JPH0360899B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention is capable of stably obtaining a structure with fine crystal grains even when carburized at a high temperature of 950°C or higher, and has various structures with outstanding strength and toughness. This relates to steel for high-temperature carburizing that can provide parts (or products) for use. (Prior art) For example, the power transmission mechanism (power train system) of transportation machinery such as automobiles, other industrial machinery, agricultural machinery, etc. includes structural parts such as various gears, bearings, and shafts. is used. These structural parts are generally formed from mechanical structural carbon steel or alloy steel, and the resulting molded parts are then subjected to surface hardening treatments such as gas carburizing and carbonitriding to complete the finished product. It had been. In this case, conventional surface hardening treatments are all 950
The process is carried out at a temperature of less than 0.degree. C., and requires a long treatment time to obtain the required depth of carburization or carbonitriding, which is one of the factors that inhibits the productivity of the above-mentioned structural parts. Therefore, against this background, a vacuum carburizing method was developed as one of the carburizing techniques that allows carburizing in a short time. This vacuum carburizing method is generally carried out at a high temperature of 950°C or higher. However, conventional mechanical structural steels such as JIS
When the above-mentioned high-temperature vacuum carburizing treatment is applied to structural parts formed from standard SCR420 and SCM420 materials, the crystal grains become coarse due to the high-temperature treatment, resulting in large heat treatment distortions and a significant decrease in the strength of the parts. There was a problem of doing something like that. Therefore,
Generally, a method is adopted in which the above-mentioned parts are subjected to vacuum carburizing treatment, then cooled to a temperature below the transformation point, and then heated again to the austenitizing temperature and quenched, which is a so-called grain refining treatment. However, in this case, the processing time increases because cooling and reheating treatments are added to the vacuum carburizing process, and the process time is not reduced as much as originally expected compared to the conventional gas carburizing process, and the carburizing process This has been a major impediment to the widespread use of the vacuum carburizing method, which has excellent performance. On the other hand, in order to solve these problems, attempts have been made to develop steel that has a dual-phase structure of austenite and ferrite at high temperatures, but in this case, although grain refinement in the core is achieved, the carburized layer The problem remained that grain refinement was still insufficient. (Purpose of the Invention) The present invention has been made by focusing on the above-mentioned conventional problems, and is particularly applicable when carburizing a part formed into a predetermined shape at a high temperature of 950°C or higher. However, unlike conventional methods, the crystal grains in the carburized layer do not become coarse, and the crystal grains are refined not only in the core of the part but also in the carburized layer, and a structure with fine crystal grains can be stably obtained. The aim is to provide high-temperature carburized steel that can be (Structure of the Invention) The high-temperature carburized steel described in claim 1 of the present invention has a carbon content of 0.03% or more and 0.2% by weight.
Below, Si: 1.5% over 3.0% or less, Mn: 0.2% or more
The basic component is 2.0% or less, Ti: 0.03% or more and 0.3% or less, the balance is Fe and impurities, and the core during high-temperature carburizing has a two-phase structure containing an austenite phase and a ferrite phase, and after high-temperature carburizing. The crystal grains in the carburized layer and the core are characterized by being fine grains with a grain size number of 6 or more. In addition, the high temperature carburized steel described in claim 2 of the present invention has C: 0.03% or more and 0.2% or less, Si: more than 1.5% and 3.0% or less, and Mn: 0.2% by weight.
% or more and 2.0% or less, Ti: 0.03% or more and 0.3% or less, Ni: 2.0% or less, Cr: 2.0%
Contains one or more of the following: Mo: 0.5% or less, the balance is Fe and impurities, the core during high temperature carburizing has a two-phase structure containing an austenite phase and a ferrite phase, and a high temperature It is characterized in that the crystal grains in the carburized layer and the core after carburizing are fine grains with a grain size number of 6 or higher. Furthermore, the high-temperature carburized steel described in claim 3 of the present invention has a carbon content of 0.03% by weight.
Over 0.2%, Si: over 1.5%, up to 3.0%, Mn:
The basic components are 0.2% or more and 2.0% or less, Ti: 0.03% or more and 0.3% or less, and Al: 0.1% or less, Nb+
Contains one or more ^of Ta: 0.5% or less, Zr: 0.1% or less, N: _0.30 % or less, and the balance is Fe.
and impurities, and the core during high-temperature carburizing has a two-phase structure containing an austenite phase and a ferrite phase, and the crystal grains in the carburized layer and core after high-temperature carburizing have a grain size number of 6 or more. It is characterized by fine grains. Furthermore, claim 4 according to the present invention
The high-temperature carburized steel described in Section 1 has a C:
0.03% or more and 0.2% or less, Si: over 1.5% and 3.0% or less,
Mn: 0.2% or more and 2.0% or less, Ti: 0.03% or more and 0.3%
The following are the basic ingredients, and furthermore, Ni: 2.0% or less,
One or more of the following: Cr: 2.0% or less, Mo: 0.5% or less, Al: 0.1% or less, Nb + Ta: 0.5
% or less, Zr: 0.1% or less, N: 0.03% or less, and the remainder consists of Fe and impurities. It is characterized by a phase structure, and the crystal grains in the carburized layer and the core after high-temperature carburizing treatment are regular grains with a grain size number of 6 or more. And to further improve weather resistance,
Cu: 5% or less, to improve machinability,
Pb: 0.4% or less, S: 0.4% or less, Te: 0.1% or less,
Contains one or more of Bi: 0.4% or less, Se: 0.4% or less, Ca: 0.01% or less, and B: 0.0005% or less to prevent coarsening of crystal grains. [O] to improve cooling properties:
It is regulated to 0.0030% or less and/or S: 0.02% or less, and further contains Sn, Sb, As, which are carburizing inhibiting elements.
It is also preferable to regulate the content of such substances. Next, the reason for limiting the component range (wt%) of the high temperature carburized steel according to the present invention will be explained. C: 0.03% or more and 0.2 or less C is added to obtain the strength required for structural parts and the surface hardness after carburizing, and at the same time, it strengthens the structure of crystal grains by obtaining a two-phase structure of austenite + ferrite at high temperatures. This element is included to improve grain size, but if the content is less than 0.03%, it will not be possible to obtain the above-mentioned required strength and two-phase structure at high temperatures, and if it exceeds 0.2%, the toughness and Since cold-melting properties deteriorate and it becomes difficult to secure a two-phase structure at high temperatures, the range is set to 0.03% or more and 0.2% or less. Si: More than 1.5% and less than 3.0% Si is effective as a deoxidizing element, and especially in the high-temperature carburized steel of the present invention, it ensures a two-phase structure of austenite + ferrite at high temperatures and prevents coarsening of core grains. It is an effective element in
% exceeding 3.0% or less. Mn: 0.2% or more and 2.0% or less Mn is an effective element for deoxidizing and desulfurizing as well as increasing strength.
%, the above-mentioned effects of deoxidation, desulfurization and strength improvement cannot be obtained, and sufficient surface hardness after carburization cannot be obtained. On the other hand, if it exceeds 2.0%, the workability and machinability deteriorate, so 0.2%
The range was set at % or more and 2.0% or less. Ti: 0.03% or more and 0.3% or less Ti combines with carbon that invades during high-temperature carburizing treatment to precipitate titanium carbide, and this precipitate inhibits the growth of austenite, resulting in coarse grains in the surface layer. It is an element that is effective in refining the crystal grains of the surface carburized layer formed after carburizing treatment, and for this purpose, it must be contained in an amount of 0.03% or more. The higher the Ti content, the better it is for crystal grain refinement, but since a large Ti content deteriorates toughness, the upper limit was set at 0.3%. One or more of the following: Ni: 2.0% or less, Cr: 2.0% or less, Mo: 0.5% or less Ni, Cr, and Mo are used to further improve the hardenability of steel and strengthen the matrix. Although it is an effective element,
