JP4906245B2 - Machine structural steel with excellent machinability - Google Patents

Description

  For example, steel used for driving parts such as gears and shafts that are parts for automobiles, etc., after hot-rolled or hot-forged material is subjected to various cutting processes and processed into a predetermined shape, then Further, the present invention relates to a free-cutting steel for machine structure having excellent machinability, which is used after being subjected to surface hardening treatment such as carburizing or quenching and tempering.

  Pb free-cutting steel is used in many respects from the viewpoint of machinability, mainly for use in drive system parts such as gears and shafts of automobiles in which mainly case hardening steel is used. However, due to the recent trend of reducing environmentally hazardous substances, reduction of the amount of Pb free-cutting steel is required. Therefore, as non-Pb free-cutting steel that replaces Pb free-cutting steel, sulfur (S) is added to the steel to produce manganese sulfide (MnS) in the steel, which acts as a stress concentration source during cutting. Therefore, S-based free-cutting steel that improves machinability is promising. And invention which adds Ca in order to improve the carbide tool workability of S system free-cutting steel is indicated (for example, refer to patent documents 1). On the other hand, the inventors of the present invention, in the S-based free-cutting steel to which Ca is added, have varying tool life and stable cutting workability despite the Ca content and S content in the steel being substantially equal. I find that I can not get it.

JP-A-57-140853

  The problem to be solved by the present invention is an S-based free-cutting steel containing Ca without containing Pb, which is an environmentally hazardous substance, especially for a mechanical structure containing chemical components and inclusion compositions excellent in machinability. Is to provide steel.

As a result of intensive studies by the inventors of the present application, even in a steel having approximately the same Ca content, a high melting point in which the oxide composition is rich in Al ₂ O ₃ by controlling Ca and Al in the steel to a predetermined mass% ratio. It was found that it was modified to a slightly lower melting point chemical composition such as CaO—Al ₂ O ₃ or CaO—Al ₂ O ₃ —SiO ₂ instead of inclusions, and the area and number of inclusions contained in the steel It was found that stable good machinability can be exhibited by limiting the ratio to a predetermined range.

  That is, in order to obtain the machinability improving effect by Ca, it is necessary to make Ca / Al 0.1 or more by mass% ratio. On the other hand, it was found that when Ca / Al exceeds 1.0 by mass ratio, the carbide tool covering protective effect is saturated and the sulfide is too hard, leading to a reduction in drill life.

Furthermore, in the steel for machine structural use of the present invention controlled to the above range of Ca / Al ratio, it is a sulfide including an oxide contained together with inevitable impurities present in the steel and a sulfide adjacent to the oxide. For a composite sulfide, when the cross-sectional area of the composite sulfide containing or adjacent to the oxide is S _S and the cross-sectional area of the oxide in the composite sulfide is S _O , the cross-sectional area of the composite sulfide is The area ratio, which is the ratio of the cross-sectional area S _O of the oxide to (S _S + S _O ), is 3 to 90%, and the composite sulfide that includes or is adjacent to the oxide with respect to the total number of sulfides in the unit area. It has been found that when the number ratio of the number of objects is 4 to 80%, it has stable and good machinability.

That is, the means of the present invention for solving the above-described problems is the mass% in the invention of the means of claim 1, C: 0.17 to 0.60%, Si: 0.05 to 2.0%. , Mn: 0.3-2.5%, S: 0.03-0.16%, Al: 0.002-0.030%, Ca: 0.0005-0.01%, O: 0.0005 -0.0029%, N: 0.0084-0.02%, balance Fe and inevitable impurities, the mass ratio of Ca and Al is Ca / Al: 0.1-1.0, and contains sulfide The area ratio which is the ratio of the cross-sectional area occupied by the oxide to the cross-sectional area occupied by the composite sulfide which is the sulfide containing the oxide contained in the steel or the sulfide adjacent to the oxide is 3 was 90%, the proportion der of the number of the composite sulfide to the more the number of the total sulphide in a unit area A machinability excellent mechanical structural steel, wherein the number ratio is 4-80%.

