CN1306056C - Low-carbon free cutting steel - Google Patents

Abstract

A low-carbon free-cutting steel containing C: less than 0.05%-0.20%, Mn: 0.4-2.0%, S: 0.21-1.0%, Ti: 0.002-0.10%, and P: 0.001-0.30% in mass % , Al: 0.2% or less, O (oxygen): 0.001-0.03%, N: 0.0005-0.02%, the balance is Fe and unavoidable impurities, and the inclusions in steel satisfy the following formulas (1) and (2) , (A+B)/C≥0.8····(1); N _A ≥5····(2); In the formula, the meanings of A, B, C and N _A are as follows: Among the inclusions within 1 mm ² on the cross-section of the direction, and the equivalent circle diameter is more than 1 μm, A: the total area occupied by the substantial MnS with Ti carbide or/and Ti carbonitride inside; B: no Ti carbide inside C: the total area occupied by all inclusions; N _A : the number of substantial MnS with Ti carbides or/and Ti carbonitrides inside.

Description

Low-carbon free-cutting steel

Technical field

The invention relates to the low-carbon free-cutting steel that does not contain lead (Pb), though particularly about not containing lead, but with in the past lead and easy-cutting steel and contain simultaneously and plumbously compare with other compound free-cutting steel that improves the element of different cutting ability, when using inserted tool to cut, have good cutting ability, hot workability and cutting post-treatment surface texture is good and production cost is low low-carbon free-cutting steel.In addition, the invention still further relates to the low-carbon free-cutting steel that except above-mentioned various performances, also has good carburizing performance.

Background technology

In the past, for the soft little component of special requirement intensity not, in order to boost productivity, often used the good steel of cutting ability, promptly so-called free-cutting steel.The most familiar free-cutting steel of people has: contain a large amount of S, utilize MnS to improve the sulphur free-cutting steel of cutting ability; Add the lead and easy-cutting steel of Pb; And the compound free-cutting steel that contains S and Pb.Especially the free-cutting steel that contains Pb has the also good characteristic of machined surface roughness of the steel surface after the good and processing of the easily broken fragility of life-span of prolonging cutter, smear metal.In addition, also has the free-cutting steel that contains Te (tellurium) and Bi (bismuth) etc. in order to improve cutting ability.These steel are used for various mechanical parts such as e-machine parts such as the peripheral components of little component such as the braking component of automobile and PC and mould in large quantities.

On the other hand, because the raising of cutting machine performance can be carried out high speed cutting, follow this trend in recent years, for the raw-material steel that constitute above-mentioned parts, the cutting ability when urgent hope improves High Speed Machining.

In addition, above-mentioned parts in order to ensure its surperficial intensity, need to carry out carburizing treatment after being processed into the shape of regulation through machining sometimes.Therefore, for the employed steel of these parts,, also wish to have good carburizing performance except requiring to have the high cutting ability.

Steel for the raw material of above-mentioned parts uses require good cutting ability, and described cutting ability not only will prolong life-span of cutter, but also the character of lay special stress on smear metal fracture in small, broken bits i.e. " smear metal processing ".Property is handled in this smear metal, is to realize machining production line Institute of Automation conditio sune qua non, also be boost productivity necessary.In addition, except cutter life and the smear metal property handled, consider that also wish the in good condition of cutting steel finished surface afterwards, promptly the roughness ratio of finished surface is less from the angle of working accuracy.In above-mentioned free-cutting steel, lead and easy-cutting steel and contain Pb and other improves the compound free-cutting steel of the element of cutting ability, these performances are all fine, cutting ability the best in existing steel.

In recent years, along with people improve constantly the degree of concern of environmental problem, the free-cutting steel that does not contain Pb is developed in urgent hope.This be because, contain steel to human body and the deleterious Pb of global environment, not only in its manufacturing processed, need the exhaust equipment that is on a grand scale, and consider from point of view of environment protection, restriction uses the cry of Pb more and more higher.

In order to adapt to above-mentioned requirements, as the substitute products of lead and easy-cutting steel, people have proposed many kind technical schemes about the low-carbon (LC) sulphur free-cutting steel that do not contain Pb.But, also do not develop up to now and can satisfy all that the free-cutting steel performance requriements that contains Pb promptly helps to prolong cutter life, the free-cutting steel that property is good, machined surface roughness is less is handled in smear metal.

In the patent documentation 1 (spy opens the 2003-49240 communique) a kind of Ti of existence is disclosed or/and the sulfide-based inclusion of the carbon of Zr, the improved free-cutting steel of cutting ability.In this free-cutting steel, owing to disperse to have Ti carbon sulfide or Zr carbon sulfide with MnS, thereby be difficult to obtain the plan lubricant effect of MnS, cutter and the friction that is cut between the material raise.As a result, resistance to cutting improves, and forms built-up-edge on the blade of cutter easily.In case the formation built-up-edge, the machined surface roughness after the smart cutting increases, and the working accuracy of parts suffers damage.

In patent documentation 1, do not see Ti content and be the embodiment below 0.1%.This shows that the goal of the invention of patent documentation 1 is to generate Ti carbon sulfide by containing a large amount of Ti, has in fact put down in writing in the document, is dispersed with the sulfide-based inclusion of granulous Ti carbon with MnS in matrix.This situation can not satisfy the cutter life, smear metal of the employed steel requirement of above-mentioned parts and handle performances such as property, machined surface roughness.

Disclose a kind of free-cutting steel in the patent documentation 2 (spy opens 2003-49241), this steel contains Ti or/and Zr, (Ti+0.52Zr)/S＜2, contains the carbon sulfide of Ti or Zr as inclusion, has improved the cutter life that turning processing and drilling add man-hour.In the invention of this patent documentation 2, make to generate Ti carbon sulfide in the steel, to improve the cutter life that turning adds man-hour.Adopt this technology, can improve the life-span of cutter to a certain extent really, but because the carbon sulfide of Ti or Zr exists, be difficult to obtain the lubricant effect of MnS, cutter and the friction that is cut between the material increase.As a result, resistance to cutting raises, and forms built-up-edge on the blade of cutter easily.Once the formation built-up-edge, the machined surface roughness after the cutting increases, and causes working accuracy to worsen.

In patent documentation 2, do not see the embodiment that contains S more than 0.21%, the free-cutting steel of Ti below 0.1% that stipulates among following the present invention.Hence one can see that, and the invention of patent documentation 2 is not to improve machined surface roughness, to improve the invention of the smear metal property handled as target.That is, in the invention of patent documentation 2, in matrix, be dispersed with the carbon sulfide of Ti or Zr, thereby can not obtain desirable machined surface roughness and the smear metal property handled with MnS.

In the patent documentation 3 (special flat 2000-319753 communique), disclose and contained the low-carbon (LC) sulphur free-cutting steel that does not add Pb that S surpasses 0.4%, increases MnS content.But it is little that this steel improves the effect in inserted tool life-span.In addition, this steel does not have to improve the smear metal property handled that comes into one's own equally with cutter life, does not improve the performance of sulphur free-cutting steel in the past significantly.

Disclose a kind of invention about free-cutting steel in the patent documentation 4 (spy opens flat 09-53147 communique), this steel contains C:0.01-0.2%, Si:0.10-0.60%, Mn:0.5-1.75%, P:0.005-0.15%, S; 0.15-0.40%, O (oxygen): 0.001-0.010%, Ti:0.0005-0.020%, N:0.003-0.03% have particularly cutter life of good cutting ability for inserted tool.In this invention, described steel must contain Ti and 0.1-0.6%Si, to improve the life-span of inserted tool.In addition, this invention is not siliceous as in the present invention, contains " inside has the Ti carbide or/and the MnS of the essence of Ti carbonitride " by making in the steel, not only improves the life-span of cutter, but also improves smear metal property handled and machined surface roughness.

A kind of invention of low-carbon (LC) sulphur free-cutting steel is disclosed in the patent documentation 5 (speciallyying permit communique No. 3390988), this steel contains C:0.02-0.15%, Mn:0.3-1.8%, S:0.225-0.5%, Ti:0.1-0.6%, Zr:0.1-0.6%, and satisfy Ti+Zr:0.3-0.6%, (Ti+Zr)/and S ratio: 1.1-1.5, improved the anisotropy of mechanical property.This invention is formed by mentioned component, and Ti that the resistance to deformation when generating hot-work is high and the sulfide of Zr have improved the anisotropy of mechanical properties and the cutting ability of steel.But, owing to there is the high sulfide of these resistance to deformation, be difficult to during cutting obtain the lubricant effect that sulfide produces, thus the resistance to cutting increase, the life-span deterioration of cutter, and the roughness of finished surface also worsens.

[patent documentation 1] spy opens the 2003-49240 communique

[patent documentation 2] spy opens the 2003-49241 communique

[patent documentation 3] spy opens the 2000-319753 communique

[patent documentation 4] spy opens flat 09-53147 communique

No. 3390988 communique of [patent documentation 5] special permission

Summary of the invention

Task of the present invention is, a kind of low-carbon free-cutting steel is provided, do not contain environmentally harmful Pb, with in the past Pb free-cutting steel and contain Pb simultaneously and compare with other compound free-cutting steel of giving the element of machinability, especially in the occasion of using inserted tool to cut, demonstrate excellent cutting ability, hot workability is good, surface texture after the cutting is also good, can be with low production cost manufacturing.In addition, task of the present invention also is, the low-carbon free-cutting steel that also has good carburizing performance except above-mentioned various performances is provided.