If the Ni content exceeds 2.0%, the Cr content exceeds 2.0%, and the Mo content exceeds 0.5%, the toughness of the steel will deteriorate, so it is necessary to further improve the hardenability of the steel and strengthen the matrix. When measuring, Ni: 2.0% or less, Cr:
One or more of these are contained within the range of 2.0% or less and Mo: 0.5% or less. Al: 0.1% or less, Nb + Ta (including cases where either one is 0): 0.5% or less, Zr: 0.1% or less,
N: One or more of 0.03% or less Al, Nb, Ta, Zr, and N are effective elements for preventing coarsening of austenite crystal grains during carburizing treatment at high temperatures. If desired effects are to be obtained, one or more of these may be added.
However, if the Al content exceeds 0.1%, the Nb+Ta content exceeds 0.5%, and the Zr content exceeds 0.1%, the effect of preventing crystal grain coarsening will actually decrease, making it difficult to ensure toughness. , when added, each was within the above range. In addition, if the N content exceeds 0.03%, the integrity of the steel ingot or slab will be damaged due to N blowholes, so the N content should be set at 0.03%.
% or less. In addition, Cu:
5% or less can be contained as appropriate, and in order to improve machinability, Pb: 0.4% or less, S: 0.4% or less, Te: 0.1% or less, Bi: 0.4% or less, Se: 0.4%.
Hereinafter, Ca may be appropriately contained in a range of 0.01% or less. Furthermore, if the B content exceeds 0.0005%, there is a risk of coarsening of austenite crystal grains during carburizing treatment at high temperatures, so the upper limit is set at 0.0005%.
It is more desirable to regulate it to below, and in order to improve fatigue strength and cold weathering property, it is also desirable to regulate [O] to 0.0030% or less and S: 0.02% or less.
Carburizing inhibiting elements Sn: 0.05% or less, Sb: 0.05
It is also desirable to restrict As to 0.05% or less. The high-temperature carburizing steel according to the present invention, whose composition has been adjusted in this manner, is used as a raw material to be molded into the shapes of various structural parts such as gears, ball joints, drive shafts, camshafts, steering parts, bearings, and bearing races, and then, for example, 950℃
By performing the above high-temperature carburizing treatment, it is possible to carburize in a short time, and the crystal grains in the carburized layer and core after high-temperature carburizing have a grain size number of 6.
As a result, structural parts with excellent toughness, fatigue strength, etc., and good dimensional accuracy can be obtained in a short time. That is, in the high temperature carburizing steel according to the present invention,
Even when carburizing at high temperatures exceeding 950℃,
During high-temperature carburizing, the core maintains a two-phase structure containing an austenite phase and a ferrite phase, so austenite and ferrite inhibit each other's crystal grain growth, and as a result, coarsening of the crystal grains is inhibited. On the other hand, the surface layer into which carbon penetrates changes from a two-phase structure of austate + ferrite to a single-phase austenite structure during high-temperature carburizing. Normally, when single-phase austenite is exposed to high temperatures of 950°C or higher, the crystal grains become coarse, but in the present invention, Ti is added at 0.03% or more.
Since it is contained in a range of 0.3% or less, the invading carbon combines with Ti dissolved in austenite,
As a result of the precipitation of titanium carbide, this precipitate inhibits the growth of austenite, so that the surface layer also does not have coarse grains. Next, examples of the present invention will be described together with comparative examples. After melting steel with the chemical composition shown in Table 1, it is ingot-formed, and a round bar with a diameter of 32 mm is manufactured by forging.
After heating at 925°C for 1 hour, normalizing is performed under air cooling conditions.
Next, it was cut to a diameter of 25 mm, and then vacuum carburized. Then, the average austenite grain size of the carburized layer and the average grain size of austenite and ferrite in the core were measured for each.
Incidentally, Table 2 shows the vacuum carburizing treatment conditions. Also,
The grain size was measured according to the "Austenite grain size test method for steel" specified in JIS G0551. The results are also shown in Table 1.

【table】

[Table] As shown in Table 1, in comparison steel Nos. 1 to 6 whose C, Si, and Ti contents are outside the range of the present invention, the average crystals of austenite in the carburized layer and austenite and ferrite in the core are None of the samples simultaneously satisfied the condition that the grain size was No. 6 or more, and it is clear that coarsening of crystal grains occurred in the carburized layer or core during high-temperature vacuum carburizing. On the contrary,
Inventive steel Nos. 7 to 22 within the scope of the present invention, the austenite in the carburized layer and the austenite and ferrite in the core have an average crystal grain size of No. 6 or higher, and are fine grains. As shown, the higher the Ti content, the better the grain refinement, and as shown in Nos. 16 to 22, Al, Nb, Ta, Zr, N
It is clear that the addition of an appropriate amount of has become effective and has significantly reduced the size of the particles, making it possible to improve the toughness, wear resistance, fatigue strength, and dimensional accuracy of structural parts such as gears. It is possible. Next, the invention steel No. 7 and the invention steel No. 10 containing one or more of Ni, Cr, and Mo.
When the mechanical properties of Nos. 12 and 15 were investigated, the results were shown in Table 3.

[Table] As shown in Table 3, by adding appropriate amounts of one or more of Ni, Cr, and Mo, it is possible to further improve the hardenability of steel and improve mechanical properties. It was clear that it could be further increased. Next, machinability tests were conducted on sample material No. 11 in Table 1 and sample materials No. 23 to 26 to which the machinability improving elements shown in Table 4 were added under the conditions shown in Table 5. As a result, the results shown in Table 6 were obtained.

【table】

【table】

[Table] As shown in Table 6, the addition of machinability-improving elements clearly resulted in a significant improvement in machinability, and an increase in the number of drilled holes was confirmed. Furthermore, weather resistance tests were conducted on sample material No. 11 in Table 1 and sample material No. 27 to which the weather resistance improving elements shown in Table 7 were added under the conditions shown in Table 8.
The results are shown in Table 9.

【table】

【table】

[Table] As shown in Table 9, sample material No. 27 is sample material No. 11.
Significant improvement in weather resistance was observed compared to In addition, the above-mentioned sulfide form controlling elements are added as appropriate, and B, O, S, Sn, Sb, As
When we conducted an experiment by selectively regulating the content of
Good results were obtained in all cases. (Effects of the Invention) As explained above, in the high-temperature carburizing steel according to the present invention, even when carburizing is performed at a high temperature of 950°C or higher, the core part during this high-temperature carburizing process contains an austenite phase and a ferrite phase. It has a two-phase structure containing
After high-temperature carburizing, the carburized layer and core grains are fine grains with a grain size number of 6 or higher, resulting in a structure with excellent strength, toughness, wear resistance, and fatigue resistance, and high dimensional accuracy. parts, such as gears,
It has a very excellent effect in that steering parts, shafts, bearing races, etc. can be manufactured by carburizing in a short time.

Claims (1)

[Claims] 1. C: 0.03% or more and 0.2% or less, Si: 1.5% by weight
% exceeding 3.0% or less, Mn: 0.2% or more and 2.0% or less,
Ti: 0.03% to 0.3% as the basic component, the balance
It consists of Fe and impurities, and the core during high-temperature carburizing has a two-phase structure containing an austenite phase and a ferrite phase, and the carburized layer and core grains after high-temperature carburizing have a grain size number of 6 or higher. A high-temperature carburized steel characterized by fine-grained grains. 2 In weight%, C: 0.03% or more and 0.2% or less, Si:
More than 1.5% and less than 3.0%, Mn: 0.2% and more and less than 2.0%,
The basic components are Ti: 0.03% or more and 0.3% or less, Ni: 2.0% or less, Cr: 2.0% or less, Mo: 0.5%
Contains one or more of the following, with the remainder
It consists of Fe and impurities, and the core during high-temperature carburizing has a two-phase structure containing an austenite phase and a ferrite phase, and the carburized layer and core grains after high-temperature carburizing have a grain size number of 6 or higher. Steel for high-temperature carburizing, characterized by its fine-grained structure. 3 In weight%, C: 0.03% or more and 0.2% or less, Si:
More than 1.5% and less than 3.0%, Mn: 0.2% and more and less than 2.0%,
The basic components are Ti: 0.03% or more and 0.3% or less, Al: 0.1% or less, Nb + Ta: 0.5% or less, Zr:
0.1% or less, N: 0.03% or less, the remainder consists of Fe and impurities, and the core during high-temperature carburizing treatment has a two-phase structure containing an austenite phase and a ferrite phase, A steel for high-temperature carburizing, characterized in that the crystal grains in the carburized layer and core after high-temperature carburizing treatment are fine grains with a grain size number of 6 or more. 4 In weight%, C: 0.03% or more and 0.2% or less, Si:
More than 1.5% and less than 3.0%, Mn: 0.2% and more and less than 2.0%,
The basic components are Ti: 0.03% or more and 0.3% or less, Ni: 2.0% or less, Cr: 2.0% or less, Mo: 0.5%
One or more of the following and Al: 0.1%
Below, Nb + Ta: 0.5% or less, Zr: 0.1% or less,
Contains one or more of N: 0.03% or less, with the remainder consisting of Fe and impurities, and the core during high-temperature carburizing has a two-phase structure containing an austenite phase and a ferrite phase, and the high-temperature carburizing process A steel for high-temperature carburizing, characterized in that the crystal grains in the subsequent carburized layer and core are fine grains with a grain size number of 6 or more.