JP6798557B2 - steel - Google Patents

Description

The present invention relates to steel.

Compared to hot forging, cold forging (including rolling) can improve the surface surface and dimensional accuracy of the product, and also has good yield, so it is a relatively small machine such as a bolt. It is widely applied as a method for manufacturing parts. When mechanical parts are manufactured by cold forging, medium carbon carbon steels and alloys for machine structural use specified in, for example, JIS G 4051, JIS G 4052, JIS G 4104, JIS G 4105, JIS G 4106, etc. Steel is often used to make the final product through manufacturing processes such as hot wire rolling-annealing (or spheroidizing annealing) -drawing-cold forging-quenching / tempering. The above-mentioned general manufacturing process is characterized in that a step of annealing or spheroidizing annealing is added before cold forging. The reason why annealing or spheroidizing annealing is added before cold forging is that medium carbon carbon steels and alloy steels are still hot-rolled (that is, when they are air-cooled without heat treatment after hot rolling). In), the hardness of the rolled material is high, and the die is significantly worn during cold forging, resulting in a high manufacturing cost. In addition, the material is insufficiently ductile with hot rolling, resulting in cracking during cold forging. This is because there is a manufacturing problem such as a decrease in yield because it becomes easy.

However, since annealing costs a lot of money, it has been required to develop a steel material that can omit the annealing process in order to reduce the manufacturing cost of parts. In response to such a request, so-called boron steel for bolts, in which a small amount of B is added to the steel material, has been developed (for example, Patent Document 1 and Patent Document 3). Boron steel is characterized by reducing the carbon content of the steel material and the amount of alloying elements such as Cr and Mo added to reduce the hardness of the wire as it is hot-rolled and improve the elongation to quench it. The purpose is to make it unnecessary and to compensate for the decrease in hardenability due to the reduction in the amount of alloying elements added by the effect of improving the hardenability by adding a small amount of B without increasing the hardness of the rolled material.

In order to exhibit the effect of improving hardenability by adding a small amount of B, it is necessary that B is in a solid solution state in austenite. On the other hand, when nitrogen in a solid solution state is present in the steel, BN is generated and the amount of the solid solution B (B dissolved in the steel) is reduced, so that the hardenability improving effect of B is exhibited. Will be lost. Therefore, in boron steel, it is generally practiced to fix N in the steel as TiN in advance by adding Ti having a strong affinity for N to suppress the formation of BN. For example, Patent Document 4 describes that precipitation of BN is suppressed by setting Ti / N (mass ratio) to 4 or more. In principle, the precipitation of BN can be suppressed by setting Ti / N to 3.42 or more.

However, as compared with conventional steels, general boron steels as described above are more likely to generate so-called coarse grains in which some austenite crystal grains grow abnormally and become coarse during quenching and heating. For parts with coarse particles, the heat treatment strain generated during quenching increases, resulting in deterioration of dimensional accuracy and deterioration of parts characteristics such as impact value, fatigue strength, and delayed fracture characteristics. Therefore, it is a big practical problem to prevent the generation of coarse particles, especially in high-strength bolts having a tensile strength of 800 MPa or more. In order to suppress the generation of coarse particles due to such abnormal grain growth, in order to pin the grain boundaries of austenite crystal grains, a large number of pinned particles (precipitates, etc.) are dispersed in the structure, that is, fine particles. It is effective to disperse a large amount of fine particles.

There are two main reasons why coarse particles are likely to occur in boron steel.

(1) When boron steel is used as a component material, the annealing step after cold forging of boron steel is omitted, so that the boron steel is heated directly from the cold working structure to the austenite region. In this case, due to the influence of cold working, excessive fineness of austenite crystal grains and partial non-uniformity of crystal grain size occur, so that some crystal grains are liable to cause abnormal grain growth.

(2) In the above-mentioned boron steel, N in the steel is fixed as TiN by adding Ti, so that AlN, which effectively acts as pinning particles, is not generated in the conventional carbon steel and alloy steel. Moreover, since TiN is coarser than AlN, it cannot be finely dispersed, and it is difficult to secure the number of pinned particles required for preventing coarse particles.

Since the factor (1) above is unavoidable in order to omit the annealing step, how to secure the number of pinned particles in boron steel to improve the factor (2) is the generation of coarse particles. It has been regarded as a point of prevention.

Under these circumstances, a technique for preventing the generation of coarse particles of boron steel has been proposed. For example, Patent Document 5 and Patent Document 6 describe that TiC and Ti (CN), which are finer precipitates than TiN, are used as pinning particles instead of AlN and TiN. In these techniques, TiC and Ti (CN) with a diameter of 0.2 μm or less are contained in the steel before quenching and heating and after hot rolling in order to secure the number of pinned particles required to prevent coarse particles. It is stipulated that the total number of and is 20 pieces / 100 μm ² or more. By dispersing a large amount of such fine precipitates in advance before quenching and heating, these precipitates function as pinning particles that pin the austenite grain boundaries during quenching and heating. Since this technology makes it possible to stably prevent the generation of coarse particles in boron steel, steels to which this technology is applied are currently widely used as inexpensive steel materials for bolts that can omit the annealing process.