As everyone knows, the state of inclusion such as sulfide is very big for the cutting ability influence of steel.In the steel that contains C, Ti, S, N, O observed inclusion have various, for example Ti sulfide, Ti carbon sulfide, Ti carbide, Ti carbonitride, Ti nitride, Ti oxide compound.In addition, if also contain Mn, also there is Mn sulfide in the steel by chemical formula " MnS " expression.Except these inclusiones,, so also there is the oxide compound of these elements if contain Al and Si in the steel.The form that these inclusiones exist is varied, the composition of inclusion and exist form very big for influences such as the cutting ability of steel and other mechanical propertys.

Before this, inventor wait the low-carbon (LC) that did not once just contain Pb the application of sulphur free-cutting steel patent (number of patent application: 2002-26368 number).The feature of this free-cutting steel is, C, the Mn, S, Ti, Si, P, Al, O and the N that contain specified amount, the content of Ti and S satisfy following (A) formula, and the atomic ratio of Mn and S satisfies following (B) formula, and, contain that there is Ti sulfide in inside or/and the MnS of Ti carbon sulfide.

Ti (quality %)/S (quality %)＜1 ... (A)

Mn/S≥1 ...(B)

Compare with the Pb free-cutting steel, the cutter life of this steel is far better, but also has the excellent chip property handled.But, there are some problems aspect the surface texture of this steel after cutting, that is, after carrying out the essence cutting, the problem that increases of the roughness of finished surface is obvious sometimes.

When the Ti sulfide that has essence in the matrix or/and during Ti carbon sulfide, be difficult to obtain the plan lubricant effect of MnS, thereby the resistance to cutting rising, built-up-edge formed easily at the blade place of cutter, thereby the surfaceness of cutting back steel is not good, the machined surface roughness deterioration." the Ti sulfide of essence is or/and Ti carbon sulfide " recited above, be meant that in an inclusion Ti sulfide and the shared area ratio of Ti carbon sulfide add up to the inclusion more than 50%, have illustrated several such inclusiones in Fig. 1 of back (a).

In order to address the above problem, the inventor studies, and the result has drawn new understanding described below.

When (1) in to matrix, existing the steel of " the Ti sulfide of essence is or/and Ti carbon sulfide " to cut, form built-up-edge on the blade of cutter, make the machined surface roughness deterioration.

(2) suppress the generation of " the Ti sulfide of essence is or/and Ti carbon sulfide " and make and have many MnS in the steel as much as possible, can suppress the formation of built-up-edge, obtain good machined surface roughness.

(3) still, do not contain Ti, only have the steel of MnS, the life-span deterioration of inserted tool.In order to improve the life-span of inserted tool, must to add Ti, and make in the steel and contain " inside has the Ti carbide or/and the MnS of the essence of Ti carbonitride ".

(4) " inside has the Ti carbide or/and the MnS of the essence of Ti carbonitride " life-span of having improved cutter, and do not damage the plan lubricant effect of MnS.

Based on above-mentioned cognition, the inventor has further furtherd investigate the relation of chemical ingredients and inclusion morphology, and the result has invented low-carbon free-cutting steel described below.This low-carbon free-cutting steel has and Pb free-cutting steel and the equal above cutting ability of compound free-cutting steel.In addition, the % that relates to component content below is quality %.

A kind of low-carbon (LC) sulphur easily cuts rolling stock, it is characterized in that, C:0.05% to 0.20% less than, Mn:0.4-2.0%, S:0.21-1.0%, Ti:0.002-0.10%, P:0.001-0.30%, below the AL:0.2%, O (oxygen): 0.001-0.03%, N; 0.0005-0.02%, surplus is Fe and unavoidable impurities, contained inclusion satisfies following (1) formula and (2) formula in the steel,

(A+B)/C≥0.8 ....(1)

N _A≥5 ....(2)

In the formula, A, B, C and N _AImplication as follows:

A: be parallel to 1mm on the section of rolling direction ²In, in the inclusion more than diameter of equivalent circle 1 μ m, inside has the Ti carbide or/and the shared total area of the MnS of the essence of Ti carbonitride;

B: be parallel to 1mm on the section of rolling direction ²In, in the inclusion more than diameter of equivalent circle 1 μ m, the inner shared total area of MnS that does not have the essence of Ti carbide and Ti carbonitride;

C: be parallel to 1mm on the section of rolling direction ²Interior, the shared total area of all inclusiones more than diameter of equivalent circle 1 μ m;

N _A: be parallel to 1mm on the section of rolling direction ²In the inclusion interior, that diameter of equivalent circle 1 μ m is above, inside has the Ti carbide or/and the number of the MnS of the essence of Ti carbonitride.

Above-mentioned low-carbon free-cutting steel can contain the composition more than a kind at least one group that is selected from following the 1st group to the 3rd group.

The 1st group:

Se:0.0005-0.10%, Te:0.0005-0.10%, Bi:0.01-0.3%, Sn:0.01-0.3%, Ca:0.0001-0.01%, Mg:0.0001-0.005%, B:0.0002-0.02% and rare earth element: 0.0005-0.02%.

The 2nd group:

Cu:0.01-1.0%, Ni:0.01-2.0%, Mo:0.01-0.5%, V:0.005-0.5% and Nb:0.005-0.5%.

The 3rd group:

Si:0.1-2.0% and Cr:0.03-1.0%.

Here said " inside has the Ti carbide or/and the MnS of the essence of Ti carbonitride ", as described below, be meant that in an inclusion the shared area occupation ratio of MnS is more than 50%, innerly also has the Ti carbide or/and there is the inclusion of (coexistence) in the Ti carbonitride.In addition, said the MnS of the essence of Ti carbide and Ti carbonitride " inner do not exist " is meant that in an inclusion the shared area occupation ratio of MnS is more than 50%, inside does not have Ti carbide and Ti carbonitride to have the inclusion of (coexistence).In addition, these " inside have the Ti carbide or/and the MnS of the essence of Ti carbonitride " and the MnS of the essence of Ti carbide and Ti carbonitride " inner do not exist ", its inside can exist sulfide beyond Ti carbide and the Ti carbonitride, carbon sulfide, carbide, nitride etc.

The principal character of the low-carbon free-cutting steel of the invention described above is as follows.

(1) contain C:0.05% to 0.20% less than, S:0.21%-1.0%, and the content of Ti is 0.002-0.1%.

(2) Ti combines with C, S, N and O, forms sulfide, carbon sulfide, carbide, carbonitride and oxide compound.Compare with Mn, the tendency that Ti forms sulfide is stronger, thereby forms Ti sulfide and Ti carbon sulfide easily.But, as long as cautiously consider the content balance of Mn and Ti, S and N, just can not generate many " the Ti carbon sulfide of essence is or/and the Ti sulfide ", can have many " inside have the Ti carbide or/and the MnS of the essence of Ti carbonitride " and " the inner MnS that does not have the essence of Ti carbide and Ti carbonitride ".

(3) occasion of the inclusion morphology described in (2) on adopt the chemical constitution described in above-mentioned (1) and obtain, be present in the inclusion in the matrix, softening in working angles, " MnS of essence " the performance lubricant effect accounted for a greater part of of all inclusiones, and this " MnS of essence " sulfide in addition i.e. " the Ti sulfide of essence is or/and Ti carbon sulfide " does not exist basically.At this moment, in order to obtain good machined surface roughness, the growing amount of " MnS of essence " must account for major part in the growing amount of all inclusiones.Specifically, 1mm on the sightingpiston of the section of rolling direction ²In, diameter of equivalent circle is more than 8 one-tenth of the total area that the total area of 1 μ m above " MnS of essence " must account for all above inclusiones of diameter of equivalent circle 1 μ m.Have only in this case, could suppress to cause that owing to the existence of " the Ti sulfide of essence is or/and Ti carbon sulfide " the blade place of cutter generates built-up-edge, obtains good machined surface roughness.

" MnS of essence " recited above, be that the shared area occupation ratio of MnS is a inclusion more than 50% in an inclusion, it is made of " inside has the Ti carbide or/and the MnS of the essence of Ti carbonitride " and " the inner MnS that does not have the essence of Ti carbide and Ti carbonitride ".

Shown in top (1) formula, " more than accounting for 8 one-tenth " be " A+B " in (1) formula.The definition of A and B is to be parallel to 1mm on the section of rolling direction ²In, in the sulfide more than diameter of equivalent circle is 1 μ m, the area that " inside has the Ti carbide or/and the MnS of the essence of Ti carbonitride " is shared and " the inner MnS that does not have the essence of Ti carbide and Ti carbonitride " shared area.

In addition, " inside has the Ti carbide or/and the MnS of the essence of Ti carbonitride ", " the inner MnS that does not have the essence of Ti carbide and Ti carbonitride ", " the Ti sulfide of essence is or/and Ti carbon sulfide " and other sulfide, carbon sulfide, carbide, nitride, oxide compound, Al ₂O ₃, SiO ₂Deng total area total amount be C in (1) formula.

(4) even contain the steel of the inclusion described in above-mentioned (3); promptly; basically there is not " the Ti sulfide of essence is or/and Ti carbon sulfide "; the inclusion major part that is contained in the steel is " MnS of essence "; if but have " inside has the Ti carbide or/and the MnS of the essence of Ti carbonitride ", when the high speed range cutting that cutting temperature raises, owing to form the TiN film of hard on the tool surface; protect cutter, just can obtain good cutter life.

