1. A blank for vacuum carburization, which has a chemical component combination containing, in mass%, C: 0.13 to 0.28%, Si: 0.01-1.20%, Mn: 0.10 to 1.50%, P: 0.030% or less, S: 0.050% or less, Cr: 0.30 to 2.20%, Mo: 0.60% or less and including 0%, Al: 0.027-0.090%, N: 0.0140 to 0.0300%, the remainder consisting of Fe and unavoidable impurities,

AlN precipitates having a diameter of more than 100nm in terms of equivalent circle in the cross section are 1.5 precipitates per 100. mu.m²The following.

2. A method for producing a vacuum carburized blank according to claim 1,

performing final hot working on a steel material having the above chemical component combination, heating to a temperature of 1150 ℃ or higher and T1 ℃ or higher calculated by the following formula (1), and then cooling to 900 ℃ at a cooling rate of 1 ℃/sec or higher,

formula (1): t1 ═ 100000 × Al × N +1090

The element symbol in formula (1) represents the value of the content of each element, and the content is expressed in mass%.

Background

For parts such as gears that require high surface hardness, generally, a steel material of low carbon alloy steel such as SCM420 is used, formed by forging or the like, and then subjected to a surface hardening treatment by carburizing, quenching, and tempering. Gas carburizing has been widely used as a method for carburizing and quenching. However, in recent years, the use of vacuum carburization has been expanding due to demands for shortening of processing time and downsizing of processing lots. Since vacuum carburizing can increase the treatment temperature as compared with gas carburizing, the treatment time can be shortened as compared with gas carburizing. As a technique for carrying out conventional vacuum carburization (reduced pressure carburization), there are, for example, techniques described in japanese patent laid-open nos. 2008-069436 and 2014-208867.

Disclosure of Invention

The problem of the high processing temperature in vacuum carburization is abnormal grain growth occurring on the surface of the workpiece. When abnormal grain growth occurs, not only the strength of the structural body such as bending fatigue strength and surface fatigue strength is reduced, but also the amount of strain (deformation) during quenching is increased, resulting in a reduction in productivity, and therefore it is necessary to suppress this.

In order to prevent abnormal grain growth on the surface of the workpiece, there is a method of finely dispersing precipitates such as AlN and suppressing movement of grain boundaries at the time of abnormal growth of the grains. However, it has been reported that, in the vacuum carburization, denitrification (denitrogenation) is more likely to occur in the surface portion of the workpiece than in the interior portion, and thus the amount of AlN precipitated is reduced, and abnormal grain growth cannot be suppressed in some cases.

For example, in Japanese patent laid-open No. 2008-069436, the case where denitrification occurs is mentioned. However, no direct countermeasures against this are disclosed, and only countermeasures for precipitating carbonitrides by cooling at a cooling rate of 5 ℃/min or less in the range of 900 to 650 ℃ in the cooling stage of the hot forging step are disclosed. Further, in japanese patent application laid-open No. 2014-208867, as a method for preventing denitrification, it is premised on supplying a nitriding gas such as ammonia in the carburizing treatment, which not only adversely affects the durability of the treatment equipment but also causes an increase in the manufacturing cost, and thus is difficult to be practically employed.

The present invention has been made in view of the above-described background, and an object thereof is to provide a blank for vacuum carburizing and a method for manufacturing the same, which can suppress abnormal grain growth during vacuum carburizing.

One embodiment of the present invention is a blank for vacuum carburization, including, in mass%, C: 0.13 to 0.28%, Si: 0.01-1.20%, Mn: 0.10 to 1.50%, P: 0.030% or less, S: 0.050% or less, Cr: 0.30 to 2.20%, Mo: 0.60% or less (including 0%), Al: 0.027-0.090%, N: 0.0140 to 0.0300%, the balance being a combination of chemical components consisting of Fe and unavoidable impurities,

AlN precipitates having a diameter of more than 100nm in terms of equivalent circle in the cross section are 1.5 precipitates per 100. mu.m²The following.

Another aspect of the present invention is a method for producing the blank for vacuum carburization,

performing final hot working on a steel material having the above combination of chemical components, heating to a temperature of 1150 ℃ or higher and T1 ℃ or higher calculated by the following formula (1), and then cooling to 900 ℃ at a cooling rate of 1 ℃/sec or higher,

T1＝100000×Al×N+1090···(1)

in the formula (1), the element symbol represents the content (mass%) of each element.

In the chemical composition of the above-described blank for vacuum carburization, in order to prevent shortage of AlN due to denitrification at the time of vacuum carburization, attention is paid to increasing the amount of AlN precipitates, and the amounts of Al and N added are increased as compared with the prior art so that a sufficient amount of fine AlN remains even if AlN is decreased due to denitrification.

Simply increasing the amounts of Al and N added results in coarse AlN being produced, and there is a possibility that fine AlN is prevented from being precipitated during vacuum carburization. The present inventors have also made experiments under various conditions and found the following findings. That is, it was found that when coarse AlN is present before the treatment for AlN precipitation, coarse AlN is preferentially and easily grown continuouslyGrowth and coarsening of the AlN nitride particles make it difficult for new fine AlN nitride particles to precipitate, and as a result, the crystal grains are easily coarsened. As a result of further investigation, it was found that AlN was sufficiently dissolved in a solid solution during the production of the raw material and that coarse AlN precipitates having a diameter of more than 100nm in an equivalent circle were 1.5/100 μm in cross section²In the following cases, under appropriate conditions described later, the coarsening of crystal grains on the surface portion of the workpiece can be reliably suppressed, and the manufacturing conditions for this can be clarified.

Thus, in the above-mentioned blank for vacuum carburization, by precipitating solid-dissolved AlN during the temperature rise of vacuum carburization, a large amount of fine AlN can be precipitated, and a state in which no coarse AlN precipitates or very few coarse AlN precipitates are present can be maintained, and even if AlN is reduced by some denitrification, sufficiently fine AlN remains, and abnormal grain growth in the treated surface portion at the time of vacuum carburization can be reliably suppressed.

In the above-described manufacturing method, the steel material is heated to a temperature of 1150 ℃ or higher and T1 ℃ or higher calculated by the formula (1) to perform the final hot working. Thus, AlN is sufficiently dissolved during hot working, and then cooled to 900 ℃ at a high cooling rate of 1 ℃/sec or more, whereby the formation of coarse AlN can be reliably suppressed. As a result, as described above, it was possible to obtain 1.5 coarse AlN precipitates having an equivalent circle diameter of 100nm or more in the cross section of the blank for vacuum carburization per 100 μm²The following conditions were used.

Thus, a blank for vacuum carburizing capable of suppressing abnormal grain growth in the treated surface portion at the time of vacuum carburizing is obtained.

Drawings

Features, advantages, and technical and industrial significance of exemplary embodiments of the present invention will be described below with reference to the accompanying drawings, in which like reference numerals refer to like elements.

Fig. 1 is an SEM image for AlN observation in example 8.

Fig. 2 is an SEM image for AlN observation in comparative example 17.

Detailed Description

First, the reasons for limiting the chemical composition of the blank for vacuum carburization will be described.

C：0.13～0.28％，

C (carbon) is an element necessary for obtaining internal hardness for securing strength in order to increase the hardness after quenching treatment. To obtain this effect, C is contained at 0.13% or more. On the other hand, since excessive addition of C causes excessive increase in hardness before machining and deterioration in workability, the upper limit of the C content is set to 0.28% in order to prevent this.

Si：0.01～1.20％，

Si (silicon) is an element that is indispensable as a deoxidizer in steel making, and is an element that suppresses the formation of carbides during tempering and improves the temper softening resistance. In order to obtain this effect, Si is contained at 0.01% or more. On the other hand, since excessive addition of Si causes excessive increase in hardness before machining and deterioration in workability, the upper limit of the Si content is set to 1.20% in order to prevent this.

