JP2001294936A - Method for strengthening steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a strengthening method of various kinds of steel such as bar steel, wire rod used for bolt, nut, etc., steel pipe, steel sheet used for structural member of automobile, and steel wire used for wire rope, steel coad, etc. SOLUTION: The steel containing 0.010-1.5 mass% C+N is subjected to cold-workings in (n) times (n is an integer of >=2) having >=5% reduction of area in each time, and is heated up to 120-500 deg.C during working at any time of the cold-working in 1-(n-1)th times. The steel to be worked may be heated in an above temperature range at least during the cold-working in (n-1)th times. Further, the steel can be heated up in the above temperature range during the cold-working at the (n)th time, and can be further heated in the above temperature range after cold-working at the (n)th time.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a method for strengthening a steel material. More specifically, for example, it relates to a method of strengthening various steel materials such as steel bars and wires used for bolts and nuts, steel pipes and steel plates used for structural members of automobiles, and further, wire ropes and steel wires used for steel cords and the like. .

[0002]

2. Description of the Related Art As a method of strengthening a steel material, that is, a method of increasing the strength of a steel material, a method of including various alloying elements in a steel material of the steel material and a method of heat-treating the steel material are generally used. Further, a technique of performing so-called “cold working” to work-harden a steel material to increase the strength is often used.

[0003] Among the above methods for strengthening steel materials, cold working is
Compared with hot working such as hot forging and hot rolling, the dimensions of steel materials can be adjusted with high accuracy. For this reason, the cold working has an advantage that a costly process such as cutting can be omitted.For example, bolts and nuts used for a vehicle's undercarriage parts, steering parts, and the like have recently been cold drawn and drawn. At the same time as molding by cold forging, it is often manufactured with increased strength.

[0004] Further, ultra-fine steel wires for steel cords used as reinforcing materials for radial tires of automobiles are subjected to strong cold drawing in the final production process, and a large tensile strength of 3000 MPa or more is secured.

[0005] However, simply performing cold working on steel does not always provide the required high strength. Therefore, there is a strong demand in the industry for a strengthening method that can surely impart higher strength to steel.

[0006] In response to such demands, a technique for increasing the strength by heat treatment after cold working is disclosed in, for example, Japanese Patent Application Laid-Open No. 10-1998.
306345 and JP-A-7-90495.

[0007] JP-A-10-306345 discloses that
A "wire and bar for cold forging having excellent strain aging characteristics and a method for producing the same" is disclosed. However, even when cold working and heat treatment (strain aging treatment) are performed by using the technique proposed in this publication, the amount of hardening is only about 35 at most in Vickers hardness as in the embodiment.

[0008] Japanese Patent Application Laid-Open No. 7-90495 discloses a "high-strength steel wire and a method for producing the same". The technique proposed in this publication is to perform wire drawing at a reduction of area of 60 to 98% and further heat to 300 to 500 ° C.
However, even when this method is used, the tensile strength that is increased by secondary curing by heat treatment (hereinafter, “tensile strength” is referred to as “TS”)
) Is as small as less than 20 kgf / mm ² as shown in FIG. 2 of the embodiment of the above publication, and it cannot be said that sufficient reinforcement is achieved.

By using the techniques proposed in the above-mentioned Japanese Patent Application Laid-Open Nos. 10-306345 and 7-90495, a high-strength steel material can be obtained. But,
As already mentioned, the high strength achieved by these techniques is not always sufficient.

[0010]

SUMMARY OF THE INVENTION The present invention has been made in view of the above situation, and has as its object to provide, for example, steel bars and wires used for bolts and nuts, and steel pipes and steel plates used for structural members of automobiles. It is still another object of the present invention to provide a method for reinforcing various steel materials such as a steel wire used for a wire rope, a steel cord, and the like. Specifically, TS as an index of the amount of reinforcement when only cold working is performed on various steel materials
(Hereinafter, TS when only this cold working is performed)
It is an object of the present invention to provide a method of strengthening a steel material capable of increasing the strength by 15% or more of the amount of increase of (と い う TS1).

[0011]

The gist of the present invention resides in a method for strengthening a steel material as shown in the following (1) to (6).

(1) A steel material containing 0.010 to 1.5% by mass of C + N by mass% is subjected to n times of cold working so that the area reduction rate is 5% or more each time. -1) During the cold work of at least one of the first cold work,
A method for strengthening a steel material, wherein the temperature is raised to 20 to 500 ° C. Here, n is an integer of 2 or more.

(2) A steel material containing 0.010 to 1.5% by mass of C + N by mass% is cold-worked n times so that a reduction in area is 5% or more each time. -1) During the cold work of at least one of the first cold work,
A method for strengthening a steel material, wherein the temperature is raised to 20 to 500 ° C, and further to 120 to 500 ° C during the n-th cold working. Here, n is an integer of 2 or more.

(3) A steel material containing 0.010 to 1.5% by mass of C + N by mass% is cold-worked n times so that the area reduction rate is 5% or more each time. -1) During the cold work of at least one of the first cold work,
A method for strengthening a steel material, wherein the temperature is raised to 20 to 500 ° C, and after the nth cold working, the temperature is further raised to 120 to 500 ° C. Here, n is an integer of 2 or more. (4) A steel material containing 0.010 to 1.5% of C + N in mass% is subjected to n cold workings in which the area reduction rate is 5% or more each time. -1) The steel material to be processed is at 120 to 500 ° C. in at least one of the intervals.
A method for strengthening a steel material, wherein the temperature is raised to a predetermined temperature. Here, n is an integer of 2 or more.

