CN1886532A - Low carbon composite free-cutting steel product excellent in roughness of finished surface and method for production thereof - Google Patents

Abstract

The present invention provides a low-carbon resulfurized free machining steel product excellent in machinability typified by finished surface roughness even though toxic Pb or special elements such as Bi or Te are not added, and a suitable production method thereof. A steel product has a specific composition, has contents of Mn and S satisfying the following conditions: 0.40 Mn*S 1.2 and Mn/S 3.0, and contains a ferrite-pearlite structure as the metallographic structure, in which the average width ( m) of sulfide inclusions in the steel product is 2.8* (log d) or more, wherein d (mm) is the diameter of the steel product, and pro-eutectoid ferrite in the metallographic structure has a hardness HV of 133 to 150 or a difference in deformation resistance at a strain of 0.3 between 200 DEG C and 25 DEG C is 110 MPa or more and 200 MPa or less, the deformation resistances being determined in a compression test at a deformation rate of 0.3 mm/min.

Description

The low-carbon (LC) revulcanization free-cutting steel product and the method for making thereof of finished surface roughness excellence

Technical field

The present invention relates to a kind of not leaded (Pb) and have low-carbon (LC) revulcanization free-cutting steel product of satisfied machinability and preparation method thereof.Here " product made from steel " is typically referred to as hot rolled rod and steel bar.

Background technology

But low-carbon (LC) revulcanization free-cutting steel product is used in and does not need high mechanical property need good machinability and can be in the finding such as for example screw and threaded connector etc. that cutting is produced in large quantities.The free-cutting steel product that also contains Pb except that containing S is widely used as the free-cutting steel product with more satisfied machinability.Lead is unhealthful objectionable impurities, has needed to reduce the lead content in this free-cutting steel product.Tellurium (Te) also is used in some free-cutting steel products, but it has toxicity and makes the hot workability variation, must reduce its content.

Carry out many researchs and improved the machinability of low-carbon (LC) revulcanization free-cutting steel product, many controls (referring to patent documentation 1,2,3,4,5 and 6) that relate to kind, size and the structure of sulfide inclusion thing in them.

Patent documentation 7 points out that the oxygen level in the product made from steel is important for the size of controlling the sulfide inclusion thing and structure.Patent documentation 8 shows that the oxygen level that is controlled in the preceding molten steel of discharging is important.

Many technology relate to the control (referring to patent documentation 9,10,11,12 and 13) of oxide inclusion.

Except that these inclusiones, structure and character (parent character) also influence machinability significantly, but only have technology seldom to note these factors.For example, only there is a spot of technology to notice them, for example is defined in the technology (patent documentation 14) of the striped pearlitic texture that prolongs continuously on the rolling direction and stipulates dissolved carbon content (patent documentation 15) in the proeutectoid ferrite.

For example patent documentation 16 proposes a kind of low-carbon (LC) revulcanization free-cutting steel product, it contains S, 0.003 weight %～N of 0.03 weight % and the oxygen of 100ppm～300ppm of 0.16 weight %～0.5 weight %, and the conventional free-cutting steel product that its nitrogen that contains (N) amount is produced than continuous casting is big.The built-up edge (build-upedge) that the free-cutting steel product that obtains forms on knife face in the time of can reducing cutting has the machinability that is equal to or higher than the ingot steel product.

Patent documentation 1: No. 1605766 communique of Japanese Patent (claims)

Patent documentation 2: No. 1907099 communique of Japanese Patent (claims)

Patent documentation 3: No. 2129869 communique of Japanese Patent (claims)

Patent documentation 4: Japanese Patent Application Publication (JP-A) 09-157791 communique (claims)

Patent documentation 5: Japanese Patent Application Publication (JP-A) 11-293391 communique (claims)

Patent documentation 6: Japanese Patent Application Publication (JP-A) 2003-253390 communique (claims)

Patent documentation 7: Japanese Patent Application Publication (JP-A) 09-31522 communique (claims)

Patent documentation 8: Japanese Patent Application Publication (JP-A) 56-105460 communique (claims)

Patent documentation 9: No. 1605766 communique of Japanese Patent (claims)

Patent documentation 10: No. 1907099 communique of Japanese Patent (Japan patent applicant announce 04-54736 number) (claims)

Patent documentation 11: No. 2922105 communique of Japanese Patent (claims)

Patent documentation 12: Japanese Patent Application Publication (JP-A) 09-71838 communique (claims)

Patent documentation 13: Japanese Patent Application Publication (JP-A) 10-158781 communique (claims)

Patent documentation 14: No. 2125814 communique of Japanese Patent (Japan patent applicant announce (JP-B) 01-11069 number) (claims)

Patent documentation 15: No. 2740982 communique of Japanese Patent (claims)

Patent documentation 16: No. 2129869 communique of Japanese Patent (Japan patent applicant announce (JP-B) 08-949 number) (claims)

Summary of the invention

The problem that the present invention solves

Although disclosed each technology plays an important role in the machinability that improves the free-cutting steel product in the above-mentioned application, they can not produce sufficient machinability in forming process, particularly the finished surface roughness.

For example, inclusion in the disclosed technology control of patent documentation 8 product made from steel makes having 5 μ m or be 50% or greater than 50% greater than the major axis of 5 μ m and 2 μ m or greater than the MnS content of the minor axis of 2 μ m in total MnS inclusion, the ratio of described major axis and minor axis is 5 or less than 5, Al in the oxide inclusion ₂O ₃Average content be 15% or less than 15%.But it is 0.2% or greater than 0.2% Pb, Bi and Te that this product made from steel must contain total content, does not add these elements and can not produce sufficient machinability.

For size and the structure (configuration) of controlling the sulfide inclusion thing, the oxygen level in patent documentation 7 and 8 disclosed technology control product made from steel or the molten steel.But in fact these product made from steel contain the high-load oxygen of 100～500ppm.So high oxygen level often produces the deleterious oxide inclusion of machinability, also causes pore, produces surface imperfection.

Finished the present invention based on these problems, an object of the present invention is to provide a kind of low-carbon (LC) revulcanization free-cutting steel product, even do not add toxicity Pb or element-specific such as Bi or Te for example, it is the machinability of representative with the finished surface roughness that described low-carbon (LC) revulcanization free-cutting steel also has satisfied, and the present invention also provides a kind of appropriate method for preparing described product made from steel.

The method of dealing with problems

In order to achieve the above object, on the one hand, the invention provides a kind of low-carbon (LC) revulcanization free-cutting steel product of finished surface roughness excellence, described low-carbon (LC) revulcanization free-cutting steel product comprises: based on mass percent, and C:0.02%～0.12%; Si:0.01% or be lower than 0.01%; Mn:1.0%～2.0%; P:0.05%～0.20%; S:0.30%～0.60%; N:0.007%～0.03%, equal amount is Fe and unavoidable impurities, the content of Mn and S satisfies following condition: 0.40≤Mn*S≤1.2 and Mn/S 〉=3.0, described product made from steel has the ferrite-pearlite structure as its structure, the width average of sulfide inclusion thing in the wherein said product made from steel (μ m) is 2.8 * (log d) or greater than 2.8 * (log d), d is the diameter (mm) of described product made from steel in the formula, and proeutectoid ferrite has 133～150 hardness HV in the structure.

On the other hand, the invention provides a kind of low-carbon (LC) revulcanization free-cutting steel product of finished surface roughness excellence, described product made from steel comprises: based on mass percent, and C:0.02%～0.12%; Si:0.01% or be lower than 0.01%; Mn:1.0%～2.0%; P:0.05%～0.20%; S:0.30%～0.60%; N:0.007%～0.03%, equal amount is Fe and unavoidable impurities, the content of Mn and S satisfies following condition: 0.40≤Mn * S≤1.2 and Mn/S 〉=3.0, described product made from steel has the ferrite-pearlite structure as its structure, the width average of sulfide inclusion thing in the wherein said product made from steel (μ m) is 2.8 * (log d) or greater than 2.8 * (log d), d is the diameter (mm) of described product made from steel in the formula, resistance to deformation difference during the strain 0.3 (strain) between 200 ℃ and 25 ℃ is 110MPa or greater than 110MPa and 200MPa or less than 200MPa, measures resistance to deformation in compression testing under 0.3mm/ minute rate of deformation.

In addition, the invention provides a kind of appropriate method for preparing the low-carbon (LC) revulcanization free-cutting steel product of described finished surface roughness excellence.Particularly, the invention provides a kind of method for preparing the low-carbon (LC) revulcanization free-cutting steel product of finished product surfaceness excellence, described method comprises the steps: to cast and has the steel of above-mentioned composition, and the content of controlling free oxygen (Of) before casting step is 30ppm or greater than 30ppm and be lower than 100ppm, the ratio Of/S of control free oxygen and S is in 0.005～0.030 scope, and free oxygen Of and S are included in the molten steel before the casting.