(5) in the steel that has " inside has the Ti carbide or/and the MnS of the essence of carbonitride ", MnS contained in the compound free-cutting steel of this " MnS of essence " and in the past JIS SUM22L-24L compares, and tiny and number increases.In this occasion, these tiny " MnS of essence " form the starting point of the stress concentration in the working angles, help crack propagation, thereby can obtain and the equal above smear metal property handled of compound free-cutting steel.

(6) steel that contains " inside has the Ti carbide or/and the MnS of the essence of Ti carbonitride ", hot workability does not have problems fully, thereby can increase effective S content that improves cutting ability, in this occasion, for adopting the continuous casting equipment manufacturing, can not produce any obstacle.In addition, just can give full play to effect, thereby manufacturing cost reduces, can be suitable for making the steel of cheapness owing to add a spot of Ti.

As mentioned above,, adjust the form of inclusion, just can obtain good cutting ability as long as limit the scope of alloying constituent.But the employed steel of trolley part except cutting ability, also wish to have good carburizing performance sometimes.Therefore, the inventor investigates the Effect on Performance of steel for Si and Cr, found that, measures by adjusting Si amount and Cr, can not damage the form of above-mentioned inclusion, thereby the cutting ability of steel can deterioration, can improve the carburizing performance.

Si and Cr solid solution improve the hardening capacity of steel in austenite, the carburized depth when increasing carburizing treatment and the hardness of cementation zone.Except Si and Cr, the element that improves hardening capacity also has Mn, Mo, P etc.But, consider from the angle of cutting ability or hot workability, with respect to the S amount, must contain the Mn of q.s, need heavy addition.In such cases, in order to improve hardening capacity, further add Mn and can cause manufacturing cost to increase.In addition, Mo also can improve the hardening capacity of steel effectively, but the price of Mo is than Si and Cr height, thereby if add the Mo of a great deal of that obtains equal effect, manufacturing cost just will increase.P also has same effect, but when adding P, the hardness of steel itself sharply increases, and causes the cutting ability deterioration.In addition, in the occasion that is not subjected to the material cost restriction, these elements also can add in the scope of not damaging cutting ability and mechanical property.But,, preferentially select Si and Cr for use as the composition that improves the carburizing performance in the occasion that not hope damages cutting ability and makes with low production cost.

Description of drawings

Fig. 1 is the synoptic diagram of the inclusion morphology of expression steel of the present invention and comparative steel.

Fig. 2 is (the A+B)/C of expression steel of the present invention and comparative steel and the figure of the relation of average machined surface roughness.

Fig. 3 is the figure of the relation of the average machined surface roughness of expression steel of the present invention and comparative steel and cutter life.

Fig. 4 is the figure that the relation of property and cutter life is handled in the smear metal of expression steel of the present invention and comparative steel.

Embodiment

1. about " inside has the Ti carbide or/and the MnS of the essence of Ti carbonitride "

Ti combines with S, C, N, O, forms by chemical formula TiS and Ti ₄C ₂S ₂The Ti sulfide and the Ti carbon sulfide of expression, Ti such as the Ti carbide of being represented by chemical formula TiC and Ti (CN), TiN, TiO, Ti carbonitride, Ti nitride, Ti oxide compound are inclusion.In addition, sometimes, the Ti solid solution is in MnS, so that (Mn, Ti) S and form exist, but the Ti of this solid solution in MnS is trace extremely, thereby this sulfide comes down to MnS.

When also having, not solid solution of Ti in MnS, in an inclusion with the MnS existence that is separated significantly.This Ti is with TiC or/and Ti (C, N) form exists, and promptly to exist with the visibly different form of the composition of MnS, the form of its existence is diversified, for example be present in a sulfide around near, perhaps exist or the like with the form that is enclosed among the MnS.

Fig. 1 is the synoptic diagram of the inclusion that exists in containing the free-cutting steel of Ti, wherein, (a) is the free-cutting steel of comparative example, (b) is free-cutting steel of the present invention.In the steel shown in Fig. 1 (a), the sulfide and the carbon sulfide that have the Ti of many Individual existences, perhaps in an inclusion be more than 50%, can be regarded as the sulfide of Ti and the inclusion of carbon sulfide, promptly foregoing " the Ti sulfide of essence is or/and Ti carbon sulfide " in fact with the shared area occupation ratio of the sulfide of the occasion Ti of MnS coexistence and carbon sulfide.On the other hand, in the steel of the present invention shown in Fig. 1 (b), there are many Ti carbide or/and the Ti carbonitride is taken in peripheral part or the inner inclusion that exists of MnS, i.e. " inside has the Ti carbide or/and the MnS of the essence of Ti carbonitride " and " the inner MnS that does not have the essence of Ti carbide and Ti carbonitride ".

Steel shown in above-mentioned Fig. 1 (b), the occasion that Ti sulfide and Ti carbon sulfide or Ti carbide and Ti carbonitride, Ti nitride, Ti oxide compound and other inclusion and MnS obviously are separated and exist, the shared area occupation ratio of MnS is 50% above person, be considered to come down to a MnS, i.e. " MnS of essence ".Otherwise, in an inclusion, these Ti are that shared area occupation ratios such as inclusion or the oxide compound that is made of other composition, nitride, carbide are the inclusion more than 50%, not " MnS of essence ", in fact be considered to a Ti and be inclusion and the oxide compound that constitutes by other composition, nitride, carbide etc.

In above-mentioned MnS, particularly with the Ti carbide or/and Ti carbonitride and MnS obviously exist separately and the shared area occupation ratio of MnS is that inclusion more than 50% is defined as " inside has the Ti carbide or/and the MnS of the essence of Ti carbonitride ".On the other hand, so-called " the inner MnS that does not have the essence of Ti carbide and Ti carbonitride ", be meant that above-mentioned Ti except Ti carbide and Ti carbonitride is that inclusion such as inclusion or the oxide compound that is made of other composition, nitride, carbide and MnS are separated significantly and exist and area occupation ratio that MnS is shared is more than 50% in an inclusion, bear the MnS of the effect of MnS in fact, and not have above-mentioned Ti fully be the MnS of inclusiones such as inclusion and the oxide compound that is made of other composition, nitride, carbide.Promptly, the total of " inside has the Ti carbide or/and the MnS of the essence of Ti carbonitride " and " the inner MnS that does not have the essence of Ti carbide and Ti carbonitride ", expression is regarded as the total of the inclusion (above-mentioned " MnS of essence ") of MnS in fact, and inclusion in addition is that Ti such as Ti sulfide, Ti carbon sulfide, Ti carbide, Ti carbonitride, Ti nitride, Ti oxide compound are inclusion and the oxide compound that is made of other element, carbide and nitride etc.

Recited above in an inclusion MnS or Ti be the shared area occupation ratio of inclusion, can be by using EPMA (electron probe microanalyzer) and EDX (energy dispersion type x-ray analysis equipment) etc. that the microscopic test sheet that cuts on the pole of being used by the confession cutting test is carried out surface analysis and quantitative analysis is determined.In addition, the affirmation that also can use the same method of " inside has the Ti carbide or/and the MnS of the essence of Ti carbonitride " in the steel and the MnS of the essence of Ti carbide and Ti carbonitride " inner do not exist " and other inclusion, its total area and number also can adopt method such as image analysis to measure.At this moment, can in many visuals field, measure, make the area summation of field of view surpass 1mm ², the total area and the number with each inclusion is converted into every 1mm then ²The average total area and mean number.

2. stipulate the foundation of (A+B)/C 〉=0.8

In above-mentioned (1) formula, A is being parallel to 1mm on the section of rolling direction ²In, in the inclusion more than diameter of equivalent circle 1 μ m, " inside has the Ti carbide or/and the MnS of the essence of Ti carbonitride " shared total area, B is being parallel to 1mm on the section of rolling direction ²In the above inclusion of diameter of equivalent circle 1 μ m in, " inside does not exist ^TiThe MnS of the essence of carbide and Ti carbonitride " the shared total area.Here said " diameter of equivalent circle " is meant the area that adopts above-mentioned image analysis method to obtain an inclusion, is converted into the diameter with bowlder of the same area.Limiting " diameter of equivalent circle is more than the 1 μ m ", is not influence basically for cutting ability because of the inclusion less than 1 μ m.

Above-mentioned (1) formula represents that the total amount of A and B must be at shared more than 80% of the total area of all inclusiones more than the diameter of equivalent circle 1 μ m.As long as just can obtain good cutting ability in this scope, preferred scope is more than 90%.In addition, as mentioned above, by A and B represent beyond inclusion, be meant the nitride, carbide, oxide compound, " the Ti sulfide of essence is or/and Ti carbon sulfide " of Individual existence etc.That is, (1) formula represents that the total area of " MnS of essence " those inclusiones in addition is less than 20% of the shared total area of all inclusiones (C in (1) formula), and more preferably this total area is less than 10%.

If, in containing the steel of a large amount of S, add Ti,, thereby form Ti sulfide and Ti carbon sulfide easily because Ti is stronger than the tendency that Mn forms sulfide in order to improve cutting ability.But though (1) formula of stipulating among the present invention is to be prerequisite to add Ti, it is intended that the generation that suppresses Ti sulfide and Ti carbon sulfide.This is because Ti sulfide and Ti carbon sulfide have hindered the plan lubricant effect of MnS in working angles.In case the plan lubricant effect of MnS suffers damage, cutter and the friction that is cut between the material will raise, and the blade part formation built-up-edge at cutter causes the machined surface roughness deterioration.Therefore, must suppress the generation of Ti sulfide and Ti carbon sulfide.Promptly, such by what stipulate in (1) formula, basically " the Ti sulfide of essence is or/and Ti carbon sulfide " that does not have Individual existence in the steel, the inclusion that is contained in the steel more than 80% for " MnS of essence ", the plan lubricant effect in the time of so just can obtaining to cut.

