CN101568655B - High carbon hot-rolled steel sheet and method for production thereof - Google Patents

High carbon hot-rolled steel sheet and method for production thereof Download PDF

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CN101568655B
CN101568655B CN2007800482110A CN200780048211A CN101568655B CN 101568655 B CN101568655 B CN 101568655B CN 2007800482110 A CN2007800482110 A CN 2007800482110A CN 200780048211 A CN200780048211 A CN 200780048211A CN 101568655 B CN101568655 B CN 101568655B
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hot
steel sheet
rolled steel
temperature
crystal grain
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CN101568655A (en
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仮屋房亮
濑户一洋
中村展之
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JFE Steel Corp
JFE Engineering Corp
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NKK Corp
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • 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
    • 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
    • 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
    • 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
    • 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
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese

Abstract

The present invention provides a high-carbon hot-rolled steel sheet having excellent bending properties after stretching and a method of producing the same. A method of producing a high-carbon hot-rolled steel sheet includes a step of hot-rolling steel having a composition containing, in % by mass, 0.2 to 0.7% of C, 2% or less of Si, 2% or less of Mn, 0.03% or less of P, 0.03% or less of S, 0.01%or less of Sol. Al, and 0.01% or less of N at a finishing temperature of (Ar3 transformation point - 20 DEG C) or more to form a hot-rolled steel sheet, a step of cooling the hot-rolled steel sheet to a temperature of 650 DEG C or less at a cooling rate of 60 DEG C/sec to less than 120 DEG C/sec, a step of coiling the hot-rolled steel sheet at a coiling temperature of 600 DEG C or less after cooling, and a step of annealing the hot-rolled steel sheet at an annealing temperature of 640 DEG C to Ac1 transformation point.

Description

High-carbon hot-rolled steel sheet and manufacture method thereof
Technical field
The present invention relates to high-carbon hot-rolled steel sheet, particularly processing after good high-carbon hot-rolled steel sheet and the manufacture method thereof of curved characteristic.
Background technology
The high carbon steel sheet that is used for instrument or trolley part (gear, variator) etc., in order to be processed into various complicated shapes, the user needs good processibility.On the other hand, the requirement that reduces the parts manufacturing cost in recent years strengthens, and carries out the omission of manufacturing procedure, the change of working method.For example,,, developed and can thicken the double-acting processing technology that is shaped and has realized shortening a large amount of operations as the processing technology of the automobile driving system parts that used high carbon steel sheet as putting down in writing in the non-patent literature 1, and by part practicability.Simultaneously, high carbon steel requires to make up stretchings no problemly, draws, a plurality of processing modes such as bulging protruding, crooked, reaming process.Particularly, crackle often occurs, so expect the good curved characteristic after the stretch process at bend owing to add man-hour in the laggard line bend of enforcement stretch process.
So far, in order to improve the processibility of high carbon steel sheet, several technology have been studied.For example, in patent documentation 1, proposed, the high carbon steel of hot rolling regulation chemical ingredients, after carrying out the deoxygenated skin, in the hydrogen atmosphere more than 95 capacity %, after annealing, cool off with the speed of cooling below 100 ℃/hour, thereby make the method for the high-carbon steel band of soft and homogeneity of structure, excellent processability by chemical ingredients predetermined heating speed, soaking time.And, in patent documentation 2, proposed, will be at (Ac 1Transformation temperature+30 ℃) steel plate after rolling under the above finishing temperature is cooled to 20～500 ℃ temperature with 10～100 ℃/second speed of cooling, keeps after 1～10 second, at 500～(Ac 1Transformation temperature+30 ℃) temperature range reheat batches, then as required at 650～(Ac 1Transformation temperature+30 ℃) soaking more than 1 hour down, thus the good high-carbon steel-sheet method of processibility made.In addition, proposed in patent documentation 3, the steel that will contain C and be 0.2～0.7 quality % is at finishing temperature (Ar 3Transformation temperature-20 ℃) after carrying out hot rolling more than, cools off below 650 ℃, batch below 600 ℃ at coiling temperature then, at annealing temperature more than 640 ℃, Ac to stop temperature greater than 120 ℃/second of speed of cooling and cooling 1Anneal below the transformation temperature, thereby make the method for the good high-carbon hot-rolled steel sheet of stretch flange.
