JPH03207833A - Nitriding steel and its manufacture - Google Patents

Abstract

PURPOSE:To obtain the nitriding steel capable of short time nitriding by incorporating specified amounts of Cr, Mo, V and Nb into a medium-high carbon steel, subjecting it to hot rolling and thereafter executing cooling under specified conditions. CONSTITUTION:At the time of manufacturing a nitriding steel for machine structural parts requiring wear resistance in civil engineering machine, industrial machine, or the like, a slab having a compsn. obtd. by incorporating, by weight, 0.50 to 3.50% Cr, 0.50 to 1.50% Mo, 0.10 to 0.50% V and 0.01 to 0.10% Nb into a steel contg. 0.30 to 0.60% C, 0.10 to 0.50% Si and 0.30 to 0.90% Mn is hot-rolled, is thereafter rapidly cooled to the temp. range of 600 to the Ar1 transformation point, is held to the above temp. for 8 to 10 hr and is thereafter air-cooled to a room temp. Or, the slab is hot-rolled, is thereafter gradually cooled to the temp. range of 450 to 550 deg.C at 35 to 50 deg.C/hr cooling rate and is thereafter air-cooled. The nitriding steel having the structure of ferrite and fine pearlite, capable of short time nitriding and excellent in mechanical characteristics and nitriding properties can be obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application This invention relates to a nitriding steel for mechanical structural parts that require fit wear resistance, such as civil engineering machinery and industrial machinery, and a method for manufacturing the same.

Conventional techniques Methods for improving the wear resistance properties of mechanical tools and mechanical parts include induction hardening and carburizing hardening, but these have the problem of deteriorating toughness due to coarsening of crystal grains, and the heat treatment distortion is large and asymmetrical. Moreover, it is not suitable for surface hardening of parts with complex shapes (eg, crankshafts for automobile parts).

Nitriding treatment is performed by heat treatment (for example, in an ammonia gas atmosphere).
The purpose is to improve wear resistance by applying nitrogen to the surface layer of the steel to form nitrides and carbonitrides. Unlike carburizing treatment, which performs surface hardening treatment at 50
Since it is carried out at a low temperature of 0 to 550°C, it is characterized in that it does not cause deterioration of toughness or heat treatment distortion due to coarsening of crystal grains. For this reason, nitriding has been widely used for tools that require wear resistance.

Problems to be Solved by the Invention However, since the nitriding process usually takes about 60 to 100 hours, it is desired to shorten the nitriding process time from the viewpoint of cost rationalization and man-hour reduction. However, the current situation is that no effective method has yet been found.

In view of these circumstances, this invention provides a high-toughness nitriding steel that increases the diffusion rate of nitrogen during nitriding by controlling the composition of the nitriding steel, and enables short-time nitriding. This is an attempt to propose a manufacturing method.

Means for Solving the Problems The inventors have conducted various studies in order to improve the above-mentioned wear resistance characteristics, and as a result, they have obtained the knowledge described below.

(2) Cr, Mo, V, and Nb are effective ingredients for increasing the depth of the nitriding layer, and are effective in shortening the nitriding time.

■Nitriding time can be shortened because the diffusion rate of nitrogen in ferrite and fine pearlite is fast.

■ Because paynite is likely to grow during cooling after rolling due to the addition of MO, it is necessary to control the cooling method after rolling in order to suppress the formation of paynite and obtain a mixed structure of ferrite and fine barite. There is.

This invention was made based on the above knowledge,
In addition to the addition of Cr, Mo, and ■, the nitriding time can be shortened by adding an appropriate amount of Nb and by controlling the cooling method after rolling to make the microstructure a mixed structure of ferrite and fine barlite. In addition to improving toughness by adding an appropriate amount of Nb.

