JPH06104864B2 - Manufacturing method of steel material for non-heat treated bolts with excellent toughness - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a steel material for a non-heat treated bolt having excellent toughness.

Conventional technology Conventionally, for example, for bolts with a tensile strength of 70 kgf / mm ² or more,
Mainly, temper bolts are used. The temper bolt is
Usually, spheroidizing annealing, cold wire drawing, cold bolt forming, thread rolling is performed using medium carbon steel wire rods or steel bars or low alloy steel wire rods or steel bars with a C content of 0.3% by weight or more. Through
Finally, it is manufactured by quenching and tempering.

However, in recent years, so-called non-heat treated bolts capable of omitting spheroidizing annealing and heat treatment for quenching and tempering have been demanded against the backdrop of increasing demands for energy saving, and non-heat treated bolts that can already manufacture such bolts have been demanded. Steel materials for use have been proposed. As one of such steel materials for non-heat treated bolts, a steel material that obtains a predetermined strength by precipitation strengthening based on low carbon steel has been proposed, but conventionally, non-heat treated bolts manufactured from this type of steel material have been proposed. Has a lower toughness than a heat-treated bolt and has a large variation in strength, and therefore has poor reliability.

More specifically, JIS 1051 stipulates mechanical properties for strength classification, and as one of the mechanical properties, wedge tensile strength or head impact strength for evaluating the toughness of bolts is specified. Has been done. By the way, when there is a change in the manufacturing process of a product, the reliability of the component including the non-heat treated bolt is generally regarded as important. Therefore, when the manufacturing process of such a product is changed, the bolt not only satisfies the above-mentioned standard characteristics, but also undergoes a strict test. Here, in the case of heat treated bolts, not only the above standard values can be satisfied, but also the values that can be satisfied can be achieved even under more severe test conditions above the standard. In the manufactured non-heat treated bolt, for example, when a wedge tensile test, which is one of the substantial tests of the bolt, is performed under strict test conditions exceeding the standard, under-neck fracture occurs when the wedge angle exceeds 10 °, Also, the impact value is low. As described above, the conventional non-heat treated bolts made of the steel material for non-heat treated bolts have a drawback of poor toughness.

Next, regarding the variation in strength, the practically used steel materials for non-heat treated bolts are mainly low carbon steels, and they are trying to obtain a predetermined strength by precipitation strengthening. Due to the variation in the cooling rate, the variation in strength is relatively large, and the variation in the strength of the steel material directly becomes the variation in the strength of the bolt.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention The inventors of the present invention have conducted extensive studies to solve the problems in the conventional steel materials for non-heat treated bolts, and as a result, set the amount of alloying elements in the steel to a predetermined range, After hot working the steel, it is cooled to a predetermined temperature or lower at a predetermined cooling rate to form a steel structure into a composite structure of ferrite and low-temperature transformation forming phase to improve its toughness, and thus spheroidizing annealing. It is possible to omit processing and quenching and tempering processing,
Furthermore, the inventors have found that a steel material capable of producing a non-heat treated bolt having strength and toughness comparable to that of the heat treated bolt can be obtained, and thus achieved the present invention.

Therefore, an object of the present invention is to provide a method for manufacturing a steel material for a non-heat treated bolt, which is excellent in toughness and is capable of producing a non-heat treated bolt with less variation in strength.

Means for Solving Problems A method for manufacturing a steel material for a non-heat treated bolt having excellent toughness according to the present invention is as follows: (a) C 0.02 to 0.10%, Si 1% or less, Mn 1.0 to 2.5%, S 0.015% or less and Al 0.01 to 0.05%, and (b) at least one element selected from the group consisting of Ti 0.01 to 0.20%, Zr 0.01 to 0.20%, and V 0.01 to 0.20%. Then, the steel consisting of the balance iron and unavoidable impurities is heated to 900 to 1100 ° C, finish rolling is performed at a finishing temperature of 850 to 1200 ° C, and then the average cooling rate is 1 to 15 ° C from the temperature range of 800 to 950 ° C.
Cools to a temperature range of 400 ° C or less in seconds, and has a volume content of 6
It is characterized by having a composite structure of 0% or more of ferrite and the balance being a low-temperature transformation forming phase composed of martensite, bainite, or a mixed structure thereof.

First, the chemical composition of the steel used in the present invention will be described.