Non-patent literature 1:Journal of the JSTP, 44,2003, p.409-413
Patent documentation 1: Japanese kokai publication hei 9-157758 communique
Patent documentation 2: Japanese kokai publication hei 5-9588 communique
Patent documentation 3: TOHKEMY 2003-13145 communique
Summary of the invention
But, though the high-carbon hot-rolled steel sheet of putting down in writing in these prior aries adds the characteristic good in man-hour with single processing modes such as stretching, reamings, but under the situation of implementing the most processing modes of combination such as bending machining after the stretch process, there is the problem that cracks etc.
The objective of the invention is to, good high-carbon hot-rolled steel sheet and the manufacture method thereof of curved characteristic after the stretch process is provided.
The curved characteristic of the inventor after to the stretch process of high-carbon hot-rolled steel sheet furtherd investigate, and found that, Sol.Al amount, cooling conditions, coiling temperature and the annealing temperature after the hot rolling of suitably controlling steel are very important.And find, by will being controlled at by the ferrite particle diameter that the aftermentioned assay method is tried to achieve below the 5.0 μ m, and be that the area occupation ratio of the ferrite crystal grain more than 4.0 is controlled at below 15% with length-to-diameter ratio, can access the good curved characteristic after the stretch process.
The present invention is based on above opinion and finishes, a kind of manufacture method of high-carbon hot-rolled steel sheet is provided, has following operation: will contain C:0.2～0.7%, the steel of composition below the Si:2%, below the Mn:2%, below the P:0.03%, below the S:0.03%, below the Sol.Al:0.01%, below the N:0.01% in quality %, at (Ar 3Transformation temperature-20 ℃) carries out hot rolling under the above finishing temperature, thereby form hot-rolled steel sheet; With described hot-rolled steel sheet to be cooled to temperature below 650 ℃ more than 60 ℃/second and less than 120 ℃/second speed of cooling; Described cooled hot-rolled steel sheet is batched under the coiling temperature below 600 ℃; And with the hot-rolled steel sheet after described the batching more than 640 ℃, Ac 1Anneal under the annealing temperature below the transformation temperature.
In the method for the invention, preferably carry out in the refrigerative operation described, with hot-rolled steel sheet being cooled to temperature below 600 ℃ more than 80 ℃/second and less than 120 ℃/second speed of cooling, and, in described operation of batching, under the temperature below 550 ℃, batch.
The present invention also provides a kind of high-carbon hot-rolled steel sheet, it is hot rolling Spheroidizing Annealing material, have in quality % contain C:0.2～0.7%, below the Si:2%, below the Mn:2%, below the P:0.03%, below the S:0.03%, the composition that Sol.Al:0.01% is following, N:0.01% is following, the ferrite particle diameter is below the 5.0 μ m, and length-to-diameter ratio is that the area occupation ratio of the ferrite crystal grain more than 4.0 is below 15%.
Wherein, the ferrite particle diameter is meant the mean value of the particle diameter that utilizes image analysis that ferrite crystal grain is approximately circle and try to achieve, in addition, length-to-diameter ratio is meant and utilizes image analysis that ferrite crystal grain is approximately oval mean value of trying to achieve (long axis of ellipse)/(oval minor axis).Particularly, grinding is parallel to the thickness of slab cross section of the rolling direction of steel plate, 1/4 position with nital (nitric acid+ethanol) corrosion thickness of slab, utilize scanning electron microscope under 1500 times of multiplying powers little tissue to be observed then, the image analysis software " ImagePro Plus Ver.4.0 " of using Media Cybemetics corporate system (TM) is obtained the length-to-diameter ratio of ferrite particle diameter, ferrite crystal grain by image analysis.And then, for each ferrite crystal grain, obtain length-to-diameter ratio and be the area occupation ratio more than 4.0, with its total area, thereby obtain the area occupation ratio in each visual field divided by the visual field, as length-to-diameter ratio the area occupation ratio of the ferrite crystal grain more than 4.0 with the mean value in 50 visuals field.
The area occupation ratio that is more preferably above-mentioned length-to-diameter ratio and is the ferrite crystal grain more than 4.0 is below 10%.In addition, in the present invention, the composition of steel can also contain at least a kind among the B, the Cr that are selected from following content range, Ni, Mo, Cu, Ti, Nb, W, V, the Zr on the basis of above-mentioned composition.In quality %, below the B:0.005%, below the Cr:.3.5%, below the Ni:3.5, below the Mo:0.7%, below the Cu:0.1%, below the Ti:0.1%, below the Nb:0.1%, W, V, Zr: amount to below 0.1%.
