JPS6411083B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for manufacturing a high-strength non-tempered steel bar. (Prior art) Conventionally, high-strength steel bars are generally produced by hot rolling or hot forging raw material steel, such as medium carbon steel or low alloy steel, and then reheating, quenching and tempering, that is, refining treatment. It is provided with strength characteristics depending on the purpose and use. However, the thermal refining treatment requires a large amount of thermal energy, and the manufacturing cost inevitably increases due to an increase in the number of processing steps, an increase in the number of products in progress, and the like. Therefore, in recent years, in the production of high-strength steel bars, small amounts of so-called precipitation-hardening alloying elements such as V, Nb, and Ti are added to medium carbon steel in order to simplify the production process, and in particular to omit the quenching-tempering process. Using the so-called non-tempered steel as a raw material, we can produce high-strength non-tempered steel bars that achieve the desired properties even when left to cool after hot working by using heating during hot working, processing, and subsequent cooling steps. Attention has been paid. For example, in Japanese Patent Publication No. 58-2243, a small amount of V is added to medium carbon steel, which is then heated to a temperature of 1100°C or higher and die-forged, and then heated to 500°C for 10-100°C.
A method for manufacturing a non-thermal forged product having a ferrite-pearlite structure in which fine V carbonitrides are precipitated in ferrite by air cooling at a cooling rate of .degree. C./min is described. However, according to such a method, although the required strength can be obtained, a decrease in toughness and ductility is inevitable as the strength increases. In particular, the impact value is almost half that of ordinary tempered steel. For example, class ² tempered steel with a tensile strength of 80Kg/mm,
It has a charpy value of 14 Kgm/cm ² at room temperature, but it is 7 Kgm/cm ² or less in the case of non-heat treated steel with a tensile strength of 80 Kg/mm ² . (Object of the Invention) As a result of intensive research in order to solve the above-mentioned problems in the production of high-strength non-tempered steel bars, the present inventors have discovered that they have added a relatively large amount of S along with a small amount of V to medium carbon steel. Regarding the hot rolling and subsequent hot forging of the raw material steel, the benefits can be achieved by specifying the start and end temperatures of hot rolling and hot forging to be lower temperatures than in conventional methods. By refining the ferrite grains in the steel bar that is produced, elongating the MnS inclusions in the working direction during these hot working processes, and controlling the cooling rate after hot forging, we are able to achieve high strength while still hot forging. The present invention was developed based on the discovery that it is possible to obtain a high-strength non-tempered steel bar with excellent toughness and ductility. (Structure of the Invention) The method for manufacturing a high-strength non-tempered steel bar according to the present invention includes:
By weight, C 0.30-0.50%, Si 0.15-0.50%, Mn 1.0-1.65%, S 0.04-0.1%, V 0.08-0.2%, Al 0.015-0.05%, the balance being substantially iron and unavoidable impurities. Heating the steel material to a temperature of 850 to 1000℃, 800 to 950℃
After hot rolling at a finishing temperature of
Hot forging is carried out at a finishing temperature of ℃, and then
It is characterized by cooling from the _A3 transformation point to 550°C at a cooling rate of 0.3 to 10°C/sec. First, the reason for limiting the chemical composition of the steel material in the method of the present invention will be explained. C ensures the strength of non-tempered steel bars, and
V is added as an indispensable element to form carbides and exhibit a strengthening effect through precipitation hardening, but if the content is less than 0.35%, the strengthening effect is poor, while if it exceeds 0.50% In this case, V carbide is excessively produced, resulting in a decrease in toughness. Therefore, the C content range is 0.35 to 0.50
%. In addition to deoxidizing, Si is an effective element for strengthening the ferrite structure of steel after cooling by rolling and forging. However, if it is less than 0.15%, the strength will be insufficient, and if it exceeds 0.50%, it will have poor cold workability. Deteriorates toughness. Therefore, the Si content range is 0.15 to 0.50%. Like C, Mn is an essential element for increasing the strength of steel, and in order to increase the tensile strength of steel after rolling and forging to 80 kg/mm ² or more within the C content range limited above, Mn needs to be added in an amount of 1.0% or more. However, when adding too much Mn, a bainite structure is generated after rolling and forging, which deteriorates the toughness of the steel, so the upper limit is set at 1.65%. In the method of the present invention, S is an essential element for increasing the impact value of steel. Generally, S is added to improve the machinability of steel, in this case,
Since S needs to be MnS, which is difficult to deform during rolling, the amount of oxygen in the steel is increased to 1100~
Rolling is carried out at high temperatures such as 950°C. It is also known that for this reason, when increasing the S content in steel, the impact value is generally lowered. However, according to the method of the present invention, S