Like this, be defined in the compositing range of steel given to this invention and satisfy the occasion of (1) formula, when essence cut, can obtain and in the past Pb free-cutting steel and the compound free-cutting steel good machined surface roughness more than on an equal basis.On the other hand, even in specified chemical composition scope of the present invention,, can not obtain good cutting ability if do not satisfy (1) formula.

3. stipulate N _A〉=5 foundation

N in above-mentioned (2) formula _A, be " to be parallel to 1mm on the section of rolling direction ²In, in the inclusion more than diameter of equivalent circle 1 μ m, inside has the Ti carbide or/and the number of the MnS of the essence of Ti carbonitride ".As mentioned above, " inside has the Ti carbide or/and the MnS of the essence of Ti carbonitride " is meant that the shared area occupation ratio of MnS is more than 50% in an inclusion.This " inside has the Ti carbide or/and the MnS of the essence of Ti carbonitride " does not damage the plan lubricant effect basically, thereby is not easy to form built-up-edge, and the machined surface roughness that is cut material can deterioration.

In addition, use inserted tool to cut the steel that contains " inside has the Ti carbide or/and the MnS of the essence of Ti carbonitride ", when examining this tool surface then, formed TiN on the surface of discovery cutter at the high-speed region that surpasses 100 meters/minute.It is believed that, in the working angles be cut the tool surface that material contacts on raise because friction causes temperature, along with temperature rises, Ti is that inclusion reacts and go bad, formation thickness is the lamellated hard TiN of a few μ m to tens μ m.The existence of TiN can be confirmed by following method, promptly, after cutting finished, the carbon element class that adopts the Ar sputter to remove tool surface was polluted (oil content etc.), with AES (Auger electron beam split) or EPMA (electron probe microanalyzer) tool surface is carried out surface analysis and point analysis.According to these analytical resultss, be the 10-80% that is cut the surface in contact of material and cutter attached to the surface-area of the TiN on the cutter, MnS that adheres to when rest part is machining and Fe or do not have the tool matrix of dirt settling.Owing to formed the TiN of this hard on the tool surface, suppressed the thermodiffusion wearing and tearing of cutter and, can obtain fabulous cutter life because the mechanical wear that the hard inclusions thing causes is compared with plumbous compound free-cutting steel with in the past sulphur free-cutting steel and sulphur.

In order to obtain above-mentioned effect, need only 1mm on the section sightingpiston of rolling direction ²In exist more than 5, preferably " inside has the Ti carbide or/and the MnS of the essence of Ti carbonitride " gets final product more than 10.

On the other hand, even in the chemical composition range of the present invention's regulation,, also can not obtain good cutting ability if do not satisfy the condition of (2) formula.

Adding Ti and forming in the steel of the inclusion morphology that satisfies (1) formula and (2) formula, MnS very slightly exists.That is, the number of MnS significantly increases.This small MnS, the focal point of stress of the smear metal that produces when becoming cutting helps the crack propagation in the smear metal, thereby has improved the smear metal property handled.

In sum, stably contain " inside has the Ti carbide or/and the MnS of the essence of Ti carbonitride ", 1mm on the sightingpiston of rolling direction section as long as make in the steel ²In 1mm on the sightingpiston of rolling direction section is arranged more than 5 ²The total total area of interior " inside has the Ti carbide or/and the MnS of the essence of Ti carbonitride " and " the inner MnS that does not have the Ti carbide and the essence of Ti carbonitride " is more than 8 one-tenth of all inclusion total areas, as described above, just can obtain and lead and easy-cutting steel and the equal above cutter life of compound free-cutting steel, machined surface roughness and the smear metal property handled.In order more stably to realize such inclusion morphology, adopt Continuous casting process to have the steel of good cutting with low production cost manufacturing, must consider the balance of Mn, Ti, S and N and content.Specifically, as long as by the following stated.

(a)Ti(％)/S(％)≤0.25

Add more Ti with respect to the S amount and promptly surpass at 0.25 o'clock than calculating Ti/S, have a lot of Ti sulfide and Ti carbon sulfide in the steel with quality %.As a result, can not satisfy (1) formula, the plan lubricant effect that is produced by MnS suffers damage.At this moment, resistance to cutting raises, and often forms built-up-edge easily at the blade part of cutter, result, surfaceness deterioration during smart cutting, working accuracy variation.

Otherwise the Ti that adds trace with respect to S amount is that the mass ratio Ti/S of the two is 0.25 when following, and there are not " the Ti sulfide of essence is or/and Ti carbon sulfide " basically individually in Ti formation Ti carbide or Ti carbonitride.

Ti carbide and Ti carbonitride are separated out with various forms, and they are present in the inside of a MnS sometimes.When using the inserted tool high speed cutting to contain the steel of " inside has the Ti carbide or/and the MnS of the essence of Ti carbonitride ", can obtain good cutter life, promptly, want to suppress the generation of " the Ti sulfide of essence is or/and the Ti carbon sulfide " of Individual existence, as long as adjust to below 0.25 Ti (%)/S (%) just passable.

(b) amount of Mn and S is with atomic ratio measuring [Mn]/[S] 〉=1

When being hot-work, causes S the element of crackle.But, as long as keep suitable composition, promptly make [Mn]/[S] 〉=1 with the ratio of atomicity (mole number) in atomic ratio, Mn will form the MnS crystallization and separate out, even Ti (%)/S (%)≤0.25, hot workability can not go wrong yet.In addition, as long as in this scope, for example under the prerequisite that adopts continuous casting to make, there is not any problem in hot workability, thereby can add more S, increase can effectively improve the MnS of cutting ability, and, even higher S content can not damage the inclusion morphology of being represented by (1) formula and (2) formula yet.

Occasion in [Mn]/[S]＜1, if the Ti that adds amount surpasses the S amount, the sulfide of a large amount of solid solutions of FeS in MnS and TiS will become main body, can not improve hot workability.In addition, even hot workability if the Ti that adds amount surpasses the S amount, also can be improved in [Mn]/[S]＜1.But in this occasion, because that Ti forms the tendency of sulfide is bigger than Mn, thereby the main sulfide that generates is not MnS, becomes main body than the Ti sulfide or the Ti carbon sulfide of MnS hard.In this case, as mentioned above, at cutter and be cut and can not obtain the plan lubricant effect that produced by soft sulfide between the material, resistance to cutting raises, the machined surface roughness deterioration during cutting.That is, with atomic ratio measuring the amount of Mn and S is defined as [Mn]/[S] 〉=1, the effect with the raising cutting ability that is produced by MnS can also obtain good high-temperature ductility energy simultaneously, thereby is desirable condition.

(c)Ti(％)/N(％)≥1.35

The principal character of free-cutting steel of the present invention is, contains " inside has the Ti carbide or/and the MnS of the essence of Ti carbonitride ".If Ti (%)/N (%)＜1.35 sometimes can not fully obtain " inside has the Ti carbide or/and the MnS of the essence of Ti carbonitride ".In this case, the Ti major part of interpolation forms the TiN crystallization in the initial stage of solidifying and separates out, thereby can not guarantee to form " inside has the Ti carbide or/and the MnS of the essence of Ti carbonitride " needed enough Ti.Therefore, Ti (%)/N (%) is being advisable more than 1.35, and in order more stably to obtain " inside has the Ti carbide or/and the MnS of the essence of Ti carbonitride ", Ti (%)/N (%) is getting final product more than 1.5.

4. the qualification foundation of chemical constitution

The following describes the foundation that limits chemical constitution in the present invention and the effect and the effect of various compositions.

C:0.05% to 0.20% less than

C is for the far-reaching important element of cutting ability.C content is 0.20% when above, and the intensity of steel improves, the cutting ability deterioration, thereby be not suitable for the purposes of special requirement cutting ability.But C content is lower than at 0.05% o'clock, and it is too soft that steel become, and produces to gnaw in working angles and cuts, and quickened the wearing and tearing of cutter, and machined surface roughness increases.Therefore, Shi Yi C content be 0.05% to 0.20% less than.In order to obtain better cutting ability, the preferable range of C content is 0.07-0.18%.

Mn：0.4-2.0％

Mn and S form sulfide-based inclusion, are the important elements that produces considerable influence for cutting ability.Its content is lower than at 0.4% o'clock, and the sulfide absolute magnitude deficiency of formation can not obtain satisfied cutting ability.In addition, Mn is the element that improves the hardening capacity of steel, thereby obtains the occasion of good carburizing performance in hope, can increase its content.But, form MnS in order to make Mn and S, in containing the steel of the present invention of a large amount of S, must contain a large amount of Mn.If add Mn in order to improve the carburizing performance, the content of Mn increases, and considers it is disadvantageous from the angle of manufacturing cost.Therefore, the upper limit with Mn content is defined as 2.0%.Surpass at 2.0% o'clock, the intensity of steel raises, and resistance to cutting increases, and the life-span of cutter reduces.In order to reduce resistance to cutting, improve cutter life, improve the smear metal property handled, improve machined surface roughness, to improve hot workability, the relation of the content of Mn and S content is important.In order to obtain these performances infalliblely, the content of Mn is 0.6-1.8% preferably.

