JPH04254547A - Steel for induction hardening - Google Patents

Abstract

PURPOSE:To obtain a steel for machine structural use capable of forming 'a surface hardened layer having sufficient hardness and free from dispersion of hardness' only by ordinary induction hardening treatment. CONSTITUTION:The steel for induction hardening is constituted so that it has a chemical composition consisting of 0.30-0.60% C, <=1.00% Si, 0.30-2.00% Mn, 0.040-0.100% S, and the balance Fe with inevitable impurities or further containing, if necessary, one or more kinds among <=3.50% Ni, <=2.00% Cr, <=2.00% MO, <=1.00% Cu, 0.0003-0.0050% B, 0.010-0.100% Al, 0.010-0.100% Ti, 0.010-0.100% Nb, 0.01-0.30% V, 0.0005-0.0100% Ca, and 0.01-0.20% Pb. Moreover, the structure of this steel is regulated to a structure where the average grain size of ferrite is <=20mum.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] This invention is a mechanical structural steel (induction hardened steel) suitable for parts that require induction hardening, such as shafts, bolts, gears, etc. used in automobiles, construction machines, and industrial machines. ).

0002

[Prior art and its problems] "Induction hardening" is used to improve the wear resistance and fatigue properties of steel materials.
This is a type of hardening method that hardens only 3mm).
It is one of the widely used processes because the processing time is very short and the workability is good.

However, in this induction hardening, the heating time is extremely short (1 to 2 seconds) compared to normal hardening (heating for 30 to 60 minutes per 25 mm diameter), so incomplete hardening (previous structure remains) (or incomplete solid solution of carbides), which often resulted in a decrease in hardness and variations in hardness.

Therefore, in order to prevent the above-mentioned disadvantages, recent measures have been taken, such as performing normal hardening and tempering once before induction hardening to completely dissolve the alloying elements and make the structure a tempered martensitic structure. is often taken.

However, performing normal hardening and tempering before induction hardening reduces workability and increases costs, so even if this pretreatment (normal hardening and tempering) is omitted, the desired performance cannot be achieved. There was a desire to develop stable induction hardened steel.

[0006]

[Means for Solving the Problems] From the above-mentioned viewpoint, the present inventors have developed a material for mechanical structures in which a "surface-hardened layer having sufficient hardness and no hardness variation" can be obtained only by ordinary induction hardening treatment. As a result of intensive research aimed at realizing steel, we were able to obtain the following knowledge.

(a) Insufficient hardness and hardness variations in the hardened layer due to induction hardening are closely related to the ferrite grain size of the steel before induction hardening. (b)
Then, the average grain size of ferrite before induction hardening is 2
When controlled to 0μm or less, ordinary induction hardening (
Heating temperature: 900-1000℃, heating time: 1-2 seconds)
By applying it alone, it becomes possible to stably form a hardened layer with sufficient hardness without any variation. (c)
By the way, in order to ensure the strength required for this type of material, steel containing 0.30% or more of C (hereinafter, % representing the component ratio is expressed as weight %) has a ferrite grain size of 20%.
In order to adjust the thickness to below μm, it is necessary to control the cooling rate after forging or rolling to 60° C./min or more, which causes considerable operational difficulties. However, when specific amounts of Mn and S are added in combination to steel, MnS, which serves as ferrite precipitation nuclei, is finely dispersed and precipitated, resulting in a cooling rate of 60°C/min after forging or rolling. Even if it is less than 20 μm, the ferrite grain size can be controlled to 20 μm or less.

[0008] The present invention was completed based on the above-mentioned findings and the like.
0.30-0.60%, Si: 1.00% or less, Mn: 0.30-2.00%, S: 0.
040 to 0.100%, or if necessary further Ni: 3.50% or less,
Cr: 2.00% or less, Mo: 2.00%
Below, Cu: 1.00% or less, B: 0
．． 0003-0.0050%, Al: 0.010
~0.100%, Ti:0.010 ~0.10
0%, Nb: 0.010 to 0.100%,
V: 0.01-0.30%, Ca: 0.0005
~0.0100%, Pb: 0.01~0.20%, and has a chemical composition with the balance consisting of Fe and unavoidable impurities, and the average grain size of ferrite is 20 μm or less The major advantage is that by adjusting the structure to a certain structure, it is possible to stably create a surface-hardened layer that has sufficient hardness with as little variation in hardness as possible using only induction hardening without the need for special pretreatment. It has characteristics. Here, the "grain size of ferrite" refers to the "grain size of individual ferrite" as shown in Figure 1.
and "grain size of aggregate ferrite".