However, the above technique has drawbacks. That is, when a large amount of fine TiC or Ti (CN) is dispersed in the structure after hot rolling, there is a side effect that the hardness of ferrite increases due to precipitation strengthening by fine precipitate particles. The problem is that the softening effect of hot-rolled material due to boron steelization is reduced. That is, when the amount of fine TiC or Ti (CN) is increased, the generation of coarse grains can be suppressed, but the life of the cold forging die is shortened because the hardness of the rolled material is increased by precipitation strengthening. .. On the contrary, if the amount of fine TiC or Ti (CN) is suppressed, the hardness of the rolled material can be suppressed, but coarse grains are generated. That is, when fine TiC or Ti (CN) is used, the suppression of the generation of coarse grains and the suppression of the hardness of the rolled material before cold forging are in a trade-off relationship. Therefore, it is difficult to completely achieve both the softening of the rolled material and the suppression of stable coarse grains by the above technique alone.

Patent Document 7 also describes the same technical idea as the above-mentioned technique for preventing the generation of coarse particles of boron steel. That is, it is a technique for dispersing the carbonitrides of these elements in steel by keeping the relationship of the contents of Ti, Nb, Al, and N within a certain range, and preventing the coarsening of crystal grains. Patent Document 7 further describes the effect of improving machinability by adding 0.01% or more of Bi. However, in Patent Document 7, only the effect of enhancing machinability is disclosed as the effect of Bi. There is no description about the relationship between Bi and the coarsening characteristics of crystal grains. Since Bi is added for the purpose of improving machinability, Patent Document 7 only considers adding a relatively large amount of Bi. In this case, as described in Patent Document 7, there is a concern that the hot workability may be lowered due to the addition of Bi.

Patent Document 8 describes a skin-baking steel that exhibits excellent grain coarsening resistance even when carburized at a higher temperature than conventional examples, and exhibits excellent cold workability without soft annealing. Annealed steels for the purpose of providing are disclosed. However, Patent Document 8 also proposes only the use of fine Ti carbides, Ti-containing composite carbides, and the like as means for ensuring grain coarsening resistance. In Patent Document 8, the hot rolling temperature is extremely low in order to ensure cold workability, and thus the productivity of the surface baking steel is impaired.

Japanese Patent Application Laid-Open No. 5-339676 Japan Special Fairness No. 5-63524 Japanese Patent Application Laid-Open No. 61-253347 Japanese Patent Application Laid-Open No. 3-47918 Japanese Patent No. 3443285 Japanese Patent No. 3490293 Japanese Patent Application Laid-Open No. 2000-328189 Japanese Patent Application Laid-Open No. 2006-265704

One of the challenges of steel for cold forging is to improve the cold forging property of the steel and the productivity of the steel without annealing after hot rolling and before cold forging. Keeping the steel soft without the use of damaging manufacturing conditions. Another challenge of cold forged steel is to exhibit high hardenability after cold forging in order to impart high strength to mechanical parts. A further problem with steel for cold forging is the generation of coarse grains during quenching after cold forging in order to prevent deterioration of mechanical parts such as dimensional accuracy, impact value, fatigue strength, and delayed fracture characteristics. Is to suppress. As mentioned above, prior art cannot solve all of these at the same time. The use of TiC and Ti (CN) proposed in the prior art as a means for suppressing the generation of coarse grains hardens the steel after hot rolling and before cold forging by precipitation strengthening, so that the cold forging property of the steel and the cold forging property and It impairs productivity.

The present invention has been made in view of the above problems. That is, the present invention suppresses the generation of coarse grains during quenching without using Ti carbides such as TiC and Ti (CN) and Ti carbonitrides, thereby producing manufacturability, cold forging property, and after quenching. The challenge is to provide steel with excellent mechanical properties.

The gist of the present invention is as follows.

(1) The steel according to one aspect of the present invention has a chemical component of unit mass%, C: 0.15% to 0.40%, Mn: 0.10% to 1.50%, S: 0. 002 to 0.020%, Ti: 0.005% to 0.050%, B: 0.0005 to 0.0050%, Bi: 0.0010% to 0.0100%, P: 0.020% or less, N: 0.0100% or less, Si: 0% or more and less than 0.30%, Cr: 0 to 1.50%, Al: 0 to 0.050%, Mo: 0 to 0.20%, Cu: 0 to 0 0.20% Ni: 0-0.20% and Nb: containing from 0 to 0.030%, the balance being Fe and impurities, N fixed index _{I FN} is 0, which is defined by equation 1 below greater than or equal and, Ti-Nb-based precipitates generated index _{I P,} defined by equation 2 below is 0.0100 or less.
_{I FN = [Ti] -3.5 ×} [N] ... ( Equation 1)
Here, [Ti] is the Ti content in the unit mass%, and [N] is the N content in the unit mass%.
_{I P = 0.3 × [Ti]} + 0.15 × [Nb] - [N] ... ( Equation 2)
Here, [Ti] is the Ti content in the unit mass%, [Nb] is the Nb content in the unit mass%, and [N] is the N content in the unit mass%.
(2) The steel according to (1) above has the chemical composition of Si: 0.01% or more and less than 0.30%, Cr: 0.01 to 1.50%, and Al: in a unit mass%. It may contain one or more selected from the group consisting of 0.001 to 0.050%.
(3) In the steel according to (1) or (2) above, the chemical composition is, in terms of unit mass%, Mo: 0.02 to 0.20%, Cu: 0.02 to 0.20%, Ni. It may contain one or more selected from the group consisting of: 0.02 to 0.20% and Nb: 0.002 to 0.030% .