By containing a relatively large amount of and performing hot rolling and forging at a low temperature, as will be described later, the room temperature impact value of the resulting non-thermal steel bar can be significantly improved. That is, in the present invention, while adding S in the range of 0.04 to 0.1%,
Thoroughly deoxidize the steel with Al, and then heat it to 1000℃.
Rolling and forging at the following low temperatures facilitates the elongation of MnS inclusions in the rolling and forging direction, thus improving the room temperature impact value in the rolling and forging direction. Thus, to improve the impact value of the steel, S requires an addition of at least 0.04%.
On the other hand, if too much is added, the increase in MnS inclusions will adversely affect the impact value, so the amount added should be 0.1% or less. In the method of the present invention, precipitation hardening of V carbides is used to add 80 kg/mm ² of V to the resulting non-tempered steel bar.
It is an essential element to provide above-mentioned strength, and in order to obtain sufficient precipitation hardening, it is necessary to add at least 0.08%, but when adding too much, the toughness will deteriorate as the strength increases. Therefore, the amount added should be 0.2% or less. Al is added for deoxidation and grain refinement, and in order to effectively exhibit these effects,
It is necessary to add at least 0.015%. However, even if it is contained in a large amount exceeding 0.05%, the effect increases only slightly, so the content range is from 0.015 to
It shall be 0.05%. Note that the content of oxygen O ₂ must be kept low in order to easily deform MnS during rolling, and in the present invention, it is preferably 0.004% or less. In the method of the present invention, steel having the above-mentioned composition is used as a material steel, which is hot-rolled at a low temperature in the austenite region, then reheated, and hot-forged at a low temperature again to improve the impact value. improve. That is, according to the present invention, by heating to a low temperature during rolling and forging, the austenite crystal grains at the time of heating are refined, and by refining the austenite crystal grains during rolling and forging, In addition to making the ferrite grains finer, as described above, the MnS inclusions are made easier to elongate in the rolling and forging direction, thereby improving the impact value at room temperature.
MnS inclusions are relatively softer than the steel itself when the steel heating temperature is below 1000℃.
Since it is easily deformed, it improves the impact value in the rolling and forging directions. When the steel heating temperature is higher than 1000℃, V is sufficiently dissolved in solid solution, so the obtained steel bar is
Although there is no particular problem in terms of strength, the toughness decreases due to coarse grains, and the MnS inclusions are harder than the steel itself, making it difficult to deform during hot rolling and forging processes. On the other hand, when the heating temperature is 850° C. or lower, the amount of solid solution of V is small, so the required strength cannot be obtained, and the deformation resistance increases, making rolling and forging difficult. Therefore, in the method of the present invention, the heating temperatures for rolling and forging are each in the range of 850 to 1000°C. In addition, the finishing temperature of rolling and forging is 800
Temperatures range from ~950°C. When the finishing temperature is lower than 800℃, unrecrystallized structure remains, so
The required strength cannot be imparted to the resulting rolled forged steel bar. On the other hand, if the temperature is higher than 950°C, the crystal grains are coarse and the toughness is low. The steel bar hot forged as described above is then cooled from the _A3 transformation point to 550°C at a cooling rate of 0.3 to 10°C/sec. When the cooling rate is slower than 0.3° C./sec, the crystal grains become coarse, so sufficient strength cannot be obtained and toughness cannot be improved. However, if the cooling rate is faster than 10° C./sec, supercooled structures such as bainite are formed, resulting in a decrease in toughness. In the present invention, an as-rolled steel bar having sufficient high speed, high toughness and ductility can be obtained by hot rolling the raw steel under the above conditions. Therefore, this rolled steel bar was further hot forged as described above, and further,
By cooling at a predetermined cooling rate, the impact value of the resulting steel bar can be further improved. (Effects of the Invention) As described above, according to the present invention, by adding a small amount of V to medium carbon steel, the steel can be strengthened by precipitation hardening, and at the same time, compared to Adding a large amount of S, this material steel is hot-rolled and forged at low temperature, and during this processing process,
Facilitates the elongation of MnS inclusions in the rolling and forging direction,
Furthermore, by controlling the cooling rate after forging, the impact value can be further improved, thus achieving a
In addition to high strength of Kg/mm ² or more, the impact value at room temperature is