[0009]

[Operation] As mentioned above, the present invention aims to ensure the desired properties by adjusting the content of C and Si in the steel, and in particular, by adjusting the amounts of Mn and S added within a specific range, the precipitation nuclei of ferrite are obtained. Finely dispersed MnS is generated to finely precipitate ferrite, thereby increasing the average ferrite grain size to 20μ.
This relates to a steel for machine structures that prevents hardness reduction and hardness variation after induction hardening by controlling the content of the constituent chemical components and the average ferrite grain size as described above. The reason for limiting the numerical value will be explained in more detail along with its effect.

(A) Chemical component C C has the effect of improving the static strength and hardness of steel, but in order to obtain the specified hardness as a steel for induction hardening, 0.
．． It is necessary to ensure a content of 30% or more. On the other hand, 0
．． Even if C is contained in an amount exceeding 60%, the hardness increasing effect is saturated. Therefore, the C content was determined to be 0.30 to 0.60%.

Si Si is an effective component that not only acts as a deoxidizing agent for steel but also imparts static strength to steel.
．． Even if it is contained in an amount exceeding 0%, not only the effect will be saturated, but also the cold workability will deteriorate. Therefore, the amount of Si added was determined to be 1.0% or less.

Mn Mn, like Si, is an effective element for deoxidizing steel.
In order to form MnS, promote finely dispersed precipitation of ferrite, and control the average ferrite grain size to 20 μm or less, which is the main aim of the present invention, ensure a Mn content of 0.30% or more along with a predetermined amount of S. There is a need to. On the other hand, 2.00
If Mn is contained in an amount exceeding %, the cold workability of the steel will be reduced. Therefore, the Mn content is between 0.30 and 2.
It was set as 00%.

SS is an effective element for improving the machinability of steel, but in order to form MnS and control the average ferrite grain size to 20 μm or less, which is the main aim of the present invention, a predetermined amount of Mn must be added. It is also necessary to ensure an S content of 0.040% or more. On the other hand, when S is contained in an amount exceeding 0.100%, the toughness of the steel decreases. Therefore, the S content is 0.040~0
．． It was set as 100%.

[00014] Ni, Cr, Mo, Cu, B, Al, Ti, Nb, V,
Ca and Pb These elements each affect the hardenability, toughness,
Since it has the effect of improving strength or machinability, one or more types may be added or contained as necessary, and the reason for limiting the content of each component is as follows.

a) Ni Ni has the effect of improving the hardenability and toughness of steel, but even if it is contained in an amount exceeding 3.50%, the effect of the above effect is saturated and the economic efficiency is reduced. The amount of Ni added was determined to be 3.50% or less since this would cause damage.

b) Cr Like Ni, Cr is an effective element for improving the hardenability of steel, but adding more than 2.00% will not only result in no commensurate improvement effect, but will also reduce the toughness. This will lead to a decline. Therefore, the amount of Cr added was determined to be 2.00% or less.

c) Mo Mo is an extremely effective element for improving hardenability and toughness, but if it is contained in an amount exceeding 2.00%, its effect will be saturated and economic efficiency will be impaired. The amount of Mo added was determined to be 2.00% or less.

d) Cu Cu is an element that is effective in improving the hardenability and static strength of steel, but when it is contained in an amount exceeding 1.00%, hot workability decreases and furthermore, static strength decreases. The Cu content was set at 1.00% or less since this would lead to a decrease in the Cu content.

e) B B is an element effective in improving the hardenability and static strength of steel, but if its content is less than 0.0003%, sufficient effects cannot be obtained. On the other hand, if B is contained in an amount exceeding 0.0050%, the crystal grains will become coarser, resulting in a decrease in toughness. Therefore, the B content is 0.0003
It was set at ~0.0050%.

f) Al Al is an effective element for refining the crystal grains of steel and improving its toughness, but in order to fully demonstrate its effect, it must be added in an amount of 0.010% or more. be. but,
If it is added in an amount exceeding 0.100%, the crystal grains will become coarser, resulting in a decrease in toughness. Therefore, the Al content was determined to be 0.010 to 0.100%.

g) Ti Like Al, Ti is an effective element for refining crystal grains and improving toughness, but in order to fully demonstrate its effect, it is necessary to add 0.010% or more. It is. However, if it is contained in an amount exceeding 0.100%, machinability decreases and the crystal grains become coarser, resulting in a decrease in toughness. Therefore, the Ti content is between 0.010 and 0.
．． It was set as 100%.

h) Nb Like Al and Ti, Nb is an effective element for refining crystal grains and improving toughness, but in order to fully demonstrate its effect, it must be contained in an amount of 0.010% or more. Addition is necessary. However, when added in excess of 0.100%, machinability deteriorates. Therefore, the Nb content is 0
．． 010% to 0.100%.