Beneficial effect

The finished surface roughness of free-cutting steel product changes significantly with generation, size, shape and the homogeneity of built-up edge.Built-up edge is the phenomenon that the part work material sticked on the tool surface and play a part the part of cutter.Its special initial finished surface roughness of damaging work material.Built-up edge only takes place under given conditions, but the free-cutting steel product usually cuts under the condition that produces built-up edge in the art.

On the other hand, thus built-up edge plays the effect that the protection blade prolongs cutter life.Therefore, consider all factors, it is disadvantageous removing (preventing its generation) these built-up edges, and it is important stably forming built-up edge and making its size and dimension homogenizing.

The present invention can stably form the uniform built-up edge of its size and dimension by the effect and the increase dissolved N content of large-sized spherical MnS inclusion.In addition, the present invention contains the hardness of the proeutectoid ferrite in the structure of steel of ferrite-pearlite composite structure by control, can further stably form the more built-up edge of homogenizing of size and dimension.

Another distinguishing feature of the present invention is, as in the control of proeutectoid ferrite hardness, by the resistance to deformation difference between high temperature and the room temperature in the compression testing of product made from steel is controlled in the suitable scope, rather than the hardness of control proeutectoid ferrite, make the built-up edge stabilization.

By these methods, the present invention can typically improve the finished surface roughness of product made from steel in forming process.

Description of drawings

Fig. 1 is the key drawing of the relation between Mn content and the S content among expression the present invention.

Fig. 2 is illustrated in the temperature variant key drawing of the resistance to deformation of product made from steel in the compression testing.

Fig. 3 is at the key drawing of room temperature (25 ℃) relation between distortion and the resistance to deformation difference in 200 ℃ the scope in the compression testing of expression product made from steel.

Embodiment

The product made from steel structure

Low-carbon (LC) revulcanization free-cutting steel of the present invention has ferrite and the pearlitic composite structure that improves machinability basically.In addition, in order to improve the finished product surfaceness in forming process, the present invention is controlled in 133～150 scopes preferred 135～145 scope with the hardness HV of the proeutectoid ferrite in the compound structure.

In forming process, this typically reduces the work hardening of free-cutting steel product when cutting, can stably form the built-up edge with uniform-dimension and shape, improves the finished product surfaceness.In these factors, the work hardening of free-cutting steel product when cutting influences the stability of built-up edge significantly.Work hardening when cutting by reducing can stably form built-up edge.Therefore, the Hardness Control of proeutectoid ferrite it is said reduce the free-cutting steel product when cutting work hardening or work hardening is reduced to the control device of optimum range.

If the hardness HV of proeutectoid ferrite is above 150 or more strictly surpass 145, then the work hardening of free-cutting steel product reduces, and is undue hard but proeutectoid ferrite becomes, and makes the cutting force increase, thereby quicken the wearing and tearing of cutter.Shorten cutter life like this and damaged the finished product surfaceness.

By contrast, if the hardness HV of proeutectoid ferrite is lower than 133, or more strictly be lower than 135, then proeutectoid ferrite becomes too soft, thereby increases the work hardening of free-cutting steel product when cutting significantly.This causes built-up edge can not stablize formation, and size and dimension is inhomogeneous, thereby damages the finished product surfaceness significantly.

The hardness of control proeutectoid ferrite has also been improved the machinability after cold drawn.Therefore, in other words, no matter these cold worked treatment rates how, in cold drawn or cold drawn silk, even shrink when reducing at area, Hardness Control also produces suitable machinability.Before the cutting of free-cutting steel product, carry out shape and/or dimensional precision and the improvement machinabilities of conventional cold working to improve the free-cutting steel product such as for example cold drawn and cold drawn silk usually.But, needing higher area to shrink and improve machinability, this will influence the shape and size precision unfriendly, be cold worked main purpose yet improve the shape and size precision, so this reduce processibility and cold worked efficient.But just to the shape and size precision of improving the free-cutting steel product, this is cold worked main purpose, and the present invention can carry out cold working.This is very big advantage.In addition, in cold working, no matter how area shrinks, perhaps even at area shrink under the reduction, the present invention can both provide suitable machinability.

Go out the structure of sample through etch exposed, and use miniature Vickers' hardness tester only to measure the hardness of the proeutectoid ferrite in the product made from steel structure that exposes at 5kg or under, measure the hardness of proeutectoid ferrite like this less than the load of 5kg.In this program, on the small part of product made from steel, measure hardness, it has some variations.Therefore, on the longitudinal direction of product made from steel sample and/or diameter (thickness) direction, measure the hardness of a plurality of points, for example about 15 points, the mean value of the hardness of mensuration is defined as the hardness of proeutectoid ferrite.Can or more measure hardness naturally on the multiple spot at 15.Owing to measure, so from the mean value of determination data, the determination data of hardness comprises too high hardness or low excessively hardness with small proeutectoid ferrite body portion.In this case, preferably after rejecting these data, determine mean value again.

Element-specific of mentioning later as for example P and N etc. and the merging of additional C u and Ni and the result of the merging of the preparation condition mentioned later such as hot-rolled temperature and the rate of cooling after the hot rolling for example are by the hardness of sosoloid strengthening effect control proeutectoid ferrite.Except above-mentioned element, these sosoloid strengthening elements also comprise Si, Mn and Cr.But because the reason of mentioning later, the present invention does not use these elements.

Compression testing

By the resistance to deformation in the compression testing of control product made from steel, rather than the hardness of control proeutectoid ferrite, or directly measure the hardness of above-mentioned proeutectoid ferrite, can stably form built-up edge.In other words, as in the hardness of proeutectoid ferrite, can determine the neoplastic stability of smear metal with the resistance to deformation in the compression testing of product made from steel.

As above-mentioned, to be the part work material adhere on the tool surface when cutting built-up edge, and the phenomenon of cutting as the part of cutter.Built-up edge is to be formed by work material, thereby the time repeatedly grows and peel off in cutting.The size of built-up edge is with the change in location of cutter, and this affects the finished surface roughness of the free-cutting steel product that obtains.Because bear bigger viscous deformation partly, built-up edge produces fragment on the interface between fragment and the built-up edge.The size of built-up edge changes with the variation of plastic deformation point.Therefore, stable in order to make built-up edge, preferably make the centrostigma of viscous deformation is the center with the interface between built-up edge and the fragment always, prevents that it from moving on to other point.

Built-up edge has temperature distribution.One of sign of concentration of plastic deformation degree is poor between the resistance to deformation under the high temperature and the resistance to deformation under the room temperature in the compression testing of product made from steel.As in the control of proeutectoid ferrite hardness, by the resistance to deformation difference between the temperature is controlled in the suitable scope, the centrostigma of viscous deformation can be the center with the interface between built-up edge and the fragment always, thereby makes the built-up edge stabilization.The present invention's regulation, the resistance to deformation difference between the temperature is that the resistance to deformation between 200 ℃ and 25 ℃ is poor in the compression verification.More specifically, it is under 0.3mm/ minute rate of deformation, and the resistance to deformation when 0.3 strain between determined 200 ℃ and 25 ℃ of the compression verification is poor.According to the present invention, the resistance to deformation difference in the compression verification between 200 ℃ and 25 ℃ should be for 110MPa or greater than 110MPa and 200MPa or less than 200MPa.

If the resistance to deformation difference between 200 ℃ and 25 ℃ is less than 110MPa, then proeutectoid ferrite becomes too soft, thereby increases the work hardening of free-cutting steel product when cutting significantly.Like this, the centrostigma of viscous deformation moves, and is not the center with the interface between built-up edge and the fragment.This makes the built-up edge instability, has uneven size and dimension, thereby damages the finished product surfaceness significantly.

On the contrary, if the resistance to deformation difference between 200 ℃ and 25 ℃ surpasses 200MPa, then proeutectoid ferrite becomes undue hard, has too high processing resistance, thereby quickens the wearing and tearing of cutter.This shortens cutter life, makes finished surface roughness variation.

Poor by room temperature (25 ℃) and the resistance to deformation between 200 ℃ in the compression verification of optimizing product made from steel, as in the Hardness Control of proeutectoid ferrite, can stably forming built-up edge.

Fig. 2 is illustrated in the resistance to deformation of product made from steel in the compression verification with variation of temperature.In Fig. 2, the data that black triangle is represented are data of the invention sample 52 of embodiment 3, and the data that black box is represented are data of Comparative Examples 38 among the embodiment 3.Resistance to deformation when Fig. 2 is illustrated in strain 0.3 in 0.3mm/ minute the rate of deformation lower compression test.

Fig. 2 is illustrated in the resistance to deformation of invention sample under the probe temperature than Comparative Examples height.In invention sample and comparative sample, resistance to deformation begins to increase from 25 ℃ of room temperatures, reaches maximum at 200 ℃, reduces significantly being higher than under 200 ℃ the temperature.

The resistance to deformation difference of product made from steel between 25 ℃ (room temperatures) and 200 ℃ influences the stable of the intensity of viscous deformation and built-up edge significantly, increases at 25 ℃ (room temperatures) and 200 ℃ of scope internal strain resistances.Therefore, the present invention is with the definition of the resistance to deformation difference between 25 ℃ (room temperatures) and 200 ℃ machinability.

