JPS6140030B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a case hardening boron steel that does not coarsen austenite grains during carburizing and has small quenching distortion. Conventionally, various case hardening steels have been known as surface hardening materials as specified in JIS/G4052, and among these, SCM and SNCM types have excellent properties and are widely used. has been used for. 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 a Ti content of 0.01 to 0.05%, which is achieved by fixing it as TiN in the steel.
This was to prevent B from forming BN and eliminating the hardenability improvement effect. However, it has been pointed out that conventional case-hardening boron steels have a problem in that austenite grains become coarser during carburizing, resulting in increased quenching strain. From the above-mentioned viewpoint, the present inventors focused on particularly low-priced case-hardened boron steel in order to find an economical case-hardened steel for mechanical structures that has small quenching distortion during carburizing. As a result of conducting research to suppress the quenching distortion by preventing the coarsening of austenite grains during carburization, the following findings (a) to (c) were obtained. 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, and fine dispersion is effective in preventing austenite grain coarsening. It has not become the form of 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. It has an indirect suppressive effect on the coarsening of austenite viscous grains during carburization, but for this purpose, the N content is 0.006%.
). (b) When the N content in the steel is chemically equivalently greater than the Ti content, the excess N combines with Al to form AlN, but AlN is generated during carburizing or during heat treatment before carburizing, and is a substitute for austenite. To suppress coarsening. However, in this case, there is an excess of N,
In addition, unless the sol.Al content is chemically equivalently larger than the excess N content, there is almost no effect of suppressing grain coarsening (in this case, it is desirable that the N content exceeds 0.006%). (c) 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: 0.08 % or less, Ti: 0.1% or less, N: more than 0.006% to 0.020%, 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: the remainder, and the Ti content and
sol.Between Al content and N content {Ti, sol,
The content of Al and N is expressed in weight%), 0.004<N-Ti/3.42<sol. Al/1
．． This is characterized in that heat treatment distortion is suppressed as much as possible by making the inequality 93 hold true. 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%, 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 deteriorate, 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 bonding force with N than B, so it has the effect of preventing B from bonding with N and eliminating the hardenability improvement effect, but 0.1%
If the content exceeds 0.1%, a large amount of Ti nitride will be generated, deteriorating machinability and toughness, so the content was limited to 0.1% or less. (h) N As mentioned above, when the content of N is 0.006% or less, it prevents the crystal grains after rolling or forging from becoming extremely coarse and forms austenite during carburizing in the subsequent process. The indirect effect of TiN that contributes to grain refinement becomes less noticeable, and the effect of AlN on preventing austenite grain coarsening also tends to decrease. Also,
In order to reduce the N content to 0.006% or less, it is necessary to significantly increase the cost of steel manufacturing. Therefore, it is preferable to adjust the N content to more than 0.006%. On the other hand, the N content is 0.020
If the N content exceeds 0.020%, the bond with B becomes excessive and there is a growing concern that the effect of B on improving hardenability will be adversely affected. Therefore, it is preferable to adjust the N content to 0.020% or less if possible. (i) 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%. (j) 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%. Furthermore, between the Ti content, sol.Al content, and N content, 0.004<N-Ti/3.42<sol. Al/1.9
3 The reason for stipulating the inequality is that when the N content is chemically equivalent to more than the Ti content, the excess N becomes Al
Similar to TiC, AlN is formed during carburizing or during heat treatment before carburizing, and suppresses the coarsening of austenite. This is because the above-mentioned effect will hardly be produced unless 0.004% or more of sol.Al (a positive value of ) is present and the sol.Al content is chemically equivalently greater than the amount of excess N. Note that the excess N is desirably present in a proportion of 0.0055% or more. Next, the steel of the present invention will be explained through examples and in comparison with comparative examples. First, inventive steels 1 to 13 having chemical compositions as shown in Table 1 and comparative steels 1 to 13 were melted by a conventional method, and the shapes were as shown in Fig. 1 (dimensions: outer diameter 55 mm x
(Inner diameter 35mm x Thickness 10mm x Eccentricity 8mm x Opening 6mm)
A C ring was manufactured. Then, the austenite coarsening temperature (temperature at which the grain size becomes JIS No. 7 or higher) after normalization of the present invention steel and comparative steel,

【table】

[Table] The C-ring shown in Fig. 1 was carburized and quenched (930°C x 4 hours).
→860℃×30mm→60℃ OQ, carbon potential = 0.8%) The amount of displacement of the opening is also shown in Table 1. Figure 2 also shows Ti-N steels shown in Table 1, with coarse graining temperatures of less than 950°C marked as ×, those of 950°C or higher as ○, and those of 1010°C or higher as ◎. It is a graph shown at a position corresponding to the amount. As is clear from FIG. 2, the range of 0.004<N-Ti/3.42 (<sol.Al/1.
93) In all regions, the coarsening temperature is 950℃ or higher, and
Alternatively, steels 9 to 11 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 amount of 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 showing a C-ring for measuring heat treatment strain, 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 , Ti: 0.1% or less, N: more than 0.006% to 0.020%, Fe and unavoidable impurities: the remainder, (more than weight %), and the Ti content (weight %) and sol 0.004<N-Ti/3.42<sol. Al
/1.93 A case-hardened boron steel with low heat treatment distortion, characterized by the following inequality: 2 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, Ti: 0.1% or less , N: more than 0.006% to 0.020%, 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, ( 0.004<N between Ti content (wt%), sol.Al content (wt%), and N content (wt%) as inevitable impurities -Ti/3.42<sol. Al
/1.93 A case-hardened boron steel with low heat treatment distortion, characterized by the following inequality: