JP7135485B2 - Carburizing steel and parts - Google Patents

Description

The present invention relates to carburizing steels and parts, and more particularly to carburizing steels used in hot forging and parts obtained from such carburizing steels.

Steel parts such as gears, pulleys and transmission shafts are used in automobiles and industrial machinery. Steel parts are manufactured, for example, by the following manufacturing method. A rolled steel bar or wire made of steel for machine structural use is prepared. Machine structural steels are, for example, JIS standard SCr420, SCM420, SNCM420, and the like. The prepared rolled steel bar or wire rod is hot forged to roughly form an intermediate product. If necessary, the intermediate product is normalized. Furthermore, cutting is performed on the intermediate product. Carburizing quenching or carbonitriding quenching is performed on the cut intermediate product, and tempering is performed at 200° C. or less. Shot peening is performed as necessary on the quenched and tempered intermediate product. A steel component having fatigue strength and wear resistance is manufactured by the manufacturing process described above.

BACKGROUND ART In recent years, for example, in order to improve the fuel efficiency of automobiles or to achieve higher engine output, steel parts are becoming lighter and smaller. Weight reduction and miniaturization have increased the load on steel components. Therefore, steel parts are required to have high bending fatigue strength and contact fatigue strength.

Here, "contact fatigue" includes "face fatigue", "line fatigue" and "point fatigue". However, in practice, line contact and point contact are less likely to occur in steel parts. In most cases, surface contact occurs. Therefore, steel parts are required to improve "surface fatigue strength" as contact fatigue strength. Pitching is a representative example of fracture mode due to surface fatigue. A high surface fatigue strength means a high pitting strength.

As described above, steel parts are required to have high bending fatigue strength and surface 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. 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

An object of the present invention is to suppress the effect on hydrogen embrittlement resistance by reducing the Mn content while having high machinability and high surface fatigue protection after quenching. An object of the present invention is to provide a carburizing steel used for hot forging, which has excellent resistance to hydrogen embrittlement in steel.

The present inventors have investigated and investigated the components and structures that affect the hot forging 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 further studied steel compositions that improve hardenability even when Mn is less than 0.40%. As a result, the hardenability is improved by increasing the Si content to 0.20 to 0.60% and the Cr content to 1.60 to 2.00%. It was found that hydrogen embrittlement resistance can be obtained.

[Hardenability 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 Mo content, the bending fatigue strength is improved.

(b) By increasing the Cr content and/or the Mo content, not only the bending fatigue strength but also the surface fatigue strength is increased.

(c) However, if the Mo content is too high, after performing hot forging on the carburizing steel, or after performing normalizing treatment on the hot forged carburizing steel, the steel A bainite structure is easily generated inside. If the bainite structure is generated, the hardness of the steel increases, so the machinability of the steel decreases. Even if Mo is not contained, if the Cr content is too high, a bainite structure is likely to be generated as well.

(d) Therefore, while improving temper softening resistance, bending fatigue strength and wear resistance of the surface layer portion after carburizing and quenching, that is, surface fatigue strength of the surface layer portion after carburizing and quenching can be increased, and machinability can be improved. The Cr content and Mo content must be adjusted as follows. Specifically, the Cr content and Mo content are adjusted so that F1 defined by the following formula falls within the range of 1.90 to 2.40.
F1 = Si + Cr + 2 x Mo
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]
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. If the ratio of Ca content to S content in steel is set within an appropriate range, the morphology of sulfides can be controlled to maintain hydrogen embrittlement resistance. Specifically, when the chemical composition of the steel for hot forged parts satisfies the formula (2), hydrogen embrittlement resistance is maintained.
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 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, the hydrogen embrittlement resistance is lowered. 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. If F2 satisfies the formula (2), excellent hydrogen embrittlement resistance 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]
Machinability depends on the metallographic structure of the steel. When the metallographic structure is mainly ferrite and the area ratio of proeutectoid ferrite is high, the machinability is excellent. Specifically, if the area ratio of ferrite is 50% or more in the metal structure of the carburizing steel, and the balance is pearlite and bainite, the hot forging processability is enhanced.