S：0.21-1.0％

S and Mn form sulfide, are the elements that effectively improves cutting ability.The effect of the raising cutting ability that MnS produced improves along with its growing amount, thereby the selection of S content is important.Its content is lower than at 0.21% o'clock, can not obtain the sulfide-based inclusion of q.s, can't obtain satisfied cutting ability; Otherwise S content surpasses at 0.35% o'clock usually, and the hot workability deterioration has promoted the S segregation of steel ingot central part, causes crackle during forging, but as long as keep suitable composition, just its upper limit can be brought up to 1.0%.In order to utilize MnS to improve cutting ability, preferably add more S, interpolation is better more than 0.35%, preferably surpasses 0.40%.But, to add when too much, the rate of recovery worsens, and cause production cost to raise, thereby the preferred higher limit of S content is 0.70%.

Ti：0.002-0.10％

Ti and N and C form the Ti carbide or/and the Ti carbonitride is to make to have the inner requisite important element of MnS that contains these compounds in the steel.As mentioned above, if when containing " inside has the Ti carbide or/and the MnS of the essence of Ti carbonitride " in the steel, when using the inserted tool high speed cutting, the life-span of cutter increases substantially.In order there to be such MnS, its content must but, stably be scattered in the steel in order to make it more than 0.002%, does not make the machined surface roughness deterioration, obtains good cutter life, the balance between the content of necessary consideration Ti and the content of S and N.In addition, Ti content surpasses at 0.10% o'clock, owing to have " the Ti sulfide of essence is or/and Ti carbon sulfide " in the steel, machined surface roughness deterioration when making smart the cutting.Therefore, will be defined as 0.10% on the Ti content.In order more stably to obtain good machined surface roughness, Ti content is preferably below 0.08%, more preferably less than 0.03%.

On the other hand, the content of Ti is lower than at 0.002% o'clock, can not generate " inside has the Ti carbide or/and the MnS of the essence of Ti carbonitride " of improving the needed q.s of cutter life.In order to generate this MnS infalliblely, improve the inserted tool life-span, wish that the content of Ti surpasses 0.01%.

P：0.001-0.30％

P improves the hardening capacity of steel, also improves the intensity of steel simultaneously.In order to obtain this effect, its content is getting final product more than 0.001%.In addition, as long as its content is below 0.30%, cutting ability just can deterioration, can guarantee hardening capacity and intensity, but, its content surpasses at 0.30% o'clock, and the intensity of steel is too high, cutting ability deterioration not only, and promoted the segregation in the steel ingot, cause hot workability to worsen.Therefore, the content with P is defined as 0.001-0.30%.For stably keeping good cutting ability and intensity, preferred content range is 0.005-0.13%.

Al:0.2% following (also can not adding)

Al is used as strong reductor, and its content just can below 0.2%.But the oxide compound that deoxidation generates is a hard, if content surpasses 0.2%, will generate a large amount of hardening oxidation things, causes the cutting ability deterioration.Thereby, preferably below 0.1%.In addition, adding the just abundant occasion of deoxidation of C and Mn, also can not add Al, its content also can be the impurity content below 0.002%.

O (oxygen): 0.001-0.03%

The effect of oxygen in steel of the present invention do not suffer damage because of the deoxidation state, contains an amount of oxygen, and solid solution can prevent the extension owing to the rolling MnS that causes in MnS, improve the anisotropy of mechanical property.In addition, oxygen also is effective for the improvement of cutting ability, hot workability and S segregation.But its content surpasses at 0.03% o'clock, the bad damage of refractory materials when causing melting.Therefore, the scope dictates with oxygen level is 0.001-0.03%.In order suitably to obtain above-mentioned effect, preferred content range is 0.0015-0.01%.

N：0.0005-0.02％

N easily with the nitride of AL and Ti formation hard, these nitride have the effect of crystal grain thinning.But, if there are these nitride in a large number, quicken the loss of cutter easily, cause the cutting ability deterioration.In steel of the present invention, because Ti is the composition that must add, thereby the content of N is few more good more, and but, in order to obtain above-mentioned effect, regulation contains more than 0.0005%.On the other hand, when N content is too much, form thick TiN, might damage cutting ability, thereby the upper content limit of N is defined as 0.02%.In order to ensure better cutting ability, the upper limit of N content preferably 0.015%.In addition, be to utilize the existence of " inside has the Ti carbide or/and the MnS of the essence of Ti carbonitride " to improve cutting ability in the present invention, stably there is such MnS in the steel in order to make, Ti and N should satisfy Ti (%)/N (%) 〉=1.35.This is because as mentioned above, Ti (%)/N (%)＜1.35 o'clock, the Ti major part of interpolation generated TiN in the initial stage of solidifying, and can not stably obtain " inside has the Ti carbide or/and the MnS of the essence of Ti carbonitride ".

Chemical constitution that constitutes by each element of adjusting content by the above respectively and the inclusion morphology of (1) formula and (2) formula defined can obtain having good cutting ability, the low-carbon free-cutting steel of hot workability and finished surface proterties.

Low-carbon free-cutting steel of the present invention can further contain the composition more than a kind that is selected from least 1 following the 1st group to the 3rd group group.

(1) the 1st group element

The 1st group element is outside above-mentioned main component, does not damage effect of the present invention, further improves the element of the cutting ability of steel.Therefore, in order to obtain better cutting ability, can contain these elements more than a kind.

Se：0.0005-0.10％、Te：0.0005-0.10％

Se and Te, with Mn generate Mn (S, Se) and Mn (S, Te).These inclusiones and MnS are same, play the plan lubrication in working angles, are the elements that effectively improves cutting ability, can contain these elements in above-mentioned scope in order further to improve cutting ability.But their content separately is lower than at 0.0005% o'clock, the effect deficiency, on the contrary the content of Se and Te surpasses at 0.10% o'clock, and effect reaches capacity, and is not only uneconomical, and the hot workability deterioration.In order more stably to have good hot workability and cutting ability concurrently, their content separately is 0.0010-0.05% preferably.

Bi：0.01-0.3％、Sn：0.01-0.3％

Bi and Sn have the effect of the cutting ability of improving steel.It is believed that this is because they are same with Pb, forms the low melting point metal inclusion, due to the effect that performance is lubricated when cutting.In order to obtain this effect infalliblely, their content separately is being advisable more than 0.01%.But its content surpasses at 0.3% o'clock respectively, and above-mentioned effect reaches capacity, and makes the hot workability deterioration.In order more stably to have good hot workability and cutting ability concurrently, their content preferably is respectively 0.03-0.1%.

Ca：0.0001-0.01％

Ca has very high avidity for S and O (oxygen), forms sulfide and oxide compound in steel.In addition, the Ca solid solution forms (Mn, Ca) S, the but wherein Ca of solid solution is micro-, thereby can not damage the effect of MnS in MnS.In addition, the oxide compound that is formed by Ca is a low melting point oxide, is the effective interpolation element that further improves cutting ability in steel of the present invention.Want to obtain to add the effect of improving cutting ability that Ca produces, the content lower limit of Ca should be 0.0001% infalliblely.But, because the interpolation effective rate of utilization of Ca is very poor,, must add a large amount of Ca in order to improve the content of Ca, consider it is disadvantageous from the production cost angle.Therefore, the upper limit of Ca content is defined as 0.01%, preferred higher limit is 0.005%.

Mg：0.0001-0.005％

Mg also has high avidity for S and O (oxygen) in steel, form sulfide or oxide compound.Contain the effect of sulfide and the nucleus that oxide compound plays MnS of Mg, have the effect that suppresses the MnS elongation.The occasion that obtains this effect at needs also can be added.In order fully to obtain this effect, the lower limit of Mg content is preferably more than 0.0001%, and still, the formed oxide compound of Mg is a hard, if the content of Mg is too much, will cause the cutting ability deterioration.Therefore the upper content limit with Mg is defined as 0.005%.In order to have the effect that suppresses the MnS elongation and good cutting ability concurrently, preferred higher limit is 0.002%.

B：0.0002-0.02％

B combines with O (oxygen) or N and forms oxide compound or nitride, has the effect that improves cutting ability, can add as required.In order to obtain this effect, its content is getting final product more than 0.0002%.In order to obtain this effect more reliably, wish that its content is more than 0.0010%.But the content of B surpasses at 0.02% o'clock, and not only above-mentioned effect reaches capacity, and causes the hot workability deterioration.

Rare earth element: 0.0005-0.02%

Rare earth element is a group element that is classified as lanthanon.When adding rare earth element, use with these elements to the mixed rare earth alloy of main component usually etc.Ree content among the present invention shows with the total scale of element more than a kind or 2 kinds in the rare earth element.Rare earth element and oxygen form oxide compound, also combine with S to form sulfide, improve cutting ability.In order to obtain this effect infalliblely, its content is getting final product more than 0.0005%.But its content surpasses at 0.02% o'clock, and above-mentioned effect reaches capacity.In addition, the interpolation effective rate of utilization of rare earth element is low, and is therefore uneconomical if will contain a large amount of rare earth elements.

(2) the 2nd group elements

The 2nd group element all is the element with effect of the intensity that improves steel.As required, can contain wherein element more than a kind.

Cu：0.01-1.0％

Cu has the effect that improves the intensity of steel by precipitation strength.In order to obtain this effect, its content must be more than 0.01%.In order to obtain this effect more infalliblely, wish to add more than 0.1%.But its content surpasses at 1.0% o'clock, and not only the hot workability deterioration perhaps because thickization of precipitate of Cu reaches capacity above-mentioned effect, and causes cutting ability to reduce.

Ni：0.01-2.0％

Ni has the effect that improves the intensity of steel by solution strengthening.In order to obtain this effect really, its content is preferably more than 0.01%.But its content surpasses at 2.0% o'clock, the cutting ability deterioration, and the while hot workability is variation also.