(5) A steel material containing 0.01 to 1.5% by mass of C + N in mass% is subjected to n times of cold working so that the area reduction rate is 5% or more each time. In at least one of the (n-1) times,
The temperature was raised to 0 to 500 ° C., and after the nth cold working,
A method for strengthening a steel material, wherein the temperature is raised to 20 to 500 ° C. Here, n is an integer of 2 or more. (6) A steel material containing 0.010 to 1.5% C + N in mass% is subjected to n cold workings in which the area reduction rate is 5% or more each time. -1) The steel material to be processed is at 120 to 500 ° C. in at least one of the intervals.
And during the n-th cold working,
A method for strengthening a steel material, wherein the temperature is raised to ° C. Here, n is an integer of 2 or more. Note that C + N indicates the sum of the contents of C and N in mass%, and the “area reduction rate” (for each time) is defined as A ₀ , the cross-sectional area before processing, and A _1, the cross-sectional area after processing. A ₀ -A ₁ ) / A ₀ is represented by 1
Multiplying by 00 results in% display. The term “cold working” as used in the present invention means that the temperature of a steel material to be processed before working is 100 ° C.
Refers to the following. In order to solve the above-mentioned problems, the present inventors have sought to strengthen steel materials capable of achieving high strength in which the final increase in TS after the strengthening treatment is 1.15 × ΔTS1 or more. In order to provide a method, various investigations and studies were conducted on the strength change of steel materials when cold working and heat treatment were combined. As a result, the following findings were obtained. (A) When low-temperature heat treatment is performed after cold working a steel material, as conventionally known, the strength increases due to strain aging. However, cold working is performed once or in multiple stages, and the cold working is performed. Compared to the case of performing low-temperature heat treatment (one time) after the end of working, cold working is divided into multiple stages, and the steel material to be processed is heated to a specific temperature range during at least one of the cold working steps. If further cold working is performed, the strength finally obtained is
Even if the total amount of cold working (reduction rate) is the same, it becomes higher. This is because, when the temperature of the steel material to be processed is raised to a specific temperature range during cold working, C (carbon) and N (nitrogen) are fixed around dislocations, and the dislocations are stabilized. By adding cold working, dislocations are complicatedly entangled with each other, so that the strength is increased even if the total cold working amount (area reduction rate) is the same. (B) The greater the number of times of cold working for raising the temperature of the steel material to be processed to a specific temperature range during the cold working, the higher the strength even if the total amount of cold working (area reduction) is the same. . (C) The cold working is divided into multiple stages, and at least one of the cold working is performed in comparison with a case where cold working is performed once or in multiple stages and low-temperature heat treatment is performed (one time) after the completion of the cold working. If the temperature of the steel material to be processed is raised to a specific temperature range during the cold working and then further cold working is performed, the finally obtained strength becomes high even with the same working amount in total. This is because, as in the case of (a), if the steel material to be processed is heated to a specific temperature range during the cold working, C and N are fixed around the dislocation, and the dislocation is stabilized. By further performing cold working in this state, dislocations are complicatedly entangled with each other, so that the strength is increased even if the total amount of cold working (area reduction ratio) is the same. (D) The greater the number of intervals between cold workings in which the temperature of the steel material to be worked is raised to a specific temperature range, the higher the strength, even if the total amount of cold working (area reduction) is the same. (E) The above phenomena (a) to (d) also occur when it is assumed that C or N is not dissolved in the steel material to be processed before cold working, and C and N are obtained by summing C and N. This phenomenon is common to steel materials containing a specific amount. (F) When the working amount (reduction rate) in cold working increases, carbides and carbonitrides in steel materials and C and N present in nitrides can move around dislocations in steel materials. become. Therefore, as long as the steel material contains a specific amount of C and N in the sum thereof, the phenomena described in the above (a) to (d) occur.

The present invention has been completed based on the above findings.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Each requirement of the present invention will be described in detail below. In addition, "%" of the content of each element means "% by mass". (A) Chemical composition The steel material to which the present invention is applied is subjected to the desired properties (strength, ductility, corrosion resistance, etc.) after hot working such as hot forging and hot rolling, and then through cold working and heating. In addition, high strength is provided so that the final increase amount of TS is 1.15 × ΔTS1 or more. Here, △ TS1 is T when only cold working is performed.
Pointing to the increase in S is as described above.

In the present invention, the above-mentioned properties are provided,
From the viewpoint of ensuring cold workability and industrial productivity, only the content of C + N as the chemical composition of the steel is limited to the following range.

C + N: 0.010-1.5% C and N are fixed around dislocations in the steel material during the cold working or after the cold working and when the temperature of the steel material is raised to a temperature described later, dislocations are formed. Is stabilized, and further cold working is performed in this state, which has the effect of greatly increasing the strength of the steel material. This effect is obtained irrespective of whether C and N are contained alone or in combination. However, when the sum of the C and N contents is less than 0.010%, the final TS increase is 1.15.
× △ High strength of TS1 or more cannot be obtained. On the other hand, when the content of C and N exceeds 1.5% in total, huge eutectic carbides and carbonitrides are generated when the steel solidifies, and the homogenization heat treatment is performed at a high temperature for a long time. However, since eutectic carbides and carbonitrides are unlikely to be lost, troubles during cold working (for example, disconnection during drawing or cracking during forging) frequently occur.
Therefore, C + N, which is the sum of the contents of C and N, is 0.0
10% to 1.5%. The amount of C + N, which is the sum of the contents of C and N, is preferably 0.10 to 1.2%.

