Patent Application
20240352562
The present invention relates to a case hardening steel for warm forging and a roughly-shaped forged material manufactured using the same.
Steel components and the like for transmissions represented by gears are ordinarily manufactured by subjecting a roughly-shaped forged material obtained by hot forging to machining and surface hardening treatment. Hot forging has a problem that energy consumption is relatively large because of its high heating temperature, a yield is poor because of generation of scale on a surface, and it is difficult to surely obtain dimensional accuracy. In addition, the roughly-shaped forged material after hot forging is not good in workability at machining as it is, because of its increased hardness, and thus it is essential to perform heat treatment for reducing the hardness before machining. Energy consumption for the heat treatment is also problematic.
Some of the above-mentioned problems in the case of selecting hot forging may be solved by substituting hot forging with warm forging to be performed at a low temperature. That is, warm forging, in which the forging temperature is lower than in the hot forging, has advantages, such as low energy consumption amount, small scale generation amount leading to a good yield, good dimensional accuracy, and small machining allowance for the succeeding steps.
However, in the case of using a conventional case hardening steel, the heat treatment before machining can hardly be omitted even when warm forging is selected.
For example, when warm forging is performed on typical case hardening steels such as SCM420 and SCr420, the hardness of the steels increases after the forging to such a degree that cutting workability deteriorates, even though the degree is lower than that in the case of hot forging. For this reason, when these typical case hardening steels are used to obtain a roughly-shaped forged material by performing warm forging thereon, and the roughly-shaped forged material thus obtained is cut without adding any work, chip treatability deteriorates and tool wear increases. Thus, the merit of selecting warm forging is limited as long as a conventional and typical case hardening steel is used, and it is difficult to solve the problems, such as energy consumption due to necessary heat treatment before machining such as normalizing or annealing, and a decrease in yield due to scale generation.
As case hardening steels for warm forging in the past, for example, the after-mentioned technologies described in Patent Documents 1 to 3 have been disclosed. Patent Document 1 refers to performing of warm forging and cutting properties, but does not describe improvement of cutting properties when cutting is performed after warm forging. Patent Document 2 refers to warm forging and cutting properties, but in which evaluation of cutting properties after warm forging is not done, and it only refers to steels that cannot be expected to finally have high strength from the viewpoint of the chemical components. Patent Document 3 refers to warm forging, but in which evaluation of the subsequent cutting properties is not done. Thus, it is not possible to derive from Patent Documents 1 to 3 how the chemical composition of steel needs to be devised to improve cutting properties without applying heat treatment after warm forging.
The present invention has been made in view of such a background, and an object of the present invention is to provide a case hardening steel having excellent cutting properties after warm forging and a roughly-shaped forged material having excellent cutting properties obtained by performing warm forging on the case hardening steel.
An aspect of the present invention is a case hardening steel for warm forging at a forging temperature of 850° C. to 1100° C.,
the case hardening steel having a chemical composition comprising, by mass %:
C: 0.15 to 0.23%; Si: 0.60 to 0.95%; Mn: 0.60 to 1.20%; P: 0.035% or less; S: 0.035% or less; Cr: 1.50% or less (excluding 0%); Al: 0.050% or less; Ti: 0.01 to 0.05%; B: 0.0005 to 0.0050%; N: 0.0020 to 0.0200%;
Mo: 0.20% or less as an optional element; and Nb: 0.01 to 0.05% as an optional element; the balance being Fe and unavoidable impurities, and satisfying
(wherein element symbols in Formulae 1 and 2 represent contents (mass %) of respective elements.
Another aspect of the present invention is a roughly-shaped forged material obtained by performing warm forging at a forging temperature of 850° C. to 1100° C. on the above-described case hardening steel for warm forging, the roughly-shaped forged material having:
a surface hardness of 200 HV or less;
a metal structure of which a ferrite rate is 80% to 90%; and
a ferrite hardness of 140 mHV to 160 mHV.
As a result of intensive studies, it has been found that it is important to control the ferrite rate and the ferrite hardness within optimal ranges, in addition to reducing macroscopic hardness, to allow sufficient machinability after warm forging. It has been also found that when warm forging is adopted and whereby the structure is refined, chips tend to be long, however, chip breaking properties can be improved by adjusting the Si content and the ferrite structure (rate, hardness) without particularly increasing the S content effective for improving chip breaking properties. Because there is no need to increase the S content when the Si content is optimized, it also becomes possible to reduce the concern about the occurrence of cracking in warm forging that may be caused by increase of the S content. Further, with a focus on Si and Mo which are ferrite stabilizing elements, the tendencies of the ferrite rate and the ferrite hardness after warm forging have been studied, and it has been found that having the above chemical composition and satisfying Formulae 1 and 2 at the same time enable control of the ferrite rate and the ferrite hardness after warm forging within optimum ranges and allow sufficient machinability.
