CN101248202A - Highly strong, thick electric resistance-welded steel pipe excellent in quenching property, hot forming processability and fatigue strength, and method for manufacture thereof - Google Patents

Abstract

The present invention provides high strength thick-gauge electric-resistance welded steel pipe excellent in hardenability, hot workability, and fatigue strength and a method of production of the same, that is, thick-gauge electric-resistance welded steel pipe containing, by mass%, C: 0.25 to 0.4%, Si: 0.01 to 0.50%, Mn: 0.8 to 1.5%, P: 0.05% or less, S: 0.05% or less, Al: 0.05% or less, Ti: 0.005 to 0.05%, B: 0.0005 to 0.01%, N: 0.001 to 0.05%, and a balance of Fe and unavoidable impurities, having a critical cooling rate Vc expressed by equation <1> of less than 30 DEG C/s, and having a ratio of thickness t and outside diameter D, t/D, of over 0.15 to 0.30 in range: logVc = 2.94 - 0.75 where, P=2.7C+0.4Si+Mn.

Description

The high-strength thick Electric Welded Steel Pipe and the manufacture method thereof of hardening capacity, hot workability and fatigue strength excellence

Technical field

The present invention relates to be suitable for to be used to guarantee running car stability the hollow stabilizer, hardening capacity, hot workability and all excellent high-strength thick Electric Welded Steel Pipe and the manufacture method thereof of fatigue strength.

Background technology

As one of countermeasure of the fuel availability that improves automobile, advancing the lightweight of car body.When motor turning, relax car body side-sway, guarantee that when running at high speed the stabilizer (Stabilizer) of the stability of car body is also enumerated out as light-weighted object.In the past, thereby stabilizer is to produce by solid materials such as bar steel are processed into required shape, but in order to seek lightweight, uses weldless steel tube and Electric Welded Steel Pipe (Electricity Sew to weld the Steel pipe; Electric resistance-welded steelpipe) situation of hollow material manufacturing such as increases.

The Electric Welded Steel Pipe of using as stabilizer, in the WO2002/070767 communique, once proposed, form by regulation, thereby the metal structure in electric welding welding zone and mother metal district was even, the difference of hardness in electric welding welding zone and mother metal district reduces, the hollow stabilizer Electric Welded Steel Pipe of excellent in workability.In addition, open in the flat 2004-011009 communique once the spy and to propose, guarantee the hollow stabilizer Electric Welded Steel Pipe of hardening capacity by the content of regulation Ti, N.

Open in the 2004-009126 communique the spy, once proposed a kind of hollow stabilizer Electric Welded Steel Pipe, the wall thickness t of its steel pipe and the ratio t/D of outer diameter D are more than 20%, and tensile strength is 400～755N/mm ², and disclose by the rolling wall thickness that makes of undergauge and increase.

In addition, open in the 2003-201543 communique the spy, it is rolling to propose once that pipe was carried out undergauge, making tensile strength is the automotive structural members High Tensile Steel Tube of the excellent workability of the hydraulic pressure resistant extrusion molding below 70% above 580MPa, yield ratio, in addition, open in the 2004-292922 communique the spy, proposed to be undertaken by the Heating temperature in rolling to undergauge, diameter reducing rate etc. specific, the manufacture method of High Tensile Steel Tube of compound secondary workability excellence that makes bending machining, undergauge processing, the flat processing of pipe end etc.

In addition, open in the 2005-076047 communique the spy, a kind of manufacture method of hollow stabilizer of excellent in fatigue characteristics was once proposed, this method be implement with the raw material pipe by clod wash shape into the forming process of stabilizer shape and to this shaping steel pipe implement to quench, the manufacture method of the hollow stabilizer of tempering heat treatment, wherein, the above-mentioned raw materials pipe is after the mother metal steel pipe is implemented heat treated, implements under 600～850 ℃ of rolling temperatures, the condition of accumulation diameter reducing rate 40% or more that the section contraction is rolling to form.

In addition, at Japanese Patent the 3rd, 653, in No. 871 communiques, a kind of excellent in workability was once disclosed, the high quenching Electric Welded Steel Pipe of retained strength rate after the hydrogen that is caused by corrosion is invaded, wherein, by quality %, contain C:0.15～0.3%, Mn:0.5～2.0%, Cu:0.05～0.30%, also contain and be selected from Si≤0.41%, P≤0.02%, Al≤0.03%, Nb≤0.020%, B≤0.001%, Ti≤0.01%, among Cr≤0.42% more than a kind, be defined as 0≤Ni+Mo＜0.15% and S≤0.003% as unavoidable impurities, and its surplus is made up of Fe.

For example, stabilizer is rolling by Electric Welded Steel Pipe further being carried out undergauge, reaches desired wall thickness/external diameter than the heavy wall Electric Welded Steel Pipe that forms, 1) the cold shaping processing by bending machining etc., be configured as desired shape, thereby be heated, after water-cooled quenches, implement tempering, perhaps 2) heating heavy wall Electric Welded Steel Pipe, thermoforming processing by compacting etc. is configured as desired shape, then carries out after water-cooled quenches, implement tempering, make thus.The latter's thermoforming method for processing is compared with the former cold shaping method for processing, shapes easily, can tackle complicated shape, sees it is excellent from this aspect point, is favourable as manufacturing process therefore.

Yet, when adopting this method, owing to form after the heating, therefore the time till quenching elongated, the temperature of the member that has been shaped reduces, generation is reduced by the temperature that compacting tool set and contacting of raw material steel pipe (Electric Welded Steel Pipe) cause, or the non-uniform temperature that causes owing to the generation of heated oxide skin, or the like, therefore guarantee the sufficient as-quenched condition difficulty that becomes on the whole, worry generation under hardening, and then need the steel for steel pipes material of hardening capacity excellence.Therefore, stabilizer with steel general use have a high-hardenability contain the B steel.Contain the B steel and lack hot workability, add in thermoforming and man-hour crackle and defective take place easily, this becomes very big problem.In addition, containing the B steel, is that the fatigue strength of very important characteristic reduces for stabilizer often.

In addition, there is the further trend of accelerationization in the lightweight of car body, and as the Electric Welded Steel Pipe that stabilizer is used, positive demand intensity is high Electric Welded Steel Pipe further.

Summary of the invention

Above-mentioned WO2002/070767 communique, spy opens flat 2004-011009 communique, spy and opens 2004-009126 communique, spy and open 2003-201543 communique, spy and open 2004-292922 communique, spy and open stabilizer that 2005-076047 communique, spy open narrations such as 2005-076047 communique, No. 3653871 communique of Japanese Patent with Electric Welded Steel Pipe and High Tensile Steel Tube, as the automotive structural members steel pipe is useful, but as described above, for the problem that produces owing to the technique change in the manufacturing process of automotive structural members, can not tackle fully.In addition, aspect fatigue characteristic, can not say so fully.

Problem of the present invention is, in view of the above-mentioned problems, provides to have sufficient hardening capacity, and the high-strength thick Electric Welded Steel Pipe and the manufacture method thereof of hot workability and fatigue strength excellence.

Heavy wall Electric Welded Steel Pipe of the present invention is finished in order to solve above-mentioned problem, by doing one's utmost to increase C content under weldability, the flexible degree intensity (hardness) improved not damaging, and limit the N content range closely, hot workability and fatigue strength are improved, and then the composition by adjusting steel is guaranteed hardening capacity so that critical cooling velocity Vc is specific scope.In addition, in the manufacturing of heavy wall Electric Welded Steel Pipe of the present invention, Heating temperature, section decrement are determined in specific scope that it is rolling that Electric Welded Steel Pipe is implemented undergauge.

Main idea of the present invention is as follows.

(1) a kind of hardening capacity, the high-strength thick Electric Welded Steel Pipe that hot workability and fatigue strength are all excellent, it is characterized in that, by quality %, contain C:0.25～0.4%, Si:0.01～0.50%, Mn:0.8～1.5%, below the P:0.05%, below the S:0.05%, below the Al:0.05%, Ti:0.005～0.05%, B:0.0005～0.01%, N:0.001～0.05%, its surplus is made up of Fe and unavoidable impurities, by formula＜1〉the critical cooling velocity Vc of expression is less than 30 ℃/second, and the ratio t/D of wall thickness t and outer diameter D is for greater than 0.15 and be no more than 0.30 scope.

logVc＝2.94-0.75β ......<1>

Wherein, β=2.7C+0.4Si+Mn

(2) according to all excellent high-strength thick Electric Welded Steel Pipe of (1) described hardening capacity, hot workability and fatigue strength, it is characterized in that, by quality %, also contain among Cr:0.1～1%, Mo:0.05～1%, V:0.01～0.5%, Ni:0.1～1% more than a kind or 2 kinds, and in formula＜1〉in, β=2.7C+0.4Si+Mn+0.45Ni+0.8Cr+2Mo

(3) according to all excellent high-strength thick Electric Welded Steel Pipe of (1) or (2) described hardening capacity, hot workability and fatigue strength, it is characterized in that,, also contain Nb:0.01～0.1% by quality %.