It is possible to obtain a high-strength non-tempered steel bar with high toughness and high ductility of 10 Kgm/cm ² or more. (Example) Examples of the present invention are listed below. Example Material steel having the chemical composition shown in Table 1 was heated to various temperatures as shown in Table 2, rolling was started and completed at the temperature shown in Table 2, and rolled steel bars with a diameter of 40 mm were obtained. Obtained. Its mechanical properties and ferrite particle size are shown in Table 2. In Table 1, steels A and B have the chemical composition specified in the present invention, steels C and D are comparative steels, and the impact value was measured using a JIS No. 3 test piece. In Table 2, steels A, B, C, and D have different rolling conditions for steel A, and steel A has a high rolling heating temperature, has a small ferrite grain size, and has a low impact value. D cannot be rolled because the rolling heating temperature is too low. B and C were heated and rolled within the temperature range defined by the present invention, and have large ferrite grain sizes and improved impact values. Within the range specified by the present invention, steel B, which has a lower C content and a higher Mn content than steel A, is used as the raw material steel and is rolled according to the conditions of the present invention, and is the same as B and C. The ferrite grain size is large and the impact value is improved. On the other hand, F is the rolling heating temperature and the rolling temperature is E above.
However, because the amount of C in the material steel is higher than the range specified by the present invention, and the amount of Mn is lower than the amount specified by the present invention, the tensile strength is maintained at 80 Kg/mm ² . However, due to the excessive amount of C, the impact value is extremely small. G has a small ferrite grain size because the material steel is a low S steel and the rolling heating temperature is high, and therefore the impact value is also small. H is obtained by rolling material steel D under the conditions specified in the present invention, and although the ferrite grains are fine, the impact value is inferior because steel D is a low S steel. Next, the hot rolled steel bar obtained above was reheated to the forging heating temperature shown in Table 3, finished forging at the finishing temperature shown in Table 3, and then cooled at a rate of 1.0°C/sec. A forged steel bar with a diameter of 30 mm was obtained. Table 3 shows the mechanical properties and ferrite grain size of the forged steel bar thus obtained. Steels I and J are obtained by reheating and forging the rolled steel bars A and B, respectively, from Steel A having the chemical composition specified in the present invention, but the forging heating temperature and forging finishing temperature are as specified in the present invention. Although the tensile strength is high, the ferrite grain size is large and the impact value is therefore small. On the other hand, Steels K and L are products forged according to the method of the present invention, and have fine ferrite grains, and while maintaining a tensile strength of 80 kg/mm ² class, their impact value is significantly lower. It has been improved. Steel M has a small S content in the material steel and a high finishing temperature, so the ferrite grain size is small and the impact value is low. Steel N is forged under the temperature conditions specified in the present invention, and although the ferrite grains are fine, the impact value is not improved because the S content in the material steel is lower than that specified in the present invention. do not have. According to the method of the present invention, the impact value can be improved by hot rolling or forging the steel material at a low temperature while containing a relatively large amount of S, as shown in FIG. 1, for example. , as shown for the case of steel B, the shock transition temperature shifts to the lower temperature side, while the low S
In the case of steel, for example, as shown for steel H,
This is due to the high impact transition temperature even with rolling or forging at low temperatures. Next, Table 4 shows that rolled steel bar B was heated to 950°C,
Forging start and end temperatures are 920℃ and 850℃, respectively.
After being forged as

【table】

【table】

[Table] (Note) The rolling temperature column indicates rolling start temperature/rolling end temperature.

【table】

[Table] Table 4 shows the mechanical properties of the obtained forged steel bar. Steel O
and P, the cooling rates of which are outside the range defined by the present invention, and therefore no improvement in impact value is observed.
On the other hand, Steel Q is a steel according to the present invention, and it is clear that it has a much better impact value than the comparative steels mentioned above.

[Brief explanation of drawings]

The drawings show that in the steel according to the invention and the comparative steel,
It is a graph showing the relationship between temperature and impact value.

Claims (1)

[Claims] 1% by weight: C 0.30-0.50%, Si 0.15-0.50%, Mn 1.0-1.65%, S 0.04-0.1%, V 0.08-0.2%, Al 0.015-0.05%, the balance being substantial A steel material made of iron and unavoidable impurities is heated to a temperature of 850 to 1000℃, and then heated to a temperature of 800 to 950℃.
After hot rolling at a finishing temperature of
Hot forging is carried out at a finishing temperature of ℃, and then
A method for manufacturing a high-strength non-thermal steel bar, characterized by cooling from the _A3 transformation point to 550°C at a cooling rate of 0.3 to 10°C/sec.