JPS62187536A - Manufacture of non-refining forgings - Google Patents

Abstract

PURPOSE:To improve the fatigue strength and toughness in a state of non- refining, after forging, by bringing steel for machine structure or low alloy steel to hot rolling, and thereafter, quenching it, forming a quench hardening depth layer on the surface by a prescribed sectional area rate, and thereafter, executing the forging in a specified temperature range. CONSTITUTION:Steel for machine structure, etc., are brought to hot rolling, and thereafter, only the surface layer is quenched by cooling in a short time by comparatively small quantity of water, and a quench hardening depth layer is generated on the surface, by 20-70% by a sectional area rate. Also, the inside is made to remain ferrite and pearlite structures. Subsequently, by bringing the steel for machine structure, etc., which have been quenched, to forging at a temperature between 550 deg.C-Ac1, a non-refining forgings whose tensile strength is >=70kgf/mm<2> and whose Charpy absorption energy at a room temperature is >=5kgfm is obtained. This non-refining forgings has characteristics which are excellent in the fatigue strength and toughness, even if the subsequent heat treatment is omitted.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for producing selts and shafts that remain unheated after forging and have excellent fatigue strength and toughness.

(Prior art) Traditionally, parts that require high fatigue strength and toughness, such as bolts and shafts used for automobiles and industrial machinery, have been processed mainly by hot processing in terms of deformability and deformation resistance during forging. It was being done.

However, since this hot forging is performed by heating to a high temperature of 1200° C. or higher, austenite crystal grains grow and become coarser, resulting in a significant decrease in toughness. For this reason, heat treatment such as quenching and tempering was essential to improve the material quality after forging.

In this way, in the case of forming by hot forging, a large amount of energy is required for high-temperature heating before forging and heat treatment of the forged product, and the dimensional accuracy is poor, so cutting must be performed after forging. As a result, the yield was low.

On the other hand, for the purpose of non-heat refining after forging and omitting the cutting process, there is a warm forging method that involves rapid cooling after rolling and then forging in a warm region with a lower processing temperature than hot forging. , for example, is disclosed in Japanese Patent Application Laid-Open No. 58-77526.

However, since this method cools the material to the inside, it requires a large amount of refrigerant and a long time for cooling, and has the disadvantages that the deformation resistance during forging is large and the life of the tool is significantly shortened.

(Problems to be Solved by the Present Invention) As described above, conventional forged product manufacturing technology requires heat treatment such as quenching and tempering after forging, or in cases where heat treatment is not required. However, the deformation resistance during molding was large and the tool life was significantly reduced.

(Means and effects for solving the problems) The gist of the present invention is to rapidly cool mechanical structural steel or low alloy steel after hot rolling, and harden the surface by 20 to 70% in cross-sectional area ratio. A tensile strength of 70Kgf/mm2 or more and a Charpy absorbed energy of 5Kg at room temperature, characterized by forming a deep layer and then forging between 550℃ and one point of Ac.
This is a method for manufacturing a non-thermal forged product having fm or more.

In the present invention, after hot rolling, only the surface layer is rapidly cooled to a quenched structure by cooling in a short time with a relatively small amount of water, while the inside remains a soft ferrite and pearlite structure.
As a result, a steel bar with low curvature resistance and excellent deformability can be obtained, and warm forging becomes possible. Therefore, a cutting process is not necessary, and a forged product with excellent fatigue strength and toughness can be obtained even if heat treatment after forging is omitted.

Next, the reason for limiting each requirement in the present invention will be described.

First, there are no particular restrictions on the steel types to which the present invention applies, as long as they fall within the range of ordinary mechanical structural steels or low alloy steels used in automobiles, industrial machinery, etc. Briefly describe the desired range of ingredients.

■ C: 0.30-0.60%, C is an element with high hardenability and greatly affects the strength of forged products, and if it is less than 0.30%, a tensile strength of 70 Kgf / mj cannot be obtained. Moreover, if the content exceeds 0.60%, the strength will be too high and the desired toughness will not be obtained.

■ St: 0.10-0.50%. St has a deoxidizing effect, and for this purpose, a minimum amount of 0.10% or more is required. However, if it exceeds 0.50%, the ferrite in particular becomes brittle and the desired toughness cannot be obtained.

■Mn: 0.30-2.00%. Like St, Mn has a deoxidizing effect, and when deoxidizing with Mn alone, a minimum amount of 0.30% or more is required. In addition, Mn is used as an element for improving hardenability with relatively little deterioration of toughness.
If it exceeds %, the cost will be high.

■ Cr: 2.00% or less. Cr is an element that improves hardenability and at the same time reduces the deformation resistance during forging by roughening the lamella spacing of the nozorite formed inside, but if it exceeds 2.00%, the cost increases.

■ Ni:5. Q% or less. Ni increases both the strength and toughness of ferrite, but the upper limit is limited by cost.

■ B: 0.0005-0.0050%. B is an element that significantly improves hardenability in small amounts, but at 0.0005%
If it is less than 0.005%, there is no effect, and if it exceeds 0.0050%, toughness and ductility deteriorate.