There is no particular limitation on the composition of the chemical components other than C + N of the steel material targeted by the present invention. Properties required in final product and final T
As long as it is possible to provide high strength in which the increase amount of S is 1.15 × △ TS1 or more, and the component range is such that the cold workability and the industrial productivity can be ensured. Good.

Specifically, for example, as elements other than C + N, Si: 2.0% or less; Mn: 3.0% or less;
u: 2.0% or less, Ni: 3.0% or less, Cr: 3.0
% Or less, Mo: 2.0% or less, W: 1.0% or less, V:
0.5% or less, Nb: 0.5% or less, Ti: 0.5% or less, Zr: 0.2% or less, Al: 0.1% or less, B:
0.005% or less, Pb: 0.3% or less, rare earth element:
0.1% or less, Ca: 0.01% or less, Mg: 0.01
% Or less, and the balance is composed of Fe and impurities, as long as P as impurities is 0.05% or less and S is 0.05% or less.

When elements other than C + N are additionally contained for the purpose of improving the properties of steel materials or final products, for each element, Si: 0.1 to 2.0%, M:
n: 0.1 to 3.0%, Cu: 0.05 to 2.0%, N
i: 0.2 to 3.0%, Cr: 0.1 to 3.0%, M
o: 0.05-2.0%, W: 0.05-1.0%,
V: 0.02 to 0.5%, Nb: 0.01 to 0.5%,
Ti: 0.01-0.5%, Zr: 0.005-0.2
%, Al: 0.005 to 0.1%, B: 0.0003 to
0.005%, Pb: 0.02 to 0.3%, rare earth element: 0.02 to 0.1%, Ca: 0.0005 to 0.0
The content is preferably 1% and Mg: 0.0005 to 0.01%. Further, P as an impurity is 0.05%
Hereinafter, S is preferably set to 0.05% or less.

The reason why the range of the content of each element other than C and N is set as described above will be described below.

Si: Si need not be added, but if added, has the effect of increasing the strength. Furthermore, it also has a deoxidizing effect. To ensure these effects, 0.1% of Si
It is desirable to set the content as described above. However, if the content exceeds 2.0%, the cold workability is greatly reduced, and disconnection during cold drawing and cracking during cold forging occur frequently. Therefore, the Si content is preferably set to 2.0% or less, and when added, the content is preferably set to 0.1 to 2.0%.

Mn: Mn may not be added, but if added, it has an effect of increasing strength and an effect of fixing S in steel as MnS to prevent hot brittleness. In order to surely obtain these effects, the content of Mn is preferably set to 0.1% or more. However, Mn is an element that tends to segregate, and if its content exceeds 3.0%, the tendency of segregation particularly to the center of the steel material becomes large, and the cold workability is reduced, so that disconnection during cold drawing is performed. And cracking during cold forging occur frequently.
Therefore, the content of Mn is preferably set to 3.0% or less, and when added, the content is preferably set to 0.1 to 3.0%.

Cu: Cu need not be added, but if added, has the effect of increasing the corrosion resistance. To ensure this effect, it is preferable that the content of Cu be 0.05% or more. However, if the content exceeds 2.0%, segregation occurs at crystal grain boundaries, and cracks and flaws become noticeable during hot working such as bulk rolling of steel ingots and hot rolling of wires. Therefore, the content of Cu is preferably 2.0% or less,
When added, the content is preferably 0.05 to 2.0%.

Ni: Ni may not be added, but if added, it has the effect of forming a solid solution in the ferrite and improving the toughness of the ferrite. To ensure this effect,
Preferably, the content of Ni is 0.2% or more. On the other hand, if the content exceeds 3.0%, the above effect is saturated, and the cost increases. Therefore, the content of Ni is preferably set to 3.0% or less, and when added, the content is preferably set to 0.2 to 3.0%.

Cr: Cr need not be added, but if added, it has the effect of reducing the lamella spacing of pearlite and increasing the strength of the steel material by forming a solid solution in the steel. In order to surely obtain such an effect, it is desirable that the content of Cr is 0.1% or more. However, Cr is an element that is easily segregated, and if it exceeds 3.0%, the tendency of segregation particularly to the center of the steel material is increased, and the cold workability is reduced, so that disconnection during cold drawing and cold forging are performed. Many cracks occur inside. Therefore, the content of Cr is preferably set to 3.0% or less, and when added, the content is preferably set to 0.1 to 3.0%.

Mo: Mo need not be added, but if added, it precipitates as fine carbides by heat treatment and has the effect of increasing strength and fatigue properties. To ensure this effect,
Mo is preferably set to a content of 0.05% or more.
However, even if Mo is contained in excess of 2.0%, the above-mentioned effects are saturated and the cost is increased. Therefore,
The content of Mo is preferably 2.0% or less, and when added, the content is preferably 0.05 to 2.0%.

