JPH09291312A - Production of high strength non-heat treated wire rod for bolt - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a non-heat treated wire rod for bolt, combining excellent strength with workability, by subjecting a steel stock, having a composition which contains specific mass percentages of elements and in which mass percentages of respective elements satisfy specific inequalities, to hot rolling and then to specific heat treatment. SOLUTION: A steel stock, which has a composition consisting of, by mass, 0.10-0.30% C, <=0.80% (including 0%) Si, 0.80-2.20% Mn, 0.5-1.5% Cr, 0.005-0.10% Al, and the balance Fe with inevitable impurities and satisfying required conditions of inequalities, 100×C+8×Si+7×Mn+11×Cr>=40 and 100×C+24×Si+15×Mn+10×Cr<=70 [where the symbols of elements represent the contents (mass%) of respective elements], is used. The high speed non-heat treated wire rod can be produced by subjecting this steel stock to hot rolling, to cooling through the temp. region from 800 deg.C to 400 deg.C at (5 to 20) deg.C/sec average cooling rate, and then to annealing treatment at 500-700 deg.C.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a 10T class (981 to 1178) by subjecting a wire rod to a desired strength and dimension by wire drawing and then bolt forming.
The present invention relates to a method for manufacturing a wire rod for a high-strength non-heat treated bolt capable of obtaining a high-strength bolt having a strength of about N / mm ² or more).

[0002]

2. Description of the Related Art Recently, in order to reduce the processing cost and simplify the manufacturing process, there has been a movement to omit the conventional spheroidizing annealing performed before bolt forming and the quenching / tempering treatment performed after bolt forming. For bolts up to the class, non-heat treatment has advanced considerably so that such heat treatment can be omitted.

However, if the strength level exceeds that level, particularly if the strength level exceeds the 10T class, not only the workability of bolts is extremely deteriorated but also the tool life is remarkably deteriorated. Therefore, conventional tempered steel is used. I often do it. However, in order to meet the demand for processing cost reduction and simplification of manufacturing process, it is desired to develop a wire rod for high strength non-heat treated bolts having a tensile strength of 900 N / mm ² level or higher.

[0004]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and its purpose is 90
It is intended to provide a wire rod for a high strength non-heat treated bolt having a tensile strength of about 0 N / mm ² or more.

[0005]

The method for producing a wire rod for a high-strength non-heat treated bolt according to the present invention which has been able to solve the above-mentioned problems is as follows: C: 0.10 to 0.30% Si: 0.80% or less Mn: 0.80 to 2.20% Cr: 0.5 to 1.5% Al: 0.005 to 0.10% or further Mo: 0.20% or less (including 0%) Ti At least one element selected from the group consisting of: 0.10% or less (including 0%) B: 0.0050% or less (including 0%) Nb: 0.1% or less (including 0%) Including, balance: Fe and unavoidable impurities, and after hot rolling a steel material satisfying the requirements of the following formula (1), (2) or (1a), (2a), 800 ℃ to 400 ℃
The point is that the cooling is performed at an average cooling rate of 5 to 20 ° C./sec and then an annealing treatment is performed at 500 to 700 ° C. X ₁ = 100 × C + 8 × Si + 7 × Mn + 11 × Cr ≥ 40 ...... (1) Y ₁ = 100 × C + 24 × Si + 15 × Mn + 10 × Cr ≤ 70 …… (2) X ₂ = 100 × C + 8 × Si + 7 × Mn + 11 × Cr + 100 × Ti + 2000 × B + 70 × Nb ≧ 40 …… (1a) Y ₂ = 100 × C + 24 × Si + 15 × Mn + 10 × Cr-150 × Ti + 1000 × B-33 × Nb ≦ 70 …… (2a) (The element symbol in the formula is the content rate of each element: mass%
Represents)

According to the above method of the present invention, 900 N / mm ²
It is possible to obtain a wire rod for high-strength non-heat treated bolts having the above-mentioned high strength and an excellent strength-ductility balance of 50% or more when drawn, and bolt formation required when using conventional heat-treated steel. It is possible to omit the previous spheroidizing annealing and the quenching / tempering treatment after forming, and it is possible to meet the demands for reduction of processing cost and simplification of manufacturing process.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION First, the reason why the composition of the steel material is determined in the present invention will be explained. C: 0.10 to 0.30% C is an element indispensable for giving a predetermined strength to the steel material by solid solution strengthening, and at least 0.10%.
The above must be contained. However, if the amount of C is too large, the workability deteriorates, so it must be suppressed to 0.30% or less. Considering both strength and workability, the more preferable C content is in the range of 0.15 to 0.25%.