In this case, the area ratio, which is the ratio of the cross-sectional area occupied by the oxide to the cross-sectional area of the composite sulfide, is an arbitrary composite sulfide in a test area 1 mm ² (longitudinal and lateral 1 mm) parallel to the forging direction of the steel material. The area ratio (%) of the oxide is obtained for 20 pieces, and the average value is used. In addition, the number ratio, which is the ratio of the number of composite sulfides containing or adjacent to the oxide to the total number of sulfides in a unit area, is a composite in a test area 1 mm ² (longitudinal and lateral 1 mm) parallel to the rolling direction of the steel material. The number of sulfides divided by the total number of sulfides.

  Further, in addition to the above means, the steel components are in mass%, Ni: 0.1 to 2.5%, Cr: 0.1 to 2.5%, Mo: 0.05 to 1.50%, V: The composition contains one or more selected from 0.01 to 0.50%, Nb: 0.001 to 0.30%, and B: 0.0003 to 0.005%. It is a steel for machine structural use with excellent machinability.

  By using the above steel for machine structure, a steel for machine structure that improves fatigue strength and strength anisotropy and exhibits stable machinability can be obtained. In particular, in the above, the Ca / Al ratio is set to 0.1 to 1.0, which is necessary to cause Ca to act on inclusion composition control effective for machinability and further includes an oxide. In the composite sulfide which is a sulfide adjacent to the oxide, the area ratio of the ratio of the area occupied by the oxide to the area occupied by the composite sulfide is 3 to 90%, and further to the total number of sulfides in the unit area It is assumed that the number ratio, which is the ratio of the number of complex sulfides containing or adjacent to oxide, is 4 to 80%. As a result, a machine structural steel that exhibits stable machinability is obtained.

  The reasons for limiting the steel components of the present invention will be described below. In addition, each% shall show the mass%.

C: 0.17 to 0.60%
C is an element necessary for ensuring the strength. For this purpose, 0.17% or more is necessary. However, if it exceeds 0.60%, the machinability deteriorates. Therefore, C is set to 0.17 to 0.60%.

Si: 0.05-2.0%
Si is an element necessary as a deoxidizer, and 0.05% or more is necessary for this purpose. However, if it exceeds 2.0%, the machinability is reduced due to an increase in the hard oxide. Further, in carburizing applications, the grain boundary oxide layer depth on the surface of the carburized layer is increased, and the fatigue life is reduced. Therefore, Si is set to 0.05 to 2.0%, preferably 0.05 to 0.5%.

Mn: 0.3 to 2.5%
Mn is an element necessary for ensuring hardenability and an element necessary for generating MnS. For this purpose, 0.3% or more is necessary. However, if it is more than 2.5%, the machinability is lowered, and in the case of carburizing, excessive Mn increases the carburizing abnormal layer depth on the surface of the carburizing layer and reduces the fatigue life. Therefore, Mn is set to 0.3 to 2.5%, preferably 0.4 to 1.5%.

S: 0.03-0.16%
S is an element necessary for ensuring machinability at the time of drilling, gear cutting, and the like, and is an element necessary for ensuring chip disposal. For this purpose, 0.03% or more is desirable. . However, if it is more than 0.16%, strength properties such as static strength and fatigue strength are lowered, and hot workability is further lowered. Therefore, S is set to 0.03 to 0.16%.

Al: 0.002 to 0.030%
Al is an element that forms nitrides and is effective in suppressing grain coarsening during carburization. For this purpose, 0.002% or more is necessary. However, since it is necessary to reduce Al ₂ O ₃ which is harmful to machinability and fatigue life, the upper limit is made 0.030%. Therefore, Al is made 0.002 to 0.030%, preferably 0.002 to 0.025%.

Ca: 0.0005 to 0.01%
Ca is an element that effectively acts on tool protection during carbide turning to improve machinability, and is also effective in improving fatigue strength and static strength by controlling sulfide morphology. For that purpose, 0.0005% or more is necessary. However, if it exceeds 0.01%, large inclusions that reduce the fatigue life and static strength are generated, and the productivity is deteriorated and the manufacturing cost is increased. Therefore, Ca is 0.0005 to 0.01%, preferably 0.0005 to 0.0050%.

