JP7135484B2 - Carburizing steel and parts - Google Patents

Description

TECHNICAL FIELD The present invention relates to carburizing steel and parts, and more particularly to carburizing steel used in cold working such as cold forging, and parts obtained from the carburizing steel.

In recent years, cold working such as cold forging has been widely used in the manufacture of parts for automobiles and building structures. This is because cold working is superior in terms of dimensional accuracy, yield, and manufacturing cost. Parts of automobiles and building structures are, for example, shafts, bolts, ball joints, inner/outer races, spiders, pinion gears and the like. Although these parts may be used as they are cold-worked, in many cases they are cold-worked to form a desired shape and then quenched and tempered to adjust the final strength.

In recent years, parts have become smaller and lighter, and there is a demand for parts with even higher strength. Conventionally, JIS G 4053 alloy steel for machine structural use, for example, has been used for parts for the above applications that have been quenched and tempered to increase their strength. This alloy steel for machine structure is, for example, chromium steel, chromium molybdenum steel, nickel chromium molybdenum steel, or the like.

These steels contain alloying elements such as molybdenum (Mo) and nickel (Ni) to enhance hardenability and temper softening resistance. When these steel materials are used to manufacture steel bars and wire rods by hot rolling, the hardness of the manufactured steel rods and wire rods is high. Therefore, cold working becomes difficult. Therefore, in order to ensure cold workability, steel bars and wire rods are subjected to multiple times of long-time spheroidizing annealing as softening heat treatment, and then formed into desired shapes by cold forging or the like. After that, it is adjusted to the desired strength and hardness by quenching and tempering.

Furthermore, steel parts are required to have high bending fatigue strength. In addition, steel parts used in automobiles and industrial machinery, such as transmission shafts, are often used in a state where lubricating oil is applied, so delayed fracture is likely to occur due to the ingress of hydrogen derived from the lubricating oil. In order to prevent such delayed fracture, delayed fracture resistance is also required.

Patent Document 1 discloses that high bending fatigue strength can be obtained by reducing the Si content to 0.01 to 0.10% and increasing the Cr content and/or Mo content. there is However, Patent Literature 1 does not consider the effect of Mn on hydrogen embrittlement resistance.

Patent Document 2 discloses that the cold workability of steel is improved by limiting the Si content to 0.20% or less. It is disclosed that if Cr is 0.55 or less, excellent hydrogen embrittlement resistance can be obtained even with high strength.

JP 2013-108144 A JP 2017-122270 A

In Patent Document 2, 0.4% or more of Mn is contained in steel in order to ensure hardenability. Mn has the effect of increasing the hardenability and strength of the steel, but it also has the drawback of lowering the hydrogen embrittlement resistance. Since the lower limit of the Mn content in the steel disclosed in Patent Document 2 is the same as the Mn content in the steel disclosed in Patent Document 1, Patent Document 2 also has hydrogen embrittlement resistance caused by Mn. There is a limit to the prevention of a decrease in

The object of the present invention is to reduce the Mn content in the steel to Mn: less than 0.40%, thereby having excellent hydrogen embrittlement resistance in a hydrogen penetration environment and reducing the Si content in the steel. Therefore, it is an object of the present invention to provide a carburizing steel having high cold workability and high bending fatigue strength after quenching even if the Mn content is reduced as described above.

The present inventors have investigated and investigated the components and structures that affect the cold workability and hydrogen embrittlement resistance of carburizing steel. The present inventors investigated the effect of Mn on the hydrogen embrittlement resistance of 1300 MPa class low alloy boron steel, and found that the critical diffusible hydrogen amount can be improved by reducing Mn.

In addition to the above findings, the present inventors have found that by adjusting the C, Si, Cr, Mo and V contents instead of increasing the Mn content in the steel, the bending fatigue strength and quenching It was found that the wear resistance of the Moreover, the inventors found that the Si content can be reduced by adjusting the Ca and S contents in the steel. Furthermore, the present inventors have found a relationship between the critical diffusion hydrogen amount ratio HR and the ratio of the Mn content to the Cr content, and have found that even if the Mn is less than 0.40%, excellent hydrogen embrittlement resistance is obtained.

[Hardenability and cold workability of carburizing steel]
(a) If the Si content is reduced, the bending fatigue strength increases, but the temper softening resistance decreases, and the surface fatigue strength becomes insufficient. On the other hand, by decreasing the Si content and increasing the Cr content and/or the Mn content, the bending fatigue strength is improved.

(b) However, if the Mn content is too high, a bainite structure tends to form in the carburizing steel after it is normalized. If the bainite structure is generated, the hardness of the steel increases, so the machinability of the steel decreases. Even if Mn is not contained, if the Cr content is too high, a bainite structure is likely to be generated as well.

(c) Therefore, the Cr content and the Mn content must be adjusted so as to improve the hardenability, increase the bending fatigue strength, and improve the cold workability. Specifically, the contents of C, Si and Cr are adjusted so that F1 defined by the following formula falls within the range of 0.48 to 0.58.
F1=C+Si/7+Mn/5+Cr/9
Here, the element symbol in the above formula for F1 is substituted with the content (% by mass) of the corresponding element, and when the corresponding element is an impurity level, 0 is substituted for the corresponding element symbol in the above formula for F1. be done.

Mo and V also have the effect of improving the hardenability and increasing the bending fatigue strength and the wear resistance of the hardened portion. When these elements are contained, the contents of C, Si, Cr, Mo and V are adjusted so that F1 defined by the following formula is in the range of 0.48 to 0.58.
0.48≦C+Si/7+Mn/5+Cr/9+2×Mo/5+V≦0.58 (1′)
Here, the element symbol in the above formula for F1 is substituted with the content (% by mass) of the corresponding element, and when the corresponding element is an impurity level, 0 is substituted for the corresponding element symbol in the above formula for F1. be done.

