JPS622023B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a case-hardened boron steel that does not coarsen austenite grains during carburizing and has small quenching distortion. [Prior art] Various case hardening steels as specified in JIS/G4052 have been known as surface hardening materials, and among these, SCM and SNCM steels have particularly excellent properties. Because of this, it has been widely used. However, these case hardening steels contain expensive Ni and Mo, which increases their price. Hardened boron steel
It has even been put into practical use as an economical alternative steel. This type of case-hardened boron steel is made of
It is characterized by containing 0.01 to 0.05% Ti, and this is to prevent B from forming BN and eliminating the hardenability improvement effect by fixing N in the steel as TiN. It was hot. [Problems to be Solved by the Invention] However, it has been pointed out that conventional case-hardened boron steels have a problem in that austenite grains become coarser during carburizing, resulting in increased quenching strain. [Means for Solving the Problems] From the above-mentioned viewpoint, the present inventors aimed to find an economical case hardening steel for machine structures that has small quenching distortion during carburizing and is particularly inexpensive. Focusing on case-hardening boron steel, we conducted research to prevent coarsening of austenite grains during carburization and suppress quenching distortion, and as a result, we obtained the findings shown in (a) to (d) below. It's ivy. In other words, (a) In general, when B combines with N to form BN, the hardenability improvement effect disappears, so conventionally, B-added steel is made by adding Ti, which has a stronger bonding force with N than B. By fixing the N inside as TiN,
However, since TiN is a compound that is extremely difficult to dissolve in solid solution, it already exists as large precipitates after rolling or forging. It has not taken shape. Therefore, from the viewpoint of suppressing austenite coarsening,
Since TiN does not contribute much, if the Ti content and N content are balanced chemically (i.e., Ti = 3.42N), the grains will naturally become coarser, and as a result, the quenching strain will increase. (However, since TiN is difficult to form a solid solution even at high temperatures, it remains even when heated at high temperatures before rolling or forging, and has the effect of preventing extreme coarsening of crystal grains after rolling or forging. (This has an indirect effect of suppressing the coarsening of austenite grains at the time of use, but for this purpose, it is desirable that the N content exceeds 0.006%). (b) Therefore, if the Ti content is chemically equivalently larger than the N content, the excess Ti will combine with C and form TiC, which, unlike TiN, is a compound that does not easily form a solid solution. Because there is no
It is not formed after rolling or forging, but is formed during carburizing or during heat treatment before carburizing, such as normalizing. Furthermore, since it is in a finely dispersed form, it is extremely effective in suppressing austenite coarsening during carburizing heating. (c) Moreover, in this case, there is almost no effect unless the excess Ti amount is 0.02% or more. (d) By adding one or more of Nb and V to the case-hardened boron steel, the austenite coarsening temperature can be further increased, making high-temperature carburizing possible and shortening the carburizing time. Therefore, the present invention has been made based on the above knowledge, and is based on the above-mentioned findings.
(The following % indicates weight %), C: 0.10 to less than 0.27%, Si: 0.35% or less, Mn: 0.30 to 0.90%, Cr: 0.90 to 1.80%, B: 0.0005 to 0.0050%, sol.Al: Contains 0.08% or less, Ti: more than 0.03% to 0.1%, and further contains one or more of the following, if necessary, Nb: more than 0.06% to 0.10%, V: 0.02 to 0.20%, Fe and unavoidable Impurities: Remaining: The composition should be made up of the following, and the inequality 0.02<Ti-3.42N should hold between the Ti content and the N content (the contents of Ti and N are expressed in weight%). In particular, it is characterized by suppressing heat treatment distortion as much as possible. Next, the reason why the component composition of the case-hardened boron steel of the present invention is limited as described above will be explained. (a) C The C component has the effect of ensuring the strength of case hardening steel for machine structures, and if its content is less than 0.10%, the desired effect cannot be obtained;
If the content exceeds 0.27%, cold workability deteriorates and the toughness of the core after carburization rapidly deteriorates.
It was limited to less than %. (b) Si The Si component has the effect of suppressing the oxidation of B, which invalidates the improvement of hardenability, and when added as a deoxidizer to steel, the above effect is also accompanied, but its content If it exceeds 0.35%, the carburizing properties will deteriorate and the abnormal carburized layer will expand, reducing fatigue strength and pitting resistance, so the content was limited to 0.35% or less. (c) Mn The Mn component is an effective deoxidizing agent to suppress the oxidation of B, and also has the effect of improving hardenability, and forms MnS to prevent hot embrittlement caused by S. However, its content is 0.30
If it is less than %, the desired effect cannot be obtained,
On the other hand, if the content exceeds 0.90%, cold workability and machinability will rapidly deteriorate, and
Since the carburized abnormal layer becomes deeper, its content should be reduced.
It was limited to 0.30-0.90%. (d) Cr The Cr component is an effective element for improving hardenability, strength, toughness, wear resistance, etc., as well as improving carburizability and suppressing abnormal carburized layers. If the content is less than 0.90%, the desired effect cannot be obtained, while if the content exceeds 1.80%, cold workability and machinability will decrease, and the surface portion will be excessively carburized during carburization. Therefore, its content was limited to 0.90 to 1.80%. (e) B Component B is an inexpensive element that significantly improves hardenability in extremely small amounts, but if its content is less than 0.0005% in acid-soluble amount, the desired effect cannot be obtained. On the other hand, if the content exceeds 0.0050%, the hardenability will decrease, so the content was limited to 0.0005 to 0.0050%. (f) sol.Al The sol.Al component is an effective deoxidizing agent for suppressing the oxidation of B, but if its content exceeds 0.08%, alumina inclusions will rapidly increase, causing damage to the workpiece. The content was limited to 0.08% or less because it deteriorates properties. (g) Ti The Ti component has a stronger binding force with N than B, and therefore has the effect of preventing B from combining with N and eliminating the hardenability improvement effect, but its content is 0.03%. If the content is less than 0.1%, the desired effect will not be obtained, and if the content exceeds 0.1%, a large amount of Ti nitride will be generated, deteriorating machinability and toughness. Therefore, the content should be reduced to 0.1%.
% or less. (h) Nb The Nb component has the effect of generating carbonitrides and further increasing the austenite coarsening temperature, but if its content is less than 0.06%, the desired effect cannot be obtained; If the content exceeds 0.10%, no improvement in the effect can be expected and the cost will also increase, so the content was limited to more than 0.06% to 0.10%. (i) V The V component has the effect of generating carbonitrides and further increasing the austenite coarsening temperature, but if its content is less than 0.02%, the desired effect cannot be obtained; If the content exceeds 0.20%, the effect cannot be expected to improve and the cost will increase, so the content should be reduced to 0.02%.
It was limited to ~0.20%. Regarding N, as mentioned above, when the content is 0.006% or less, it prevents the crystal grains after rolling or forging from becoming extremely coarse, and improves the fineness of the austenite grains during carburizing in the subsequent process. At the same time, the indirect effect of TiN that contributes to the formation of austenite grains becomes less noticeable, and the effect of AlN on preventing austenite grain coarsening also tends to decrease. Further, in order to reduce the N content to 0.006% or less, it is necessary to significantly increase the cost in steel manufacturing. Therefore, it is preferable to adjust the N content to more than 0.006%. On the other hand, if the N content exceeds 0.020%, the bond with B becomes excessively large, increasing the concern that it will have a negative effect on the hardenability improvement effect of B.
It is preferable to adjust the content to 0.020% or less. Furthermore, the reason for specifying the inequality 0.02<Ti-3.42N between Ti content and N content is that the Ti content is chemically equivalently larger than the N content, that is, O<Ti- 3.42N, the excess Ti forms TiC, and its fine dispersion suppresses the coarsening of austenite during carburizing heating. In this case, unless the excess Ti content is 0.02% or more, almost no effect can be seen, so 0.02<Ti-3.42N is specified. Note that the excess Ti is desirably present in a proportion of 0.022% or more. [Example] Next, the steel of the present invention will be explained through examples and in comparison with comparative examples. First, inventive steels 1 to 15 and comparative steels 1 to 13 having the chemical compositions shown in Table 1 were melted by a conventional method.
Shape as shown in Figure 1 (Dimensions: outer diameter 55mm x inner diameter
35mm x thickness 10mm x eccentricity 8mm x opening 6mm) C
manufactured a ring. Then, the austenite coarsening temperature (temperature at which the grain size becomes JIS No. 7 or higher) after normalization of the inventive steel and comparative steel, and the carburizing and quenching of the C ring shown in Fig. 1 (930°C x 4 hr → 860℃×
30min→60℃ 0Q, carb

