CN101903547B - Steel sheets and process for manufacturing the same - Google Patents
Steel sheets and process for manufacturing the same Download PDFInfo
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- CN101903547B CN101903547B CN2008801214570A CN200880121457A CN101903547B CN 101903547 B CN101903547 B CN 101903547B CN 2008801214570 A CN2008801214570 A CN 2008801214570A CN 200880121457 A CN200880121457 A CN 200880121457A CN 101903547 B CN101903547 B CN 101903547B
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
Abstract
Provided are steel sheets which are soft and have excellent workability and excellent hardenability and a process for manufacturing the same. A steel sheet characterized by having a composition which contains by mass C: 0.3 to 0.7%, Si: 0.1% or below, Mn: 0.20% or below, P: 0.01% or below, S: 0.01% or below, Al: 0.05% or below, and N: 0.0050% or below with the balance being Fe and unavoidable impurities and a structure which comprises ferrite, graphite and cementite and in which the total volume fraction of ferrite, graphite and cementite accounts for at least 95% of the whole structure and the ratio of the volume of graphite to the total volume of graphite and cementite (degree of graphitization) is 5% or above, wherein the mean particle diameter of graphite and cementite is 5[mu]m or below; and a steel sheet characterized by having the above composition and the above structure, wherein the sum of the volume fractions of graphite and cementite present in ferrite grains accounts for 15% or below of the total of graphite and cementite.
Description
Technical field
The present invention relates to be fit to steel plate, particularly processibility and the good steel plate and the method for manufacture thereof of hardening capacity of purposes such as trolley part.
Background technology
Instrument or trolley part employed steel plates such as (gear, variators) is many after being processed into predetermined shape, and thermal treatments such as enforcement tempering are used again.This steel plate requires to have good processibility, to be processed into the shape of various complicacies.In recent years; Strong request reduces the manufacturing cost of this parts; Therefore developing to omit manufacturing procedure or to change working method is the processing technology of purpose; For example can drive the heavy wall processing of base part through the automobile that has used high carbon steel sheet, realize significantly shortening the processing technology repeatedly of operation, and part technology practicability.As far as employed steel plates such as above-mentioned trolley parts, strict day by day along with what processibility was required, need softer and have the steel plate of high ductibility.For example under the situation of processing, need lower yielding stress through cold forging.And, after stamping-out processing, carry out needing good stretch flangeability under the situation of reaming processing (burring).
In order to satisfy above-mentioned requirements, and send as an envoy to C greying in the steel of research realizes the technology of the raising of processibility.For example; Patent documentation 1 discloses processibility, toughness, good steel plate that is suitable for the cultivator pawl and the method for manufacture thereof of hardening capacity; This steel plate have contain below C:0.40～0.80%, Si:0.20%～2.00%, Mn:0.20～1.50%, Al:0.001～0.150%, P:0.018% in mass, below the S:0.010%, below the N:0.0050%, surplus is made up of what Fe and unavoidable impurities constituted; Have the tissue that is the main body mutually with ferritic phase and graphite, and have TS≤60kgf/mm
2Soft material.In addition, patent documentation 2 discloses the method for manufacture of the intermediate carbon steel sheet of excellent processability, it is characterized in that, with hot-rolled steel sheet at Ac
1～Ac
3Keep after 0.1～10 hour in the TR of point; Speed of cooling with 20～100 ℃/hour is cooled to normal temperature; Then in 650 ℃～750 ℃ TR, carry out closed annealing; Make the above greying of 50 area % of the cementite in the steel thus; Above-mentioned steel plate contains C:0.10～0.45%, Si:0.05%～1.00%, Mn:0.05～0.50%, Nb:0.005～0.1%, Al:0.01～1.00%, N:0.002～0.010%, B:3～50ppm, Ca:0.001～0.01% and Ni:0～2.00% in quality %, and surplus is made up of Fe and unavoidable impurities, below the P:0.012% in the impurity, below the S:0.008%.And; Patent documentation 3 discloses good high-carbon tole and the method for manufacture thereof of processibility; Have following chemical constitution and the tissue that comprises ferritic, graphite and cementite; Above-mentioned chemical constitution is: contain C:0.20～1.00%, Si in quality %: greater than 0.20% and below 1.20%, Mn:0.05～0.50%, N:0.005～0.015%, B:0.2 * N%～0.8 * N% and Al: less than 0.05%; And satisfy the amount of 1.0 * (N-B) %～5.0 * (N-B) %, surplus is made up of Fe and unavoidable impurities, below the P:0.020% as impurity, below the S:0.010%.
Patent documentation 1: japanese kokai publication hei 1-025946 communique
Patent documentation 2: japanese kokai publication hei 7-258743 communique
Patent documentation 3: japanese kokai publication hei 4-202744 communique
Summary of the invention
All the time, improve processibility, as records such as patent documentation 1,3, need to add a large amount of Si in order to make the C greying in the steel.But, owing to add Si, so the hardization of ferritic own, becoming is difficult to obtain good processibility.In addition, as patent documentation 2, develop the composition system that has added B and Nb through formation; And under predetermined condition, carry out 2 times and anneal; Even may not heavy addition Si, also can realize the technology of greying and high ductibility, can cause cost to rise but carry out 2 annealing.Here, patent documentation 2 is the graphited technology more than 50% that make the cementite in the steel, is grouped into as the one-tenth of the disclosed steel of embodiment of patent documentation 2, and Si content is many, reaches 0.20% the amount that surpasses.In addition; The steel plate of record is soft in the patent documentation 1～3, though the extension characteristics in bendability and the tension test is good, when the quench treatment of steel plate; Occur the not fully fusion of graphite, cementite sometimes according to different heating conditions, the situation of off-temper takes place.In addition, though the steel plate of record be soft in the patent documentation 1～3, there is the problem that the stretch flangeability of the bore expanding workability index after processing as stamping-out may not be good.