[Regarding sulfide control in carburizing steel]
The cold workability of carburizing steel is further affected by sulfide-based inclusions (hereinafter referred to as sulfides) represented by MnS. Specifically, if the sulfides contained in the vicinity of the surface of the carburizing steel are fine and have a nearly spherical shape, the number of coarse sulfides that cause forging cracks can be reduced. The cold workability of

Ca dissolves in sulfides and promotes spheroidization of sulfides. However, if the Ca content is too high with respect to S, Ca that has not dissolved in the sulfide forms coarse oxides, degrading the hydrogen embrittlement resistance of the steel. By setting the ratio of the Ca content to the S content in the steel within an appropriate range, it is possible to control the morphology of sulfides, improve cold workability, and maintain hydrogen embrittlement resistance. Specifically, when the chemical composition of the carburizing steel satisfies the formula (2), excellent cold workability is obtained while maintaining hydrogen embrittlement resistance, making it possible to form more complicated parts. .
0.03≦Ca/S≦0.15 (2)
Here, the content (% by mass) of the corresponding element is substituted for each element symbol in formula (2).

Define F2=Ca/S. F2 is an index of cold workability and hydrogen embrittlement resistance. Ca forms a solid solution in sulfides, finely disperses the sulfides, and spheroidizes the sulfides. If F2 is too low, that is, if the Ca content relative to the S content is too low, it is difficult for Ca to form a solid solution in the sulfide, and the sulfide is difficult to spheroidize. Therefore, cold workability is deteriorated. On the other hand, if F2 is too large, the Ca content is too high relative to the S content. In this case, Ca not dissolved in the sulfide forms coarse oxides, and the hydrogen embrittlement resistance of cold-worked parts such as cold-forged parts deteriorates. If F2 satisfies the formula (2), excellent cold workability and resistance to hydrogen embrittlement can be obtained.

[About hydrogen embrittlement resistance of carburizing steel]
In addition to limiting the amount of Mn, by satisfying formula (3) on the premise of formula (2) described above, excellent hydrogen embrittlement resistance can be obtained even with high strength.
Mn/Cr≦0.20 (3)
Here, the content (% by mass) of the corresponding element is substituted for each element symbol in formula (3). Define F3=Mn/Cr.

FIG. 1 is a diagram showing the relationship between the critical diffusion hydrogen amount ratio HR and F3 (=Mn/Cr). FIG. 1 was obtained from the examples described later.

The vertical axis in FIG. 1 indicates the critical diffusible hydrogen amount ratio HR. The critical diffusible hydrogen amount ratio HR is defined by the following formula (A) based on the critical diffusible hydrogen amount Href of steel M having a chemical composition corresponding to SCM435 of JIS G4053 (2016). The critical diffusible hydrogen amount ratio HR is an index of hydrogen embrittlement resistance.
HR=Hc/Href (A)
Hc is the critical amount of diffusion hydrogen for each test number in Examples described later. The critical diffusion hydrogen amount Hc means the maximum amount of hydrogen in a test piece that did not break when a constant load test was performed on test pieces to which various concentrations of hydrogen were introduced in each test number.

Referring to FIG. 1, as F3 decreases, that is, as the ratio of Mn content to Cr content decreases, the critical diffusible hydrogen amount ratio HR remarkably increases. When F3 is lower than 0.20, HR is higher than 1.00 and excellent hydrogen embrittlement resistance is obtained.

[Metal structure of carburizing steel]
In addition to the above matters, cold workability also depends on the metallographic structure of the steel. When the metal structure is mainly ferrite and the area ratio of proeutectoid ferrite is high, the cold workability is excellent. Specifically, if the area ratio of ferrite is 60% or more in the metal structure of the carburizing steel, and the balance is pearlite and bainite, the cold workability is enhanced. In this case, the parts can be molded even if the spheroidizing annealing treatment is omitted or shortened.

The present invention was made based on the above findings, and the gist thereof is as follows.
(1) in mass %,
C: 0.15-0.25%, Si: less than 0.20, Mn: less than 0.20-0.40%, Cr: 1.60-2.00%, Al: 0.005-0.060 %, N: 0.0015 to 0.0080%, Ca: 0.0003 to 0.0050%, and S: 0.010 to 0.020%,
P: 0.020% or less, O: limited to 0.0020% or less,
the balance consists of Fe and impurities,
Having a chemical composition satisfying the following formulas (1) to (3),
Carburizing steel characterized by having a ferrite area ratio of 60% or more.
0.48≦C+Si/7+Mn/5+Cr/9≦0.58 (1)
0.03≦Ca/S≦0.15 (2)
Mn/Cr≦0.20 (3)
Here, the content (% by mass) of the corresponding element is substituted for each element symbol in formulas (1) to (3), and when Si is at the impurity level, 0 is assigned to Si in formula (1). assigned.
(2) Furthermore, in % by mass,
(1) characterized by containing one or more selected from the group consisting of Sb: 0.100% or less, Sn: 0.100% or less, and Bi: 0.100% or less carburizing steel.
(3) Furthermore, in % by mass,
Ti: 0.010 to 0.050%, B: 0.0003 to 0.0040%, Cu: 0.50% or less, Ni: 0.30% or less, Mo: 0.05% or less, V: 0.05% or less. 050% or less, and Nb: 0.050% or less, containing one or more selected from the group consisting of
The carburizing steel according to (1) or (2), which satisfies the following formula (1′).
0.48≦C+Si/7+Mn/5+Cr/9+2×Mo/5+V≦0.58 (1′)
Here, the content (% by mass) of the corresponding element is substituted for each element symbol in formula (1′), and when the corresponding element is an impurity level, the corresponding element symbol in formula (1′) is 0 is substituted.
(4) A component comprising a core portion having the composition according to any one of (1) to (3) and a surface layer portion having a carbon content higher than that of the core portion.

