JPH02153019A - Production of steel member - Google Patents

Abstract

PURPOSE:To produce the steel member having a bainite structure and high strength and high toughness by subjecting a steel stock contg. at least C, Si, Mn, and Cr to hot forging and subjecting this steel stock to a specific cooling treatment from the austenitization temp. or above then to a tempering treatment. CONSTITUTION:The steel stock consisting of the steel products contg. at least the C, Si, Mn and Cr is subjected to hot forging. The above-mentioned steel stock is then continuously cooled at 0.4 to 10.0 deg.C/sec cooling rate from the austenitization temp. accompanied with the hot forging or above or from the temp. at which the steel stock is overheated again to the austenitization temp. or above after the hot forging. The steel stock is made into the substantially bainite or bainite + martensite structure in this way. The above-mentioned steel stock is thereafter subjected to the tempering treatment in the temp. range where the martensite softens to obtain the substantially bainite or bainite + sorbite structure. The steel member having the excellent strength and toughness is obtd. in this way.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for manufacturing steel members, and particularly relates to a method for manufacturing high strength and high toughness steel members mainly having a bainite or bainite + sorbite structure. be.

[Prior art]

Conventionally, 70 to 100 kgf/mm” class (equivalent hardness Hv
Steel members that require a tensile strength of 220 to 320) are often manufactured by subjecting carbon steel or alloy steel to quenching and tempering treatment (refining treatment).

On the other hand, in recent years, from the perspective of reducing heat treatment costs, non-tempered steel with the addition of vanadium has been increasingly used, so that it can achieve the same strength as tempered steel simply by air cooling after hot forging. It has come to be like this.

By the way, if non-heat-treated steel is simply hot-forged and air-cooled, the grain size will be coarse and it will essentially have a pearlite or ferritic pearlite structure, resulting in lower toughness (impact strength) than heat-treated steel. This problem limits its use.

On the other hand, Japanese Patent Publication No. 61-31184 discloses that by changing the structure of non-tempered steel to a ferritic pearlite structure suitable for nitriding through controlled cooling,
A technique for manufacturing steel parts with excellent fatigue strength, wear resistance, bitching resistance, and spalling resistance is disclosed.

[Problem to be solved by the invention]

The main reasons why ordinary non-tempered steel has low toughness are (1) that the steel structure is pearlite or ferrite + pearlite structure, (2) that the crystal grain size of the steel structure is coarse.

As a countermeasure for the above reason (1), it is known that it is effective to change the steel structure to bainite or ferrite-decade bainite structure.

In order to form a bainite structure, the austenitization temperature (
It is possible to perform constant temperature cooling (austempering treatment) from a temperature of about 730° C. or higher, but this is impractical in terms of processing time and processing cost.

Therefore, in order to form a bainitic structure through continuous cooling, it is also being considered to increase the amount of M7 and C1, which are elements that improve hardenability, to improve hardenability. However, in this case, it is difficult to set the cooling rate, and it is very difficult to stably form a bainite structure. That is, when the cooling rate is too low, a pearlite structure is formed, and when the cooling rate is too high, a martensitic structure is formed.

As a countermeasure for the above reason (2), it is also effective to normalize the steel material after hot forging to make the grain size finer than NO.6. However, when the grain size is made finer, the hardenability of the steel decreases, and it becomes difficult to stably form a bainite or ferrite-decade bainite structure in a normal steel structure.

Therefore, in the previous application, the present inventors proposed that an appropriate amount of Mn be used in each case.
After hot forging, a steel cable material made of a steel material containing %Cr and We proposed a method for producing a steel member having a substantially bainite or ferritic-decainite structure by controlled cooling at ~4.0°C/5ec).

However, in this manufacturing method of steel members, expensive Mo is added, which increases the material cost of the steel material, and in many cases, it tends to become a multiple phase of ferrite + bainite instead of a single bainite phase, resulting in a slightly lower strength. In addition, since the adjustable cooling rate range is narrow, there are problems such as difficulty in controlling the cooling rate when actually manufacturing steel members of various sizes.

An object of the present invention is to establish a technology that substantially changes the steel structure to bainite or bainitic martensitic structure by continuous cooling, and further changes the steel structure to substantially bainite or bainite + sorbite structure by tempering, thereby improving strength and strength. An object of the present invention is to provide a method for manufacturing a steel member having excellent toughness.