The objective of the invention is to, provide soft, the processibility that tool is good, and have good hardening capacity, have the steel plate and the method for manufacture thereof of the excellent processability of good stretch flangeability.
The inventor has carried out wholwe-hearted research to above-mentioned prior art problems, and its result finds, in the high carbon steel; Even it is very low at Si content; Particularly be under the situation below 0.1%, through the distribution of control graphite and cementite, though the greying ratio is improved; But still can access good processibility, and then can guarantee good hardening capacity and stretch flangeability.That is, carried out wholwe-hearted research for the influence of intensity, hardening capacity and the stretch flangeability of the steel plate that contains C:0.3～0.7 quality %, consequently draw following opinion taeniae telarum.
(1) for softization; Formation comprises the tissue of ferritic, graphite and cementite; And the summation that makes ferritic, graphite and cementite shared volume fraction in whole tissue is more than 95%, and graphite shared volume in graphite and cementite are overall is that method more than 5% is effective.
(2) in order to improve hardening capacity, the median size that also need make graphite and cementite is below the 5 μ m.
(3) in order to control the particle diameter of graphite and cementite, the cooling conditions after the hot rolling is very important.
(4) in order to improve stretch flangeability, also need make the graphite that exists in the ferrite crystal grain and cementite is below 15% in the summation of graphite and cementite shared volume fraction in totally.
(5) in order to control the graphite that exists in the ferrite crystal grain and the volume fraction of cementite, the cooling conditions after the hot rolling is very important.
The present invention is based on above-mentioned opinion and accomplish; A kind of steel plate is provided; It is characterized in that; Have in quality % contain C:0.3～0.7%, below the Si:0.1%, below the Mn:0.20%, below the P:0.01%, below the S:0.01%, below the Al:0.05%, below the N:0.0050% and surplus form by what Fe and unavoidable impurities constituted; Have the tissue that comprises ferritic, graphite and cementite, and the summation of ferritic, graphite and cementite shared volume fraction in whole tissue is more than 95%; Graphite in graphite and cementite are overall shared volume fraction, be that the graphite rate is more than 5%, the median size of graphite and cementite is below the 5 μ m.
Steel plate of the present invention, in quality %, also contain be selected from below the Ni:3.0%, in below the B:0.005%, below the Cu:0.1% at least a kind.
Steel plate of the present invention can be made through following method; The steel that will have above-mentioned composition hot rolling and process hot-rolled sheet under 800～950 ℃ finishing temperature; Be cooled to after cooling below 500 ℃ stops temperature with the hot-rolled sheet of the above average cooling rate of 50 ℃/s after said hot rolling; Under the coiling temperature below 450 ℃, batch, and the hot-rolled sheet after said the batching is annealed under the annealing temperature below 720 ℃.
The present invention provides a kind of steel plate; It is characterized in that; Have in quality % contain C:0.3～0.7%, below the Si:0.1%, Mn: less than 0.15%, below the P:0.01%, below the S:0.01%, below the Al:0.05%, below the N:0.0050% and surplus form by what Fe and unavoidable impurities constituted; Has the tissue that comprises ferritic, graphite and cementite; And; The summation of ferritic, graphite and cementite shared volume fraction in whole tissue is more than 95%, graphite in graphite and cementite are overall shared volume fraction, be that the graphite rate is more than 5%, the graphite that exists in the ferrite crystal grain and cementite are below 15% in the summation of graphite and cementite shared volume fraction in totally.
Steel plate of the present invention, preferably in quality %, also contain be selected from below the Ni:3.0%, in below the B:0.005%, below the Cu:0.1% at least a kind.
Steel plate of the present invention can be made through following method; The steel that will have above-mentioned composition hot rolling and process hot-rolled sheet under 800～950 ℃ finishing temperature; Be cooled to after cooling below 600 ℃ stops temperature with the hot-rolled sheet of the above average cooling rate of 50 ℃/s after said hot rolling; Under the coiling temperature below 550 ℃, batch, and the hot-rolled sheet after said the batching is annealed under the annealing temperature below 720 ℃.
According to the present invention, can make softly, have good processibility, and have the steel plate of good hardening capacity.Particularly, get final product owing to only need control the cooling conditions after composition and the hot rolling, therefore steel plate of the present invention can easily be made with low cost.In addition, steel plate of the present invention is soft, and excellent processability; Therefore be fit to the heavy wall processing that automobile drives base part; Even be applied to the parts of complicated shape, yet need do not process and weld, and then realize that the productivity of trolley part improves and the cost reduction a plurality of parts.And, the off-temper that steel plate of the present invention does not exist when heating through high frequency etc. the not fusion by graphite and cementite to cause yet.