The carburizing steel according to the present invention has excellent bending fatigue strength, excellent cold workability, and excellent resistance to hydrogen embrittlement in a hydrogen penetration environment.

FIG. 2 is a diagram showing the relationship between the critical diffusible hydrogen amount ratio and Mn/Cr in carburizing steel. FIG. 2 is a side view of a roller pitching small roller test piece produced in an example. FIG. 3 is a side view of a notched Ono-type rotary bending fatigue test piece prepared in an example. FIG. 4 is a diagram showing carburizing and quenching conditions in Examples. FIG. 4 is a side view of a test specimen with an annular V-notch;

The carburizing steel according to this embodiment will be described in detail below. "%" for elements means % by weight unless otherwise specified.

[Chemical composition]
The chemical composition of the carburizing steel of this embodiment contains the following elements.

[C: 0.15 to 0.25%]
Carbon (C) increases the strength of steel. Specifically, C enhances the strength of the core of the steel part that has been carburized and quenched or carbonitrided and quenched. However, if C is contained excessively, the area ratio of pearlite increases, the area ratio of ferrite decreases, and the hardness of the steel increases, resulting in poor cold workability. Therefore, the C content is 0.15-0.25%. A preferable upper limit of the C content is 0.23%. A preferable lower limit of the C content is 0.13%.

[Si: less than 0.20%]
Silicon (Si) strengthens ferrite through solid solution strengthening. When it is desired to lower the tensile strength of steel, the lower the Si content is, the better. However, it may be contained when increasing the temper hardness of cold-worked parts. On the other hand, if the Si content is too high, the strength of the steel will be too high and the cold workability of the steel will be reduced. In this case, a softening heat treatment is required for a long period of time, increasing the cost. Therefore, the Si content is less than 0.20%. A preferable upper limit of the Si content is 0.18%, more preferably 0.16%.

[Mn: less than 0.20 to 0.40%]
Manganese (Mn) increases hardenability and strength of steel. If the Mn content is too low, this effect cannot be obtained. On the other hand, if the Mn content is too high, the starting temperature of ferrite transformation is lowered during cooling after finish rolling, and the area ratio of ferrite is lowered. Moreover, Mn combines with Si to form inclusions (MnO—SiO ₂ ). These inclusions are soft and are stretched and broken during cold rolling to be finer, so that the density of MnO—SiO ₂ is reduced and the resistance to hydrogen embrittlement is increased. If the Mn content is too low, this effect cannot be obtained. On the other hand, if the Mn content is too high, it segregates at the grain boundaries and promotes intergranular fracture, resulting in rather low hydrogen embrittlement resistance. Therefore, the Mn content is less than 0.20-0.40%. A preferred lower limit for the Mn content is 0.22%, more preferably 0.25%. A preferable upper limit of the Mn content is 0.38%, more preferably 0.35%.

[Cr: 1.60 to 2.00%]
Chromium (Cr) enhances the hardenability and temper softening resistance of steel, and enhances the bending fatigue strength and surface fatigue strength of steel. However, if Cr is contained excessively, a bainite structure tends to form in the steel after normalizing treatment. If the bainite structure is generated, the cold workability of the steel is deteriorated. Therefore, the Cr content is 1.60-2.00%. A preferable upper limit of the Cr content is 1.90%. A preferable lower limit of the Cr content is 1.80%.

[Al: 0.005 to 0.060%]
Aluminum (Al) deoxidizes steel. This effect cannot be obtained if the Al content is less than 0.005%. Al further combines with N to form AlN, which suppresses coarsening of austenite grains during carburizing heating. If coarsening of austenite grains is suppressed, the bending fatigue strength of steel increases. On the other hand, if the Al content exceeds 0.060%, coarse oxide-based inclusions are formed and the bending fatigue strength of the steel is lowered. Therefore, the Al content is 0.005-0.060%. A preferable lower limit of the Al content is 0.010%. A preferred upper limit for the Al content is 0.055%.

[N: 0.0015 to 0.0080%]
Nitrogen (N) combines with Ti in steel to form nitrides and refine austenite grains during hot rolling. This effect cannot be obtained if the N content is less than 0.0015%. On the other hand, if the N content exceeds 0.0080%, the effect is saturated. Furthermore, N combines with B to form nitrides, which reduces the solid solution B amount. In this case, the hardenability of steel deteriorates. Therefore, the N content is 0.0015-0.0080%. A preferable lower limit of the N content is 0.0020%. A preferable upper limit of the N content is 0.0070%.

[Ca: 0.0003 to 0.0050%]
Calcium (Ca) forms a solid solution in sulfide to make the sulfide fine and spherical. Thereby, Ca enhances the cold workability of steel. If the Ca content is too low, this effect cannot be obtained. On the other hand, if the Ca content is too high, coarse oxides are formed. Coarse oxides reduce the cold forgeability of steel. Therefore, the Ca content is 0.0003-0.0050%. A preferable lower limit of the Ca content is 0.005%, more preferably 0.0007%. A preferable upper limit of the Ca content is 0.0035%, more preferably 0.0030%, and still more preferably 0.0025%.

