JP2018012874A - Method of manufacturing steel wire for bolt - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a bolt steel wire excellent in delayed fracture resistance and fatigue characteristic and a method of manufacturing a bolt using the same.SOLUTION: There is provided a method of manufacturing a steel wire for bolt including a rolled material preparation process for preparing a rolled material containing C:0.20 to 0.60 mass%, Si:1.0 to 3.0 mass%, Mn:0.10 to 1.5 mass%, P:0.015 mass% or less, S:0.015 mass% or less, Cr:0.3 to 1.5 mass%, Al:0.001 to 0.10 mass%, N:0.001 to 0.02 mass% and the balance iron with inevitable impurities, a spheroidizing process for conducting spheroidizing on the rolled material in atmosphere containing carbon monoxide and carbon dioxide at temperature of 700 to 830°C for holding time of 5 to 15 hours, a cold working process for conducting cold working on the rolled material and a low temperature annealing process for conducting annealing on the rolled material under atmosphere with inert gas of 90 vol.% or more at a temperature of 600 to 700°C for holding time of 3 to 10 hours.SELECTED DRAWING: None

Description

  The present invention relates to a method of manufacturing a steel wire for bolts, a method of manufacturing a bolt using the steel wire for bolts, particularly a method of manufacturing a steel wire for high-strength bolts exhibiting high delayed fracture resistance and fatigue characteristics, and the bolt The present invention relates to high-strength bolts using steel wires.

  Delayed fracture resistance is required for bolt steel wires used in automobiles and various industrial machines and bolts obtained using the bolt steel wires. In particular, in order to obtain high fatigue resistance, for example, a high strength bolt steel wire that requires a high tensile strength such as 1200 MPa or more, and a high strength bolt obtained using the high strength bolt steel wire, delayed fracture resistance Sex is required.

Regarding the cause of delayed fracture that occurs after a certain amount of time has passed since the steel material is stressed, it is considered that various factors are intertwined in a complex manner, and it is difficult to identify the cause. However, there is generally a common recognition that the hydrogen embrittlement phenomenon is involved.
On the other hand, although factors such as tempering temperature, metal structure, material hardness (hardness), crystal grain size and various alloy elements (composition) are recognized as factors affecting delayed fracture phenomenon, means for preventing delayed fracture However, the fact is that various methods have only been proposed by trial and error.

Delayed fracture generally tends to occur as the amount of C increases. Therefore, delayed fracture resistance can be improved by intentionally decarburizing the surface layer of the steel material. However, there is a problem that surface hardness decreases due to decarburization and fatigue resistance decreases.
Therefore, it is difficult to achieve both delayed fracture resistance and fatigue resistance at a high level.
In fact, attempts have been made to improve various characteristics of bolt steel, but even if one characteristic can be improved, one or both of delayed fracture resistance and fatigue resistance are often not satisfied.

  For example, Patent Document 1 proposes a steel material whose scale peelability is improved by optimizing the Si content. However, since the Si content is reduced, sufficient delayed fracture resistance may not be obtained. In addition, due to the atmosphere during low-temperature annealing, sufficient fatigue resistance may not be ensured.

  Patent Document 2 proposes a steel material in which both the softening characteristics and the coarse grain resistance are improved without containing Nb and Ti by controlling the metal structure. However, since Cr is not added, the corrosion resistance is inferior, and it may be difficult to ensure sufficient delayed fracture resistance. Moreover, since low temperature annealing is not implemented, cold forging may worsen and bolt molding may be difficult.

JP 2002-241889 A JP 2007-023310 A

  This invention is made | formed in view of the above situations, The objective is to provide the manufacturing method of a bolt steel wire excellent in delayed fracture resistance and fatigue characteristics, and this.

The method for manufacturing a steel wire for bolts according to the present invention includes:
C: 0.20-0.60 mass%
Si: 1.0-3.0 mass%
Mn: 0.10 to 1.5% by mass
P: 0.015 mass% or less S: 0.015 mass% or less Cr: 0.3-1.5 mass%
Al: 0.001 to 0.10% by mass
N: 0.001 to 0.02 mass%
A rolled material preparation step of preparing a rolled material, the balance of which is iron and inevitable impurities,
In the atmosphere containing carbon monoxide and carbon dioxide, the spheroidizing annealing step for performing spheroidizing annealing at a temperature of 700 to 830 ° C. and a holding time of 5 to 15 hours,
A cold working step for cold working the rolled material;
A low temperature annealing step in which the rolled material is annealed at a temperature of 600 to 700 ° C. and a holding time of 3 to 10 hours in an atmosphere of 90% by volume or more of an inert gas;
including.

  In the method for manufacturing a steel wire for bolts according to the present invention, the carbon potential of the atmosphere in the spheroidizing annealing step is 0.6 to 1.1 times the C content of the rolled material, if necessary. Good.