(4) according to all excellent high-strength thick Electric Welded Steel Pipe of each described hardening capacity, hot workability and the fatigue strength of (1)～(3), it is characterized in that,, also contain Ca:0.0002～0.005% by quality %.

(5) manufacture method of the high-strength thick Electric Welded Steel Pipe of the equal excellence of a kind of hardening capacity, hot workability and fatigue strength, it is characterized in that, to have (1)～Electric Welded Steel Pipe of each described composition of (4) is heated to 800～1200 ℃, is that to carry out undergauge under 40～80% the scope rolling at the section decrement.

Heavy wall Electric Welded Steel Pipe of the present invention, hardening capacity is extremely excellent, therefore use in the manufacturing of member at vehicle structures such as stabilizers, the occasion of for example carrying out carrying out immediately after the thermoforming processing quench treatment also can access sufficient quenching effect, in addition, the quenching means also are not limited to water-cooled, adopt speed of cooling also can access sufficient quenching effect than the little oil quenching of water-cooled.

In addition, owing to have superior hot workability, therefore when making automobile component, also be difficult to take place crackle and defective even carry out thermoforming.In addition, because the fatigue strength excellence, therefore with respect to the weather resistance height of cyclic loading.In addition, because intensity is higher, therefore can make vehicle structure such as stabilizer with member lightweight more.

Description of drawings

Fig. 1 is the figure of expression heavy wall Electric Welded Steel Pipe with the relation of hardening capacity, the hardness after the tempering and the C content of steel.

Fig. 2 is the figure that is illustrated in the relation of section shrinkage value under 850 ℃ and N content.

Fig. 3 is the figure that is used to illustrate the method for fatigue test.

Embodiment

Present inventors have discussed the hardening capacity, hot workability and the fatigue strength that make the heavy wall Electric Welded Steel Pipe that stabilizer uses and have improved, and further strengthen the method for intensity.

At first, intensity is discussed, steel of using for the Electric Welded Steel Pipe of the composition shown in the table 1 that changes C content (test material A, B, C) and steel (existing material A, B) have relatively been investigated the hardness after cold quenching, the tempering.Fig. 1 represents the changes in hardness after cold quenching, the tempering and the relation of C content.

Distinguish by Fig. 1, increase, thereby hardness increases by C content, for example high about more than 10% than the firmness level of current material, show intensity is improved.In addition we know, its effect is at C: be 0.25% more remarkable when above.

Table 1 (quality %)

	C	Si	Mn	Cr	Mo	B	N	Ti
									Existing material A	0.22	0.20	0.55	0.35	-	0.0015	0.0050	0.015
Existing material B	0.26	0.20	0.83	0.36	0.15	0.0016	0.0038	0.016
									Test material A	0.28	0.25	0.80～ 1.1	0～0.35	0.15～ 0.30	0.0011	0.004	0.014
Test material B	0.33	0.25	0.80～ 1.1	0～0.35	0.15～ 0.30	0.0011	0.004	0.015
									Test material C	0.37	0.25	0.80～ 1.1	0.15～ 0.30	0.15～ 0.30	0.0011	0.004	0.015

Secondly, hardening capacity is discussed.

Hardness when Fig. 1 also is presented as 100% martensitic stucture simultaneously and is 90% martensitic stucture and the relation of C content, if test material A, B, C are quenched to and are at least 90% martensitic stucture as can be known, then can guarantee than the high hardness more than 10% of existing material.Therefore, as the index of hardening capacity, for example according to " iron and steel, 74 (1988), P.1073 use in the past with regard to the known critical cooling velocity Vc (℃/second) that obtains 90% martensitic stucture to get final product.This is usually by following＜1〉formula represents.

logVc＝2.94-0.75β ......<1>

Wherein, β=2.7C+0.4Si+Mn or β=2.7C+0.4Si+Mn+0.45Ni+0.8Cr+2Mo.

Clearly show that by Fig. 1 when adopting water quenching, can obtain the martensitic stucture more than 90%, but in oil-quenched occasion, compare with water quenching, speed of cooling reduces greatly.The speed of cooling that is reached when usually, the steel pipe of the employed size of stabilizer being carried out oil quenching is 30 ℃/second.Therefore, even also guarantee 90% martensitic stucture, in the present invention critical cooling velocity Vc is defined as less than 30 ℃/second for oil quenching.