Si: 0.80% or less Si is an element useful as a solid solution strengthening element and a deoxidizing element, but since those effects are also exhibited by C and Mn, depending on the content ratio of these elements. The case where Si is not contained is also acceptable. However, if the amount of Si is too large, the toughness will be adversely affected, so it must be suppressed to 0.80% or less. The more preferable Si content is in the range of 0.20 to 0.60%.

Mn: 0.8 to 2.2% Mn is an element having a deoxidizing / desulfurizing effect, and contributes to strengthening by increasing hardenability and further lowering the transformation temperature to form a structure. It contributes to refinement and improvement of toughness. It is necessary to contain 0.8% or more in order to effectively exhibit such effects, but if it is too much, workability is adversely affected, so 2.2% is made the upper limit.

Cr: 0.5 to 1.5% Cr is an effective element that contributes to strengthening, and the effect is effectively exhibited by containing 0.5% or more, but if it is too much, the toughness is increased. As it becomes worse, it must be kept below 1.5%. The more preferable content ratio of Cr is 0.
It is in the range of 8 to 1.3%.

Al: 0.005 to 0.10% Al is 0.005% as a deoxidizer added during melting.
The element is inevitably mixed in a certain amount or more, but if it is too large, it becomes a source of oxide inclusions, which lowers the cleanliness and adversely affects toughness, so 0.10% is made the upper limit. The more preferable Al content is in the range of 0.01 to 0.06%.

The essential elements of the steel material used in the present invention are as described above, and the balance is iron and unavoidable impurities, but by adding the following elements as necessary, the characteristics as a wire rod for high strength non-heat treated bolts It is also effective to further improve.

Mo: 0.20% or less, Ti: 0.10%
Hereinafter, at least one element selected from the group consisting of B: 0.0050% or less and Nb: 0.1% or less (both include 0%). All of these elements enhance hardenability and have high strength. In addition to such hardenability, Ti fixes the solid solution nitrogen during austenitization and suppresses the formation of BN, thereby ensuring the hardenability improving effect of B addition. Nb also plays a role and forms carbonitrides to refine austenite crystal grains to contribute to improvement of toughness. The effect of the above elements is 0.0 in Mo.
3% or more, Ti 0.02% or more, B 0.0
It is effectively exhibited by containing about 005% or more and about 0.01% or more of Nb, but those effects are
Mo 0.20%, Ti 0.10%, B 0.005
It is economically wasteful because 0% and Nb are almost saturated at 0.1%, and further modification effects cannot be obtained even if the content is further increased. More preferable contents of each of the above selection elements are Mo: 0.05 to 0.15% and Ti: 0.05 to.
0.07%, B: 0.0015 to 0.0030%, N
b: 0.025 to 0.050%.

In the steel material used in the present invention, in addition to the absolute amounts of the above-mentioned contained elements, the content of each of the above-mentioned elements is represented by the above formula (1),
The steel material satisfying the relationship of (2) and containing the above-mentioned selective element must satisfy the relationships of the above formulas (1a) and (2a).

That is, the above formulas (1) and (1a) are defined as the requirements for securing the strength of 900 N / mm ² level or more from the degree of influence of each element on the strength as the wire rod for non-heat treated bolts. The value of X ₁ or X ₂ is 40
If the amount is less than 900 N / mm, the absolute amount of each element is 900 N / mm, which is intended in the present invention, even if all of them satisfy the above-mentioned preferable requirements.
It is impossible to secure strength of ² or more. By the way, Figure 1
Is a graph showing the relationship between the above X ₁ or X ₂ and the strength out of a large amount of experimental data including the examples described later. If X ₁ or X ₂ is less than 40, the strength is insufficient. It can be seen that the tensile strength above 900 N / mm ² level cannot be secured.

The above formulas (2) and (2a) are defined as requirements for ensuring sufficient workability from the degree of influence of each element on the cold workability as a wire rod for non-heat treated bolts. However, if the value of Y ₁ or Y ₂ exceeds 70, the cold workability intended in the present invention cannot be obtained even if the absolute amounts of the respective elements all satisfy the above-mentioned preferable requirements. . Incidentally, FIG. 2 is a graph showing the relationship between the above X ₁ or X ₂ and the cold workability (drawing ratio), arranged out of a large amount of experimental data including the examples described below.
_{When 1} or Y ₂ exceeds 70, the drawing ratio becomes less than 50%, and it can be confirmed that bolting becomes difficult.

In the present invention, in addition to specifying the above-mentioned component compositions, the cooling rate after hot rolling a steel material satisfying these requirements is properly controlled, and thereafter, an annealing treatment is carried out in an appropriate temperature range. Is an essential requirement. That is, after hot rolling, the material is cooled between 800 ° C and 400 ° C at an average cooling rate of 5 to 20 ° C / sec, and then 500
It is extremely important to carry out the annealing treatment in the temperature range of up to 700 ° C. in order to secure excellent strength and workability at the level intended in the present application.

In the present invention, however, in addition to specifying the above-mentioned component composition, the cooling temperature and cooling rate after hot rolling are properly controlled to secure an optimal metallographic structure in terms of strength and workability. Therefore, it is necessary to make the structure a metal structure mainly composed of bainite. For that purpose, the temperature between 800 ° C. and 400 ° C. after hot rolling is 5 to 20 ° C.
It is necessary to cool at an average rate of / sec, and when the average cooling rate in the temperature range is less than 5 ° C / sec, the amount of ferrite in the metal structure increases to soften, and satisfactory strength characteristics are obtained. I will not be able to. On the other hand, if it exceeds 20 ° C./sec, the amount of martensite increases and it becomes excessively hardened, resulting in poor workability, which makes bolt forming difficult.

By adopting the above-mentioned proper cooling conditions, a wire rod for non-heat treated bolts having high strength and good workability can be obtained, but the cooling rate becomes slow at the portion where the wire rod contacts during cooling, In addition, the cooling rate becomes faster in the portion exposed to the atmosphere in the non-contact state, which causes variations in strength within the lot, and eventually, the strength characteristics of the product bolt may become unstable. Therefore, in the present invention, in order to eliminate such strength variations and provide stable strength characteristics and workability, it is necessary to subsequently perform an annealing treatment for homogenization under a temperature condition of 500 to 700 ° C. If the temperature at this time is less than 500 ° C., a satisfactory homogenizing effect cannot be obtained, and conversely, if an annealing condition of more than 700 ° C. is adopted, the proper metallographic structure given by the setting of the above cooling conditions is altered. 90 softened and contemplated by the present invention
It will not be possible to secure strength above the 0 N / mm ² level.

However, as described above, after hot rolling at 800 ° C.
From 400 to 400 ° C. at an average cooling rate of 5 to 20 ° C./sec and then annealed at a temperature of 500 to 700 ° C., a stable 900 N can be obtained without causing the above problems. It is possible to obtain a wire rod for a non-heat treated bolt having a high strength of / mm ² or more and an excellent workability of a drawing rate of 50% or more.

[0021]

EXAMPLES Next, examples of the present invention will be described. However, the present invention is not limited by the following examples, and the present invention can be practiced with appropriate modifications within a range that can conform to the spirit of the preceding and following examples. Of course, it is possible, and all of them are included in the technical scope of the present invention.

Example Steel materials having the chemical compositions shown in Tables 1 and 2 were melted and then forged to manufacture a square billet of 155 mm, and then a wire rod having a diameter of 8.3 mm was manufactured by hot rolling. During this period, 2 to 25 ° C / sec between 800 ° C and 400 ° C
The average cooling rate was variously changed during the cooling. Then
Annealing treatment was performed at a temperature of 500 to 700 ° C., and the obtained wire rods had tensile strength and drawing (both JI
S Z2241) was measured. Tables 3 and 4 show the average cooling rate, the annealing temperature, and the tensile strength and drawing value of the obtained steel wire.