According to the present invention, can make softly, have the steel plate of hardening capacity of the excellent processability of good stretch flangeability.Particularly, get final product owing to only need control the cooling conditions after composition and the hot rolling, therefore steel plate of the present invention can easily be made with low cost.In addition; Steel plate of the present invention is soft; Therefore excellent processabilities such as stretch flangeability are fit to the heavy wall processing that automobile drives base part, even be applied to the parts of complicated shape; Yet need not process and weld, and then realize that the productivity of trolley part improves and the cost reduction a plurality of parts.
Description of drawings
Fig. 1 is the figure of relation of median size d and the Δ Hv of expression cementite and graphite.
Fig. 2 is the figure of relation of volume fraction S and the average lambda of the cementite that exists in the expression ferrite crystal grain and graphite.
Embodiment
Below, the steel plate and the method for manufacture thereof of excellent processability of the present invention is elaborated.And short of specified otherwise is then represented " % " expression " quality % " of the amount of composition.
1) forms
C：0.3～0.7％
C is the element that forms graphite.If C amount is less than 0.3%, the hardness after then can not guaranteeing to quench, if surpass 0.7%, even then the greying steel plate also can hardization, and processibility reduces.Therefore, making the C amount is 0.3～0.7%.
Below the Si:0.1%
If Si amount surpasses 0.1%, ferritic hardization then, processibility reduces.Therefore, making the Si amount is 0.1%, is preferably below 0.05%.
Below the Mn:0.20%
If the Mn amount surpasses 0.20%, then hinder graphite and form, therefore Mn below 0.20%, is preferably below 0.10%.
Below the P:0.01%
P locates segregation at crystal boundary etc. reduces processibility, have in addition make cementite stable, hinder the effect that graphite forms, therefore preferably reduce as far as possible.Therefore, making the P amount is below 0.01%, to be preferably below 0.008%.
Below the S:0.01%
S forms the MnS sulfides reduces processibility, in addition, has the effect that cementite is stable, obstruction graphite forms that makes, therefore preferred reduction as far as possible.Therefore, making the S amount is below 0.01%, to be preferably below 0.007%.
Below the Al:0.05%
Al combines with solid solution N to form AlN, makes the detrimentally affect of solid solution N innoxious, and AlN is promoted the element that graphite forms as nuclear, and wherein, solid solution N has the effect that graphite forms that hinders.
Therefore, preferably making the Al amount is more than 0.003%, but if surpass 0.05%, then the cleanliness factor of steel reduces, and makes the processibility variation, and therefore making the Al amount is below 0.05%, to be preferably below 0.04%.
Below the N:0.0050%
If the N amount surpasses 0.0050%, the effect that makes the cementite stabilization of solid solution N is become significantly, graphite forms and is hindered.Therefore, making the N amount is below 0.0050%, to be preferably below 0.0040%.
Surplus is Fe and unavoidable impurities, but because following reason, it is contained be selected from below the Ni:3.0%, in below the B:0.005%, below the Cu:0.1% at least a kind.
Below the Ni:3.0%
Ni is the element that promotes that graphite forms, and is the element that helps to improve hardening capacity, preferably contain more than 0.1% in order to obtain such effect event, but if the Ni amount surpasses 3.0%, then this effect is saturated.Therefore, making Ni amount is below 3.0%, is preferably 0.1～3.0%, more preferably 0.3～1.0%.
Below the B:0.005%
B combines to form BN with N; Be the useful element that plays a role as the nuclear that graphite forms, and be the element that plays a role effectively aspect the raising hardening capacity, in order to obtain such effect so preferably contain more than 0.0005%; But if the B amount surpasses 0.005%, then this effect is saturated.Therefore, making B amount is below 0.005%, is preferably 0.0005～0.005%, more preferably 0.0010～0.0040%.
Below the Cu:0.1%
Cu is the element that promotes that graphite forms, and is the element that helps to improve hardening capacity, preferably contains more than 0.01% in order to obtain such effect, more preferably contain more than 0.02%, but if the Cu amount surpasses 0.1%, then its effect is saturated.Therefore, making the Cu amount is below 0.1%, more preferably below 0.07%.
2) tissue
In order to realize softization of steel plate; And extension characteristics is improved; Need to form the tissue that comprises ferritic, graphite and cementite; And the summation that makes ferritic, graphite and cementite shared volume fraction in whole tissue is more than 95%, and graphite shared graphite rate in graphite and cementite are overall is more than 5%.At this moment, in the present invention, even the graphite rate be 100%, be the whole greyings of cementite, also obtain same effect, therefore make content as above.If the summation of the volume fraction of ferritic, graphite and cementite is less than 95%, promptly in addition the volume fraction of phase surpasses 5%, and then processibility reduces.In addition, less than 5%, then processibility reduces as if the graphite rate.
Here, the volume fraction of ferritic, graphite and cementite is obtained as follows.After the thickness of slab 1/4 position grinding with the thickness of slab cross section on the rolling direction of steel plate; Corrode with nital; Utilize opticmicroscope under 400 times multiplying power, to observe 5 places in each visual field; Totally 10 visuals field (amounting to 50 places); Image analysis software " Image ProPlus ver.4.0 " through Media Cybernetics society system is carried out image analysis processing to above-mentioned image, obtains the area of ferritic, graphite and cementite, and with ferritic, graphite and cementite in the total inspection area shared ratio (area occupation ratio) as they volume fractions separately.In addition, with the area (Sgr) of graphite the area (Scm) of the area (Sgr) of graphite and cementite with in shared ratio (area occupation ratio) as the volume fraction (graphite rate) of graphite.That is, graphite rate (%) is represented through following formula.