According to the present invention, even after having implemented processing such as stretch process, also can make the good high-carbon hot-rolled steel sheet of curved characteristic.
Description of drawings
Fig. 1 is the figure that the relation of area occupation ratio that length-to-diameter ratio is the ferrite crystal grain more than 4.0 and the curved characteristic after the stretch process is shown.
Embodiment
Below, describe high-carbon hot-rolled steel sheet of the present invention and manufacture method thereof in detail.In addition, as the unit " % " of component content, short of specified otherwise is all represented " quality % ".
The composition of steel
The C amount: carbon is the important element of the hardness after forming carbide and giving quenching.When C amount less than 0.2% the time, can not obtain after the quenching as the full intensity of physical structure with parts.On the other hand, when C amount greater than 0.7% the time, even be below the 5.0 μ m and length-to-diameter ratio is 15% when following at the area occupation ratio of the ferrite crystal grain more than 4.0 for example, can not obtain the curved characteristic after the sufficient stretch process at the ferrite particle diameter.And, because the hardness after the hot rolling significantly uprises, steel plate becomes fragile, becoming inconvenience of operation, and also saturated with the intensity of parts after quenching as physical structure.Therefore, the C gauge is fixed on 0.2～0.7%.In addition, under the situation of the hardness after paying attention to quenching more, preferably make the C amount greater than 0.5%, and, under the situation of paying attention to processibility more, preferably make the C amount below 0.5%.
Si amount: Si makes the carbide greying and has the tendency that hinders hardenability, and therefore the gauge with it is decided to be below 2%, preferably is defined as below 0.5%.
Mn amount: when containing Mn, have the tendency that causes that ductility reduces when excessive, therefore the gauge with it is decided to be below 2%, preferably is defined as below 1%.
P amount: when containing P, ductility such as stretch flange reduce when excessive, and become and occur crackle easily, therefore its content are defined as below 0.03%, preferably are defined as below 0.02%.
S amount: when containing S, same with P, ductility such as stretch flange reduce when excessive, and become and occur crackle easily, therefore its content are defined as below 0.03%, preferably are defined as below 0.007%.
The Sol.Al amount: Sol.Al is a most important element among the present invention.That is, inventor's latest find is when the Sol.Al amount surpasses 0.01%, use how as the nitrogen of nonoxidizing atmosphere gas, when in nitrogen atmosphere gas hot-rolled steel sheet being annealed, forms AlN because of less expensive on the steel plate top layer, the steel plate case-hardening, thus the curved characteristic after the stretch process is significantly reduced.Therefore, regulation Sol.Al amount is below 0.01%.
N amount: when containing N, ductility reduces when excessive, and therefore the gauge with it is decided to be below 0.01%, preferably is defined as below 0.005%.
Here, reduce above each element measure to specified amount, for example less than 0.0001% o'clock, will cause cost to increase, therefore preferably contain and have an appointment more than 0.0001%.
Surplus is Fe and unavoidable impurities, even but for example to improve hardenability, to improve anti-temper softening as purpose, even further in the scope of adding usually, add at least a element among B, Cr, Ni, Mo, Cu, Ti, Nb, W, V, the Zr etc., also can't damage effect of the present invention.These elements that particularly, can contain following content: B:0.005% is following, Cr:3.5% following, Ni:3.5% is following, Mo:0.7% is following, Cu:0.1% is following, Ti:0.1% is following, Nb:0.1% is following, W, V, Zr: amount to below 0.1%.In addition, because above-mentioned purpose, preferably contain: B:0.0005% is above, Cr:0.05% is above, Ni:0.05% is above, Mo:0.05% is above, Cu:0.01% is above, Ti:0.01% is above, Nb:0.01% is above, W, V, Zr: amount to more than 0.01%.And, do not influence effect of the present invention even in manufacturing processed, sneak into elements such as Sn, Pb as impurity yet.
Create conditions
The hot rolled finishing temperature: finishing temperature is less than (Ar 3Transformation temperature-20 ℃) time, by partly rolling, the ferrite particle diameter after the annealing is greater than 5.0 μ m, so the curved characteristic variation after the stretch process in the ferrite zone.Therefore, make the hot rolled finishing temperature at (Ar 3Transformation temperature-20 ℃) more than.In addition, Ar 3Transformation temperature can be calculated by following formula (1), but also can use the temperature of practical measurement.
Ar 3Transformation temperature=910-203 * [C] 1/2+ 44.7 * [Si]-30 * [Mn] ... (1)
Wherein, the content (%) of [M] expression element M.In addition,, also can introduce correction term according to containing element, for example, when containing Cr, Mo, Ni, can add on the right of formula (1) correction term-11 * [Cr] ,+31.5 * [Mo] ,-15.2 * [Ni].
Cooling conditions after the hot rolling:, but still realized the micronize of ferrite crystal grain though Sol.Al amount is low and be difficult to take place the inhibition crystal grain-growth that fixing (pinning) by AlN causes among the present invention.Infer this be because, by cooling off fast after hot rolling, the strain of giving austenite crystal in rolling becomes and accumulates easily, after annealing in the strain of accumulating work as the nucleation site of ferrite crystal grain.When the speed of cooling after the hot rolling during less than 60 ℃/second, the strain of giving austenite crystal in rolling becomes and is difficult to accumulate, therefore after annealing in the nucleation site of ferrite crystal grain reduce the ferrite crystal grain growth easily that becomes.The ferrite particle diameter is greater than 5.0 μ m as a result, the curved characteristic variation after the stretch process.On the other hand, when speed of cooling more than 120 ℃/second the time, the ferrite particle diameter after the annealing is below the 5.0 μ m, but length-to-diameter ratio at the area occupation ratio of the ferrite crystal grain more than 4.0 greater than 15%, therefore same as described above, the curved characteristic variation after the stretch process.Infer this be because, speed of cooling is given strain to austenite crystal in rolling in the rolling excessive existence in back more than 120 ℃/second the time, therefore after annealing in, equiaxial ferrite crystal grain is difficult to growth.As known from the above, making the speed of cooling after the hot rolling is more than 60 ℃/second and less than 120 ℃/second.Preferably make speed of cooling on be limited to 115 ℃/second.
By such speed of cooling carry out the terminal temperature of refrigerative hot-rolled steel sheet, when promptly cooling stops temperature and is higher than 650 ℃, the strain of accumulating in the austenite to the cooling of batching till the hot-rolled steel sheet is released.Ferrite particle diameter after the result annealing is greater than 5.0 μ m, the curved characteristic variation after the stretch process.Therefore, making cooling stop temperature is below 650 ℃, is preferably below 600 ℃.In addition, owing to have problems on the accuracy of measurement of temperature, therefore preferably making cooling stop temperature is more than 500 ℃.
Reach the cooling that cooling stops after the temperature and there is no need special stipulation, can naturally cooling, also can weaken cooling Power and proceed to force cooling.From the viewpoints such as homogeneity of steel plate, preferred degree of forcing to be cooled to the inhibition backheat.
Coiling temperature: cooled hot-rolled steel sheet is batched, but this moment is when coiling temperature during greater than 600 ℃, because the strain of accumulating in the austenite during hot rolling is released, therefore the ferrite particle diameter after the annealing after is greater than 5.0 μ m, the curved characteristic variation after the stretch process.Therefore, make coiling temperature below 600 ℃.In addition, in order fully to obtain above-mentioned quenching effect, preferably make coiling temperature be lower than above-mentioned cooling and stop temperature.In addition, because the shape variation of hot-rolled steel sheet, therefore preferably making coiling temperature is more than 200 ℃, is more preferably more than 350 ℃.
Making length-to-diameter ratio is 10% when following at the area occupation ratio of the ferrite crystal grain more than 4.0, curved characteristic further improves, but this must make speed of cooling more than 80 ℃/second and less than 120 ℃/second, and cooling stops temperature below 600 ℃, and coiling temperature is below 550 ℃.
Remove descaling: the hot-rolled steel sheet after batching removed descaling usually before the hot-rolled steel sheet annealing of carrying out subsequently.The method of removing descaling does not have particular restriction, but preferably carries out pickling by usual method.
The annealing temperature of hot-rolled steel sheet: remove hot-rolled steel sheet behind the descaling by pickling etc.,, implement annealing as Spheroidizing Annealing in order to realize the nodularization of carbide.At this moment, when annealing temperature during less than 640 ℃, ferrite crystal grain is grown up and is become insufficient, length-to-diameter ratio at the area occupation ratio of the ferrite crystal grain more than 4.0 greater than 15%, thereby the curved characteristic variation after the tensile deformation.On the other hand, annealing temperature is greater than Ac 1During transformation temperature, part is carried out austenitizing, and generates perlite in cooling, so the curved characteristic variation after the stretch process.Therefore, the annealing temperature that makes hot-rolled steel sheet more than 640 ℃, Ac 1Below the transformation temperature.In order to obtain better stretch flange, the annealing temperature that preferably makes hot-rolled steel sheet is more than 680 ℃.In addition, Ac 1Transformation temperature can be calculated by following formula (2), but also can use the temperature of practical measurement.