Room temperature (25 ℃) is consistent satisfactorily with the resistance to deformation difference and the machinability of the determined product made from steel of proeutectoid ferrite hardness between 200 ℃.In other words, in the expression compression verification 200 ℃ with 25 ℃ between the scope of resistance to deformation difference meet or consistent with the scope of the hardness of the proeutectoid ferrite of the compound structure of representing 133～150 HV satisfactorily.

Along with strained in the compression verification increases, room temperature (25 ℃) and the resistance to deformation difference between 200 ℃ become and merit attention.Fig. 3 represents that respectively under 0.1,0.2 and 0.3 strain, invention sample and comparative sample are poor in room temperature (25 ℃) and the resistance to deformation between 200 ℃.In Fig. 3, the data that blank bar is represented are data of Comparative Examples, and the data that secret note is represented are data of invention sample.Since room temperature (25 ℃) and the resistance to deformation difference between 200 ℃ 0.3 strain be higher than between 0.3 the strain variation not quite, so the strain in the compression verification is set at 0.3.

As in the Hardness Control of proeutectoid ferrite, can control in the compression verification determine and the present invention's regulation poor in the resistance to deformation between 200 ℃ and 25 ℃ under 0.3 the strain.More specifically, in the combination of element-specific that for example P and N etc. mention later and additional C u and Ni and for example under the appropriate combination of the preparation condition mentioned later such as hot-rolled temperature and hot rolling postcooling speed, strengthen by sosoloid that can to control this resistance to deformation poor.

The composition of product made from steel

Based on mass percent, the composition of low-carbon (LC) revulcanization free-cutting steel product of the present invention and stipulate that the reason of each element is described below.

Free-cutting steel product of the present invention generally typically is applied to not to be needed high mechanical property but needs machinability and can pass through to cut on the finding of producing in large quantities, for example screw and threaded connector.Except these use needed machinability, in the production of for example product made from steel such as wire rod and bar, the free-cutting steel product also must have the performance and the workabilitys such as for example intensity of certain level.In it was produced, except the working condition of mentioning later, the chemical constitution of product made from steel also played an important role to producing the ferrite-pearlite composite structure.

In order to satisfy structure and performance demands, product made from steel of the present invention comprises following component by mass percent as its basic chemical constitution: C:0.02%～0.12%; Si:0.01% or be lower than 0.01%; Mn:1.0%～2.0%; P:0.05%～0.20%; S:0.30%～0.60%; N:0.007%～0.03%, equal amount are Fe and unavoidable impurities, and the content of Mn and S satisfies following condition: 0.40≤Mn*S (=Mn * S)≤1.2 and Mn/S 〉=3.0.

When needing, in above-mentioned composition, the content of Cr is controlled to 0.04% or less than 0.04%, the total content of Ti, Nb, V, Al and Zr is controlled to 0.020% or less than 0.020%, these elements are controlled to be impurity.

If desired, described composition also optionally comprises greater than 0.30% and is equal to or less than 1.0% Cu and greater than 0.20% and be equal to or less than among 1.0% the Ni one or both.

C：0.02％～0.12％

It is poor with the resistance to deformation between the hardness of guaranteeing its intensity, proeutectoid ferrite and 200 ℃ and 25 ℃ that ladle contains C.If the content of C is less than 0.02%, described steel has inadequate intensity and inadequate proeutectoid ferrite hardness.In addition, described steel shows the machinability of too high toughness and ductility and reduction.On the contrary, if C content surpasses 0.12%, described steel shows the too high intensity and the hardness of proeutectoid ferrite, and this makes the machinability variation, rather than improves it.Therefore, the lower limit set of C content is 0.02% or preferred 0.03%, and its upper limit is set at 0.12% or preferred 0.07%.

Mn：1.0％～2.0％

Mn combines with S and forms sulfide M nS in described steel, thereby improves machinability.It also prevents because the FeS that forms produces hot short circuit.In order to show these advantages, the lower limit set of Mn is 1.0%.But Mn has desoxydatoin, if its content surpasses 2.0%, works to remove free oxygen (Of) in its molten steel before casting, described steel is lacked produce the needed free oxygen of large-sized spherical MnS.In addition, described steel has too high intensity, thereby reduces machinability on the contrary.Therefore the upper limit of Mn is set at 2.0%.In conjunction with the S that mentions later, further control or stipulate the content of Mn, thereby prevent the deoxidation behavior, make Mn mainly promote the formation of sulfide M nS.

P：0.05％～0.20％

By the sosoloid strengthening effect, it is poor that P is that the hardness HV with proeutectoid ferrite is controlled in 133～150 scope and/or the control compression verification resistance to deformation between 200 ℃ and 25 ℃, thus the important element of improvement machinability.More specifically, strengthen the Cu that strengthens with the sosoloid of N or comprise with selectivity and/or the sosoloid of Ni is strengthened by the sosoloid that makes up P suitably, further combine with rate of cooling after hot-rolled temperature of mentioning later and the hot rolling, the present invention is controlled at the resistance to deformation difference between 200 ℃ and 25 ℃ in the hardness of proeutectoid ferrite and the compression verification in the top specified scope.In order to show these advantages, described product made from steel must contain 0.05% or greater than 0.05% P.On the contrary, even because product made from steel contains the P that surpasses 0.20% amount, these advantages also reach capacity, so the upper limit of P is set at 0.20%.

S：0.30％～0.60％

S is by forming the element that sulfide improves machinability with Mn.Less than 0.30% o'clock, this advantage was excessively little at S content.On the contrary, surpass 0.60% S content and may make the hot workability variation.Therefore, its lower limit set is 0.30% or preferred 0.35%, and its upper limit is set at 0.60% or preferred 0.50%.

From the relation between S and the Mn, should set S content the content of Mn and S is satisfied condition: 0.40≤Mn*S (=Mn * S)≤1.2 and Mn/S 〉=3.0.Fig. 1 represents the relation between the content of Mn and S among the present invention, and X-coordinate is represented Mn content (%), and ordinate zou is represented S content (%).In Fig. 1, extend to the lower limit that upper right straight line is represented Mn/S from the lower-left, promptly Mn/S is 3.0 straight line, extends to these upper left curves from the bottom right and represents Mn * S respectively.Those curves of expression Mn * S represent respectively that from the left side of figure Mn * S is 0.40,0.45,0.5,0.8,1.0 and 1.2 curve.

In Fig. 1, the zone of the Mn/S that satisfies condition 〉=3.0 is that Mn/S is the zone below 3.0 the straight line.Mn * S is 0.40 or is that Mn * S is the upper area of 0.40 curve greater than 0.40 zone.Mn * S is 1.2 or is that Mn * S is the zone of 1.20 curve below less than 1.2 zone.The content of Mn and S satisfies all above-mentioned content requirement and following condition among the present invention, 0.40≤Mn * S≤1.2 and Mn/S 〉=3.0, scope be the shadow region, diagonal angle.0.45 Mn * S and Mn * S of 0.5 preferred and preferred lower limit of representing Mn * S respectively.1.0 Mn * S and Mn * S of 0.8 preferred and preferred upper limit of representing Mn * S respectively.

If the content of Mn and S makes Mn * S surpass above-mentioned stated limit 0.40～1.2, preferable range 0.45～1.0 and the more preferably upper limit of scope 0.5～0.8, then the S too high levels is so that reduce the needed free oxygen of control MnS shape.This makes the machinability variation.On the contrary, if Mn * S less than above-mentioned lower limit, the absolute content of MnS reduces, thereby machinability variation or free oxygen content are increased, thereby increases the danger that forms pore.

Mn/S less than 3.0 forms FeS than causing, and reduces processibilities such as for example hot rolling processibility, thereby can not produce described product made from steel.

Si:0.01% or less than 0.01%

Si has desoxydatoin, removes the free oxygen (Of) in the preceding molten steel of casting, thereby the needed free oxygen of the spherical MnS of formation large size is lacked.If the Si content of described product made from steel surpasses 0.01%, then this disadvantageous effect is significantly, forms hard oxide compound, thereby makes the remarkable variation of machinability.Therefore, Si content reduces to 0.01% or less than 0.01%.

N：0.007％～0.02％

The same with above-mentioned P, N is the important element of 133～150 scopes that proeutectoid ferrite hardness HV are controlled at by the sosoloid strengthening effect.Become to merit attention plays an important role N to the dynamic load strain aging (dynamic strain aging) that makes product made from steel by the sosoloid strengthening effect.The dynamic load strain aging of product made from steel can be stablized the formation of built-up edge.If described product made from steel has noticeable like this dynamic load strain aging, then built-up edge stably forms, and has uniform size and dimension.In addition, this noticeable dynamic load strain aging of product made from steel makes in the compression verification resistance to deformation difference between 200 ℃ and 25 ℃ increase so that be controlled in the top specified scope.It is the machinability of representative with the surfaceness that N also plays improvement.