The case hardening steel having a specific chemical composition within the range of the above basic chemical composition and satisfying Formulae 1 and 2 makes it possible to obtain an excellent roughly-shaped forged material which allows cutting properties with no manufacturing problem even when heat treatment after warm forging is omitted.
The case hardening steel for warm forging is a steel to be subjected to warm forging at a forging temperature of 850° C. to 1100° C. When the forging temperature is too low, deformation resistance at the time of forging increases and it becomes difficult to mold a desired shape, and when the forging temperature is too high, an energy saving effect as compared with hot forging deteriorates, therefore the forging temperature of the warm forging is set to 850° C. or higher and 1100° C. or lower.
The case hardening steel for warm forging has, as a basic chemical composition, a chemical composition including, by mass %, C: 0.15 to 0.23%, Si: 0.60 to 0.95%, Mn: 0.60 to 1.20%, P: 0.035% or less, S: 0.035% or less, Cr: 1.50% or less (excluding 0%), Al: 0.050% or less, Ti: 0.01 to 0.05%, B: 0.0005 to 0.0050%, and N: 0.0020 to 0.0200%, the balance being Fe and unavoidable impurities.
C (Carbon) is contained in an amount of 0.15% or more to surely obtain necessary strength after quenching and to prevent deterioration of chip treatability. When the C content is too high, the macro hardness becomes too high, so that the workability at machining performed after forging may deteriorate. Thus, the C content is set to 0.23% or less.
Si (silicon) is an element necessary for securing machinability. When the Si content is too low, the ferrite hardness decreases, and the chip treatability deteriorates, which may promote tool wear. Thus, the Si content is set to 0.60% or more. When the Si content is too high, the hardness may excessively increase to thereby deteriorate workability at machining performed after forging. Thus, the Si content is set to 0.95% or less.
Mn (manganese) is contained in an amount of 0.60% or more to surely obtain sufficient internal hardness strength after carburization. When the Mn content is too high, retained austenite increases, and there is a concern that the hardness of the carburized layer may decrease, and the hardness after forging may increase, leading to deterioration of machinability. Thus, the Mn content is set to 1.20% or less.
When the content of P (phosphorus) is too high, P segregates at grain boundaries to cause a decrease in fatigue strength. Thus, the content of P is set to 0.035% or less.
When the content of S (sulfur) is too high, the amount of sulfide-based inclusions increases to cause a decrease in fatigue strength. Thus, the S content is set to 0.035% or less.
Cr (chromium) is effective in surely obtaining internal hardness by improving hardenability, but when the content is too high, the hardness after warm forging may increase and machinability may decrease. Thus, the Cr content is set to 1.50% or less.
When the content of Al (aluminum) is too high, coarse precipitates of AlN may increase to deteriorate toughness. Thus, the Al content is set to 0.050% or less.
Ti (titanium) is contained in an amount of 0.01% or more because Ti is effective in obtaining a so-called N-kill action, that is, an action of consuming N as TiN to prevent N from combining with B. On the other hand, when the content of Ti is too high, there is a concern that the strength may decrease because of the generation of TiN and abnormal wear of the tools during cutting may accelerate. Thus, the Ti content is set to 0.05% or less.
B (boron) is contained in an amount of 0.0005% or more to obtain a strength improving effect through grain boundary strengthening. On the other hand, the above-described effect becomes saturated when the B content is excessively high, thus the upper limit is set to 0.0050%.
N (nitrogen) is contained in an amount of 0.0020% or more because it becomes AlN and has an effect of reducing crystal grain coarsening due to a pinning effect. On the other hand, when the N content is excessively high, coarse precipitates of AlN may disadvantageously increase to thereby deteriorate toughness. Thus, the N content is set to 0.0200% or less.
Mo (molybdenum) is an optional additive element and, thus, it needs not be positively contained. Although the Mo content may be 0%, a small amount of Mo may be contained as an impurity in some cases. Since Mo contained in the steel acts as an element effective for improving hardenability, a small amount of Mo may be added when needed. On the other hand, when the Mo content is excessively high, the cost may increase and the cutting workability may deteriorate. Thus, the Mo content is limited to 0.20% or less.