Graphite rate={ Sgr/ (Sgr+Scm) } * 100
If only control the summation or the graphite rate of the volume fraction of ferritic, graphite and cementite, then may not necessarily access good hardening capacity, the hardening capacity when particularly carrying out high-frequency quenching.That is, in the present invention, in order to ensure good hardening capacity, the median size that need make cementite and graphite is below the 5 μ m.More preferably below the 3 μ m.
The inventor has carried out various researchs in order to obtain good hardening capacity.Below show an example of research.Promptly; To have after the steel billet of forming that C:0.55%, Si:0.01%, Mn:0.10%, P:0.003%, S:0.0006%, Al:0.005%, N:0.0018%, Ni:0.50%, B:0.0013%, surplus be made up of Fe and unavoidable impurities is heated to 1150 ℃; Carry out the roughing of 5 passages; Under 880 ℃ finishing temperature, carry out the finish to gauge of 7 passages, process the hot-rolled sheet of thickness of slab 4.0mm, after batching under 430 ℃ the coiling temperature; Pickling is at 720 ℃ of bell-type annealings of implementing 40 hours down.At this moment, in order to change the particle diameter of cementite and graphite, the TR up to coiling temperature after finish to gauge is cooled off, and make average cooling rate air cooling (5 ℃/s)～200 ℃/change in the scope of s.Then, following operation is investigated tissue and hardening capacity.
In addition; With likewise above-mentioned, after the grinding of thickness of slab 1/4 position in cross section that will be parallel, the nital corrosion, under 1500 times multiplying power with rolling direction; Observe through 5 places of sem pair cross-section, 10 visuals field, every place (adding up to 50 visuals field); Use above-mentioned image analysis software, per 2 degree are measured 2 on the periphery of passing cementite or the graphite diameters with the center of gravity of the equivalent ellipsoidal of cementite or graphite (identical with the area of cementite and graphite, and primary moment and the equal ellipse of second moment); Calculating mean value, and then obtain particle diameter separately.Observe 50 visuals field then and calculate the cementite try to achieve and the MV of the particle diameter of graphite, with its median size as cementite and graphite.
Hardening capacity: cutting diameter is the plectane test film of 100mm, uses high-frequency heat treating system, with the frequency of 100kHz the peripheral end of plectane test film is heated to 1000 ℃ after, carry out water-cooled immediately.Then,, measure Vickers' hardness Hv apart from the surfaces externally and internally of the inboard of peripheral end 1.5mm [load: 49N (=5kgf)], obtain the poor Δ Hv of maximum Hv and minimum Hv in the position, 8 place of the circumferential direction of the plectane test film after thermal treatment.If this Δ Hv is below 8, we can say that then hardening capacity is good.
Fig. 1 representes the median size d of cementite and graphite and the relation between Δ Hv.Can know that then Δ Hv reaches below 8, can access good hardening capacity if the median size d of cementite and graphite reaches below the 5 μ m.
Carry out various researchs based on above-mentioned research, the result finds that the inventor is in order to ensure good hardening capacity, and the median size that need make cementite and graphite is below the 5 μ m, more preferably below the 3 μ m.Through like this tissue being stipulated, can access good hardening capacity, and think and the reasons are as follows.That is, this is because if the median size of cementite and graphite reaches below the 5 μ m, and then cementite and graphite almost completely fuse the homogenizing of the hardness after can realizing quenching during ratio-frequency heating.
3) create conditions
Below, preferably creating conditions of steel plate of the present invention is shown.And the method for manufacture of steel plate of the present invention is not limited to following method.
Finishing temperature during hot rolling: 800～950 ℃
Finishing temperature during hot rolling is if be lower than 800 ℃, and then rolling load enlarges markedly, if surpass 950 ℃, the scale that then generates thickens the reduction of pickling property, and has the situation that generates Decarburized layer on the steel plate top layer, and therefore making finishing temperature is 800～950 ℃.
Average cooling rate after the hot rolling: 50 ℃/more than the s
Steel plate after the hot rolling is cooled to following cooling with the above average cooling rate of 50 ℃/s immediately and stops temperature.If average cooling rate less than 50 ℃/s, then causes the growth of ferrite crystal grain easily in the cooling, form big ferrite crystal grain.When annealing afterwards, because graphite or cementite be through forming ferritic crystal boundary, inclusion etc. as nucleus, therefore if ferrite crystal grain is big, then be that the nucleus graphite, the cementite chap that form are big with the crystal boundary, hardening capacity reduces.In addition, if average cooling rate is slow, then generate thick perlite, graphite or cementite form via pearlitic fracture, aggegation, thickization, so graphite or cementite become thick, and hardening capacity reduces.And; If make average cooling rate be 50 ℃/more than the s; The rolling strain that then imports in the austenite through hot rolling remains in the microstructure after phase transformation easily, and dislocation desity is increased, and then when annealing, has above rheme mistake promotes graphite formation as nucleus advantage.From foregoing, make average cooling rate be 50 ℃/more than the s, be preferably 80 ℃/more than the s.The upper limit of average cooling rate does not need special stipulation, but in order to suppress steel plate shape variation, to guarantee the shape of steel plate, preferably make average cooling rate be 200 ℃/below the s.