W: W need not be added, but if added, it has the effect of precipitating as fine carbides by heat treatment and increasing the strength. W also effectively acts to improve corrosion resistance.
In order to surely obtain these effects, the content of W is preferably set to 0.05% or more. However, even if W is contained in excess of 1.0%, the above effects are saturated and the cost is only increased. Therefore, the content of W is preferably set to 1.0% or less, and when added, 0.05 to 1.0%.
Is preferably set as the content.

V: V may not be added. However, if V is added, it has an effect of generating carbides and nitrides, making austenite crystal grains fine, and improving ductility and toughness. To ensure this effect, it is preferable that the content of V be 0.02% or more. However, even if V is contained in an amount exceeding 0.5%, the above effect is saturated and the cost is increased. Therefore, the content of V is preferably 0.5% or less, and when added, the content is preferably 0.02 to 0.5%.

Nb: Nb does not have to be added, but if added, it has the effect of generating carbides and nitrides, making austenite crystal grains finer, and improving ductility and toughness. In order to surely obtain this effect, the content of Nb is preferably set to 0.01% or more. However, if Nb is 0.5
%, The effect is saturated and the cost is increased. Therefore, the content of Nb is 0.5
% Or less, and 0.01 to
The content is preferably 0.5%.

Ti: Ti need not be added, but if added, it has the effect of generating carbides and nitrides, making austenite crystal grains finer, and improving ductility and toughness. To ensure this effect, it is preferable that the content of Ti be 0.01% or more. However, even if the content of Ti exceeds 0.5%, the above effect is saturated and the cost is increased. Therefore, the content of Ti is 0.5
% Or less, and 0.01 to
The content is preferably 0.5%. Zr: Zr may not be added, but when added, it has an effect of generating carbides and nitrides, making austenite crystal grains finer, and improving ductility and toughness. To ensure this effect, the content of Zr is preferably set to 0.005% or more. However, even if Zr is contained in an amount exceeding 0.2%, the above effect is saturated, and the cost is only increased. Therefore, the content of Zr is preferably 0.2% or less, and when added, the content is preferably 0.005 to 0.2%.

Al: Al need not be added, but if added, it has a deoxidizing effect. To ensure this effect,
Preferably, the content of Al is 0.005% or more. However, when the Al content exceeds 0.1%, disconnection frequently occurs during wire drawing. Therefore, the content of Al is preferably 0.1% or less, and when it is added, it is 0.0%.
The content is preferably in the range of 0.5 to 0.1%. B: B need not be added, but if added, it has the effect of promoting the growth of cementite in pearlite and increasing the ductility of the wire. To ensure this effect, the content of B is preferably set to 0.0003% or more. However, if the content exceeds 0.005%, cracks are likely to occur during hot or warm working. Therefore, the content of B is preferably set to 0.005% or less, and when added, the content is preferably set to 0.0003 to 0.005%. Pb: Pb may not be added, but if added, has the effect of improving machinability. To ensure this effect,
It is preferable that the content of Pb is 0.02% or more.
However, if the content exceeds 0.3%, the hot workability is reduced, and cracks are likely to occur during hot working. Therefore, the content of Pb is preferably 0.3% or less,
When added, the content is preferably 0.02 to 0.3%.

Rare earth element: The rare earth element need not be added, but if added, it has the effect of increasing hot workability. To ensure this effect, the rare earth element should be 0.02
% Is preferable. However, even if the rare earth element is contained in an amount exceeding 0.1%, the above effect is saturated and the cost is increased. Therefore, the content of the rare earth element is preferably 0.1% or less, and when added, the content is preferably 0.02 to 0.1%. The “content of the rare earth element” in the present invention indicates “the total content of the rare earth element”.

Ca: Ca may not be added, but if added, has the effect of enhancing hot workability. It also has the effect of enhancing machinability. In order to surely obtain these effects, the content of Ca is preferably 0.0005% or more. However, even if Ca is contained in an amount exceeding 0.01%, the above-mentioned effect is saturated, and the cost is increased. Therefore, the content of Ca is preferably set to 0.01% or less, and when added, the content is preferably set to 0.005 to 0.01%.

Mg: Mg need not be added, but if added, has the effect of improving hot workability. It also has the effect of enhancing machinability. In order to ensure these effects, it is preferable that the content of Mg is 0.0005% or more. However, even if Mg is contained in an amount exceeding 0.01%, the above effect is saturated and the cost is increased. Therefore, the content of Mg is preferably set to 0.01% or less, and when added, the content is preferably set to 0.0005 to 0.01%. When the steel material contains Mo, W, V, Nb, Ti or Zr among the above-described elements,
According to the contents of N and Al, it is preferable that the content satisfy the following relational expression.

The value of N (%)-0.52Al (%) is 0
In the above case: C + 0.86 (N-0.52Al) -0.
13Mo-0.065W-0.24V-0.13Nb-
0.25Ti-0.13Zr ≦ 0.01. Here, each element symbol in the above relational expression represents the content of that element.

The value of N (%)-0.52 Al (%) is 0
If less than: C-0.13Mo-0.065W-0.2
4V-0.13Nb-0.25Ti-0.13Zr ≦
0.01. Here, each element symbol in the above relational expression also indicates the content of that element.

