JPH0576523B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to a method for producing case hardened steel. More specifically, the present invention relates to a method for manufacturing steel for case hardening, which can be subjected to surface effect treatment while suppressing coarsening of crystal grains. Conventional technology Carbon case-hardened steel and alloy case-hardened steel are used for power transmission parts and sliding parts of industrial machinery, construction machinery, automobiles, etc., but they have excellent wear resistance, high fatigue strength,
Since high surface pressure strength is required, case hardening treatment, that is, surface hardening treatment, is generally performed. There are various methods for surface hardening, but carburizing and quenching or carbonitriding are still the most popular methods because they have a remarkable effect on improving the properties of steel. However, in carburizing or carbonitriding, austenite crystal grains tend to become coarser because heating is usually performed at 900 to 950°C for several hours. Therefore, as a countermeasure to this problem, a method has been considered in which Al and N are added in a well-balanced manner to prevent austenite crystal grains from becoming coarser by the effect of suppressing crystal grain growth (pinning effect) by Al nitride. However, in reality, such measures alone are not sufficient, and it becomes extremely difficult to prevent coarsening of austenite grains, especially when high-temperature carburizing is performed at 950° C. or higher. Therefore, various studies have been carried out in the past, and for example, Tokko No. 52-5003, Tokko No. 1647, Tokko No. 1647, and Tokko No.
Steels to which Nb is added have been proposed, as described in Publication No. 57-1657. The basic idea behind these proposals is
This is an attempt to utilize the pinning effect at austenite grain boundaries by precipitating Nb carbonitrides. However, in this case, the effect varies greatly depending on the history of thermal processing before carburizing or carbonitriding, and is extremely unstable. Problems to be Solved by the Invention The purpose of the present invention is to solve the problems of the prior art as described above, and more specifically, to make full use of the effect of Nb addition to ensure coarsening of austenite grains. The object of the present invention is to realize case hardening treatment of structural carbon steel and alloy steel while preventing the above. Means to Solve the Problems In other words, the inventors of the present invention conducted extensive experiments and studies to provide a case-hardened steel that does not cause coarsening of austenite grains. It has been found that the pinning effect of Nb can be maximized by precipitating all the Nb contained in the steel finely and uniformly as Nb carbide so that no molten Nb remains.
In addition, for this purpose, Nb carbonitrides precipitated by simple heat treatment, including carburizing or carbonitriding, have little effect, and fine particles that precipitate during processing in the temperature range of 750℃ to 1000℃ It was found that only Nb carbide is effective. Furthermore, in order to precipitate all the Nb contained in the steel as Nb carbide, which is effective in preventing coarsening of austenite grains,
It is necessary to process at a working degree of 20% or more in the temperature range of It was found that it is necessary to heat most of the Nb nitride to a solid solution in advance. In addition, the present inventors have repeatedly studied and found the following. That is, Nb carbide has a higher solid solution temperature than Nb nitride, and does not dissolve until high temperatures. on the other hand,
When Nb nitride is subjected to processing strain, its solid solution temperature decreases. Therefore, when performing normal carburizing or high-temperature carburizing on processed cold-forged or warm-forged materials, Nb carbide suppresses grain coarsening better than Nb nitride. It has become clear that it is effective. Therefore, in the present invention, in addition to regulating the N content, Al, Ti, and
Add Zr to fix N (i.e. Al nitride,
(Ti nitride, Zr nitride) and Nb carbide. Furthermore, as a result of the inventors' investigation, the total amount of solid solute Nb (that is, the amount of Nb after subtracting the chemical equivalent equivalent to the amount of solid solute N) that can form Nb carbide.