[S: 0.010 to 0.020%]
Sulfur (S), which is an impurity, combines with Mn to form MnS and enhances the machinability of steel. However, if S is contained excessively, coarse MnS tends to be generated, and the fatigue strength (bending fatigue strength and surface fatigue strength) of steel is lowered. Sulfur (S) forms sulfides to reduce the cold workability of steel and further reduces the hydrogen embrittlement resistance. Therefore, the S content is 0.010% or more and 0.020% or less.

[P: 0.020% or less]
Phosphorus (P) is an impurity. P segregates at grain boundaries to embrittle the grain boundaries, and further lowers hydrogen embrittlement resistance. Therefore, the smaller the P content, the better. The P content is 0.025% or less. A preferable P content is 0.020% or less.

[O: 0.0020% or less]
Oxygen (O) is an impurity. O combines with Al to form hard oxide inclusions and lowers the bending fatigue strength of steel. If the O content exceeds 0.0020%, a large amount of oxide is produced and MnS is coarsened. Therefore, the O content is 0.0020% or less. A preferred upper limit of the O content is 0.0018%. It is preferable that the O content is as low as possible.

[0.48≦C+Si/7+Mn/5+Cr/9≦0.58]
As described above, the chemical composition of the carburizing steel of the present embodiment further satisfies the formula (1) in order to ensure bending fatigue strength and cold workability. In addition, the value of the F1 formula defined in the following formula (1) is preferably 0.51 or more and 0.55 or less.
0.48≦F1 (=C+Si/7+Mn/5+Cr/9)≦0.58 (1)

Further, the chemical composition of the carburizing steel of the present embodiment can contain at least one of Mo and V as an optional additive element, in which case the formula (1′) is satisfied in addition to the formula (1).
0.48≦C+Si/7+Mn/5+Cr/9+2×Mo/5+V≦0.58 (1′)

[0.03≦Ca/S≦0.15]
As described above, the chemical composition of the carburizing steel of the present embodiment further satisfies formula (2) in order to obtain excellent cold workability while maintaining hydrogen embrittlement resistance.
0.03≦F2 (=Ca/S)≦0.15 (2)

F2 is an index of hydrogen embrittlement resistance. Ca forms a solid solution in sulfides, finely disperses the sulfides, and spheroidizes the sulfides. If F2 is too low, that is, if the Ca content relative to the S content is too low, Ca is less likely to form a solid solution in the sulfide, and the sulfide is less likely to be spheroidized. Therefore, cold workability is deteriorated. On the other hand, if F2 is too large, the Ca content is too high relative to the S content. In this case, Ca that did not form a solid solution in the sulfide forms coarse oxides, degrading hydrogen embrittlement resistance. Therefore, F2 is between 0.03 and 0.15. A preferred lower limit for F2 is 0.04. A preferred upper limit for F2 is 0.12, more preferably 0.10.

[Mn/Cr≦0.20]
The chemical composition of the carburizing steel described above further satisfies equation (3).
F3 (=Mn/Cr)≦0.20 (3)
Mn and Cr improve hardenability. Furthermore, as mentioned above, the proper ratio of Mn to Cr provides excellent resistance to hydrogen embrittlement. When F3 is 0.20 or less, the critical diffusible hydrogen amount ratio HR becomes higher than 1.0 as shown in FIG. 1, and excellent hydrogen embrittlement resistance is obtained.

[Optional element]
The carburizing steel according to the present embodiment may optionally contain at least one of the following elements instead of part of Fe.

[Sb: 0.100% or less; Sn: 0.100% or less; Bi: 0.100% or less]
In the present invention, in addition to the above essential components, one or more of antimony (Sb), tin (Sn), and bismuth (Bi) may be added within the range of 0.100% or less. . These elements exhibit the effect of suppressing hydrogen penetration by adding 0.001% or more. Although the lower limit for exhibiting the effect of suppressing hydrogen penetration is set to 0.001% or more for each, the preferable lower limit for exhibiting the effect sufficiently is set to 0.005% or more for each. Furthermore, each is preferably 0.010% or more. On the other hand, if the upper limit of each content exceeds 0.100%, the cold workability of the steel deteriorates, making continuous casting difficult. Also, preferably, the total concentration of Sb, Sn, and Bi should be 0.030 to 0.100%.

[Ti: 0.010 to 0.050%]
Titanium (Ti) combines with N in steel to form nitrides (TiN). The formation of TiN suppresses the formation of BN and increases the amount of dissolved B. As a result, the hardenability of the steel is enhanced. Ti further combines with C to form carbides (TiC) to refine grains. This enhances the hydrogen embrittlement resistance of the steel. These effects cannot be obtained if the Ti content is less than 0.010%. On the other hand, if the Ti content exceeds 0.050%, a large amount of coarse TiN is produced. In this case, the fatigue strength and hydrogen embrittlement resistance of the steel are lowered. Therefore, the Ti content is 0.010-0.050%. A preferred lower limit for the Ti content is 0.015%. A preferred upper limit for the Ti content is 0.045%.

[B: 0.0003 to 0.0040%]
Boron (B) enhances the hardenability of steel. B further suppresses the grain boundary segregation of P and enhances the hydrogen embrittlement resistance of steel. If the B content is less than 0.0003%, these effects cannot be obtained. On the other hand, if the B content exceeds 0.0040%, the effect of improving hardenability is saturated. In addition, coarse BN is produced to deteriorate cold workability. Therefore, the B content is 0.0003-0.0040%. A preferable lower limit of the B content is 0.0005%. A preferable upper limit of the B content is 0.0025%.