As described above, for the intensity that makes the raw material steel pipe improves, when increasing C content, selection component is so that by formula＜1〉the critical cooling velocity Vc of expression is less than 30 ℃/second.

Secondly, present inventors discuss the method that improves hot workability and fatigue strength.

Present inventors investigate in the result who implements the bad reason of hot workability that hot formed 600～900 ℃ humidity province contains the B steel, find out that N content has a significant impact the thermal distortion drag in the steel.Promptly, present inventors are for the 0.3C-1.1Mn-0.020Ti-0.0013B steel, made making the test materials of N content, carried out uniaxial tension test under 850 ℃ in hot formed temperature range, measured the section shrinkage value of this moment as reality in 0.01%～0.001% range.

Fig. 2 expresses the section shrinkage value under 850 ℃ and the relation of N content.

Known that by Fig. 2 N content is low more, the section shrinkage value is big more, and promptly hot workability improves.N content was reduced to 0.005% o'clock, and the section shrinkage value rises to as roughly can carrying out 40% of hot formed benchmark, when N contains quantity not sufficient 0.004%, the section shrinkage value reach can implement no problemly hot formed more than 50%.Find that its reason is owing to reduce by N content, the cause that reduces in the amount of the TiN that carries out separating out hot formed humidity province.Promptly find out: usually, contain in the B steel, the BN that reduces for the effect of the raising hardening capacity that suppresses to make B separates out, and must contain the high Ti of effect of fix N, therefore, separate out TiN owing to contain the B steel carrying out hot formed humidity province, so hot workability is bad.

Find that in addition the separating out in a large number of TiN also makes the fatigue strength as the key property of stabilizer reduce.In addition, TiN separate out in a large number toughness also unfavorable.On the other hand, can suppress austenite (γ) grain growth, help toughness to improve by there being TiN in right amount.Therefore, by the stringent regulations N content of stringent regulations less in the past, can make the hot workability, fatigue strength, the toughness that contain the B steel reach preferred.

Like this, heavy wall Electric Welded Steel Pipe of the present invention is by improving C content and N content being suppressed for a small amount of, make intensity improve, make hot workability and fatigue strength to improve, and by suitably controlling other composition, reduce critical cooling velocity Vc, hardening capacity is improved.

Below, the chemical ingredients of heavy wall Electric Welded Steel Pipe of the present invention is described.

C: be that solid solution is separated out in matrix or with the carbide form, make the element of the intensity increase of steel.

As the high-strength vehicle structure member more than in the past, requirement is 90% martensitic stucture, the hardness of Hv400 at least, therefore it is necessary it being contained more than the C:0.25%, when content surpasses 0.4%, therefore processibility and weldability deterioration are defined as 0.25～0.4% scope with C content.

Si: be the alloying element that helps solution strengthening,, need contain more than 0.01% in order to obtain its effect.In addition, have the effect that improves anti-temper softening,, need to add more than 0.25% in order to obtain its effect.On the other hand, when addition surpassed 0.5%, toughness reduced.Therefore, Si content is defined as 0.01～0.50% scope.Moreover, be preferably 0.25～0.35%.

Mn: be the element that hardening capacity is improved, when containing quantity not sufficient 0.8%, can not fully guarantee to improve the effect of hardening capacity, in addition, when surpassing 1.5%, therefore the viability generation detrimentally affect to weldability and welding zone is defined as 0.8～1.5% scope with Mn content.

Al: be as the deoxidation material of molten steel essential element, also be the element of fix N in addition, so its content produces a very large impact crystal particle diameter and mechanical properties.When content surpasses 0.05%, thickization of crystal particle diameter, toughness reduces, and perhaps non-metallic inclusion increases, and surface imperfection takes place in goods easily, so its content is defined as below 0.05%.Moreover, be preferably below 0.03%.

B: be the element that the hardening capacity of steel is increased substantially, also have the effect of grain-boundary strengthening in addition.When it contains quantity not sufficient 0.0005%, can not expect to make the effect of hardening capacity raising, on the other hand, when surpassing 0.01%, exist to generate the thick tendency that contains the B phase, also cause embrittlement easily.Therefore, its content is defined as 0.0005～0.01%.Moreover, be preferably more than 0.0010% and be no more than 0.0020%.