It has been found that it is necessary for Nb ^* to remain in the steel in an amount of at least 0.01%. Therefore, the present invention stipulates that the total amount of solid solution Nb, Nb ^* , defined by the Nb content, Sol.Al content, Ti content, N content, and Zr content, is greater than 0.01%. . i.e. Nb content, sol.Al
The following relational expression was required to be satisfied among the content, Ti content, N content, and Zr content. Nb (%) - 93/14 [N (%) - 14/27 Sol.Al
(%) - 14/48Ti (%) - 14/91Zr (%)]≧0.01% Therefore, according to the present invention, C: 0.10 to 0.45%, Si: 1.5% or less, Mn: 0.1 to 1.5%, Nb : 0.01 to 0.14%, N: 0.009% or less, and further contains one or more of the following: Sol.Al: 0.05% or less, Ti: 0.04% or less, Zr: 0.05% or less, and the remainder is Consisting of Fe and unavoidable impurities, and Nb content, sol.Al
After heating the steel whose chemical composition is 0.01% or more of Nb ^* determined by the following formula according to the content, Ti content, Zr content and N content to 1050℃ or more,
Provided is a method for manufacturing case-hardened steel that is less likely to coarsen grains and is characterized by performing processing at a working degree of 20% or more in a temperature range of 750°C to 1000°C. Nb ^* = Nb (%) - 93/14 [N (%) - 14/27s
ol.Al (%) - 14/48Ti (%) - 14/91Zr (%)] In the present invention, the above steel composition further contains at least one selected from the following group: It's okay. Group: Cr: 1.5% or less, Mo: 0.3% or less, Ni: 2.0% or less. Group: S: 0.04-0.13%, Pb: 0.03-0.35%, Ca: 0.001-0.01%, Te: 0.005-0.05%. Effect The reason for limiting the range of steel components and manufacturing conditions in the method of the present invention will be described below. (1) C C is a basic component to ensure strength, and as a case hardening steel, it is necessary to have a center hardness of at least 20 H _R C after carburizing, carbonitriding and quenching, and tempering. is the amount of C at least
It is necessary to contain 0.05% or more. Also,
Fatigue strength, which is a necessary property for structural members, is greatly influenced by the compressive residual stress introduced into the surface layer, but this depends on the difference in hardness between the surface and the center after surface hardening. The greater the hardness difference, the greater the compressive residual stress and the greater the fatigue strength. Therefore, if the center hardness is increased too much, the difference in hardness from the surface becomes small, which is not only disadvantageous in terms of fatigue strength, but also significantly deteriorates impact resistance. This limit is generally set at a center hardness of about H _R C45, and for this purpose the amount of C needs to be suppressed to 0.40% or less. However, in the present invention, considering that the amount of solid solute C decreases due to the formation of Nb carbide,
Both the lower limit and upper limit of the amount of C need to be increased by that amount. Therefore, the amount of C required to generate Nb carbide was considered to be 0.05%, and the lower limit of the content was set to 0.10% and the upper limit to 0.45%. (2) Si Si is usually added as a deoxidizing agent, but in some cases, just adding Mn or Al can be sufficient for deoxidizing. Si is an element that dissolves in ferrite and reduces the ductility, cold workability, and machinability of steel, so the lower the amount, the better; however, the adverse effects are small up to 1.5%, so the upper limit of the Si content should be set.
It was set at 1.5%. (3) Mn Mn is not only essential as a deoxidizing agent, but also effective in improving the strength and toughness of steel. Further, it is the most effective element for improving hardenability, which is an important property for case hardening steel, and it is preferable to add at least 0.10% in consideration of synergistic effects with other hardenability improving elements. However, when added in excess of 1.5%, hot workability deteriorates and machinability begins to decline rapidly. Therefore,
The lower limit of Mn content was set to 0.1%, and the upper limit was set to 1.5%. (4) Nb
It combines with Nb to form fine and uniform Nb carbide.
This Nb carbide suppresses the coarsening of austenite grains, but in order to exhibit this effect, it is necessary to contain at least 0.01% Nb. However, if Nb is added in an amount exceeding 0.14%, the solid solution C will be significantly reduced, which may lead to a decrease in strength. Therefore, the lower limit of the Nb content was set to 0.01%, and the upper limit was set to 0.14%. (5) Sol.Al Al is usually added as a deoxidizing agent, but in some cases, just adding Si or Mn can be sufficient to deoxidize. In the present invention, as mentioned above, Nb
The purpose is to suppress the coarsening of austenite grains by the pinning effect of carbides, and therefore, it is necessary to fix all N, where the bonding force of Nb is stronger than that of C. For this purpose, it is effective to add Al, which has a stronger bonding force with N than Nb, but as the amount added increases, the number of alumina-based inclusions increases, which increases the number of defects on the surface of the steel. Al nitrides aggregate and coarsen,
It has a negative effect on the fine dispersion of Nb carbide. Therefore, the smaller the amount of Sol.Al added, the better.
The harmful effects are slight up to 0.05%, so
The upper limit of Sol.Al content was set to 0.05%. (6) Ti Ti, like the aforementioned Al, is also extremely effective in fixing N for Nb carbide formation. However, the TiN produced has a large shape and deteriorates machinability, so the smaller the amount of Ti added, the better. Therefore, 0.04 is a small range of machinability deterioration.
