JPH0534403B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing a directly quenched hot forged product. Conventionally, mechanical structural parts used in automobiles and construction machinery that require high strength have been produced by hot forging steel materials, then reheating, and then undergoing heat treatment such as quenching and tempering to meet the needs of the purpose and use. It is manufactured and put into use with added strength properties. In this way, in the method of performing the above-mentioned heat treatment after forging, alloying elements such as Cr and Mo are added to the steel material used to ensure hardenability, and the austenite crystal grains are further refined. In order to ensure toughness after quenching and tempering, precipitation hardening alloy elements such as Al, Nb, Ti, and V are added. However, the above heat treatment after hot forging inevitably involves an increase in processing steps and an increase in work in progress, and
This requires a large amount of energy, and furthermore, it is necessary to add various alloying elements to the steel material used, as described above, which inevitably increases manufacturing costs. For this reason, in recent years, in order to reduce manufacturing costs and simplify the process, there has been a demand for a method to obtain forged products by directly quenching steel materials with as little added amount of expensive alloying elements as possible after hot forging. It has reached the point where However, when directly quenching the above-mentioned steel material after hot forging, the hardness of the forged product after quenching varies depending on the heating temperature and forging conditions before forging. In other words, if a steel material containing the above alloying elements is heated to a high temperature such as 1250°C and forged, the austenite grains of the steel material become coarse due to the high temperature heating, and then the austenite grains of the steel material become coarse due to the hot forging. Although the grains become fine, they immediately grow and become coarse again, so the hardenability is good even if straight hardening is performed after forging. However, the forging temperature is 1050℃
If the drop is below or if forging is repeated, the austenite grains will not grow into uniform coarse grains,
If the forged product is quenched directly after forging, the hardenability of the forged product will be non-uniform, resulting in areas where the hardness is not sufficiently high, resulting in uneven hardness. In order to avoid such uneven hardness, conventionally, steel materials are heated to a high temperature of 1200°C or higher before forging, and quenching after forging is also started at the highest possible temperature. However, high-temperature forging shortens the life of the die, and there is also the problem that an oxide layer is formed before quenching, resulting in uneven quenching on the surface. As a result of intensive research to solve the above-mentioned problems, the inventors of the present invention found that the chemical composition of the steel material should be regulated, especially the amounts of Al and N, and the heating temperature and quenching temperature before forging should be within a predetermined range. The inventors have discovered that a forged product with good hardenability and uniformity can be obtained by directly quenching after hot forging, leading to the present invention. The method for manufacturing a direct quenched forged product according to the present invention includes:
Steel materials containing C 0.26 to 0.50%, Si 0.50% or less, and Mn 0.30 to 1.8% in weight percent, regulated to P 0.020% or less, S 0.01 to 0.2%, Al 0.006% or less, and N 0.005% or less 1200℃
It is characterized by being forged after being heated to a temperature of 850°C or higher, and then quenched at a temperature of 850°C or higher. First, the reasons for limiting the composition of the steel material used as the steel material in the method of the present invention will be explained. C is added as an essential element to ensure the strength of forged products through direct quenching and tempering treatment after hot forging, but if its content is less than 0.26%, this strength effect is poor; When it exceeds 0.50%, quench cracking or placement cracking occurs during direct quenching after forging. Therefore, the range of addition amount of C is
Set to 0.26-0.50%. Si is added for deoxidation, but when added in excess, it deteriorates ductility. Therefore, the amount of Si added should be 0.5% or less. It is necessary to add at least 0.3% of Mn to prevent surface cracking during solidification, and
It is effective in improving hardenability and increasing strength. However, if the content exceeds 1.8%, it may cause cracking, so the upper limit is
The rate shall be 1.8%. Since P deteriorates the ductility of the steel material, promotes quench cracking, and deteriorates the toughness during tempering, the content is set to 0.020% or less. S is effective for improving the machinability of forged products, but on the other hand, it reduces ductility. Therefore, when ductility is important, the upper limit is preferably 0.03%, and when machinability is important, the upper limit is 0.20%. Al is added as a strong deoxidizing agent, but 0.006
If it is added in excess of %, austenite grains may be mixed during quenching after forging, resulting in uneven quenching, so the upper limit should be set as above.
It shall be 0.006%. N has a strong bonding force with Al, so if it is contained in large amounts in steel, it will bond with Al as a deoxidizing agent.
After all, mixed austenite grains are produced during quenching after forging, resulting in uneven quenching. For this reason,
In the present invention, the amount of N is 0.006% or less. Furthermore, the material steel used in the present invention may optionally contain at least one member selected from Cr and Mo in order to improve various mechanical properties, such as increasing hardenability when surface hardening a forged product. Seeds can be added. The upper limit of the addition amount of these elements is 1.2% and 1.2% for Cr and Mo, respectively.
Set at 0.3%. This is because if the amounts added exceed the above-mentioned upper limits, quench cracking will occur. In addition, in order to improve the machinability of forged products, Pb is added in an amount of 0.05 to 0.30 in addition to or in place of S.
It may be added within a range of %. According to the present invention, by suppressing the amounts of Al and N in the steel to a very small amount, it is possible to prevent uneven quenching caused by mixed grains of austenite grains during quenching of the steel after hot forging. So,
Forged after heating to a temperature of 900-1200℃, then quenched at a temperature of 850℃ or higher, and then processed according to conventional methods.
By tempering at about 450 to 650°C, a forged product with good and uniform hardenability and excellent toughness can be obtained. When the quenching temperature is lower than 850°C, the quenching property is lowered and uneven quenching may occur, which is not preferable. When using conventional steel materials, if they are forged in the temperature range of 1100 to 1000℃ and then directly quenched, the austenite grains become mixed and uneven quenching occurs, resulting in uneven quenching; If direct quenching is performed after forging at a low temperature of 1,100 to 900 degrees Celsius, the austenite grains will become finer and the hardenability will deteriorate, but according to the present invention, forging can be performed at a low temperature of 1100 to 900 degrees Celsius. Furthermore, a forged product with good and uniform hardenability can be obtained. Furthermore, according to the method of the present invention, it is not necessary to add expensive alloying elements to the raw material steel used, or it is sufficient to add a very small amount of alloying elements especially to improve hardenability, so the material cost is reduced. is also reduced. The present invention will be explained below with reference to Examples. Example 1 A 30 mm diameter steel bar having a chemical composition shown in Table 1 A to F is heated to 900°C, 1000°C, 1100°C or 1200°C, then die forged to a diameter of 20mm, and then
Water cooling from temperatures of 920℃, 950℃ and 1030℃,
The hardness distribution in the cross section of the obtained forged product was measured. The results are shown in Figure 1. According to the steel of the present invention,
Since the austenite crystal grains coarsen immediately after forging, even if the heating temperature before forging is low, the hardenability after forging is good, and the core is well hardened. However, according to conventional steels in which the amount of Al is greater than the amount specified in the present invention, when the heating temperature before forging is 900°C, 1000°C, and 1100°C, the austenite grains become fine during quenching, and When the temperature is 1200℃, the grains become mixed, while in comparison steel, where the amount of N is higher than the amount specified in the present invention, the temperature is 900℃.
When heated to a temperature higher than that, it becomes fine grains, and when heated to a higher temperature, it becomes mixed grains, so in these cases, the hardenability after forging is significantly non-uniform. Next, 40 mm having the chemical composition shown in Table 1 A to D
A steel bar with a diameter of It was tempered at 420°C for 2 hours. A tensile test piece and a Shapey test piece were taken from the center of each round bar from each of the forged products thus obtained and tested. The results are shown in Table 2. When the strength is equivalent, the steel of the present invention has excellent toughness. Example 2 A 75 mm square bar having the chemical composition shown in Table 1 E and F was heated to 1050°C, hot forged into a 50mm diameter steel bar, and then oil quenched at a temperature of 950°C to determine the hardness distribution in the cross section. It was measured. The results are shown in Figure 2. Steel with improved hardenability by adding Cr and Mo