C is necessary to give the required strength to the steel by solid solution strengthening, and requires at least 0.02% addition in the present invention. However, if it exceeds 0.10%, the C concentration in the low temperature transformation forming phase becomes too high and the toughness deteriorates.

Similar to C, Si has a great effect of increasing the strength of steel by solid solution strengthening, but when it is added in an excessive amount, it causes a remarkable deterioration in toughness and further significantly increases decarburization. Therefore, in the present invention, the upper limit of the amount of Si added is 1%.

Mn is added in order to obtain a desired structure during toughening of steel and cooling after hot rolling, but when the addition amount is less than 1.0%, the above effect cannot be sufficiently obtained. ,
On the other hand, when it exceeds 2.5%, not only the toughness but also the workability at the time of forming the wire rod and the bar deteriorates as Mn segregation increases during manufacturing. Therefore, the amount of Mn added is in the range of 1.0 to 2.5%.

Ti, Zr and V are also added in order to improve the toughness by refining the crystal grains and reducing the solute N. However, for any element, if the addition amount is less than 0.02%, the above effect cannot be sufficiently obtained, while 0.2%
Even if added in excess, the effect will be saturated. Therefore, in the present invention, any of the above elements 0.02 ~ 0.2
% Is added.

S is an element that easily segregates, and is 0.01% to reduce the amount of MnS-based non-metallic inclusions and improve toughness and workability.
It should be 5% or less.

Al is necessary to improve the toughness by refining the crystal grains and reducing the solid solution N. However, if the amount added exceeds 0.05%, the cleanliness decreases and the toughness is impaired.

If O content is 25 ppm or less, it is possible to prevent a large amount of oxide inclusions such as Al ₂ O _{3 from being} produced, and the toughness and workability are further improved.

In the present invention, Nb can be added to steel in addition to the above-mentioned elements. Nb is added here in the range of 0.1% or less for the purpose of not only precipitation hardening but also for improving the toughness by refining the crystal grains and reducing the solid solution N. Even if added in excess of 0.1%, the above effect will be saturated, so for economic reasons, the addition amount should be 0.1% or less.

In the present invention, if necessary, at least one element selected from the group consisting of Cr 1.5% or less, Mo 0.3% or less, and Ni 1% or less can be added to steel.

By adding these elements, it is possible to further strengthen the toughness of the steel, and it is possible to more easily obtain the desired structure described below during cooling after hot working.

According to the present invention, the steel having the above chemical composition is 900-1
After heating to 100 ° C and finishing rolling at a finishing temperature of 850 to 1200 ° C, an average cooling rate of 1 to 15 from the temperature range of 800 to 950 ° C.
By cooling to a temperature range of 400 ° C or less at ℃ / sec to form a composite structure of a ferrite having a volume content of 60% or more and a low temperature transformation forming phase consisting of martensite or bainite or the mixed structure of the remainder. As a result, a steel material for non-heat treated bolts having excellent toughness and less variation in strength can be obtained.

First, in the method of the present invention, the steel is heated to a temperature in the range of 900 to 1100 ° C. and then hot rolled. Heating temperature is 90
When the temperature is lower than 0 ° C, the deformation resistance increases during hot rolling, and the productivity decreases. On the other hand, the steel heating temperature is 1100 ℃
When it exceeds, coarsening of crystal grains and decarburization increase. Furthermore, the solid solution temperature of Nb carbides and nitrides is relatively high, but when the steel heating temperature exceeds 1100 ° C., the solid solution amount rapidly increases, resulting in variation in strength.
A particularly preferred heating temperature is in the range of 900 to 1050 ° C.

In the present invention, the thus heated steel is hot-rolled, but the rolling finish temperature is in the range of 850 to 1200 ° C, preferably 900 to 1150 ° C. Rolling finish temperature is 1200 ℃
When the temperature is higher than, the crystal grains become coarse,
For example, the workability at the time of forming the obtained steel material such as a wire rod and a steel bar into a bolt is not good, and the obtained bolt also has poor characteristics. On the other hand, the rolling finish temperature, in view of the behavior of the crystal grains, the lower the temperature, the finer the crystal grains, the toughness of the steel is improved, on the other hand, since the productivity is significantly reduced, in the present invention, The rolling finishing temperature is 850 ° C or higher.

In the present invention, the cooling start temperature after the hot rolling
The temperature range is 800 to 950 ° C, and cooling is performed from this temperature range to a temperature range of 400 ° C or less at an average cooling rate of 1 to 15 ° C / sec. When the cooling start temperature is higher than 950 ° C,
Since it is difficult to control the amount of ferrite deposited and the crystal grains become large, it is not possible to obtain a predetermined toughness. On the other hand, even when the cooling start temperature is lower than 800 ° C., it is difficult to control the amount of ferrite precipitation, and it is difficult to keep the quality of the obtained product constant.

Then, from the cooling start temperature, the average cooling rate is 1 to 15 ° C /
By cooling to a temperature range of 400 ° C or less in seconds,
The obtained steel structure has a composite structure of ferrite having a volume content of 60% or more and the balance of a low temperature transformation forming phase composed of martensite, bainite or a mixed structure thereof. When the cooling rate is slower than 1 ° C / sec, the pearlite phase may be precipitated subsequent to the precipitation of the ferrite phase, which is not preferable. On the other hand, when the cooling rate exceeds 15 ° C / sec, the required ferrite volume content is It becomes difficult to obtain a composite structure having this, and as a result, the tendency that the variations in strength and toughness become large increases. Therefore, in the method of the present invention, the average cooling rate is in the range of 1 to 15 ° C / sec.

In the method of the present invention, cooling to a temperature of 400 ° C. or lower is because when the cooling is stopped at a temperature of higher than 400 ° C., the strength variation after hot rolling is small, but the variation in the self-tempering effect of the low-temperature transformation forming phase is caused. This is because work hardening during cold working such as cold drawing and thread rolling is different, which causes variations in the strength of the final product, for example, a body bolt.

The steel used in the present invention is melted, for example, in an LD converter, but the melting method is not limited at all.

The steel material according to the method of the present invention obtained as described above,
Besides bolts, it can be suitably used for manufacturing pins, rivets, and the like.

EFFECTS OF THE INVENTION According to the present invention, a low alloy steel having an alloying element content in a predetermined range is hot worked, and then cooled to a predetermined temperature or lower at a predetermined cooling rate to transform a steel structure into a ferrite and a low temperature transformation. By forming a composite structure with the produced phase and improving its toughness, spheroidizing annealing treatment and quenching and tempering treatment are omitted,
It is possible to obtain a steel material which has strength and toughness comparable to that of a heat-treated bolt and which can be used to manufacture a non-heat-treated bolt with little variation in strength.

EXAMPLES The present invention will be described below with reference to examples, but the present invention is not limited to these examples.

Steel having the chemical composition shown in Table 1 was melted in an LD smelting furnace and rolled into a wire having a diameter of 10.3 mm under the rolling conditions shown in Table 2.

These wires were subjected to a tensile test and a microscopic test to observe their mechanical properties and metallic structure. The results are shown in Table 3. As shown in Table 3, it is clear that in the steel material produced by the method of the present invention, the variation in strength is extremely small.

Next, among the test materials in Table 1, the numbers 2, 3, 4, 6,
Rolled materials of 7, 8, 11, 12, 14, 15, 16, and 17 are drawn to a diameter of 9.05 mm, formed into hexagonal upset bolts with a screw size of M10, and then subjected to bluing treatment and non-tempered. I made a bolt. As for No. 12, a temper bolt was manufactured by the conventional manufacturing process. No cracks occurred during the production of the bolt.

A bolt substance test was performed on the bolt manufactured as described above. According to the JIS standard, the R under the neck of a hexagon bolt having a screw size of M10 (Fig. 1) is a minimum of 0.4 mm, but in this test, the R was 0.15 mm.

As the test, a wedge tensile test, a head impact test and a product tensile test were performed. In this test, the wedge angle α (Fig. 2) in the wedge tensile test was set to 15 ° (maximum 10 ° in JIS standard) and head in order to make the test conditions extremely strict compared to JIS standard. The angle β (Fig. 3) in the impact test was set to 50 ° (JIS standard: 60 to 80 °).

Further, a micro impact test piece was sampled from the manufactured bolt and subjected to an impact test at room temperature.