% or less was defined as the Ti content range. (7) Zr Zr, like Sol.Al and Ti, is a very effective element for fixing N. However, when added in a large amount, machinability deteriorates, and the Zr nitride aggregates and coarsens, which adversely affects the fine dispersion of the Nb carbide. Since the limit of Zr content that does not cause such adverse effects is 0.05%, the upper limit was set at 0.05%. (8) N In the present invention, as mentioned above, the formation of Nb nitride is undesirable, so in order to fix N,
Although the addition of Sol.Al, Ti, and Zr is prescribed, the smaller the amount of these elements added, the better. Therefore, it is preferable that the content of N to be fixed is also small. Therefore, by limiting the N content to 0.009% or less, it is added to fix N.
The negative effects of Sol.Al, Ti, and Zr become completely negligible. Therefore, the upper limit of N content is set to 0.009.
%. (9) Cr Cr improves the hardenability of steel and
It is effective in improving carburizing properties and improving wear resistance after carburizing. However, if Cr is added in excess of 1.5%, there is a strong tendency for excessive carburization to occur, and strip-like cementite is formed in the surface layer, which significantly deteriorates fatigue strength and pitting resistance.
The upper limit of Cr content was set at 1.5%. (10) Mo Like Cr, Mo is also an element that improves hardenability and carburizability. Also, unlike Si, Mn, and Cr, it has a lower oxidation potential than Fe, so it is less likely to be oxidized during carburizing, and therefore has the effect of suppressing the formation of a so-called abnormal carburized layer (incompletely quenched layer) that forms on the surface layer. Furthermore, it has the effect of preventing Mo from becoming embrittled during quenching and tempering. Therefore, it is preferable to add a large amount, but since it is an expensive element, if you consider adding it in combination with other hardenability-improving elements such as Mn and Cr, it can be used as a case hardening steel.
It is not a good idea to add Mo in excess of 0.3%. Therefore, the upper limit of Mo content was set to 0.3%. (11) Ni Ni is also an element that improves hardenability and has the effect of significantly improving toughness. Further, like Mo, it suppresses carburized abnormal layers, but it is an expensive element and has the effect of inhibiting carburization, so it is not preferable to add a large amount. Therefore, the upper limit of the Ni content was set to 2.0%. Note that C, Si, Mn, Cr, Mo, and Ni do not affect the coarsening behavior of austenite grains. (12) S, Pb, Ca, Te These elements are optionally added elements and are effective in improving machinability when cutting is performed before carburizing or carbonitriding. Cutting method, cutting speed,
It is preferable to add one or more of these elements depending on the cutting depth and the like. Note that these elements have no effect on the austenite grain coarsening tendency and carburizability. The minimum addition amounts necessary to improve the machinability of structural steel are S: 0.04%, Pb: 0.03%, Ca:
0.001%, Te: 0.005%. Also, S is 0.13
%, when Pb exceeds 0.35%, strength and toughness decrease significantly, Ca is difficult to add above 0.001% due to steelmaking technology, and when Te exceeds 0.05%, hot workability decreases rapidly. For S, the lower limit is set to 0.04%, the upper limit is set to 0.13%, and Pb
For, the lower limit is 0.3%, the upper limit is 0.35%,
For Ca, the lower limit is 0.001% and the upper limit is 0.01.
%, for Te, the lower limit is 0.005% and the upper limit is
It was set as 0.05%. (13) Relational expression among Nb, Sol.Al, Ti, and N In the present invention, as mentioned above, it is necessary to precipitate Nb carbide, and for this purpose, the N content that can be combined with Nb is regulated, and the Sol. .Al, Ti,
Zr is added to fix N. Therefore,
In order to prevent coarsening of austenite grains, the amount of Nb that can be bonded with C, Nb ^*, is required to be at least 0.01%, so the lower limit of Nb ^* was set to 0.01%. The reason for limiting the following manufacturing conditions will be explained. (1) Heating temperature The present invention aims to prevent coarsening of austenite grains by using uniformly dispersed fine Nb carbides. For this purpose, Nb carbide precipitated by simple heat treatment (including carburizing or carbonitriding) has a small effect and is insufficient. On the other hand, Nb carbide precipitated due to deformation is used to prevent coarsening of austenite grains. It is extremely effective. In addition, if a large amount of Nb carbonitride remains during heating before processing, the remaining carbonitride will grow and become coarse, so the 1050
It is necessary to heat most of the Nb carbonitride to a solid solution in advance by heating to a temperature of ℃ or higher. Therefore, in order to sufficiently precipitate fine Nb carbides through processing, the pre-existing Nb carbonitrides must be dissolved in solid solution, and for this purpose it is necessary to heat the material to a temperature of 1050°C or higher. . (2) Processing temperature range As mentioned above, the present invention is capable of producing processing-induced precipitation.