According to the present invention, it is possible to provide a steel capable of achieving both softening before cold forging and suppression of generation of coarse particles during quenching after cold forging. Further, the steel according to the present invention is excellent in manufacturability because it is not cracked during casting, rolling, etc., and can be manufactured under conditions within a range that does not impose a load on the manufacturing equipment. By applying the steel according to the present invention to cold forged parts, wear of the die during cold forging can be suppressed and the life of the die can be improved. Further, by applying the steel according to the present invention to cold forged parts, the cost of expensive dies can be reduced, which can contribute to the reduction of the manufacturing cost of high-strength bolts having a tensile strength of 800 MPa or more. it can. Further, the steel according to the present invention is also excellent in machinability. Therefore, the present invention has an extremely large industrial contribution.

The steel according to the embodiment of the present invention will be described. The steel according to this embodiment has the following characteristics.

(A) The steel according to the present embodiment has a chemical component of unit mass%, C: 0.15% to 0.40%, Mn: 0.10% to 1.50%, S: 0.002 to 0.020%, Ti: 0.005% to 0.050%, B: 0.0005 to 0.0050%, Bi: 0.0010% to 0.0100%, P: 0.020% or less, N: 0.0100% or less, Si: 0% or more and less than 0.30%, Cr: 0 to 1.50%, Al: 0 to 0.050%, Mo: 0 to 0.20%, Cu: 0 to 0. It contains 20%, Ni: 0 to 0.20%, and Nb: 0 to 0.030%, and the balance consists of Fe and impurities.
(B) In the steel according to (a) above, the chemical composition is, in unit mass%, Si: 0.01% or more and less than 0.30%, Cr: 0.01 to 1.50%, and Al: It may contain one or more selected from the group consisting of 0.001 to 0.050%.
(C) In the steel according to (a) or (b) above, the chemical composition is, in terms of unit mass%, Mo: 0.02 to 0.20%, Cu: 0.02 to 0.20%, Ni. It may contain one or more selected from the group consisting of: 0.02 to 0.20% and Nb: 0.002 to 0.030%.
(D) The steel according to any one of (a) to (c) above may have an N fixed index _IFN defined by the following formula 1 of 0 or more.
_IFN = [Ti] -3.5 x [N] ... (Equation 1)
Here, [Ti] is the Ti content in the unit mass%, and [N] is the N content in the unit mass%.
(E) above (a) ~ according to any one of (d) steel, Ti-Nb-based precipitates generated index _{I P,} which is defined by Equation 2 below may also be 0.0100 or less ..
_IP = 0.3 x [Ti] + 0.15 x [Nb]-[N] ... (Equation 2)
Here, [Ti] is the Ti content in the unit mass%, [Nb] is the Nb content in the unit mass%, and [N] is the N content in the unit mass%.
Further, by bolting, quenching, and tempering the steel according to the present embodiment by a known method, a bolt having excellent productivity and no coarse grains can be obtained.

The present inventors have made particles such as TiC and Ti (CN), which are particles that cause a remarkable increase in ferrite hardness due to precipitation strengthening, and thus cause an increase in steel hardness, which impairs the cold workability of steel. We investigated a technique for suppressing the generation of coarse particles, which is different from the conventional technique for finely dispersing the grains. The above characteristics are based on the following findings obtained by the present inventors diligently researching a technique for suppressing abnormal grain growth of austenite crystal grains during quenching and heating of steel.

(1) With an extremely small amount of Bi of 0.0100% or less, abnormal grain growth of austenite crystal grains during quenching and heating can be suppressed, and cold-worked parts having excellent dimensional accuracy and mechanical properties can be obtained.

(2) Due to the above-mentioned effect of Bi, austenite does not depend on the precipitates (TiC, Ti (CN), NbC) conventionally used as pinning particles (that is, without impairing the cold workability of steel). Abnormal grain growth of crystal grains can be suppressed. As a result, the hardness of the rolled material after hot rolling can be suppressed, and the cold workability of the steel can be improved.

(3) On the other hand, when the Bi content exceeds 0.0100%, the hot ductility of the steel is lowered, so that cracks and scratches are likely to occur in the steel manufacturing process (casting, rolling process, etc.), and the steel It was found that the yield decreased. Furthermore, it was also found that when the Bi content exceeds 0.0100%, grain boundary embrittlement occurs in the hardened steel and the mechanical properties of the steel are impaired. Therefore, it was also found that although the content of Bi is essential in the steel according to the present embodiment, its content needs to be suppressed to an extremely low level.

Hereinafter, the steel according to this embodiment will be described in detail.
First, the chemical composition of the steel of the present invention will be described. Hereinafter, the unit "%" relating to the chemical component indicates "mass%".

[C: 0.15 to 0.40%]
C is an element required to increase the strength of steel having a tempered martensite structure. In order to make the tensile strength after quenching 800 MPa or more, the C content needs to be 0.15% or more. The lower limit of the preferred C content is 0.17%, 0.19%, or 0.23%.
On the other hand, if the C content exceeds 0.40%, the hardness of the rolled material after hot rolling becomes too high, so that the life of the cold forging die is significantly shortened. Therefore, the upper limit of the C content is set to 0.40%. The upper limit of the preferred C content is 0.35%, 0.34%, 0.33%, or 0.30%.