[Cu: 0.50% or less]
Copper (Cu) enhances the hardenability of steel and enhances the bending fatigue strength of steel. However, when Cu is contained excessively, the hardenability becomes too high, bainite is formed after finish rolling, and the cold workability of the steel is significantly deteriorated. Therefore, the Cu content is 0-0.50%. A preferable lower limit of the Cu content is 0.015%, more preferably 0.020%. A preferable upper limit of the Cu content is 0.30%, more preferably 0.20%.

[Ni: 0.30% or less]
Nickel (Ni) enhances the hardenability of steel and enhances bending fatigue strength. However, if the Ni content is too high, a bainite structure tends to form after finish rolling. Furthermore, the effect of increasing the bending fatigue strength by improving the hardenability is saturated. Therefore, the Ni content is 0-0.30%. A preferable lower limit of the Ni content is 0.01%, more preferably 0.03%. A preferable upper limit of the Ni content is 0.20%, more preferably 0.10%.

Mo: 0.05% or less Molybdenum (Mo) is an optional element. That is, Mo does not have to be contained. Mo increases the hardenability and temper softening resistance of steel, and increases the bending fatigue strength and surface fatigue strength of steel. However, if Mo is contained excessively, a bainite structure tends to form in the steel after normalizing treatment, and the cold workability of the steel is deteriorated. Therefore, Mo content is 0.05% or less. The lower limit of Mo content is preferably 0.005%, more preferably 0.008%. A preferred upper limit for the Mo content is 0.03%, more preferably 0.02%.

V: 0.050% or less Vanadium (V) enhances the hardenability of steel. V further combines with C and N to form carbides, nitrides or carbonitrides to refine grains. However, if the V content is too high, carbides and carbonitrides increase the strength of the steel and reduce the cold workability of the steel. Therefore, the V content is 0-0.050%. A preferable lower limit of the V content is 0.003%, more preferably 0.004%. A preferable upper limit of the V content is 0.030%, more preferably 0.020%.

Nb: 0.050% or less Niobium (Nb) combines with C and N to form carbides, nitrides or carbonitrides (called carbonitrides and the like). Nb carbonitride or the like refines austenite grains during hot rolling due to the pinning effect, suppresses the formation of bainite during the cooling process after finish rolling, and increases the area ratio of pro-eutectoid ferrite. Nb carbonitride and the like further suppress coarsening of austenite grains during carburizing heating. However, if Nb is contained excessively, Nb carbides, Nb nitrides, and Nb carbonitrides coarsen, and coarsening of austenite grains cannot be suppressed. Therefore, the Nb content is 0.050% or less. If even a small amount of Nb is contained, the above effect can be obtained to some extent. A preferable lower limit of the Nb content is 0.010%. A preferable lower limit of the Nb content for obtaining the above effect more effectively is 0.005%. A preferable upper limit of the Nb content is 0.040%, more preferably 0.030%.

[About metal structure]
The metal structure of the carburizing steel of this embodiment is mainly composed of ferrite. More specifically, in the metallographic structure of carburizing steel, the area ratio of ferrite is 60% or more, and the balance is pearlite and bainite. The ferrite in the above "area ratio of ferrite" is proeutectoid ferrite and does not include ferrite between lamellar cementites of pearlite.

Proeutectoid ferrite and pearlite are softer than bainite and have excellent cold workability. Furthermore, pro-eutectoid ferrite is superior to pearlite in cold workability. If the area ratio of pro-eutectoid ferrite is 60% or more and less than 100%, excellent cold workability can be obtained.

The metallographic structure is measured by the following method. Samples are taken from the inside of the carburizing steel (D/4 parts for steel bars or wire rods, t/4 parts for plate materials or steel pipes). Among the surfaces of the collected samples, the surface perpendicular to the rolling direction of the carburizing steel is used as the observation surface. After polishing the observation surface, it is etched with 3% nitric acid alcohol (nital etchant) for 5 to 15 seconds. Observe the etched viewing surface with a 500x optical microscope to generate photographic images of any 5 fields of view.

In each field of view, each phase such as proeutectoid ferrite, pearlite, and bainite has a different contrast for each phase. Therefore, each phase is identified based on the contrast. Among the identified phases, the ratio of the total area of pro-eutectoid ferrite in all fields of view to the total area of all fields of view is defined as the pro-eutectoid ferrite area ratio (%).

[Production method]
As an example of the method for producing the carburizing steel of the present invention, a method for producing a steel bar or wire (bar and wire) will be described. The method for producing carburizing steel according to the present embodiment includes a step of producing a billet (blooming rolling step) and a step of rolling the produced billet into bars (finish rolling step). Each step will be described in detail below.

[Blooming rolling process]
First, a material having the above chemical composition is prepared. For example, the material is manufactured in the following way. Molten steel having the chemical composition described above is produced using a converter or an electric furnace, and cast slabs or ingots are produced by continuous casting or ingot casting.

After heating the prepared material (slab, ingot), it is bloomed, and if necessary, further rolled by a continuous rolling mill after blooming to produce a billet.

[Finish rolling process]
The billet produced by the blooming process is further subjected to hot rolling to produce carburizing steel such as bars and wires. The rolling here is, for example, continuous rolling using a continuous rolling mill in which horizontal roll stands and vertical roll stands are alternately arranged in a row.

First, the billet is put into a heating furnace and heated. A preferable heating temperature is 1000 to 1100° C. or less. If the heating temperature during product rolling is too high, the fine carbides and carbonitrides precipitated after the blooming step will dissolve again. In this case, carbides and carbonitrides precipitate coherently during ferrite transformation during cooling after product rolling. Precipitated carbonitrides and carbides increase the strength of the steel after product rolling and reduce cold workability. Note that Ti carbides and Ti carbonitrides are difficult to form a solid solution by heating the billet. Therefore, the strength after rolling of the product is hardly affected, and the cold workability can be maintained.

