CN101624679B - Steel wire material for spring and its producing method - Google Patents

Abstract

The steel wire material for a spring of the invention contains; C: 0.37-0.54%, Si: 1.7-2.30%, Mn: 0.1-1.30%, Cr: 0.15-1.1%, Cu: 0.15-0.6%, Ti: 0.010-0.1%, Al: 0.003-0.05%, and the balance including iron with inevitable impurities, wherein ferrite decarburized layer depth is 0.01 mm or less, whole decarburized layer depth is 0.20 mm or less, and fracture reduction of area is 25% or more. It alternately may contain; C: 0.38-0.47%, Si: 1.9-2.5%, Mn: 0.6-1.3%, Ti: 0.05-0.15%, Al: 0.003-0.1%, and the balance including iron with inevitable impurities, wherein ferrite decarburized layer depth is 0.01 mm or less.

Description

Steel wire material for spring and manufacture method thereof

Technical field

The present invention relates to not exist in fact the ferrite Decarburized layer, and good steel wire material for spring and the manufacture method thereof of processibility.

In addition, the invention still further relates to the useful spring steel (spring steel) of helical spring former material that uses as under the state of modified (Q-tempering), relate to the steel wire material for spring of corrosion fatigue excellent in more detail.

Background technology

In the steel wire material for spring that requires high fatigue strength, refer generally to high-alloying, in addition for the endurance that improves the spring original material wire behind the Q-tempering than and add a large amount of Si.But the heavy addition of Si can make the austenite region in the average phasor narrow, and therefore the ferrite decarburization takes place easily.

In order to widen austenite region, add so-called Ni, Cu, the such alloying element of Mn gets final product to suppress the ferrite decarburization.But when only adding these alloying elements, the hardenability of wire rod excessively increases, and in the process of cooling after hot rolling, the so cold tissue of mistake of bainite and martensite takes place easily.This crosses cold tissue the Wire Drawing of wire rod (the particularly wire rod of particle size) is caused detrimentally affect, broken string (カ Star ピ one disconnected Line) and transverse crack fracture generation that the V-arrangement crackle of wire rod length direction is caused.

Therefore, keeping good processibility frontier defense for the limit ends the ferrite decarburization and has proposed various technology.For example opening the spy and disclose a kind of technology in 2002-194432 number, is to begin from hot rolled to the whole process that finishes, by with the steel temperature maintenance than A ₃The temperature field that transformation temperature is high, and the speed of cooling after the hot rolling is in more than 0.5 ℃/second, thus prevent the ferrite decarburization.Same in addition patent documentation is also open, processibility is improved, and make speed of cooling below 3.0 ℃/second.

In addition, opening the spy and to disclose a kind of technology in 2007-9300 number, is in the process of cooling of wire coil, chilling A ₃Transformation temperature and A ₁The temperature range in the decarburization zone between the transformation temperature (eutectoid transformation temperature) prevents the ferrite decarburization thus.Also disclosing a kind of technology with patent documentation in addition, is by Xu Leng behind described chilling, with the promotion pearlitic transformation, thus the normal temperature processibility of raising wire rod.

, in employed springs such as automobile, require lightweight in order to reduce tail gas and raising fuel efficiency, as wherein one encircling and the sensing high strength.In the spring (tensile strength behind the Q-tempering for example is the spring more than the 1900MPa) of high strength, generally become problem because of losing in early days of causing of hydrogen embrittlement and corrosion fatigue.

In order to solve such problem, proposed to have various technology in the past.Known for example have with Cr as general corrosion-resistant raising element, but the spy opens the 2002-47539 communique and proposes, during tension test under the low rate of straining after carrying out the brine spray round-robin test, add the situation that Cr has erosion resistance to reduce on the contrary, in order to improve erosion resistance at this moment, Cu and Ni are that effectively the total amount that makes Cu and Ni is more than 2 times of Cr.

The spy opens 2004-10965 number and points out, C is the reason that corrosion fatigue strength reduces, it is effective for improving erosion resistance to reduce C, and because the reduction of the anti-permanent strain that the reduction of C may cause, can prevent by the interpolation of Si, Cu, Ni etc., Cu and Ni be used to improve on the erosion resistance also effective.

But these 2 its state of the art of patent documentation all be cannot say for sure abundant height, and corrosion fatigue strength still has the leeway of further raising.For example in those patent documentations, only too recognize its excellent corrosion resistance about Ni, studying in great detail of the mechanism of action that it is detailed and merits and demerits two aspects is then not enough.External same, also be like this about the element beyond the Ni.

In order to prevent the ferrite decarburization, though propose to have above-mentioned various prior art, its effect is also insufficient.For example the spy opens 2002-194432 number, though can reach ferrite decarburizaed depth 0mm in the embodiment hurdle, at this moment the Si of the steel of Shi Yonging amount is 1.79 fewer weight %.Though the spy opens 2007-9300 number and also can reach ferrite decarburizaed depth 0mm in addition, at this moment the C of the steel of Shi Yonging amount is 0.48 many weight %.The C amount is measured after a little while with Si for a long time, the ferritic-pearlitic attenuation in the continuous cooling curve (Continuous-Cooling-Transformation:CCT), so ferrite decarburization ratio is easier to.The spy opens 2002-194432 number and the special technology of opening 2007-9300 number, because the composition that can be suitable for system is restricted, so expect that further the ferrite decarburization prevents technology.

Summary of the invention

Therefore the objective of the invention is to, provide a kind of and can suppress the ferrite decarburization more to heavens, and also can improve the manufacture method of the spring wire rod of processibility, and manufacture method and the steel wire material for spring that obtains in view of the above.

Another object of the present invention is to, be that the Si amount is high even provide a kind of, and during the low hypoeutectoid steel of C amount, also can prevent the ferrite decarburization, and also can improve the manufacture method of the steel wire material for spring of processibility, and the steel wire material for spring of creating conditions in view of the above and obtaining.

Another other purposes of the present invention are, provide a kind of can be at the steel wire material for spring that improves corrosion fatigue strength (corrosion fatigue strength after particularly modified) on the higher level.

Present inventors are research with keen determination repeatedly in order to solve aforementioned problems, it found that, open the transformation temperature that the 2002-194432 grade is only considered steel as the spy, only control hot-rolled condition, what can prevent really only is the decarburization of core, the C amount that is the steel surface in the hot rolling is lacked than core, and when therefore distinguishing the transformation temperature of only considering steel, decarburization is further carried out on acceleration ground.Therefore,, set rolling temperature, still can prevent the ferrite decarburization more to heavens, thereby finish this first invention if avoid the ferrite territory under this state even hypothesis C=0 quality % is carried out in the decarburization of steel surface.

This first invention is a steel wire material for spring, wherein contains the C:0.37～0.54% (meaning of quality %.Si:1.7～2.30%, Mn:0.1～1.30%, Cr:0.15～1.1%, Cu:0.15～0.6%, Ti:0.010～0.1%, Al:0.003～0.05% down together),, surplus is made of iron and unavoidable impurities, ferrite decarburizaed depth is below the 0.01mm, the total decarburized layer degree of depth is below the 0.20mm, and the fracture shrinking percentage is more than 25%.

In above-mentioned steel wire material for spring, also can suit to make up and contain at least a of Ni:0.15～0.7%, V:0.07～0.4% and Nb:0.01～0.1%, Mo:0.01～0.3%.

In above-mentioned steel wire material for spring, preferred P is below 0.020%, below the S:0.020%, below the N:0.0070%, below the O:0.0015%.

The manufacture method of above-mentioned steel wire material for spring is with steel hot rolling and after batching, and uses cooling bed cooling, the A in the equilibrium phase diagram of described steel during C=0 quality % ₁Transformation temperature, A ₃Transformation temperature and A ₄Transformation temperature is called A _{1 (C=0)}Transformation temperature, A _{3 (C=0)}Transformation temperature, A _{4 (C=0)}During transformation temperature, the Heating temperature that makes the steel before the hot rolling is more than 900 ℃, A _{4 (C=0)}Below the transformation temperature, making the Da Wendu that is up to of the steel in the hot rolled finish to gauge is A _{3 (C=0)}Transformation temperature is above, A _{4 (C=0)}Below the transformation temperature, being rolled onto cooling bed mounting temperature is A _{1 (C=0)}Transformation temperature is above, A _{1 (C=0)}Below transformation temperature+50 ℃, according to the continuous cooling curve of the grain size number of corresponding austenite crystal numbers 8.0～11, the temperature range that the cooling ferrite is separated out is more than 1.0 ℃/second in coiled material compact part speed of cooling, is below 8 ℃/second in coiled material part of rarefaction speed of cooling.

The manufacture method of the above-mentioned steel wire material for spring of design temperature condition more specifically, be hot-strip and batch after, cool off with cooling bed, making the Heating temperature of the preceding steel of hot rolling is more than 900 ℃, below 1250 ℃, the Da Wendu that is up to of the steel in the hot rolled finish to gauge is more than 1050 ℃, below 1200 ℃, the temperature of coiled material mounting during to cooling bed go up is more than 900 ℃, below 980 ℃, temperature range territory 750～600 ℃ of temperature, in coiled material compact part speed of cooling is more than 1.0 ℃/second, is below 8 ℃/second in coiled material part of rarefaction speed of cooling.

In the manufacture method of above-mentioned steel wire material for spring, do not carry out the water-cooled of the preceding steel of finish to gauge, but make steel processing heating by finish to gauge, also the Da Wendu that is up to of the steel in the finish to gauge can be controlled at aforementioned range thus.

In the manufacture method of above-mentioned steel wire material for spring, the ideal critical diameter DCI by following formula (1) expression of described steel for example is 75～135mm.

DCI(mm)＝25.4×(0.171+0.001[C]+0.265[C] ²)×

(3.3333[Mn]+1)×(1+0.7[Si])×

(1+0.363[Ni])×(1+2.16[Cr])×

(1+0.365[Cu])×(1+1.73[V])×

(1+3[Mo]) …(1)

(in the following formula, the content (quality %) of each element in [] expression steel.)

Also have, in this manual, the A in the equilibrium phase diagram of steel during C=0 quality % ₁Transformation temperature, A ₃Transformation temperature and A ₄Transformation temperature is called A _{1 (C=0)}Transformation temperature, A _{3 (C=0)}Transformation temperature, A _{4 (C=0)}Transformation temperature.

Equilibrium phase diagram for example can utilize Thermo-Calc (selecting BCC-A2, FCC-A1, LIQUID, a CEMENTITE4 phase) mapping.

According to this first invention, suppose the state of the C=0 quality % that takes place at steel surface and set rolling condition, therefore the ferrite decarburization can be suppressed more to heavens, and also processibility can be improved.

In addition, present inventors are research with keen determination repeatedly in order to solve aforementioned problems, it found that, in order to improve corrosion fatigue strength, except preventing the ferrite decarburization, also need to improve intensity (hardness), corrosion pit shape, this 3 point of hydrogen embrittlement resistance of steel, and illustrated the influence that various elements are given these 3 complexity of bringing, thereby finished this second invention.

This second invention is a steel wire material for spring, wherein, contain C:0.38～0.47%, Si:1.9～2.5%, Mn:0.6～1.3%, Ti:0.05～0.15% and Al:0.003～0.1%, surplus is made of iron and unavoidable impurities, ferrite decarburizaed depth is below the 0.01mm, by the Ceq1 shown in the following formula (1) is more than 0.580, is below 0.49 by the Ceq2 shown in the following formula (2), is below 0.570 by the Ceq3 shown in the following formula (3).

Ceq1＝[C]+0.11[Si]-0.07[Mn]-0.05[Ni]+0.02[Cr]…(1)

Ceq2＝[C]+0.30[Cr]-0.15[Ni]-0.70[Cu]…(2)

Ceq3＝[C]-0.04[Si]+0.24[Mn]+0.10[Ni]+0.20[Cr]-0.89[Ti]-

1.92[Nb]…(3)

(in the following formula, the content (quality %) of each element in [] expression steel.)

In above-mentioned steel wire material for spring, also can contain Cr:0.1～0.4%, Cu:0.1～0.7%, Ni:0.1～0.7% or Nb:0.01～0.1% as required again.

In above-mentioned steel wire material for spring, preferred P is below 0.02%, below the S:0.02%, below the N:0.007%, below the O:0.0015%.

In above-mentioned steel wire material for spring, after carrying out the corrosion test shown in following, among the surperficial viewed corrosion pit of test film, select corrosion pit more than 5 from big to small successively by the degree of depth, preferably these corrosion pits are below 0.9 by the mean value of the long-width ratio shown in the following formula (4).

Long-width ratio=(degree of depth of corrosion pit * 2)/(width of corrosion pit) ... (4)

Corrosion test:

After 10 minutes, the oil cooling with 70 ℃ of temperature carries out oil quenching with 925 ℃ temperature heating spring steel wires, then carried out tempering in 60 minutes with 400 ℃ temperature heating after, with No. 800 sand paper lapped face, the test film that the making corrosion test is used.

To this test film, follow the NaCl aqueous solution 8 hour of JIS Z 2371 with 35 ℃ of spraying 5 quality %, thereafter, test film was kept 16 hours in the moist environment of 35 ℃ of humidity 60% and temperature, this as 1 circulation, is carried out 14 circulations with it.

Thereafter, except that after derusting, with the corrosion pit on laser microscope viewing test sheet surface.

According to this second invention, after preventing the ferrite decarburization, because suitably controlled various alloying elements, so can improve the hardness of the steel after modified (Q-tempering), can make the corrosion pit planarization, and can improve patience, consequently, can realize excellent corrosion fatigue strength for hydrogen embrittlement.Steel wire material for spring of the present invention in addition, alloying element obtain saving, and economy is also excellent.

Description of drawings

Fig. 1 is the graphic representation of the relation of the Vickers' hardness measured among the expression embodiment 2 and Ceq1.

Fig. 2 is the graphic representation of the relation of the long-width ratio of the corrosion pit measured among the expression embodiment 2 and Ceq2.

Fig. 3 is the graphic representation of the relation of the hydrogen embrittlement crackle life-span measured among the expression embodiment 2 and Ceq3.

Embodiment

At first, describe for this first invention.

Present inventors are found that of research with keen determination repeatedly, creates conditions by suitable control, can prevent the ferrite decarburization, and the spring that can make excellent in workability made steel wire rod.Below, at first describe for this first creating conditions of invention (following only be called the present invention).For the chemical ingredients of steel form and describe thereafter.

In the present invention, set at the state of supposition C=0 quality % on the point of rolling condition and have maximum feature.Even the state of C=0 quality % still is rolled with the condition that ferrite is difficult to decarburization, can prevent the decarburization of steel surface thus more to heavens.

If in more detail, then be in rolling, steel are remained on the A that calculates according to the assembly component ₃The temperature that transformation temperature is above, the carbon diffusion in the steel that prevents to cause by phase transformation, the carbon concentration of steel surface also can reduce slowly.And in hypoeutectoid steel, if the minimizing of C amount, then A ₃Transformation temperature rises.On the other hand, rolling temperature (steel temperature) can reduce slowly in roughing and intermediate rolling stage especially.And, if the rolling temperature that reduces becomes the A on the steel surface of rising slowly slowly ₃Below the transformation temperature, then undergo phase transition, carry out rapidly by the ferrite decarburization that the carbon diffusion causes on the steel surface.Therefore in the present invention, carry out in this case, also improved and created conditions, make not residual ferrite Decarburized layer in the wire rod that finally obtains in such ferrite decarburization.

That is, in manufacture method of the present invention, after described roughing and intermediate rolling, carry out final rolling in, what make steel is up to Da Wendu at A _{3 (C=0)}Transformation temperature is above (to be preferably A _{3 (C=0)}More than transformation temperature+50 ℃, A more preferably _{3 (C=0)}More than transformation temperature+70 ℃), A _{4 (C=0)}Transformation temperature is following (to be preferably A _{4 (C=0)}Below-50 ℃, A more preferably _{4 (C=0)}Below-100 ℃).If in finish to gauge, be heated to A _{3 (C=0)}More than the transformation temperature, (for example intermediate rolling stage after roughing) is even be in A then before this _{3 (C=0)}The ferrite decarburization takes place below the transformation temperature, because the counter-diffusion of C also can make the ferrite Decarburized layer disappear.Also have, why make be up to temperature on be limited to A _{4 (C=0)}Below the transformation temperature, be owing to, then generate delta ferrite, so the ferrite decarburization is carried out on the contrary on the steel surface if surpass this temperature.In addition, if be in A _{4 (C=0)}More than the transformation temperature, then become very high temperature, therefore, still can carry out total decarburization (all decarburizations).

Open in 2002-194432 number openly the spy, " ferrite decarburization, it is because in the body tissue difficult to understand ferrite transformation takes place under two phase region temperature " is therefore " in hot rolling by remaining on A with steel ₃More than the transformation temperature, avoid two phase region temperature, the ferrite decarburization is taken place ".But the spy opens 2002-194432 number about making in a single day because of being reduced to A ₃Transformation temperature is following and point that ferrite Decarburized layer that produce disappears, and how the transformation temperature that makes carbon concentration become the steel surface of 0 quality % to be reflected as and to create conditions, then not open also hint not.

If the concrete Da Wendu that is up to that represents the steel in the finish to gauge then for example is (to be preferably more than 1100 ℃) more than 1050 ℃, (be preferably below 1150 ℃) below 1200 ℃.This temperature is than common finishing temperature height.

The method that is used to make finishing temperature be in described scope is not particularly limited, but omits the water-cooled (comprise and weaken water-cooled) carry out usually before finish to gauge, and the processing when utilizing finish to gauge is generated heat and improved the steel temperature and get final product.

Rolling temperature (for example outlet temperature of intermediate rolling) before the finish to gauge is not specially limited, and as above-mentioned, can be A yet _{3 (C=0)}Transformation temperature is following (to be preferably A _{3 (C=0)}Below-50 ℃, A more preferably _{3 (C=0)}Below-100 ℃.Perhaps below 1000 ℃, be preferably below 950 ℃, more preferably below 930 ℃).Be in A _{3 (C=0)}Below the transformation temperature,, also can this ferrite glue carbon be disappeared by finish to gauge even have the ferrite decarburization to carry out at steel surface.Rolling temperature before the finish to gauge is generally more than 850 ℃, is preferably more than 860 ℃, more preferably more than 870 ℃.

In addition in manufacture method of the present invention, the condition before and after the hot rolling (heating condition, the cooling conditions after batching) is as follows.

The Heating temperature of the steel before the hot rolling is (to be preferably more than 1000 ℃, more preferably more than 1100 ℃) A more than 900 ℃ _{4 (C=0)}Transformation temperature following (be preferably below 1250 ℃, be preferably below 1200 ℃).The preferred especially A of described Heating temperature _{3 (C=0)}More than the transformation temperature.If Heating temperature is low excessively, then the hot rolled productivity reduces.Long in the residence time in ferritic-austenitic territory in addition.On the other hand, if Heating temperature surpasses A _{4 (C=0)}Ferrite decarburization that δ-ferrite transformation causes then because of taking place, and being undertaken by the full decarburization that heat causes in transformation temperature.

Cooling conditions after importantly batching in the condition of (after the finish to gauge) after the hot rolling.The wire rod of finish to gauge is after batching, and by cooling bed cooling, this cooling conditions can make a big impact to the processibility of decarburized layer deepness and wire rod.

Cooling beginning temperature can be set at volume (round wire rod) to cooling bed mounting temperature.This mounting temperature is A _{1 (C=0)}Transformation temperature is above (to be preferably A _{1 (C=0)}More than transformation temperature+5 ℃, A more preferably _{1 (C=0)}More than transformation temperature+10 ℃.Perhaps more than 900 ℃, be preferably more than 920 ℃, more preferably more than 925 ℃), A _{1 (C=0)}(be preferably A below transformation temperature+50 ℃ _{1 (C=0)}Below transformation temperature+45 ℃, A more preferably _{1 (C=0)}Below transformation temperature+40 ℃.Perhaps below 980 ℃, be preferably below 975 ℃, more preferably below 970 ℃).If the mounting temperature is low excessively, then elongated in the residence time in the single-phase territory of ferrite, ferrite decarburization and full decarburization are easy to generate.Otherwise if the mounting temperature is too high, then thickization of austenite crystal (austinite grain size number for example will be lower than 8.0) retreats at CCT line chart Medium pearlite nose.Consequently, in the cooling after mounting, cold tissue (bainite and martensite) will take place easily, the processibility deterioration of wire rod.If thickization of crystal grain, the crystal grain boundary (ternary point crystal boundary) that then becomes the nuclear of pearlitic transformation tails off in addition, and pearlitic transformation begins temperature and reduces, so ferrite increases easily, and the control of ferrite decarburization is difficulty also.

In cooling bed, be divided into the compact part (the width two ends of cooling conveyer belt) of wire coil and the part of rarefaction (the but width central authorities of travelling belt) of wire coil and come controlled chilling speed very important.The compact part of volume is compared with part of rarefaction, and its speed of cooling is slack-off easily, if its speed of cooling is slow excessively, then decarburization (particularly ferrite decarburization) produces.Therefore the speed of cooling of coiled material compact part is more than 1.0 ℃/second, is preferably more than 1.3 ℃/second, more preferably more than 1.5 ℃/second.On the other hand, the part of rarefaction of volume is compared with compact part, and its speed of cooling accelerates easily, if its speed of cooling is too fast, then is easy to generate cold tissue.Therefore the speed of cooling of part of rarefaction is below 8 ℃/second, is preferably below 7 ℃/second.

The management of described speed of cooling, it carries out the CCT curve and is advisable.In the present invention, number be about 8.0～11 in the austenitic grain size in cooling bed stage preferably in mounting, consider that therefore the pairing CCT curve of this grain size number manages speed of cooling.In the present invention promptly, in described CCT curve, make the speed of cooling of the temperature range that ferrite separates out (for example separating out beginning temperature (Fs) to perlite from ferrite separates out between the beginning temperature (Ps)) be in above-mentioned scope, so come controlled chilling speed.In addition, estimating with arbitrary CCT curve of austenitic grain size numbers 8.0～11, all is to make speed of cooling be in above-mentioned scope to come controlled chilling speed.

The temperature range of management speed of cooling also can be set by concrete numerical range, and this management temperature range for example is 750～600 ℃.

The speed of cooling of coiled material compact part and part of rarefaction for example can be controlled respectively by regulating its air quantity everywhere of collision.

Also have, to the condition of batching, its design is to make the wire rod after batching directly supply to cooling bed with the mounting temperature of regulation after finish to gauge.Usually, the mounting temperature that is chilled to regulation by water-cooled or air-cooled (preferred water-cooled) after the finish to gauge is batched afterwards.By chilling, the cooling on cooling bed begins, and can prevent that also the ferrite decarburization from beginning.

According to described manufacture method, can suppress the ferrite decarburization more to heavens, and also can improve processibility.Therefore, even the Si amount is high, and the steel of this easy ferrite decarburization of C amount, also can prevent the ferrite decarburization.

Can either prevent the ferrite decarburization according to described manufacture method, the composition of steel that can improve processibility again is as follows.

C：0.37～0.54％、

Si：1.7～2.30％、

Mn：0.1～1.30％、

Cr：0.15～1.1％、

Cu：0.15～0.6％、

Ti：0.010～0.1％、

Al：0.003～0.05％，

Surplus: iron and unavoidable impurities,

Below, be described in detail with regard to the qualification reason of composition.

C：0.37～0.54％

If the C amount is superfluous, then hardenability excessively increases, and cold tissue took place in the process of cooling after rolling, the processibility deterioration of wire rod, so the C amount is below 0.54%.And adopting manufacture method of the present invention, the C amount further reduces, and also can prevent the ferrite decarburization.C amount is few more in addition, further improve on the point of processibility favourable more.Therefore, preferred C amount is below 0.48%, particularly preferably in below 0.42%.On the other hand, if C amount excessively reduces, then ferrite is separated out the zone and is increased, the ferrite decarburization prevent difficulty.Intensity behind the Q-tempering (hardness) reduces in addition.Therefore the C amount is decided to be more than 0.37% and (is preferably more than 0.38%).

Si：1.7～2.30％

Si helps intensity to improve (for example matrix strength raising) as the solution strengthening element, and endurance is improved, so the Si amount is more than 1.7%.And adopt manufacture method of the present invention, and further make the Si increment, also can prevent the ferrite decarburization.Therefore in the present invention, the lower limit set of Si amount can be got higherly, for example also can be set at more than 1.75%, even particularly the Si amount also can prevent the ferrite decarburization at (for example more than 2.0%) more than 1.9%, this is narrowed is advantage of the present invention.But if Si amount is superfluous, then ferrite is separated out the zone and is increased, the preventing difficulty of ferrite decarburization.Therefore the Si amount is decided to be below 2.30%.Described Si amount is preferably below 2.1%, more preferably below 1.9%.

Mn：0.1～1.30％

Mn improves the hardenability of steel, is effective elements on the hardness after being used to guarantee Q-tempering.If the Mn amount is very few, then be difficult to reach the desired hardenability of spring wire rod.Otherwise if Mn amount is superfluous, cold tissue took place, the processibility deterioration of wire rod in the cooling after then rolling.Therefore the Mn amount is decided to be more than 0.1% and (is preferably more than 0.12%, more preferably more than 0.2%), (is preferably below 1.0%, more preferably below 0.9%, more preferably below 0.8%) below 1.30.

Cr：0.15～1.1％

Cr makes the matrix strengthening of steel by solution strengthening, still improves the element of hardenability in addition.If the Cr amount is very few, then be difficult to reach the desired hardenability of spring wire rod.Otherwise if Cr amount surplus, then the processibility deterioration of wire rod.Therefore the Cr amount is (to be preferably more than 0.2%, more preferably more than 0.5%, to be preferably more than 1.0% especially) more than 0.15%, (is preferably below 1.05%) below 1.1%.

Cu：0.15～0.6％

Cu has the effect of the erosion resistance that improves steel, when still suppressing hot rolling in addition and the element of ferrite decarburization during the thermal treatment in the spring processing.If but the Cu surplus, then thermal crack produces.Therefore the Cu amount is decided to be more than 0.15% and (is preferably more than 0.20%), (be preferably below 0.5%) below 0.6%.

Ti：0.010～0.1％

Ti makes the old austenite crystal miniaturization behind the Q-tempering, is effective elements in the raising of atmosphere weather resistance and hydrogen embrittlement resistance.Ti forms the Ti carbide in addition, prevents thickization of austenite crystal when mounting, and it is also effective that cold organizational aspects took place the cooling after being used for preventing mounting.But basic Ti amount is superfluous, and then thick Ti nitride is separated out, the processibility deterioration.Therefore the Ti amount is decided to be more than 0.010% and (is preferably more than 0.020%), (be preferably below 0.09%) below 0.1%.

Al：0.003～0.05％

Al is the reductor when handling as molten steel and the element that plays a role.Al forms fine Al nitride in addition, under its pinning effect and have the effect that makes the crystal grain miniaturization.But if Al amount is superfluous, then form thick Al oxide compound, bring disadvantageous effect to fatigue characteristic etc.Therefore the Al amount is decided to be more than 0.003% and (is preferably more than 0.005%), (be preferably below 0.03%) below 0.05%.

The basal component of the steel that use among the present invention (and the steel wire material for spring that obtains thus) is formed as mentioned above, and surplus comes down to iron.But the unavoidable impurities that certainly allows the situation of goods and materials, producing apparatus etc. because of iron material (containing scrap iron), auxiliary material etc. to sneak into is included in the steel (wire rod).Also can this unavoidable impurities of strict control, for example P, S, O, N etc. can be controlled in the following scope.

Below the P:0.020%

P makes embrittlement of grain boundaries in old austenite grain boundary segregation, is the element that fatigue characteristic are reduced.Therefore the P amount is low as far as possible more for preferred, for example can be controlled at below 0.020% (to be preferably below 0.010%).

Below the S:0.020%

S makes embrittlement of grain boundaries in old austenite grain boundary segregation, is the element that fatigue characteristic are reduced.Therefore the S amount is low as far as possible more for preferred, for example can be controlled at below 0.020% (to be preferably below 0.010%).

Below the N:0.0070%

N amount is many more, can form thick nitride with Ti and Al more, brings disadvantageous effect to fatigue characteristic etc.Therefore the N amount is the least possible more for preferred, for example can be controlled at below 0.0070% (to be preferably below 0.005%).On the other hand, if excessively reduce the N amount, then productivity significantly reduces.N forms nitride with Ti and Al and helps the miniaturization of crystal grain in addition.If from this viewpoint, then preferably N amount is set at more than 0.001% and (is preferably more than 0.002%).

Below the O:0.0015%

If the O amount is superfluous, then thick oxide based inclusion (Al ₂O ₃Deng) form, bring disadvantageous effect to fatigue characteristic etc.Therefore (be preferably below 0.0010%) being defined as below 0.0015% on the O amount.On the other hand, the lower limit of O amount (is preferably more than 0.0004%) being generally on the industrial production more than 0.0002%.

In addition, steel of the present invention also can contain following selection element as required.

Ni：0.15～0.7％

Ni have suppress rolling before and rolling in the effect of ferrite decarburization, be element in addition with the flexible effect that improves the former material of spring behind the Q-tempering.Therefore as required, recommend to contain the amount that Ni is preferably (more preferably more than 0.2%) more than 0.15%.But if the Ni amount is superfluous, then because Q-tempering causes the retained austenite amount to increase, tensile strength reduces.Therefore making it to contain sometimes Ni amount is decided to be below 0.7% and (is preferably below 0.65%, more preferably below 0.6%).

V:0.07～0.4% and/or Nb:0.01～0.1%

V and Nb form fine compound (V carbide, nitride and their complex chemical compound, Nb carbide, nitride, sulfide and their complex chemical compound), has the effect that improves hydrogen embrittlement resistance and fatigue characteristic, its performance crystal grain micronized effect also is the element with the effect that improves toughness and endurance in addition.V also helps the raising of weather resistance in addition.Therefore as required, recommend to contain the preferred amount of (more preferably more than 0.10%) more than 0.07% of V, contain the amount that Nb is preferably (more preferably more than 0.02%) more than 0.01%.

But if V and Nb surplus then do not have the amount of the carbide of solid solution to increase in austenite when the heating of quenching, thereby can not get full intensity.Except this drawback, if the V amount is superfluous, then the retained austenite amount increases, and spring hardness reduces, and encourages the ferrite decarburization in the hot rolling in addition.If the Nb amount is superfluous, then form thick Nb nitride in addition, counter trade produces fatigue and loses.Therefore make it to contain sometimes V amount and be decided to be 0.4% below and (be preferably below 0.3%), Nb measures to be decided to be below 0.1% and (is preferably below 0.05%).

Mo：0.01～0.3％

Mo guarantees hardenability, and softening impedance is improved, and is to be used to improve effective elements on the weather resistance.Therefore recommend to contain the amount that Mo is preferably (more preferably more than 0.02%) more than 0.01%.But if Mo amount is superfluous, cold tissue, processibility and deteriorated ductility took place then during the cooling after hot rolling.Therefore making it to contain sometimes Mo amount is decided to be below 0.3% and (is preferably below 0.2%).

B：0.0003～0.005％

B is the grain boundary segregation that prevents P, to improving hydrogen embrittlement resistance, toughness and ductility effective elements.In wire rod, can contain B as required.In addition, B does not rely on the interpolation of a large amount of alloying elements, can increase hardenability on a small quantity.Therefore, the ferritic of wire surface that suppresses to generate among the Xu Leng of rolling back separated out, and the hardness the during quenching when guaranteeing that spring is made is to very dark.B recommends preferably to contain the amount of (more preferably more than 0.0005%) more than 0.0003%.When but B content is excessive, form Fe ₂₃(CB) ₆Deng B compound free B reduce, therefore, prevent that the effect of grain boundary segregation of P is saturated.In addition, because this B compound is more when thick, therefore, become the starting point of fatigue damage, fatigue characteristic reduce.Therefore, when containing B, qualifying bit on it (is preferably below 0.004%) below 0.005%.

In the steel of Shi Yonging (and steel wire material for spring of the present invention), can make the ideal critical diameter DCI shown in the following formula (1) in the method for the invention, be preferably 80～120mm, more preferably 85～110mm for for example 75～135mm.If making DIC is more than the 75mm, guarantee that then spring strength becomes easy.By making DCI below 130mm, guarantee that processibility becomes easy in addition.

If use above-mentioned steel and follow method of the present invention and make steel wire material for spring, then can prevent decarburization, and can improve processibility.More particularly, according to the present invention, can make the ferrite decarburizaed depth of wire rod for example be essentially 0mm (specifically for below the 0.01mm, be preferably 0.00mm), full decarburized layer deepness for example (is preferably below the 0.18mm for 0.20mm is following, more preferably 0.15mm is following), can make the fracture shrinking percentage is (to be preferably more than 28%, more preferably more than 30%) more than 25%.Also have, tensile strength for example is 1000MPa above (being preferably about 1100～1500MPa, more preferably about 1200～1400MPa).

Steel wire material for spring of the present invention, not only the ferrite decarburization is prevented, and processibility is also excellent, therefore also can be used in the drawing processing of being undertaken by particle size under rolling state.The line of steel wire material for spring of the present invention footpath for example is that 5～25mm (is preferably 7～23mm, more preferably 10～20mm).

Secondly, describe for this second invention.

The steel wire rod of this second invention (following only be called the present invention) has characteristic following on some: the ferrite decarburization is prevented, and the hardness height after modified, and the pit that produces because of corrosion is smooth, and hydrogen embrittlement resistance improves.Its corrosion fatigue strength excellence of such steel.Preventing of described ferrite decarburization can be reached by preparing to create conditions.Hardness after modified, the shape of corrosion pit, hydrogen embrittlement resistance after having prevented the ferrite decarburization, just can be reached by suitable control alloying element (promptly by making above-mentioned Ceq1～3 suitable).Next coming in order describe.

In the present invention, prevent the ferrite decarburization by preparing manufacture method.Though the ferrite decarburization also can reduce by the control alloying element, but the addition of alloying element increases in this case, economy might reduce, and the control with Ceq1～3 is difficult to exist side by side in addition, therefore prevents the ferrite decarburization by preparing of manufacture method.

The basic consideration direction of the manufacture method of this second invention is identical with this first invention with step.Therefore, in the following description, only record and narrate and the different thin portion condition of this first invention.

The preferred range of finish to gauge is (preferred especially more than 1050 ℃) more than 1000 ℃, (preferred especially below 1200 ℃) below 1250 ℃.

Rolling temperature (for example outlet temperature of intermediate rolling) before the finish to gauge is not particularly limited, and (is preferably more than 860 ℃) but be generally more than 850 ℃.

The Heating temperature of the steel before the hot rolling is (to be preferably A more than 900 ℃ _{3 (C=0)}More than the transformation temperature), A _{4 (C=0)}Transformation temperature following (being preferably below 1250 ℃).

To cooling bed mounting temperature is more than 900 ℃, is preferably more than 940 ℃.

When cooling bed,, be divided into the compact part of volume and the part of rarefaction of volume and come controlled chilling speed 600～750 ℃ temperature range.The speed of cooling of coiled material compact part is (to be preferably more than 1.2 ℃/second) more than 1.0 ℃/second.The speed of cooling of the part of rarefaction of volume is (to be preferably below 7 ℃/second) below 8 ℃/second.

Steel wire material for spring of the present invention, not only the ferrite Decarburized layer is lowered, but also following some feature: 1) the hardness height after modified (Q-tempering); 2) pit that produces because of corrosion is smooth, and 3) the hydrogen embrittlement resistance raising.Except the preventing of ferrite decarburization,, therefore can improve corrosion fatigue strength owing to also have both this 3 features.And maximum of the present invention is characterised in that following point: clearly illustrate and give 1) hardness, 2) pit shape, 3) complex relationship of alloying element of hydrogen embrittlement resistance, finding has high correlationship (with reference to Fig. 1～3) with Ceq1, Ceq2, Ceq3 respectively.Ceq1 is big more hard more, and Ceq2 small rut shape more is smooth more, and the more little hydrogen embrittlement resistance of Ceq3 improves more, and it is advantageously being brought into play on the raising corrosion fatigue strength respectively and is acting on.

Ceq?1＝[C]+0.11[Si]-0.07[Mn]-0.05[Ni]+0.02[Cr]…(1)

Ceq2＝[C]+0.30[Cr]-0.15[Ni]-0.70[Cu]…(2)

Ceq3＝[C]-0.04[Si]+0.24[Mn]+0.10[Ni]+0.20[Cr]-0.89[Ti]-

1.92[Nb]…(3)

Hold as following formula, for example Ni is from the viewpoint of Ceq1 (hardness) and Ceq3 (hydrogen embrittlement resistance), concerning corrosion fatigue strength, play disadvantageous effect,, then concerning corrosion fatigue strength, bringing into play favourable effect from the viewpoint of Ceq2 (pit shape).There is complicated relation in other alloying elements too each other.According to the present invention, be not individually to control each element, but be based on the viewpoint of Ceq1～3 and carry out controlling comprehensively, can positively improve corrosion fatigue strength thus.

The scope of Ceq1 is more than 0.580, is preferably more than 0.59, more preferably more than 0.60.Ceq2 is below 0.49, is preferably below 0.47, more preferably below 0.45, is preferably below 0.43 especially.Ceq3 is below 0.570, is preferably below 0.54, more preferably below 0.52.

The hardness of steel wire material for spring of the present invention for example is 540HV above (for example about 540～580HV).Also having described hardness if be converted into HRC and tensile strength, then is about 52～54HRC, to be equivalent to about 1900～2000MPa.

The pit shape of being reached by steel wire material for spring of the present invention in addition can be specific according to long-width ratio, and this long-width ratio is tried to achieve by implementing following corrosion test, this Aspect Ratio as be below 0.9 about (for example about 0.3～0.85).

Corrosion test:

(a) with 925 ℃ temperature heating spring steel wire after 10 minutes, carry out oil quenching with the oil cooling of 70 ℃ of temperature, then carried out in 60 minutes after the tempering (thereafter with 400 ℃ temperature heating, carry out (for example making diameter shorten back about 0.25mm) behind the undergauge if need just cutting surface), next use No. 800 sand paper lapped face, make the test film that corrosion test is used.

(b) to this test film, follow the NaCl aqueous solution 8 hour of JIS Z 2371 with 35 ℃ of spraying 5 quality %, thereafter, test film was kept 16 hours in the moist environment of 35 ℃ of humidity 60% and temperature, this as 1 circulation, is added up to 14 circulations with it.

(c) be in the liquid of 10 quality % with distilled water with ammonium citrate (ammonium citrate) dilution, dip test sheet under the normal temperature is removed the rust that takes place because of brine spray.Then with the corrosion pit on laser microscope viewing test sheet surface, from the viewed corrosion pit in test film surface this, the dual-depth corrosion pit of select progressively more than 5 from big to small calculates the long-width ratio of these corrosion pits according to following formula (4).

Long-width ratio=(degree of depth of corrosion pit * 2)/(width of corrosion pit) ... (4)

The hydrogen crackle life-span of steel wire material for spring of the present invention for example is (for example about 800～1200 seconds) more than 720 seconds.The hydrogen crackle life-span can try to achieve as follows.

After 10 minutes, carry out oil quenching with 925 ℃ temperature heating spring steel wires, then carried out tempering in 60 minutes, become test film with 400 ℃ temperature heating with the oil cooling of 70 ℃ of temperature.The limit plays a role the stress of 1400MPa by 4 bendings, and the limit is immersed in test film in the mixed aqueous solution of sulfuric acid (0.5mol/L) and potassium sulfocyanate (0.01mol/L).With potentiostat (potentiostat) apply than SCE electrode low-voltage of 700mV, measure the time of taking place until crackle.

The corrosion fatigue strength of steel wire material for spring of the present invention for example is 290MPa above (being preferably about 300～400MPa).Corrosion fatigue strength for example can be tried to achieve in the following manner.

(a) after 10 minutes, carry out oil quenching with 925 ℃ temperature heating spring steel wire with the oil cooling of 70 ℃ of temperature, then with 400 ℃ temperature heat carried out tempering in 60 minutes after, be processed into JIS test film (fatigue test piece).

(b) grind the parallel portion of this fatigue test piece with No. 800 sand paper.Do not make it corrosion with the bend of tunicle protection test sheet; afterwards; follow JIS Z 2371 with 35 ℃ to the spray NaCl aqueous solution 8 hours of 5 quality % of this test film; thereafter; test film was kept 16 hours in the moist environment of 35 ℃ of humidity 60% and temperature; with this as 1 circulation, it is added up to 14 circulations after, carry out fatigue test with little wild formula rotary bending fatigue test.The limit makes bearing strength test increase with 10MPa at interval, and 5 test films enforcement fatigue tests are adopted on the limit under each bearing strength test, and 5 test films all reach 1,000 ten thousand times and the stress do not lost is corrosion fatigue strength.

The composition that can be suitable for steel wire material for spring of the present invention is composed as follows.

C：0.38～0.47％、

Si：1.9～2.5％、

Mn：0.6～1.3％、

Ti:0.05～0.15% reaches

Al：0.003～0.1％、

Surplus: iron and unavoidable impurities

Qualification reason for alloying element amount (composition) describes in detail.

C：0.38～0.47％

C must contain in steel, and it helps the raising of the intensity (hardness) behind the Q-tempering.But if C amount is too much, then the long-width ratio of corrosion pit increases, and causes the concentrated increase of stress to corrosion pit, the toughness deterioration of matrix in the steel in addition, and hydrogen embrittlement resistance is deterioration also.Consequently, if the C amount is superfluous, then corrosion fatigue deterioration in characteristics.Therefore the C amount is decided to be more than 0.38% and (is preferably more than 0.39%), (be preferably below 0.45%, more preferably below 0.43%) below 0.47%.

Si：1.9～2.5％

Si helps intensity to improve as the solution strengthening element, endurance is improved.Therefore, if Si amount is very few, matrix strength deficiency then.Carbide when Si makes tempering is in addition separated out temperature and is moved to high temperature side, makes the temper brittleness territory move to high temperature side, also has the effect that improves hydrogen embrittlement resistance thus.But if the Si surplus, the dissolving in of the carbide when then hindering modified heating, intensity reduces.Therefore the Si amount is decided to be more than 1.9% and (is preferably more than 1.95%), (be preferably below 2.3%, more preferably below 2.2%) below 2.5%.

Mn：0.6～1.3％

Mn is the element (austenite former) of the austenite region in the expanding equilibrium phasor, and is effective to the decarburization of stable inhibition ferrite.But if the Mn amount is superfluous, then the toughness of matrix reduces in the steel, hydrogen embrittlement resistance deterioration, consequently corrosion fatigue deterioration in characteristics.Therefore the Mn amount is decided to be more than 0.6% and (is preferably more than 0.65%, more preferably more than 0.7%), (be preferably below 1.1%, more preferably below 0.9%) below 1.3%.

Ti：0.05～0.15％

Ti makes the old austenite crystal miniaturization behind the Q-tempering, and is effective to the raising of atmosphere weather resistance and hydrogen embrittlement resistance.But if the Ti amount is superfluous, then thick Ti nitride is separated out, the fatigue characteristic deterioration.Therefore the Ti amount is decided to be more than 0.05% and (is preferably more than 0.06%, more preferably more than 0.07%), (be preferably below 0.1%, more preferably below 0.09%, be preferably especially below 0.085%) below 0.15%.

Al：0.003～0.1％

Al is the reductor when handling as molten steel and the element that plays a role.Al forms fine Al nitride in addition, under its pinning effect and have the effect that makes the crystal grain miniaturization.But if Al amount is superfluous, then form thick Al oxide compound, bring disadvantageous effect to fatigue characteristic etc.Therefore the Al amount is decided to be more than 0.003% and (is preferably more than 0.005%), (be preferably below 0.05%, more preferably below 0.03%) below 0.1%.