【table】

〔Effect of the invention〕

As is clear from Fig. 2, in the 0.02<Ti-3.42N region, which is the region of the steel of the present invention, the grain coarsening temperature is all 950°C or higher, and the Nb
Alternatively, the steels 9 to 12 of the present invention containing V have an even higher temperature of 1010°C or higher, and as a result, as is clear from Table 1, the displacement of the C-ring opening is greater than that of the comparative steels. The strain of the steel of the present invention is significantly smaller, which proves that the strain of the steel of the present invention is smaller due to heat treatment. As mentioned above, the case-hardened boron steel of the present invention is inexpensive and generates very little heat treatment distortion, so it can be carburized at high temperatures and has industrially useful effects such as significantly shortening carburizing time. .

[Brief explanation of the drawing]

Figures 1a and b are front and side views of a C-ring for measuring heat treatment strain, and Figure 2 is a
2 is a graph showing the coarse graining temperature range of steel with respect to Ti and N amounts.

Claims (1)

[Claims] 1 C: 0.10 to less than 0.27%, Si: 0.35% or less, Mn: 0.30 to 0.90%, Cr: 0.90 to 1.80%, B: 0.0005 to 0.0050%, sol.Al: 0.08% or less, Contains Ti: more than 0.03% to 0.1%, Fe and unavoidable impurities: the remainder (more than % by weight), and contains Ti content (wt%) and N content as unavoidable impurities ( A case-hardened boron steel with low heat treatment distortion, characterized by the inequality 0.02<Ti-3.42N (weight%). 2 C: 0.10~0.27% or less, Si: 0.35% or less, Mn: 0.30~0.90%, Cr: 0.90~1.80%, B: 0.0005~0.0050%, sol.Al: 0.08% or less, Ti: More than 0.03%~ 0.1%, and further contains one or more of the following: Nb: more than 0.06% to 0.10%, V: 0.02 to 0.20%, Fe and unavoidable impurities: the remainder (more than % by weight) A skin with small heat treatment distortion, characterized by having the following: Ti content (wt%) and N content (wt%) as an unavoidable impurity, the inequality 0.02<Ti-3.42N holds true. Hardened boron steel.