JPS61133366A - Case hardening steel for cold forging provided with free-machinability - Google Patents

Abstract

PURPOSE:To obtain a case hardening steel for cold forging provided with free- machinability and superior resistance to grain coarsening by reducing the amount of Al and adding Nb which is more effective in inhibiting the growth of austenite grains than Al so as to satisfy a specified relation. CONSTITUTION:The composition of a case hardening steel for cold forging is composed of, by weight, 0.1-0.3% C, <0.5% Si, 0.3-1.8% Mn, 0.3-1.8% Cr, 0.001-0.025% N, 0.005-0.02% acid-sol.Al, 0.0115-0.1% Nb (Nb/acid sol.Al>=2.3), one or more among 0.04-0.4% S, 0.005-0.2% Te, 0.05-0.4% Pb, 0.005-0.2% Bi and 0.001-0.02% Ca, and the balance Fe with inevitable impurities.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field) The present invention relates to a case hardening steel, particularly to a case hardening steel for cold forging which has excellent resistance to coarse graining and has free machinability.

(Background of the Invention) Case hardened steel is widely used in power transmission parts for automobiles and machinery in order to obtain the mechanical properties required for the purpose. Surface hardening treatment such as carburizing or carbonitriding is applied to the shaped parts to ensure strength and wear resistance of the surface, impact resistance of the core, and the toughness of the entire part is rounded. . When performing this surface hardening treatment, the material after cold working is heated for 7xJ at an austenite region temperature higher than the transformation point and in a surface hardening treatment atmosphere such as carburizing or carbonitriding. During the treatment process, a small number of austenite crystal grains may grow abnormally, causing the crystal grains to partially coarsen in the steel structure, resulting in a mixed grain structure. This coarse grained portion has better hardenability than the other portions, but causes a ninth-larger heat treatment strain, and furthermore, there is a problem in that the toughness is reduced.

Compared to solving these problems, conventional methods
Case-hardening steel was used that prevents coarsening of austenite grains by sufficiently precipitating At-T.
Even if 11t is finely precipitated, Mut11 tends to aggregate and coarsen, so the effect of suppressing austenite crystal grain growth will be weakened.Furthermore, by finely precipitating Mutn, the initial austenite grain size will decrease. As the crystal grain coarsening temperature decreases, the effect of suppressing austenite grain coarsening cannot be sufficiently achieved. Various improvements have been made to obtain steel (for example, JP-A-58-110657, JP-A-58-11).
(See Publication No. 3318). In addition, the carburizing temperature of case hardening steel is conventionally 92
In recent years, high-temperature carburizing (950-1000°C) or vacuum carburizing (950-1000°C) has been used to improve efficiency.
50℃) and other high-temperature, short-time processing conditions\6
Ru. For this reason, it is desired that the austenite crystal grains do not coarsen even at high carburizing temperatures, and since case hardening steel is used for precision parts such as gears,
There are many machining processes such as cutting and shaving, and it is desired to improve machinability.

(Summary of the Invention, Detailed Description) The present invention was developed in view of the above-mentioned current situation, and can prevent austenite crystal grains from becoming coarse after cold working such as cold forging, and can improve machinability. We are proposing a hardened steel with a weight of Cα10 to α50181 (L50
Mn [13 to 1.8%% Or 13
0~1.80%, N(LO01~(LO25%,
5oAAA [LO05 ~ (Lower than LO296, M'b
[LO115 ~ (Included in 11%1, ° Kudashi Nb
/1sot Aj≧2-3, and further IIC
Day 104~IIL40%, Pk α05~(L40
Arrogance, Bi(L005~α 20 Te(LO0
5~IIL21) 96%Oa 11001~α02(1
Case hardening steel for cold forging with excellent coarse grain resistance and machinability, containing one or more of KMo cL0 with the remainder being Fe and unavoidable impurities, and in addition to the above components, KMo cL0
5~(L896t) - Contains a case hardening steel for cold forging which has excellent toughness, coarse grain resistance and machinability. Namely, the present invention is concerned with case hardening steel for cold forging which contains L896t and has excellent roughening resistance and machinability. -6 In order to increase the At content, the At content is lowered, and Nb f Nb/5otAt≧z3 has an even stronger suppressing effect on austenite grain growth than Mut.
It is characterized by preventing the coarsening of orstenite crystal grains during carburizing treatment after cold working or carbonitriding treatment by adding so as to satisfy the relationship of S, P'b, Bi, To, (Each time a is added, it is assumed that free machinability is imparted.) Also, Mof is added in order to improve toughness after carburizing, quenching and tempering.