[Mn: 0.10 to 1.50%]
Mn is an element effective for improving the hardenability of steel. In order to secure the hardenability required for obtaining martensite by quenching, the Mn content needs to be 0.10% or more. The lower limit of the preferred Mn content is 0.20%, 0.35%, or 0.40%.
On the other hand, if the Mn content exceeds 1.50%, the hardness of the rolled material after hot rolling and before cold forging becomes too high, so that the life of the cold forging die is significantly shortened. Therefore, the upper limit of the Mn content is set to 1.50%. The upper limit of the preferred Mn content is 1.30%, 1.00%, or 0.80%.

[S: 0.002-0.020%]
S exists in steel as MnS, TiS, and Ti ₂ C ₂ S, and has an effect of suppressing abnormal grain growth of austenite crystal grains by acting as pinning particles during quenching and heating. Therefore, the S content needs to be 0.002% or more. The lower limit of the preferred S content is 0.003%.
However, in the steel according to the present embodiment, since Bi is used to suppress abnormal grain growth, the S content may be smaller than that of the prior art. Further, when the S content exceeds 0.020%, S embrittles the old austenite grain boundaries of the hardened steel and lowers the delayed fracture resistance (hydrogen embrittlement resistance). In addition, since the above-mentioned Ti ₂ C ₂ S is a particle that impairs the machinability of steel, if the S content exceeds 0.020%, the machinability of steel may deteriorate. Therefore, it is necessary to limit the S content to 0.020% or less. Preferably, the upper limit of the S content is 0.015%, 0.010%, or 0.005%.

[Ti: 0.005% to 0.050%]
Ti forms a compound with C, N, and S in the steel and exists in the steel as Ti-based inclusions such as TiN, Ti (CN), TiC, TiS, and Ti ₂ C ₂ S, and is pinned during quenching and heating. By acting as a stop particle, it has the effect of suppressing abnormal grain growth of austenite crystal grains. Further, since Ti has a strong affinity with the solid solution N in the steel, it is an extremely effective element for fixing the solid solution N in the steel as TiN in advance and suppressing the formation of BN. In boron steel, it is necessary to suppress the formation of BN in order to secure the content of solid solution B which is effective for improving hardenability. Therefore, the Ti content needs to be 0.005% or more. The lower limit of the preferred Ti content is 0.010%, 0.015%, or 0.020%.
However, in the steel according to the present embodiment, since Bi is used to suppress abnormal grain growth, the Ti content may be smaller than that of the prior art. Further, when the Ti content exceeds 0.050%, the Ti-based inclusion particles cause precipitation strengthening, and the hardness of the rolled material after hot rolling becomes too high. Therefore, the mold for cold forging Life is significantly reduced. In order to suppress the hardness of the rolled material after hot rolling while increasing the content of Ti-based inclusion particles, it is necessary to lower the hot rolling temperature, which is due to productivity, equipment life, etc. It is not preferable in that respect. Further, when the Ti content is increased, a large amount of Ti ₂ C ₂ S, which is a particle that impairs the machinability of the steel, is generated and the machinability is deteriorated. Therefore, the cutting process is applied to the steel according to the present embodiment. Becomes difficult. Therefore, the upper limit of the Ti content is set to 0.050%. Preferred Ti content is 0.040% or less, 0.030% or less, less than 0.030%, or 0.025% or less.

[B: 0.0005 to 0.0050%]
B is an element that contributes to the improvement of hardenability of steel when contained in a small amount, and improves the hardenability without increasing the hardness of the rolled material after hot rolling and before cold forging. The effect can be obtained and the hardness after cold forging and quenching can be increased. B is an essential element especially for boron steel for bolts. Further, B has an effect of suppressing grain boundary destruction by segregating at the former austenite grain boundaries and strengthening the former austenite grain boundaries. In order to obtain the above effect, the B content needs to be 0.0005% or more. Preferably, the lower limit of the B content is 0.0010%, 0.0012%, or 0.0015%.
On the other hand, when the B content exceeds 0.0050%, the effect is saturated. Therefore, the B content is set to 0.0050% or less. Preferably, the upper limit of the B content is 0.0030%, 0.0025%, 0.0020%, or 0.0018%.

[Bi: 0.0010% to 0.0100%]
The effect of a small amount of Bi of about 0.0010% to 0.0100% on the structure during quenching of steel has not been investigated in detail so far. The present inventors have found that a small amount of Bi has an effect of preventing the generation of coarse grains by suppressing the abnormal grain growth of austenite crystal grains during quenching and heating. Further, since the Bi content required to suppress abnormal grain growth is very small, the above-mentioned effect of Bi that suppresses the generation of coarse grains during quenching and heating reduces the hardness of the rolled material after hot rolling. The present inventors have also found that it can be obtained without increasing the amount. In order to obtain the above effects, the Bi content needs to be 0.0010% or more. The lower limit of the Bi content is preferably 0.0020%, 0.0025%, or 0.0030%.
On the other hand, when the Bi content exceeds 0.0100%, not only the effect is saturated, but also the hot ductility of the steel is lowered, so that cracks and scratches occur in the steel manufacturing process (casting, rolling process, etc.). It becomes easier and the yield decreases. Further, when the Bi content exceeds 0.0100%, grain boundary embrittlement occurs in the hardened steel, and the mechanical properties of the steel are impaired. Therefore, the Bi content is set to 0.0100% or less. The Bi content is preferably less than 0.0100%, 0.0080% or less, or 0.0060% or less.