[Means to solve the problem]

The method for manufacturing a steel member according to the present invention includes at least C, Sl.
, Mfi, and C1, and then the steel material is heated to a temperature higher than the austenitizing temperature associated with hot forging, or at a temperature higher than the austenitizing temperature after hot forging. From the temperature reheated to 0.4~
The steel material is continuously cooled at a cooling rate of 10.0'C/sec to form a substantially bainite or bainitic martensitic structure, and then the steel material is tempered in a temperature range where martensite softens to substantially form bainite or martensite. This is to create a bainite + sorbite structure.

[Effect]

In the method for manufacturing a steel member according to the present invention, a steel cable material made of a steel material containing small amounts of C, st, M, and C is used. It is necessary to precipitate a bainite or bainitic martensitic structure by cooling from a temperature of
It is also necessary to enable tempering treatment. S and C are necessary to ensure base hardness.

Ml and Cr are elements that improve hardenability and are necessary to promote martensitic transformation and bainite transformation.

In addition, in addition to the above, it is desirable to use a steel material containing V, M, , S, and P. (2) improves the base hardness by precipitation of carbonitrides, and Mo improves the hardness of the matrix by precipitation of carbonitride. By promoting transformation, S and P improve the machinability of bainite structure, which is difficult in terms of machinability.

A steel cable material made of the above-mentioned steel material is hot-forged, and from a temperature higher than the austenitizing temperature associated with hot forging, or from a temperature reheated to the austenitizing temperature after hot forging, 0
．． Continuous cooling is performed at a cooling rate of 4 to 10.0°C/sec to substantially form a bainite or bainite+martensite structure.

The temperature above the austenitizing temperature includes alloying elements (M,..., CrM) that are involved in hardenability.

) is set at a temperature at which sufficient solid solution is achieved, but in the case of a steel composition that achieves the strength and hardness targeted by the present invention, the temperature is set at a temperature of 850 to 95.
0°C is appropriate.

Regarding the cooling rate, in order to obtain a bainite or bainite + martensitic structure, the cooling rate is 0.4 to 10.0"C/
A cooling rate of sec is required. If it is less than 0.4'C/sec, pearlite will precipitate over a considerable part. On the other hand, if it is faster than 10.0'C/sec, martensite will be precipitated in large quantities.In particular, bainitic martensite Even if it is a multi-phase, the martensite phase has an area ratio of 5
If it exceeds 0%, it is not preferable because a large amount of thermal energy is required for tempering, and in order to precipitate a bainite phase with an area ratio of 50% or more, a cooling rate of 10.0° C./sec or less is required.

Next, the steel cable having the bainite or bainite + martensitic structure is tempered at a temperature range in which martensite softens to substantially form bainite or bainite + martensite.
Make it a sorbite organization.

In the above steel material, the temperature range in which the bainite phase does not soften and only the martensitic phase softens is 150 to 600°
At temperatures lower than 150'C, martensite does not become sorbite, and at temperatures higher than 600°C, bainite softens and becomes tempered bainite, resulting in a decrease in hardness.

The structure of the steel member obtained by the above manufacturing method is a bainite or bainite+sorbite structure, and has excellent strength, hardness, and toughness.

〔Effect of the invention〕

According to the method for manufacturing a steel member according to the present invention, the above [effect]
As explained in the above section, by adjusting cooling from a temperature above the austenitizing temperature, the addition of Mo is no longer essential to create a bainite or bainite + martensite structure, so the material cost of the steel material does not increase. , and the adjustable cooling speed range is 0.4~10.0''
C/sec, it is easy to control the cooling rate when manufacturing steel members of various sizes and shapes, which can reduce manufacturing costs accordingly, and even in the case of a dual phase of bainite + martensite, the area of martensite is small. Since the ratio is relatively small, the thermal energy required for tempering is relatively small, so the cost of tempering can be reduced, and other effects can be obtained.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings.

In this embodiment, at least C, St, M, , and C,.

After hot forging or reheating after hot forging, a bainite or on-bainiite + martensitic structure is formed by continuous cooling, and then the steel cable material is The present invention relates to a method for manufacturing a steel member having excellent strength, hardness, and toughness by tempering the steel to substantially form a bainite or bainite + sorbite structure.

First, a method for manufacturing this rope member will be explained based on FIG.

The composition of the steel material used is preferably one containing the following alloying elements. However, it is shown in weight%.

C: 0.15-0.40%, Sz: 0.50%
Below, Mo: 0.50% or less, M: 0.5
0-1.50%, C: 0.10-1.30%, V
: 0.20% or less, N: 0.02% or less, A
j2: 0.10% or less, S in addition to the above as necessary:
0.20% or less, Pb: 0.35% or less.