In order to show these advantages, described product made from steel must contain 0.007% or greater than 0.007% N, when N content less than 0.007% the time, these advantages are too small.On the contrary, if described product made from steel contains the N that surpasses 0.02% amount, then the processibility that becomes in undue height and/or the hot rolling of the hardness of proeutectoid ferrite is typically reducing.Therefore, the lower limit and the upper limit with N is set at 0.007% and 0.02% respectively.

Dissolved nitrogen

In order to show the advantage of N fully, especially in order to increase the dynamic load strain aging of described product made from steel, except top specified preferred total N content, described product made from steel preferably has 70ppm or greater than dissolved nitrogen (dissolving N) content of 70ppm.If product made from steel contains the dissolved nitrogen content less than 70ppm, even Zong N content height, it does not have the dynamic load strain aging of abundant increase yet, and it is poor to increase in the compression verification resistance to deformation between 200 ℃ and 25 ℃.

In order to increase the dissolved nitrogen content in the described product made from steel, mention as the back, should reduce for example amount of the element of formation nitride such as Ti, Nb, V, Al and Zr.Rate of cooling after increasing the Heating temperature in the last hot-work (hot rolling or forge hot) and/or increasing hot-work also is effective.

By measuring N (total nitrogen) total content in the product made from steel, from total nitrogen, remove the content of compound nitrogen (sedimentary nitrogen), calculate the dissolved nitrogen content of determining product made from steel according to following equation.From product made from steel, electrolytically remove these fixed nitrogens, analyze this content, determine the content of compound nitrogen quantitatively by indophenyl absorption measurement method.

Dissolved nitrogen content (ppm)=(total nitrogen content)-(compound nitrogen content)

Oxygen

During the casting of the product made from steel of on have, specify forming,, the free oxygen (Of) in the molten steel before the casting controlled to 30ppm or, the Of/S ratio of free oxygen and S is controlled at 0.005～0.030 greater than 30ppm and less than 100ppm according to the present invention.The term " MnS " that the present invention uses except comprising the compound that mainly comprises S, also comprise to dissolved oxygen wherein with the MnS that forms sosoloid and with oxide compound compound MnS, be representative with MnS.Be dissolved among the MnS or influence size and the structure of MnS significantly with MnS compound oxygen.These MnS materials are formed in the preceding molten steel of casting.Therefore, the oxygen level in the product made from steel that control obtains is unessential, and the free oxygen content before the control casting in the molten steel is important.More specifically, determine the structure of MnS with the free oxygen content in the molten steel before the casting, by the free oxygen in the molten steel before the control casting, MnS can have big size and spherical form, thereby improves machinability.

If less than 0.005, then MnS can not have big size and spherical form to free oxygen less than 30ppm and Of/S in the molten steel before casting, unable to get up improves the effect of machinability.On the contrary, if free oxygen surpasses 100ppm and Of/S surpasses 0.030, then excessive free oxygen will cause pore.

Select one or more method rightly, for example control strong deoxidation such as MnS content, control example such as Al and Si etc. element, the obducent composition of control slag, after forcing to add FeO and before reaching balance, cast, can control the free oxygen content in the molten steel.

By measuring electromotive force, with computing unit electromotive force is converted into oxygen level, thereby determines free oxygen, determine the free oxygen content in the molten steel.Use commercially available comprise the oxygen concn pond and consume molten steel product oxygen sensor as the dipping of the thermopair of temperature sensor determine electromotive force.Use YAMARI-ELECTRONITE CO., the HY-OP DIGITAL INDICATORMODEL of LTD (HY-OP digital indicator type) carries out emf measurement and calculating.

Cr and Ti, Nb, V, Al, Zr

Cr, Ti, Nb, V, Al and Zr are fixing effectively to dissolve N to improving machinability, thereby forms nitride.These elements reduce dissolving N content, thereby make the machinability variation.When described steel contains the Cr of 0.04% the amount of surpassing and/or contains total amount when surpassing 0.020% Ti, Nb, V, Al and Zr, this unfavorable effect merits attention.These elements should preferably minimize in the present invention.Therefore, Cr content is controlled to preferred 0.04% or less than 0.04%, more preferably 0.020% or less than 0.020%.The total content of Ti, Nb, V, Al and Zr is controlled to preferred 0.020% or less than 0.020%, more preferably 0.015% or, further preferred 0.010% or less than 0.010% less than 0.015%.

Cu and Ni

Cu and Ni are dissolved in and form sosoloid in the ferrite, thus reinforced ferrite.It is effectively that these elements are controlled at for the hardness HV with proeutectoid ferrite in 133～150 the scope, and can be used in combination with above-mentioned N.When Cu and/or Ni optionally were included in the described product made from steel, in order to show this advantage, the content of Cu was greater than 0.30% and be equal to or less than 1.0%, and the content of Ni is greater than 0.20% and be equal to or less than 1.0%.If Cu content be 0.30% or less than 0.30% or Ni content be 0.20% or less than 0.20%, then described product made from steel can not show these advantages.If Cu content surpass 1.0% or Ni content surpass 1.0%, this advantage also no longer increases.

The structure of MnS

At length explain the structure of MnS in the product made from steel (sulfide inclusion thing) below.As above-mentioned, composition by product made from steel and fusion and casting condition are determined amount and the distribution of MnS basically, but it is configured in hot rolling after the casting or the hot forging process and also changes.If MnS has large-sized spherical form, it is unfavorable for flattening, even after the processing, its structure also changes in wide region.The width of MnS influences the machinability that carries out cold worked product made from steel such as for example wire drawing after hot-rolled steel product or the hot rolling significantly.Machinability generally increases with the width of MnS.But, can be according to the width average of the required MnS of the diameter change of product made from steel.For example, machinability increases with the diameter reduction of product made from steel, increases with its diameter and reduces, and its precondition is that the MnS with equal volume, number and structure (width) is included in the product made from steel.Note this structure,, also can improve machinability by making MnS have sufficient width even the diameter of product made from steel is big.

At the width average of MnS and influence in the relation between the diameter (standard specifications) of the product made from steel of machinability, the width average that needs should be 2.8 * (log d) or greater than 2.8 * (log d), d represents the diameter of product made from steel (wire rod after rolling or bar) in the formula.If the maximum width of MnS is worth less than this, machinability reduces.

As above-mentioned, the term that the present invention uses " MnS " except comprising the compound that mainly comprise S of MnS as representative, also comprise with oxygen be dissolved in the MnS that wherein forms sosoloid and with oxide compound compound MnS.These sulfide also are effective for improving machinability.100 times of observed images are amplified in analysis under opticmicroscope, determine the maximum width of each MnS.View-point is important, should observe the zone of mentioning below.To the most important zone of machinability is zone from the dark 0.1mm of the outer surface of product made from steel to dark d/8, should observe this zone.Should observe with plane that rolling direction parallels in 6mm ²Or greater than 6mm ²The zone of area.The outer surface of polished steel product is just enough before observation, does not need etching.Remove after the MnS of main shaft less than 1 μ m, measure and analyze maximum width.This is because of the MnS generation bigger error at measurment of main shaft less than 1 μ m, and little to the machinability influence.

In this relation, above-mentioned patent documentation 10 points out that minor axis is 2 μ m or greater than the regulation factor of 2 μ m as MnS.But unless the maximum width of MnS increases, otherwise when product made from steel has bigger diameter, regardless of the diameter of product made from steel how, this even regulation all can not promote the improvement of machinability.

The preparation method

The preferred preparation condition of product made from steel of the present invention is described below.

At first, in order to make MnS have big size and spherical form to improve machinability, when product made from steel fusion with above-mentioned composition and casting, according to the present invention, free oxygen (Of) in the molten steel before the casting is controlled to be 30ppm or greater than 30ppm and less than 100ppm, the Of/S ratio of free oxygen and S is controlled to be 0.005～0.030.

In order to control the maximum width of MnS, at preferred 1000 ℃ or be higher than 1000 ℃, more preferably 1040 ℃ or be higher than and heat steel billet (thigh) in 1040 ℃ of hot rollings under the temperature.When from process furnace, pulling out steel billet, measure the Heating temperature of steel billet.

In order to make low-carbon (LC) revulcanization free-cutting steel product of the present invention have ferrite and pearlitic composite structure and the hardness HV of proeutectoid ferrite to be controlled in 133～150 the scope obtaining higher machinability, subsequently hot-rolled temperature is set in ferrite zone or ferritic-austenitic zone effectively.

Be controlled at for the hardness HV with proeutectoid ferrite in 133～150 the scope or with the resistance to deformation difference between 200 ℃ and 25 ℃ in the compression verification and be controlled in the top specified scope, the rate of cooling after the control hot rolling is important.The hardness that air blast is cooled off and/or for example acceleration such as water cooling or spray cooling cooling can increase proeutectoid ferrite effectively in rod rolling controlled chilling line (Stelmor line) after the hot rolling.By after ferritic transformation, increasing rate of cooling immediately, can not change under ferrite and the pearlitic composite structure, only increase the hardness of proeutectoid ferrite.This can be controlled at the resistance to deformation difference between 200 ℃ and 25 ℃ in the compression verification in the scope of stipulating above.