Mn：0.10～1.50％，

Mn (manganese) is an element that acts as a deoxidizer during steel making and is effective for improving quenching performance. In order to obtain this effect, Mn is contained at 0.10% or more. On the other hand, since excessive addition of Mn excessively increases the hardness before machining and deteriorates the workability, the upper limit of the Mn content is set to 1.50% in order to prevent this.

P: less than 0.030 percent of the total weight of the composition,

p (phosphorus) is an element contained as an impurity, is an element that easily segregates at an austenite grain boundary, and is an element that causes a reduction in bending fatigue strength when segregated. Therefore, the upper limit of the allowable content of P is set to 0.030%.

S: the content of the active carbon is less than 0.050 percent,

s (sulfur) is an element contained as an impurity. It is known that S is an element for improving machinability, but when S is contained in a large amount, sulfide-based nonmetallic inclusions increase, which causes a reduction in fatigue strength. Therefore, the upper limit of the allowable content of S is set to 0.050%.

Cr：0.30～2.20％，

Cr (chromium) is an element that improves hardenability. In order to obtain this effect, Cr is contained at 0.30% or more. On the other hand, excessive addition of Cr excessively increases the hardness before machining and lowers the workability, so in order to prevent this, the upper limit of the Cr content is set to 2.20%.

Mo: less than 0.60% (including 0%),

mo (molybdenum) is an element having an effect of improving the quenching property and an element of improving the temper softening resistance, and therefore is an optional additive element that can be added as needed. On the other hand, if Mo is excessively contained, the hardness before machining is excessively increased, the workability is lowered, and the cost is increased, so in order to prevent this, the upper limit of the allowable content of Mo is set to 0.60%.

Al：0.027～0.090％，

Al (aluminum) is an element used as a deoxidizer in steel making, and when present as fine AlN in combination with N, it exerts an effect of suppressing abnormal grain growth in carburization. In order to precipitate AlN, which is necessary for suppressing abnormal grain growth on the treated surface portion, the Al content needs to be 0.027% or more. On the other hand, the upper limit of the Al content is set to 0.090%, because coarse AlN is generated by excessive addition of Al, or AlN cannot be sufficiently dissolved during hot working (hot rolling or hot forging) before carburizing treatment, so that fine AlN cannot be sufficiently precipitated at the subsequent vacuum carburizing temperature rise, and abnormal grain growth cannot be suppressed.

N：0.0140～0.0300％，

N (nitrogen) is an element that exhibits an effect of suppressing abnormal grain growth during carburization when it is bonded to Al and present as AlN. In order to precipitate AlN in a sufficient amount, it is effective to make the N content 0.0140% or more. On the other hand, since coarse AlN is easily produced by adding an excessive amount of N, the upper limit of the N content is set to 0.0300%.

Further, it is necessary that the number of AlN precipitates having an equivalent circle diameter of 100nm or more in the cross section of the blank for vacuum carburization is 1.5/100. mu.m²The following. That is, need toThe number of coarse AlN precipitates having an equivalent circle diameter of 100nm or more is reduced to at least 1.5 precipitates per 100 μm²The following. Thus, in the temperature raising process during vacuum carburization, fine AlN can be deposited in a sufficiently large amount while maintaining the state in which coarse AlN is not present or is very little, if any.

Further, the blank for vacuum carburization preferably has a ferrite-pearlite structure containing almost no bainite in the internal structure. This improves machinability, and reduces the driving force for grain growth during vacuum carburization, thereby suppressing abnormal grain growth.

Next, in order to obtain the above excellent blank for vacuum carburization, the following production method can be employed. That is, a method of manufacturing a blank for vacuum carburizing can be used, in which a steel material having the above combination of chemical components is subjected to final hot working, the hot working is performed while heating to a temperature described later, and then, the steel material is cooled to 900 ℃ at a cooling rate of 1 ℃/sec or more.

The final hot working is referred to as hot working if the hot working is performed once, and is finally performed when the hot working is performed a plurality of times. The final hot working is typically hot forging, but hot rolling or other thermoplastic working methods may be used.