Ac 1Transformation temperature=754.83-32.25 * [C]+23.32 * [Si]-17.76 * [Mn] ... (2)
Wherein, the content (%) of [M] expression element M.In addition,, also can introduce correction term according to containing element, for example, when containing Cr, Mo, V, can add on the right of formula (2) correction term+17.3 * [Cr] ,+4.51 * [Mo] ,+15.62 * [V].
The annealing time of hot-rolled steel sheet preferably about 8 hours～about 80 hours.Carbide spheroidization in the gained steel plate, average aspect ratio are about below 5.0 (in the value of the position finding of thickness of slab about 1/4).
When high carbon steel of the present invention is carried out melting, can use any in converter, the electric furnace.And, formed steel billet by the high carbon steel of melting by ingot casting-split rolling method or continuous casting like this.Steel billet usually after heating by hot rolling.In addition, under the situation that is the steel billet made by continuous casting, can use directly or with suppress temperature be reduced to purpose carry out Bao Re and be rolled directly rolling.And the heating steel billet carries out under the hot rolled situation, for fear of the condition of surface variation that is caused by oxide skin, preferably makes billet heating temperature below 1280 ℃.Hot rolling also can be omitted roughing and only be carried out finish to gauge.In addition, in order to ensure finishing temperature, also can in hot rolling, be rolled material heating by heating units such as sheet material strip well heaters.In addition, because nodularization promotion or hardness reduce, also can batch the back by device insulation coiled materials such as slow cooling covers.Just do not have particular restriction as long as the thickness of slab of hot-rolled steel sheet can be kept of the present invention creating conditions, but in operation the hot-rolled steel sheet of preferred especially 1.0～10.0mm.
The annealing of hot-rolled steel sheet can be undertaken by in pack annealing, the continuous annealing any.After the annealing, carry out temper rolling as required.Because this temper rolling does not influence hardenability, therefore its condition there is not particular restriction.
By the hot-rolled steel sheet of the method manufacturing of the invention described above is the hot-rolled steel sheet of implementing after the hot rolling Spheroidizing Annealing, as mentioned above, is that to have average aspect ratio be about below 5.0 and by the hot-rolled steel sheet of the carbide of nodularization.
And the ferrite particle diameter of hot-rolled steel sheet of the present invention is below the 5.0 μ m.Curved characteristic after the ferrite grain diameter influence stretch process, when ferrite particle diameter during greater than 5.0 μ m, separate out small carbide in the ferrite crystal grain in a large number, crack owing to the small space that produces at the carbide and the interface of parent phase (ferrite) in the stretch process is connected in bending machining.By making the ferrite particle diameter is below the 5.0 μ m, and the small carbide in the ferrite crystal grain tails off, and is difficult to connect owing to become in the bending of small space after stretch process that stretch process produces, and therefore can suppress crackle and produce.
And in hot-rolled steel sheet of the present invention, length-to-diameter ratio is that the area occupation ratio of the ferrite crystal grain more than 4.0 is below 15%.The shape of ferrite crystal grain and ferrite particle diameter similarly influence the curved characteristic after the stretch process, when the length-to-diameter ratio of this ferrite crystal grain is 4.0 when above, in length-to-diameter ratio was ferrite crystal grain and crystal boundary less than 4.0 the ferrite crystal grain that waits the axle shape more than 4.0, becoming in the stretch process was easy to generate small crackle.When this length-to-diameter ratio be the area occupation ratio of the ferrite crystal grain more than 4.0 greater than 15% the time, as starting point, crackle produces in bending machining with the small crackle in the stretch process.By making this length-to-diameter ratio is that the area occupation ratio of the ferrite crystal grain more than 4.0 is below 15%, suppresses crackle in can the bending after stretch process to produce.The area occupation ratio that is more preferably length-to-diameter ratio and is the ferrite crystal grain more than 4.0 is below 10%.
Embodiment 1
To have the steel A～E of chemical ingredients shown in the table 1 and the continuous casting steel billet of Z and be heated to 1250 ℃, under condition shown in the table 2, carry out hot rolling, after the pickling, under condition shown in the table 2, carry out the annealing of hot-rolled steel sheet equally, steel plate No.1～20 of making thickness of slab 5.0mm.In addition, annealing is at nitriding atmosphere gas (N 2Atmosphere gas) carry out in.
Wherein, steel plate No.1～10th, example of the present invention, steel plate No.11～20th, comparative example.And the length-to-diameter ratio of ferrite particle diameter, ferrite crystal grain and area occupation ratio are measured with following method.In addition, the curved characteristic after the stretch process is estimated with following method.
The length-to-diameter ratio of ferrite particle diameter, ferrite crystal grain and area occupation ratio: here, the ferrite particle diameter is meant the mean value of the particle diameter that utilizes image analysis that ferrite crystal grain is approximately circle and try to achieve, in addition, length-to-diameter ratio is meant and utilizes image analysis that ferrite crystal grain is approximately oval mean value of trying to achieve (long axis of ellipse)/(oval minor axis).Particularly, grinding is parallel to the thickness of slab cross section of the rolling direction of steel plate, 1/4 position with nital (nitric acid+ethanol) corrosion thickness of slab, utilize scanning electron microscope under 1500 times of multiplying powers little tissue to be observed then, the image analysis software " Image Pro Plus Ver.4.0 " of using Media Cybemetics corporate system (TM) is obtained the length-to-diameter ratio of ferrite particle diameter, ferrite crystal grain by image analysis.And then, for each ferrite crystal grain, obtain the area occupation ratio of length-to-diameter ratio more than 4.0, with its total area, thereby obtain the area occupation ratio in each visual field divided by the visual field, as length-to-diameter ratio the area occupation ratio of the ferrite crystal grain more than 4.0 with the mean value in 50 visuals field.
And grinding is parallel to the thickness of slab cross section of the rolling direction of steel plate, with picric acid ethanolic soln (picric acid; Ethanol) 1/4 position of corrosion thickness of slab utilizes scanning electron microscope under 3000 times of multiplying powers little tissue to be observed then, utilizes above-mentioned image analysis software to obtain the length-to-diameter ratio of carbide (major diameter)/(minor axis).Then, the length-to-diameter ratio that average (number is average) tries to achieve each carbide is obtained average aspect ratio, confirms by Spheroidizing Annealing.
Curved characteristic after the stretch process: use edge and the rectangular direction of rolling direction width that collect, parallel portion No. 5 test films of JIS as 30mm, carry out tension test by method according to JIS Z 2241, after giving 15% prestrain, carry out pliability test by extruding bending method according to JIS Z 2248.To make punch diameter in the pliability test be 1mm, carry out 3 tests, is zero with the steel plate that does not all crack for 3 times, and the steel plate that produces 1 time and 2 times crackle is △, with the steel plate that all cracks for 3 times for *.In addition, with zero situation as example.
The result is illustrated in table 3.As steel plate No.1～10 of example of the present invention, the ferrite particle diameter all below 5.0 μ m, and length-to-diameter ratio be the area occupation ratio of the ferrite crystal grain more than 4.0 below 15%, the curved characteristic after the stretch process is good.In addition, in arbitrary example of the present invention, the average aspect ratio of having confirmed carbide is below 5.0, and makes carbide spheroidization by Spheroidizing Annealing.
It is 5.0 μ m when following, the length-to-diameter ratio relation at the area occupation ratio and the curved characteristic after the stretch process of the ferrite crystal grain more than 4.0 that Fig. 1 shows the ferrite particle diameter.As steel plate No.1～10 of example of the present invention, making the ferrite particle diameter is below the 5.0 μ m as can be known, and to make length-to-diameter ratio be below 15% at the area occupation ratio of the ferrite crystal grain more than 4.0, can access the curved characteristic after the good stretch process.