P: P as an impurity lowers the deformability during cold working. In particular, when the content of P is 0.05
%, The deformability during cold working is significantly reduced. Therefore, the content of P as an impurity element is 0.1.
It is good to make it below 05%.

S: S as an impurity also reduces the deformability during cold working. In particular, when the content of S is 0.05
%, The deformability during cold working is significantly reduced. Therefore, the content of S as an impurity element is 0.
It is good to make it below 05%.

For applications of steel bars and wires used for bolts and nuts, for example, C: 0.1 to 0.1.
5%, Si: 0.1 to 1.0%, Mn: 0.1 to 1.5
%, Cu: 0.5% or less, Ni: 2.0% or less, Cr:
1.5% or less, Mo: 0.5% or less, W: 0.5% or less, V: 0.2% or less, Nb: 0.1% or less, Ti:
0.1% or less, Al: 0.005 to 0.05%, N:
0.003 to 0.02%, B: 0.003% or less, P:
Steel having a chemical composition of 0.05% or less and S: 0.05% or less may be used as the material steel.

For applications of steel pipes and steel plates used for structural members of automobiles, for example, C: 0.03 to 0.03
0.3%, Si: 0.1 to 1.5%, Mn: 0.1 to
2.0%, Cu: 0.5% or less, Ni: 2.0% or less,
Cr: 1.5% or less, Mo: 1.0% or less, W: 1.0
%, V: 0.2% or less, Nb: 0.1% or less, T
i: 0.1% or less, Al: 0.003 to 0.03%,
N: 0.003 to 0.02%, B: 0.005% or less,
Steel having a chemical composition of P: 0.05% or less and S: 0.05% or less may be used as the material steel.

Further, for applications of steel wires used for wire ropes, steel cords, etc., for example, C: 0.
6 to 1.2%, Si: 0.1 to 1.5%, Mn: 0.1
1.0%, Cu: 0.5% or less, Ni: 1.0% or less, Cr: 1.0% or less, Mo: 0.5% or less, W:
0.5% or less, V: 0.2% or less, Nb: 0.1% or less, Ti: 0.1% or less, Al: 0.03% or less, N:
0.02% or less, B: 0.002% or less, P: 0.05
% Or less, S: steel having a chemical composition of 0.05% or less may be used as the material steel. (B) Cold work In order to impart high strength to a steel material such that the final increase in TS is 1.15 × △ TS1 or more, the cold work is divided into n times and at least one of the cold work is performed. During the hot working, the temperature of the steel material to be processed is raised to a temperature range described later, or at least one of
It is necessary to raise the temperature of the steel material to be processed to a temperature range to be described later between cold workings, and to perform further cold working after the above-mentioned temperature raising. Therefore, n, which is the number of times of cold working, is an integer of 2 or more.

The reason why the surface reduction rate in each of the n times of cold working is set to 5% or more is that if it is less than 5%, the desired high strength (final strength) can be obtained even by a method of combining cold working and heating. This is because a high strength at which the amount of increase in TS is 1.15 × △ TS1 or more cannot be provided. In addition, (each time)
"Reduction of area" is the cross-sectional area before processing A _0, the cross-sectional area after working as _{A 1 (A 0 -A 1)} / A refers to those represented by _0, which 100 multiplied by Invite% The display is as described above.

There is no particular upper limit on the number of times of cold working and the area reduction rate. The number of working times and the area reduction rate may be appropriately determined in consideration of the size and shape of the steel material before the first cold working, the size and shape of the final product, and the equipment capacity. The method of the cold working is not particularly limited, and any method such as cold drawing by a die, cold rolling by a rolling mill, and cold forging using a mold may be used as necessary. (C) Temperature raising process In the present invention, the temperature of the temperature raising process performed during the cold working or after the cold working needs to be 120 to 500 ° C. When the temperature in the heating process is lower than 120 ° C.,
Desired high strength (final TS increment is 1.15 × ΔT
S1 or higher strength) is extremely difficult to obtain. Even when a desired high strength is obtained, it is necessary to maintain the steel material at that temperature for a long time, which is extremely inferior in industrial productivity.
On the other hand, when the temperature in the temperature raising treatment exceeds 500 ° C., even if the holding time at that temperature is shortened, the strength is greatly reduced, and the desired high strength cannot be obtained. Therefore, the temperature of the heating process performed during or after cold working is set to 1
20 to 500 ° C. It should be noted that the temperature of the heating process is 3
If the temperature exceeds 50 ° C., the strength may vary greatly depending on the holding time at that temperature. Therefore, the temperature of the temperature raising treatment performed during or after cold working is 120 to 3 ° C.
The temperature is preferably set to 50 ° C. As described in the section of (B) Cold working, the final increase in TS is 1.1.
In order to impart a high strength of 5 × △ TS1 or more to the steel material, the cold working is divided into n times, and the steel material to be processed is heated to 120 to 500 ° C. during at least one of the cold working. Alternatively, it is necessary to raise the temperature of the steel material to be processed to 120 to 500 ° C. during at least one of the cold workings, and to perform further cold working after the above-mentioned temperature rise. The temperature raising process performed between the cold workings may be performed any number of times from one to (n-1) times. However, in the case where the temperature raising treatment is performed between the cold workings, the number of times is preferably four or less from the viewpoint of industrial productivity. In addition, when the temperature raising process is performed during the cold working, 120 ° is also used during the final n-th cold working.
The temperature may be raised to 500 ° C.