Using the heated billet, hot forging is performed in a row of finishing rolling mills so that the forging ratio is 5.5 or more to form a bar wire having a predetermined diameter. A finishing mill train includes a plurality of stands arranged in a row. Each stand includes multiple rolls arranged around the pass line.

The manufacturing conditions for finish rolling using a row of finishing rolling mills are as follows.
Finishing temperature: 750-850°C
The finishing temperature means the billet temperature (° C.) at the delivery side of the stand that finally reduces the billet (hereinafter referred to as finishing stand) among the plurality of stands in the row of finishing rolling mills. Finishing temperatures are measured using an infrared thermometer located on the exit side of the finishing stand.

If the finishing temperature is less than 750° C., ferrite transformation starts from unrecrystallized austenite grains, and the matrix structure after cooling becomes too fine. In this case, the strength of the steel increases and the cold workability decreases. On the other hand, if the finishing temperature exceeds 850° C., the austenite grains after recrystallization become coarse, and the starting temperature of ferrite transformation becomes low. Therefore, the area ratio of the pro-eutectoid ferrite after cooling becomes small, and the cold workability deteriorates.

If the finishing temperature is 750 to 850° C., the area ratio of proeutectoid ferrite in the metal structure becomes 60% or more on condition that the cooling conditions described later are satisfied.

Cooling rate: less than 5.0°C/sec The cooling rate of steel after finish rolling affects the matrix structure. If the cooling rate is 5.0° C./second or more, hard bainite or the like is likely to form in the steel, and the area ratio of pro-eutectoid ferrite is less than 40%. If the cooling rate is less than 5.0° C./sec, the area ratio of proeutectoid ferrite in the metallographic structure of the steel after cooling is 40% or more.

The lower limit of the cooling rate is not particularly limited. However, considering actual production operations, the lower limit of the cooling rate is, for example, 0.2° C./sec.

The steel for carburizing of the present embodiment (bar and wire in this example) is manufactured by the manufacturing process described above. That is, the carburizing steel of this embodiment is a so-called as-rolled material (as-rolled material). In this case, the chemical composition that satisfies the formulas (1) to (3) improves the bending fatigue strength of the carburizing steel. Furthermore, by satisfying the manufacturing conditions (heating temperature, finishing temperature and cooling rate) in the finish rolling, the area ratio of proeutectoid ferrite in the metal structure of the steel becomes 60% or more. Therefore, excellent fatigue strength, cold workability and hydrogen embrittlement resistance can be obtained.

In the manufacturing method described above, a rod wire is manufactured. However, like the bar and wire, it is also possible to manufacture sheet materials and steel pipes of carburizing steel by carrying out blooming and finish rolling processes.

[Production of machined parts using carburizing steel of the present embodiment]
The steel parts are manufactured by cold working the steel bars and wire rods manufactured by the manufacturing process described above. In the conventional method for manufacturing cold-worked parts, softening heat treatment is performed multiple times before wire drawing and before cold working for the purpose of softening a steel material having too high strength. However, since the steel for cold-worked parts of the present embodiment is excellent in cold-workability, such softening heat treatment can be omitted or simplified. As a result, it is possible to suppress an increase in manufacturing cost due to the softening heat treatment.

The steel part has a core part whose metal structure and carbon content do not change before and after the carburizing and quenching process or the carbonitriding and quenching process, and a surface layer part whose carbon content increases after the process. The core portion is a region between the 1/4 depth position of the plate thickness or wall thickness from the surface of the part and the center of the plate thickness or wall thickness from the surface, and the surface layer is 0.5 to 0.5 from the surface of the part. 1.0 mm depth region. Moreover, the metal structure of the surface layer is composed of martensite.

Steel No. having the chemical compositions in Tables 1-1 and 1-2. Molten steel of A to S was produced. Steel no. A billet having a cross section of 162 mm×162 mm was produced by continuous casting and blooming using each of the molten steels A to S (blooming rolling step). At this time, the rolling ratio in the blooming step, which is the value obtained by dividing the cross-sectional area of the slab by the cross-sectional area of the steel slab, was 5.5. Chemical composition no. The billets A to S were once cooled to room temperature, and the presence or absence of surface cracks in the slab was visually determined. The results are shown in Tables 2-1 and 2-2.

Next, chemical composition no. Finish rolling (hot rolling) was performed on the A to S billets to produce carburizing bars and wires having a diameter of 14 mm (finish hot rolling process). The heating temperature (°C) of the billet, the finishing temperature (°C) in finish rolling, and the cooling rate (°C/sec) after finish rolling were as shown in Tables 2-1 and 2-2, respectively. The processing speed was 5 to 15/sec in any test number.

[Microstructure Observation Test]
A steel wire rod for carburizing was cut in a direction perpendicular to the rolling direction to obtain a 10 mm sample. The cut surface of the sample was filled with resin so that the surface to be inspected was mirror-polished. After that, the microstructure was observed by the method described above, and the area ratio (%) of pro-eutectoid ferrite was determined. "Ferrite area ratio (%)" in the "Microstructure" column of Tables 2-1 and 2-2 indicates the area ratio (%) of pro-eutectoid ferrite.