In the above-described manufacturing method, the final hot working is performed by heating the steel material to a temperature of 1150 ℃ or higher and T1 ℃ or higher calculated by the formula (1). T1 ═ 100000 × Al × N +1090 … (1)

By heating at the above temperature during the hot working, coarse AlN present in the steel material can be dissolved in a solid solution, and the number of coarse AlN can be reduced so as to fall within the above range. The reason why 1150 ℃ C is set to one lower limit temperature is that in order to sufficiently achieve the requirement for solid solution of AlN in the steel material containing Al and N in the above-mentioned predetermined ranges, the reason why T1 (. degree. C.) is set to the other lower limit temperature is that even in the steel material composed of the components in the above-mentioned predetermined ranges, a higher temperature is required to sufficiently make solid solution as the Al and N contents are higher.

Next, after the steel material is formed into a billet of a desired shape by the above-described final hot working, the condition for cooling the billet is set to a relatively rapid cooling condition in which cooling is performed at a cooling rate of 1 ℃/sec or more (preferably 1.5 ℃/sec or more) to 900 ℃. In particular, when a large-sized component is used, this condition cannot be satisfied by simply placing cooling in the atmosphere, and therefore, it is necessary to intentionally increase the cooling rate by fan cooling or the like. This suppresses the formation of coarse AlN after hot working, and ensures the conditions for AlN in the vacuum carburization blank.

As a specific production method for producing the above-described blank for vacuum carburization, there is a method of preparing an ingot by casting after performing dissolution of raw materials and adjustment of components, performing rough processing such as hot rolling on the ingot, and then performing hot forging, hot rolling, or the like as the above-described final hot processing.

Here, after the final hot working, annealing may be added. By adding annealing, the effect of making it easier to machine into the final part shape can be obtained.

The annealing itself is a conventionally known heat treatment, and can be performed under various known conditions, for example, after the final hot working, the blank for vacuum carburizing cooled to about room temperature after being cooled to 900 ℃ under the above-described cooling conditions, is heated to a temperature range of 850 ℃ to 900 ℃, is then slowly cooled to about 600 ℃ to 700 ℃, and is then left to stand and cooled to room temperature.

The annealing may be performed by heating at the final hot working. Specifically, annealing conditions may be employed in which the steel sheet is cooled to 900 ℃ under the above-described cooling conditions, and then kept at a temperature of 600 to 680 ℃ for 40 to 120 minutes during further cooling, and then left to cool to room temperature.

The obtained blank for vacuum carburization is usually subjected to cutting, then to vacuum carburization, and then to finishing. Here, vacuum carburization is also referred to as reduced pressure carburization, and is performed, for example, in a treatment furnace maintained at a treatment temperature while introducing a carburizing gas such as acetylene in a pulse form, and the ambient pressure is reduced to a pressure lower than the atmospheric pressure. The pressure reduction condition is preferably in the range of 50 to 3000 Pa. As the carburizing gas, for example, a hydrocarbon gas, particularly acetylene, can be used.

Here, from the viewpoint of improving productivity, the treatment temperature of vacuum carburization is preferably 1000 ℃ or higher, and is performed at a temperature lower than T2 (deg.c) represented by the following formula (2),

t2 is 300 × (Al-0.027) +1000 … (2) (where the element symbol in formula (2) represents the value of the content (mass%) of each element).

That is, although the vacuum carburizing treatment can be performed at a higher treatment temperature as described above, in order to reliably suppress the coarsening of crystal grains in the treated surface portion and perform the carburizing treatment at the same time, it is necessary to determine an appropriate temperature corresponding to the value of the Al content. The formula (2) is derived from the results of experiments under a wide range of conditions based on the idea described above.

Therefore, by performing the vacuum carburization at the vacuum carburization process temperature lower than T2, abnormal grain growth during carburization can be reliably suppressed.

(Experimental example 1)

Examples of the above-described blank for vacuum carburization and the method for producing the same will be described.

In this example, as shown in table 1, various test pieces were produced using samples (examples 1 to 13, comparative examples 14 to 19, and reference example 20) composed of 20 kinds of steel materials different in chemical composition, and evaluated.