If the temperature is further raised to 120 to 500 ° C. after all the cold working has been completed n times, the strength can be further increased. For this reason, after the n-th cold working,
The temperature may be raised to 20 to 500 ° C. A great effect can be obtained by performing the heating process to 120 to 500 ° C. after the n-th cold working if the temperature is not raised to 120 to 500 ° C. during the n-th cold working. After the temperature was raised to 120 to 500 ° C. during the n-th cold working,
Although the temperature raising treatment to 500 ° C. may be performed, in this case, the effect of the temperature raising treatment to 120 to 500 ° C. after processing is small. Heating in the temperature raising treatment performed between the cold workings may be performed by a normal method such as heat treatment furnace heating, high frequency heating, electric heating, and infrared heating.

Heating during the cold working during the temperature raising treatment may be achieved by utilizing the heat generated by working. However, in order to heat the steel material stably and uniformly, for example, the roll or It is preferable to perform auxiliary temperature rise through a die.

Hereinafter, the present invention will be described in detail with reference to examples.

[0050]

EXAMPLES Steels A to M having the chemical compositions shown in Table 1 were melted by a usual method using a 150 kg vacuum furnace.

In Table 1, steels B to M are examples of the present invention in which the chemical composition is within the range specified in the present invention, and steel A is a comparative example in which the chemical composition is out of the range of the content specified in the present invention.

[0052]

[Table 1] (Example 1) Among the steels smelted as described above, Table 1
After heating the steel ingots of the steels A to C shown in (1) to 1200 ° C., they were hot forged by a usual method to obtain a steel slab having a thickness of 40 mm. Next, each of the above steel pieces was heated to 1100 ° C., and then hot-rolled to a rolling finish temperature of 800 ° C. and a winding temperature of 650 ° C. to obtain a steel sheet having a thickness of 4.0 mm.

The steel sheet having a thickness of 4.0 mm thus obtained was pickled by a usual method, then cold-rolled by 0.5 mm per pass to reduce the thickness, and cold-rolled by a total of 6 passes. A cold-rolled steel sheet having a thickness of 1.0 mm was obtained. Cold rolling speed is 0.5m
/ Min or less was controlled so that the temperature of the steel sheet did not become 100 ° C. or more during rolling. During cold rolling, the temperature of the steel sheet immediately before entering the rolling mill and immediately after leaving the rolling mill was measured with a radiation thermometer, and the temperature of the steel sheet measured immediately after leaving the rolling mill was measured. It was controlled as the temperature of the steel sheet during rolling.

The area reduction rate in each of the 6-pass cold rolling passes is 13%, 14%, 17%, and 20% in order from the first pass.
%, 25% and 33%.

In performing the above cold rolling, heat treatment was performed under the conditions shown in Table 2 after some passes. A normal electric furnace was used for this heat treatment.

A 13B tensile test piece described in JIS Z 2201 was collected from a hot-rolled steel sheet having a thickness of 4.0 mm and a steel sheet having a thickness of 1.0 mm subjected to the treatment shown in Table 2. Then, a tensile test was performed. Table 2 also shows the results of the tensile test. Note that ΔTS in Table 2 indicates the difference between the TS of the steel sheet after the final treatment and the TS of the steel sheet before cold rolling (that is, while hot rolling). ΔTS of Test Nos. 1, 5, and 10 subjected to only heat treatment, that is, only cold rolling (rolling in which the temperature of a steel material to be rolled before rolling is 100 ° C. or less) defined in the present invention without performing a temperature raising treatment. △ TS already mentioned
Equivalent to 1.

[0057]

[Table 2] From Table 2, cold rolling and heat treatment (heating treatment) under the conditions according to the present invention, using a steel whose C + N amount satisfies the conditions of the present invention.
It is clear that in the case of Test Nos. 7 to 9 and 12 to 14 in which the test was performed, a desired high strength (high strength not less than 1.15 × ΔTS1) was obtained. On the other hand, since the amount of C + N of the steel A of the comparative example is less than 0.010%, in the case of Test Nos. 2 to 4 using this steel, cold rolling and heat treatment (elevation) were performed under the conditions according to the present invention. (Test Nos. 3 and 4) cannot achieve the desired high strength.

Test Nos. 6 and 10 used steels B and C in which the amount of C + N satisfies the conditions of the present invention. However, the desired high strength can be achieved because the processing conditions do not satisfy the requirements of the present invention. Not. (Example 2) Among the steels melted as described above, Table 1
After heating the steel ingots of steels D to I shown in (1) to 1200 ° C., they were hot forged by a usual method to obtain round bars having a diameter of 80 mm. Next, each of the above-mentioned round bars was heated to 1150 ° C., and then hot-rolled so that the rolling finishing temperature was 880 ° C., and the diameter was 10.
A 5 mm wire was used. The cooling after the finish rolling in the case of using the steels D, G, H, and I was allowed to cool (natural cooling), and the cooling after the finish rolling in the case of using the steels E and F was air-cooled. The thus obtained wire having a diameter of 10.5 mm was pickled by a usual method, subjected to a phosphate coating treatment, and finished with a die having a diameter of 7 mm using two dies having finish diameters of 8.8 mm and 7.6 mm. Cold drawing was performed in two passes to 0.6 mm. In addition,
Each of the above two dies is a carbide die having a dice angle of 14 degrees.