【table】

According to the method of the present invention using [Table], the hardenability during forging and hardening is further improved. Example 3 A 30 mm square piece with free machinability given the chemical composition shown in Table 1 G and H was heated to 1000°C, heated and forged into a 20 mm diameter steel bar, and then cooled with water from a temperature of 900°C.
The hardness distribution in the cross section was measured. The results are shown in Figure 3.

[Brief explanation of the drawing]

Figure 1 A is a graph showing the hardness distribution of a directly quenched forged product obtained by the present invention, B is a similar graph for a directly forged product obtained by a conventional method, and the horizontal axis is the center of the forged product. Indicates distance from.
Figure 2 is a graph showing the hardness distribution of a forged product made by directly quenching a steel containing Cr and Mo at 1050°C, and Fig. 3 is a graph showing the hardness distribution of a forged product made by directly quenching a steel containing S and Pb at 1000°C after hot forging. It is a graph showing the hardness distribution of a forged product.

Claims (1)

[Claims] Contains 0.26-0.50% C, 0.50% Si or less, and 0.30-1.8% Mn in 1% by weight, P 0.020% or less, S 0.01-0.2%, Al 0.006% or less, and N 0.005% or less. Steel materials regulated to 900 to 1200℃
A method for producing a directly quenched hot forged product, which is characterized by forging after heating to a temperature of 850°C or higher, and then quenching at a temperature of 850°C or higher. 2 Contains at least one selected from C 0.26 to 0.50%, Si 0.50% or less, Mn 0.30 to 1.8%, Cr 1.2% or less, and Mo 0.3% or less, P 0.020% or less, S 0.01 to 0.2%, Steel materials regulated to less than 0.006% Al and less than 0.005% N at 900 to 1200℃
A method for producing a directly quenched hot forged product, which is characterized by forging after heating to a temperature of 850°C or higher, and then quenching at a temperature of 850°C or higher.