O: 0.0005 to 0.0029%
O is an element necessary for the production of Ca-based oxides effective for tool coating protection, and for this purpose, 0.0005% or more is necessary. However, if it is more than 0.0029%, a large amount of oxide inclusions are generated and the fatigue strength is lowered. Therefore, O is set to 0.0005 to 0.0029%.

N: 0.0084 to 0.02%
N is an element that forms nitrides with Al and is effective in suppressing coarsening of crystal grains. However, even if it exceeds 0.02%, the effect of suppressing the coarsening of crystal grains is saturated. Therefore, N is set to 0.0084 to 0.02%.

Ca / Al by mass% ratio: 0.1 to 1.0
In Ca / Al, the lower limit of 0.1 is necessary for the control of oxides and sulfides by Ca. If the upper limit of 1.0 is exceeded, the tool covering protection effect is saturated and the sulfide becomes too hard. This will reduce the drill life. Therefore, Ca / Al is 0.1 to 1.0, preferably 0.1 to 0.8. More desirably, it is 0.1 to 0.6.

{S _O / (S _S + S _O )} × 100 = 3 to 90 (%) (1)
(Number of complex sulfides containing oxide or adjacent sulfides / total number of sulfides) × 100 = 4 to 80 (%) (2)
When the cross-sectional area of the sulfide containing the oxide or adjacent to the oxide is S _S and the cross-sectional area of the oxide is S _O , the cross-sectional area of this composite sulfide (S _S + S _O ) The area ratio, which is the ratio of the cross-sectional area S _O occupied by the oxide, is 3 to 90%, and the number ratio, which is the ratio of the number of composite sulfides to the total number of sulfides in a unit area, that is, 1 mm 2, is 4. It shall be ~ 80%. That is, if the formula (1) is less than 3%, there is no effect in suppressing carbide tool wear, and if it exceeds 90%, the contact frequency between the tool and the oxide during drilling increases and the amount of tool wear increases. Furthermore, if the formula (2), which is the ratio of the number of composite sulfides to the total number of sulfides, is less than 4%, the effect of suppressing carbide tool wear is insufficient, and if it exceeds 80%, composite sulfides harder than ordinary MnS are larger. Since it occupies a large number, the amount of tool wear in drilling increases. Therefore, the above equations (1) and (2) are used.

Ni: 0.1 to 2.5%
Ni is an element necessary for ensuring hardenability and toughness, and for this purpose, 0.1% or more is necessary. However, if it is more than 2.5%, the machinability is lowered, and the cost is increased because it is an expensive element. Therefore, Ni is set to 0.1 to 2.5%.

Cr: 0.1 to 2.5%
Cr is an element necessary for ensuring the hardenability of the base, and for this purpose, 0.1% or more is necessary. However, if it is more than 2.5%, the machinability is lowered. Therefore, Cr is set to 0.1 to 2.5%, preferably 0.6 to 1.5%.

Mo: 0.05 to 1.50%
Mo is an element necessary for ensuring the hardenability of the base, and 0.05% or more is necessary for this purpose. However, if it is more than 1.50%, the machinability is lowered and the manufacturing cost is increased. Therefore, Mo is set to 0.05 to 1.50%.

V: 0.01 to 0.50%
V is an element necessary for ensuring the hardenability of the base, and for this purpose, it is necessary to be 0.01% or more. However, if it is more than 0.50%, the machinability is lowered and the production cost is increased. Therefore, V is set to 0.01 to 0.50%.

Nb: 0.001 to 0.30%
Nb is an element effective in suppressing the coarsening of crystal grains during carburization. For this purpose, 0.001% or more is necessary. However, if it is more than 0.30%, the cost is increased, and the production of NbC that impairs the machinability cannot be suppressed. Therefore, Nb is set to 0.001 to 0.30%.

B: 0.0003 to 0.005%.
B is an element necessary for ensuring hardenability and strengthening grain boundaries. For this purpose, B is required to be 0.0003% or more. However, when it exceeds 0.005%, the effect is saturated. Therefore, B is set to 0.0003 to 0.005%.