Table 1 (quality %)
Steel C Si Mn P S Sol.Al N The Ar that obtains by formula (1) 3Transformation temperature (℃) The Ac that obtains by formula (2) 1Transformation temperature (℃)
A 0.26 0.22 0.83 0.010 0.0025 0.007 0.0031 791 737
B 0.34 0.20 0.74 0.015 0.0018 0.005 0.0033 778 735
C 0.35 0.02 0.15 0.009 0.0030 0.006 0.0036 786 741
D 0.49 0.19 0.76 0.011 0.0027 0.010 0.0032 754 730
E 0.66 0.21 0.75 0.014 0.0045 0.003 0.0030 732 725
Z 0.36 0.21 0.73 0.013 0.0022 0.032 0.0032 776 735
Table 2
Table 3
Steel plate No. Ferrite particle diameter (μ m) Length-to-diameter ratio is the area occupation ratio (%) of the ferrite crystal grain more than 4.0 Curved characteristic after the stretch process Remarks
1 3.5 13.1 ○ Example of the present invention
2 3.2 8.8 ○ Example of the present invention
3 2.8 12.4 ○ Example of the present invention
4 2.6 9.2 ○ Example of the present invention
5 4.4 11.6 ○ Example of the present invention
6 3.3 7.5 ○ Example of the present invention
7 4.1 13.9 ○ Example of the present invention
8 3.7 8.7 ○ Example of the present invention
9 4.5 14.6 ○ Example of the present invention
10 3.1 9.2 ○ Example of the present invention
11 6.4 13.3 △ Comparative example
12 6.2 14.4 △ Comparative example
13 4.4 15.8 × Comparative example
14 5.6 14.1 △ Comparative example
15 4.7 18.4 × Comparative example
16 5.9 14.6 △ Comparative example
17 5.4 13.9 △ Comparative example
18 5.5 14.2 △ Comparative example
19 4.1 22.4 × Comparative example
20 4.0 13.5 × Comparative example
Embodiment 2
With F steel (C:0.31%, Si:0.18%, Mn:0.68%, P:0.012%, S:0.0033%, Sol.Al:0.005%, N:0.0040%, Ar 3Transformation temperature: 785 ℃, Ac 1Transformation temperature: 737 ℃),
G steel (C:0.23%, Si:0.18%, Mn:0.76%, P:0.016%, S:0.0040%, Sol.Al:0.008%, N:0.0028%, Cr:1.2%, Ar 3Transformation temperature: 785 ℃, Ac 1Transformation temperature: 759 ℃),
H steel (C:0.32%, Si:1.2%, Mn:1.5%, P:0.025%, S:0.010%, Sol.Al:0.006%, N:0.0070%, Ar 3Transformation temperature: 804 ℃, Ac 1Transformation temperature: 746 ℃),
I steel (C:0.35%, Si:0.20%, Mn:0.68%, P:0.012%, S:0.0038%, Sol.Al:0.005%, N:0.0033%, Mo:0.17%, Cr:0.98%, Ar 3Transformation temperature: 773 ℃, Ac 1Transformation temperature: 754 ℃) and
E steel shown in the table 1 carries out continuous casting and after becoming steel billet, is heated to 1230 ℃, carries out the annealing of hot rolling and hot-rolled steel sheet under condition shown in the table 4, makes steel plate No.21～37 of thickness of slab 4.5mm.In addition, annealing is at nitriding atmosphere gas (N 2Atmosphere gas) carry out in.For the hot-rolled steel sheet of gained, adopt method similarly to Example 1, measure the length-to-diameter ratio and the area occupation ratio of ferrite particle diameter, ferrite crystal grain, the curved characteristic after the evaluation stretch process.And, same with embodiment, confirmed the nodularization situation of carbide.
In addition, the Ar of F steel～I steel 3Transformation temperature, Ac 1Transformation temperature is tried to achieve by above-mentioned formula (1) or formula (2), for the G steel that contains Cr or Mo, I steel, then uses above-mentioned correction term and tries to achieve.
The results are shown in table 5.In steel plate No.21～27 that condition beyond making speed of cooling is certain, the curved characteristic after the stretch process of speed of cooling No.22～26 within the scope of the invention is significantly good as can be known.In steel plate No.23～26, can make length-to-diameter ratio is below 10% at the area occupation ratio of the ferrite crystal grain more than 4.0 as can be known.And, make speed of cooling certain and in steel plate No.28～33 studied as can be known, cooling stops temperature, coiling temperature, and all the curved characteristic after within the scope of the invention the stretch process of steel plate No.30～33 is good especially.Making cooling as can be known to stop temperature be below 600 ℃ and make coiling temperature is steel plate No.33 below 550 ℃, and can make length-to-diameter ratio is below 10% at the area occupation ratio of the ferrite crystal grain more than 4.0.In addition, in example of the present invention, the average aspect ratio of having confirmed arbitrary carbide is below 5.0, and carbide nodularization by Spheroidizing Annealing.
Steel consist of in E～I steel in the scope of the present invention any and also comprise G steel and the I steel that has added the alloying element beyond the basal component, show the curved characteristic after the good stretch process.