That is, the gist of this invention is that, in terms of weight ratio, C 0.30
~0.60%, Si0.10-0.50%, Mn 0.
30-0.90%, Cr 0.50-3.5
0%, Mo 0.50-1.50%, V 0.10-
0.50%, Nb 0.01-0.10%,
It is a nitriding steel characterized by having the remainder Fe and unavoidable impurities, and having a mixed structure of ferrite and fine barite, and as a manufacturing method, the steel of the above-mentioned assembly is hot-rolled and then heated at 600 ° C. It is characterized in that it is rapidly cooled to a temperature range of Ar + transformation point, held at that temperature for 8 to 10 hours, and then air cooled, and the above-mentioned steel is heated to a temperature of 35 to 50% after hot rolling.
It is characterized in that it is cooled to 450 to 550°C at a cooling rate of °C/Hr and then air-cooled.
Function The reasons for limiting the chemical precepts of this invention steel are as follows.

C: 0.30 to 0.10% 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 steel for nitriding, a balance between static strength and toughness is required, and 0.30% or more is required to obtain the minimum static strength. However, if it exceeds 0.60%, the toughness decreases rapidly, so the C content is preferably 0.30 to 0.60%.

Si: 0.10-0.50% Si is an element necessary for deoxidizing molten steel, and is also an element necessary for imparting a certain static strength to steel, but 0.10%
%, the desired deoxidizing effect cannot be obtained; on the other hand, if it is less than 0.
If it exceeds 50%, workability deteriorates, so 0.10 to 0
．． It was set at 50%.

Mn: 0.30-0.90%Mn, like Si, is an element necessary for deoxidizing molten steel, and is also an effective element for imparting hardenability to steel, but less than 0.30%. If the content exceeds 0.90%, the toughness deteriorates, so the content was set at 0.30 to 0.90%.

Cr: 0.50 to 3.50% Cr is an effective element for improving nitriding properties and imparting hardenability to steel, but if it is less than 0.50%, it does not improve nitriding properties and hardenability. The desired effect cannot be obtained, and 3.
If it exceeds 50%, the toughness will decrease, so 0.50~
It was set at 3.50%.

Mo: 0.50 - 1.50% Mo is an effective element for improving nitriding properties and hardenability like Cr, but
．． If it is less than 50%, the nitriding properties and hardenability cannot be sufficiently improved, while if it exceeds 1.50%, the effect on the nitriding properties is saturated and the economic efficiency is impaired.
．． It was set at 50%.

V:0.10~0. 50% ■ is an effective element for improving the nitriding properties and at the same time increasing the high-temperature strength of steel, but in order to exhibit its effect, it must be at least 0.10%, and on the other hand, 0.5
If it exceeds 0%, the effect on nitriding properties will be saturated and economic efficiency will be impaired, so the content was set at 0.10 to 0.50%.

Nb: 0.01-0.10% Nb is an effective element for improving nitriding properties, static strength, and toughness, but if it is less than 0.01%, these effects cannot be sufficiently obtained, and on the other hand, If it exceeds 0.10%, machinability during machining will be impaired, so it was set at 0.01 to 0.10%.

The reason why the microstructure of the nitrided steel is a mixed structure of ferrite and fine barlite is as follows.

Although ferrite and fine barlite structures are softer in hardness than payinite and martensitic structures, they have significantly better toughness and less dislocations that impede N movement. is easy to spread,
Nitriding is possible for a short time. Therefore, it is necessary to have a mixed structure of ferrite and fine barlite.

In order to make the microstructure of the nitriding steel a mixed structure of ferrite and fine barlite, it is necessary to control cooling after hot rolling the nitriding steel. In this invention, the following two methods are adopted as the control method.

(1) After hot rolling, it is rapidly cooled to a temperature range of 600°C to A r + transformation point, and maintained at the temperature for 8 to 10 hours.

The reason for this is that rapid cooling is performed after hot rolling to suppress the amount of ferrite and barlite produced and to increase the survival rate of fine barlite. Moreover, when the cooling target temperature after hot rolling is less than 600° C., there is a possibility that paynite will be formed due to subsequent air cooling.