Mo：0.01-0.5％

Mo is the element that can improve hardening capacity, and is but very expensive because its price is higher than Si and Cr if add the Mo of a great deal of of the equal effect of interpolation of acquisition and Si and Cr in order to improve the carburizing performance, thereby the shortcoming that has manufacturing cost to increase.But Mo also has thinning microstructure, improves the flexible effect, and the occasion that obtains these effects in hope also can be added.In order to obtain this effect really, its content is wished more than 0.01%.But surpass at 0.5% o'clock, not only above-mentioned effect reaches capacity, and the production cost of steel increases.

V：0.005-0.5％

V-arrangement becomes tiny nitride and carbonitride to separate out, and improves the intensity of steel.Its content can obtain this effect 0.005% when above, obtains the occasion of this effect more reliably in hope, preferably contains more than 0.01%.But its content surpasses at 0.5% o'clock, and not only above-mentioned effect reaches capacity, and generates too much nitride and carbide, causes cutting ability to reduce.

Nb：0.005-0.5％

Nb forms tiny nitride and carbonitride is separated out, and improves the intensity of steel.Its content can obtain this effect 0.005% when above, obtains the occasion of this effect more reliably in hope, preferably contains more than 0.01%.But its content surpasses at 0.5% o'clock, and not only above-mentioned effect reaches capacity, and generates too much nitride and carbide, causes cutting ability to reduce, nor economical.

(3) the 3rd group elements

The 3rd group element be hope improve the carburizing performance of steel occasion, can contain the element of wherein a kind of or two kinds in following ranges.

Si：0.1-2.0％

In the described free-cutting steel of the present invention of claim 1-4, do not add Si wittingly.Therefore, Si is a kind of impurity, and its content is less than 0.1%.In addition, in the described free-cutting steel of the present invention of claim 1-4, sometimes for the oxygen in the steel is existed in right amount, also add Si as deoxidant element, but also do not need residually wittingly in this case, residual Si is an impurity in the steel, and its content is less than 0.1%.

In addition, the Si solid solution improves the intensity of steel in ferrite, also have the effect of the hardening capacity that improves steel simultaneously.By improving the hardening capacity of steel, can reach purpose as the needed raising carburizing of trolley part performance.Only limit to this occasion, can contain the Si more than 0.1%.Improve the occasion of carburizing performance in hope more infalliblely, wish that its content surpasses 0.6%, but content surpasses at 2.0% o'clock, the hot workability deterioration perhaps because the solution strengthening ferritic phase improves resistance to cutting, produces adverse influence to cutting ability.In addition, even,, also can make the oxygen level in the steel reach suitable scope by suitable interpolation C, Mn and Al less than 0.1% impurity content.

Cr：0.03-1.0％

Cr improves the hardening capacity of steel, and a small amount of the interpolation just can be improved the carburizing performance.The steel that contains Cr, the carburizing performance improves, and the carburized layer hardness after the carburizing treatment is higher, can also improve Effective case depth.In order to obtain this effect, the content of Cr is getting final product more than 0.03%.In addition, in order to improve the carburizing performance more infalliblely, wish that its content is higher than 0.05%.But Cr content surpasses at 1.0% o'clock, and not only cutting ability deterioration, and manufacturing cost increases.

Contain above-mentioned Si or/and during Cr, can obtain to have the steel of good cutting ability and hot workability and good carburizing performance.

[embodiment]

1. the preparation of test portion

Use the preparation of ratio-frequency heating induction furnace to have the 150kg steel ingot (diameter: about 220mm) of the various compositions shown in table 1 and the table 2.Shown in the table 1 is steel of the present invention, and shown in the table 2 is in the past steel or comparative steel.In order stably to generate " inside has the Ti carbide or/and the MnS of the essence of Ti carbonitride ", these ingot castings are heated to 1250 ℃ high temperature, keep more than 2 hours in this temperature then, afterwards in order to simulate the described operation of rolling, make carrying out finish forge more than 1000 ℃, obtain the pole of diameter 65mm behind the air cooling.Then, should extend forged material and be heated to 950 ℃, keep air cooling after 1 hour, carry out normalizing treatment.In addition, the hot workability of the steel № 51-53 of comparative example is very poor, cracks during forging, can not make the extension forged material, thereby does not carry out following investigation.

Table 1

Steel No	Chemical constitution (quality %, surplus: Fe and impurity)											[Mn]/[S] (atomic ratio)	Ti/S (quality % ratio)	Ti/N (quality % ratio)	(A+B)/C	N A≥5
																	C	Si	Mn	P	S	Al	Ti	Cr	N	O	Other
	1	0.05	＜0.01	0.89	0.015	0.42	＜0.001	0.092	-	0.0012	0.0096																	-	1.11	0.22	17.00	0.95	○
2												0.06	＜0.01	1.13	0.019	0.41	＜0.001	0.060	-	0.0085	0.0089	-	1.61	0.15	7.02	0.96	○
	3	0.09	0.06	0.97	0.032	0.44	0.002	0.009	-	0.0028	0.0018																	-	1.28	0.02	3.21	0.99	○
4												0.10	0.04	1.49	0.029	0.55	0.001	0.061	-	0.0047	0.0033	-	1.58	0.11	12.98	0.98	○
	5	0.10	0.05	1.58	0.032	0.49	＜0.001	0.018	-	0.0047	0.0027																	-	1.88	0.04	3.83	0.97	○
6												0.10	＜0.01	0.96	0.016	0.36	＜0.001	0.087	-	0.0063	0.0046	-	1.56	0.24	13.79	0.88	○
	7	0.10	＜0.01	0.85	0.018	0.35	＜0.001	0.080	-	0.0085	0.0079																	-	1.42	0.23	9.41	0.94	○
8												0.12	0.04	1.10	0.030	0.38	＜0.001	0.026	-	0.0125	0.0018	-	1.69	0.07	2.08	0.96	○
	9	0.13	＜0.01	1.59	0.032	0.48	＜0.001	0.024	-	0.0050	0.0043																	-	1.95	0.05	4.80	0.98	○
10												0.15	0.07	1.49	0.030	0.49	0.025	0.014	-	0.0047	0.0020	-	1.77	0.03	2.98	0.99	○
	11	0.18	0.03	1.52	0.058	0.47	0.002	0.021	-	0.0130	0.0036																	-	1.89	0.04	1.62	0.96	○
12												0.09	0.02	1.17	0.022	0.40	0.005	0.053	0.20	0.0054	0.0035	-	1.71	0.13	9.81	0.95	○
	13	0.11	0.46	1.60	0.020	0.60	＜0.001	0.014	-	0.0101	0.0026																	-	1.56	0.02	1.39	0.98	○
14												0.12	1.28	1.46	0.015	0.44	0.002	0.024	-	0.0058	0.0027	-	1.93	0.05	4.14	0.98	○
	15	0.15	0.04	1.12	0.026	0.40	＜0.001	0.027	0.50	0.0125	0.0038																	-	1.63	0.07	2.16	0.96	○
16												0.18	0.85	1.36	0.040	0.39	＜0.001	0.021	-	0.0134	0.0031	-	2.04	0.05	1.57	0.97	○
	17	0.18	0.01	1.46	0.028	0.46	0.002	0.025	0.15	0.0048	0.0064																	-	1.85	0.05	5.21	0.98	○
18												0.14	0.06	1.65	0.028	0.45	0.002	0.018	-	0.0049	0.0024	Se：0.010	2.14	0.04	3.67	0.99	○
	19	0.08	0.13	1.60	0.030	0.42	＜0.001	0.014	-	0.0095	0.0028																	Te：0.015	2.22	0.03	1.47	0.98	○
20												0.12	0.03	1.50	0.031	0.45	0.002	0.022	-	0.0120	0.0032	Bi：0.05	1.95	0.05	1.83	0.98	○
	21	0.16	0.01	1.35	0.025	0.42	0.002	0.028	-	0.0059	0.0055																	Sn：0.04	1.88	0.07	4.75	0.98	○
22												0.06	0.01	0.88	0.017	0.49	0.001	0.082	-	0.0062	0.0073	Ca：0.0029	1.05	0.17	13.27	0.99	○
	23	0.10	0.05	1.02	0.020	0.46	0.004	0.025	-	0.0037	0.0026																	Mg：0.0015	1.29	0.05	6.76	0.97	○
24												0.10	0.01	1.48	0.027	0.47	0.002	0.021	-	0.0095	0.0048	B：0.0025	1.84	0.04	2.21	0.97	○
	25	0.11	0.01	1.22	0.029	0.37	0.002	0.022	-	0.0124	0.0025																	Cu：0.10	1.92	0.06	1.77	0.98	○
26												0.15	0.02	1.38	0.035	0.43	0.002	0.025	-	0.0045	0.0056	V：0.05	1.87	0.06	5.56	0.98	○
	27	0.12	＜0.01	1.55	0.030	0.46	＜0.001	0.026	-	0.0047	0.0050																	Nb：0.12	1.97	0.06	5.53	0.98	○
28												0.16	0.07	1.00	0.025	0.25	0.002	0.060	-	0.0101	0.0092	Ni：0.10	2.33	0.24	5.93	0.89	○
	29	0.10	0.03	1.40	0.038	0.41	0.002	0.029	-	0.0052	0.0064																	Mo：0.10	1.99	0.07	5.58	0.97	○
30												0.11	0.01	1.25	0.035	0.46	＜0.001	0.025	-	0.0130	0.0048	Ca：0.0015，Mg：0.0018	1.59	0.05	1.92	0.97	○
	31	0.09	0.05	1.44	0.027	0.43	0.003	0.022	-	0.0112	0.0030																	Bi：0.07，Nb：0.10	1.95	0.05	1.96	0.98	○
32												0.10	0.02	1.46	0.026	0.42	0.003	0.022	-	0.0132	0.0024	Sn：0.07，V：0.10	2.03	0.05	1.67	0.98	○
	33	0.12	0.05	0.99	0.031	0.40	0.001	0.029	-	0.0085	0.0030																	Ca：0.001，Mo：0.12	1.44	0.07	3.41	0.97	○
34												0.10	0.03	1.52	0.032	0.45	＜0.001	0.019	-	0.0090	0.0035	Te：0.012，Ni：0.11	1.97	0.04	2.11	0.98	○