Refrigerative cooling after the hot rolling stops temperature: below 500 ℃
Need carry out the refrigerative minimum temperature with above-mentioned such speed of cooling; I.e. cooling stops temperature if be higher than 500 ℃, then in the cooling till batch, generates proeutectoid ferrite, and generates thick perlite; Cementite, graphite become thick during annealing after batching; Cause hardening capacity to reduce, therefore making cooling stop temperature is below 500 ℃, is preferably below 470 ℃.The lower limit that cooling stops temperature not needing special stipulation, but in order to ensure the shape of steel plate, it is more than 200 ℃ that preferred cooling stops temperature.
Coiling temperature: 450 ℃
Cooled hot-rolled sheet is batched immediately, at this moment, if coiling temperature is higher than 450 ℃, then generates thick perlite, and cementite, graphite become thick during annealing, and hardening capacity reduces.Therefore, making coiling temperature is below 450 ℃.And in order to obtain the cooling performance after the above-mentioned hot rolling fully, preferred coiling temperature is lower than cooling and stops temperature.In addition, because the easy variation of shape of hot-rolled sheet, therefore preferably making coiling temperature is more than 200 ℃.
Annealing temperature: below 720 ℃
After hot-rolled sheet after batching carried out pickling etc. and remove scaling, implement annealing promoting the balling or the greying of cementite, and then realize softization.At this moment, annealing temperature then generates thick perlite if surpass 720 ℃ in the cooling, cause hardening capacity to reduce, and therefore is below 720 ℃.In addition, if annealing temperature is lower than 600 ℃, then annealing time significantly prolongs, and therefore preferably making annealing temperature is more than 600 ℃.
And though annealing time does not need special qualification, in order to form graphite so be more than 8 hours, in addition, excessive thickization of ferrite crystal grain might cause ductility to reduce, and therefore preferably making annealing time is below 100 hours.
For melting steel of the present invention, can use in converter and the electric furnace any.As above the steel of melting is made into steel billet through ingot casting-split rolling method or continuous casting.Usually, after steel billet heating (reheat), carry out hot rolling.And, under the situation of the steel billet of making through continuous casting, can directly use or carry out directly rolling, directly rolling is to be reduced to purpose and in insulation, to be rolled to suppress temperature.With the steel billet reheat and carry out under the hot rolled situation,, be below 1280 ℃ and preferably make billet heating temperature for fear of the variation of the condition of surface that causes by scale.Hot rolling can be omitted roughing and only carried out finish to gauge.In order to ensure finishing temperature, can in hot rolling, utilize thin slab process furnace heating means such as (sheet bar heater) to heat to being rolled material.As long as can keep of the present invention creating conditions, the then not special restriction of the thickness of slab of hot-rolled sheet, but preferred 1.0～10.0mm.Steel plate after the annealing can carry out surface light as required and prick.Embodiment is shown in the embodiment 1.
If only controlled the summation and the graphite rate of the volume fraction of ferritic, graphite and cementite, then may not necessarily access good stretch flangeability.That is, in the present invention, in order to ensure good stretch flangeability, need make the cementite of existence in the ferrite crystal grain and the TV rate of graphite is below 15%.More preferably below 10%.
The inventor obtains good stretch flangeability to have carried out various researchs.One example of research below is shown.To have after the steel billet of forming that C:0.55%, Si:0.01%, Mn:0.10%, P:0.003%, S:0.0006%, Al:0.005%, N:0.0018%, Ni:0.50%, B:0.0013%, surplus be made up of Fe and unavoidable impurities is heated to 1150 ℃; Carry out the roughing of 5 passages; Under 870 ℃ finishing temperature, carry out the finish to gauge of 7 passages; Process the hot-rolled sheet of thickness of slab 4.0mm; After batching under 520 ℃ the coiling temperature, carry out pickling, at 720 ℃ of bell-type annealings of implementing 40 hours down.At this moment, for the amount and the distribution that change cementite and graphite, the TR up to coiling temperature after finish to gauge is cooled off, and make average cooling rate air cooling (5 ℃/s)～200 ℃/change in the scope of s.Then, following operation is investigated tissue and stretch flangeability.
In addition; With above-mentioned likewise to thickness of slab 1/4 position in the cross section parallel with rolling direction grind, nital corrosion; Through 5 places of opticmicroscope on observation cross section under 400 times the multiplying power, 10 visuals field, every place (amounting to 50 visuals field) used above-mentioned image analysis software then; The cementite and the graphite that exist in the cementite that exists on the identification ferritic crystal boundary and graphite and the ferrite crystal grain, the cementite that exists on the mensuration ferritic crystal boundary and the occupied area S of graphite
On, and ferrite crystal grain in the cementite that exists and the occupied area S of graphite
In, obtain the cementite that exists in the ferrite crystal grain and the area occupation ratio of graphite by following formula, and with above-mentioned area occupation ratio as the cementite that exists in the ferrite crystal grain and graphite shared volume fraction S (%) in cementite and graphite are overall.That is, S (%) representes through following formula.