N: be to make nitride or carbonitride separate out, have the element of the effect that improves intensity.But, for containing the B steel, have following problems: owing to separating out of BN causes the reduction of hardening capacity, add Ti as described above in order to prevent separating out of BN, reduce owing to separating out of TiN causes hot workability and fatigue strength thus, and then toughness reduces.On the other hand, TiN also has the alligatoring of austenite (γ) particle diameter when suppressing high temperature, the effect that toughness is improved.Therefore, reach best in order to make hot workability, fatigue strength and flexible balance, its content is defined as 0.001～0.005% scope.Moreover, be preferably 0.002%～less than 0.004%.

Ti: by N in the steel is fixed with the TiN form, and suppress separating out of BN, come stable and improve the hardening capacity that the interpolation by B brings effectively, thereby play a role.Therefore, in order to meet the stoichiometric ratio of TiN, the Ti addition more than 3.42 times of N content is the minimum requirement, and according to the scope of above-mentioned N content, the scope of Ti content is also determined automatically.But, owing to also have a part of separating out with the carbide form, therefore for fix N more effectively, with Ti be defined as than theoretical value high more than 0.005, on the other hand, when surpassing 0.05%, therefore the tendency that has the toughness deterioration is defined as 0.005～0.05% scope.Moreover, be preferably 0.01～0.02%.

P: be that anti-welding crack and toughness are produced the element of bad influence, therefore be defined as below 0.05%.Moreover, be preferably below 0.03%.

S: influence the formation of the non-metallic inclusion of steel, make the processibility deterioration of the bendability of steel pipe, flat property etc., become toughness deterioration, anisotropy and reheat cracking susceptibility cause of increased simultaneously.In addition, the viability to the welding zone also produces bad influence.Therefore, its content is limited to below 0.05%.Moreover, be preferably below 0.01%.

Heavy wall Electric Welded Steel Pipe of the present invention can contain more than one of one or more and/or Ca, Nb among Cr, Mo, V, the Ni as requested.

Cr: be the element that hardening capacity is improved, have in addition and separate out M in the matrix of making ₂₃C ₆The effect of type carbide has when improving intensity, makes the effect of carbide miniaturization.When containing quantity not sufficient 0.1%, can not fully expect these effects and effect, when surpassing 1%, defective take place easily during electric welding in addition.Therefore, its content is defined as 0.1～1% scope.Moreover, be preferably 0.1～0.6%.

Mo: being the element with effect of the hardening capacity of making raising, also is the element with effect of the solution strengthening of bringing.Can not fully expect these effects when it contains quantity not sufficient 0.05%, when surpassing 1%, separate out thick carbide easily on the other hand, make the toughness deterioration, so its content be defined as 0.05～1% scope.Moreover, be preferably 0.1～0.5%.

Ni: be element with effect of hardening capacity of making and toughness raising.Can not expect its effect when it contains quantity not sufficient 0.1%, when surpassing 1%, after quenching, have the possibility that has residual austenite on the other hand, make the fatigue durability deterioration.Therefore, its content is defined as 0.1～1% scope.Moreover, be preferably 0.015～0.5%.

V: being the element with effect of the hardening capacity of making raising, also is the element with effect of the precipitation strength that is brought by the V carbonitride.Can not fully expect these effects when it contains quantity not sufficient 0.01%, when surpassing 0.5%, separate out thick carbide easily on the other hand, make the toughness deterioration, so its content be defined as 0.01～0.5% scope.Moreover, be preferably 0.02～0.05%.

Nb: have the effect of the precipitation strength that brings by the Nb carbonitride, have the miniaturization of the original austenite of making particle diameter in addition, the effect that toughness is improved.Has the effect that suppresses surface decarburization in addition.

When it contained quantity not sufficient 0.01%, it was insufficient to improve intensity and flexible effect, and carbide increases when content surpasses 0.1%, and toughness reduces, so its content is defined as 0.01～0.1% scope.Moreover, be preferably 0.02～0.04%.

Ca: be have the shape that makes oxide compound and sulfide become spherical, thereby make the element of the effect that processibility improves.Can not fully expect these effects when it contains quantity not sufficient 0.0002%, oxide compound increase in the steel when surpassing 0.005% on the other hand make the toughness deterioration, so its content is defined as 0.0002～0.005% scope.Moreover, be preferably 0.002～0.004%.

Secondly, in heavy wall Electric Welded Steel Pipe of the present invention, be defined as greater than 0.15 and be no more than 0.30 reason and describe for the scope of the wall thickness t (mm) of steel pipe and the ratio t/D of the outer diameter D (mm) of steel pipe.