The present invention was made based on the above findings, and the gist thereof is as follows.
(1) In mass%, C: 0.15 to 0.25%, Si: 0.20 to 0.60%, Mn: 0.20 to less than 0.40%, Cr: 1.60 to 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,
Having a chemical composition satisfying the following formulas (1) to (3),
Carburizing steel characterized by having a ferrite area ratio of 50% or more.
1.90≦Si+Cr≦2.40 (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).
(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 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
The carburizing steel according to (1) or (2), which satisfies the following formula (1′).
1.90≦Si+Cr+2×Mo≦2.40 (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 at the impurity level, the corresponding element symbol in formula (1′) contains 0 is substituted.
(4) The steel for carburizing according to any one of (1) to (3), characterized in that the metallographic structure of the steel has a ferrite area ratio of 65% to 90%.
(5) A part 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 surface fatigue strength and machinability, and excellent resistance to hydrogen embrittlement under a hydrogen penetration environment.

FIG. 3 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. It is a front view of a large roller used in a roller pitching test 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, thereby deteriorating the machinability. 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: 0.20 to 0.60%]
Silicon (Si) enhances hardenability. Si also increases the temper softening resistance of the steel. Therefore, Si increases the surface fatigue strength of steel. However, if Si is contained excessively, a grain boundary oxide layer is excessively formed by carburizing quenching or carbonitriding quenching, and the bending fatigue strength of the manufactured parts is lowered. On the other hand, silicon (Si) suppresses precipitation of cementite and increases temper softening resistance. Si also deoxidizes the steel. The deoxidation product, MnO—SiO ₂ , is a vitrified soft inclusion, which is refined by stretching and breaking during hot rolling. Therefore, the hydrogen embrittlement resistance is enhanced. From the viewpoint of bending fatigue strength and hydrogen embrittlement resistance of the manufactured parts, the Si content is set to 0.20% or more and 0.60% or less. If the Si content is less than 0.20%, the above effects cannot be obtained. On the other hand, if the Si content exceeds 0.60%, the strength becomes too high. In this case, the machinability of the steel deteriorates. The lower limit of the Si content is preferably over 0.35%, more preferably 0.40%, still more preferably 0.45%, still more preferably over 0.50%.

[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, the area ratio of ferrite is lowered, and the machinability of the steel after hot forging is lowered. Moreover, Mn combines with Si to form inclusions (MnO—SiO ₂ ). Since these inclusions are soft and are stretched and split during hot rolling to be finer, the density of MnO—SiO ₂ is reduced and hydrogen embrittlement resistance 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 hot forging or normalizing treatment. If the bainite structure is generated, the machinability of the steel is lowered. 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 0.0025%, more preferably 0.0030%, and still more preferably 0.0035%.

[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 and lowers the hot forgeability of steel and further lowers 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.

[1.90≤Si+Cr≤2.40]
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, surface fatigue strength and machinability.
1.90≦F1 (=Si+Cr)≦2.40 (1)

Further, the chemical composition of the carburizing steel of the present embodiment can contain Mo as an optional additive element, but in this case, the formula (1′) is satisfied in addition to the formula (1).
1.90≦F1 (=Si+Cr+2×Mo)≦2.40 (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 hot forging 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, the hydrogen embrittlement resistance is lowered. 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. The lower limit for exhibiting the effect of suppressing hydrogen penetration is 0.001% or more for each, and the preferable lower limit for exhibiting the effect sufficiently is 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 hot 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 generated to deteriorate hot forgeability. 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 excessively contained, hardenability becomes too high, bainite is formed after finish rolling, and hot ductility and hot workability are 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 in the steel after hot forging or normalizing treatment. 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 hot forging or normalizing treatment, and the machinability of the steel deteriorates. 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 machinability 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 0.002%. 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. Here, in the metallographic structure of carburizing steel, the area ratio of ferrite is 50% or more, and the balance is pearlite and bainite. The ferrite having a ferrite area ratio of 50% or more is proeutectoid ferrite, and does not include ferrite between lamellar cementites of pearlite.

Proeutectoid ferrite is softer than pearlite and bainite and has excellent machinability. Furthermore, pro-eutectoid ferrite has better machinability than pearlite and bainite. If the area ratio of pro-eutectoid ferrite is 65% or more and less than 100%, excellent machinability 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. The precipitated carbonitrides and carbides increase the strength of the steel after product rolling and reduce the hot forgeability. 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 hot forgeability 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 lower than 750° C., ferrite transformation starts from unrecrystallized austenite grains, and the metal structure after cooling becomes too fine. In this case, the strength of the steel increases and the hot forgeability deteriorates. 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 hot forgeability deteriorates.

If the finishing temperature is 750 to 850° C., the area ratio of proeutectoid ferrite in the metal structure becomes 50% 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 metallographic 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 50%. 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 50% 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 tensile strength of the carburizing steel having the chemical composition satisfying the formulas (1) to (3) is 650 MPa or less. 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 50% or more. Therefore, excellent fatigue strength, machinability and hydrogen embrittlement resistance are 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 bars and wire rods manufactured by the above-described manufacturing processes are turned into steel parts by the following manufacturing processes. First, a prepared rolled steel bar or wire rod is hot forged to roughly form an intermediate product. If necessary, the intermediate product is normalized. Cutting is performed on the intermediate product. Carburizing quenching or carbonitriding quenching is performed on the cut intermediate product, and tempering is performed at 200° C. or less. A steel component is manufactured by the manufacturing process described above.

The steel component includes a core part whose carbon content does not change before and after the carburizing and quenching process or the carbonitriding and quenching process, and a surface 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 wire rods with a diameter of 14 mm (finish hot rolling step). 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. The obtained results are shown in the column of "ferrite area ratio" in Tables 2-1 and 2-2.

[Area fatigue strength test piece and 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.

[Surface fatigue strength test]
The surface fatigue strength was determined by a roller pitting test. The roller pitting test was conducted under the conditions shown in Table 3 by combining the small roller test piece and a large roller having the shape shown in FIG.

The large roller shown in FIG. 5 is made of steel that satisfies the JIS standard SCM420H, and the general manufacturing process includes normalizing, test piece processing, eutectoid carburizing in a gas carburizing furnace, low temperature tempering and polishing. made by

For each test number, the number of tests in the roller pitting test was 6. After the test, an SN diagram was prepared with the vertical axis representing the surface pressure and the horizontal axis representing the number of repetitions until pitching occurred. The highest surface pressure among those in which pitting did not occur up to the number of repetitions of 2.0×10 ⁷ was taken as the surface fatigue strength. Pitching was defined as occurrence of pitting when the area of the largest damaged portion on the surface of the small roller test piece was 1 mm ² or more.

Tables 2-1 and 2-2 show the surface fatigue strength ratio obtained by the test. For the surface fatigue strength ratios in Tables 2-1 and 2-2, the surface fatigue strength ratio of Test No. 19 having a chemical composition corresponding to SCM420 of JIS G4053 (2016) was used as the reference value (1.00). The surface fatigue strength of each test number is shown as a ratio to the reference value. It was judged that excellent surface fatigue strength was obtained when the surface fatigue strength ratio was 1.10 or more.

[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 ⁷ repetitions, the highest stress was taken as the high cycle rotating bending fatigue strength.

Tables 2-1 and 2-2 show high-cycle bending fatigue strength. For the high-cycle bending fatigue strength, the high-cycle bending fatigue strength of Test No. 19 having a chemical composition corresponding to SCM420 of JIS G4053 (2016) was used as the reference value (1.00). The high-cycle bending fatigue strength of each test number is shown as a ratio to the reference value. If the high-cycle bending fatigue strength was 1.10 or more, it was determined that excellent bending fatigue strength was obtained.