S＝{S
in/(S
on+S
in)}×100
And; Here; Have cementite crystal grain or graphite grains that part is present in the part on the ferritic crystal boundary; The entire area of each cementite crystal grain or graphite grains is measured as the cementite crystal grain that exists on the ferritic crystal boundary or the occupied area of graphite grains, in addition, measured as the cementite crystal grain that exists in the ferrite crystal grain or the occupied area of graphite grains not having the cementite that is present in the part on the ferritic crystal boundary or the area of graphite grains.
Stretch flangeability: cut the test film that drifiting test uses (100 * 100mm), use punch diameter 10mm, (spacing: punching tool thickness of slab 20%) carries out punching at the center of test film to punch die diameter 11.6mm.Then; Utilize cylinder flat-bottom punch (diameter 50mm Φ, shoulder R8mm) pushing to dash good hole and carry out reaming processing; Aperture d (mm) when the mensuration bore edges produces the crackle that connects thickness of slab, and according to computes hole expansibility λ (%), implement 6 identical tests and obtain average lambda (%).
λ＝100×(d-10)/10
Fig. 2 is the figure of relation of volume fraction S and the average lambda of the cementite that exists in the expression ferritic and graphite.If the cementite that exists in the ferrite crystal grain and the volume fraction S of graphite reach below 15%, then can access the average lambda more than 60%, and then can access good stretch flangeability.
The result who carries out various researchs based on above-mentioned research is that the inventor finds that in order to ensure good stretch flangeability need make the cementite of existence in the ferritic and the TV rate of graphite is below 15%, more preferably below 10%.To obtain the reason of good stretch flangeability following through as above tissue being carried out regulation.Promptly; If cementite or graphite are present in the ferrite crystal grain in a large number; Then produce fine crackle at cementite or graphite and ferritic crystal boundary easily during punching processing, the initial stage from drifiting test begins to propagate, combine, and then causes the crackle of perforation thickness of slab easily.On the other hand; The velocity of diffusion of carbon is fast in the ferritic crystal boundary; Obtain stronger promotion with thickization of ferrite crystal grain internal-phase ratio aggegation degree; Cementite on the ferritic crystal boundary or graphite are than cementite in the ferrite crystal grain or easy thickization of graphite, and the interval of each cementite crystal grain, graphite grains enlarges easily.Therefore, compare cementite on the ferritic crystal boundary or the graphite crack propagation that slows down with cementite or graphite in the ferrite crystal grain.
3) create conditions
Preferably creating conditions of steel plate of the present invention below is shown.And the method for manufacture of steel plate of the present invention does not receive the qualification of following content.
Finishing temperature during hot rolling: 800～950 ℃
If the finishing temperature during hot rolling is lower than 800 ℃, then rolling load enlarges markedly, if be higher than 950 ℃, and the scale thickening pickling property reduction that then generates, and have the situation that produces Decarburized layer on the steel plate top layer, therefore making finishing temperature is 800～950 ℃.
Average cooling rate after the hot rolling: 50 ℃/more than the s
If with the steel plate of the above average cooling rate of 50 ℃/s after to hot rolling cool off immediately up to after the cooling stated stop temperature, then the generation of proeutectoid ferrite is suppressed, ferritic and cementite are separated out imperceptibly.Therefore, when annealing after batching, C becomes and spreads at the ferritic crystal boundary easily, and aggegation/thickization and the greying of the cementite on the ferritic crystal boundary are promoted that cementite in the ferrite crystal grain or graphite reduce, and stretch flangeability improves.In addition, import rolling strain in the austenite through hot rolling and become and remain in easily in the microstructure after phase transformation, cause that dislocation desity increases, processibility improves.When consequently annealing be that the graphite of nucleus forms and becomes easily with the dislocation, softization takes place, processibility improves.From above content, make average cooling rate be 50 ℃/more than the s, be preferably 80 ℃/more than the s.The upper limit of average cooling rate does not need special stipulation, but for the shape variation that suppresses steel plate, the shape of guaranteeing steel plate, preferably be 200 ℃/below the s.
Refrigerative cooling after the hot rolling stops temperature: below 600 ℃
Cool off needed minimum temperature according to above-mentioned speed of cooling; I.e. cooling stops temperature if be higher than 600 ℃, then generates proeutectoid ferrite in the cooling till batching, and generates perlite; Cementite that exists in the ferrite crystal grain during annealing after batching or graphite increase; Cause stretch flangeability to reduce, therefore be below 600 ℃, be preferably below 550 ℃.The lower limit that cooling stops temperature not needing special stipulation, but in order to ensure the shape of steel plate, preferably is more than 200 ℃.
Coiling temperature: below 550 ℃
Cooled hot-rolled sheet is batched immediately, and at this moment, coiling temperature then generates perlite if be higher than 550 ℃, and cementite that exists in the ferrite crystal grain during annealing or graphite increase, and stretch flangeability reduces.Therefore, making coiling temperature is below 550 ℃.And,, preferably make coiling temperature be lower than cooling and stop temperature in order fully to obtain the cooling performance after the above-mentioned hot rolling.In addition, because therefore the easy variation of shape of hot-rolled sheet is more than 200 ℃ guaranteeing that the steel plate vpg connection preferably makes coiling temperature, more preferably be higher than 450 ℃.
Annealing temperature: below 720 ℃
After hot-rolled sheet after batching implemented pickling etc. and remove scaling, implement annealing promoting the balling and the greying of cementite, and then realize softization.At this moment,, then generate perlite in the cooling, cause stretch flangeability to reduce, therefore be below 720 ℃ if annealing temperature is higher than 720 ℃.In addition, if annealing temperature is lower than 600 ℃, the cementite or the graphite that then exist in the ferrite crystal grain increase, and have the tendency of stretch flangeability variation, and therefore preferably making annealing temperature is more than 600 ℃.
And annealing time does not need special qualification, but in order to form graphite and to reduce cementite or the graphite in the ferrite crystal grain; And preferably be more than 8 hours, in addition, excessive thickization of ferrite crystal grain; Possibly cause ductility to reduce, therefore preferably be below 100 hours.
For melting steel of the present invention, can use in converter and the electric furnace any.As above the steel of melting is made into steel billet through ingot casting-split rolling method or continuous casting.Usually, after steel billet heating (reheat), carry out hot rolling.And, under the situation of the steel billet of making through continuous casting, can directly use or carry out directly rolling, directly rolling is to be reduced to purpose and in insulation, to be rolled to suppress temperature.With the steel billet reheat and carry out under the hot rolled situation,, be below 1280 ℃ and preferably make billet heating temperature for fear of the condition of surface variation that causes by scale.Hot rolling can be omitted roughing and only carried out finish to gauge.In order to ensure finishing temperature, can in hot rolling, utilize heating means such as thin slab process furnace to be rolled material heating.As long as can keep of the present invention creating conditions, the then not special restriction of the thickness of slab of hot-rolled sheet, but preferred 1.0～10.0mm.After utilizing pickling or sandblast etc. to remove the scale on surface, hot-rolled sheet is carried out hot-rolled sheet annealing.Steel plate after the annealing can carry out surface light as required and prick.Embodiment is shown in the embodiment 2.
Embodiment
Embodiment 1
The steel billet of the steel No.A～S that forms shown in the table 1 is heated to 1250 ℃, after carrying out hot rolling, pickling under the condition shown in the table 2, likewise under condition shown in the table 2, anneals, processing thickness of slab is steel plate No.1～22 of 4.0mm.Then, utilize aforesaid method, the Δ Hv that obtains the median size of graphite rate, cementite and graphite and be used to estimate hardening capacity.In addition, cut No. 5 tension test sheets of JIS, implement tension test, obtain yielding stress YP, tensile strength TS and unit elongation El along rolling direction.
The result is shown in table 3.But the steel plate of knowledge capital invention example is low YP, low TS, high El, low Δ Hv, soft and excellent processability, and also hardening capacity is also good.And confirmed the present invention's example steel plate organize as shown in table 3ly, mainly constitute by ferritic, cementite and graphite, their TV rate is more than 95%.
Table 2
Table 3
*: F ferritic, G graphite, C cementite
Embodiment 2
The steel billet of the steel No.AA～AS that forms shown in the table 4 is heated to 1250 ℃, after carrying out hot rolling, pickling under the condition shown in the table 5, likewise under condition shown in the table 5, anneals, processing thickness of slab is steel plate No.101～122 of 4.0mm.Then, utilize aforesaid method, obtain the cementite that exists in the graphite rate, ferrite crystal grain and graphite shared volume fraction S and in cementite and graphite are overall as the average lambda of the index of stretch flangeability.In addition, cut No. 5 tension test sheets of JIS, implement tension test, obtain yielding stress YP, tensile strength TS and unit elongation El along rolling direction.And, each sample is carried out 2 identical tests and obtains MV, the characteristic value of this MV as this steel plate.
The result is shown in table 6.But the steel plate of knowledge capital invention example is low YP, low TS, high El, high λ, and is soft and comprise the excellent processability of stretch flangeability.And confirmed the present invention's example steel plate organize as shown in table 6ly, mainly constitute by ferritic, cementite and graphite, their TV rate is more than 95%.
Table 5
Table 6
*: F ferritic, G graphite, C cementite
Claims (3)
1. steel plate; It is characterized in that; Have in quality % contain C:0.3～0.7%, below the Si:0.1%, Mn: less than 0.15%, below the P:0.01%, below the S:0.01%, below the Al:0.05%, below the N:0.0050% and surplus form by what Fe and unavoidable impurities constituted; Has the tissue that comprises ferritic, graphite and cementite; And; The summation of ferritic, graphite and cementite shared volume fraction in whole tissue is more than 95%, graphite in graphite and cementite are overall shared volume fraction, be that the graphite rate is more than 5%, the graphite that exists in the ferrite crystal grain and cementite are below 15% in the summation of graphite and cementite shared volume fraction in totally.
2. steel plate as claimed in claim 1 is characterized in that, in the composition that is had, in quality %, also contain be selected from below the Ni:3.0%, in below the B:0.005%, below the Cu:0.1% at least a kind.
3. the method for manufacture of a steel plate; It is characterized in that; The steel that will have claim 1 or 2 described compositions hot rolling and process hot-rolled sheet under 800～950 ℃ finishing temperature; Be cooled to after cooling below 600 ℃ stops temperature with the hot-rolled sheet of the above average cooling rate of 50 ℃/s after, under the coiling temperature below 550 ℃, batch, and the hot-rolled sheet after said the batching is annealed under the annealing temperature below 720 ℃ said hot rolling.