[Measurement test of spheroidization rate of sulfide in wire]
A steel wire rod for carburizing was cut in a direction perpendicular to the rolling direction to obtain a 10 mm sample. The sample was longitudinally cross-sectioned, and the sample was embedded in a resin so that the surface including the rolling direction of the wire (longitudinal cross section of the sample) was the test surface, and mirror-polished.

Using SEM-EDS, multiple fields of view were observed at a depth of D/8 from the surface of the wire (D is the diameter of the wire) from the surface of the test surface so that the total area of the field of view was ^{5 mm 2 .} The average area and average aspect ratio of the sulfides in the field of view were obtained. Here, the aspect ratio is the ratio (major axis/minor axis) of the length (major axis) and width (minor axis) of the sulfide. Specifically, 100 arbitrary observation regions within the test surface were selected at a magnification of 500 times. The total area of the observation area was 5 mm ² as described above. A backscattered electron image was created in each observation area, and all sulfides in the field were identified based on the contrast determined by the backscattered electron image.

Among the identified sulfides, the areas and aspect ratios of the sulfides that could be observed entirely within the field of view were measured, and the equivalent circle diameter (the diameter when the area was converted to a circle) was determined from the area. The total area of sulfides identified as sulfides and having an equivalent circle diameter of 1 μm or more in 100 observation regions was defined as A _all (μm ² ). Furthermore, the total area of sulfides having an aspect ratio of 3 or less (hereinafter referred to as spherical sulfides) among sulfides having an equivalent circle diameter of 1 μm or more was defined as A (μm ² ). Using the obtained A _all and A, the spheroidization rate SPH (%) was calculated based on the formula (6).
SPH=A/A _all (6)

The SPH obtained are shown in Tables 2-1 and 2-2. When the SPH was 0.40 or more, it was judged that the sulfide was sufficiently spheroidized.

[Cold workability evaluation test]
Ten cylindrical test pieces were produced by machining from the central part of the wire of each test number. Each cylindrical test piece had a diameter of 10 mm and a length of 15 mm, and the longitudinal direction of the test piece was the rolling direction of the wire rod.

A cold compression test was performed with a 500 ton hydraulic press. At this time, cold compression was performed using 10 test pieces and increasing the compression rate stepwise. Specifically, first, 10 test pieces were cold-pressed at an initial rolling reduction. After the cold compression, it was visually confirmed whether or not cracks occurred in each test piece. After removing the test pieces in which cracks were confirmed, the remaining test pieces (that is, the cylindrical test pieces in which no cracks were observed) were subjected to cold compression again at a higher compression ratio. After the test, the presence or absence of cracks was checked. After removing the test pieces in which cracks were confirmed, the remaining test pieces were again subjected to cold compression at a higher compression ratio. The above steps were repeated until 5 of the 10 test pieces were found to have cracks. The compression ratio when cracks were confirmed in 5 cylindrical test pieces out of 10 test pieces was defined as the "critical compression ratio" (%). In addition, when the number of round bar test pieces in which cracks were confirmed after the cold compression was performed at a compressibility of 80% was 5 or less, the limit compressibility of the steel was defined as "80%". When the critical compressibility was 70% or more, it was judged that the cold workability was excellent.

[Bending fatigue strength test piece]
The steel bars of each test number were heated at 1200° C. for 30 minutes. Next, hot forging was performed at a finishing temperature of 950° C. or higher to produce a round bar with a diameter of 35 mm. A round bar with a diameter of 35 mm was machined, and the roller pitching small roller test piece shown in FIG. 2 and the notched Ono type rotating bending fatigue test piece shown in FIG. The unit is mm).

Carburizing and quenching were performed on each of the prepared test pieces using a gas carburizing furnace under the conditions shown in FIG. After quenching, tempering was performed at 170° C. for 1.5 hours. For the purpose of removing heat treatment distortion, finishing of the grip portion was performed on the small roller test piece for the roller pitching test and the Ono type rotating bending fatigue test piece.

[Bending fatigue strength test]
The bending fatigue strength was determined by Ono's rotary bending fatigue test. The number of tests in the Ono-type rotary bending fatigue test was 8 for each test number. The number of revolutions during the test was set to 3000 rpm, and the test was carried out by a normal method otherwise. Among those that did not break up to 1.0×10 ⁴ repetitions, the highest stress was taken as the middle cycle rotating bending fatigue strength.

Tables 2-1 and 2-2 show the bending fatigue strength at medium cycles. For the medium-cycle bending fatigue strength, the medium-cycle bending fatigue strength of Test No. 19 was used as the reference value (1.00%). The middle cycle bending fatigue strength of each test number is shown as a ratio to the reference value. It was judged that excellent bending fatigue strength was obtained when the bending fatigue strength at medium cycle was 1.10 or more.

[Hydrogen embrittlement resistance evaluation test]
Quenching and tempering were performed on the wire of each test number to adjust the tensile strength of the wire to about 1200 MPa.

However, when the tempering temperature for obtaining a tensile strength of about 1200 MPa was less than 435°C, it was determined that the strength was insufficient, the hydrogen embrittlement resistance was not evaluated, and it was determined to be out of the scope of the present invention.

A plurality of annular V-notch test pieces shown in FIG. 5 were produced for each test number wire by machining the wire having the adjusted tensile strength. Numerical values with no units shown in FIG. 5 indicate the dimensions (unit: mm) of the corresponding portions of the test piece. The "φ value" in the figure indicates the diameter (mm) of the designated portion. "60°" indicates that the V notch angle is 60°. "0.175R" indicates that the V-notch bottom radius is 0.175 mm.

Various concentrations of hydrogen were introduced to the specimens of each steel using the electrolytic charging method. The electrolytic charging method was carried out as follows. A test piece was immersed in an ammonium thiocyanate aqueous solution. While the test piece was immersed, an anodic potential was generated on the surface of the test piece and hydrogen was taken into the test piece.

After hydrogen was introduced into the test piece, a galvanized film was formed on the surface of the test piece to prevent hydrogen from escaping from the test piece. Subsequently, a constant load test was performed in which a constant load was applied so that a tensile stress with a nominal stress of 1080 MPa (90% of the tensile strength) was applied to the V-notch cross section of the test piece. A temperature programmed analysis method using a gas chromatograph was performed on the test piece that was broken during the test and the test piece that was not broken, and the amount of hydrogen in the test piece was measured. After the measurement, the maximum amount of hydrogen in the test pieces that did not break was defined as the critical diffusible hydrogen amount Hc in each test number.

Furthermore, the limit diffusible hydrogen amount of steel M having a chemical composition corresponding to SCM435 of JIS G4053 (2016) was used as the reference (Href) for the limit diffusible hydrogen amount ratio HR. Using the critical diffusible hydrogen amount Href as a reference, the critical diffusible hydrogen amount ratio HR was obtained using the formula (A).
HR=Hc/Href (A)
It was judged that if the ratio HR was higher than 1.00, the hydrogen embrittlement resistance was excellent.

[Test results]
Test results are shown in Tables 2-1 and 2-2.

The chemical compositions of the carburizing steels of test numbers 1 to 13 were appropriate and satisfied the formulas (1) to (3). Furthermore, the total area ratio of proeutectoid ferrite and pearlite in the metal structure inside the wire was 95% or more, and the area ratio of ferrite was 60% or more. In addition, the medium cycle fatigue strength ratio of these test numbers was 1.10 or more, and the critical compressibility was all 75% or more, indicating excellent cold workability. Furthermore, the ratio HR was 1.05 or more, indicating excellent hydrogen embrittlement resistance.

On the other hand, test numbers 14 to 16 contained more than 0.10% of any of Sb, Sn and Bi, and cracks occurred during rolling. Since Test Nos. 14 to 16 are in an unusable state, the cold workability evaluation test, the measurement of the spheroidization rate SPH (%), and the measurement of the medium cycle fatigue strength ratio were not performed.

Test numbers 17 and 18 are examples that did not satisfy the condition of formula (1). Test No. 14 is inferior in medium cycle fatigue strength ratio, and Test No. 15 is inferior in cold workability.

Test numbers 19 and 20 are examples that did not satisfy the condition of formula (2). Test No. 16 is inferior in medium cycle fatigue strength ratio, and Test No. 17 is inferior in hydrogen embrittlement resistance.

Test No. 21 is an example that does not satisfy the condition of formula (3), and is inferior in hydrogen embrittlement resistance.

The wire of Test No. 22 does not contain Ca. Therefore, it is inferior in hydrogen embrittlement resistance.

Test numbers 23 to 26 are examples in which the area ratio of ferrite is less than 60% even though the formulas (1) to (3) are satisfied. Both are inferior in cold workability.

FIG. 1 is a graph summarizing the relationship between the F3 (Mn/Cr) value and the critical diffusible hydrogen amount ratio HR for the inventive examples and comparative examples having an F2 value of 0.03 or more and 0.150 or less. As shown in this graph, on the premise that the F2 value is 0.03 or more and 0.150 or less, if the F3 is lower than 0.20, the HR is higher than 1.00, and excellent hydrogen resistance It can be seen that embrittlement properties are obtained.

The embodiments of the present invention have been described above. However, the above-described embodiments are merely examples for implementing the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment without departing from the spirit of the present invention.

The carburizing steel of the present invention has high bending fatigue strength and cold workability, and has excellent resistance to hydrogen embrittlement in an environment where hydrogen penetrates. It is suitable for use in steel parts such as gears, pulleys, and transmission shafts.

Claims (4)

in % by mass,
C: 0.15 to 0.25%,
Si: less than 0.20,
Mn: less than 0.20 to 0.40%,
Cr: 1.60-2.00%,
Al: 0.005 to 0.060%,
N: 0.0015 to 0.0080%,
Ca: 0.0003 to 0.0050%, and S: 0.010 to 0.020%,
P: 0.020% or less,
O: limited to 0.0020% or less,
the balance consists of Fe and impurities,
A carburizing steel characterized by having a chemical composition satisfying the following formulas (1) to (3) and having a ferrite area ratio of 60% or more.
0.48≦C+Si/7+Mn/5+Cr/9≦0.58 (1)
0.03≦Ca/S≦0.15 (2)
Mn/Cr≦0.20 (3)
Here, the content (% by mass) of the corresponding element is substituted for each element symbol in formulas (1) to (3). Furthermore, in mass %,
Sb: 0.100% or less,
2. The carburizing steel according to claim 1, containing one or more selected from the group consisting of Sn: 0.100% or less and Bi: 0.100% or less. Furthermore, in mass %,
Ti: 0.010 to 0.050%,
B: 0.0003 to 0.0040%,
Cu: 0.50% or less,
Ni: 0.30% or less,
Mo: 0.05% or less,
V: 0.050% or less, and Nb: 0.050% or less, containing one or more selected from the group consisting of
3. The carburizing steel according to claim 1, wherein the following formula (1') is satisfied.
0.48≦C+Si/7+Mn/5+Cr/9+2×Mo/5+V≦0.58 (1′)
Here, the content (% by mass) of the corresponding element is substituted for each element symbol in formula (1′). A component comprising a core portion having the composition according to any one of claims 1 to 3 and a surface layer portion having a higher carbon content than the core portion.

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