Next, the reason why the depth of the surface hardening layer is limited to 20 to 70% in cross-sectional area ratio will be described. In general, a method for increasing the fatigue strength of shafts and the like is to harden the surface layer of these parts by high-frequency waves or the like after forming them, thereby producing a hardened layer of several percent. The present invention is a method for increasing fatigue strength without heat-refining after warm forging, in which a hardened layer is generated on the surface by rapid cooling after hot rolling. The forging was carried out under conditions such that the remaining If the quench depth layer is less than 20%, the fatigue strength of the forged product will not be sufficient;
This is because if the martensitic structure exceeds 50%, the deformation resistance during forging increases and the tool life deteriorates.
% or more is defined as the hardened depth layer. ).

In addition, the lower limit temperature for forging was set at 550°C.
This is because if it is less than that, the deformation resistance will increase and the toughness and ductility will decrease. Furthermore, the upper limit of the forging temperature was set to Ac1 point because
This is because when the Ac value exceeds 1 point, the surface portion becomes austenite again, and the hardness and hardened structure of the surface layer indented during hot rolling disappear.

(Example) Next, examples of the present invention will be described.

Table 1 shows the types and chemical components of the test materials. Carbon steel for mechanical structures (S53C) and low alloy steel (80
A billet with chemical components compatible with M440,5AE15B36) was used.

Each sample billet was heated to 1000 to 1050°C and rolled into a 30IaIφ steel bar. The finish rolling temperatures at this time are shown in Table 2 (symbols A, B, C, E, F, G
.

I, G, K), 345 rr after rolling.
? /hr (553C) and 210 vl/hr
(80M440 and 5AE15B36) for 5 seconds, and at the same time as comparative materials Table 2 (symbols,
It was manufactured by air cooling from the finishing temperature shown in (H,N).

FIG. 1 shows the hardness distribution in the cross section of 553C when the rolling was completed at a finishing temperature of 745° C. and cooled after rolling (symbol C in Table 2). As is clear from the figure, the hardness decreases closer to the center of the steel bar, and the quenched depth layer in this case is 64.0%.

Next, in order to investigate the material properties of the steel bars produced in this way after warm forging, we used 30Wφ x 550-length steel bars at forging temperatures of 600°C and 700°C (553C, SCM4
40. and Ac1 point of 5AE15B36 is 7 respectively.
It was forged to a thickness of 11.2 ms at 25°C, 744°C and 710°C [corresponding to forging just below the Ac1 point]. From the forged material warm-forged in this way, JIS
A No. 1 rotary bending fatigue test piece, a 6φ x 32L small tensile test piece, and a JIS No. S Charpy impact test piece were collected and used in each test. In this case, the material of the present invention, which was water-cooled after rolling, remained unheated without any special heat treatment, and the comparative material, which was air-cooled after rolling, was quenched at 850°C after forging and quenched at 600°C. The above test piece was taken after the back treatment.

Each specimen was mainly taken from the center, but 5
Regarding the 600° C. forged material of the present invention made of 530 steel and symbol C in Table 2, fatigue and Charpy impact test pieces were taken from the surface layer and tested.

Table 2 shows the results of tests on fatigue, tensile strength, and toughness taken from these central areas.

The fatigue and tensile strength of the specimens subjected to water cooling after finishing rolling and warm forging at 600° C. are much higher than those of the specimens subjected to quenching and tempering after warm forging. Also, AC
Fatigue and tensile strength are slightly lower when warm forging is performed at 700°C near the 1st point, but fatigue strength is still 3.
5 km! The tensile strength is at the level of 70 kg. In addition, the absorbed energy of the material warmly forged at 600℃ is slightly inferior to that of the heat-treated material after warm forging, but it is still at the level of 5kg.
The 700C warm forged material has a high value equivalent to or higher than that of the heat-treated material. ! In addition, the fatigue strength of the surface layer test piece of 600℃ forged material of 553C steel of the present invention with symbol C is 55K.
gf/-1 and absorbed energy was 9 Kgfm.

(Effects of the Invention) As described above, the present invention rapidly cools the steel material after hot rolling to generate a hard hardened layer on the surface, thereby achieving excellent fatigue strength and toughness even if the subsequent heat treatment is omitted. This is a method that can produce forged products, and its effects are great.

[Brief explanation of drawings]

The first figure is a diagram showing the cross-sectional hardness distribution of an S53C30wφ steel bar that was rapidly cooled after rolling. Agent: Patent attorney Masamitsu Akizawa and 2 others

Claims (1)

[Claims] (1) Machine structural steel or low alloy steel is hot-rolled and then rapidly cooled to form a hardened depth layer with a cross-sectional area ratio of 20 to 70% on the surface, and then forged between 550℃ and Ac_1 point. A method for producing a non-thermal forged product having a tensile strength of 70 Kgf/mm^2 or more and a Charpy absorbed energy of 5 Kgfm or more at room temperature, characterized in that: