes the resistance to deformation of steel plate increase by solid solution in steel, is therefore the element needing to do one's utmost to reduce.Therefore, in order to suppress the increase of resistance to deformation, the Si content in steel plate is limited in less than 0.5%, preferably below 0.45%, more preferably below 0.4%, particularly preferably in less than 0.3%.But if the content of Si is few, then the deoxidation in molten system becomes difficulty, and is difficult to the intensity after meeting cold working, hardness, is therefore preferably more than 0.005%, is more preferably more than 0.008%, is particularly preferably more than 0.01%.

＜Mn：0.2～1％＞

Mn is the element with deoxidation and desulfidation in steelmaking process.If improve the content of the N in steel in addition, then the dynamic strain aging that the heat release in processing causes easily causes crackle to occur, and but then, Mn makes processibility at this moment improve, and has the effect suppressing crackle.In order to effectively play these effects, the Mn content in steel is more than 0.2%, is preferably more than 0.22%, is more preferably more than 0.25%.But if Mn content becomes superfluous, then resistance to deformation is excessive, and segregation causes the ununiformity of tissue to occur, and is therefore less than 1%, is preferably less than 0.98%, is more preferably less than 0.95%.

Below < P:0.05% (not containing 0%) >

P is unavoidably by the impurity element contained in steel, if it is contained in ferrite, then making cold-workability deterioration in ferrite grain boundaries segregation, in addition, also make higher content strengthen, is the element forming the reason that resistance to deformation increases.Therefore, the content of P wishes to do one's utmost to reduce from the viewpoint of cold-workability, but extreme minimizing can cause the increase of steel-making cost, therefore considers process capability and as less than 0.05%, is preferably less than 0.03%.

Below < S:0.05% (not containing 0%) >

S is also same with P is inevitable impurity, as FeS in the membranaceous precipitation of crystal grain boundary, is the element making processibility deterioration.In addition, also have and cause hot short effect.Therefore, from the view point of raising deformability, make S content be less than 0.05% in the present invention, be preferably less than 0.03%.But, make S content be 0 industrially difficult.Further, because S has the effect making machinability improve, so from the view point of raising machinability, recommend preferably containing more than 0.002%, more preferably containing more than 0.006%.

＜Al：0.01～0.1％＞

Al is to the effective element of deoxidation at steelmaking process.In order to obtain the effect of this deoxidation, the Al content in steel is more than 0.01%, is preferably more than 0.015%, is more preferably more than 0.02%.But if the content of Al is superfluous, then making toughness reduce, easily crackle occurs, is therefore less than 0.1%, is preferably less than 0.09%, is more preferably less than 0.08%.

＜N：0.008～0.025％＞

N is the important element for being obtained set intensity by the static strain timeliness after processing.Therefore, the N content in steel is more than 0.008%, is preferably more than 0.0085%, is more preferably more than 0.009%.But if the content of N is superfluous, then, except static strain timeliness, the impact of the dynamic strain aging in processing becomes remarkable, resistance to deformation increases and inappropriate, is therefore less than 0.025%, is preferably less than 0.023%, be more preferably less than 0.02%.

The above > of < solid solution N:0.007%

Then, in steel plate, ensured by solid solution N given amount (hereinafter referred to as " solid solution N measures ".), thus less can improve resistance to deformation, and static strain timeliness is promoted.In order to ensure intensity required after cold working, solid solution N measures to be needed more than 0.007%.But, if solid solution N measures surplus, then cold-workability deterioration, and the fixed amount that solid solution N strains to processing also becomes many, and SS trace becomes easily generation and surface property is also deteriorated, is therefore preferably less than 0.03%.Further, because the content of the N in steel is less than 0.025%, so solid solution N amount does not reach more than 0.025% in fact.

At this, solid solution N in the present invention amount is according to JISG1228, deducts total N compound amount and the amount of trying to achieve from the total N content steel.Below illustrate the assay method of the practicality that this solid solution N measures.

(a) inactive gas scorification-thermal conductivity method (mensuration of total N content)

Put into crucible by from for the sample that cuts of examination material, melting in inactive gas air-flow and extract N, extract is delivered to thermal conductivity unit, measures the change of thermal conductivity, try to achieve total N content.

(b) ammonia fractionation by distillation indophenol blue absorption photometry (mensuration of total N compound amount)

By from the sample cut for examination material, be dissolved in 10%AA system electrolytic solution, carry out constant-current electrolysis, measure the total N compound amount in steel.The 10%AA system electrolytic solution used is the electrolytic solution of the non-aqueous solvent that methyl alcohol is formed by 10% acetone, 10% Tetramethylammonium chloride, surplus, is the solution that can not make steel Surface Creation passive state epithelium.

Sample for examination material is about 0.5g, makes it to be dissolved in this 10%AA system electrolytic solution, by the undissolved residue (N compound) generated, with the metre filter of the polycarbonate in 0.1 μm, aperture.By the undissolved residue thermal degradation in sulfuric acid, potassium sulfate and pure copper chip (チ Star プ) obtained, make resolvent consistent with filtrate.After making this solution become alkalescence with sodium hydroxide, carry out wet distillation, make the ammonia distillated be absorbed in dilute sulphuric acid.In addition, add phenol, clorox and five cyanogen one nitrosyl and close sodium ferrite and blue complex is generated, use extinction photometer measure absorbancy and try to achieve total N compound amount.

Then, from the total N content of trying to achieve according to the method for above-mentioned (a), deduct total N compound amount of being tried to achieve by the method for above-mentioned (b), thus solid solution N can be tried to achieve measure.

The content of < C and N meets the relation > of 10C+N≤3.0

In steel of the present invention, solid solution C makes resistance to deformation greatly increase, and less contributes to static strain timeliness, on the other hand, solid solution N can less make resistance to deformation rise and promote static strain timeliness, therefore has the effect that the hardness after can making processing increases.Therefore, in steel of the present invention, rise to less make the resistance to deformation in processing, and the hardness after processing is increased, the content of C and the content of N must meet the relation of 10C+N≤3.0, be preferably 0.009≤10C+N≤2.8, be more preferably 0.01≤10C+N≤2.5, be particularly preferably 0.01≤10C+N≤2.0.From the view point of the crystal grain miniaturization made in hot-rolled steel sheet and the plasticity guaranteeing this steel plate, need C content to a certain degree and solid solution C to measure, but during 10C+N > 3.0, the amount of C and/or N is superfluous, and resistance to deformation is excessive.At this, in above-mentioned inequality, why make the coefficient of C content be 10 times of the coefficient of N content, even if be consider that solid solution C is same amount compared with solid solution N, the degree making the Strength and Dformation resistance of hot-rolled steel sheet of the present invention increase still can about large 1 order of magnitude (10 times).

Steel of the present invention is substantially containing mentioned component, and surplus is iron and inevitable impurity, but in addition, in the scope not damaging effect of the present invention, also can add following permission composition.

Below < Cr:2% (not containing 0%) and/or below Mo:2% (not containing 0%) >

Cr is the intensity had by improving crystal grain boundary, making the deformability of steel improve the element of this effect, in order to make such effect effectively play, preferably making Cr contain more than 0.2%.But if make Cr contain superfluously, then resistance to deformation increases, and cold-workability likely reduces, and therefore recommends its content to be less than 2%, is more recommended as less than 1.5%, and special recommendation is less than 1%.

In addition, Mo has the element that the hardness of the steel after making processing and deformability increase this effect, in order to make such effect effectively play, preferably makes Mo contain more than 0.04%, more preferably containing more than 0.08%.But if make Mo contain superfluously, then cold-workability is likely deteriorated, therefore recommends its content to be less than 2%, be more recommended as less than 1.5%, special recommendation is less than 1%.

At least one > that < selects from the group that below Ti:0.2% (not containing 0%), below Nb:0.2% (not containing 0%), below V:0.2% (not containing 0%) are formed

The avidity of these elements and N is strong, coexists and forms N compound, make the crystal grain miniaturization of steel with N, and the toughness of the processed goods obtained after making cold working improves, and in addition, still has the element making resistance to anti-thread breakage this effect of raising.But, even if each element contains higher than higher limit, also can not get characteristic improving effect.Recommend the content of each element to be respectively less than 0.2%, be more recommended as 0.001 ~ 0.15%, special recommendation is 0.002 ~ 0.1%.

Below < B:0.005% (not containing 0%) >

B is same with above-mentioned Ti, Nb and V, strong with the avidity of N, coexists and forms N compound, make the crystal grain miniaturization of steel with N, and the toughness of the processed goods obtained after making cold working improves, and in addition, is have the element improving resistance to this effect anti-thread breakage.Therefore, due to steel plate of the present invention contain B time, can guarantee that required solid solution N measures, the intensity after cold working be improved, so its content is recommended as less than 0.005%, is more recommended as 0.0001 ~ 0.0035%, special recommendation is 0.0002 ~ 0.002%.

At least one > that < selects from the group that below Cu:5% (not containing 0%), below Ni:5% (not containing 0%), below Co:5% (not containing 0%) are formed

These elements all have makes steel generation strain aging, makes it the effect of hardening, and is for the effective element of intensity after raising processing.In order to make such effect effectively play, these elements preferably containing more than 0.1%, more preferably contain more than 0.3% respectively.But, if the content of these elements is superfluous, then make the effect of steel generation strain aging and sclerosis, make processing in addition after the effect that improves of intensity saturated, in addition, likely promote crackle, be therefore recommended as less than 5% respectively, more be recommended as less than 4%, special recommendation is less than 3%.

At least one > that < selects from the group that below Ca:0.05% (not containing 0%), below REM:0.05% (not containing 0%), below Mg:0.02% (not containing 0%), below Li:0.02% (not containing 0%), below Pb:0.5% (not containing 0%), below Bi:0.5% (not containing 0%) are formed

Ca makes the vulcanizing compounds system inclusion ballings such as MnS, improves the deformability of steel, and is the element of the raising contributing to machinability.In order to make such effect effectively play, Ca is preferably made to contain more than 0.0005%, more preferably containing more than 0.001%.But even if contain superfluously, its effect is also saturated, can not expect the effect conformed to content, therefore be recommended as less than 0.05%, more be recommended as less than 0.03%, special recommendation is less than 0.01%.

REM and Ca is same, makes the vulcanizing compounds system inclusion ballings such as MnS, improves the deformability of steel, and is the element of the raising contributing to machinability.In order to make such effect effectively play, REM is preferably made to contain more than 0.0005%, more preferably containing more than 0.001%.But even if contain superfluously, its effect is also saturated, can not expect the effect conformed to content, therefore be recommended as less than 0.05%, more be recommended as less than 0.03%, special recommendation is less than 0.01%.

It should be noted that, in the present invention, so-called REM is the meaning containing lanthanon (15 kinds of elements from La to Lu) and Sc (scandium) and Y (yttrium).Among these elements, the element preferably containing at least one selected the group formed from La, Ce and Y, more preferably containing La and/or Ce.

Mg and Ca is same, makes the vulcanizing compounds system inclusion ballings such as MnS, improves the deformability of steel, and is the element of the raising contributing to machinability.In order to make such effect effectively play, Mg is preferably made to contain more than 0.0002%, preferably containing more than 0.0005%.But even if contain superfluously, its effect is also saturated, can not expect the effect conformed to content, therefore be recommended as less than 0.02%, more be recommended as less than 0.015%, special recommendation is less than 0.01%.

Li and Ca is same, makes the vulcanizing compounds system inclusion ballings such as MnS, can improve the deformability of steel, and in addition, Al system oxide compound eutectic being revealed and innoxious, is the element of the raising contributing to machinability.In order to make such effect effectively play, Li is preferably made to contain more than 0.0002%, more preferably containing more than 0.0005%.But even if contain superfluously, its effect is also saturated, can not expect the effect conformed to content, therefore be recommended as less than 0.02%, more be recommended as less than 0.015%, special recommendation is less than 0.01%.

Pb is being effective element for making in machinability raising.In order to make such effect effectively play, Pb is preferably made to contain more than 0.005%, more preferably containing more than 0.01%.But if contain superfluously, then produce problem roll mark etc. occurring and manufactures, be therefore recommended as less than 0.5%, be more recommended as less than 0.4%, special recommendation is less than 0.3%.

Bi and Pb is same, is being effective element for making in machinability raising.In order to make such effect effectively play, Bi is preferably made to contain more than 0.005%, more preferably containing more than 0.01%.But even if contain superfluously, the effect that machinability improves is also saturated, is therefore recommended as less than 0.5%, is more recommended as less than 0.4%, be particularly recommended as less than 0.3%.

Next, tissue steel plate of the present invention being given to feature is described.

(tissue of steel plate of the present invention)

As mentioned above, the feature of steel plate of the present invention is, based on bainite ferrite-polygonal ferrite-perlite multiphase steel, particularly by the size control of bainite ferrite grain at specified range.

< bainite ferrite: more than 5%, perlite: lower than 20%, surplus: polygonal ferrite >

The tissue of steel plate of the present invention is made up of bainite ferrite, polygonal ferrite and pearlitic heterogeneous structure.Bainite ferrite has improve processibility in cold working, and improves hardness after processing, suppresses the effect of the generation of tension strain trace on the other hand, in order to effectively play these effects, count more than 5% with area occupation ratio, be preferably more than 10%, more preferably more than 15%.In addition, if perlite exists superfluously, then make the plasticity deterioration of steel plate, therefore perlite is in area occupation ratio lower than 20%, is more preferably less than 19%, and more preferably less than 18%, be particularly preferably less than 15%.Surplus is polygonal ferrite.

It should be noted that, in the tissue of steel plate of the present invention, except above-mentioned tissue, also there is cementite phase, its area occupation ratio is at most denier below about 1%, therefore in this manual, bainite ferrite, polygonal ferrite, pearlitic each area occupation ratio are defined as the standardized area occupation ratio of mode becoming 100% according to the total area occupation ratio of this three-phase.

The average crystal grain diameter of bainite ferrite described in <: the scope > of 3 ~ 50 μm

In order to make the processibility of steel plate improve, and meet the surface property after processing, the average crystal grain diameter of the bainite ferrite of formation bainite ferrite tissue needs the scope at 3 ~ 50 μm.If bainite ferrite grain is too tiny, then resistance to deformation becomes too high, and therefore its average crystal grain diameter is more than 3 μm, is preferably more than 4 μm, is more preferably more than 5 μm.On the other hand, if ferrite too coarsening, then the surface property deterioration after processing, the deterioration such as toughness, fatigue characteristic in addition, therefore its average crystal grain diameter is less than 50 μm, is preferably less than 45 μm, is more preferably less than 40 μm.

(measuring method of the area occupation ratio of each phase)

About the area occupation ratio of above-mentioned each phase, carry out nital corrosion to each for examination steel plate, utilize scanning electron microscope (SEM; Multiplying power 1000 times) shooting 5 visuals field, bainite ferrite, polygonal ferrite and pearlitic each ratio can be tried to achieve with enumeration algorithm.

At this, bainite ferrite is defined as the ferrite particles of the shape major axis of the crystal grain existed in bainite (general name of top bainite and bottom bainite) tissue (with reference to Gu Yuanzhong, the understanding of " definition of the bainite structure of iron steel "-present situation-, No. 1st, thermal treatment the 50th volume, Heisei in February, 22, p.22-27), its long-width ratio (ratio of major axis/minor axis) is more than 2.In addition, the shape that polygonal ferrite is defined as crystal grain such as is at the ferrite particles of shaft-like, and its long-width ratio (ratio of major axis/minor axis) is less than 2.

(measuring method of average crystal grain diameter)

About the average crystal grain diameter of above-mentioned bainite ferrite, can measure as follows.That is, the crystal grain diameter of bainite ferrite being present in most skin section, thickness of slab 1/4, this 3 place of thickness of slab central part is respectively measured.About the particle diameter of 1 bainite ferrite particle, be that nital corrosion is carried out, by scanning electron microscope (SEM for the side surface part of the rolling direction respectively located; Multiplying power 1000 times) 5 visuals field are taken to this position, utilize and the center of gravity diameter that image analysis draws is carried out, as average crystal grain diameter to the crystal grain of bainite ferrite.

Then, below explanation is used for obtaining the preferred manufacture method of the invention described above steel plate.