C: 0.15 to 0.40% C is an element that is the basis of matrix hardness, and in order to obtain a hardness of Hv22o to 320 in a bainite or bainite + sorbite (tempered martensite) structure, it must be at least 0.15% to 0.40%.
15% or more is required, but if it exceeds 0.40%, toughness and workability will deteriorate, which is not preferable.

Sl: 0.50% or less Si, together with C, is an effective element for ensuring base hardness, but if it exceeds 0.50%, workability deteriorates, which is not preferable.

M,: 0.50-1.50% M is 5! It is an element necessary for deoxidizing steel, and at the same time it is an element that improves the hardenability during martensitic transformation and bainite transformation.In order to obtain the hardenability to precipitate bainite, it must be at least 0.50%. Although necessary, 1
．． If it exceeds 50%, the hardenability becomes excessive, precipitation of martensite increases, and workability also decreases.

C: 0.10 to 1.30% C1 is an element that improves the hardenability during martensitic transformation and bainite transformation, similar to M and l, and when nitrided, it combines with N and forms an element in the surface layer. It is an element that increases hardness by forming nitrides. In order to obtain the effect of improving hardenability, the content is required to be 0.10% or more, but if it exceeds 1.30%, the hardenability becomes excessive and a large amount of martensite is precipitated, which is not preferable.

Mo: 0.50% or less Mo, like M7 and Cr, is an element that improves hardenability, and is an important element that particularly improves hardenability during bainite transformation and promotes bainite transformation. If it exceeds 0.50%, not only the effect of promoting bainite formation is saturated, but also workability deteriorates, which is not preferable.

V: 0.20% or less ■ combines with C and N in the steel material to generate carbonitrides,
It has the effect of improving base hardness, and if it exceeds 0.20%, it is not preferable because it causes a decrease in toughness and workability.

Heg: o, io% or less A2 combines with N in the steel material to produce hard AffiN, and has the effect of preventing coarsening of grain size when heated above the austenitizing temperature, but 0.10% If the amount is too large, the effect will be saturated and the processability will deteriorate, which is not preferable.

N: 0.02% or less N combines with Affi in the steel material to produce hard AλN, and also combines with ■ to precipitate a hard compound, improving the hardness of the matrix, and also reducing the coarseness of the steel crystals. This contributes to improving toughness as it prevents corrosion. However, if it exceeds 0.02%, the above-mentioned effects become saturated and workability deteriorates, which is not preferable.

S: 0.20% or less and Pb: 0.35% or less Both S and Pb are elements to be improved in rigidity. Since the bainite structure has some difficulties in machinability, the addition of these materials is effective in improving machinability. S: 0.20% or less, Pb: 0.35%
If it is below, the above effects can be obtained without significantly impairing the mechanical properties of the steel.

Next, a steel cable material made of a steel material having the above composition is cut into a predetermined shape for forging, and the steel material is hot forged. In the case of adjusting cooling following this hot forging, adjust cooling as follows from the temperature of 850 to 950°C associated with this hot forging, and If the temperature has dropped to 850 to 950° C., reheating is performed as normalization after hot forging, and cooling is performed from that temperature in the following manner.

The reason why the controlled cooling is performed from the austenitic state in the range of 850 to 950'C is as follows.

If the temperature is less than 850°C, solid solution of alloying elements such as M, %, Cr, MO, etc. is insufficient, and sufficient heat treatment characteristics cannot be obtained. Further, at a temperature higher than 950° C., sufficient hardness cannot be obtained because part of the finely precipitated carbonitrides (①) after hot forging are dissolved again. In addition, coarse grains occur and toughness decreases.

Next, as mentioned above, the steel cable material of the austenitic Mi collar at a temperature of 850 to 950 °C is heated at 0.4 to 10.0 °C/se.
The steel material is continuously adjusted and cooled to room temperature at a cooling rate of c, so that the steel material has a substantially bainite or bainitic martensite structure. In the structure of the bainitic martensite, when the area ratio of martensite is greater than 50%, a large amount of thermal energy is required for the later-described tempering process, resulting in an increase in the cost of the tempering process. Therefore, it is desirable that the area ratio of the bainite phase is 50% or more.

If the cooling rate of this controlled cooling is less than 0.4°C/sec, pearlite will precipitate in a considerable part of the steel cable, and if it is faster than 10.0°C/sec, martensite will be deposited in most of the steel material. This is not preferred because precipitation occurs.