When the hot rolled steel wire rod cools off in rod rolling controlled chilling line, preferably just after wire rod is placed on the rod rolling controlled chilling line basically and parts reach 500 ℃ or when being lower than 500 ℃ between, with 1.0 ℃/second or greater than 1.0 ℃/second average rate of cooling V (℃/second), cool off wire rod with air-cooling.Word " is placed basically " and is meant first point that wire rod is placed on the layout air cooling apparatus.More strictly speaking, because these speed are different between the thick and thin part of wire rod coil, so when cooling off in rod rolling controlled chilling line, " rate of cooling " of wire rod is meant the mean value of the rate of cooling of wire rod.

Wire rod after the hot rolling and steel bar carry out for example cold working such as wire drawing or elongation and machining as required, thereby obtain product.

Embodiment 1

The explained later embodiments of the invention.At first in embodiment 1 and 2, prove the effect of improving the machinability of steel wire by the hardness of control proeutectoid ferrite.

Under various hot-rolled conditions, prepare a series of steel wires with physical device with various compositions.Estimate machinability and other performance of steel bar respectively.Specifically, through fusion and casting, under 20 ℃/second casting solidified rate of cooling, preparation has the low-carbon (LC) steel billet of the composition 1～14 shown in following table 1 and 2.Table 2 is continuous tables of table 1, has also expressed free oxygen content and the Of/S in the molten steel before the casting.

These steel billets heat and hot rolling under the condition shown in the following table 3, thereby make the steel wire rod with the footpath of silk shown in the table 3.Rate of cooling behind rolling shown in the table 3 is meant the sample steel wire rod after the finish rolling is being placed on the Stelmor conveyer, begins the air blast cooling then and steel wire rod is cooled to average rate of cooling in 500 ℃ the situation, do not comprise the situation of rolling pattern C.Among the rolling pattern C that is marked with asterisk (*) in table 3, steel wire rod is cooled to 600 ℃ with 0.8 ℃/second average rate of cooling, and quickens cooling in the temperature below 600 ℃ with 2.5 ℃/second.Control by for example ring spacing of coil wire rod, use the isoparametric combination of control of volume of air and direction in slow cool down cover (slow-cooling cover) and the air cooling, control the rate of cooling after the hot rolling suitably.

Relation between the diameter (d) (2.8 * (log d)) of the width average of the MnS of the steel wire rod that table 3 expression makes, the width average of MnS and product made from steel and the hardness (HV) of proeutectoid ferrite.These are determined with aforesaid method.The structure of observable steel wire rod finds that they all are the ferrite-pearlite structures.

The steel wire rod that makes carries out the machining property testing.In the machining property testing, cutting or centerless grinding sample wire rod are removed scale, be fixed to make it center on axle center rotation on the lathe, with rapid steel product cutter (SKH4) vertically otch in the wire rod to be shaped, determine the finished surface roughness after the cutting.Form with the cutter feed rate of 92m/ minute cutting speed, 0.03mm/rev and the depth of cut of 1.0mm.Here, the finished surface roughness is defined as the medullary ray center line average Ra (μ m) that determines with the surface roughness measurement method of stipulating among the B0601 of Japanese Industrial Standards (JIS).

The material steel 2,3 and 6 of the steel wire rod that is used for invention sample 2～11 and 14 shown in table 1～3 expression tables 1 has the chemical constitution in the stated limit of the present invention and has Mn and S content: 0.40≤Mn * S＜1.2 and Mn/S 〉=3.0 of satisfying following condition.In the molten steel before casting, each all has 30ppm or greater than 30ppm and less than the Of/S ratio in free oxygen in the 100ppm scope and 0.005～0.030 scope these steel wire rods.In the preferable range that its rolling condition is stipulated in the above.

Each the width average (μ m) of sulfide inclusion thing of the steel wire rod that obtains is 2.8 * (log d) or greater than 2.8 * (log d), the hardness HV of proeutectoid ferrite is 133～150 in structure.Therefore, they have 33.6 μ m or less than the finished surface roughness Ra of 33.6 μ m (27.9～33.6 μ m).This finished surface roughness is better than 34.8～40.3 μ m in the above mentioned patent documentation 6, as in the present invention, also controls number, size and the structure of sulfide inclusion thing in this patent documentation.

On the contrary, comparative sample 1,12,15 and 19～22 each all have the finished surface roughness Ra of 37.5～48.2 μ m, show than the remarkable inferior machinability of invention sample.In comparative sample 13 and 16～18, owing to when rolling, break, so can not obtain steel wire rod.

For example, as shown in table 1, the material steel 1 that is used for comparative sample 1 has the low Mn * S less than 0.40 lower limit.As shown in table 2, in the molten steel before casting, the material steel 4 that is used for comparative sample 12 has than little low free oxygen of lower limit 30ppm and the low Of/S littler than lower limit 0.005.Therefore, comparative sample 12 has the harmonic(-)mean width (μ m) less than 2.8 * (log d) MnS.

The material steel that before casting, has the low free oxygen lower in the molten steel 7 preparations shown in the comparative sample 15 usefulness tables 2 than lower limit 30ppm, thus the harmonic(-)mean width of MnS (μ m) is less than 2.8 * (logd).

Comparative sample 19 usefulness are as shown in table 2 has to have in molten steel less than the low free oxygen of lower limit and the material steel 11 of Of/S before 2.2% high high Mn content of the ratio upper limit 2.0% shown in the table 1 and the casting and prepares.

Comparative sample 20 usefulness have material steel 12 preparations of 0.28% the S content lower than lower limit 0.3%, thereby have the width average (μ m) that is lower than 2.8 * (log d) MnS.

Comparative sample 21 and 22 is respectively with material steel 13 with low N content littler than lower limit 0.007% and 14 preparations, thereby has the soft HV less than 133 proeutectoid ferrite.

These results show the crucial meaning of requirement of the present invention.

Table 1

Sequence number	The chemical constitution of steel (mass percent, surplus are Fe and impurity)
																C	Si	Mn	P	S	N	Cr	Cu	Ni	Ti	Al	V	Nb	Zr	Ti, Al, V, the total content of Nb and Zr
	1 2 3 4 5 6 7 8 9 10 11 12 13 14	0.05 0.04 0.06 0.07 0.08 0.05 0.04 0.06 0.08 0.07 0.08 0.08 0.07 0.05	0.005 0.005 0.005 0.005 0.005 0.006 0.005 0.005 0.005 0.005 0.007 0.005 0.007 0.005	1.2 1.5 1.8 1.9 1.3 1.5 1.8 1.1 1.5 0.8 2.2 1.1 1.3 1.5	0.08 0.07 0.08 0.08 0.07 0.07 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.07	0.33 0.4 0.5 0.55 0.45 0.4 0.55 0.38 0.52 0.35 0.56 0.28 0.38 0.45	0.008 0.008 0.011 0.008 0.007 0.009 0.015 0.014 0.009 0.011 0.008 0.007 0.004 0.005	0.03 0.02 0.03 0.03 0.04 0.03 0.02 0.01 0.02 0.03 0.02 0.03 0.03 0.03	0.05 0.03 0.02 0.03 0.04 0.03 0.02 0.03 0.03 0.02 0.02 0.02 0.03 0.02	0.02 0.01 0.01 0.01 0.02 0.01 0.01 0.02 0.01 0.01 0.02 0.01 0.01 0.01	0.001 0.001 0.002 0.001 0.002 0.002 0.002 0.002 0.001 0.002 0.001 0.002 0.002 0.002	0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001	0.006 0.003 0.003 0.003 0.002 0.003 0.003 0.004 0.003 0.003 0.003 0.003 0.003 0.003	0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001	0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001																0.010 0.007 0.008 0.007 0.007 0.008 0.008 0.009 0.007 0.008 0.007 0.008 0.008 0.008

Table 2 (continuous table 1)

Sequence number	The chemical constitution of steel (mass percent)
					Of	Of/S	Mn/S	Mn*S
	1 2 3 4 5 6 7 8 9 10 11 12 13 14	0.0053 0.0048 0.0036 0.0026 0.0052 0.0065 0.0028 0.0065 0.0039 0.0105 0.0019 0.007 0.0063 0.0048	0.0161 0.012 0.0072 0.0047 0.0116 0.0163 0.0051 0.0171 0.0075 0.03 0.0034 0.025 0.0166 0.0107	3.6364 3.75 3.6 3.4545 2.8889 3.75 3.2727 2.8947 2.8846 2.2857 3.9286 3.9286 3.4211 3.3333					0.396 0.6 0.9 1.045 0.585 0.6 0.99 0.418 0.78 0.28 1.232 0.308 0.494 0.675