The surplus of the spring steel that uses among the present invention comes down to iron.But the unavoidable impurities that certainly allows the situation of goods and materials, producing apparatus etc. because of iron material (containing scrap iron), auxiliary material etc. to sneak into is included in the steel (wire rod).Also can this unavoidable impurities of strict control, for example P, S, O, N etc. can be controlled in the following scope.

Below the P:0.02%

P makes embrittlement of grain boundaries in old austenite grain boundary segregation, is the element that fatigue characteristic are reduced.Therefore the P amount is low as far as possible more for preferred, for example can be controlled at below 0.02% (to be preferably below 0.01%).

Below the S:0.02%

S makes embrittlement of grain boundaries in old austenite grain boundary segregation, is the element that fatigue characteristic are reduced.Therefore the S amount is low as far as possible more for preferred, for example can be controlled at below 0.02% (to be preferably below 0.01%).

Below the N:0.007%

N amount is many more, can form thick nitride with Ti and Al more, brings disadvantageous effect to fatigue characteristic etc.Therefore the N amount is the least possible more for preferred, for example can be controlled at below 0.007% (to be preferably below 0.005%).On the other hand, if excessively reduce the N amount, then productivity significantly reduces.N forms nitride with Al and helps the miniaturization of crystal grain in addition.If from this viewpoint, then preferably N amount is set at more than 0.001% and (is preferably more than 0.002%).

Below the O:0.0015%

If the O amount is superfluous, then thick oxide based inclusion (Al ₂O ₃Deng) form, bring disadvantageous effect to fatigue characteristic etc.Therefore (be preferably below 0.0010%) being defined as below 0.0015% on the O amount.On the other hand, the lower limit of O amount (is preferably more than 0.0004%) being generally on the industrial production more than 0.0002%.

Spring steel of the present invention in addition also can contain following selection element as required.

Cr：0.1～0.4％

Cr makes the matrix strengthening of steel by solution strengthening, also has the effect that improves hardenability in addition.In order to bring into play such effect, be recommended in and contain the amount that Cr is preferably (more preferably more than 0.15%, more preferably more than 0.220%) more than 0.1% in the steel.But have the pH value that reduces the corrosion pit bottom, make the long-width ratio of corrosion pit increase (becoming sharp keen) such effect, it causes disadvantageous effect to corrosion fatigue.Therefore in the present invention, (be preferably below 0.3%, more preferably below 0.25%) being defined as below 0.4% on the Cr amount.

Cu：0.1～0.7％

Cu is the element more expensive than iron on electrochemistry, has the effect of the erosion resistance that improves steel.Cu form to increase the amorphous of the rust that produces in the corrosion in addition, has inhibition as the effect in the corrosion pit bottom densified of the Cl element of one of corrosion.Under this effect, the long-width ratio of corrosion pit is restricted, and stress concentration is relaxed, and the corrosion fatigue characteristic improves.In order to bring into play such effect, recommend to make to contain the amount that Cu is preferably (more preferably more than 0.15%, more preferably more than 0.22%) more than 0.1% in the steel.But, can cause hot-rolled crackle to produce owing to add Cu.Therefore in the present invention, (be preferably below 0.5%, more preferably below 0.4%, be preferably especially below 0.35%) being defined as below 0.7% on the Cu amount.

Ni：0.1～0.7％

Ni is the same with Cu, has the effect that improves erosion resistance, and makes the amorphous of rust form increase, thereby reduces the effect of the long-width ratio of corrosion pit.In order to bring into play such effect, recommend to make to contain the amount that Ni is preferably (more preferably more than 0.15%, more preferably more than 0.20%) more than 0.1% in the steel.But Ni has the effect that the retained austenite amount in the matrix that makes after modified (Q-tempering) increases, and the hardness (tensile strength) after modified is reduced.Hydrogen embrittlement resistance is reduced.Therefore in the present invention,, be preferably below 0.5%, more preferably below 0.4%, be preferably below 0.35% especially being defined as below 0.7% on the Ni amount.

Spring steel of the present invention, above-mentioned Cr, Cu and Ni can not comprise yet, but preferably contain among Cr, Cu and the Ni at least a kind with above-mentioned amount, and more preferably the amount of above palm fibre contains Cr and Ni any a kind.

Nb：0.01～0.1％

Nb forms fine compound (Nb carbide, nitride, sulfide and their complex chemical compound), is to have the element that improves the hydrogen embrittlement resistance effect.The Nb performance has the crystal grain micronized effect in addition, also has the effect that improves toughness and endurance.Therefore as required, recommend to contain the preferred amount of (more preferably more than 0.02%) more than 0.01% of Nb.But if the Nb amount is superfluous, in austenite, do not have the carbide amount of solid solution to increase when then quenching heating, can not get full intensity.If the Nb amount is superfluous, then form thick Nb nitride in addition, fatigue takes place easily lose.Therefore making this contain sometimes Nb amount is decided to be below 0.1% and (is preferably below 0.05%).

B：0.0003～0.005％

B is the grain boundary segregation that prevents P, to improving hydrogen embrittlement resistance, toughness and ductility effective elements.In wire rod, can contain B as required.In addition, B does not rely on the interpolation of a large amount of alloying elements, can increase hardenability on a small quantity.Therefore, the ferritic of wire surface that suppresses to generate among the Xu Leng of rolling back separated out, and the hardness the during quenching when guaranteeing that spring is made is to very dark.B recommends preferably to contain the amount of (more preferably more than 0.0005%) more than 0.0003%.When but B content is excessive, form Fe ₂₃(CB) ₆Deng B compound free B reduce, therefore, prevent that the effect of grain boundary segregation of P is saturated.In addition, because this B compound is more when thick, therefore, become the starting point of fatigue damage, fatigue characteristic reduce.Therefore, when containing B, qualifying bit on it (is preferably below 0.004%) below 0.005%.

The line footpath of spring steel of the present invention for example is that 9～25mm (is preferably 10～20mm).

[embodiment]

Below, enumerate embodiment and be described more specifically the present invention, but the present invention is not limited by the examples below, in the scope that can meet above-mentioned, following aim, can certainly be changed enforcement, these all are included in the technical scope of the present invention.

Embodiment 1

Displaying is about this first inventive embodiment.

(steel grade SA～SL), make the steel ingot at 400mm angle by continuous casting carries out the slab that split rolling method becomes the 155mm angle to the steel of forming with the chemical ingredients shown in 80 tons the converter melting table 1 again.Carry out hot rolling after heating this slab, water-cooled is extremely near batching after the mounting temperature, mounting is in cooling bed (travelling belt) of Stelmor cooling apparatus, and the limit is regulated the air quantity limit that supplies to coiled material compact part and coiled material part of rarefaction and carried out the air blast cooling, makes 2 tons of the spring wire rods of diameter 14.3mm thus.Detailed creating conditions is as shown in table 2.In addition in the table 2, speed of cooling is the speed between 750～600 ℃ of the temperature.

Investigate tensile strength, fracture shrinking percentage, the decarburized layer deepness of the wire rod that obtains as follows.In addition, implement following test for the austinite grain size of the cooling of investigating wire rod before beginning number.

(1) tension test (tensile strength, fracture shrinking percentage)

From the top ends (rolling) of wire coil and terminal part (rolling end) the 5th volume of breaking into pieces.1 volume of tip side and end side is divided into 8 five equilibriums respectively, adds up to and make 16 wire rod sheets.After correcting routine wire rod sheet and become rectilinear form by roller, make No. 2 test films (between chuck apart from 200mm) of JIS Z2201, carry out tension test, measure tensile strength and fracture shrinking percentage by each wire rod sheet.Among 16 test films, the minimum value of the maximum value of tensile strength and fracture shrinking percentage is the tensile strength and the fracture shrinking percentage of this wire rod.Tensile strength height, fracture shrinking percentage little example (particularly being lower than at 25% o'clock) were judged as the influence of cold tissue, for defective.

(2) decarburized layer deepness

In 16 wire rod sheets that obtain by described tip side and end side, cut off about 10mm from the next door of extracting the tension test sheet, obtain sample.The limit makes the cut surface exposing surface that resin is imbedded on the sample limit, with sand paper and silicon carbide particle this sample is carried out wet grinding, then carries out etching with picrol (picral), makes to add up to 16 decarburized layer deepness mensuration test films.Observe these test films with opticmicroscope with 200 times observation multiplying power, measure the full decarburized layer deepness and the ferrite decarburizaed depth on top layer.This assay method is followed the microscopical mensuration of JIS G 0558.Among 16 samples, the maximum value of full decarburized layer deepness and ferrite decarburizaed depth is that " full decarburized layer deepness " of the present invention reaches " ferrite decarburizaed depth ".

(3) austinite grain size number

Same with above-mentioned experimental example, implement melting batching to wire rod from steel.Not with the cooling conditions shown in the table 2, but cold by high wind, cool off this wire rod with about 20 ℃/second speed of cooling and be rolled onto straight 200 ℃ temperature, making based on the wire rod of martensitic stucture (promptly, when observing the degree of depth of top layer 0.1mm with opticmicroscope with 200 times observation multiplying power, 95 area % of tissue are above to be the wire rod of martensitic stucture) 2 tons.1 volume with tip side and end side is divided into 8 five equilibriums respectively, adds up to and makes 16 wire rod sheets.Sample from each wire rod sheet wet type is cut off the length about 20mm carries out annealing in 500 ℃ * 2 hours with it.The limit makes cut surface exposing surface limit that sample is imbedded resin, with sand paper and silicon carbide particle this sample is carried out wet grinding, then carries out etching with picrol (picral), make to add up to the mensuration test film of 16 austinite grain sizes number.With these test films of observation by light microscope, measure from the surface to the austinite grain size of the position of the degree of depth of 0.1mm number.This assay method is followed the micrographic test method of the grain size number of JIS G 0551.Among 16 samples, adopt the minimum value of austinite grain size number.

Measurement result is presented in the table 2.Also record and narrate with good grounds chemical ingredients in addition in the table 2 and form the A that (wherein C=0%) calculated by Thermo-Calc ₁Transformation temperature, A ₃Transformation temperature and A ₄Transformation temperature (is A _{1 (C=0)}Transformation temperature, A _{3 (C=0)}Transformation temperature, A _{4 (C=0)}Transformation temperature) also has steel grade J, near A C=0% ₃And A ₄Toe-in closes, A _{3 (C=0)}Transformation temperature and A ₄(C=0) transformation temperature disappears.

Satisfy wire rod SA-1, SB-1, SC-1, SD-1, SE-1, SF-1 and the SG-1 of important document of the present invention, ferrite decarburizaed depth is 0.00mm, and the value of broken face shrinking percentage is more than 25%.

On the other hand, wire rod SA-3 and SE-2 that Heating temperature is high, be up to Da Wendu low wire rod SA-2, SC-3 and SL-1 in the finish to gauge, being up in the finish to gauge reaches temperature high wire rod SB-1 and SF-2, wire rod SD-2, SG-2 that the mounting temperature is low, wire rod SB-3 and SE-3 that the speed of cooling of coiled material compact part is little, and the ferrite decarburization takes place at extraneous wire rod SH-1 of the present invention and SJ-1 in C or Si amount.

Also have, for example the A of A steel ₃Transformation temperature (C=0.42%) is about about 840 ℃, and in the example of SA-2, rolling middle typical temperature remains on this A ₃More than the transformation temperature.But in the example of this SA-2, produced the ferrite decarburization.Shown in SA-1, be A by making the Da Wendu that is up in the finish to gauge _{3 (C=0)}More than the transformation temperature, can prevent the ferrite decarburization.

In addition, the wire rod C-2 that the mounting temperature is high, the wire rod SI-1 and the SK-1 of wire rod SD-3 that the speed of cooling of coiled material part of rarefaction is big and SF-3 and alloying element amount surplus, the fracture shrinking percentage is low to moderate and is lower than 25%.

Embodiment 2

Displaying is about this second inventive embodiment.

Steel with the chemical ingredients shown in the small size vacuum calciner melting table 3 of 150kg is formed carries out forge hot, makes the slab at 155mm angle.Forming Ceq1～3 of calculating according to chemical ingredients is presented in the table 5.Carry out hot rolling after heating described slab, water-cooled is extremely near batching after the mounting temperature, mounting is on cooling bed (travelling belt) of Stelmor cooling apparatus, and the limit is regulated the air quantity limit that supplies to coiled material compact part and coiled material part of rarefaction and carried out the air blast cooling, makes the spring steel (wire rod) of diameter 13.5mm thus.Detailed creating conditions is as shown in table 4.In addition in the table 4, speed of cooling is the speed between 600～750 ℃ of the temperature.Also have in table 4, record and narrate with good grounds chemical ingredients and form the A that (wherein C=0%) calculated by Thermo-Calc _{1 (C=0)}Transformation temperature, A _{3 (C=0)}Transformation temperature, A _{4 (C=0)}Transformation temperature.

The length and width of the decarburized layer deepness of the steel wire material for spring that obtains of investigation, the fatigue strength after modified (Q-tempering), Vickers' hardness, corrosion pit hydrogen embrittlement crackle life-span when as follows.

(1) decarburized layer deepness

From the terminal part (rolling end) of wire coil the 3rd, 4 and 5 volumes of breaking into pieces, 1 volume is divided into 8 five equilibriums respectively, makes to add up to 24 wire rod sheets.Cut off respectively about 10mm from the wire rod sheet, obtain sample.The limit makes cut surface exposing surface limit that test portion is imbedded resin, with sand paper and silicon carbide particle this sample is carried out wet grinding, then carries out etching with picrol (picral), makes to add up to 24 decarburized layer deepness mensuration test films.Observe these test films with opticmicroscope with 200 times observation multiplying power, identical with the situation of embodiment 1, try to achieve full decarburized layer deepness (DmT) and ferrite decarburizaed depth (DmF).

(2) fatigue strength

Described wire rod sheet is implemented drawing processing (frotton processing) and cut off the sample of making diameter 12.5mm * long 70mm.Heat this sample after 10 minutes with 925 ℃ temperature, carry out oil quenching, then carried out tempering in 60 minutes with 400 ℃ temperature heating with the oil cooling of 70 ℃ of temperature.The steel that will carry out Q-tempering is processed into bend diameter 12mm, No. 1 test film (fatigue test piece) of parallel portion diameter 8mmJIS Z2274.

Grind the parallel portion of this fatigue test piece with No. 800 sand paper.Do not make it corrosion with the bend of tunicle protection test sheet; afterwards; follow JIS Z 2371 with 35 ℃ to the spray NaCl aqueous solution 8 hours of 5 quality % of this test film; thereafter; test film was kept 16 hours in the moist environment of 35 ℃ of humidity 60% and temperature; this as 1 circulation, is added up to 14 circulations with it.The test film keeping of having carried out this corrosion test is in vacuum drier, until supplying little wild formula rotary bending fatigue test.The limit makes bearing strength test increase with 10MPa at interval, and the limit adopts 5 test films to implement little wild formula rotary bending fatigue test under each bearing strength test, and 5 test films all reach 1,000 ten thousand times and the stress do not lost is corrosion fatigue strength.The result is presented in the table 5.

(3) Vickers' hardness

Described wire rod sheet is carried out frotton processing and cuts off the sample of making diameter 12.5mm * long 60mm.With the condition identical this sample is carried out Q-tempering, make the test film that Vickers' hardness is measured usefulness with fatigue test.This test film is imbedded resin and cross section is exposed, carry out the hardness mirror finish after, carry out Vickers hardnes test with the load of 10kg at the position of the dark 0.1mm in distance top layer, measure Vickers' hardness.The result is presented in the table 5.The graphic representation of the relation of record expression Vickers' hardness and Ceq1 among Fig. 1.In table 5, record the tensile strength (in table 5, being recited as " conversion TS ") that converts by Vickers' hardness in addition.This converts and uses following formula (5):

TS＝58.33×(-9.751+0.16491×HV-9.4457×10 ^-5×HV ²)-1135.7

…(5)

[in the following formula, TS represents tensile strength (MPa), and HV represents Vickers' hardness.]

(4) long-width ratio of corrosion pit

Described wire rod sheet is carried out frotton processing and cuts off the sample of making diameter 12.5mm * long 120mm.With the condition identical this sample is carried out Q-tempering, be machined into the shape of diameter 10mm * long 100mm, make the test film that long-width ratio is measured usefulness with fatigue test.Surface with No. 800 sand paper grinding test sheets; do not make it corrosion with the two ends 10mm of this test film of enamel paint (enamel) lining protection; follow JIS Z 2371 with 35 ℃ to the spray NaCl aqueous solution 8 hours of 5 quality % of this test film; thereafter; test film was kept 16 hours in the moist environment of 35 ℃ of humidity 60% and temperature; this as 1 circulation, is added up to 14 circulations with it.Thereafter, be in the liquid of 10 quality % with distilled water with ammonium citrate (98.7%) dilution, dip test sheet under the normal temperature is removed the rust that takes place because of brine spray, corrosion pit with laser microscope (Lasertec society system " 1LM21W ", multiplying power: 100～200 times) viewing test sheet surface.Each steel grade uses 5 test films.From 5 test film surface observations to corrosion pit among, by 10 corrosion pits of degree of depth select progressively from big to small,, try to achieve long-width ratio with the degree of depth and the width substitution following formula (4) of each corrosion pit, try to achieve their mean value.The result is presented in the table 5.Record the graphic representation of long-width ratio (mean value) with the relation of Ceq2 of expression corrosion pit among Fig. 2.

(5) the hydrogen embrittlement crackle life-span

Described wire rod sheet is carried out frotton processing and cuts off the sample of making diameter 12.5mm * long 70mm.With the condition identical this sample is carried out after the Q-tempering, downcut the test film of wide 10mm * thick 1.5mm * long 65mm with fatigue test.For this test film, the limit plays a role the stress of 1400MPa by 4 bendings, and the limit is immersed in test film in the mixed aqueous solution of sulfuric acid (0.5mol/L) and potassium sulfocyanate (0.01mol/L).With potentiostat apply than SCE electrode low-voltage of 700mV, measure the time of taking place until crackle.The result is presented in the table 5.Record the graphic representation of the relation of hydrogen embrittlement crackle life-span and Ceq3 among Fig. 3.

By the result of table 3～5 as can be known, the ferrite decarburization does not take place, and the steel grade A～K that satisfies the important document of Ceq1 of the present invention～3 demonstrates good corrosion fatigue strength (for example more than the 290MPa).On the other hand, Ceq1 is lower than its Vickers' hardness of steel grade L of 0.580, so fatigue strength is low.Ceq2 surpasses steel grade N～P of 0.49, and the long-width ratio in its corrosion Hangzhoupro is big, so fatigue strength is low.Ceq3 surpasses steel grade M～T of 0.570, and its hydrogen embrittlement life-span is short, so fatigue strength is low.

Shown in the result of table 3～5, in the steel that has prevented the ferrite decarburization, when the hydrogen embrittlement crackle life-span can have influence on corrosion fatigue strength to the length and width of its Vickers' hardness, corrosion pit in addition.And shown in the curve of Fig. 1～3, when there is high correlationship in the hydrogen embrittlement crackle life-span with Ceq1～3 respectively to the length and width of these Vickers' hardnesses, corrosion pit.Therefore adjust the chemical ingredients of steel and form, make it to satisfy the important document of Ceq1 of the present invention～3, the length and width that can control Vickers' hardness, corrosion pit are the hydrogen embrittlement crackle life-span when, can reach good corrosion fatigue strength.

Claims (15)

1. the manufacture method of a steel wire material for spring, it is characterized in that, described steel wire material for spring contains C:0.37～0.54%, Si:1.7～2.30%, Mn:0.1～1.30%, Cr:0.15～1.1%, Cu:0.15～0.6%, Ti:0.010～0.1%, Al:0.003～0.05% in quality %, surplus is made of iron and unavoidable impurities

And ferrite decarburizaed depth is below the 0.01mm, and the total decarburized layer degree of depth is below the 0.20mm, and the fracture shrinking percentage is more than 25%,

In the manufacture method of described steel wire material for spring, to steel hot rolling and after batching, use cooling bed cooling,

A in the equilibrium phase diagram of described steel during with C=0 quality % ₁Transformation temperature, A ₃Transformation temperature and A ₄Transformation temperature is called A _{1 (C=0)}Transformation temperature, A _{3 (C=0)}Transformation temperature, A _{4 (C=0)}During transformation temperature,

Making the Heating temperature of the preceding steel of hot rolling is A more than 900 ℃ _{4 (C=0)}Below the transformation temperature,

Making the Da Wendu that is up to of the steel in the hot rolled finish to gauge is A _{3 (C=0)}The above A of transformation temperature _{4 (C=0)}Below the transformation temperature,

The temperature of coiled material mounting during to cooling bed go up is A _{1 (C=0)}The above A of transformation temperature _{1 (C=0)}Below transformation temperature+50 ℃,

By the continuous cooling curve corresponding with the grain size number of austenite crystal numbers 8.0～11, the temperature range that can separate out at ferrite, carrying out speed of cooling at the coiled material compact part is cooling more than 1.0 ℃/second, and carrying out speed of cooling at the coiled material part of rarefaction is cooling below 8 ℃/second.

2. the manufacture method of steel wire material for spring according to claim 1, it is characterized in that, do not carry out the water-cooled of the steel before the finish to gauge, make it produce heat but in finish to gauge steel are processed, the Da Wendu that is up to the steel in the finish to gauge is controlled at described scope thus.

3. the manufacture method of steel wire material for spring according to claim 1 is characterized in that, the ideal critical diameter DCI by following formula (1) expression of described steel is 75～135mm,

DCI(mm)＝25.4×(0.171+0.001[C]+0.265[C] ²)×

(3.3333[Mn]+1)×(1+0.7[Si])×

(1+0.363[Ni])×(1+2.16[Cr])×

(1+0.365[Cu])×(1+1.73[V])×

(1+3[Mo]) …(1)

In the following formula, the mass percentage content of each element in [] expression steel.

4. the manufacture method of a steel wire material for spring, it is characterized in that, described steel wire material for spring contains C:0.37～0.54%, Si:1.7～2.30%, Mn:0.1～1.30%, Cr:0.15～1.1%, Cu:0.15～0.6%, Ti:0.010～0.1%, Al:0.003～0.05% in quality %, surplus is made of iron and unavoidable impurities

And ferrite decarburizaed depth is below the 0.01mm, and the total decarburized layer degree of depth is below the 0.20mm, and the fracture shrinking percentage is more than 25%,

In the manufacture method of described steel wire material for spring, to steel hot rolling and after batching, cool off with cooling bed,

The Heating temperature that makes the steel before the hot rolling is more than 900 ℃ below 1250 ℃,

Making the Da Wendu that is up to of steel in the hot rolled finish to gauge is more than 1050 ℃ below 1200 ℃,

The temperature of coiled material mounting during to cooling bed go up is more than 900 ℃ below 980 ℃,

In temperature is 750～600 ℃ temperature province, and carrying out speed of cooling at the coiled material compact part is cooling more than 1.0 ℃/second, and carrying out speed of cooling at the coiled material part of rarefaction is cooling below 8 ℃/second.

5. according to the manufacture method of claim 1 or 4 described steel wire material for spring, it is characterized in that described steel wire material for spring also contains Ni:0.15～0.7% in quality %.

6. according to the manufacture method of claim 1 or 4 described steel wire material for spring, it is characterized in that described steel wire material for spring also contains at least a of V:0.07～0.4% and Nb:0.01～0.1% in quality %.

7. according to the manufacture method of claim 1 or 4 described steel wire material for spring, it is characterized in that described steel wire material for spring also contains Mo:0.01～0.3% in quality %.

8. according to the manufacture method of claim 1 or 4 described steel wire material for spring, it is characterized in that described steel wire material for spring is in quality %, P is below 0.020%, S is below 0.020%, N is below 0.0070%, O is below 0.0015%.

9. a steel wire material for spring is characterized in that, contains C:0.38～0.47%, Si:1.9～2.5%, Mn:0.6～1.3%, Ti:0.05～0.15%, reaches Al:0.003～0.1% in quality %, and surplus is made of iron and unavoidable impurities,

And ferrite decarburizaed depth is below the 0.01mm,

By the Ceq1 shown in the following formula (1) is more than 0.580,

By the Ceq2 shown in the following formula (2) is below 0.49,

By the Ceq3 shown in the following formula (3) is below 0.570,

Ceq1＝[C]+0.11[Si]-0.07[Mn]-0.05[Ni]+0.02[Cr]…(1)

Ceq2＝[C]+0.30[Cr]-0.15[Ni]-0.70[Cu]…(2)

Ceq3＝[C]-0.04[Si]+0.24[Mn]+0.10[Ni]+0.20[Cr]-0.89[Ti]-

1.92[Nb]…(3)

In the following formula, the mass percentage content of each element in [] expression steel.

10. steel wire material for spring according to claim 9 is characterized in that, also contains Cr:0.1～0.4% in quality %.

11. steel wire material for spring according to claim 9 is characterized in that, also contains Cu:0.1～0.7% in quality %.

12. steel wire material for spring according to claim 9 is characterized in that, also contains Ni:0.1～0.7% in quality %.

13. steel wire material for spring according to claim 9 is characterized in that, also contains Nb:0.01～0.1% in quality %.

14. steel wire material for spring according to claim 9 is characterized in that, in quality %, P is below 0.02%, below the S:0.02%, below the N:0.007%, below the O:0.0015%.

15. steel wire material for spring according to claim 9, it is characterized in that, after carrying out the corrosion test shown in following, from the surperficial viewed corrosion pit of test film, select corrosion pit more than 5 from big to small successively by the degree of depth, the mean value by the long-width ratio shown in the following formula (4) of these corrosion pits is below 0.9

Long-width ratio=(degree of depth of corrosion pit * 2)/(width of corrosion pit) ... (4)

Corrosion test:

With 925 ℃ temperature heating spring steel wires after 10 minutes, be that 70 ℃ oil cooling carries out oil quenching with temperature, then carried out tempering in 60 minutes with 400 ℃ temperature heating after, with No. 800 sand paper lapped face, the test film that the making corrosion test is used,

Follow JIS Z 2371, at 35 ℃ to the spray NaCl aqueous solution 8 hours of 5 quality % of this test film, thereafter, with test film humidity be 60% and temperature be to keep 16 hours in 35 ℃ the moist environment, this as 1 circulation, is added up to 14 circulations to this test film

Thereafter, except that after derusting, with the corrosion pit on laser microscope viewing test sheet surface.

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