i) V V is an effective element for precipitating carbonitrides in steel and increasing the high-temperature strength of steel, but in order to fully exhibit its effect, it must be present in an amount of 0.01% or more. Addition is necessary. but,
Addition of more than 0.30% leads to a decrease in hot workability. Therefore, the V content is 0.01-0.30%
It was determined that

j) Ca Ca is an element that improves the machinability of steel, but in order to fully exhibit its effect, it must be added in an amount of 0.0005% or more. However, if the Ca content exceeds 0.0100%, it will reduce the toughness of the steel.
．． 0005 to 0.0100%.

k) Pb Pb is also an effective element for improving the machinability of steel, but in order to fully exhibit its effect, it is necessary to ensure a content of 0.01% or more. However, P content exceeding 0.20% reduces the toughness of steel.
The b content was determined to be 0.01 to 0.20%.

(B) Ferrite average grain size If the ferrite average grain size (individual ferrite average grain size and collective ferrite average grain size) is larger than 20 μm, the induction hardening layer can be formed without pretreatment such as normal hardening and tempering. of"
It becomes difficult to prevent "a decrease in hardness" or "occurrence of variations in hardness."Therefore, the average ferrite grain size was set to 20 μm or less.

Next, the effects of the present invention will be explained in more detail with reference to Examples.

[Example] Steel with each component composition shown in Tables 1 and 2 was 50k
After melting in an atmospheric furnace of g, each has a diameter of 50mmφ.
A sample material was prepared by forging and elongating into a bar with a diameter of 25 mm.

[Table 1]

[Table 2]

[00028] Next, each of the above test materials was heated to 800 to 900°C.
After heating to a temperature range of 1.5 hours, air cooling (50 m
Cooling rate of mφ material: 30-40℃/min, 25mmφ
Material cooling rate: 60-80°C/min) Normalizing was performed. After that, these test materials were
× length 50mm” test piece, frequency: 200k
Hz, heating temperature: 950°C, movement speed: 3mm/sec
Induction hardening was performed under the following conditions (time from heating to cooling: approximately 2 seconds).

Next, the hardness of the sample material after induction hardening was measured to investigate the "hardness variation", and Table 1, It is also listed in Table 2. In addition, the evaluation of hardness variation was performed using a 25 mmφ forged and drawn material (as shown in Fig. 2).
Because the cooling rate during normalization was fast, the ferrite grain size has become fine, so there is almost no variation in hardness.) Draw a smooth insertion line within the hardness distribution curve, and compare that insertion line with the actual hardness distribution. A method was used to calculate the degree of variation by determining the area caused by the difference from the hardness distribution curve showing the hardness distribution curve. Then, the degree of variation is A = [area of 50mmφ material]/
[Area of 25 mmφ material] Area ratio (A
).

As is clear from the results shown in Tables 1 and 2, the degree of hardness variation after induction hardening for all of the steels of the present invention is 2.0 or less, whereas the degree of variation for the comparison steel is less than 2.0. It can be seen that the hardness varies from 2.1 to 7.1, and the variation in hardness is extremely large.

[00031]

[Summary of Effects] As explained above, according to the present invention, which has as its important point the combined addition of appropriate amounts of Mn and S in order to particularly control the ferrite grain size of induction hardened steel, induction hardening It is possible to significantly reduce the variation in hardness of the layer, and it is also possible to stably secure sufficient hardness, making it possible to omit "quenching and tempering" as a preliminary treatment before induction hardening, which is extremely useful in industry. Useful effects are produced.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing the definition of ferrite grain size.

FIG. 2 is an explanatory diagram of a method for determining "hardness variation degree."

Claims (2)

[Claims] [Claim 1] Weight percentage: C: 0.30-0.60%, Si: 1.0
0% or less, Mn: 0.30-2.00%,
A steel for induction hardening, characterized in that it contains 0.040 to 0.100% S, the balance consists of Fe and unavoidable impurities, and the average ferrite grain size is 20 μm or less. [Claim 2] Weight percentage: C: 0.30-0.60%, Si: 1
．． 00% or less, Mn: 0.30-2.00
%, S: 0.040 to 0.100%,
Furthermore, Ni: 3.50% or less, Cr: 2
．． 00% or less, Mo: 2.00% or less, Cu: 1.00% or less, B: 0
．． 0003-0.0050%, Al: 0.010
~0.100%, Ti:0.010 ~0.10
0%, Nb: 0.010 to 0.100%,
V: 0.01-0.30%, Ca: 0.0005
~0.0100%, Pb: 0.01~0.20%, the balance is Fe and unavoidable impurities, and the average ferrite grain size is 20 μm or less. Steel for induction hardening.