TABLE 1

The diameter of each steel material cast by melting in an electric furnace was produced by forging and drawingThe steel bar of (1). A test piece having a diameter of 8mm and a height (longitudinal direction) of 12mm was produced by machining from this steel bar.

Each test piece was subjected to a processing corresponding to the final heat treatment. Specifically, the steel sheet was heated at the heating temperature shown in table 2 using a "thermemassator" device manufactured by fuji electric wave machine (inc.) and then subjected to upsetting with a compressibility of 25%. After the implementation, after cooling to 900 ℃ at the cooling rate shown in table 2, annealing treatment was then performed by either of the following two methods.

In table 2, the annealing treatment denoted by "IA" is a treatment in which the test piece is once cooled to room temperature, then heated to 900 ℃, heated and held for 60 minutes, then slowly cooled to 600 ℃, and left to cool to room temperature.

In table 2, the annealing treatment denoted as "FIA" was carried out after the above-described final hot working, after cooling the test piece to 900 ℃ at the cooling rate described in table 2, continuously leaving the test piece to cool to a surface temperature of 650 ℃, heating to 650 ℃ for 60 minutes, and then leaving the test piece to cool to room temperature.

The test piece before the vacuum carburization treatment to which the above-described annealing treatment was applied was subjected to metal structure observation, and the structure state was confirmed, and the equivalent circle diameter and the number density of the precipitated AlN were calculated. The results are shown in Table 2. In table 2, the expression "F + P" refers to a ferrite-pearlite structure.

Regarding the AlN precipitated state, a surface perpendicular to the longitudinal direction of the annealed test piece was cut out, buried and polished, and then the polished surface was etched and observed by an FE-SEM (field emission scanning electron microscope). The measurement was performed by observing 10 visual fields with 2 ten thousand times of visual field, and SEM images were taken. The SEM images were subjected to image analysis using image analysis software "Quick Grain Standard", and the equivalent circle diameter and number density of AlN were calculated. EDX (energy dispersive X-ray analysis) was also performed to confirm that the precipitates were AlN.

For reference, fig. 1 and 2 show an example of SEM images of example 8 and comparative example 17. In example 8 shown in fig. 1, coarse AlN precipitates were not observed at all in the SEM image. In comparative example 17 shown in fig. 2, at least two AlN (particles indicated by white arrows) were confirmed in one visual field shown in fig. 2 in the SEM image, and the equivalent circle diameters thereof were all 100nm or more, and were about 170nm (upper part of fig. 2) and 295nm (lower part of fig. 2), respectively.

Next, the annealed test piece was subjected to vacuum carburization at a temperature shown in table 2. Further, the pressure in the furnace was controlled to 100Pa, and acetylene (C) was used₂H₂) The carburizing time was 1.5 hours as a carburizing gas. In table 2, the temperatures calculated by the formula (2) are shown as the temperatures at which the upper limit of the carburizing treatment temperature should be limited. The calculation result of the formula (2) is valid only when the combination of chemical components is within the above-described appropriate range.

The metal structure of each test piece after vacuum carburization was observed. Specifically, a surface parallel to the longitudinal direction and passing through the center of the test piece was cut out, and after etching with picric alcohol, 10 fields of view were arbitrarily observed at a magnification of 100 times using an optical microscope. In this case, since the problematic site is the treated surface portion as described above, the carburized layer is observed to be limited to the influence of carburization. When there were more than 20% of grain growth regions with a grain size number 3 or more larger than that of the other regions in the observed range, the mixed crystal grain was judged to have abnormal grain growth. The measurement of the crystal particle size was carried out in accordance with JIS G0551.

TABLE 2

As shown in tables 1 and 2, in examples 1 to 13, the combination of chemical components was within an appropriate range, and in the state of the blank for vacuum carburization before the vacuum carburization treatment, the number of coarse AlN precipitates having a circle equivalent diameter of 100nm or more was 1.5/100 μm²The following. Therefore, abnormal grain growth was not observed in the treated surface portion as a problem even after the vacuum carburization treatment. In addition to this, the present invention is,for the sake of prevention, the inside was observed at the same time, but no coarsening of crystal grains was observed at all.