[P: 0.020% or less]
P is an impurity, which is an element that embrittles the old γ grain boundaries and lowers the delayed fracture resistance (hydrogen embrittlement resistance) of steel. Therefore, it is necessary to limit the P content to 0.020% or less. Preferably, the upper limit of the P content is 0.015%, 0.013%, or 0.010%.
Since P is not required to solve the steel problem of this embodiment, the lower limit of the P content is 0%. However, in order to suppress the cost of the refining step for reducing the P content, the lower limit of the P content may be 0.001%.

[N: 0.0100% or less]
When N forms a compound with B and exists in the steel as BN, it reduces the amount of solid solution B and impairs the effect of improving hardenability by B. Since N is harmful in the steel according to the present embodiment, the lower limit of the N content is 0%. However, the lower limit of the N content may be 0.0001%, 0.0005%, or 0.0010% in order to reduce the cost of the refining step for reducing the N content.
When the N content is high, the Ti content required to fix N in the steel as TiN increases, so it is desirable to reduce the N content as much as possible. Therefore, it is necessary to limit the N content to 0.0100% or less. Preferably, the upper limit of the N content is 0.0070%, 0.0050%, or 0.0040%.

If necessary, the spring steel according to the present embodiment may further contain one or more selected from the group consisting of Si, Cr, and Al in the range described later. However, since Si, Cr, and Al are not essential, the lower limit of the contents of each of Si, Cr, and Al is 0%.

[Si: 0% or more and less than 0.30%]
As described above, in the steel according to the present embodiment, the lower limit of the Si content is 0%. However, Si is an element effective for improving the hardenability of steel and improving the temper softening resistance of martensite. In order to obtain the above effects, the Si content is preferably more than 0% or 0.01% or more. The lower limit of the Si content may be 0.05% or 0.15%.
However, when the Si content is 0.30% or more, the amount of increase in hardness of the steel (rolled material) after hot rolling and before cold forging increases, so that the life of the cold forging die is shortened. To do. Therefore, the Si content is set to less than 0.30%. The upper limit of the preferred Si content is 0.27%, 0.25%, or 0.20%.

[Cr: 0 to 1.50%]
As described above, in the steel according to the present embodiment, the lower limit of the Cr content is 0%. However, Cr is an effective element for improving the hardenability of steel and improving the temper softening resistance of martensite. When the above effect is obtained, the Cr content is preferably more than 0% or 0.01% or more. The lower limit of the Cr content may be 0.10%, 0.20%, or 0.30%.
On the other hand, if the Cr content exceeds 1.50%, the hardness of the rolled material after hot rolling and before cold forging becomes too high, so that the life of the cold forging die is significantly shortened. Therefore, the upper limit of the Cr content is set to 1.50%. The upper limit of the preferred Cr content is 1.20%, 1.00%, or 0.80%.

[Al: 0 to 0.050%]
Al is an element effective for deoxidizing steel, but it does not necessarily have to be contained when deoxidizing with another element (Si, Ti, etc.). Therefore, the lower limit of the Al content is 0%. However, in order to obtain the deoxidizing effect of Al, it is preferable to contain 0.001% or more, 0.005% or more, or 0.010% or more.
On the other hand, when the Al content exceeds 0.050%, problems such as the formation of coarse inclusions and a decrease in the toughness of the steel become remarkable. Therefore, even when Al is contained, the upper limit of the Al content is set to 0.050%. The upper limit of the Al content is preferably 0.040%, 0.030%, or 0.025%.

If necessary, the spring steel according to the present embodiment may further contain one or more selected from the group consisting of Mo, Cu, Ni, and Nb within the range described later. However, since Mo, Cu, Ni, and Nb are not essential, the lower limit of the contents of each of Mo, Cu, Ni, and Nb is 0%.

[Mo: 0 to 0.20%]
As described above, in the steel according to the present embodiment, the lower limit of the Mo content is 0%. However, Mo is an element that contributes to the improvement of hardenability of steel even if its content is small. In order to obtain the above effects, the Mo content is preferably 0.02% or more. More preferably, the lower limit of the Mo content is 0.03%, 0.04%, or 0.05%.
On the other hand, since Mo is an expensive alloying element, if the Mo content exceeds 0.20%, it is disadvantageous in terms of manufacturing cost. Therefore, even when Mo is contained, the Mo content is set to 0.20% or less. Preferably, the upper limit of the Mo content is 0.16%, 0.13%, or 0.10%.

[Cu: 0 to 0.20%]
As described above, in the steel according to the present embodiment, the lower limit of the Cu content is 0%. However, Cu is an element that improves the corrosion resistance of steel. In order to obtain the above effects, the Cu content is preferably 0.02% or more. More preferably, the lower limit of the Cu content is 0.05%.
On the other hand, when the Cu content exceeds 0.20%, the hot ductility of the steel is lowered, and problems such as impairing the manufacturability during continuous casting become remarkable. Therefore, even when Cu is contained, the Cu content is set to 0.20% or less. Preferably, the upper limit of the Cu content is 0.15%, 0.10%, or 0.08%.

[Ni: 0 to 0.20%]
As described above, in the steel according to the present embodiment, the lower limit of the Ni content is 0%. However, Ni is an element that improves the corrosion resistance of steel and is also an effective element for improving the toughness of steel. When the above effect is obtained, the Ni content is preferably 0.02% or more. More preferably, the lower limit of the Ni content is 0.03%, 0.04%, or 0.05%.
On the other hand, since Ni is an expensive alloying element, if the Ni content exceeds 0.20%, it is disadvantageous in terms of manufacturing cost. Therefore, even when Ni is contained, the Ni content is set to 0.20% or less. Preferably, the upper limit of the Ni content is 0.15%, 0.12%, 0.10%, or 0.08%.

[Nb: 0 to 0.030%]
As described above, in the steel according to the present embodiment, the lower limit of the Nb content is 0%. However, Nb forms a compound with C in the steel and exists in the steel as Nb-based inclusions such as NbC or TiNb (CN), and suppresses abnormal grain growth of austenite crystal grains as pinning particles during quenching and heating. Has the effect of When the above effect is obtained, the Nb content is preferably 0.002% or more. More preferably, the lower limit of the Nb content is 0.003%, 0.005%, or 0.006%.
On the other hand, when the Nb content exceeds 0.030%, not only the effect is saturated, but also Nb-based inclusions cause precipitation strengthening, so that the manufacturability during continuous casting is impaired. Alternatively, in this case, the Nb-based inclusions cause precipitation strengthening, so that the hardness of the rolled material after hot rolling becomes too high. Therefore, when the Nb content exceeds 0.030%, problems such as a decrease in manufacturability and a significant decrease in the life of the cold forging die become remarkable. Therefore, even when Nb is contained, the Nb content is set to 0.030% or less. Preferably, the upper limit of the Nb content is 0.015%, 0.013%, or 0.010%.

The steel according to the present embodiment contains the above alloy components, and the balance of the chemical components contains Fe and impurities. In the present embodiment, the impurity is a component mixed by raw materials such as ore and scrap and other factors when the steel material is industrially manufactured, and is a level that does not impair the action and effect of the steel according to the present embodiment. Means what is the quantity of.

[N fixed index _IFN : preferably 0 or more]
In order to obtain the effect of the B content described above, it is necessary to suppress the formation of BN by reducing the N (solid solution N) dissolved in the steel. Therefore, it is desirable to reduce the content of N in the steel and to stably fix N in the form of TiN by containing Ti in the steel, thereby reducing the amount of solid solution N. In order to fix N with Ti and obtain the above effect, it is preferable that the N fixed index _IFN defined by the following formula 1 is 0 or more. The lower limit of the N fixed index _{I FN} 0.0005,0.0010,0.0014, or may be 0.0050. However, even if the N fixed index _IFN is not particularly limited, as long as the Ti content and the N content are controlled within the above-mentioned ranges, the steel according to the present embodiment is softened before cold forging. , The generation of coarse grains during quenching can be suppressed.
_IFN = [Ti] -3.5 x [N] ... (Equation 1)
[Ti] and [N] in the above formula 1 indicate the Ti content and the N content in the steel in a unit mass%, and are 0% when these elements are not contained.

[Ti-Nb-based precipitates generated index _{I P:} preferably 0.0100 or less]
As described above, it is preferable to fix N as TiN using Ti to reduce the amount of solid solution N. However, it is not preferable to contain an amount of Ti in excess of the amount required to fix TiN. As described above, Ti also combines with C, S and the like to form fine precipitates, and these fine precipitates may adversely affect the characteristics of the steel according to the present embodiment. In addition, the present inventors have found that Nb also has the same function as Ti.

Specifically, fine TiC, Ti (CN), NbC, TiNb (CN), Ti ₂ C ₂ S and other Ti-Nb-based precipitates present in steel are pinned during quenching and heating. As a stop particle, it has the effect of suppressing the generation of coarse grains by suppressing the abnormal grain growth of austenite crystal grains. However, when a large amount of these Ti-Nb-based precipitate particles are dispersed in the structure after hot rolling, there is a side effect that the hardness of ferrite increases due to precipitation strengthening by fine precipitate particles. .. Therefore, when these Ti-Nb-based precipitate particles are dispersed in an excessively large amount in the steel, the hardness of the rolled material after hot rolling becomes too high, so that the die for cold forging Problems such as a significant decrease in the life of the steel become noticeable. Further, as described above, Ti ₂ C ₂ S causes deterioration of machinability. Therefore, in the steel according to the present embodiment, it is preferable to limit the amount of these Ti-Nb-based precipitate particles.

In order to suppress the hardness after rolling after the hot rolling, it is preferable that the Ti-Nb-based precipitates generated index I _P, which is calculated by the following equation 2 and 0.0100 or less. The Ti-Nb-based precipitates generated index _{I P} 0.0075 or less, less than 0.0050, 0.0045 or less, 0.0040 or less, or 0.0035 may be less. However, even without particularly limiting the Ti-Nb-based precipitates generated index I _P, Ti content within the range described above, Nb content, and N as long as the content is controlled, according to the present embodiment The steel is softened before cold forging and can suppress the generation of coarse grains during quenching.
_IP = 0.3 x [Ti] + 0.15 x [Nb]-[N] ... (Equation 2)
[Ti], [N], and [Nb] in the above formula 2 indicate the Ti content, N content, and Nb content in the steel in a unit mass%, and when these elements are not contained, It is set to 0%.