On the other hand, even if it is cooled to a temperature higher than the Ar + transformation point and held at that temperature, ferrite is only recovered from austenite and fine barlite is not produced, and not only is it possible to produce paynite along with fine barite during subsequent air cooling. be alive.

Therefore, the target temperature for cooling after hot rolling is 600.
℃~A r + transformation point.

If the holding time is less than 8 hours, the fine barlite will not be completely preserved, while if the holding time exceeds 10 hours, a mixed structure of ferrite and fine pearlite will be obtained. It was set as 10 hours.

(2) 45 at a cooling rate of 35-50℃/Hr after hot rolling
Cool (slowly cool) to 0 to 550°C.

The reason is that if the steel is cooled at a rate higher than 50° C./Hr after hot rolling, paynite will be lost. On the other hand, 35℃/}
Slow cooling at a cooling rate of less than 1 r completes the transformation of ferrite and fine barite, but the time required for cooling becomes longer, which goes against the reduction in man-hours.

Furthermore, if the material is cooled to a temperature higher than 550° C. and then air cooled, paynite is formed during air cooling. On the other hand, cooling to less than 450° C. takes a long time.

For this reason, the cooling rate was set at 35-50°C/Hr, and the cooling target temperature was set at 450-550°C.

Examples Example 1 After melting steel having the chemical precepts shown in Table 1, each
A mm square steel billet was heated to 1100°C, hot rolled at a finishing temperature of 950°C to form a 30mm round bar, and the cooling rate was adjusted to 40°C/Hr. It was cooled to 500°C in an atmospheric furnace.

After cooling, a JIS No. 3 (2 mm notch) Charby impact test piece and a static bending test piece (L: 55 mm) shown in Figure 1 were used.
, t + : 10 mm, t 2 : 8 mm), gas nitriding treatment was performed at 530° C. for 24 hours in NHs gas, and Charby impact test and static bending test were performed. In the static bending test, static bending strength (crack initiation load) was examined at a strain rate of 10-''/s.

For Charpy impact test pieces, after the impact test, the hardness near the white layer (hardness at 0.1 mm from the steel surface),
The hardness of the base material (core) and the depth of the nitrided layer were measured, and the microstructure of the base material was observed.

The above measurement results are also shown in Table 1.

As is clear from Table 1, test steel No. The invention steels Nos. 1 to 10 have the target values for Charby absorbed energy, static bending strength, near-white layer hardness, base metal hardness, and nitrided layer depth (15. OK gf-m, 2000 Kgf or more, and HV9, respectively).
00 or more, HV250 or more, and 0.70mm or more).

On the other hand, test steel No. Among the comparison steels of No. 11 to 29 and current steels A and B, steels with higher C or Mn and Cr content than specified (No.
．． No. 11, 15, 17, 25, 28) and steels containing less Nb than specified (No. 24, A, B), the Charby absorbed energy is greatly reduced, and deterioration of toughness is observed. In addition, the static bending strength of all steels, including the other comparison steels in which no decrease in Charby absorbed energy was observed, was significantly decreased.

The precepts (Cr, Mo, V, N
If b is less than the standard (No. 18, 20, 22
, 24, 25, 26, 27, 28, 29, A), the nitrided layer depth is shallow.

From the above results, it can be seen that even if the manufacturing conditions are satisfied, if the content of precepts is different, the target characteristics cannot be obtained.

Next, the results will be shown below in the case of a steel having a certain alloy content the same as that of the present invention, containing payinite, and a tempered martensitic steel having a microstructure.

A steel slab (160 mm square) having the same chemical properties as shown in Table 1 was used as a rolled material, heated to 1100°C, and hot rolled at a finishing temperature of 950°C to form a 30 mm round bar. Immediately after that, the following treatment (a) or (b) was performed.

(a) Cooling was performed in an atmospheric furnace adjusted to have a cooling rate of 55° C./Hr.

(b) After quenching in oil, tempering was performed at 650°C for 1 hour.

Test steel No. in Table 1. For 1-5, 11-19 (
a), test steel No. For samples 6-to and 20-29, the process (b) was performed. Furthermore, for the current steels A and B, both treatments (a) and (b) were carried out.

After the treatment in (a) or (b), the specimen was processed and nitrided in the same manner as in Example 1, and then the mechanical properties, nitriding properties, and base material microstructure were investigated.

The above results are shown in Table 2.

As is clear from Table 2, all target values were met. There is no added steel, especially for conventional treatments (b).
) Comparison processing No. In samples No. 36-40 and No. 50-59, although the base material hardness was high, the Charpy absorbed energy decreased significantly.

In this way, in the case of steel containing payinite in the microstructure or tempered martensitic steel, even if the content of alloying elements satisfies the specified value, the mechanical properties and nitriding properties deteriorate. I understand that.

Example 2 Test steel No. shown in Table 1. After hot-rolling the 160 mm square steel piece of Example 1, it was cooled using the heat pattern shown in Figure 2, and the test piece was processed and nitrided in the same manner as in Example 1 to determine the mechanical properties, nitriding properties, and base material microstructure. conducted a survey.

Further, after cooling the same steel piece in the beat pattern shown in FIG. 3, the test piece was processed, nitrided, and the base material microstructure was investigated in the same manner as in Example 1.

The results of the above two investigations are shown in Tables 3 and 4 in comparison with those conducted under conditions different from those of the present invention.

Table 3 shows that among the present invention and comparative examples, the holding time is longer than the specified value (Test No. 6, 9), and Table 4 shows that the cooling rate is smaller than the specified value (Test No. 19). and those whose cooling end point temperature is lower than the specified value (Test No. 1
5 and 18), the microstructure is a mixture of ferrite and fine barite, and the characteristic values of mechanical properties and nitriding properties all meet the target values.

On the other hand, among the comparative examples, the holding temperature was outside the specified range (N
o. 5, 8), those whose retention time is shorter than specified (No.
7, 10), those whose cooling rate is higher than the specified one (No. 1)
6), cooling end point temperature higher than specified (No. 17,
20) is Charby absorbed energy, static bending strength,
It can be seen that the nitrided layer depth has not reached the target value.

Effects of the Invention As is clear from the above examples, the nitriding steel according to the present invention has excellent mechanical properties and nitriding properties,
In addition, by controlling the amount of alloy and creating a mixed structure of ferrite and fine barite, we can produce nitriding steel that can be nitrided in a short time and has excellent mechanical properties and nitriding properties at a low cost. It is possible to manufacture.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing a static bending test piece in Example 1 of the present invention, and FIGS. 2 and 3 are diagrams showing cooling heat patterns in Example 2.

Claims (1)

[Claims] 1 Weight ratio: C0.30-0.60%, Si0.10-0
．． 50%, Mn0.30~0.90%, Cr0.50~
3.50%, Mo0.50~1.50%, V0.10~
0.50%, Nb 0.01 to 0.10%, the remainder consisting of Fe and inevitable impurities, and having a mixed structure of ferrite and fine pearlite. 2 Weight ratio: C0.30-0.60%, Si0.10-0
．． 50%, Mn0.30~0.90%, Cr0.50~
3.50%, Mo0.50~1.50%, V0.10~
After hot rolling, a steel containing 0.50% and 0.01~0.10% of Nb, and the balance consisting of Fe and unavoidable impurities is rapidly cooled to a temperature range of 600°C to Ar_1 transformation point, and then A method for producing steel for nitriding, which comprises holding the steel for 8 to 10 hours and then cooling it in air. 3 Weight ratio: C0.30-0.60%, Si0.10-0
．． 50%, Mn0.30~0.90%, Cr0.50~
3.50%, Mo0.50~1.50%, V0.10~
After hot rolling, a steel containing 0.50%, 0.01 to 0.10% Nb, and the balance consisting of Fe and unavoidable impurities was heated.
A method for producing steel for nitriding, characterized by cooling to 450 to 550°C at a cooling rate of 5 to 50°C/Hr, and then cooling in air.