For the lightweight of stabilizer, t/D is more little preferred more.But t/D is more little, and the principle stress that applies during use is big more, so fatigue characteristic reduce.On the other hand, when t/D increased, light-weighted effect diminished, and the manufacturing of Electric Welded Steel Pipe becomes difficult.In order to ensure the fatigue strength of minimum, the lower limit of t/D is defined as greater than 0.15, and from manufacturing and light-weighted viewpoint, the upper limit is defined as 0.30.

Manufacture method to heavy wall Electric Welded Steel Pipe of the present invention describes.

With being smelted into molten steel with required chemical constitution, be made into strand through casting, perhaps be made into steel ingot earlier after, carry out hot rolling processing, make steel billet, this strand or steel billet are carried out hot rolling, make hot-rolled steel sheet.

The manufacture method of the Electric Welded Steel Pipe that this hot-rolled steel sheet employing is common, for example the resistance welding under hot or cold conditions is made Electric Welded Steel Pipe.

Heavy wall Electric Welded Steel Pipe of the present invention, as described above, the ratio t/D of the wall thickness/external diameter of steel pipe is defined as greater than 0.15 and is no more than 0.30, the ability of Electric Welded Steel Pipe tubing machine, have the occasion of tubulation ability of Electric Welded Steel Pipe of the wall thickness/external diameter ratio of such scope, can use above-mentioned hot-rolled steel sheet directly to make heavy wall Electric Welded Steel Pipe of the present invention.

Yet, Electric Welded Steel Pipe, its wall thickness is thick more, and the external diameter of pipe is more little, and the intensity of steel for steel pipes material is high more in addition, makes difficult more.Usually, wall thickness/external diameter is a Electric Welded Steel Pipe below 0.15 than t/D, can adopt common Electric Welded Steel Pipe tubing machine to make, but when t/D surpasses 0.15, just exceed manufacturing capacity, therefore when adopting common Electric Welded Steel Pipe tubing machine, be difficult to directly make t/D mostly and surpass 0.15 and be no more than 0.30 heavy wall Electric Welded Steel Pipe of the present invention.

Therefore, adopt common Electric Welded Steel Pipe tubing machine to make wall thickness/external diameter than being Electric Welded Steel Pipe (being referred to as female pipe) 0.15 below, down it to be implemented undergauge rolling hot again, makes wall thickness/external diameter ratio above 0.15 and be no more than 0.30 heavy wall Electric Welded Steel Pipe.

Undergauge is rolling to be adopted stretching undergauge milling train to wait to carry out.

Stretching undergauge milling train, be on rolling axle, in series to possess a plurality of reduction units that around rolling axle, have the rolling support of 3 rollers or 4 rollers, the roller revolution of each the rolling support by adjusting this reduction unit and depress power, the tension force of tube axial direction (rolling direction) of control steel pipe and the force of compression of circumferential direction, the undergauge that the ratio of wall thickness/external diameter is increased is rolling.

That is, in undergauge was rolling, because the power of depressing of outer diameter of steel pipes, external diameter was reduced, and wall thickness increases, and but then, because the tension force that acts on the tube axial direction of steel pipe, wall thickness reduces, and therefore can determine final wall thickness according to the balance of the two.Carried out the wall thickness of the rolling steel pipe of such undergauge, mainly decide by the tension force between the rolling support, therefore try to achieve tension force between the rolling support that is used to obtain the target wall thickness according to rolling therory etc., setting the roller rotating speed of each rolling support is necessary so that its tension force plays a role.

As described above, the present invention is heated to 800～1200 ℃ with above-mentioned Electric Welded Steel Pipe (female pipe), is that to implement hot undergauge 40～80% times rolling at the section decrement, makes wall thickness/external diameter than for surpassing 0.15 and be no more than 0.30 heavy wall Electric Welded Steel Pipe.

At this, so-called section decrement is outer diameter of steel pipes * 100 (%) before (outer diameter of steel pipes behind the outer diameter of steel pipes-undergauge before the undergauge)/undergauge.

When the Heating temperature of the Electric Welded Steel Pipe when undergauge is rolling, 800 ℃ of less thaies, resistance to deformation is big, on the other hand when surpassing 1200 ℃, the heated oxide skin become significantly the surface texture deterioration.Therefore, Heating temperature is defined as 800～1200 ℃ scope.

In addition, during section decrement less than 40% when undergauge is rolling, force of compression is insufficient, is Electric Welded Steel Pipe (female pipe) below 0.15 ratio of making wall thickness/external diameter for surpassing 0.15 and to be no more than 0.30 heavy wall Electric Welded Steel Pipe be difficult by the ratio of wall thickness/external diameter.On the other hand, when the section decrement surpasses 80%,, be difficult to guarantee uniform shape in addition by becoming significantly of the surface imperfection of the rolling steel pipe that causes of undergauge.Therefore, the section decrement of undergauge in rolling is defined as 40～80%.

In addition, whether heavy wall Electric Welded Steel Pipe of the present invention can be made by undergauge is rolling, can judge perpendicular to the state or the mensuration wall thickness at the angle of the inner face of the section (C section) of tube axial direction by observation.

For example, the stretching undergauge milling train that in undergauge is rolling, uses, as described above, be on rolling axle, in series to possess a plurality of reduction units that around rolling axle, have the rolling support of 3 rollers or 4 rollers, the phase shifting of the roller of usually adjacent rolling support (for example N and N+1 support), in the occasion of 3 rolling system supports, 60 ° of angles of phase shifting, in the occasion of 4 rolling system supports, the phase place 45 that only staggers.

Therefore, the interior shape of the section perpendicular to tube axial direction (C section) by the rolling heavy wall Electric Welded Steel Pipe of making of undergauge, the occasion that possesses 3 rolling system supports at stretching undergauge milling train is a hexagon, is octagon in the occasion that possesses 4 rolling system supports.

In addition, when stretching undergauge milling train is 4 rolling supports of successive (for example N, N+1, the rolling support of N+2, N+3), it is 30 °, 60 °, 90 ° that the phase place of roller staggers in the occasion of 3 rolling system supports, staggering in the occasion of 4 rolling system supports is 22.5 °, 45 °, 67.5 ° occasion, the interior shape of the section perpendicular to tube axial direction of the heavy wall Electric Welded Steel Pipe after undergauge is rolling (C section), being dodecagon in the occasion that possesses 3 rolling system supports respectively, is ten hexagons in the occasion that possesses 4 rolling system supports.

So as can be known, the interior shape perpendicular to the section of tube axial direction of heavy wall Electric Welded Steel Pipe forms polygonal occasion as described above, and this heavy wall Electric Welded Steel Pipe can be made by undergauge is rolling.

Embodiment

Melting has the various steel of the composition shown in the table 2, is cast as strand.This strand is heated to 1150 ℃, under 630 ℃ of 890 ℃ of rolling processing temperatures, coiling temperature, carries out hot rolling, make the steel plate of thickness of slab 6mm.With the width that this hot-rolled steel sheet slitting becomes to stipulate, adopt high frequency welding to make the Electric Welded Steel Pipe of external diameter 90mm (female pipe).Then by high-frequency induction heating, it is rolling to implement undergauge after this steel pipe is heated to 980 ℃, makes the heavy wall Electric Welded Steel Pipe of wall thickness 7mm, external diameter 35mm.

In addition,, the section decrement of undergauge in rolling changed, thereby made the heavy wall Electric Welded Steel Pipe of wall thickness 5～7.5mm, external diameter 30～35mm for the Electric Welded Steel Pipe that the No.1 steel of employing table 2 is made.

Resulting heavy wall Electric Welded Steel Pipe is heated to 960 ℃ carries out water-cooled, thereby quench, and carry out the tempering of 300 ℃ * 1 hour and 350 ℃ * 1 hour.Produce test film from steel pipe and carry out various tests, confirm the characteristic of heavy wall Electric Welded Steel Pipe of the present invention.

About hardness, be to measure 5 points of heavy wall central part with Hv9.8N, obtain mean value.

About hot workability, the diameter that is to use parallel zone is the uniaxial tensile test sheet of 6mm, stretches down at 850 ℃, estimates according to the decrement of the basal area of fracture portion.

In addition, about fatigue characteristic, according to the described method of Japan's " spring collection of thesis; 28 (1983) P.46 ", produce the fatigue test piece with radius-of-curvature 60mm bending shown in Figure 3, one side is fixed, and the solid material that adopts same diameter is the fatigue test of implementing alternation under the stress condition of 600MPa at the 1st principle stress amplitude, tries to achieve the fracture cycle number.

These characteristics the results are shown in table 2 and table 3.

Table 3

Know the No.1 shown in the table 2～11 steel, have excellent characteristic aspect hardness, hot workability and the fatigue strength with chemical ingredients of the present invention.

Therefore relative therewith, the No.12 steel is because critical cooling velocity Vc is big, fails fully to quench, and the C amount is also low to be 0.22%, can not obtain the example of sufficient hardness.The No.13 steel is because the example that the N amount is too high so hot workability is poor and fatigue characteristic also reduce a little.The No.14 steel is because the C quantity not sufficient can not obtain as the example of vehicle structure with the required MIN hardness of member even therefore carry out tempering under 300 ℃.

Know steel pipe No.a～e of the present invention that table 3 is represented, have the fracture cycle number and surpass 50 * 10 ³Inferior sufficient fatigue strength.

Relative therewith, the No.f steel pipe is that t/D is too small, can not obtain the example of sufficient fatigue strength.

Among the present invention the expression numerical range " more than " and " following " include given figure.

Claims (according to the modification of the 19th of treaty)

1. (revise afterwards) a kind of hardening capacity, the high-strength thick Electric Welded Steel Pipe that hot workability and fatigue strength are all excellent, it is characterized in that, by quality %, contain C:0.25～0.4%, Si:0.01～0.50%, Mn:0.8～1.5%, below the P:0.05%, below the S:0.05%, below the Al:0.05%, Ti:0.005～0.05%, B:0.0005～0.01%, more than the N:0.001% and less than 0.004%, its surplus is made up of Fe and unavoidable impurities, by formula＜1〉expression critical cooling velocity Vc less than 30 ℃/second, the ratio t/D of wall thickness t and outer diameter D is for greater than 0.15 and be no more than 0.30 scope

logVc＝2.94-0.75β <1>

Wherein, β=2.7C+0.4Si+Mn.

2. the high-strength thick Electric Welded Steel Pipe that hardening capacity according to claim 1, hot workability and fatigue strength are all excellent, it is characterized in that, by quality %, also contain among Cr:0.1～1%, Mo:0.05～1%, V:0.01～0.5%, Ni:0.1～1% more than a kind or 2 kinds, and in formula＜1〉in, β=2.7C+0.4Si+Mn+0.45Ni+0.8Cr+2Mo.

3. the high-strength thick Electric Welded Steel Pipe that hardening capacity according to claim 1 and 2, hot workability and fatigue strength are all excellent is characterized in that, by quality %, also contains Nb:0.01～0.1%.

4. according to all excellent high-strength thick Electric Welded Steel Pipe of each described hardening capacity, hot workability and the fatigue strength of claim 1～3, it is characterized in that,, also contain Ca:0.0002～0.005% by quality %.

5. the equal manufacture method of excellent high-strength thick Electric Welded Steel Pipe of a hardening capacity, hot workability and fatigue strength, it is characterized in that, the Electric Welded Steel Pipe that will have each described composition of claim 1～4 is heated to 800～1200 ℃, is that to carry out undergauge under 40～80% the scope rolling at the section decrement.

Claims (5)

1. hardening capacity, the high-strength thick Electric Welded Steel Pipe that hot workability and fatigue strength are all excellent, it is characterized in that, by quality %, contain C:0.25～0.4%, Si:0.01～0.50%, Mn:0.8～1.5%, below the P:0.05%, below the S:0.05%, below the Al:0.05%, Ti:0.005～0.05%, B:0.0005～0.01%, N:0.001～0.005%, its surplus is made up of Fe and unavoidable impurities, by formula＜1〉expression critical cooling velocity Vc less than 30 ℃/second, the ratio t/D of wall thickness t and outer diameter D is for greater than 0.15 and be no more than 0.30 scope

logVc＝2.94-0.75β<1>

Wherein, β=2.7C+0.4Si+Mn.

2. the high-strength thick Electric Welded Steel Pipe that hardening capacity according to claim 1, hot workability and fatigue strength are all excellent, it is characterized in that, by quality %, also contain among Cr:0.1～1%, Mo:0.05～1%, V:0.01～0.5%, Ni:0.1～1% more than a kind or 2 kinds, and in formula＜1〉in, β=2.7C+0.4Si+Mn+0.45Ni+0.8Cr+2Mo.

3. the high-strength thick Electric Welded Steel Pipe that hardening capacity according to claim 1 and 2, hot workability and fatigue strength are all excellent is characterized in that, by quality %, also contains Nb:0.01～0.1%.

4. according to all excellent high-strength thick Electric Welded Steel Pipe of each described hardening capacity, hot workability and the fatigue strength of claim 1～3, it is characterized in that,, also contain Ca:0.0002～0.005% by quality %.

5. the equal manufacture method of excellent high-strength thick Electric Welded Steel Pipe of a hardening capacity, hot workability and fatigue strength, it is characterized in that, the Electric Welded Steel Pipe that will have each described composition of claim 1～4 is heated to 800～1200 ℃, is that to carry out undergauge under 40～80% the scope rolling at the section decrement.

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