On the other hand, in comparative example 14, since the N content was too low, the number of fine AlN precipitates was too small, and abnormal grain growth occurred in the treated surface portion.

It is seen that in comparative example 15, the Al content is too low, and the fine AlN precipitates are too small, so that abnormal grain growth occurs in the surface layer.

In comparative examples 16 and 17, although the chemical composition was within the appropriate range, the heating temperature during final hot working was lower than T1 (. degree. C.), and therefore, the solution of coarse AlN did not sufficiently proceed, and more than 1.5 coarse AlN precipitates having an equivalent circle diameter of 100nm or more were present per 100 μm²Abnormal grain growth occurs.

In comparative examples 18 and 19, the combination of chemical components was proper, but the cooling rate to 900 ℃ after the final hot working was too slow, and the number of coarse AlN precipitates having a circle equivalent diameter of 100nm or more exceeded 1.5 AlN precipitates per 100 μm²Abnormal grain growth occurs.

In reference example 20, a test was conducted using the same steel material as in example 7, and an excellent blank for vacuum carburization was obtained because of appropriate chemical composition and production method, but the treatment temperature of the subsequent vacuum carburization treatment exceeded the condition of T2 (deg.c) as the calculation result of formula (2), and therefore the occurrence of abnormal grain growth could not be prevented.

(Experimental example 2)

In this example, tests 71 to 73 were carried out using the steel material of example 7, with the heating temperature of the final hot working set at 1250 ℃ and the cooling rate after the working to 900 ℃ being varied, and the influence on the precipitation state of coarse AlN and the like were examined.

TABLE 3

As shown in Table 3, when the cooling rate to 900 ℃ after the final hot working was less than 1 ℃/sec (test 71), the equivalent weight was obtainedMore than 1.5 AlN precipitates having a diameter of 100nm or more in a coarse size per 100 μm²Abnormal grain growth was observed after the vacuum carburization treatment. On the other hand, when the cooling rate to 900 ℃ after the final hot working is 1 ℃/sec or more (tests 72 to 73), the number of coarse AlN precipitates having an equivalent circle diameter of 100nm or more is 1.5/100 μm²Hereinafter, no abnormal grain growth was observed even after the vacuum carburization treatment. Based on the results, it is found that, in addition to selecting an appropriate combination of chemical components, it is effective to suppress the generation of coarse AlN by appropriately controlling the heating temperature for final hot working to 1150 ℃ or higher and T1(° c) or higher and the cooling rate to 900 ℃ after working, thereby suppressing abnormal grain growth in the vacuum carburization process.

(Experimental example 3)

In this example, tests 91 to 93 in which the heating temperature for the final hot working was changed were carried out using the steel material of example 9, and the influence on the precipitation state of coarse AlN and the like were examined.

TABLE 4

As shown in Table 4, in the case where the heating temperature in the final hot working was lower than T1 (. degree. C.) calculated from the formula (1) (test 91), even when the cooling rate to 900 ℃ after the working was sufficiently high, the number of coarse AlN precipitates having a circle equivalent diameter of 100nm or more exceeded 1.5 AlN precipitates per 100. mu.m²Abnormal grain growth was observed after the vacuum carburization treatment. On the other hand, when the heating temperature in the final hot working is 1150 ℃ or higher and higher than T1 (DEG C) calculated by the formula (1), and when the cooling rate to 900 ℃ after the final hot working is 1 ℃/sec or higher (tests 92 to 93), the number of coarse AlN precipitates having an equivalent circle diameter of 100nm or more is 1.5/100 μm²No abnormal grain growth was observed even after the vacuum carburization treatment. Based on the results, it was found that the heating temperature in the final heat treatment was 1150 ℃ after selecting an appropriate combination of chemical componentsThe above T1 (c) or more and the cooling rate to 900 c after the working is appropriately controlled are effective for suppressing the generation of coarse AlN, whereby abnormal grain growth in the treated surface portion in the vacuum carburization treatment can be suppressed.