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| JP2007326868A JP5157416B2 (en) | 2007-12-19 | 2007-12-19 | Steel sheet and manufacturing method thereof |
| JP2007326869A JP5157417B2 (en) | 2007-12-19 | 2007-12-19 | Steel sheet and manufacturing method thereof |
| JP2007-326868 | 2007-12-19 | ||
| JP2007-326869 | 2007-12-19 | ||
| PCT/JP2008/071597 WO2009078261A1 (en) | 2007-12-19 | 2008-11-20 | Steel sheets and process for manufacturing the same |
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| KR (2) | KR20100076073A (en) |
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| CN101906597A (en) * | 2010-08-14 | 2010-12-08 | 武汉钢铁(集团)公司 | Environment-friendly high-performance graphitized free cutting steel |
| JP5594226B2 (en) * | 2011-05-18 | 2014-09-24 | Ｊｆｅスチール株式会社 | High carbon steel sheet and method for producing the same |
| JP5338873B2 (en) * | 2011-08-05 | 2013-11-13 | Ｊｆｅスチール株式会社 | High-strength hot-dip galvanized steel sheet excellent in workability with a tensile strength of 440 MPa or more and its production method |
| EP2801635B1 (en) * | 2012-01-05 | 2017-09-20 | JFE Steel Corporation | High carbon hot-rolled steel sheet with excellent hardenability and minimal in-plane anisotropy, and method for producing same |
| JP6479538B2 (en) * | 2015-03-31 | 2019-03-06 | 株式会社神戸製鋼所 | Steel wire for machine structural parts |
| CN106048179B (en) * | 2016-07-15 | 2017-09-15 | 北京科技大学 | A kind of preparation method of graphitization hot rolled steel plate |
| CN113862609B (en) * | 2021-09-03 | 2022-05-27 | 北京科技大学 | Method for improving wear resistance and friction reduction of medium-low carbon steel workpiece by utilizing carburization and surface graphitization |
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| US5310432A (en) * | 1992-06-30 | 1994-05-10 | Sumitomo Metal Industries, Ltd. | Toothed wheel for use in automobiles and its manufacturing method |
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| JPH0830241B2 (en) | 1987-07-20 | 1996-03-27 | 川崎製鉄株式会社 | Steel sheet having excellent workability and toughness and good hardenability, and a method for producing the same |
| JPH02107742A (en) * | 1988-10-14 | 1990-04-19 | Kawasaki Steel Corp | Steel stock excellent in workability and hardenability |
| JPH04124216A (en) * | 1990-09-12 | 1992-04-24 | Sumitomo Metal Ind Ltd | Production of high carbon steel sheet having superior formability |
| JPH04202744A (en) | 1990-11-30 | 1992-07-23 | Sumitomo Metal Ind Ltd | High carbon thin steel sheet good in formability and its manufacture |
| JPH0913142A (en) * | 1991-01-17 | 1997-01-14 | Kawasaki Steel Corp | Graphite precipitated hot rolled steel sheet excellent in bending workability and heat treatability and its production |
| JP3241748B2 (en) * | 1991-04-11 | 2001-12-25 | 川崎製鉄株式会社 | Steel material excellent in workability and hardenability and its manufacturing method |
| JPH07258743A (en) | 1994-03-18 | 1995-10-09 | Sumitomo Metal Ind Ltd | Production of medium carbon steel sheet excellent in workability |
| JPH08246051A (en) * | 1995-03-07 | 1996-09-24 | Sumitomo Metal Ind Ltd | Production of medium carbon steel sheet excellent in workability |
| JPH08291362A (en) * | 1995-04-21 | 1996-11-05 | Sumitomo Metal Ind Ltd | Steel material excellent in cold workability |
| JP3848444B2 (en) * | 1997-09-08 | 2006-11-22 | 日新製鋼株式会社 | Medium and high carbon steel plates with excellent local ductility and hardenability |
| JP3879459B2 (en) * | 2001-08-31 | 2007-02-14 | Ｊｆｅスチール株式会社 | Manufacturing method of high hardenability high carbon hot rolled steel sheet |
| US20090126836A1 (en) * | 2005-05-29 | 2009-05-21 | Nobusuke Kariya | High Carbon Hot Rolled Steel Sheet and method for manufacturing same |
| JP5011846B2 (en) * | 2005-06-29 | 2012-08-29 | Ｊｆｅスチール株式会社 | High carbon hot rolled steel sheet and manufacturing method thereof |
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| US5310432A (en) * | 1992-06-30 | 1994-05-10 | Sumitomo Metal Industries, Ltd. | Toothed wheel for use in automobiles and its manufacturing method |
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| JP特开2002-121647A 2002.04.26 |
| JP特开平8-260049A 1996.10.08 |
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| EP2246450A1 (en) | 2010-11-03 |
| WO2009078261A1 (en) | 2009-06-25 |
| KR20100076073A (en) | 2010-07-05 |
| CN101903547A (en) | 2010-12-01 |
| KR20130035276A (en) | 2013-04-08 |
| EP2246450B1 (en) | 2013-09-04 |
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