JPH0285342A - High temperature/short time carburizing case hardening steel - Google Patents

Abstract

PURPOSE:To obtain the title steel capable of permitting high temp. and short time carburization and at low cost by specifying the compsn. constituted of C, Si, Mn, Nb, N, Ni, Ti, Cr, Mo, Al, V, Cu, B and Fe. CONSTITUTION:The high temp. and short time case hardening steel contains, by weight, 0.15 to 0.25% C, <=0.50% Si, <=2.00% Mn, 0.010 to 0.100% Nb and 0.0050 to 0.0200% N, furthermore contains, at need, one or more kinds among <=3.50% Ni, 0.010 to 0.100% Ti, 0.10 to 2.00% Cr, 0.10 to 1.00% Mo, 0.010 to 0.100% Al, 0.01 to 0.30% V, <=1.00% Cu and 0.0003 to 0.0050% B and the balance Fe with inevitable impurities. In the steel, the roughening of the grains is prevented and heat treating strains are not generated.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to case hardening steel used for mechanical parts such as gears and shafts used in automobiles, civil engineering construction machines, industrial machines, etc. .

(Prior Art) Case hardened steel is usually carburized and quenched and tempered after being machined and used as gears and shafts for automobile parts.

However, carburizing and quenching is used for modern steel (JIS 5Cr420, SC
M420, etc.), the limit is 925 to 930°C, and the soaking time for carburizing usually takes 5 to 6 hours (approximately 10 hours if the heating and cooling time is added).

In recent years, there has been an increasing need to shorten carburizing time in order to reduce costs, but high-temperature heating is required to shorten carburizing time.

(Problem to be solved by the invention) However, as mentioned above, the current steel (JIS 5Cr
420, SCM420, etc.), 925-930°C
When heated to a higher temperature, grains become coarser and heat treatment distortion occurs, which is a problem.

It is an object of the present invention to solve the above-mentioned problems and provide a case hardening steel that can shorten the carburizing time.

(Means for Solving the Problems) As a result of various studies in order to solve the problems of the conventional technology, the present inventors found that by strictly controlling the chemical composition of case hardening steel, the problem could not be achieved with the conventional technology. They discovered a case-hardened steel that could be carburized at high temperatures (1000-1050°C), which had previously been impossible.

That is, an important improvement of the present invention is to simultaneously add two elements, Nb and N, to the steel.

The inventors set the carburizing temperature to 925.950.1000.
As a result of investigating the influence of alloying elements on coarse graining characteristics by changing the temperature to 1050°C and 1100°C, it was found that by simultaneously adding two elements, Nb and N, to the steel as described above,
It was found that the grains did not become coarser up to 050°C.

Note that among Nb and N2 elements, when each element is added individually,
Coarsening cannot be prevented.

In order to achieve the purpose of the present invention of heating to 1000°C and shortening the carburizing time (soaking for 3 hours, total of 5 hours), Nb
It is essential to add Nb and N at the same time, and Nb nitride or Nb
They discovered that carbonitrides are effective in preventing grain coarsening.

The present invention was made based on the above findings, and in weight percent, C: 0.15 to 0.25%, Si: 0.5
0% or less, Mn: 2.00% or less, Nb: 0.
010-0.100%, N: 0.0050-0.0
200%, and if necessary, Ni: 3.5Q% or less,
Ti: 0.010-0.100%, Cr: 0.
10-2.00%, Mo: 0.10-1.00%,
^1: 0.010-0.100%, v: o, o
t ~0.30%, Cu: 1.00% or less, B; 0
．． It is a high-temperature, short-time carburized case-hardening steel that contains one or more of 0003 to 0.0050%, with the balance consisting of Fe and unavoidable impurities.

(Function) The present invention achieves the following by strictly controlling the chemical composition in steel.
This resulted in the emergence of a case-hardened steel that can be carburized at high temperatures and in a short period of time by preventing coarse grain formation at high temperatures.

The reasons for limiting the chemical composition will be explained below.

C: C is an element necessary to impart a certain static strength to steel, but on the other hand, it is an element that deteriorates toughness. Particularly in case-hardening steel that undergoes carburizing treatment, a balance between static strength and toughness is required, and to obtain the minimum static strength, 0.15
% or more is required. On the other hand, if the content exceeds 0.25%, the toughness usually decreases, so the upper limit is set to 0.25%.

Si: Si is an element necessary for deoxidizing steel, and is also an element necessary for imparting a predetermined static strength to steel. However, if it is added in an amount exceeding 0.50%, the carburizing properties will be poor, which will lead to disadvantageous conditions in case hardening steel that requires carburizing treatment, so the upper limit is set at 0.50%.

Mn: Like Si, Mn is an element necessary for deoxidizing steel, and is also an effective element for imparting hardenability to steel.

However, if it is added in excess of 2.00%, the high temperature softening resistance decreases, resulting in a decrease in static strength, so the upper limit is set at 2.00%.
00%.

Nb: Nb is an effective element for refining crystal grains and imparting hardenability to carburized layers, but if Nb is added under conditions that satisfy the conditions imposed on N described below, it will reduce roughness during high-temperature treatment. It has an anti-granulation effect. In order to exhibit this effect, it is necessary to add at least 0.010% or more.

However, if it is added in an amount exceeding 0.100%, the machinability during machining of parts will be impaired. Furthermore, the crystal grains of the steel become coarser, the toughness deteriorates, and the effect of preventing grain coarsening during high-temperature treatment is lost.

NUN imparts hardenability to steel and improves static strength, but adding N while satisfying the requirements imposed on Nb mentioned above is effective in suppressing grain coarsening during high-temperature treatment. Effective in preventing heat treatment distortion.

In order to have that effect, at least o, ooso is required.
It is necessary to add more than %. However, with the current melting technology, it is difficult to add more than 0.0200%, so the upper limit is set at 0.0200%.

The present invention is directed to the above-mentioned C-, Si, Mns Nb5N
Although the main provisions are as follows, the following components can be added within the following limited ranges as necessary.

Ni: Ni is an element effective in imparting a certain hardenability to steel and increasing static strength. Further, since it improves toughness, it can be added arbitrarily to ensure predetermined hardenability and toughness.

However, if it is added in excess of 3.50%, the effect will be saturated and the economic efficiency will be impaired, so 3.50% is set as the upper limit.

Cr: Cr is an element effective in imparting hardenability to steel. Furthermore, it is generally added to case hardening steel in order to improve carburizability.

In order to fully demonstrate its effect, at least 0.1
It is necessary to add 0% or more. However, even if it is added in an amount exceeding 2.00%, the effect will be saturated and the economical efficiency will be impaired, so the upper limit is set at 2,00%.

Mo: Mo is an element effective in imparting hardenability to steel, improving static strength, and improving toughness.

In order to fully demonstrate their effects, at least 0
．． It is necessary to add 10% or more. However, 1.00%
Even if it is added in excess of 1.00%, the effect will be saturated and the economic efficiency will be impaired, so the upper limit is set at 1.00%.

^1: Al has the effect of making the crystal grains of the steel finer and improving its toughness. In order to bring out the effect, 0.01
It is necessary to add 0% or more. On the other hand, when added in excess of o, ioo%, the cleanliness of the steel deteriorates and machinability is impaired. Furthermore, the addition of excessive AI will actually coarsen the crystal grains of the steel and deteriorate the toughness, so the upper limit is set at 0.100%.

Ti: Similar to At, Ti refines the crystal grains of steel,
It is an effective element for improving toughness. In order to exhibit this effect, it is necessary to add at least 0.010% or more. However, if it is added in excess of 0.100%, the crystal grains will become coarser and the toughness will deteriorate.
The upper limit is 0%.

■:■ is an element effective in precipitating carbonitrides in steel and increasing the high-temperature strength of steel. When assuming that the temperature of gears used as case hardening steel for high-output engines increases, V is effective in improving the static strength when the temperature rises, and in order to achieve this effect, 0.01 It is necessary to add more than %.

However, if added in excess of 0.30%, hot workability deteriorates, so the upper limit is set at 0.30%.

Cu: Cu is an element effective in imparting a certain hardenability to steel and increasing static strength. In order to exert its effect, it can be added as needed, but if it is added in an amount exceeding 1%, the hot workability of the steel will deteriorate.

Furthermore, since it also deteriorates static strength, the upper limit is set at 1.00%.

BIB is an element effective in improving the hardenability of steel and increasing its static strength. In order to exhibit this effect, it is necessary to add 0.0003% or more. However, 0.
If added in excess of 0.0050%, the crystal grains of the steel will become coarser and the toughness will deteriorate, so the upper limit is set at 0.0050%.

(Example) Table 1 below shows the chemical composition of steel regulated by the present invention and the chemical composition of comparative steel.

The test material was melted in a 150 kg vacuum melting furnace, heated to 1250°C for 1 hour, and made into a 100 mm diameter, 30 mm steel ingot.
It was forged and drawn to a diameter of 11111, and investigated using the following investigation process.

First, a steel material with a diameter of 30 mm was heated to 925° C. for 1 hour, cooled in air, and normalized, then turned to a diameter of 25 mm, carburized under the temperature conditions shown in Fig. 1, and a roughening investigation was conducted.

In addition, the carburization depth (depth from the surface at the position of hardness Hv550) was also measured.

These results are shown in Table 2.

Coarseness is determined by measuring the particle size using a comparative method, and determining whether there is even one particle whose maximum particle size number is 3 or more smaller than the average particle size number (the smaller the particle size number, the larger the particle size). If so, it was judged as coarse graining. Also, the average particle size number is 5
The following items were also judged to be coarse grained.

Next, a steel material with a diameter of 100 m was heated to 925°C for 5 hours, cooled in air, and normalized, and then processed into the gear test piece 1 shown in Fig. 2, with a carbon potential of 1.0 and a carburizing temperature of 925°C and 110°C.
Carburizing treatment was carried out under the conditions of 00°C1 soaking time of 6 hours and 3 hours. After that, it was tempered at 170°C for 60 minutes, and then shot peened (0.6 mm diameter, 47 m/mm).
S, 15 minutes), and the fatigue strength was investigated using a power circulation gear fatigue tester.

The results are shown in Table 2.

Furthermore, the fatigue strength is 10? The strength that did not cause destruction at times (
Fatigue limit).

In addition, the carburization depth was also investigated.

For the static bending test, the forged and stretched material of 30ma + diameter was
After heating for 1 hour at
Carburizing was performed under the carburizing conditions of 925°C for 6 hours, oil quenching, and then tempering at 170°C for 60 minutes, followed by shishit beaning (0.6tm).
diameter, 47m/S, 15 minutes) and 10-”/
Static bending strength was investigated at a strain rate of S.

Incidentally, the static bending strength was evaluated using the crack initiation load.

The results are shown in Table 2 below.

For the Charpy impact test, a forged material with a diameter of 30 mm was
After heating to 25°C for 1 hour, air cooling and normalizing, cutting to a diameter of 25mm, water quenching at 925°C for 1 hour, further tempering at 170°C for 1 hour, and then JIS
A No. 3 (2 mm U-notch) Charpy test piece was processed and its impact properties were investigated at room temperature.

The results are shown in Table 2 below.

As is clear from Table 2, the results of the carburization coarsening test showed that among the inventive steel and comparative steel having the chemical composition specified by the present invention, the steel that satisfied the N and Nb specifications (Nα16.17
．． It can be seen that steel No. 18) does not become coarse grained up to 1050°C, whereas other comparative steels become coarsened at 950°C.

Among the comparison steels, the steels (Nα22.25.26.27) with Ti, Nb, B, and AI that deviated from the specified specifications were already coarse-grained even at 925°C. Further, steels (No., 1B, 20) in which Si is higher than the specification of the present invention have poor carburizing properties and have a shallow carburizing depth compared to other steels.

In a gear fatigue test, the steel of the present invention, which has the chemical composition specified by the present invention, had a carburizing depth of 925' compared to the conventional one, even at a carburizing temperature of 1000°C and a carburizing soaking time of 3 hours (1/2 of the conventional value). The carburizing depth is equivalent to that of C16 hour carburizing treatment, and the fatigue strength (fatigue limit) of the gear is also equivalent to that of the conventional carburizing method for conventional steel.

In addition, steel that satisfies the regulations for Nb and N among comparative steels (N
With the exception of α16, 17, and 18), when carburized at 1000''C, heat treatment distortion occurred due to grain coarsening, and testing was not possible.

In a static bending test, the steel of the present invention having the chemical composition specified by the present invention was compared with the conventional t shown in comparison @ (Nα29.30).
! It shows the same value as A (SCr420, S0M420).

In addition, among the comparison steels, steel with a low C content (Nα
16) has inferior static bending strength compared to other steels.

Regarding the Charpy impact value, the steel of the present invention having the chemical composition specified by the present invention shows a value equivalent to that of the conventional steel.

Among the comparative steels, steels with a C content higher than this regulation (
No. 17) is a steel whose toughness has deteriorated and whose Ti, Nb, B, and hel contents are far outside the specified specifications (Nα22.25.26.
No. 27) is similarly inferior in toughness due to coarse graining.

As is clear from the above examples, adding Nb and N,
By strictly manipulating the chemical composition, they created a steel that could be carburized at high temperatures and in a short time.

(Effect of the invention) As explained above, the case hardening steel according to the present invention is made of Nb.

By adding the two elements N at the same time and strictly controlling the chemical composition in the steel, it is possible to carburize at high temperatures and in a short time, which has the effect of reducing costs.

[Brief explanation of the drawings] Figure 1 is a diagram showing the heat pattern of the carburization coarsening test;
FIG. 2 is a diagram showing the shape of a gear test piece, and FIG. 3 is a diagram showing the shape of a static bending test piece. 1 is a gear test piece, 2 is a static bending test piece. Figure 3

Claims (1)

[Claims] (1) In weight%, C: 0.15-0.25%, Si: 0
．． 50% or less, Mn: 2.00% or less, Nb: 0.01
0-0.100%, N: 0.0050-0.0200%
, further as necessary, Ni: 3.50% or less, Ti: 0
．． 010~0.100%, Cr:0.10~2.00%
, Mo: 0.10-1.00%, Al: 0.010-0
．． 100%, V: 0.01-0.30%, Cu: 1.0
0% or less, B: 0.0003 to 0.0050%, and the remainder is Fe and unavoidable impurities.