(the preferred manufacture method of steel plate of the present invention)

The manufacture of steel plate of the present invention, as long as the method that the raw steel with mentioned component composition can be configured as the thickness of slab of expectation, carrying out according to any method can.Such as, can carry out in the following way, by condition shown below, there is with converter preparation the molten steel of mentioned component composition, after becoming slab by ingot casting or continuous casting, be rolled into the hot-rolled steel sheet expecting thickness of slab.

[preparation of molten steel]

About the content of the N in molten steel, by adding the raw material containing N compound in molten steel when carrying out melting with converter, and/or the control climate of converter can be become N ₂atmosphere adjusts.

[heating]

Heating before hot rolling is carried out with 1100 ~ 1300 DEG C.Add at this and hanker, in order to not make N compound generate, and make more N solid solution as far as possible, then need the heating condition of high temperature.The preferred lower limit of Heating temperature is 1100 DEG C, preferred lower limit 1150 DEG C.On the other hand, the temperature higher than 1300 DEG C is operationally difficult.

[hot rolling]

Hot rolling is that the mode of more than 880 DEG C is carried out to make finishing temperature.If by finishing temperature too low temperature, ferrite transformation is then made at high temperature to occur, carbide precipitate coarsening in ferrite (general name of bainite ferrite and polygonal ferrite), fatigue strength deterioration, therefore needs to a certain degree above finishing temperature.In order to the particle diameter making austenite grain coarsening increase bainite ferrite to a certain extent, finishing temperature is more preferably more than 900 DEG C.Further, because guarantee temperature difficulty, so the upper limit of finishing temperature is 1000 DEG C.

The thickness of slab of hot-rolled steel sheet of the present invention is 3 ~ 20mm, but in order to make the miniaturization of bainite ferrite crystal grain, its average crystal grain diameter is controlled in set particle size range, not only need to control above-mentioned rolling temperature, also need the final draft making the tandem rolling of finish to gauge to be more than 15%.Usually, the tandem rolling of 5 ~ 7 passages is implemented in finish to gauge, but sets passage planning chart based on the viewpoint controlled of nipping of plate, and final draft is by the end of about 12 ~ 13%.Above-mentioned final draft is preferably more than 16%, is more preferably more than 17%.Above-mentioned final draft, up to 20%, 30%, more can obtain the effect of further for crystal grain miniaturization, but from the view point of roll control, the upper limit is defined as about 30%.

[chilling after hot rolling]

After above-mentioned finish to gauge terminates, within 5s, carry out chilling with the speed of cooling of 20 DEG C/more than s (the first chilling speed), stop chilling with more than 550 DEG C and lower than the temperature (chilling stopping temperature) of 650 DEG C.This is the bainite ferrite-polygonal ferrite-perlite heterogeneous structure in order to obtain set phase fraction.When speed of cooling (chilling speed) is lower than 20 DEG C/s, pearlitic transformation is promoted, or when chilling stops temperature lower than 550 DEG C, bainitic transformation is suppressed, bainite ferrite-polygonal ferrite-the perlitic steel obtaining set phase fraction all becomes difficulty, the surface quality deterioration after cold-workability, processing.On the other hand, if chilling stops temperature reaching more than 650 DEG C, then the carbide precipitate coarsening in ferrite, fatigue strength deterioration.Chilling stops temperature being preferably 560 ~ 640 DEG C, is more preferably 580 ~ 620 DEG C.

[slow cooling after chilling stopping]

After above-mentioned chilling stops, by let cool or air cooling with the speed of cooling of 10 DEG C/below s (slow cooling speed) slow cooling 5 ~ 20s.The formation of polygonal ferrite is carried out fully thus, and makes the carbide precipitate moderately miniaturization in ferrite.Speed of cooling higher than 10 DEG C/s or slow cooling time lower than 5s time, the formation volume of polygonal ferrite is not enough.On the other hand, if the slow cooling time is more than 20s, then the non-coarsening of carbide precipitate, fatigue strength deterioration.

[chilling after slow cooling, batch]

After above-mentioned slow cooling, again with the speed of cooling of 20 DEG C/more than s (the second chilling speed) chilling, batch with 500 ~ 600 DEG C.This is in order to the tissue by forming bainite ferrite+ferrite body, guarantees cold-workability.Speed of cooling (the second chilling speed) lower than 20 DEG C/s or coiling temperature higher than 600 DEG C time, perlite is formed in a large number and cold-workability is deteriorated, on the other hand, during lower than 500 DEG C, the formation volume of bainite ferrite is not enough and surface quality after processing is deteriorated.