Next, the reason for limiting the proportion of case hardened steel according to the present invention will be explained.

C is a case hardening steel in which this invention steel is used after carburizing or carbonitriding, followed by quenching and low-temperature restoring, and the upper limit is set to 100% in order to minimize the quenching strain and residual stress of 1L, which is not carburized. . On the other hand, if it is less than 11%, the hardenability will be insufficient and the strength of the 6th part will not be ensured, so the lower limit was set as (1
Let's say 10chi.

SL is necessary as a deoxidizing agent, but (if L exceeds 50%, 810! system inclusions will increase in the steel (crystal grains tend to become coarser and the toughness will deteriorate by 7kff), so if IIL exceeds 50%) The following was made.

Mn is an essential element as a deoxidizing desulfurizing agent and ensuring the toughness of steel, but it is set at rLsa ~ 1.80 to prevent it from promoting segregation and increasing brittleness at layers less than 15015.

Or is an effective element for improving the hardenability, strength, toughness, carburizability, etc. of steel, but its effect is small when the α coefficient is less than 7! Also, if the content exceeds 1.80%, carbonization occurs during austenitization of crystal grains, IiJ is not sufficiently dissolved in solid solution, and the mechanical properties after carburization deteriorate.
1.80%.

N combines with At in the steel to form AM f:, and some of it also combines with Ib, but if it is less than 1001%, the precipitation of A11i is small], and the lower limit 1j (L
O01% and -t''le.Also, if lLo exceeds 259G, cold forgeability deteriorates, so the upper limit is set to 1025.

However, if it exceeds 102%, undissolved MutN will precipitate, lowering the coarsening temperature of austenite crystal grains during carburizing and carbonitriding, so the upper limit is set to less than 102%. Also, the reason why the lower limit was set to uooss is that
Taking into account the segregation of cLaas*oaoth, it can be sufficiently deoxidized if cLaas*oaoth is dissolved in solid solution.

Ml) is an extremely effective element for suppressing coarsening of austenite crystal grains. In Figure 1 Il'1esa-r
hs%81-[L8%Mn-1,2%Or-(L O1
G % N -11To S -Nb -eoL At
After cold working the steel at a low rolling reduction rate, it is austenitized at 960C for 6 hours to determine whether or not grains have coarsened. Nb/aoA At≧2-3 is required for grain refinement9, so the lower limit of Mb is (10115%,
Further, even if more than rL1s is added, N'b(ON) becomes coarse and the effect decreases, so the upper limit is set to l11qb. Therefore, to prevent grain coarsening, sobmut and lA130
Between aotAtl OO5~lO29, Nb
α0115~IIL1-However, Bib/aotmut
A relationship of ≧2.3 is required.

IJTe combines with Mn in steel to form inclusions and improve the stiffness of steel, but
If it is less than 005, the effect will not increase and B (L40
cs, To 11L20s'j'', it would be uneconomical to increase the amount by saturation, so it was limited to the above range.

Since Bi does not have a molten S degree in steel, Bi improves free machinability by dispersing it as fine particles in steel.
b (1051Bi CL00) If the amount is less than 5%, the effect will not be satisfactory, and if it is added in excess of P'b[L40%, B1 α20, the effect will be saturated and uneconomical, so it was limited to the above range.

Oa has the effect of spheroidizing nonmetallic inclusions and improves free machinability, but if Oa [1L001% is less than 1%, the effect is not good, and even if Ca[LO2%] is added, the effect is saturated and it becomes unworkable. Since it is economical, the above range (limited friend).

Mold is added to improve toughness after carburizing, quenching, and tempering, but since it is an expensive element, it is desirable to add it in as small a quantity as possible, so that its toughness improving effect is not fully demonstrated [
11aa with L05% as the lower limit and the effect almost saturated
lt upper limit.

(Example> Next, embodiments of the present invention will be explained.

Table 1 shows the composition of the steels used in this example. Steels-1 to 6 are conventional steels, and fI47 to 25 are steels of the present invention. Note that steels N&25 to 25 are examples in which MO is added to improve toughness after carburizing, quenching, and tempering.