Next, as described above, the steel cable having a substantially bainite or bainitic martensite structure is tempered at a temperature range in which martensite softens (150 to 600"C) to substantially have a bainite or bainitic sorbite structure. At temperatures below 150°C, martensite does not change to sorbite, and at temperatures above 600°C, bainite also softens and becomes tempered bainite, which is not preferable.

Next, as described above, the steel cable having the structure of bainite or bainite + sorbite is machined to form a steel member in a predetermined shape.

As described above, the base structure of the steel member is changed to bainite or bainitic martensitic structure by continuous controlled cooling rather than constant temperature cooling, and then tempered to form bainite or bainite + sorbite composite, which increases the strength. It can be made into a steel material with excellent hardness and toughness, and its toughness (impact strength) can be significantly improved compared to ordinary non-tempered steel.

This method of manufacturing steel members can be used to manufacture, for example, connecting rods for engines, crankshafts, and gears for various machines, but it is desirable to subject gears to nitriding or nitrocarburizing as necessary.

According to the above-mentioned manufacturing method of a steel member, a steel member having a bainite or bainite-de-sorbite structure is produced by continuous cooling instead of constant temperature cooling, which is expensive for heat treatment, to form a bainite or bainitic martensitic structure, and then by performing a tempering treatment. This makes it possible to produce steel members with significantly improved toughness compared to conventional non-tempered steel.

Through the above-mentioned controlled cooling, it is possible to create a bainite or bainitic martensitic structure instead of a bainite or bainitic ferrite structure, so the cooling rate range is wide, making it possible to control the cooling rate when manufacturing steel parts of various sizes and shapes. is easy and can reduce manufacturing costs.

In addition, Beinai! - Since the area ratio of martensite in the + martensite dual phase is less than 50%, the cost of tempering treatment is not very high.

Moreover, since the steel member contains appropriate amounts of Cr, ■, and At, which combine strongly with N to precipitate hard nitrides, when nitriding or nitrocarburizing the steel member,
By forming a hardened layer on the surface and deep inside the steel member, fatigue strength can be significantly increased.

Moreover, the base fibers are strong bainite or bainite +
Since it is sorbite 'fA', the propagation of cracks within the hardened layer is suppressed. In addition, since ■ and Mo are elements that increase resistance to temper softening, the base hardness is less likely to decrease during nitriding.

Next, an example of an experiment conducted to obtain a correlation between tempering temperature and base hardness will be described.

Experimental Example: Refer to Figure 2 Using a steel material containing the various alloying elements shown in Table 1, a round bar of 30 mm diameter was produced by hot forging (1100 to 1250°C). 5.0 from the temperature of 850 to 950"C associated with hot forging.
The steel was cooled at a cooling rate of /sec to form a bainite or bainitic martensite structure. This steel structure had a bainite area ratio of 80% and a martensite area ratio of 20%.

Next, the round bars with the above structure were subjected to tempering treatment for 1.5 hours at various temperatures, and the surface hardness of each round bar was measured, and the results shown in FIG. 2 were obtained.

As can be seen from FIG. 2, the hardness decreases gradually in tempering at a temperature of 150 to 600°C, and sharply decreases in tempering at a temperature higher than 600°C. That is, 1
Tempering treatment at temperatures below 50°C causes almost no softening of martensite, and martensite does not transform into sorbite.
Furthermore, tempering at a temperature higher than 600°C causes softening of martensite and softening of bainite, resulting in a significant decrease in hardness.

In other words, when tempering is performed in a temperature range of 150 to 600°C, bainite does not soften, and only martensite softens and becomes sorbite, which becomes bainite or bainite +
It can be made into a sorbite structure. Furthermore, by appropriately selecting the tempering treatment temperature within the range of 150 to 600°C, it is possible to obtain a steel member having a desired hardness within a predetermined range.

[Brief explanation of the drawing]

The drawings relate to embodiments of the present invention; FIG. 1 is an explanatory diagram of the manufacturing process of a steel member, and FIG. 2 is a diagram showing the relationship between tempering temperature and hardness obtained through experiments.

Claims (1)

[Claims] (1) Hot forging a steel material containing at least C, Si, Mn, and Cr; 0.4 to 10 from the temperature reheated to a temperature higher than the austenitizing temperature after hot forging
．． The steel material is continuously cooled at a cooling rate of 0°C/sec to form a substantially bainite or bainite + martensite structure, and then the steel material is tempered in a temperature range where martensite softens to form a substantially bainite or bainite + martensite structure.
A method for manufacturing a steel member, characterized by forming it into a sorbite structure.