Moreover, the manufacturing method of the steel wire for bolts which concerns on this invention may further contain one or more selected from the group which the said rolling material consists of the following (a)-(d) as needed.
(A) At least one selected from the group consisting of Cu: more than 0% and 0.5% or less and Ni: more than 0% and 1.0% or less (b) Mo: more than 0% and 2% or less (c) V: More than 0% and 1% or less (d) at least one selected from the group consisting of Ti: more than 0% and 0.1% or less and Nb: more than 0% and 0.1% or less

  The bolt manufacturing method according to the present invention includes a cold forging step of cold forging into a bolt shape using the bolt steel wire, and the bolt steel wire before or after the cold forging step. A quenching and tempering step of performing quenching and tempering by heating to 950 ° C.

  The manufacturing method according to the present invention can provide a bolt steel wire excellent in delayed fracture resistance and fatigue characteristics. Furthermore, by using the steel wire for bolts, it is possible to provide a bolt excellent in delayed fracture resistance and fatigue characteristics.

As a result of intensive studies, the present inventors set the composition of steel materials such as rolled materials used in the manufacture of steel wires for bolts in an appropriate range, as will be described in detail later, and spheroidizing annealing under conditions appropriate for these wires. It was found that a bolt steel wire excellent in delayed fracture resistance and fatigue resistance can be obtained by annealing under appropriate conditions after performing cold working. Further, it has been found that a bolt excellent in delayed fracture resistance and fatigue resistance can be obtained by using this bolt steel wire and adjusting the heating temperature during quenching to an appropriate range.
The manufacturing method of the steel wire for bolts which concerns on this invention first is explained in full detail.

1. Composition The composition of the steel wire for bolts according to the present invention will be described below. The composition of the steel wire for bolts is substantially the same as the composition of steel materials such as billets obtained by melting and casting and rolled materials obtained by processing these.
The steel wire for bolts of the present invention has C: 0.20 to 0.60 mass%, Si: 1.0 to 3.0 mass%, Mn: 0.10 to 1.5 mass%, P: 0.015. Less than mass%, S: 0.015 mass% or less, Cr: 0.3-1.5 mass%, Al: 0.001-0.10 mass%, N: 0.001-0.02 mass% The balance is iron and inevitable impurities.

In addition to this, one or more selected from the group consisting of the following (a) to (d) may be further contained. That is, the elements described in the following (a) to (d) are elements that are selectively added.
(A) At least one selected from the group consisting of Cu: more than 0% by mass and 0.5% or less and Ni: more than 0% by mass and 1.0% by mass or less (b) Mo: more than 0% by mass and less than 2% by mass (C) V: more than 0% and 1% by weight or less (d) Ti: at least one selected from the group consisting of more than 0% by weight and 0.1% by weight or less and Nb: more than 0% by weight and 0.1% by weight or less In addition, the composition of the rolled steel used for the steel wire for bolts and the steel wire for bolts can be calculated | required using the method generally used for composition analysis like the ICP emission spectral analysis method, for example. Moreover, when the raw material composition used for mixing is clear, a value calculated from the raw material composition and the amount used may be used.

Details of these components will be described below.
(1) C: 0.20 to 0.60 mass%
C is added in order to ensure hardenability and strength in a quenching and tempering process that is performed when a bolt is manufactured using the obtained steel wire for bolts. In order to obtain the target strength, the lower limit was set to 0.20% by mass. A preferable lower limit is 0.25% by mass, and a more preferable lower limit is 0.30% by mass. On the other hand, when C is added excessively, toughness deteriorates and delayed fracture resistance deteriorates, so the upper limit was made 0.60% by mass. A preferable upper limit is 0.50 mass%, and a more preferable upper limit is 0.45 mass%.

(2) Si: 1.0 to 3.0% by mass
Si acts as a deoxidizer and contributes to ensuring the strength of the steel. Moreover, Si suppresses precipitation of coarse cementite and improves delayed fracture resistance. In order to obtain these effects, the lower limit was made 1.0 mass%. A preferable lower limit is 1.3% by mass, and a more preferable lower limit is 1.5% by mass. On the other hand, since Si is a ferrite stabilizing element, if excessively added, ferrite precipitates during quenching and tempering during bolt manufacturing and the fatigue strength decreases, so the upper limit was made 3.0 mass%. A preferable upper limit is 2.5 mass%, and a more preferable upper limit is 2.0 mass%.

(3) Mn: 0.10 to 1.5% by mass
Mn functions as a deoxidizer and desulfurizer and is added to improve hardenability. In order to obtain this effect, the lower limit was made 0.10% by mass. A preferable lower limit is 0.13% by mass, and a more preferable lower limit is 0.15% by mass. On the other hand, when Mn is added excessively, coarse MnS is generated and the fatigue strength is lowered, so the upper limit was made 1.5 mass%. A preferable upper limit is 1.0 mass%, and a more preferable upper limit is 0.8 mass%.

(4) P: 0.015 mass% or less (excluding 0 mass%)
P is concentrated at the grain boundaries to lower the toughness of the steel and deteriorate the delayed fracture resistance. Delayed fracture resistance is greatly improved when the P content is 0.015 mass% or less. Preferably it is 0.013 mass% or less, More preferably, it is 0.010 mass% or less. The smaller the content of P, the better. However, it is difficult to make it zero, and about 0.003% by mass is contained.