Nb (niobium) is an optional additive element and it needs not be positively contained. However, when 0.01% or more of Nb is contained, the effect of grain refinement can be obtained. On the other hand, when the Nb content is excessively high, carburizing properties may deteriorate. Thus, the Nb content is limited to 0.05% or less.
Next, on the premise that the basic chemical composition described above is satisfied, it is important to adjust the chemical components to satisfy both of the following Formulae 1 and 2. By satisfying Formulae 1 and 2, the ferrite rate and the ferrite hardness after warm forging can be controlled to fall within optimum ranges to thereby surely obtain sufficient machinability.
Formula 1 is a relational formula effective for estimating the ferrite rate in the metal structure after warm forging. Although the value of the formula does not equal the ferrite rate exactly, the ferrite rate tends to be higher as the value of Formula 1 becomes larger, and when the value is in the range of 80 or more and 90 or less, it becomes easy to control the ferrite rate after warm forging to fall within an optimum range.
Formula 2 is a relational formula effective for estimating the ferrite hardness in the metal structure after warm forging. Although the value of the formula does not equal the ferrite hardness exactly, the ferrite hardness tends to be higher as the value of Formula 2 becomes larger, and when the value is in the range of 145 or more and 160 or less, it becomes easy to control the ferrite hardness after warm forging to fall within an optimum range.
Next, the roughly-shaped forged material obtained by performing warm forging at a forging temperature of 850° C. to 1100° C. on the above-described case hardening steel for warm forging can possess the properties such as a surface hardness of 200 HV or less, a metal structure of which a ferrite rate is 80% to 90%, and a ferrite hardness of 140 mHV to 160 mHV.
By setting the surface hardness, that is, the macro hardness of the roughly-shaped forged material to be 200 HV or less, cutting work can be performed without a heat treatment after forging.
The ferrite rate set to the range of 80% to 90% and the ferrite hardness set to the range of 140 mHV to 160 mHV in the metal structure of the roughly-shaped forged material make it possible to reliably obtain chip treatability, prevent tool wear amount from increasing, and so on to thereby achieve an effect of improving cutting properties.
On the other hand, when the ferrite rate is less than the lower limit value mentioned above, the pearlite area percentage increases, and the macro hardness (surface hardness) increases, so that the effect of preventing tool wear amount from increasing, which is expected due to the change from hot forging to warm forging, may decrease. In addition, when the ferrite rate exceeds the above upper limit value, the macro hardness (surface hardness) becomes too low, and the chip treatability may disadvantageously deteriorate.
When the ferrite hardness is lower than the above lower limit value, the chip treatability may deteriorate, and when the ferrite hardness exceeds the above upper limit value, the effect of preventing an increase of tool wear amount may decrease.
Examples of the case hardening steel for warm forging and the roughly-shaped forged material of this example will be described.
In this example, as shown in Tables 1 and 2, 29 types of steel materials (steel types 1 to 29) having different chemical components were used to prepare roughly-shaped forged materials, and various evaluations were performed. Among the steels shown in Tables 1 and 2, the steel types 1 to 16 are Examples satisfying the requirements of the present invention, the steel types 17 to 28 are Comparative Examples not satisfying some requirements, and the steel type 29 is a conventional steel, JIS SCr420.
In manufacturing each roughly-shaped forged material, a steel ingot obtained by melting various steel materials in an electric furnace was forged and extended to prepare a billet having a diameter of 65 mmφ. For the steel types 1 to 28, warm forging was performed at forging temperatures described in Table 3 described later to obtain the roughly-shaped forged materials. The roughly-shaped forged materials subjected to warm forging (steel types 1 to 28) were not subjected to a heat treatment after the forging. The steel type 29 was prepared for comparison to confirm the effect of optimizing components suitable for warm forging and applying warm forging. Specifically, SCr420, which is a conventional steel, was subjected to hot forging, which has been conventionally performed, and then subjected to a heat treatment in which the steel was held at 900° C. for 1 hour for improving workability. In the examples performed this time, a small amount of Mo, which is an optional additive element, was contained as an impurity owing to the dissolved base material. Thus, in Tables 1 and 2, analysis values of Mo contained as an impurity are also shown.
Assuming machining of a gear equivalent portion after forging, a section in the vicinity of the surface of each roughly-shaped forged material corresponding to the position of the gear equivalent portion was subjected to nital corrosion then observed using an optical microscope. The area percentage of ferrite was obtained by image analysis, and this value was taken as the ferrite rate. The value of the micro-Vickers hardness measured at a ferrite structure portion of the section was taken as the ferrite hardness.