This makes maximum use of the pinning effect of Nb carbide. Temperature range over 1000℃ and 750℃
Machining at temperatures lower than ℃ cannot achieve the target uniform and fine precipitation of Nb carbides, so
The lower limit of processing temperature was set to 750°C, and the upper limit was set to 1000°C. (3) Working degree Furthermore, if the working degree is less than 20%, some fine Nb carbides will precipitate, but solid solution Nb will remain.
If solid solution Nb remains, fine Nb carbides will aggregate and coarsen during heat treatment in the post-process, reducing the effect of inhibiting austenite grain growth. For this reason, the content
It is necessary to precipitate all Nb as carbide, and the range of working ratio was specified to be 20% or more. The method and invention will be explained below using examples.
These examples are merely illustrative of the invention;
This is not intended to limit the technical scope of the invention in any way. Examples Thirty-nine types of steel shown in Table 1 were melted in a high-frequency induction melting furnace and formed into 50 mm by forging to prepare sample materials. After heating these specimens to 1150℃ for 1 hour,
It was air-cooled to 900°C, and then immediately rolled to 30φmm (area reduction rate: 43.8%) using a bar rolling mill. Also, for specimen No. 5, 39φmm (4.9%), 37φmm
(14.4%), 35φmm (23.4%), 32φmm (36%), and further rolled to 28φmm (51%). For specimen No. 5, after further heating to 1150℃, 1100℃, 1050℃, 980℃
℃, 800℃, 770℃, 700℃, and 650℃, respectively, and then immediately rolled to 30φmm. After rolling,
Each piece was cut to a thickness of 10 mm, held at 950°C for 4 hours, cooled with water, and then the prior austenite grain boundaries were exposed using a corrosive solution containing a saturated aqueous solution of picric acid with a surfactant added, and the austenite grain size was determined. Table 1 shows the amount of Nb that can form Nb carbide,
The calculated value of Nb ^* and the austenite grain size of the specimen rolled to 30φmm at 900°C are shown together. Test pieces No. 1 to No. 27 are steels according to the present invention, and the test pieces
Nos. 28 to 30 are specimens in terms of Nb content or Nb ^* amount.
Nos. 31 and 32 have N content, and specimens No. 33 and 34 have Sol.
Sample Nos. 35 to 37 are comparative steels that are outside the scope of the present invention in terms of Al content and Ti content, and sample Nos. 38 and 39 are out of the scope of the present invention in terms of Zr content.

【table】

[Table] As can be seen from Table 1, all of the steels of the present invention have fine austenite grains of No. 8 or higher, whereas specimens Nos. 28 to 34, 38, and 39 of the comparative steels all have fine austenite grain sizes of No. 8 or higher. The grain size has also become coarser than No. 5. In addition, Figure 1 shows specimens Nos. 3, 7, 8, 35, 36, and 37, which have substantially the same composition except for the Ti content, with a cutting depth of 0.8 mm and a feed rate of 0.25 mm/min using a cemented carbide P20 cutting tool.
rev., the time required for flank wear of the cutting tool to reach 0.2 mm when performing a cutting test without lubrication, organized by Ti content. As shown in Figure 1,
When the Ti content exceeds 0.05%, the tool life decreases rapidly. Next, FIG. 2 shows the relationship between the austenite grain size and rolling temperature for sample No. 5, and it can be seen that the austenite grains suddenly become coarser when the rolling temperature is outside the range of the present invention. Furthermore, FIG. 3 shows the relationship between the austenite grain size and rolling degree (area reduction rate) of specimen No. 5, which also shows that the grains begin to coarsen rapidly once the range of the present invention is exceeded. Effects As explained above, in order to maximize the pinning effect of Nb, the present invention is heated to a temperature of 1050°C or higher to completely dissolve Nb, and then
By processing 20% or more in the temperature range from ℃ to 750℃, Nb carbide is finely processed and precipitated, and by further adjusting the chemical composition of the steel.
This method is characterized in that Nb is precipitated as Nb carbide with a high solid solution temperature. For this reason, Nb
Total amount of Nb that can become Nb carbide in the content Nb ^*
is defined as 0.01% or more. In this way, the steel treated by the method of the present invention does not cause coarsening of austenite grains even when subjected to high-temperature surface hardening heat treatment, has small heat treatment strain after quenching, and has excellent base material toughness. Case hardened steel with wear resistance, fatigue strength and surface pressure strength can be provided.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between Ti content and flank wear when a cutting test was conducted using a cemented carbide cutting tool. FIG. 2 is a graph showing the relationship between austenite grain size and rolling temperature. FIG. 3 is a graph showing the relationship between austenite grain size and rolling degree.

Claims (1)

[Claims] 1 C: 0.10 to 0.45%, Si: 1.5% or less, Mn: 0.1 to 1.5%, Nb: 0.01 to 0.14%, N: 0.009% or less, furthermore, sol.Al: 0.05 % or less, Ti: 0.04% or less, Zr: 0.05% or less, the remainder consists of Fe and unavoidable impurities, and the Nb content, sol.Al
After heating the steel whose chemical composition is 0.01% or more of Nb ^* determined by the following formula according to the content, Ti content, Zr content and N content to 1050℃ or more,
A method for producing case-hardened steel that is resistant to coarsening of crystal grains, characterized by performing processing at a working degree of 20% or more in a temperature range of 750°C to 1000°C. Nb ^* = Nb (%) - 93/14 [N (%) - 14/27s
ol.Al (%) - 14/48Ti (%) - 14/91Zr (%)] 2 C: 0.10-0.45%, Si: 1.5% or less, Mn: 0.1-1.5%, Nb: 0.04-0.14%, N : 0.009% or less, further contains one or more of the following: sol.Al: 0.05% or less, Ti: 0.04% or less, Zr: 0.05% or less, furthermore, Cr: 1.5% or less, Mo : 0.3% or less, Ni: 2.0% or less, contains one or more of the following, with the balance consisting of Fe and unavoidable impurities, and Nb content, sol.Al
Steel with a chemical composition of 0.01% or more of Nb ^* determined by the following formula according to the content, Ti content, Zr content, and N content is heated to 1050°C or higher, and then heated to 750°C to 1000°C. A method for manufacturing case-hardened steel that is resistant to coarsening of crystal grains, which is characterized by performing processing at a working degree of 20% or more in the temperature range of °C. Nb ^* = Nb (%) - 93/14 [N (%) - 14/27s
ol.Al (%) - 14/48Ti (%) - 14/91Zr (%)] 3 C: 0.10 to 0.45%, Si: 1.5% or less, Mn: 0.1 to 1.5%, Nb: 0.04 to 0.14%, N : 0.009% or less, further contains one or more of the following: sol.Al: 0.05% or less, Ti: 0.04% or less, Zr: 0.05% or less, furthermore, S: 0.04 to 0.13%, Contains one or more of the following: Pb: 0.03 to 0.35%, Ca: 0.001 to 0.01%, Te: 0.005 to 0.05%, the balance consists of Fe and inevitable impurities, and Nb content, sol.Al
After heating the steel whose chemical composition is 0.01% or more of Nb ^* determined by the following formula according to the content, Ti content, Zr content and N content to 1050℃ or more,
A method for producing case-hardened steel that is resistant to coarsening of crystal grains, characterized by performing processing at a working degree of 20% or more in a temperature range of 750°C to 1000°C. Nb ^* = Nb (%) - 93/14 [N (%) - 14/27s
ol.Al (%) - 14/48Ti (%) - 14/91Zr (%)] 4 C: 0.10 to 0.45%, Si: 1.5% or less, Mn: 0.1 to 1.5%, Nb: 0.04 to 0.14%, N : 0.009% or less, further contains one or more of the following: sol.Al: 0.05% or less, Ti: 0.04% or less, Zr: 0.05% or less, furthermore, Cr: 1.5% or less, Mo Contains one or more of the following: 0.3% or less, Ni: 2.0% or less, S: 0.04~0.13%, Pb: 0.03~0.35%, Ca: 0.001~0.01%, Te: 0.005~ 0.05%, sol.Al
After heating the steel whose chemical composition is 0.01% or more of Nb ^* determined by the following formula according to the content, Ti content, Zr content and N content to 1050℃ or more,
A method for producing case-hardened steel that is resistant to coarsening of crystal grains, characterized by performing processing at a working degree of 20% or more in a temperature range of 750°C to 1000°C. Nb ^* = Nb (%) - 93/14 [N (%) - 14/27s
ol.Al (%) - 14/48Ti (%) - 14/91Zr (%)]