(5) S: 0.015 mass% (excluding 0 mass%)
S, like P, concentrates on the grain boundaries to reduce the toughness of the steel and deteriorate the delayed fracture resistance. Delayed fracture resistance is greatly improved when the S content is 0.015 mass% or less. Preferably it is 0.013 mass% or less, More preferably, it is 0.010 mass% or less. The smaller the content of S, the better. However, it is difficult to make it zero, and about 0.003% by mass is contained.

(6) Cr: 0.3 to 1.5% by mass
Cr is added to improve the corrosion resistance of the steel and ensure delayed fracture resistance. In order to obtain an effect, the lower limit was set to 0.3% by mass. A preferable lower limit is 0.5% by mass, and a more preferable lower limit is 0.7% by mass. On the other hand, if Cr is added excessively, the effect is saturated and the cost is increased, so the upper limit was made 1.5 mass%. A preferable upper limit is 1.3% by mass, and a more preferable upper limit is 1.1% by mass.

(7) Al: 0.001 to 0.10% by mass
Al acts as a deoxidizing agent and forms nitrides to prevent crystal grains from becoming coarse due to the pinning effect, thereby improving toughness and improving delayed fracture resistance and fatigue resistance. In order to obtain these effects, the lower limit was made 0.001% by mass. A preferable lower limit is 0.01% by mass, and a more preferable lower limit is 0.03% by mass. On the other hand, when Al is added excessively, coarse nitrides are formed, and the cold headability and fatigue characteristics are deteriorated, so the upper limit was made 0.1 mass%. A preferable upper limit is 0.08 mass%, and a more preferable upper limit is 0.06 mass%.

(8) N: 0.001 to 0.02 mass%
N forms nitrides with Al, Ti and Nb and prevents coarsening of the crystal grains by the pinning effect, so that toughness is improved and delayed fracture resistance and fatigue resistance are improved. In order to obtain these effects, the lower limit was made 0.001% by mass. A preferable lower limit is 0.003% by mass, and a more preferable lower limit is 0.004% by mass. On the other hand, when N is contained excessively, free N that does not form nitrides increases, and the cold heading property deteriorates. A preferable upper limit is 0.015 mass%, and a more preferable upper limit is 0.01 mass%.
Nitrogen is inevitably contained in steel in the production process at 0.003% by mass. Therefore, if the content is about 0.003 mass%, N inevitably contained may be utilized. On the other hand, when it is going to obtain N content about 0.004 mass% or more, it can implement | achieve by adding intentionally in a steelmaking process.

(9) Remainder The steel wire for bolts of this invention contains the said component, and a remainder is iron and an unavoidable impurity. Inevitable impurities are elements that are brought in depending on the status of raw materials, materials, manufacturing equipment, and the like.
In addition, for example, like P and S, it is usually preferable that the content is small. Therefore, although it is an unavoidable impurity, there is an element that separately defines the composition range as described above. For this reason, in this specification, the term “inevitable impurities” constituting the balance is a concept that excludes elements whose composition ranges are separately defined.
The steel wire for bolts of this invention may further contain one or more selected from the following (a)-(d) as needed. By adding these components, delayed fracture resistance and fatigue resistance can be further improved.

(A) At least one selected from Cu: more than 0% by mass and 0.5% by mass or less and Ni: more than 0% by mass and 1.0% by mass or less Cu and Ni improve the corrosion resistance of the steel and hydrogen on the surface Delayed fracture resistance can be improved by reducing the amount generated. In order to obtain these effects, it is preferable to add more than 0% by mass of Cu. More preferably, it is 0.1 mass% or more, More preferably, it is 0.2 mass% or more. Ni is preferably added in an amount of 0% by mass or more. More preferably, it is 0.2 mass% or more, More preferably, it is 0.4 mass% or more. On the other hand, when it adds excessively, an effect will be saturated and it will lead to a cost increase. Therefore, Cu is preferably 0.5% by mass or less. More preferably, it is 0.4 mass% or less, More preferably, it is 0.3 mass% or less. Ni is preferably 1.0% by mass or less. More preferably, it is 0.8 mass% or less, More preferably, it is 0.6 mass% or less.

(B) Mo: more than 0% by mass to 2% by mass or less Mo forms composite precipitates with Fe, Cr, V, Ti, etc. during tempering, and increases the tempering softening resistance to ensure strength and at the hydrogen trap site. As a result, the delayed fracture resistance is improved. In order to obtain these effects, it is preferable to add more than 0% by mass of Mo. More preferably, it is 0.2 mass% or more, More preferably, it is 0.5 mass% or more. On the other hand, when added in excess, the hot ductility decreases and the productivity of steel decreases. Therefore, the amount of Mo added is preferably 2% by mass or less. More preferably, it is 1.5 mass% or less, More preferably, it is 1.1 mass% or less.

(C) V: more than 0% by mass and 1% by mass or less V forms fine precipitates during tempering and becomes a hydrogen trap site, thereby improving delayed fracture resistance. In order to obtain this effect, it is preferable to add more than 0% by mass of V. More preferably, it is 0.05 mass% or more, More preferably, it is 0.1 mass% or more. On the other hand, when it adds excessively, an effect will be saturated and it will lead to cost increase. Therefore, V is preferably 1% by mass or less. More preferably, it is 0.5 mass% or less, More preferably, it is 0.3 mass% or less.

(D) At least one selected from Ti: more than 0% by weight and less than 0.1% by weight and Nb: more than 0% by weight and not more than 0.1% by weight Ti and Nb are fine precipitates in the rolling process and annealing process. By forming and suppressing coarsening of austenite grains during quenching heating, toughness improves and contributes to improved delayed fracture resistance. In order to obtain these effects, it is preferable to add more than 0% by mass of Ti. More preferably, it is 0.03 mass% or more, More preferably, it is 0.05 mass% or more. Nb is preferably added in an amount exceeding 0% by mass. More preferably, it is 0.03 mass% or more, More preferably, it is 0.05 mass% or more. On the other hand, when Ti and Nb are added excessively, coarse carbonitrides are formed, and the cold heading property is lowered. Therefore, Ti is preferably 0.1% by mass or less. More preferably, it is 0.08 mass% or less, More preferably, it is 0.06 mass% or less. Nb is preferably 0.1% by mass or less. More preferably, it is 0.08 mass% or less, More preferably, it is 0.06 mass% or less.

2. Production of rolled material A rolled material having the same composition as that of the bolt wire described above is produced. The rolled material can be obtained, for example, by obtaining a billet by melting / casting and split rolling and hot rolling the billet. The conditions for these steps may be the same as the normal production conditions for the rolled material used for the steel wire for bolts.
However, in order to improve delayed fracture resistance and fatigue characteristics, the steel is heated to 950 ° C. or higher during billet reheating during hot rolling (hereinafter sometimes referred to as “billet reheating temperature”), and 900 to 1100 ° C. After finishing and rolling into a wire or steel bar shape in the temperature range, it is desirable to subsequently cool at an average cooling rate of 0.5 to 13 ° C./second.
Details of these preferable conditions are shown below.

(1) Billet reheating temperature: 950 ° C. or higher In billet reheating during hot rolling, the billet reheating temperature is preferably 950 ° C. or higher, more preferably 1000 ° C. or higher in order to reduce deformation resistance during hot rolling. And When this temperature is less than 950 ° C., the deformation resistance during hot rolling increases. On the other hand, when the billet reheating temperature becomes too high, it becomes close to the melting temperature of steel. Accordingly, the billet reheating temperature is preferably 1400 ° C. or lower, more preferably 1300 ° C. or lower, and further preferably 1250 ° C. or lower.

(2) Finish rolling temperature: 900-1100 ° C
If the finish rolling temperature is too low, it becomes a ferrite-austenite two-phase region, and decarburization is promoted. Further, the ferrite crystal grains become too fine and the strength is increased, and the cold heading property is deteriorated. Therefore, the finish rolling temperature is preferably 900 ° C. or higher, more preferably 950 ° C. or higher. On the other hand, if the finish rolling temperature becomes too high, the ferrite crystal grains become coarse and the cold heading property deteriorates. Therefore, the finish rolling temperature is preferably 1100 ° C. or lower, more preferably 1050 ° C. or lower.

  In addition, when it contains additional elements, such as Ti and Nb, the temperature range similar to the said finish rolling temperature may be sufficient. The finish rolling temperature is preferably 900 ° C. or higher, more preferably 950 ° C. or higher, so that the additive element can be precipitated in the steel as fine carbon / nitride. On the other hand, the finish rolling temperature is preferably 1100 ° C. or lower, more preferably 1050 ° C. or lower. Within this range, carbon / nitride can be sufficiently precipitated.

(3) Average cooling rate after finish rolling: 0.5 to 13 ° C./second If the average cooling rate after finish rolling becomes too fast, martensite is generated in the surface layer, and pickling properties deteriorate. Therefore, the average cooling rate after finish rolling is preferably 13 ° C./second or less, more preferably 8 ° C./second or less. On the other hand, when the average cooling rate after rolling becomes too slow, productivity deteriorates. Therefore, the average cooling rate after finish rolling is preferably 0.5 ° C./second or more, more preferably 1.0 ° C./second or more.

  Since the scale is formed on the surface of the rolled material obtained by hot rolling, the scale on the surface of the rolled material may be removed using means such as pickling or shot blasting as necessary.

3. Spheroidizing annealing Next, the rolled material is subjected to spheroidizing annealing. Spheroidizing annealing is performed in an atmosphere containing carbon monoxide and carbon dioxide at a temperature (soaking temperature) of 700 to 830 ° C. and a holding time of 5 to 15 hours.
Details of the spheroidizing annealing are shown below.
In this specification, “3. Spheroidizing annealing”, “4. Cold working”, and “5. Low temperature annealing” are described in this order. In one preferred embodiment, the above-mentioned rolled material is subjected to spheroidizing annealing, hot working and low temperature annealing in this order. However, it is not limited to this order. For example, after low-temperature annealing is performed on the rolled material, it may be performed in any order such as cold working and then spheroidizing annealing.

(1) Temperature (soaking temperature): 700 to 830 ° C
Spheroidizing annealing is performed as a pretreatment when forging a steel wire for bolts into a bolt shape. By performing spheroidizing annealing, carbides such as cementite are spheroidized, and necessary forgeability can be ensured. In the steel having the above composition, the soaking temperature needs to be 700 to 830 ° C. in order to spheroidize the carbide. When the soaking temperature is lower than 700 ° C., the carbide is not sufficiently dissolved due to insufficient heating and spheroidization is not achieved. In addition, when the soaking temperature is 830 ° C. or higher, an austenite single layer is formed, so that insoluble carbides that become spheroidizing nuclei disappear and spheroidization does not occur. The minimum with a preferable soaking temperature of spheroidizing annealing is 730 degreeC, and a more preferable minimum is 750 degreeC. On the other hand, the upper limit with preferable soaking temperature of spheroidizing annealing is 820 degreeC, and a more preferable upper limit is 810 degreeC.
In addition, you may manage temperature using the furnace temperature of the heat processing furnace which normally performs spheroidizing annealing. Moreover, you may manage by arrange | positioning temperature measuring means, such as a thermocouple, on the surface of the rolling material which spheroidizes annealing as needed, and measuring material temperature.

(2) Holding time: 5 to 15 hours In the steel having the above composition, the holding time at the soaking temperature (soaking time) needs to be 5 to 15 hours in order to spheroidize the carbide. When the holding time is shorter than 5 hours, a sufficient carbon diffusion time cannot be secured, and spheroidization is not promoted. Moreover, when holding time becomes longer than 15 hours, an effect will be saturated and productivity will fall. The preferable lower limit of the holding time is 6 hours, and the more preferable lower limit is 7 hours. On the other hand, the preferable upper limit of the holding time is 14 hours, and the more preferable upper limit is 13 hours.
Here, the holding time refers to a time during which the temperature is within a predetermined soaking temperature range.

(3) In-furnace atmosphere: atmosphere containing carbon monoxide and carbon dioxide The atmosphere in the furnace during spheroidizing annealing is performed so that carbon monoxide (gas) and carbon dioxide ( Gas). In a preferred form, the atmospheric gas consists of carbon monoxide and carbon dioxide. However, the present invention is not limited to this, and in addition to carbon monoxide and carbon dioxide, an inert gas such as hydrogen, nitrogen, or argon may be included.
Moreover, it is preferable that the ratio (mixing ratio) of carbon monoxide and carbon dioxide is such that the carbon potential (CP value) is 0.6 to 1.1 times the C content of the steel material. More preferably, it is 0.8-1.0 times, More preferably, it is 0.8-0.95 times. By setting the ratio of the carbon potential to the C content in such a range, it is possible to obtain an effect of more reliably suppressing a decrease in fatigue resistance due to excessive decarburization and a decrease in delayed fracture resistance due to excessive carburization. The carbon potential that can be obtained can be determined, for example, by measuring the carbon content of a coiled piano wire (CP coil) placed in the furnace.

Preferred conditions for spheroidizing annealing include the following. It is preferable to apply one or more selected from these.
Extraction temperature: 680 ° C. or lower Although the extraction temperature does not affect delayed fracture resistance and fatigue properties, it is preferable to extract at 680 ° C. or lower in order to prevent red scale (hematite) formation on the steel surface. More preferably, it is 660 degreeC, More preferably, it is 650 degreeC. Although there is no particular lower limit for the extraction temperature, it is generally preferably 550 ° C. or higher from the viewpoint of productivity. More preferably, it is 570 degreeC, More preferably, it is 570 degreeC or more. The extraction temperature may be managed by using the furnace temperature of the heat treatment furnace in the same manner as the soaking temperature, and may be managed by measuring the material temperature as necessary. Input temperature: 500 to 700 ° C.
・ Rise rate from charging temperature to soaking temperature: 0.3 to 5 ° C./min ・ Cooling rate from soaking temperature to extraction temperature: 0.3 to 5 ° C./min These temperatures are the same as soaking temperature May be managed using the furnace temperature of the heat treatment furnace, and may be managed by measuring the material temperature if necessary.

4). Cold work For example, cold work is performed on a rolled material such as a rolled material that has been subjected to spheroidizing annealing. An example of cold working is wire drawing.
When wire drawing is performed, pickling or shot blasting may be performed as necessary in order to remove scale formed on the surface of the rolled material by spheroidizing annealing or the like. Further, a lubricating film may be formed on the surface of the rolled material. A lubricant film may be formed on the surface during wire drawing.
The area reduction rate by wire drawing is preferably 10 to 30%. In order to obtain these area reduction ratios, for example, a plurality of wire drawing dies may be passed, and the wire may be drawn in multiple times.

5. Low-temperature annealing A cold-worked rolled material is subjected to low-temperature annealing for the purpose of removing strain by cold working.
The low-temperature annealing conditions are as follows: temperature (soaking temperature): 600 to 700 ° C., holding time: 3 to 10 hours, furnace atmosphere: inert gas is 90% by volume or more.
When cold working is performed after low-temperature annealing, and then spheroidizing annealing is performed, the low-temperature annealing plays a role in refining the rolled material into a structure suitable for cold working, and spheroidizing annealing is cold. It will play the role of strain removal by hot working. Also in this case, the low-temperature annealing conditions are as follows: temperature (soaking temperature): 600 to 700 ° C., holding time: 3 to 10 hours, furnace atmosphere: inert gas is 90% by volume or more.
Details of the low-temperature annealing are shown below.

(1) Temperature (soaking temperature): 600 to 700 ° C
In low-temperature annealing, it is necessary to set the temperature (soaking) to 600 ° C. or higher in order to sufficiently remove cold-working strain such as wire drawing. Preferably it is 620 degreeC or more, More preferably, it is 630 degreeC or more. On the other hand, when the temperature becomes too high, the spherical carbide dissolves and the matrix is austenitized, so that it precipitates as pearlite during cooling and increases the hardness. Therefore, the temperature needs to be 700 ° C. or lower. Preferably it is 690 degrees C or less, More preferably, it is 680 degrees C or less.
In addition, you may manage temperature using the furnace temperature of the heat processing furnace which normally performs low temperature annealing. Moreover, you may manage by arrange | positioning temperature measuring means, such as a thermocouple, on the surface of the cold-rolled rolled material which performs low temperature annealing, and measuring material temperature as needed.

(2) Holding time: 3 to 10 hours In order to sufficiently remove the strain of cold working, the holding time (soaking time) needs to be 3 hours or more. Preferably it is 3.5 hours or more, More preferably, it is 4 hours or more. On the other hand, if the holding time becomes too long, the effect is saturated and the productivity is lowered, so the upper limit was set to 10 hours. A preferable upper limit of the holding time is 8 hours, and a more preferable upper limit is 6 hours.
Here, the holding time refers to a time during which the temperature is within a predetermined soaking temperature range.

(3) In-furnace atmosphere: 90% by volume or more of inert gas In-furnace atmosphere during low-temperature annealing is nitrogen, hydrogen, argon, or the like in order to maintain the carbon gradient generated by spheroidizing annealing of the cold-worked rolled material It is necessary to mainly use the inert gas. The concentration of the inert gas is 90% by volume or more. Preferably, the inert gas is 95% by volume or more, more preferably 98% by volume or more, and even more preferably only the inert gas. As an example of the contained gas other than the inert gas, inevitably contained oxygen and carbon dioxide can be mentioned.

Preferred conditions for low-temperature annealing include the following. It is preferable to apply one or more selected from these.
-Input temperature: 500-700 ° C
-Temperature increase rate from charging temperature to soaking temperature: 0.3 to 10 ° C / min Cooling rate from soaking temperature to extraction temperature: 0.3 to 10 ° C / min Extraction temperature: 500 to 700 ° C
These temperatures may be managed using the furnace temperature of the heat treatment furnace in the same manner as the soaking temperature, and may be managed by measuring the material temperature as necessary.

6). Skin pass wire drawing It is preferable to perform skin pass wire drawing (drawing with reduced surface ratio) on a rolled material subjected to low temperature annealing or the like. Thereby, the dimensional accuracy more suitable for bolt processing can be obtained.
The area reduction rate of the skin pass drawing is preferably 3 to 10%.
In addition, when performing skin pass drawing, in order to remove the scale formed on the surface of the material subjected to low temperature annealing, pickling or shot blasting may be performed as necessary. Moreover, you may form a lubricating film on the surface of the rolling material (for example, cold-worked rolling material) which performed low temperature annealing. A lubricant film may be formed on the surface during wire drawing.

Through the above steps, the bolt steel wire according to the present invention can be obtained.
Next, a method for processing a bolt using the steel wire for bolt will be described.

7). Bolt processing The bolt according to the present invention can be manufactured by forming the above-described steel wire for bolts into a bolt shape by cold forging (bolt forming) and further quenching and tempering.
The cold heading conditions are not particularly limited, and general conditions used for bolt molding may be applied.

On the other hand, in the quenching and tempering treatment, in order to control the austenite crystal grain size, the heating temperature during quenching is preferably 950 ° C. or less, more preferably 930 ° C. or less, and further preferably 910 ° C. or less. On the other hand, if the heating temperature at the time of quenching is too low, the spherical carbide is not sufficiently dissolved, and the required strength cannot be obtained. Therefore, the heating temperature during quenching is preferably 870 ° C. or higher, more preferably 890 ° C. or higher, and still more preferably 900 ° C. or higher.
Although the other heating conditions at the time of quenching are not specifically limited, The following conditions are illustrated.
Heating time during quenching: 10 to 45 minutes Cooling method: Oil cooling Quenching temperature: Room temperature to 70 ° C
Furnace atmosphere: Mixed atmosphere of carbon monoxide and carbon dioxide, nitrogen atmosphere, hydrogen atmosphere or air atmosphere, etc.

  Tempering conditions such as temperature and time can be appropriately changed according to the required strength.

If the rolling process is performed before quenching and tempering (pre-rolling), deformation is easy, so that the mold life can be improved. On the other hand, if it is carried out after quenching and tempering (post-rolling), compressive residual stress is imparted, and delayed fracture resistance and fatigue resistance are further improved. The timing of rolling is not particularly limited, and may be appropriately selected depending on the purpose and application.
The bolt of the present invention obtained by using the bolt steel wire of the present invention described above exhibits excellent delayed fracture resistance and fatigue resistance.

  Hereinafter, the present invention will be described more specifically by way of examples. However, the present invention is not limited by the following examples, and should be implemented with appropriate modifications within a range that can meet the purpose described above and below. Of course, these are all possible and are included in the technical scope of the present invention.

1. Sample preparation A steel material having a chemical composition shown in Table 1 was melted, cast, and hot-rolled to produce a rolled material (wire material) having a diameter of 12 mmφ. At that time, billet reheating, finish rolling, and cooling after finish rolling were performed under the conditions shown in Table 2 to obtain a rolled material (wire). “CP / C amount” in Table 2 indicates the ratio of the CP value to the C content of each sample.

Thereafter, descaling (P), film treatment (L), spheroidizing annealing (SA), low-temperature annealing (LA) and wire drawing (Dr) were performed in the steps shown in Table 2, and a 9.06 mmφ bolt steel A wire was manufactured. About each process, the conditions which are not described in Table 2 were implemented below.
It should be noted that the conditions underlined in Tables 1 and 2 are outside the conditions defined by the present invention.
Experiment No. Except for 17, 18 and 19, the first annealing was spheroidizing annealing, and the second annealing was low temperature annealing. On the other hand, Experiment No. For No. 17, the first annealing was low temperature annealing, the second annealing was spheroidizing annealing, For No. 18, the two annealings were both low-temperature annealing, and Experiment No. For No. 19, the two annealings were both spheroidizing annealing.

・ Descaling treatment (P)
The scale was removed by immersion in 25% hydrochloric acid (70 ° C.) for 10 minutes.

・ Film treatment process (L)
Lime film treatment was performed by immersing in a lime soap bath for 10 minutes.

・ Spheroidizing annealing (SA)
Input temperature: 530 ° C
Temperature increase rate from charging temperature to soaking temperature: 3 ° C / min Cooling rate from soaking temperature to extraction temperature: 0.5 ° C / min Extraction temperature: 640 ° C
Except for Experiment No23, the atmosphere was a mixed gas of carbon monoxide and carbon dioxide. The carbon potential (CP value) of the mixed gas was measured by putting a piano wire coil (CP coil) in the furnace and analyzing the C amount of the CP coil after heat treatment.

・ Low temperature annealing (LA)
Input temperature: 500 ° C
Temperature increase rate from charging temperature to soaking temperature: 3 ° C / min
Cooling rate from soaking temperature to extraction temperature: 3 ° C / min
Extraction temperature: 600 ° C
Experiment No. Except for 27, the atmosphere was nitrogen gas. Experiment No. In 27, the atmosphere was air.

・ Drawing process (Dr)
As the first wire drawing, the wire was drawn from φ12.0 mm to 9.3 mmφ using a wire drawing die at a wire drawing speed of 1 m / sec.
Thereafter, after heat treatment, descaling, and film treatment, the second wire drawing and finish wire drawing (skin pass wire drawing) to 9.06 mmφ were performed.

2. Sample Evaluation About each steel wire sample for bolts, a flange bolt of M10 × 1.5 Pmm and length 80 mm was formed by cold heading using a multistage former. M represents the diameter of the shaft, and P represents the pitch.

(1) Cold forging When cold forging described above, cold forging was evaluated based on the presence or absence of flange cracking. The cold heading property was evaluated as “P” (Pass) when no cracking occurred and “F” (Failure) when cracking occurred.

  Each bolt sample was quenched and tempered under the conditions shown in Table 3. At this time, the holding time during quenching was 20 minutes, the furnace atmosphere was a nitrogen atmosphere, and the quenching was oil-cooled at 60 ° C. Moreover, the holding time of tempering was 45 minutes, and the rolling process was performed after tempering. In addition, the quenching and tempering process was not performed about the sample in which the evaluation result of cold forgeability failed.

(2) Tensile strength The sample after quenching and tempering was subjected to a tensile test according to JIS B 1051 (2014) to measure the tensile strength of the bolt.

(3) Delayed fracture resistance After quenching and tempering the bolt sample to the jig aiming at the yield point, (a) the jig is immersed in 1% HCl for 15 minutes, (b) exposed in air for 24 hours, (c ) Confirmation of the presence or absence of breakage was defined as one cycle, and this was repeated for 10 cycles for evaluation. Ten bolts were evaluated with respect to one level. When none of the bolts broke, the test was “P”, and when one was broken, the test was “F”.

(4) Fatigue resistance A tensile-tensile fatigue test was performed with 0.6 times the tensile strength obtained in the tensile test as an average stress and an amplitude stress of 100 MPa. 3 bolts per level, and up to 10 7 times, if none breaks, pass “P”, and if even one breaks, reject “F” did.

  From these results, it can be considered as follows. Test No. Examples 1 to 17 are examples that satisfy the requirements defined in the present invention. All of these were excellent in cold heading, delayed fracture resistance and fatigue resistance.

Experiment No. 18 to 37 are comparative examples that do not satisfy the requirements defined in the present invention.
Experiment No. No. 18 is an example in which low-temperature annealing was performed twice without performing spheroidizing annealing at the time of steel wire production. Since the metal structure was not spheroidized, the cold forgeability was poor, and a crack occurred in the flange part during bolt molding.

Experiment No. No. 19 is an example in which spheroidizing annealing was performed twice without performing low temperature annealing at the time of manufacturing the steel wire. Carburization annealing was promoted by carrying out spheroidizing annealing twice, and fatigue resistance deteriorated.
Experiment No. 20 is an example in which the temperature during spheroidizing annealing was low. Since the metal structure was not spheroidized, the cold forgeability was poor, and a crack occurred in the flange part during bolt molding.

Experiment No. No. 21 is an example in which the temperature during spheroidizing annealing was high. Spheroidization was not promoted by becoming uniform austenite at the time of soaking, so that cold forgeability was poor and cracking occurred in the flange portion during bolt molding.
Experiment No. 22 is an example in which the holding time during spheroidizing annealing was short. Since sufficient carbon diffusion time could not be secured, spheroidization did not progress, cold forging was poor, and cracks occurred in the flange part during bolt molding.

Experiment No. No. 23 is an example in which the atmosphere in the furnace at the time of spheroidizing annealing is air. Since the atmosphere was not appropriate, decarburization progressed and fatigue resistance deteriorated.
Experiment No. 24 is an example in which the temperature during low-temperature annealing was low. The strain at the time of wire drawing could not be removed sufficiently, and the cold forgeability deteriorated because the strength of the steel wire increased.
Experiment No. 25 is an example in which the temperature during low-temperature annealing was high. Cementite dissolved during soaking and austenite was formed, so that pearlite transformation occurred during cooling, and the strength of the steel wire increased, resulting in deterioration of cold heading.

Experiment No. No. 26 is an example in which the holding time at the time of low-temperature annealing was short. The strain at the time of wire drawing could not be removed sufficiently, and the cold forgeability deteriorated because the strength of the steel wire increased.
Experiment No. 27 is an example in which the atmosphere in the furnace at the time of low-temperature annealing is air. Since it was not an inert environment, decarburization progressed and fatigue resistance deteriorated.

Experiment No. 28 is an example in which the amount of C is too small. Since the hardenability was low, martensitic transformation did not occur during quenching, and the required strength could not be obtained.
Experiment No. 29 is an example in which the amount of C is too large. Delayed fracture resistance deteriorated as the toughness decreased.

Experiment No. 30 is an example in which the amount of Si is too small. Delayed fracture resistance deteriorated due to the formation of coarse cementite during tempering.
Experiment No. 31 is an example in which the amount of Si is too large. Fatigue characteristics deteriorated because ferrite was generated during quenching and became the starting point of fracture.

Experiment No. 32 is an example in which the amount of Mn is too large. The generation of coarse MnS became a stress concentration source, and fatigue resistance deteriorated.
Experiment No. 33 is an example in which the amount of P is too large. Delayed fracture resistance deteriorated as the toughness of steel decreased.

Experiment No. 34 is an example in which the amount of S is too large. Delayed fracture resistance deteriorated as the toughness of steel decreased.
Experiment No. 35 is an example in which the amount of Cr is too small. As the corrosion resistance of the steel decreased, the amount of hydrogen in the steel surface layer increased and the delayed fracture resistance deteriorated.

Experiment No. 36 is an example in which the amount of Al is too large. Due to the generation of coarse AlN, the cold forgeability was poor, and cracking occurred in the flange portion.
Experiment No. 37 is an example in which the amount of N is too large. Free N which does not form nitrides increased, and cracking occurred in the flange portion due to a decrease in cold heading.

Claims (4)

C: 0.20-0.60 mass%
Si: 1.0-3.0 mass%
Mn: 0.10 to 1.5% by mass
P: 0.015 mass% or less S: 0.015 mass% or less Cr: 0.3-1.5 mass%
Al: 0.001 to 0.10% by mass
N: 0.001 to 0.02 mass%
A rolled material preparation step of preparing a rolled material, the balance of which is iron and inevitable impurities,
In the atmosphere containing carbon monoxide and carbon dioxide, the spheroidizing annealing step for performing spheroidizing annealing at a temperature of 700 to 830 ° C. and a holding time of 5 to 15 hours,
A cold working step for cold working the rolled material;
A low temperature annealing step in which the rolled material is annealed at a temperature of 600 to 700 ° C. and a holding time of 3 to 10 hours in an atmosphere of 90% by volume or more of an inert gas;
Of steel wire for bolts including   The method for producing a steel wire for a bolt according to claim 1, wherein the carbon potential of the atmosphere in the spheroidizing annealing step is 0.6 to 1.1 times the C content of the rolled material. The manufacturing method of the steel wire for bolts of Claim 1 or 2 in which the said rolling material further contains 1 or more selected from the group which consists of the following (a)-(d).
(A) At least one selected from the group consisting of Cu: more than 0% and 0.5% or less and Ni: more than 0% and 1.0% or less (b) Mo: more than 0% and 2% or less (c) V: More than 0% and 1% or less (d) at least one selected from the group consisting of Ti: more than 0% and 0.1% or less and Nb: more than 0% and 0.1% or less Cold forging step of cold forging into a bolt shape using the steel wire for bolts according to any one of claims 1 to 3,
A quenching and tempering step in which the bolt steel wire is heated to 870 ° C. to 950 ° C. and quenched and tempered before or after the cold forging step;
Of bolts including