Next, a preferred method for producing the steel of the present embodiment will be described.
In order to produce the steel of the present embodiment, the steel having the above-mentioned chemical composition is melted in a converter, and if necessary, a secondary refining step is performed, and slabs are produced by continuous casting. By reheating this slab and performing block rolling, a material (steel piece) for rolling a wire rod having a cross section of, for example, 162 mm square (length 162 mm × width 162 mm) is obtained. Next, the steel piece is heated at a temperature of about 1000 to 1280 ° C., and then the wire rod is rolled to obtain a wire rod shape having a diameter of 6 to 20 mm. After that, it is wound into a coil shape by a winding device while hot, and then cooled to room temperature. In this way, the steel of the present embodiment is obtained.

In the steel according to the present embodiment, the amount of Ti-based precipitated particles that cause precipitation strengthening is suppressed. Therefore, in the steel manufacturing method according to the present embodiment, hot spreading is performed in order to suppress the hardness of the steel. It is not necessary to lower the temperature to put a load on the heat spreading equipment, and defects such as cracks and flaws due to the increase in hardness are less likely to occur in the steel. Further, the hardness of the steel according to the present embodiment is suppressed without annealing after hot rolling. Therefore, the steel according to this embodiment is also excellent in terms of high productivity.

According to the steel of the present embodiment, it is possible to achieve both softening before cold forging and suppression of generation of coarse particles during quenching. In addition, the steel of the present embodiment does not crack during casting or rolling, and is excellent in manufacturability.

The hardness of the steel according to the present embodiment is not particularly limited as it can be appropriately adjusted according to the intended use. However, when it is necessary to ensure cold forging property, the hardness of the steel according to this embodiment is preferably Hv180 or less, and more preferably Hv170 or less, or Hv160 or less. .. The lower limit of the hardness of the steel according to the present embodiment is not particularly limited, but it is considered that the hardness is substantially Hv130 or about Hv140 in view of its chemical composition. The hardness of the steel according to the present embodiment can be kept within the above-mentioned preferable range without annealing after hot rolling. Further, the steel according to this embodiment is also excellent in machinability.

Further, the steel according to the present embodiment is heated to a temperature of, for example, 840 ° C to 1100 ° C, held for 30 minutes, and then quenched under the conditions of water cooling or oil cooling, and further, a temperature range of 150 ° C to 450 ° C. When the tempering treatment of heating and holding in the above is performed, the tensile strength thereof can be set to 800 MPa or more. Therefore, the steel according to this embodiment is suitable as a material for parts that require high strength. However, when the steel according to the present embodiment is used as the quenching steel, the heat treatment conditions are not particularly limited and can be appropriately selected depending on the application.

The use of the steel according to the present embodiment is not particularly limited, but it is preferably applied to high-strength mechanical parts manufactured by cold forging and quenching, particularly high-strength bolts. When the steel according to the present embodiment having high cold forging property is used as a material for high-strength mechanical parts, wear of the die during cold forging can be suppressed and the life of the die can be improved. Further, since the cost of an expensive mold can be reduced, it is possible to contribute to the reduction of the manufacturing cost of a high-strength bolt having a tensile strength of 800 MPa or more.

Next, the present invention will be described with reference to examples, but the present invention is not limited to the following examples.

First, the steels having the chemical components shown in Tables 1-1 and 1-2 were melted by a converter and further made into slabs by continuous casting. In Tables 1-1 and 1-2, for elements whose content is below the impurity level, the display of the content is left blank, and the N fixed index _IFN and the Ti-Nb-based precipitate formation index _IP Was regarded as "0% by mass" when calculating. Further, in Table 1-1 and Table 1-2, values outside the specified range of the present invention are underlined. It was confirmed whether or not the slab obtained by this was cracked on the surface of the slab. In the confirmation of cracks on the surface of the slab, after removing the scale on the surface of the slab with a check scarf, the surface of the slab was observed and the crack depth was investigated. When a crack with a depth of 1 mm or more was detected on the surface of the slab, it was determined that there was a crack on the surface of the slab during continuous casting, and it was determined that the manufacturability was "failed". The manufacturability evaluation results are shown in Tables 2-1 and 2-2.

If necessary, the slab was subjected to soaking heat diffusion treatment and block rolling to obtain a material (steel piece) for wire rod rolling having a cross section of 162 mm square (length 162 mm × width 162 mm). Next, the steel piece was heated at a temperature of about 1000 to 1280 ° C., and then the wire rod was rolled to obtain a wire rod (steel for spring) having a diameter of 10 mm.

A test piece for measuring Vickers hardness was cut out from the rolled wire. Specifically, a test piece having a cross section including the central axis of the wire rod was cut out in a direction parallel to the rolling direction. After polishing the cut out cross section, the Vickers hardness of a portion (1/4 part) at a depth of 1/4 of the diameter of the wire from the surface of the wire was measured. The test load is 10 kgf, and the average value measured at 4 points is shown in Table 2-1 and Table 2-2 as "hardness after rolling", and this is used as an index for predicting the life of the mold for cold forging. did. When the hardness of the rolled material exceeds HV180, it is determined that the "cold forging property" is "failed" because the effect of sufficiently improving the life of the cold forging die cannot be obtained. The evaluation results of cold forging property are shown in Tables 2-1 and 2-2.