Table 4 shows the above test results. It is understood that the non-heat treated bolt manufactured from the steel material according to the present example has characteristics comparable to the heat treated bolt in terms of toughness and strength variation. In addition to the results shown in Table 4, a bolt fatigue test, a relaxation test, and a proof load test were also performed. The non-heat treated bolt according to the present invention may have characteristics comparable to those of the heat treated bolt. confirmed.

[Brief description of drawings]

FIG. 1 is a side view showing a bolt neck R of a bolt,
FIG. 2 is a sectional view showing the wedge angle in the wedge tensile test, and FIG. 3 is a sectional view showing the angle in the head impact test.

Claims (4)

[Claims] 1. By weight%, (a) C 0.02 to 0.10%, Si 1% or less, Mn 1.0 to 2.5%, S 0.015% or less, and Al 0.01 to 0.05% are contained, and (b) Ti 0.01 ~ 0.20%, Zr 0.01 ~ 0.20%, and V 0.01 ~ 0.20% containing at least one element selected from the group consisting of the balance iron and inevitable impurities steel heated to 900 ~ 1100 ℃, finish After finish rolling at a temperature of 850-1200 ℃, the average cooling rate is 1-15 ℃ / in the temperature range of 800-950 ℃.
Cools to a temperature range of 400 ° C or less in seconds, and the volume content is 6
A method for producing a steel material for a non-heat treated bolt having excellent toughness, which comprises a composite structure of 0% or more of ferrite and the balance of a low temperature transformation forming phase composed of martensite or bainite or a mixed structure thereof. 2. By weight%, (a) C 0.02 to 0.10%, Si 1% or less, Mn 1.0 to 2.5%, S 0.015% or less, and Al 0.01 to 0.05% are contained, and (b) Ti 0.01 To 0.20%, Zr 0.01 to 0.20%, and V 0.01 to 0.20%, and at least one element selected from the group consisting of (c) Nb 0.1% or less and the balance iron and unavoidable impurities. To 900-1100 ° C, finish rolling at finishing temperature 850-1200 ° C, and then average cooling rate 1-15 ° C from the temperature range 800-950 ° C.
Cools to a temperature range of 400 ° C or less in seconds, and has a volume content of 6
A method for producing a steel material for a non-heat treated bolt having excellent toughness, which comprises a composite structure of 0% or more of ferrite and the balance of a low temperature transformation forming phase composed of martensite or bainite or a mixed structure thereof. 3. By weight%, (a) C 0.02 to 0.10%, Si 1% or less, Mn 1.0 to 2.5%, S 0.015% or less, and Al 0.01 to 0.05% are contained, and (b) Ti 0.01 To 0.20%, Zr 0.01 to 0.20%, and V 0.01 to 0.20%, and at least one element selected from the group consisting of (c) Cr 1.5% or less, Mo 0.3% or less, and Ni 1% or less. After heating the steel containing at least one element selected from the group consisting of balance iron and unavoidable impurities to 900 to 1100 ° C and finishing rolling at a finishing temperature of 850 to 1200 ° C, a temperature of 800 to 950 ° C. Average cooling rate from range 1 to 15 ℃ /
Cools to a temperature range of 400 ° C or less in seconds, and has a volume content of 6
A method for producing a steel material for a non-heat treated bolt having excellent toughness, which comprises a composite structure of 0% or more of ferrite and the balance of a low temperature transformation forming phase composed of martensite or bainite or a mixed structure thereof. 4. By weight%, (a) C 0.02 to 0.10%, Si 1% or less, Mn 1.0 to 2.5%, S 0.015% or less, and Al 0.01 to 0.05% are contained, and (b) Ti 0.01 To 0.20%, Zr 0.01 to 0.20%, and V 0.01 to 0.20%, and at least one element selected from the group consisting of (c) Nb 0.1% or less, (d) Cr 1.5% or less, and Mo 0.3% or less. , And at least one element selected from the group consisting of 1% or less of Ni, the balance iron and inevitable impurities are heated to 900 to 1100 ° C and the finishing temperature is 850 to 1200 ° C. After that, from the temperature range of 800-950 ℃, average cooling rate 1-15 ℃ /
Cools to a temperature range of 400 ° C or less in seconds, and has a volume content of 6
A method for producing a steel material for a non-heat treated bolt having excellent toughness, which comprises a composite structure of 0% or more of ferrite and the balance of a low temperature transformation forming phase composed of martensite or bainite or a mixed structure thereof.