The cold drawing speed is set to 0.5 m / min or less.
During the drawing, the temperature of the wire was controlled so as not to be 100 ° C. or higher. During cold drawing, the temperature of the wire immediately before entering the die and immediately after leaving the die was measured with a radiation thermometer, and the temperature of the wire measured immediately after leaving the die was cold drawn. It was controlled as the temperature of the middle wire.

In performing the above cold drawing, heat treatment was performed under the conditions shown in Tables 3 and 4 after some passes. A normal electric furnace was used for this heat treatment.

A wire rod having a diameter of 10.5 mm as hot rolled and a diameter of 7.6 subjected to the treatment described in Tables 3 and 4
A 9A tensile test piece described in JIS Z 2201 was sampled from each mm wire and subjected to a tensile test. In Tables 3 and 4,
The results of the tensile test are also shown. Note that ΔTS in Tables 3 and 4 also indicates the difference between the TS of the wire after the final treatment and the TS of the wire before cold drawing (that is, as it is hot-rolled). ΔTS of Test Nos. 15, 19, 24, 29, 32 and 35 in which only cold drawing was performed without heat treatment (that is, temperature increase treatment)
Corresponds to ΔTS1 already described.

[0062]

[Table 3]

[Table 4] From Tables 3 and 4, from the test numbers 17, 22, 27, which were subjected to cold drawing and heat treatment (heating treatment) under the conditions according to the present invention, using steel whose C + N amount satisfies the conditions of the present invention. 30, 3
In the case of 1, 33, 36, 37, the desired high strength (1.
It is clear that high strength (15 × △ TS1 or more) can be obtained. On the other hand, even when using steel whose amount of C + N satisfies the conditions of the present invention, test numbers 16, 18, 20, 21, 23, 25, where the processing conditions do not satisfy the requirements of the present invention.
In the case of 26, 28 and 34, the desired high strength was not achieved. (Example 3) Of the steels produced as described above, Table 1
After heating the steel ingots of steel J and steel K shown in
A round bar having a diameter of 80 mm was formed by hot forging by a usual method.
Next, each of the above-mentioned round bars was heated to 1150 ° C., and then hot-rolled to a rolling finish temperature of 880 ° C.
A 0.5 mm wire was used. The cooling after finish rolling when steel K was used was allowed to cool (natural cooling), and the cooling after finish rolling when steel J was used was air cooling. The thus obtained wire having a diameter of 10.5 mm is pickled by a usual method, subjected to a phosphate coating treatment, and subjected to a cold pass of 6 passes on a pass schedule in which the reduction in area at each die is 19%. Drawing was performed to obtain a wire having a diameter of 5.5 mm. The six dies used for wire drawing were all carbide dies having a dice angle of 14 degrees.

The cold drawing speed was 0.5 to 400 m / min. During cold drawing, the temperature of the wire immediately before entering the die and immediately after leaving the die was measured with a radiation thermometer, and the temperature of the wire measured immediately after leaving the die was cold drawn. The temperature of the middle wire was used. In any of the test numbers shown in Table 5, the temperature of the wire immediately before entering each die was 2
The temperature was as low as 0 to 50 ° C.

A wire having a diameter of 10.5 mm as hot-rolled, and a diameter 5.5 having been cold-drawn under the conditions shown in Table 5
A 9A tensile test piece described in JIS Z 2201 was sampled from each mm wire and subjected to a tensile test. Table 5 also shows the results of the tensile test. Note that ΔTS in Table 5 also indicates the difference between the TS of the wire after the wire drawing treatment and the TS of the wire before the cold drawing (that is, as it is hot-rolled). The ΔTS of Test Nos. 38, 39, and 42 in which the workpiece does not reach a temperature exceeding 100 ° C. during cold drawing corresponds to ΔTS1 described above. In Table 5, test number 3 for steel J was used.
The cases of Test No. 38 out of 8 and 39 were described as ΔTS1.

[0065]

[Table 5] From Table 5, in the case of Test Nos. 40, 41, 43, and 44 in which the amount of C + N satisfies the conditions of the present invention and the temperature was raised during cold drawing under the conditions of the present invention, the desired values were obtained. It is clear that high strength (high strength of 1.15 × ΔTS1 or more) can be achieved. On the other hand, even if a steel having an amount of C + N that satisfies the conditions of the present invention is used, the desired high strength cannot be achieved in the case of Test No. 45 in which the processing conditions do not satisfy the requirements of the present invention. (Example 4) Among the steels produced as described above, Table 1
After heating the steel ingots of steel L and steel M shown in
A round bar having a diameter of 80 mm was formed by hot forging by a usual method.
Next, each of the above-mentioned round bars was heated to 1150 ° C., and then hot-rolled so that the rolling finishing temperature was 880 ° C., to obtain a wire rod having a diameter of 5.5 mm. The cooling after finish rolling when steel K was used was allowed to cool (natural cooling), and the cooling after finish rolling when steel L was used was air cooling. The thus obtained wire having a diameter of 5.5 mm is pickled by a usual method,
A phosphate film treatment and cold drawing were performed to obtain a steel wire having a diameter of 1.2 mm, and then a patenting treatment was performed. The patenting treatment was performed by heating the steel wire at 980 ° C. for 20 seconds in a heating furnace and then immersing the steel wire in a 570 ° C. lead bath for 30 seconds. After the patenting treatment, the sheet was pickled and then brass-plated by a usual method. Then, 20 passes of wet drawing were performed on a pass schedule in which the area reduction rate in each die was 16%, and the diameter was 0.20 mm. Of steel wire. Each of the 20 dice used in the wet drawing was a diamond dice having a dice angle of 14 degrees.

The wet drawing speed was 1 m / min at the winding section.
The steel wire temperature was prevented from becoming 100 ° C. or higher due to the heat generated during the drawing. During wet drawing, the temperature of the steel wire immediately before entering the die and immediately after exiting the die was measured with a radiation thermometer, and the temperature of the steel wire measured immediately after exiting the die was used for wet drawing. The temperature of the middle steel wire was used.

In performing the above-mentioned wet drawing, a high-frequency heating device was installed at a position behind some of the dies, and a heat treatment was appropriately performed between drawing operations under the conditions shown in Table 6.

Diameter 1.2 as patented
from a steel wire of 0.2 mm in diameter and a steel wire of 0.2 mm in diameter which was wet drawn under the conditions shown in Table 6.
A No. A tensile test piece was collected and subjected to a tensile test. In Table 6,
The results of the tensile test are also shown. Note that ΔT in Table 6
S indicates a difference between the TS of the steel wire after the wet drawing and the TS of the steel wire before the wet drawing (that is, as it is after the patenting). The ΔTS of Test Nos. 46 and 50 in which only the wet drawing was performed without performing the heat treatment (that is, the temperature raising treatment) is the ΔT described above.
It corresponds to S1.

[0069]

[Table 6] From Table 6, it is understood that the wet drawing and the heat treatment (temperature increasing treatment) are performed under the conditions according to the present invention using the steel in which the amount of C + N satisfies the conditions of the present invention.
In the case of test numbers 48, 49, 52, and 53 where
Desired high strength (high strength of 1.15 × △ TS1 or more)
It is clear that is obtained. On the other hand, C + N
Even if a steel having an amount satisfying the conditions of the present invention is used, in the case of Test Nos. 47 and 51 in which the processing conditions do not satisfy the requirements of the present invention, a desired high strength cannot be achieved.

[0070]

According to the method of the present invention, various steel materials can be strengthened relatively easily and greatly, so that the industrial effect is great.

Continued on the front page F-term (reference) 4K032 AA01 AA04 AA05 AA06 AA07 AA11 AA12 AA14 AA16 AA19 AA21 AA22 AA23 AA24 AA27 AA29 AA31 AA32 AA35 AA36 AA37 AA39 BA01 BA02 BA03 CA03 EA02 CB02 CC03 EA02 EA13 EA14 EA15 EA16 EA17 EA18 EA19 EA20 EA23 EA24 EA25 EA27 EA28 EA31 EA32 EA33 EA35 EA36 EB06 EB07 EB08 EB09 FA02 FC03 FE02 FG10 FJ04 JA02

Claims (6)

[Claims] (1) C + N of 0.010 to 1.5% by mass
Is performed n times on the steel material containing at least 5% or more of the area reduction rate during each cold working in at least one of the first to (n-1) th cold working. 120 to steel
A method for strengthening a steel material, comprising raising the temperature to 500 ° C. Here, n is an integer of 2 or more. 2. A C + N content of 0.010 to 1.5% by mass.
Is performed n times on the steel material containing at least 5% or more of the area reduction rate during each cold working in at least one of the first to (n-1) th cold working. 120 to steel
The temperature was raised to 500 ° C and 12
A method for strengthening a steel material, wherein the temperature is raised to 0 to 500 ° C. Here, n is an integer of 2 or more. 3. C + N of 0.010 to 1.5% by mass%
Is performed n times on the steel material containing at least 5% or more of the area reduction rate during each cold working in at least one of the first to (n-1) th cold working. 120 to steel
After the temperature was raised to 500 ° C., and after the nth cold working,
A method for strengthening a steel material, wherein the temperature is raised to 500 ° C. Here, n is an integer of 2 or more. 4. An amount of C + N of 0.010 to 1.5% by mass.
The steel material containing is subjected to n times of cold working such that the reduction in area of each time becomes 5% or more, and the steel material to be processed is provided at least during any of the (n-1) times of the cold working. 120-5
A method for strengthening a steel material, wherein the temperature is raised to 00 ° C.
Here, n is an integer of 2 or more. 5. C + N of 0.010 to 1.5% by mass
The steel material containing is subjected to n times of cold working such that the reduction in area of each time becomes 5% or more, and the steel material to be processed is provided at least during any of the (n-1) times of the cold working. 120-5
After the temperature was raised to 00 ° C., and after the nth cold working,
A method for strengthening a steel material, comprising raising the temperature to 500 ° C. Here, n is an integer of 2 or more. 6. 0.01 to 1.5% of C + N by mass%
The steel material containing is subjected to n times of cold working such that the reduction in area of each time becomes 5% or more, and the steel material to be processed is provided at least during any of the (n-1) times of the cold working. 120-5
The temperature was raised to 00 ° C, and during the n-th cold working,
A method for strengthening a steel material, comprising raising the temperature to 500 ° C. Here, n is an integer of 2 or more.