[0023]

[Table 1]

[0024]

[Table 2]

[0025]

[Table 3]

[0026]

[Table 4]

From Tables 1 to 4, the following analyzes can be performed. In the examples satisfying the requirements of the component composition specified in the present invention, and after performing the annealing treatment after cooling at an appropriate speed after hot rolling, all have a drawing of 50% or more and 900 N / mm ² level or more. It can be seen that it has excellent tensile strength and excellent workability.

On the other hand, in Comparative Steels 15 to 32 in which the composition of the steel material deviates from the prescribed requirements of the present invention, the hot rolling is followed by cooling at a preferable rate and annealing treatment under an appropriate temperature condition. However, the tensile strength is 900 N / mm
^It is less than ² or the drawing does not reach 50%, and at least one of strength and workability cannot be satisfied. Further, even if the chemical composition of the steel material satisfies the specified requirements, if the cooling rate after hot rolling deviates from the specified requirements, or if the subsequent annealing temperature is out of the specified range, It can be seen that either the tensile strength or the drawing becomes poor and the object of the present invention cannot be satisfied.

[0029]

EFFECTS OF THE INVENTION The present invention is configured as described above, and specifies the composition of the steel material to be used, and after hot rolling,
Average cooling rate of 5 to 20 ° C / s between 00 ° C and 400 ° C
By setting it to ec and then annealing it at a temperature of 500 to 700 ° C., it has a tensile strength of 900 N / mm ² or more and a drawing of about 50% or more, and has excellent strength and workability. It has become possible to manufacture a wire rod for non-heat treated bolts that also has the above.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the values of X ₁ and X ₂ obtained from the chemical composition of the steel materials used in the examples and the tensile strength.

FIG. 2 is a graph showing the relationship between the values of Y ₁ and Y ₂ obtained from the chemical composition of the steel materials used in the examples and the drawing.

Claims (3)

[Claims] 1. C: 0.10 to 0.30% (mass%)
The same shall apply hereinafter) Si: 0.80% or less (including 0%) Mn: 0.80 to 2.20% Cr: 0.5 to 1.5% Al: 0.005 to 0.10% The balance: Fe and unavoidable impurities, and after hot rolling a steel material that satisfies the requirements of the following formulas (1) and (2), it is cooled between 800 ° C and 400 ° C at an average cooling rate of 5 to 20 ° C / sec. Then 500-700
A method for producing a wire rod for high-strength non-heat treated bolts, characterized by being annealed at ℃. X ₁ = 100 x C + 8 x Si + 7 x Mn + 11 x Cr ≥ 40 ...... (1) Y ₁ = 100 x C + 24 x Si + 15 x Mn + 10 x Cr ≤ 70 ...... (2) (in the formula The element symbol is the content rate of each element: mass%
Represents) 2. C: 0.10 to 0.30% Si: 0.80% or less Mn: 0.80 to 2.20% Cr: 0.5 to 1.5% Al: 0.005 to 0. In addition to containing 10%, Mo: 0.20% or less (including 0%) Ti: 0.10% or less (including 0%) B: 0.0050% or less (including 0%) Nb: 0. A steel material containing at least one element selected from the group consisting of 1% or less (including 0%), the balance: Fe and inevitable impurities, and satisfying the requirements of the following formulas (1a) and (2a) is hot-worked. 8 after rolling
Average cooling rate of 5 to 20 ° C / s between 00 ° C and 400 ° C
A method for producing a wire rod for a high-strength non-heat treated bolt, which comprises cooling at ec and then annealing at 500 to 700 ° C. X ₂ = 100 x C + 8 x Si + 7 x Mn + 11 x Cr + 100 x Ti + 2000 x B + 70 x Nb ≥ 40 (1a) Y ₂ = 100 x C + 24 x Si + 15 x Mn + 10 x Cr -150 × Ti + 1000 × B-33 × Nb ≦ 70 …… (2a) (The element symbol in the formula is the content rate of each element: mass%
Represents) 3. Tensile strength of 900 N / mm ² or more, drawing of 5
It is 0% or more, The manufacturing method of the wire rod for high strength non-heat treated bolts according to claim 1 or 2.