Below, illustrate in greater detail the present invention by embodiment, but following embodiment does not limit character of the present invention, in the scope that can meet the forward and backward purport stated, also suitably can change enforcement, these are all included in technical scope of the present invention.

[embodiment]

By the steel that the one-tenth described in the following table 1 of vacuum melting method melting is grouped into, be cast as the steel ingot of thick 120mm, with the condition shown in following table 2, hot rolling is implemented to it, make hot-rolled steel sheet.Further, in any one test, the speed of cooling terminating the stopping of rear chilling to finish to gauge is 20 DEG C/more than s, and the cooling conditions after chilling stops is, with the speed of cooling slow cooling 5 ~ 20s of 10 DEG C/below s.

For the hot-rolled steel sheet so obtained, by each measuring method illustrated in above-mentioned [embodiment], try to achieve the area occupation ratio of each phase and the average crystal grain diameter of bainite ferrite organized in solid solution N amount, steel plate.

In addition, for above-mentioned hot-rolled steel sheet, evaluate the surface quality after cold-workability and processing and hardness in such a way.

(evaluation of cold-workability)

The work hardening amount that cold-workability defines with the difference of the hardness by the test portion central part before and after processing is evaluated.Therefore, first for above-mentioned hot-rolled steel sheet, use processing reproduction test device (Fuji's electric wave work machine (strain) system, ThermecmasterZ), in processing temperature: room temperature, draft: carry out processing experiment under the condition of 70%, rate of straining: 10/s.The shape of processing experiment sheet, according to thickness of slab, is set to φ 8mm × 12mm, φ 6mm × 10mm, φ 4mm × 6mm etc. to make the ratio of diameter/height substantially certain.Measurement of hardness uses Vickers pyramid hardness testing machine, locate 1/4 (position of the middle of center and periphery) in circular diameter direction of the compression direction central part being set to processing experiment sheet, at load: 500g, mensuration number of times: under the condition of 5 times, Vickers' hardness (Hv) is measured respectively for the processing experiment sheet before and after above-mentioned processing experiment, using respective mean value as the front hardness of processing and the rear hardness of processing.

And, as mentioned above, evaluate cold-workability with the work hardening amount defined by " after processing the front hardness of hardness-processing ".Work hardening amount larger expression processibility is better, using more than 80Hv as qualified.

(evaluation of the hardness after processing)

In addition, as the evaluation of the hardness after processing, evaluate with hardness after the above-mentioned processing measured after above-mentioned processing experiment, the conduct of more than 250Hv is qualified.

(evaluation of the surface quality after processing)

In addition, the presence or absence that the surface quality after processing produces with the SS trace after tension test is evaluated.For this reason, extraction is No. 5 test films (25mm × 50mm × [plate surface ~ thickness of slab center (considering the capacity adjusting thickness of slab of tensile testing machine)]) of the JISZ2201 of right angle orientation relative to pressing direction, based on JISZ2241 (1980) (Tensile Testing Method of Metallic Materials), under the condition that room temperature 20 DEG C, initial stage pinblock speed 600mm/ divide, carry out tension test to dependent variable 15%.Then, for the generation with or without SS trace on the surface of the test film after tension test, impose whetslate polishing on the surface of test film and make on the basis of its distinctness, carry out visual valuation.The conduct not producing SS trace is qualified, and the conduct producing SS trace is defective.

These measurement results are presented in following table 3.

[table 1]

(-: is without interpolation, underscore: scope of the present invention is outer)

[table 2]

(underscore=scope of the present invention is outer, *=recommended range is outer)

[table 3]

(underscore=scope of the present invention is outer, *=recommended range is outer,

BF=bainite ferrite, PF=polygonal ferrite, P=perlite,

One: owing to breaking to set draft in processing experiment undetermined,

Invention steel: " not producing SS trace ", " processing after hardness >=250Hv " and " work hardening amount >=80Hv ",

Comparative steel: the situation not meeting the condition of foregoing invention steel)

As shown in table 3, steel No.1 ~ 3,7 ~ 14,25 ~ 28 all use and meet the steel grade that one-tenth of the present invention is grouped into the important document of regulation, with the hot-rolled condition manufacture recommended, its result can confirm, it is the invention steel of the important document abundance of organization prescribed of the present invention, after processing rear surface characteristic, processing, hardness and work hardening amount all meet criterion of acceptability, both work in-processes can be obtained and demonstrate good cold-workability, demonstrate again the hot-rolled steel sheet of set surface quality and hardness (intensity) after processing.