#t-100 with the composition shown in Table 1 was melted in a 9 vacuum melting furnace to form a 100-square steel ingot, and then heated to 401φX1 at 1200℃.
Forged into a 000mm round bar. After that, after heating at 750°C for 2 hours, spheroidizing annealing was performed by furnace cooling, and 20
After performing cold forging at rolling reduction ratios of 50% and 80%, it was turned into austenite and the austenite grain coarsening temperature was measured. The results are shown in Table 2. In this experiment, austenitization was carried out at 930 to 1050℃, assuming carburization.
Keep it in the temperature range for 6 hours and cool it with water. After that, the austenite grain size of the cross section of each sample was measured, and the temperature at which the coarse grains with the austenite crystal grain size of Na5 or less exceeded I4 in area ratio,
Or, the area ratio of crystal grains with three or more different grain sizes is 21) J
The lower limit of the austenite grain coarsening temperature is the temperature exceeding ft.

Next, in order to confirm the effect of adding MO to this inventive steel, it was melted, forged, and spheroidized two times under the same conditions as the nine samples used in the above experiment. After cold forging at a reduction rate of 50%, the temperature was heated to 950°C assuming carburizing treatment.
After heating for 6 hours, oil quenching, tempering at 180℃ for 2 hours, and Charpy impact test.

The results are shown in Table 2 as impact values.

Note that the impact value is the average value of three test results using a FiJI No. 83 test piece and a 2■U notch test piece.

Table 2 Next, the effect of the free-cutting element IkfIf & addition of 7-rank wear on the wear of the steel of the present invention is shown in FIG. 2 and Table 3. The sample material used for the cutting test was a rolled material with a diameter of 115 mm and heated at 900℃.
After simple Xt5hrjl, 106φX 500wm
Process it into a machinability test piece of L.

As the tool material, cemented carbide P20 [chip for cutting tool, hardness of 90 or more (Rockwell school), transverse rupture strength of 110 or more (kli/wm'')] was used, (-51-5°, 5
°, 5', 15°, 15°, (L B m): Depth of cut 2.0 ■; Feed (L25 cabinet/r: Lubrication-dry type, and flank wear was used as a life judgment (L2 m t- was used as the standard).

Table 3 shows the tool life (min) fe at a cutting speed of 120 m/m1n. Pb, B, To,
It is shown that the tool life is approximately 1.2 to 1.5 times longer than that of conventional steel by adding Bi, (!a alone, and Pb, B, To,
Bi, C! It is shown that the composite addition system a shows an elongation of about 1.5 to 5 times.

FIG. 2 shows the effect # of free-cutting element addition on tool life on cutting speed.

It is clear that the machinability of the nine-invention steel with the addition of free-cutting elements is worse than that of the conventional steel at all cutting speeds, and as is clear from Table 2, the grain roughening temperature This shows that it has no effect on the

Therefore, for case-hardening steel for cold forging, which is often subjected to machining, steel with excellent resistance to coarse graining and excellent rigidity is optimal.

Table 3

[Brief explanation of the drawing]

In the accompanying drawings, Fig. 1 shows the relationship between the amounts of aotAA and Bib and coarsening of austenite crystal grains, and Fig. wX2 is a chart showing the results of a free-cutting test of the steel of the present invention. Small f1 thirst 4-7t

Claims (1)

[Claims] 1. By weight: C 0.10-0.30%, Si 0.50% or less, Mn 0.3-1.8%, Cr 0.30-1.80%
, N0.001-0.025%, solAl0.005
~ less than 0.02%, Nb 0.0115 ~ 0.1%, but satisfying the relationship of Nb/solAl≧2.3, and S
0.04-0.40%, Te 0.005-0.20%,
Pb0.05-0.40%, Bi0.005-0.20
%, Ca0.001 to 0.020%, and the remainder is Fe and unavoidable impurities, and has excellent coarse grain resistance and free machinability. Case hardened steel. 2. By weight: C0.10-0.30%, Si0.50% or less, Mn0.3-1.8%, Cr0.30-1.80%
, N0.001~0.025%, Mo0.05~0.8
%, solAl0.005 to less than 0.02%, Nb0.
0115-0.1%, provided that Nb/solAl≧2.
3 is satisfied, and S0.04~0.40%, T
e0.005-0.20%, Pb0.05-0.40%
, Bi0.005-0.20%, Ca0.001-0.
A case hardening steel for cold forging which has excellent coarse grain resistance and free machinability, and is characterized by containing one or more of 0.020% and the remainder consisting of Fe and unavoidable impurities.