Assuming machining of a gear equivalent portion after forging, the Vickers hardness measured in the section in the vicinity of the surface of the roughly-shaped forged material corresponding to the position of the gear equivalent portion was taken as the macro hardness.
The surface of each roughly-shaped forged material was cut under the following conditions to evaluate the cutting properties.
The tool wear amount was evaluated by measuring the wear amount of a flank surface of a cutting tool. The result of the tool wear amount of the steel type 29 (additionally heat-treated product after hot forging) equivalent to SCr20 as a reference was taken as 100%. The steel with a tool wear amount of 110% or less was evaluated as “passed (∘)”, and the steel with a tool wear amount exceeding 110% was evaluated as “failed (x)”. In the chip evaluation, the steel with a chip having a chip length equal to or less than the result of the steel type 29 as a reference was evaluated as “good”, and the steel with a chip having a chip length longer than the reference was evaluated as “poor”.
<Strength after Carburization>
A test piece material was produced by the same manufacturing method as in manufacturing the roughly-shaped forged materials. Thereafter, a test piece having a size of 12 square×110 in length was prepared by machining (notched depth of 2 mm, angle of 60 degrees, notch base R1.0 at the center of the test piece), and was subjected to a carburizing heat treatment, and then the surface on the notched side was polished by 0.2 mm to thereby complete the test piece. The conditions for the carburizing heat treatment were as follows: a carburization treatment was performed under conditions of a carburization temperature: 950° C.×150 min and Cp: 0.85, followed by oil cooling-quenching, and then a tempering treatment at 150° C.×1 Hr was performed.
The evaluation test for strength after carburization was performed with a three-point bending fatigue test. The fatigue test was performed under the condition of a frequency of 1 Hz, and evaluation was made by determining the low cycle bending fatigue strength with which fracture occurs at the number of repetitions of 100 times. The steel with a result equal to or more than the result of the steel type 29 as a reference was evaluated as “passed (∘)”, and the steel with a result lower than the reference was evaluated as “failed (x)”.
The evaluation results are shown in Table 3. The steel with the results of “passed (∘)”, “good”, and “passed (∘)” for the tool wear amount, the chip, and the strength after carburization, respectively, was determined as “passed (∘)” as a comprehensive evaluation, and the steel with other results was determined as “failed (x)”.
As can be seen from Table 3, the steel types 1 to 16, which were subjected to warm forging at a forging temperature of 900 to 1050° C. and not subjected to a heat treatment after the forging, had properties equal to or more than those of the steel type 29, which was subjected to a heat treatment after conventional hot forging, in cutting properties and strength after carburization.
On the other hand, in the steel type 17, the strength after carburization was too low because of its low carbon (C) content.
In the steel type 18, the macro hardness was too high because of its high carbon (C) content, and the tool wear amount increased accordingly.
In the steel type 19, the ferrite hardness was too low because of its low silicon (Si) content, and its chip treatability was poor.
In the steel type 20, the macro hardness was too high because of its high silicon (Si) content, and the tool wear amount increased accordingly.
In the steel type 21, the hardness after carburization and quenching was insufficient because of its low manganese (Mn) content, and the strength after carburization was failed accordingly.
In the steel type 22, the macro hardness was increased to thereby increase the tool wear amount because of its high carbon (C) content and high manganese (Mn) content, and the strength after carburization was failed because the hardness of the carburized layer was reduced due to increase in the amount of retained austenite, which was caused by its high Mn content.
In the steel type 23, the macro hardness was increased because of its high molybdenum (Mo) content, and the tool wear amount increased accordingly.
In the steel type 24, the macro hardness was increased because of its high chromium (Cr) content, and the tool wear amount increased accordingly.
In the steel type 25, because the chemical composition did not satisfy Formula 1 and fell outside the lower limit, the ferrite rate decreased, the macro hardness was increased, and the tool wear amount increased accordingly.
In the steel type 26, because the chemical composition did not satisfy Formula 2 and fell outside the lower limit, the ferrite hardness decreased, and the chip treatability was poor accordingly.
In the steel type 27, because the chemical composition did not satisfy Formula 1 and fell outside the upper limit, the ferrite rate and macro hardness decreased, and the chip treatability was poor accordingly.
In the steel type 28, because the chemical composition did not satisfy Formula 2 and fell outside the upper limit, the ferrite hardness increased, and the tool wear amount increased accordingly.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2020-212173 | Dec 2020 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2021/035738 | 9/29/2021 | WO |