  The steel for machine structural use according to the present invention can obtain stable and excellent machinability with no variation in tool life despite the fact that it does not contain Pb, which is an environmental load element, as a free-cutting component. It has excellent effects such as contributing to the reduction of load substances.

  The best mode for carrying out the present invention will be described below with reference to a table according to a first embodiment.

  By melting steel of chemical composition shown in Table 1 in a 100 kg vacuum induction furnace, casting into an ingot, heating to 1200 ° C., forging into φ65 mm material and 40 mm square material, holding at 900 ° C. for 1 hour and air cooling. Normalization was performed and used as test materials for the following tests.

  The evaluation conditions shown in Table 2 were obtained by turning a φ65 mm material subjected to the above normalization of comparative steels 1 to 3, 5 to 16 and 18 to 21 and 23 to 31 shown in Table 1 and 23 to 31 steel into a φ60 mm material. Was used for a carbide tool turning test.

  Furthermore, the 40 mm square material which carried out the above-mentioned normalization of the comparative steel of 1 to 3, 5 to 16 and 18 to 21, and 23 to 31 shown in Table 1 and the inventive steel was milled into a 35 mm square material and Table 3 was obtained. It was subjected to the drill life test shown. When the ratio of the number of composite sulfides exceeds the upper limit for the Ca / Al ratio, the oxide area ratio in the composite sulfides, and the number of composite sulfides exceeds the upper limit, the tool wear in drilling increases, and the characteristic deterioration is Evaluation is based on the lifetime (number of holes until drilling is impossible).

  The composite sulfidation with respect to the Ca / Al ratio, the area ratio of the oxide in the composite sulfide, and the total number of sulfides of these comparative steels 1 to 3, 5 to 16 and 18 to 21, and 23 to 31 and the invention steel Number ratio of objects (excluding Pb-containing steels among comparative steels), flank wear amount and rake face wear amount of carbide tool wear as a result of these carbide tool turning test and drill life test, and Table 4 shows the drill life. Furthermore, the shape of the composite sulfide of the structure of comparative steels 2 and 3 and invention steels 5, 6, and 7 is shown in FIG. Inventive steels 5, 6, and 7 are particularly excellent in machinability.

  In Table 4, 5-7, 11-13, 18, 23-25, and 29-31 of the steels of the present invention are all excellent in flank wear amount, rake face wear amount, and drill life. 1-3, 8-9, 14-15, 19-20, 26-27 are inferior to the present invention in terms of carbide tool wear, and the comparative steels 10, 16, 21, 28 have a drill life of It turns out that it is inferior to this invention by a point.

It is the SEM photograph of the structure | tissue which shows the shape of the composite sulfide of the comparative steel 2, the comparative steel 3, the invention steel 5, the invention steel 6, and the invention steel 7.

Claims (2)

In mass%, C: 0.17 to 0.60%, Si: 0.05 to 2.0%, Mn: 0.3 to 2.5%, S: 0.03 to 0.16%, Al: 0.002 to 0.030%, Ca: 0.0005 to 0.01%, O: 0.0005 to 0.0029%, N: 0.0084 to 0.02%, balance Fe and inevitable impurities, A steel having a mass ratio of Ca and Al of Ca / Al: 0.1 to 1.0 and containing sulfide, and sulfide containing oxides contained in the steel or sulfide adjacent to the oxide. The area ratio, which is the ratio of the cross-sectional area occupied by the oxide to the cross-sectional area occupied by the composite sulfide, is 3 to 90%, and the ratio of the number of the composite sulfide to the total number of sulfides in the unit area A machine structural steel excellent in machinability, wherein the number ratio is 4 to 80%. In addition to the above steel components, the steel components are in mass%, Ni: 0.1 to 2.5%, Cr: 0.1 to 2.5%, Mo: 0.05 to 1.50%, V: 0 It contains one or more selected from 0.01 to 0.50%, Nb: 0.001 to 0.30%, and B: 0.0003 to 0.005%. Machine structural steel with excellent machinability.