Table 3

Sequence number	Steel sequence number in the table 1	Hot-rolled condition				Steel wire				Machinability finished surface roughness Ra (μ m)	Remarks	Catalogue
													Heating temperature (℃)	Final rolling temperature (℃)	Rate of cooling ℃/minute	Rolling pattern	Silk footpath (mm)	MnS		The hardness of proeutectoid ferrite (HV)
		2.8*(log d)	Width average (μ m)
				1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22	1 2 2 2 2 2 2 3 3 3 3 4 5 6 7 8 9 10 11 12 13 14	1010 1010 1010 1010 1010 1005 1020 1010 1010 1005 1020 1010 1010 1010 1010 1010 1010 1010 1010 1010 1010 1010	850 850 855 855 855 860 705 850 850 860 705 850 850 850 850 850 850 850 850 850 850 850	0.8 0.8 1.8 1.8 1.8 * 1.3 0.8 0.8 * 1.3 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8	A A B B B C D A A C D A A A A A A A A A A A									8.0 8.0 6.2 10.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0	2.53 2.53 2.21 2.80 2.53 2.53 2.53 2.53 2.53 2.53 2.53 2.53 2.53 2.53 2.53 2.53 2.53 2.53 2.53 2.53 2.53 2.53		2.82 2.72 2.39 3.15 2.73 2.79 2.71 2.56 2.59 2.57 2.61 2.03 2.83 2.91 2.22 2.83 2.29 2.84 1.85 2.85 2.89 2.93	132 134 136 137 135 140 142 135 137 142 144 134 135 135 138 135 135 132 138 135 129 127	37.5 33.6 30.2 29.7 30.1 28.6 27.9 34.9 33.6 30.1 29.8 42.6 - 32.5 33.9 - - - 47.0 46.3 48.2 47.6	Break	Contrast invention invention invention invention invention invention invention invention invention invention contrast contrast invention contrast contrast contrast contrast contrast contrast contrast contrast

In the table 3,

^*Be cooled to 600 ℃ with 0.8 ℃/second, after this quicken cooling with 2.5 ℃/second

Break: break in rolling

Contrast: comparative sample; Invention: invention sample

Embodiment 2

Below with the same way as of embodiment 1 through fusion prepare a series of have form 15～26 low-carbon (LC) steel billet shown in table 4 and 5.Table 5 is continuous tables of table 4, free oxygen content and Of/S ratio before the expression casting in the molten steel.Carry out hot rolling according to the system B in the table 3 of embodiment 1.Use machinability and other performance of the steel wire of physical device preparation with the same way as evaluation of embodiment 1.

The width average of the MnS of the silk footpath of the steel wire rod of table 6 expression preparation and the width average of MnS, product made from steel and the hardness (HV) of relation between the diameter (d) [2.8 * (log d)] and proeutectoid ferrite.The finished surface roughness that the steel wire rod that table 6 is also expressed preparation is determined in the machining property testing.The structure of observable steel wire rod finds that they all are the ferrite-pearlite structures.

Table 4～6 show that steel 15～18 and 23～26 that the conduct shown in the table 1 is used for the material of invention sample 23～26,31～34 and 36 has the chemical constitution in the specialized range of the present invention, has Mn and S content: 0.40≤Mn * S≤1.2 and Mn/S 〉=3.0 of satisfying following condition.In addition, free oxygen is controlled at 30ppm or greater than in 30ppm and the scope less than 100ppm, and the Of/S before the casting in the molten steel is than being controlled in 0.005～0.030 the scope.In the preferable range that its rolling condition is stipulated in the above.

Each the width average (μ m) of sulfide inclusion thing of the steel wire rod that obtains is 2.8* (log d) or greater than 2.8* (log d), the hardness HV of proeutectoid ferrite is 133～150 in the structure.Therefore, they have 37.6 μ m or less than the finished surface roughness Ra of 37.6 μ m (30.9～37.6 μ m).

On the contrary, each all has the finished surface roughness Ra of 43.6～48.3 μ m comparative sample 27～30, shows than the remarkable inferior machinability of invention sample.

For example, steel 19 preparations shown in the comparative sample 27 usefulness tables 4, described steel 19 has Ti, Nb, V, Al and the Zr of high total content, surpasses 0.020% the upper limit.

20 preparations of the steel with low N content lower shown in the comparative sample 28 usefulness tables 4 than lower limit 0.007%.

The steel with the high N content that surpasses 0.035% upper limit 21 shown in the comparative sample 29 usefulness tables 4 prepares, thereby has the surface quality of variation after cutting, can not determine its finished surface roughness Ra.

Comparative sample 30 has high proeutectoid ferrite hardness, and it surpasses the upper limit.

These results show the crucial meaning of requirement of the present invention.

Table 4

Sequence number	The chemical constitution of steel (mass percent, surplus are Fe and impurity)
																C	Si	Mn	P	S	N	Cr	Cu	Ni	Ti	Al	V	Nb	Zr	Ti, Al, V, the total content of Nb and Zr
	15 16 17 18 19 20 21 22 23 24 25 26	0.05 0.05 0.04 0.05 0.05 0.04 0.04 0.05 0.05 0.05 0.05 0.05	0.005 0.006 0.005 0.006 0.006 0.006 0.005 0.005 0.004 0.007 0.006 0.005	1.2 1.15 1.2 1.3 1.2 1.15 1.2 1.5 1.2 1.15 1.2 1.2	0.08 0.07 0.08 0.08 0.08 0.07 0.08 0.07 0.15 0.08 0.09 0.07	0.35 0.36 0.35 0.35 0.34 0.35 0.34 0.45 0.35 0.35 0.36 0.34	0.012 0.010 0.012 0.010 0.010 0.005 0.035 0.011 0.012 0.012 0.018 0.009	0.03 0.03 0.05 0.015 0.025 0.01 0.015 0.025 0.03 0.025 0.025 0.03	0.03 0.02 0.03 0.02 0.01 0.03 0.02 0.02 0.03 0.35 0.03 0.36	0.02 0.02 0.01 0.02 0.02 0.01 0.01 0.02 0.01 0.01 0.40 0.26	0.001 0.001 0.002 0.001 0.005 0.001 0.001 0.002 0.002 0.002 0.001 0.002	0.001 0.001 0.001 0.001 0.002 0.001 0.001 0.001 0.001 0.001 0.001 0.001	0.007 0.005 0.011 0.002 0.015 0.008 0.008 0.005 0.005 0.005 0.006 0.005	0.001 0.001 0.001 0.001 0.002 0.001 0.001 0.001 0.001 0.001 0.001 0.001	0.001 0.001 0.001 0.001 0.002 0.001 0.001 0.001 0.001 0.001 0.001 0.001																0.011 0.009 0.016 0.006 0.026 0.012 0.012 0.010 0.010 0.010 0.010 0.010

Table 5 (continuous table 4)

Sequence number	The chemical constitution of steel (mass percent)
					Of	Of/S	Mn/S	Mn*S
	15 16 17 18 19 20 21 22 23 24 25 26	0.0056 0.0057 0.0065 0.0061 0.0056 0.0057 0.0058 0.0048 0.0059 0.0068 0.0056 0.0055	0.016 0.0158 0.0186 0.0174 0.0165 0.0163 0.0171 0.0101 0.0169 0.0194 0.0156 0.0162	3.4286 3.1944 3.4286 3.7143 3.5294 3.2857 3.5294 3.3333 3.4286 3.2857 3.3333 3.5294					0.42 0.414 0.42 0.455 0.408 0.4025 0.408 0.675 0.42 0.4025 0.432 0.408

Table 6

Sequence number	Steel preface in the table 4	The hot-rolled condition system	Steel wire				Machinability finished surface roughness Ra (μ m)	Remarks	Catalogue
										Silk footpath (mm)	MnS		The hardness of proeutectoid ferrite (HV)
			2.8*(log D)	Width average (μ m)
					23 24 25 26 27 28 29 30 31 32 33 34	15 16 17 18 19 20 21 22 23 24 25 26					B B B B B B B B B B B B	8.0 8.0 6.2 10.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0		2.53 2.53 2.21 2.80 2.53 2.53 2.53 2.53 2.53 2.53 2.53 2.53	2.78 2.77 2.85 2.88 2.74 2.72 2.75 2.68 2.77 2.96 2.73 2.75	132 135 132 138 127 127 128 152 136 140 139 142	34.1 31.6 37.6 32.8 48.3 47.3 - 43.6 35.2 31.6 30.9 31.9	Surface quality reduces	Invention invention invention invention contrast contrast contrast contrast invention invention invention invention

Contrast: comparative sample; Invention: invention sample

Embodiment 3

Proof is improved the effect of the machinability of steel wire by the resistance to deformation difference between high temperature and the room temperature in the compression verification of control product made from steel.

Same way as with embodiment 1 prepares a series of low-carbon (LC) steel billets with the composition 27～41 shown in table 7 and 8 through fusion.Table 8 is continuous tables of table 7, free oxygen content and Of/S ratio before the expression casting in the molten steel.The low-carbon (LC) steel billet uses physical device to carry out hot rolling under the Heating temperature shown in the table 9, final rolling temperature and rate of cooling, thereby obtains the steel wire that each has the 8.0mm diameter.Estimate machinability and other performance of steel wire respectively.

Rate of cooling behind rolling shown in the table 9 is meant the sample steel wire rod after the finish rolling is being placed on the Stelmor conveyer, begins the air blast cooling then and steel wire rod is cooled to average rate of cooling in 500 ℃ the situation, do not comprise rolling pattern C.In the rolling pattern C shown in the table 9, with 0.8 ℃/second average rate of cooling steel wire rod is cooled to 600 ℃, quicken to be cooled to room temperature with 2.5 ℃/second from the temperature below 600 ℃.Make up the ring spacing of coil wire rod (ring pitch) for example control, use the volume of air in slow cool down cover and the air cooling and the parameters such as control of direction, control the rate of cooling after the hot rolling suitably.

The poor and dissolved N content of resistance to deformation in relation [2.8 * (log d)] between the diameter (d) of the width average of the MnS of the steel wire rod of table 10 expression preparation, the width average of MnS and product made from steel, the compression verification between 200 ℃ and 25 ℃.Observe the structure of steel wire rod, find that they all are the ferrite-pearlite structures.

By the cylindrical steel wire test sheet of diameter 8mm and high 12mm is carried out compression verification under 200 ℃ of 25 ℃ (room temperatures) and the temperature that raise, estimate resistance to deformation.In compression verification, a slice carbide is clipped between steel wire rod testing plate and the compression clamp to reduce friction, in the compression of steel wire rod testing plate, determining under 0.3mm/ minute the rate of deformation under said temperature and resistance to deformation during 0.3 strain.

Width average and dissolved N content with the MnS of aforesaid method working sample steel wire rod.

By under the same test condition of embodiment 1, measuring the finished surface roughness, estimate the machinability of the steel wire rod of preparation.These results are also illustrated in the table 10.

Steel 41 shown in the table 7 and 8 has the chemical constitution in the specialized range of the present invention, have Mn and S content: 0.40≤Mn * S≤1.2 and Mn/S 〉=3.0 of satisfying following condition, the molten steel before its casting has 30ppm or greater than 30ppm and less than the Of/S ratio in Of in the 100ppm scope and 0.005～0.030 scope.

Table 10 shows in the steel wire rod that uses steel 41 preparations, and invention sample 49,51 and 52 is time rolling at the preferred rolling and cooling conditions (B, C and E) shown in the table 9 respectively, and they have in the preferable range is 70ppm or greater than the dissolved N content of 70ppm.Stipulate as the present invention, using each the width average (μ m) of sulfide inclusion thing of steel wire rod that the invention sample obtains is 2.8 * (log d) or greater than 2.8 * (log d), and the resistance to deformation difference in the compression verification between 200 ℃ and 25 ℃ is 110MPa or greater than 110MPa and 200MPa or less than 200MPa.Their finished surface roughness Ra is that about 27.6 μ m are to about 31.5 μ m.

Invention sample 49,51 and 52 each also have 136～142 proeutectoid ferrite hardness HV, in specialized range of the present invention.

On the contrary, steel 41 preparations that comparative sample 50 usefulness are identical, but under the A of rolling condition shown in the table 9, cool off with too low rate of cooling.Like this, the width average (μ m) of to the greatest extent let it be sulfide inclusion thing is 2.8 * (log d) or greater than 2.8 * (log d), but the dissolving N content of comparative sample 50 is low, be 63ppm, resistance to deformation difference in the compression verification between 200 ℃ and 25 ℃ is low, is 103, less than lower limit.Therefore, comparative sample 50 has about 36.8 finished surface roughness Ra, shows the machinability more inferior than invention sample 49,51 and 52.

As shown in table 8, comparative sample 35 is rolling under the preferred rolling and cooling conditions B shown in the table 9, but its material steel 27 has low Mn * S (lower limit less than 0.40), has low dissolving N content, is 52ppm.It is poor that the comparative sample 35 that obtains has in 95 the low compression verification resistance to deformation between 200 ℃ and 25 ℃, less than lower limit, has poor finished surface roughness Ra, is about 38.9, shows the machinability more inferior than invention sample.

With steel 28 with the chemical constitution in the specialized range of the present invention, rolling and preparation invention sample 36 under the preferred rolling and cooling conditions B shown in the table 9, it has in the preferable range is 70ppm or greater than the dissolving N content of 70ppm.The width average of the sulfide inclusion thing of the steel wire rod that obtains (μ m) is 2.8 * (log d) or greater than 2.8 * (log d), resistance to deformation difference in the compression verification between 200 ℃ and 25 ℃ is 110MPa or greater than 110MPa and 200MPa or less than 200MPa, in the scope of the present invention's regulation.Be about the angle of 33.6 μ m from finished surface roughness Ra, it shows satisfied machinability.

Material steel 29 preparation shown in the comparative sample 37 usefulness tables 8 has in the molten steel before the casting of described material steel 29 than low low free oxygen content of lower limit 30ppm and the low Of/S ratio littler than lower limit 0.005.Therefore, although the steel wire rod that obtains is rolling under the preferred rolling and cooling conditions B shown in the table 9, it has less than the width average (μ m) of 2.8 * (log d) sulfide inclusion thing and the low dissolving N content of 60ppm.Thereby it is poor that comparative sample 35 shows in the low compression verification resistance to deformation between 200 ℃ and 25 ℃, is 102, less than lower limit, thereby has poor about 42.6 finished surface roughness Ra, shows the machinability more inferior than invention sample.

Have chemical constitution in the specialized range of the present invention shown in table 7 and 8 as the steel 30 of the material of comparative sample 38, under preferred rolling and cooling conditions B, be rolled, but it has the low dissolving N content of 67ppm.Therefore, it is poor that the comparative sample 35 that obtains has in the low compression verification resistance to deformation between 200 ℃ and 25 ℃, is 108, less than lower limit, thereby has poor about 38.7 finished surface roughness Ra, shows the machinability more inferior than invention sample.

As shown in table 8, in the molten steel before casting, have than little low free oxygen content of lower limit 30ppm and the low Of/S ratio littler than lower limit 0.005 as the steel 31 of the material of comparative sample 39.Therefore, although the steel wire rod that obtains is rolled under the preferred rolling and cooling conditions B shown in the table 9, the width average of the sulfide inclusion thing of the comparative sample 39 that obtains (μ m) is less than 2.8 * (log d).Thereby comparative sample 39 has poor about 39.2 finished surface roughness Ra, shows the machinability more inferior than invention sample.

As shown in table 8, have the low Mn/S ratio littler as the steel 32 of the material of comparative sample 40 than lower limit 3.0.Although be rolled under the preferred rolling and cooling conditions B shown in the table 9, this causes when rolling and breaks, and can not estimate finished surface roughness Ra and other performance.

As shown in table 8, have the low Mn/S ratio littler as the steel 33 of the material of comparative sample 41 than lower limit 3.0.Although be rolled under the preferred rolling and cooling conditions B shown in the table 9, this causes when rolling and breaks, and can not estimate finished surface roughness Ra and other performance.

As shown in table 7, have the low Mn content littler as the steel 34 of the material of comparative sample 42 than lower limit 1.0%.Although be rolled under the preferred rolling and cooling conditions B shown in the table 9, this causes when rolling and breaks, and can not estimate finished surface roughness Ra and other performance.

As shown in table 7, the steel 35 that is used as the material of comparative sample 43 has the high Mn content that surpasses 2.0% upper limit.In addition, in the molten steel before casting, free oxygen content is less than the lower limit of 30ppm, and Of/S compares the lower limit less than 0.005.Therefore, although under the preferred rolling and cooling conditions B shown in the table 9, be rolled, it is poor that but comparative sample 43 has in width average, dissolved N content and the compression verification of the low sulfide inclusion thing in steel wire rod the resistance to deformation between 200 ℃ and 25 ℃, have the finished surface roughness Ra of about 47.0 difference, show the machinability more inferior than invention sample.

As shown in table 7, the steel 36 that is used as the material of comparative sample 44 has 0.28% low S content, the lower limit less than 0.3%.Although therefore under the preferred rolling and cooling conditions B shown in the table 9, be rolled, but the comparative sample that obtains 44 has low Mn * S as shown in table 8, it is less than 0.40 lower limit, therefore it is poor to have in low dissolving N content and the compression verification resistance to deformation between 200 ℃ and 25 ℃, have the finished surface roughness Ra of about 46.3 difference, show the machinability more inferior than invention sample.

As shown in table 7, the steel 37 that is used as the material of comparative sample 45 has low N content, the lower limit less than 0.007%.Therefore, although the steel wire rod that obtains is rolled under the preferred rolling and cooling conditions B shown in the table 9, it is poor that but the comparative sample that obtains 45 has in low dissolving N content and the compression verification resistance to deformation between 200 ℃ and 25 ℃, have the finished surface roughness Ra of about 48.2 difference, show the machinability more inferior than invention sample.

As shown in table 8, as having free oxygen and the Of/S that surpasses the upper limit respectively in the molten steel before the casting of the steel 38,39 of comparative sample 46,47 and 48 material and 40.Therefore, although under the preferred rolling and cooling conditions B shown in the table 9, be rolled, it is poor that but the comparative sample that obtains 46,47 and 48 has in low dissolving N content and the compression verification resistance to deformation between 200 ℃ and 25 ℃, have the finished surface roughness Ra of about 36.8 to about 48.7 difference, show the machinability more inferior than invention sample.

All comparative sample all have 133～150 extraneous proeutectoid ferrite hardness HV of the present invention's regulation, yet the invention sample has the proeutectoid ferrite hardness in the specialized range.Therefore, the regulation of proeutectoid ferrite hardness (requirement) meet or satisfactorily with 200 ℃ and 25 ℃ between the regulation of resistance to deformation difference consistent.These results have shown the crucial meaning of requirement of the present invention.

Table 7

Sequence number	The chemical constitution of steel (mass percent, surplus are Fe and impurity)
																C	Si	Mn	P	S	N	Cr	Cu	Ni	Ti	Al	V	Nb	Zr	Ti, Al, V, the total content of Nb and Zr
	27 28 29 30 31 32 33 34 35 36 37 38 39 40 41	0.05 0.05 0.07 0.05 0.04 0.06 0.08 0.07 0.08 0.08 0.07 0.05 0.05 0.04 0.07	0.005 0.005 0.005 0.006 0.005 0.005 0.005 0.005 0.007 0.005 0.007 0.005 0.006 0.005 0.005	1.2 1.8 1.9 1.5 1.8 1.1 1.5 0.8 2.2 1.1 1.3 1.2 1.2 1.2 1.8	0.08 0.08 0.08 0.07 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.07 0.08 0.08	0.33 0.5 0.55 0.4 0.55 0.38 0.52 0.35 0.56 0.28 0.38 0.35 0.36 0.35 0.49	0.008 0.011 0.008 0.007 0.015 0.014 0.009 0.011 0.008 0.007 0.004 0.012 0.010 0.012 0.012	0.03 0.03 0.03 0.03 0.02 0.01 0.02 0.03 0.02 0.03 0.03 0.03 0.03 0.05 0.02	0.05 0.02 0.03 0.03 0.02 0.03 0.03 0.02 0.02 0.02 0.03 0.03 0.02 0.03 0.02	0.02 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.02 0.01 0.01 0.02 0.02 0.01 0.01	0.001 0.002 0.001 0.003 0.002 0.002 0.001 0.002 0.001 0.002 0.002 0.001 0.001 0.002 0.002	0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001	0.006 0.003 0.003 0.003 0.003 0.004 0.003 0.003 0.003 0.003 0.003 0.007 0.005 0.011 0.003	0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001	0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001																0.010 0.008 0.007 0.008 0.008 0.009 0.007 0.008 0.007 0.008 0.008 0.011 0.009 0.016 0.008

Table 8 (continuous table 7)

Sequence number	The chemical constitution of steel (mass percent)
					Of	Of/S	Mn/S	Mn*S
	27 28 29 30 31 32 33 34 35 36 37 38 39 40 41	0.0053 0.0042 0.0026 0.0063 0.0028 0.0065 0.0039 0.0105 0.0019 0.007 0.0063 0.016 0.0158 0.0186 0.0036	0.01606 0.00840 0.00473 0.01575 0.00509 0.01711 0.00750 0.03000 0.00339 0.02500 0.01658 0.04571 0.04398 0.05306 0.00735	3.636 3.600 3.455 3.750 3.273 2.895 2.885 2.286 3.929 3.929 3.421 3.429 3.194 3.429 3.673					0.396 0.900 1.045 0.600 0.990 0.418 0.780 0.280 1.232 0.308 0.494 0.420 0.414 0.420 0.882

Table 9

Rolling pattern	Hot-rolled condition			Catalogue
					Heating temperature (℃)	Final rolling temperature (℃)	Rate of cooling (℃/minute)
	A	1010	850					0.8	Comparative sample
B				1010	855	1.8	The invention sample
	C	1005	860					Be cooled to 600 ℃ with 0.8 ℃/second, after this quicken cooling with 2.5 ℃/second	The invention sample
E				1150	855	1.8	The invention sample

Table 10

Sequence number	Steel preface in the table 7 and 8	System in the hot-rolled condition table 9	Steel wire					Machinability finished surface roughness Ra (μ m)	Catalogue
										Silk footpath (mm)	The MnS width average		Resistance to deformation poor (MPa) in the compression verification	Dissolved nitrogen (ppm)
			2.8*(log d)	(μm)
					35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52	27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 41 41 41	B B B B B B B B B B B B B B 8 A C E				8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0	2.53 2.53 2.53 2.53 2.53 2.53 2.53 2.53 2.53 2.53 2.53 2.53 2.53 2.53 2.53 2.53 2.53 2.53			2.53 2.59 2.03 2.91 2.22 2.83 2.29 2.84 1.85 2.85 2.89 2.78 2.77 2.85 2.86 2.92 3.01 3.09	95 125 102 93 113 125 111 124 99 87 65 78 72 59 115 103 116 133	52 87 60 53 103 115 70 88 60 48 18 49 34 8 72 63 76 78	38.9 33.6 42.6 38.7 39.2 - - - 47.0 46.3 48.2 38.2 36.8 48.7 31.5 36.8 29.2 27.6	Contrast invention contrast contrast contrast contrast contrast contrast contrast contrast contrast contrast contrast contrast invention contrast invention invention

Contrast: comparative sample: invention: invention sample

Industrial applicability

As above-mentioned, the invention provides a kind of low-carbon (LC) and vulcanize again free-cutting steel product and suitable preparation method thereof, even do not add toxicity Pb or element-specific such as Bi or Te, also have the excellent machinability take the finished surface roughness as representative. Product made from steel of the present invention needing typically to be used for excellent machinability and to cut in large quantities the findings such as screw and nipple of producing.

Claims (6)

1. the low-carbon (LC) revulcanization free-cutting steel product of a finished surface roughness excellence, based on mass percent, it comprises;

C：0.02％～0.12％；

Si:0.01% or be lower than 0.01%;

Mn：1.0％～2.0％；

P：0.05％～0.20％；

S：0.30％～0.60％；

N：0.007％～0.03％；

Equal amount is Fe and unavoidable impurities,

The content of Mn and S satisfies following condition: 0.40≤Mn*S≤1.2 and Mn/S 〉=3.0, and described product made from steel has the ferrite-pearlite structure as its structure, the width average of sulfide inclusion thing in the wherein said product made from steel (μ m) is 2.8* (logd) or greater than 2.8* (logd), d is the diameter (mm) of described product made from steel in the formula, and proeutectoid ferrite has 133～150 hardness HV in the described structure.

2. the low-carbon (LC) revulcanization free-cutting steel product of a finished surface roughness excellence, based on mass percent, it comprises:

C：0.02％～0.12％；

Si:0.01% or be lower than 0.01%;

Mn：1.0％～2.0％；

P：0.05％～0.20％；

S：0.30％～0.60％；

N：0.007％～0.03％；

Equal amount is Fe and unavoidable impurities,

The content of Mn and S satisfies following condition: 0.40≤Mn*S≤1.2 and Mn/S 〉=3.0, and described product made from steel has the ferrite-pearlite structure as its structure, the width average of sulfide inclusion thing in the wherein said product made from steel (μ m) is 2.8* (logd) or greater than 2.8* (logd), d is the diameter (mm) of described product made from steel in the formula, and the difference of the resistance to deformation when 0.3 strain between 200 ℃ and 25 ℃ is 110MPa or higher and for 200MPa or lower, described resistance to deformation is to measure under 0.3mm/ minute rate of deformation in compression testing.

3. as the low-carbon (LC) revulcanization free-cutting steel product of described finished surface roughness excellence one of in claim 1 and 2, wherein said product made from steel contains 70ppm or greater than the dissolved nitrogen of 70ppm.

4. as the low-carbon (LC) revulcanization free-cutting steel product of each described finished surface roughness excellence in the claim 1～3, wherein Cr content is not more than 0.04%, and the total content of Ti, Nb, V, Al and Zr is not more than 0.020%.

5. as the low-carbon (LC) revulcanization free-cutting steel product of each described finished surface roughness excellence in the claim 1～4, described product made from steel also comprises greater than 0.30% and is equal to or less than 1.0% Cu and greater than 0.20% and be equal to or less than among 1.0% the Ni one or both.

6. method for preparing the low-carbon (LC) revulcanization free-cutting steel product of finished product surfaceness excellence, described method comprises the steps: to cast and has the steel of the composition that each limited in the claim 1～5; And before described casting step, the content of control free oxygen Of is 30ppm or higher and less than 100ppm, and the ratio Of/S of control Of and S is in 0.005～0.030 scope, and described Of and S are included in the molten steel before the casting.

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