Table 2

Steel No	Chemical constitution (quality %, surplus: Fe and impurity)											[Mn]/[S] (atomic ratio)	Ti/S (quality % ratio)	Ti/N (quality % ratio)	(A+B)/C	N A≥5
																	C	Si	Mn	P	S	Al	Ti	Cr	N	O	Other
	35	0.07	＜0.01	1.02	0.070	0.32	0.002	-	-	0.0052	0.0160																	*Pb：0.31	1.86	-	-	-	-
36												0.08	0.01	1.12	0.060	0.31	0.002	-	-	0.0084	0.0145	*Pb：0.18	2.11	-	-	-	-
	37	0.08	0.01	1.02	0.067	0.33	0.002	-	-	0.0066	0.0150																	-	1.80	-	-	-	-
38												0.06	0.06	0.86	0.020	0.42	0.001	*0.280	-	0.0050	0.0068	-	1.20	*0.67	56.0	*0.75	○
	39	0.08	0.02	1.15	0.033	0.35	0.001	*0.250	-	0.0079	0.0052																	-	1.92	*0.71	31.6	*0.70	○
40												0.10	0.01	1.12	0.028	0.36	＜0.001	*0.330	-	0.0085	0.0053	-	1.82	*0.92	38.8	*0.65	○
	41	0.18	0.03	0.47	0.016	0.29	0.003	*0.420	-	0.0059	0.0019																	-	*0.95	*1.45	71.2	*0.20	○
42												0.10	0.01	1.05	0.013	0.39	0.001	0.006	-	0.0099	0.0035	-	1.57	0.02	*0.6	0.99	×*
	43	0.09	0.35	1.21	0.025	0.30	0.001	0.009	-	0.0138	0.0084																	-	2.35	0.03	*0.7	0.98	×*
44												*0.52	0.17	0.52	0.016	0.21	＜0.001	0.050	-	0.0079	0.0180	-	1.45	0.24	6.4	0.96	○
	45	*0.45	＜0.01	0.85	0.019	0.32	0.002	0.078	-	0.0046	0.0058																	-	1.55	0.24	17.0	0.95	○
46												*0.01	＜0.01	0.98	0.016	0.33	0.002	0.067	-	0.0048	0.0048	-	1.73	0.20	14.0	0.96	○
	47	0.10	0.02	1.45	0.016	0.49	*0.35	0.028	-	0.0057	0.0017																	-	1.73	0.06	4.9	0.97	○
48												0.07	0.01	0.92	0.015	0.47	＜0.001	0.056	*2.50	0.0075	0.0036	-	1.14	0.12	7.5	0.99	○
	49	0.09	*2.55	1.56	0.020	0.45	0.001	0.065	-	0.0058	0.0024																	-	2.02	0.14	11.2	0.98	○
50												0.05	＜0.01	0.48	0.015	*0.15	＜0.001	0.022	-	0.0095	0.0193	-	1.87	0.15	2.3	0.92	○
	51	0.09	0.01	1.85	0.018	*1.14	0.001	0.102	-	0.0078	0.0128																	-	*0.95	0.09	13.1	-	-
52												0.06	＜0.01	*0.21	0.016	0.33	0.001	0.074	-	0.0073	0.0078	-	*0.37	0.22	10.1	-	-
	53	0.08	＜0.01	1.25	0.025	0.44	0.002	0.072	-	0.0050	0.0049																	*Te：0.12	1.66	0.16	14.4	-	-
54												0.06	＜0.01	1.03	0.016	0.46	＜0.001	0.045	-	0.0059	0.0085	*V：2.0	1.31	0.10	7.6	0.99	○
	55	0.15	＜0.01	1.11	0.015	0.46	0.002	0.080	-	0.0078	0.0063																	*Mo：1.50	1.41	0.17	10.3	0.98	○

* unabridged version invention defined terms with thumb down

2. the investigation of inclusion morphology

Be parallel to observed inclusion on the section of rolling direction, along in the majority with uncertain shape of machine direction elongation.When the number of investigating inclusion and area, (Df: the diameter that extends forged material) the longitudinal section direction at position cuts the test film that is used to observe microstructure from the Df/4 that extends forged material, it is imbedded in the resin, carry out mirror polish then, with 400 times observation by light microscope and take a picture, adopt method such as image analysis to obtain the number and the area of inclusion.At this moment, to as if the diameter (diameter of equivalent circle) that is converted into bowlder with same area be the above inclusion of 1 μ m.As mentioned above, diameter of equivalent circle being defined as more than the 1 μ m is because almost do not have what influence less than the inclusion of 1 μ m for cutting ability.

In addition, press the composition of following described these inclusiones of affirmation.Promptly, as mentioned above, (Df: the diameter that extends forged material) the longitudinal section direction at position cuts the microscopic test sheet from the Df/4 that extends forged material, this test film is imbedded in the resin, carry out mirror polish then, carry out surface analysis and timing analysis with EPMA (electron probe microanalyzer) and EDX (energy dispersion type x-ray analysis equipment) etc.The observation magnification of this moment can be selected in being no more than 10000 times scope, observing MnS and Ti carbide in an inclusion under this observation magnification or/and the Ti carbonitride obviously is separated and the area occupation ratio of MnS is a inclusion more than 50%, " inside has the Ti carbide or/and the MnS of the essence of Ti carbonitride " exactly.Result according to such observation, obtain the area of diameter of equivalent circle 1 μ m above each " inside has the Ti carbide or/and the MnS of the essence of Ti carbonitride " and " the inner MnS that does not have the essence of Ti carbide and Ti carbonitride ", calculate 1mm on the rolling direction section ²The total area of these interior inclusiones, and then calculate 1mm on the rolling direction section ²The total amount of the area that all interior inclusiones are shared is obtained (A+B)/C.

According to the number of The above results mensuration " inside has the Ti carbide or/and the MnS of the essence of Ti carbonitride ", for every 1mm on the rolling direction section ²Interior mean number is the steel more than 5, is labeled as " zero ".Otherwise, less than 5 steel, be labeled as " * " for " inside has the Ti carbide or/and the MnS of the essence of Ti carbonitride ".In addition, the steel № 35-37 of the comparative example shown in the table 2 is Pb free-cutting steel and the S free-cutting steel that does not contain Ti, does not contain " inside has the Ti carbide or/and the MnS of the essence of Ti carbonitride " in fact, thereby does not carry out these calculating.

3. cutting ability test

In the cutting ability test, use the pole that above-mentioned extension forged material outside surface is cut to diameter 60mm, investigate the test of cutter life and machined surface roughness.Cutter life test is used and is not carried out P20 inserted tool coating processing, that stipulate among the JIS, in cutting speed: 150 meters/minute, the amount of feed: 0.10mm/ change, carry out turning under the condition of depth of cut: 2.0mm and DRY CUTTING, and average flank abrasion loss is measured in the cutting beginning after 30 minutes.In addition, reach test materials more than the 200 μ m, measure its time of arrival and average flank abrasion loss (VB) at that time for average flank abrasion loss in 30 minutes.

The time that reaches 100 μ m with average thick crack surface wear amount (VB) is estimated as the index of cutter life.For good owing to wear resistance in process of the test, the minimum not enough situation of test materials that causes of the rate of wear, adopt regression analysis to calculate the time that average thick crack surface wear amount reaches 100 μ m according to turning time one tool wear curve.In addition, in the smear metal of discharging, choose 200 representational smear metals, measure its weight, calculate the number of per unit weight, estimate the smear metal property handled.

Surfaceness after machined surface roughness cuts by essence is estimated, and uses the surface that is cut material after the evaluation of contact pin type roughness tester is cut by following condition.Described machining condition is to use the JISK inserted tool through the TiALN multiple coating, cutting speed: 100 meters/minute, the amount of feed: 0.05mm/ changes, depth of cut: 0.5mm, turning under the wet condition that uses water-soluble emulsion type lubricating oil.For the test film of steel after 1 minute that be for experiment of cutting under these conditions, use the contact pin type roughness tester, make contact pilotage along the moving axially of test film, measure average machined surface roughness (Ra), estimate machined surface roughness.

4. hot workability test

Use creating conditions of continuous casting equipment in order to simulate, so that (Di: the position diameter of steel ingot) is the center according to the Di/8 with the close surface element of above-mentioned 150kg steel ingot with quadrat method preparation, cut the high temperature tension test sheet of diameter 10mm, length 130mm from the short transverse of steel ingot, fixed intervals are 110mm, directly energising is heated to 1250 ℃, keep being cooled to 1100 ℃ with 10 ℃/second speed of cooling after 5 minutes, keep after 10 seconds with 10 ^-3The rate of straining of/second carries out tension test.At this moment, measure the relative reduction in area of fracture location, estimate hot workability.