Relative to this, steel No.4 ~ 6,15 ~ 24,29 not to meet among important document that one-tenth given to this invention is grouped into and organizes at least any one comparative steel, not meet after processing rear surface characteristic, processing at least some criterion of acceptability among hardness and work hardening amount.

Such as, although steel No.4 meets into the important document be grouped into, Heating temperature before hot rolling departs from recommended range and too low, solid solution N quantity not sufficient, difference of hardness after processing.

In addition, although steel No.5 meets into the important document be grouped into, thickness of slab after hot rolling departs from specialized range and excessive, and bainite ferrite is not enough, on the other hand more coarsening, hardness, work hardening amount inequality after processing.

In addition, although steel No.6 meets into the important document be grouped into, final draft during hot rolling departs from recommended range and too small, and bainite ferrite is not enough, on the other hand coarsening, hardness, work hardening amount inequality after processing.

In addition, steel No.15 (steel grade j), although hot-rolled condition is in recommended range, N content is too low, hardness, work hardening amount inequality after processing.

On the other hand, steel No.16 (steel grade k), although hot-rolled condition is in recommended range, N content is too high, and not only cold-workability is poor, and processing rear surface characteristic is also poor.

In addition, steel No.17 (steel grade l), although hot-rolled condition is in recommended range, C content is too high, and does not meet the important document of 10C+N≤3.0, and perlite is formed superfluously, and not only cold-workability is poor, and processing rear surface characteristic is also poor.

In addition, steel No.18 (steel grade m), although hot-rolled condition is in recommended range, Si too high levels, at least cold-workability is poor.

In addition, steel No.19 (steel grade n), although hot-rolled condition is in recommended range, Mn content is too low, hardness, work hardening amount inequality after processing.

On the other hand, steel No.20 (steel grade o), although hot-rolled condition is in recommended range, Mn too high levels, at least cold-workability is poor.

In addition, steel No.21 (steel grade p), although hot-rolled condition is in recommended range, P too high levels, at least cold-workability is poor.

In addition, steel No.22 (steel grade q), although hot-rolled condition is in recommended range, S too high levels, at least cold-workability is poor.

In addition, steel No.23 (steel grade r), although hot-rolled condition is in recommended range, Al content is too low, and at least cold-workability is poor.

On the other hand, steel No.24 (steel grade s), although the hot-rolled condition beyond final draft is in recommended range, Al too high levels, at least cold-workability is poor.

On the other hand, steel No.29 (steel grade x), although hot-rolled condition is in recommended range, do not meet the important document of 10C+N≤3.0, not only cold-workability is poor, and processing rear surface characteristic is also poor.

By confirming suitability of the present invention above.

In detail and describe the present invention with reference to specific embodiment, but do not depart from the spirit and scope of the present invention and can in addition various changes and modifications, this is clear and definite for a person skilled in the art.

The Japanese patent application (Japanese Patent Application 2013-183091) that the application applied for based on September 4th, 2013, its content is incorporated herein as reference.

Utilizability in industry

Hot-rolled steel sheet of the present invention is useful in the various parts (drive disk assembly, the shell etc. such as gear) of such as automobile, can realize lightweight and high strength.

Claims

1. a hot-rolled steel sheet for cold-workability and the surface property after processing and hardness excellence, is characterized in that,

Thickness of slab is 3 ~ 20mm,

Become to be grouped into and be in mass %,

Below C:0.3% and containing 0%,

Below Si:0.5% and containing 0%,

Mn：0.2～1％、

Below P:0.05% and containing 0%,

Below S:0.05% and containing 0%,

Al：0.01～0.1％、

N：0.008～0.025％、

Surplus is made up of iron and inevitable impurity,