In addition, in order to simulate the effects of wire drawing and cold forging (cold machining) when machining the wire into a bolt shape, the wire is cold drawn with a surface reduction rate of 70%, and then The austenite structure was frozen as the former austenite grain boundaries of the martensite structure by heating to a temperature of 840 ° C. to 1100 ° C. for 30 minutes and quenching by water cooling. After that, the hardened test piece is tempered in a temperature range of A1 point or less as necessary, and a test piece having a cross section including the center of the drawn material is cut out in a direction parallel to the rolling / drawing direction. It was. After polishing the cross section of the cut-out test piece, the former austenite grain boundaries were exposed by corrosion, and the grain size of the former austenite after quenching and tempering was measured by observing with an optical microscope. The measurement of the former austenite crystal grain size was performed according to JISG0551. The measurement visual field was set to 10 visual fields or more at a magnification of 400 times, and it was determined that coarse grains were generated in the test piece in which even one large crystal grain having an old austenite particle size of 5 or less was present. The limit (minimum) heating temperature at which coarse grains are generated, which is clarified by observing and measuring the old austenite grain size of test pieces heated to various temperatures, is defined as the crystal grain coarsening temperature of the test piece. It was defined and used as an index of grain coarsening resistance. Those having a grain coarsening temperature of 900 ° C. or lower were judged to be "failed" because they were inferior in crystal grain coarsening resistance. The results of grain coarsening temperature measurement are shown in Table 2-1 and Table 2-2.

From Tables 2-1 and 2-2, A1 to A32, which are examples of the present invention, have low hardness of the wire after rolling and can be expected to improve the life of the cold forging die. Therefore, cold forging can be expected. It has excellent properties, and even if it is heated above 900 ° C during quenching and heating after cold working, coarse particles do not occur, and surface cracks of the slab do not occur during continuous casting, so the slab scraping rate It is clear that it is low and therefore excellent in manufacturability. In addition, all of Examples A1 to A32 of the present invention after the heat treatment for measuring the above-mentioned old austenite crystal particle size had a tensile strength of 800 MPa or more.

On the other hand, in the case of the comparative example, any of the cold forging property, the coarse grain prevention property, and the manufacturability is inferior. That is, since the amount of Bi added to B1 to B4 was too large, the hot ductility was lowered and the manufacturability was inferior. B5 to B7 were inferior in coarse grain prevention characteristics because Bi was not added or the amount added was too small. In B8 and B9, the amount of Ti added was too large, or the N content was small relative to the amount of Ti added and the Ti-Nb-based precipitate formation index _IP was exceeded, so the hardness of the wire after rolling was high and it was cold. Inferior in forgeability.

According to the present invention, it is possible to provide a steel capable of achieving both softening during cold forging and suppression of generation of coarse particles during quenching after cold forging. Further, the steel according to the present invention is excellent in manufacturability because it does not crack during casting or rolling and can be manufactured under conditions within a range that does not impose a load on the manufacturing equipment. By applying the steel according to the present invention to cold forged parts, wear of the die during cold forging can be suppressed and the life of the die can be improved. Further, by applying the steel according to the present invention to cold forged parts, the cost of expensive dies can be reduced, which can contribute to the reduction of the manufacturing cost of high-strength bolts having a tensile strength of 800 MPa or more. it can. Further, the steel according to the present invention is also excellent in machinability. Therefore, the present invention has an extremely large industrial contribution.

Claims (3)

The chemical composition is in units of mass%
C: 0.15% to 0.40%,
Mn: 0.10% to 1.50%,
S: 0.002-0.020%,
Ti: 0.005% to 0.050%,
B: 0.0005 to 0.0050%,
Bi: 0.0010% to 0.0100%,
P: 0.020% or less,
N: 0.0100% or less,
Si: 0% or more and less than 0.30%,
Cr: 0 to 1.50%,
Al: 0 to 0.050%,
Mo: 0-0.20%,
Cu: 0-0.20%,
It contains Ni: 0 to 0.20% and Nb: 0 to 0.030%.
The remainder Ri is Do Fe and impurities,
The N fixed index _IFN defined by Equation 1 below is 0 or greater.
Steel following defined by equation 2 Ti-Nb-based precipitates generated index _{I P} is characterized <br/> is at 0.0100 or less.
_{I FN = [Ti] -3.5 ×} [N] ... ( Equation 1)
Here, [Ti] is the Ti content in the unit mass%, and [N] is the N content in the unit mass%.
_{I P = 0.3 × [Ti]} + 0.15 × [Nb] - [N] ... ( Equation 2)
Here, [Ti] is the Ti content in the unit mass%, [Nb] is the Nb content in the unit mass%, and [N] is the N content in the unit mass%. The chemical composition is in units of mass%.
Si: 0.01% or more and less than 0.30%,
Cr: 0.01 to 1.50%, and Al: 0.001 to 0.050%
The steel according to claim 1, wherein the steel contains one kind or two or more kinds selected from the group consisting of. The chemical composition is in units of mass%.
Mo: 0.02 to 0.20%,
Cu: 0.02 to 0.20%,
Ni: 0.02 to 0.20%, and Nb: 0.002 to 0.030%
The steel according to claim 1 or 2, characterized in that it contains one or more selected from the group consisting of.