Table 4
Table 5
Steel plate No. Ferrite particle diameter (μ m) Length-to-diameter ratio is the area occupation ratio (%) of the ferrite crystal grain more than 4.0 Curved characteristic after the stretch process Remarks
21 5.4 13.2 △ Comparative example
22 4.7 11.3 ○ Example of the present invention
23 3.6 8.8 ○ Example of the present invention
24 3.1 7.9 ○ Example of the present invention
25 2.9 8.5 ○ Example of the present invention
26 3.2 7.6 ○ Example of the present invention
27 4.5 21.9 × Comparative example
28 6.4 14.2 △ Comparative example
29 5.8 14.6 △ Comparative example
30 4.3 12.1 ○ Example of the present invention
31 4.0 13.3 ○ Example of the present invention
32 4.5 10.7 ○ Example of the present invention
33 3.9 8.4 ○ Example of the present invention
34 3.8 9.5 ○ Example of the present invention
35 3.7 8.7 ○ Example of the present invention
36 4.8 9.6 ○ Example of the present invention
37 3.5 8.8 ○ Example of the present invention

Claims (6)

1. the manufacture method of a high-carbon hot-rolled steel sheet has following operation:
To contain C:0.2～0.7% in quality %, below the Si:2%, below the Mn:2%, below the P:0.03%, below the S:0.03%, Sol.Al: the steel of composition less than 0.01%, below the N:0.01%, at (Ar 3Transformation temperature-20 ℃) carries out hot rolling under the above finishing temperature, thereby make hot-rolled steel sheet;
With described hot-rolled steel sheet to be cooled to temperature below 650 ℃ more than 60 ℃/second and less than 120 ℃/second speed of cooling;
Described cooled hot-rolled steel sheet is batched under the coiling temperature below 600 ℃; With
With the hot-rolled steel sheet after described the batching more than 640 ℃, Ac 1Anneal under the annealing temperature below the transformation temperature.
2. the manufacture method of high-carbon hot-rolled steel sheet as claimed in claim 1, wherein, carry out in the refrigerative operation described, with hot-rolled steel sheet to be cooled to temperature below 600 ℃ more than 80 ℃/second and less than 120 ℃/second speed of cooling, and, in described operation of batching, under the temperature below 550 ℃, batch.
3. the manufacture method of high-carbon hot-rolled steel sheet as claimed in claim 1 or 2, wherein, the composition of steel also contains at least a kind among the B, the Cr that are selected from following content range, Ni, Mo, Cu, Ti, Nb, W, V, the Zr on the basis of above-mentioned composition,
In quality %, below the B:0.005%, below the Cr:3.5%, below the Ni:3.5%, below the Mo:0.7%, below the Cu:0.1%, below the Ti:0.1%, below the Nb:0.1%, W, V, Zr: amount to below 0.1%.
4. a high-carbon hot-rolled steel sheet is a hot rolling Spheroidizing Annealing material, it is characterized in that,
Have in quality % contain C:0.2～0.7%, below the Si:2%, below the Mn:2%, below the P:0.03%, below the S:0.03%, Sol.Al: the composition less than 0.01%, below the N:0.01%,
The ferrite particle diameter below 5.0 μ m, and length-to-diameter ratio be the area occupation ratio of the ferrite crystal grain more than 4.0 below 15%,
Wherein, the ferrite particle diameter is meant the mean value of the particle diameter that utilizes image analysis that ferrite crystal grain is approximately circle and try to achieve, in addition, length-to-diameter ratio is meant and utilizes image analysis that ferrite crystal grain is approximately oval mean value of trying to achieve (long axis of ellipse)/(oval minor axis).
5. high-carbon hot-rolled steel sheet as claimed in claim 4, wherein, length-to-diameter ratio is that the area occupation ratio of the ferrite crystal grain more than 4.0 is below 10%.
6. as claim 4 or 5 described high-carbon hot-rolled steel sheets, wherein, the composition of steel also contains at least a kind among the B, the Cr that are selected from following content range, Ni, Mo, Cu, Ti, Nb, W, V, the Zr on the basis of above-mentioned composition,
In quality %, below the B:0.005%, below the Cr:3.5%, below the Ni:3.5%, below the Mo:0.7%, below the Cu:0.1%, below the Ti:0.1%, below the Nb:0.1%, W, V, Zr: amount to below 0.1%.
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