[Cutting test]
A cutting test was performed to evaluate the machinability. Cutting test pieces were obtained by the following method. A steel bar with a diameter of 70 mm of each test number was heated at a heating temperature of 1200° C. for 30 minutes. The heated steel bar was hot forged at a finishing temperature of 950° C. or higher to obtain a round bar with a diameter of 50 mm. This round bar was subjected to normalizing treatment. Specifically, the round bar was heated at a heating temperature of 950° C. for 1 hour and then allowed to cool. A cutting test piece with a diameter of 46 mm and a length of 400 mm was obtained by machining from the normalized round bar. A cutting test was performed using the cutting test piece under the following conditions.

Cutting test (turning)
Tip: Base material P20 grade carbide, no coating Conditions: Circumferential speed 200m/min, feed 0.30mm/rev, depth of cut 1.5mm, water-soluble cutting oil used Measurement items: Flank after 10 minutes of cutting wear amount of the main cutting edge

The obtained main cutting edge wear amount is shown in the column of "flank wear amount" in Tables 2-1 and 2-2. In Tables 2-1 and 2-2, the standard value (1.00) is the major cutting edge wear amount on the flank surface of the normalized test piece of Test No. 21 (using steel E). The major cutting edge wear amount for each test number is shown as a ratio to the reference value. If the main cutting edge wear amount was 0.85 or less, it was determined that excellent machinability was obtained.

[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. 6 were produced for each wire rod of each test number by machining the wire rod whose tensile strength was adjusted. Numerical values without units in FIG. 6 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 S having a chemical composition corresponding to SCM420 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 15 were appropriate and satisfied formulas (1) to (3). Furthermore, the total area ratio of proeutectoid ferrite and pearlite in the metal structure inside the wire was 90% or more, and the area ratio of ferrite was 50% or more. Moreover, the fatigue strength of these test numbers was 1.10 or more, and the plane fatigue strength ratio was 1.10 or more. Furthermore, the main cutting edge wear amount was 0.85 or less. Furthermore, the ratio HR exceeded 1.00, indicating excellent hydrogen embrittlement resistance.

On the other hand, test numbers 16 to 18 contained more than 0.10% of any of Sb, Sn and Bi, and cracks occurred during rolling. Since Test Nos. 16 to 18 are in an unusable state, the surface fatigue strength test, bending fatigue strength test, cutting test, and hydrogen embrittlement resistance evaluation test were not conducted.

Test numbers 19 and 20 are examples that did not satisfy formula (1). Test No. 19 is inferior in high cycle fatigue strength ratio and surface fatigue strength, and Test No. 15 is inferior in machinability.

Test numbers 21 and 22 are examples that did not satisfy formula (2). Test No. 21 is inferior in high cycle fatigue strength ratio, and Test No. 22 is inferior in hydrogen embrittlement resistance.

Test numbers 23 to 25 are examples that did not satisfy the formula (3), and were inferior in hydrogen embrittlement resistance. In particular, as a result of the surface fatigue strength test after quenching of test numbers 24 and 25, the surface fatigue strength ratio of both test numbers was less than 1.10 (failed). Since the Si content in Test Nos. 24 and 25 was less than 0.2%, the temper softening resistance of each test piece was low, and the wear resistance of the quenched portion was considered insufficient. .

The wire of Test No. 26 does not contain Ca. Therefore, it is inferior in hydrogen embrittlement resistance.

Test numbers 27 to 30 are examples in which the formulas (1) to (3) are satisfied, but the area ratio of ferrite is less than 50%. Both are inferior in machinability.

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 surface fatigue strength and machinability, and excellent resistance to hydrogen embrittlement in a hydrogen penetration environment. 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: 0.20 to 0.60%,
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 50% or more.
1.90≦Si+Cr≦2.40 (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.
1.90≦Si+Cr+2×Mo≦2.40 (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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