5. cementation test

Carry out cementation test by the following stated.That is, use the columniform steel of diameter 24mm, length 50mm as test film.This test film is to cut on the position by the R/2 of the normalizing material of above-mentioned diameter 65mm.This test film is heated to 900 ℃ carries out carburizing treatment, then 850 ℃ of DIFFUSION TREATMENT.At this moment, the carbon potential during carburizing (C.P.) value is 0.8%, and the treatment time is 75 minutes, and the C.P. value during diffusion is 0.7%, and the treatment time is 20 minutes.The oil that test film after the carburizing treatment end is put into 80 ℃ cools off, and carries out quench treatment.At last, test film is heated to 190 ℃, under this temperature, keeps carrying out in 60 minutes temper.The evaluation method of carburizing performance is as described below.

On transverse section through the position apart from end 25mm (being the central authorities of length direction) of the test film of carburizing and quenching, temper, the Vickers' hardness of mensuration from the surface to inside distributes, obtain the Effective case depth of Hv400, judge that this value ratio plumbous compound free-cutting steel in the past is greatly still little.Plumbous compound free-cutting steel in the past is the steel № 35 in the table 2, and its Effective case depth is 0.25mm.As the evaluation of carburizing performance, Effective case depth with respect to steel № 35 ± 0.05mm is that Effective case depth is the occasion of 0.20-0.30mm, is evaluated as equal; In occasion, be evaluated as and be not so good as less than 0.20mm; When surpassing 0.30mm, be evaluated as and be better than.In table 3 and table 4 with zero, * and ◎ represent The above results.In equal occasion is " zero ", and the occasion that is not so good as is " * ", and the occasion that is better than is " ◎ ".

Above test-results gathers and is shown in table 3 and the table 4.In addition, (the A+B)/C of (1) shown in Fig. 2 formula and the relation of machined surface roughness, the relation of machined surface roughness shown in Fig. 3 and cutter life, the relation of property handled of smear metal shown in Fig. 4 and cutter life.

Table 3

Steel No	Relative reduction in area (%)	Tool abrasion after 30 minutes (μ m)	The time (branch) of VB=100 μ m	The smear metal property handled (number/g)	Finished surface mean roughness (μ m)	Carburizing performance evaluation
							1	61.8	45	90	18	0.7	○
2	64.3	39	98	14	0.6	○
							3	64.5	48	85	12	0.2	○
4	80.2	32	125	18	0.5	○
							5	76.8	43	96	21	0.3	○
6	63.6	44	91	14	0.7	○
							7	67.2	38	96	16	0.7	○
8	65.2	42	98	17	0.3	○
							9	81.4	36	121	20	0.4	○
10	76.8	45	93	19	0.4	○
							11	73.4	40	103	16	0.4	○
12	71.8	42	100	14	0.5	◎
							13	64.7	45	93	18	0.2	◎
14	82.9	46	90	18	0.2	◎
							15	65.2	42	98	17	0.4	◎
16	72.8	44	95	18	0.4	◎
							17	80.1	42	95	12	0.4	◎
18	71.0	38	110	18	0.3	○
							19	69.5	40	98	19	0.3	○
20	62.9	45	92	24	0.3	○
							21	64.8	46	93	13	0.2	○
22	60.8	29	120	19	0.5	○
							23	62.8	43	95	22	0.6	○
24	81.2	38	105	16	0.5	○
							25	75.9	41	101	20	0.4	○
26	78.7	43	97	16	0.3	○
							27	75.4	40	105	22	0.3	○
28	80.5	42	93	14	0.8	○
							29	78.8	36	115	18	0.3	○
30	64.2	37	105	24	0.4	○
							31	60.9	46	91	20	0.3	○
32	61.3	44	90	18	0.3	○
							33	64.3	24	126	19	0.5	○
34	67.2	41	99	17	0.3	○

Table 4

Steel No	Relative reduction in area (%)	Tool abrasion after 30 minutes (μ m)	The time (branch) of VB=100 μ m	The smear metal property handled (number/g)	Finished surface mean roughness (μ m)	Carburizing performance evaluation
							35	49.8	98	36	9	0.4	○
36	49.6	99	30	8	0.7	○
							37	55.4	165	17	6	0.7	○
38	74.3	35	113	15	1.3	×
							39	73.0	40	98	17	1.4	×
40	68.2	45	92	18	1.5	×
							41	64.2	71	68	16	1.7	×
42	67.5	89	38	15	0.5	○
							43	79.0	85	42	13	0.6	◎
44	52.8	93	36	12	1.3	◎
							45	66.9	88	39	15	1.0	◎
46	57.5	103	29	8	1.6	×
							47	78.7	101	30	16	0.3	○
48	56.0	232	12	12	1.8	◎
							49	59.8	206	16	14	1.9	◎
50	65.3	129	20	9	1.3	○
							51	6.4	-	-	-	-	-
52	4.3	-	-	-	-	-
							53	3.4	-	-	-	-	-
54	59.5	198	17	15	1.8	○
							55	58.5	264	10	10	1.9	◎

Steel № 35 and 36 among Fig. 2 is compound free-cutting steels, and steel № 37 is sulphur free-cutting steels, and they are best steel of cutting ability up to now.By table 3, table 4, Fig. 2 and Fig. 3 as can be seen, the cutter life of steel of the present invention and machined surface roughness are all good.In addition, steel № 1-34 of the present invention has good hot workability, the relative reduction in area of the high temperature tension test that the practicality of simulation use continuous casting equipment is created conditions, as shown in table 3 and compound free-cutting steel and sulphur free-cutting steel are equal or higher, without any problem.

Steel № 12-17 in the table 1 contains at least a kind steel among Si and the Cr for improving the carburizing performance in specialized range.In steel of the present invention, these steel demonstrate excellent especially carburizing performance.On the other hand, steel № 35-55 has a side to drop on steel outside the scope of the present invention's regulation in the form of inclusion and the chemical constitution, its cutter life, machined surface roughness, smear metal are handled in property and the hot workability has at least a kind of performance poorer than steel of the present invention.

Free-cutting steel of the present invention does not contain Pb, have and Pb free-cutting steel in the past and the equal above cutting ability of compound free-cutting steel, and the finished surface proterties after the cutting is also good.In addition, contain Si or/and the steel of Cr also has good carburizing performance.The hot workability of this steel is also good, can adopt Continuous casting process with low production cost manufacturing.Owing to do not contain Pb, do not worry environmental pollution.Therefore, free-cutting steel of the present invention is the raw-material steel that are fit to very much be used as various mechanical parts.

Although more than described typical specific embodiments more of the present invention in detail, it is apparent to those skilled in the art that do not break away from aim of the present invention in itself scope in can carry out some modifications to specific embodiment.Therefore, all such modifications all should comprise within the scope of the present invention.

Claims (5)

1. a low-carbon (LC) easily cuts rolling stock, it is characterized in that, in quality % contain C:0.05% to 0.20% less than, Mn:0.4-2.0%, S:0.21-1.0%, Ti:0.002-0.10%, P:0.001-0.30%, below the Al:0.2%, O (oxygen): 0.001-0.03% and N:0.0005-0.02%, surplus is Fe and unavoidable impurities, contained inclusion satisfies following (1) formula and (2) formula in the steel

(A+B)/C≥0.8····(1)

N _A≥5····(2)

In the formula, A, B, C and N _AImplication as follows:

A: be parallel to 1mm on the section of rolling direction ²In, in the inclusion more than diameter of equivalent circle 1 μ m, inside has the Ti carbide or/and the shared total area of the MnS of the essence of Ti carbonitride;

B: be parallel to 1mm on the section of rolling direction ²In, in the inclusion more than diameter of equivalent circle 1 μ m, the inner shared total area of MnS that does not have the essence of Ti carbide and Ti carbonitride;

C: be parallel to 1mm on the section of rolling direction ²The shared total area of all inclusiones interior, that diameter of equivalent circle 1 μ m is above;

N _A: be parallel to 1mm on the section of rolling direction ²In, in the inclusion more than diameter of equivalent circle 1 μ m, inside has the Ti carbide or/and the number of the MnS of the essence of Ti carbonitride.

2. low-carbon (LC) according to claim 1 easily cuts rolling stock, it is characterized in that, the element more than a kind that a part of Fe is selected from down in the group replaces

Se:0.0005-0.10%, Te:0.0005-0.10%, Sn:0.01-0.3%, Ca:0.0001-0.01%, B:0.0002-0.02% and rare earth element: 0.0005-0.02%.

3. low-carbon (LC) according to claim 1 easily cuts rolling stock, it is characterized in that, the element more than a kind that a part of Fe is selected from down in the group replaces

Cu:0.01-1.0%, Ni:0.01-2.0%, Mo:0.01-0.5%, V:0.005-0.5% and Nb:0.005-0.5%.

4. low-carbon (LC) according to claim 1 easily cuts rolling stock, it is characterized in that, the element more than a kind that a part of Fe is selected from down in the group replaces

Se:0.0005-0.10%, Te:0.0005-0.10%, Sn:0.01-0.3%, Ca:0.0001-0.01%, B:0.0002-0.02% and rare earth element: 0.0005-0.02%, and, the element more than a kind that is selected from down in the group contained

Cu:0.01-1.0%, Ni:0.01-2.0%, Mo:0.01-0.5%, V:0.005-0.5% and Nb:0.005-0.5%.

5. easily cut rolling stock according to each described low-carbon (LC) among the claim 1-4, it is characterized in that, a part of Fe by a kind among Si:0.1-2.0 quality % and the Cr:0.03-1.0 quality % or 2 kinds replace.

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