KR20110083688A - Low temperature annealing steel wire and manufacturing method thereof - Google Patents

Abstract

The low temperature annealing steel wire is, in mass%, C: 0.10 to 0.60, Si: 0.01 to 0.40%, Mn: 0.20 to 1.50%, P: 0 to 0.040%, S: 0 to 0.050%, N: 0.0005 to 0.0300 %, Containing at least one of Cr: 0.03 to 0.4%, V: 0.03 to 0.2%, and Mo: 0.03 to 0.2%, the remainder being made of Fe and unavoidable impurities, and the volume fraction of the pearlite structure. 1.40 x (C%) x 100% or more and 100% or less, the volume fraction of the cornerstone ferrite is 0% or more, [1-1.25 x (C%)] x 50% or less, and the volume rate of the bainite structure is It is 0% or more and 40% or less, and tensile strength is 480 + 850xCeq.MPa or more, and 580 + 1130xCeq.MPa or less.

Description

Low temperature annealing steel wire and manufacturing method thereof {STEEL WIRE FOR LOW TEMPERATURE ANNEALING AND PRODUCING METHOD THEREOF}

The present invention relates to a steel wire for low temperature annealing used as a raw material for mechanical parts such as bolts, screws, nuts, and the like, which are formed by cold forging, and a method of manufacturing the same. In particular, it relates to the low temperature annealing steel wire which is excellent in the soft-nitriding characteristic and softness by annealing, and which can be annealed at lower temperature, and its manufacturing method.

This application claims priority in 2009/11/17 based on Japanese Patent Application No. 2009-262158 for which it applied to Japan, and uses the content for it here.

In cold forging, since the dimensional accuracy and productivity of a characteristic are excellent, the transition from the hot forging conventionally performed at the time of the molding of mechanical parts, such as a forcible bolt, a screw, a nut, is expanding. On the other hand, in cold forging, as compared with hot forging, the deformation resistance of the steel is increased and the deformation capacity of the steel is lowered, so that the load on the mold is increased. Therefore, in cold forging, abrasion and damage of a metal mold | die generate | occur | produce, and a problem such as work cracking generate | occur | produces in a molded part tends to arise.

In order to avoid these problems, the steel used for cold forging requires very high workability. Therefore, conventionally, the hot rolled material is softened by heat treatment such as spherical annealing to improve the workability of the steel material.

Spherical annealing is a process which improves workability by making cementite into a spherical shape, and is widely used as soft nitriding treatment of cold forging steel. Since the spheroidizing annealing requires a heat treatment time of about 20 hours, in recent years, in order to improve the productivity and the cost of parts, the demand for shortening the heat treatment time, reducing the annealing temperature, or omitting annealing has increased.

In addition, in mechanical structural steel, alloy elements, such as Cr, Mo, or V, may be added in order to ensure the strength required as a mechanical component. It is known that when these alloying elements are added to steel, spheroidization of cementite is delayed at the time of soft annealing, the strength after annealing is increased, the ductility is lowered, and the cold forging is deteriorated. Therefore, when these alloying elements are added to steel, methods, such as performing spheroidization annealing twice or more, are performed in order to improve cold forging property.

In addition, in recent years, the shape of components has also become complicated for the purpose of reducing component manufacturing costs and improving the functionality of components. For this reason, the demand for the workability of steel materials used for cold forging is increasing. The workability of the cold forging steel includes the deformation resistance affecting the load on the mold and the ductility affecting the generation of work cracking, and both or one of them is required as the workability of the cold forging steel. The characteristic (strain resistance or ductility) required for the workability of this cold forging steel differs according to each use.

Under such a background, various methods have conventionally been proposed as a technique for improving cold forging of steel materials. For example, softening techniques such as a method of accelerating spheroidization of cementite to soften steels by performing preliminary drawing having a cross-sectional reduction rate of 20 to 30% before spheroidizing annealing, or a method of softening steels by performing spheroidization annealing a plurality of times, have long been used. It is well known for a long time and has been widely practiced conventionally.

In addition, Patent Literature 1 discloses a spheroidizing annealing after preliminary drawing at low temperature and in a short time by setting the ferrite structure fraction of the hot-rolled wire rod to 30 area% or more and the total of bainite structure and martensite structure to 50 area% or more of the remainder. A method of making it processable is disclosed. In this method, although the processing temperature of spheroidizing annealing can be reduced or the processing time can be shortened, cold forging properties, such as hardness after annealing and limit compression ratio, are equivalent to the conventional spheroidizing annealing material, and are inadequate in terms of workability. Do.

In addition, Patent Document 2 discloses a spheroidizing annealing process after drawing a sheet having a cross-sectional reduction rate of 28% or more for a steel material composed of a ferrite pearlite structure having a bainite volume fraction of 50% or less as a method for producing a surface hardened steel. A method is disclosed. In this method, although the hardness after spheroidization annealing becomes low and homogeneous and softens steel materials, the ductility of steel materials is still inadequate.

In addition, Patent Document 3 discloses a method of shortening spheroidization treatment time and reducing deformation resistance of steel by defining the area ratio of pseudo pearlite, bainite and ferrite in the steel structure. In this method, since it is necessary to contain 10% or more of pseudo pearlite in steel structure, in the case of steel grade with a low content of alloying elements, low hardenability, and a large wire diameter, it is necessary to raise cooling rate after winding. There is a problem that the manufacturing cost increases.

Japanese Patent Application Publication No. 2006-37159 Japanese Patent Application Publication No. 2006-124774 Japanese Patent Application Publication No. 2006-225701

An object of this invention is to provide the steel wire excellent in the cold forging property which can reduce the temperature in soft-annealing annealing before cold forging, and becomes soft and ductility becomes high after this soft-annealing annealing, and its manufacturing method.

In order to improve the cold forging of steel wire, the present inventors investigated the relationship between the structure before annealing of steel materials and the mechanical properties when this steel is annealed after preliminary drawing.

The inventors of the present invention have suppressed the cornerstone ferrite structure and bainite structure, and when the structure in the steel wire is controlled to become a structure mainly containing pearlite structure, an element that inhibits spheroidization of cement, such as Cr, Mo, and V, is contained in the steel wire. Even in this case, it was found that preliminary drawing is performed under specific conditions, and the strength of the steel wire is controlled to a specific value, whereby the strength decreases when the steel wire is annealed at low temperature, and the ductility is remarkably improved. When the cementite is spheroidized by preliminary drawing and low temperature annealing, suppressing the volume fraction of the cornerstone ferrite in the structure before the preliminary drawing results in a structure in which cementite is uniformly dispersed after annealing, thereby significantly improving the ductility of the steel wire. In addition, low temperature annealing is annealing performed below A _c1 point in order to soften steel materials.

In addition, the inventors of the present invention tend to make the size of spherical cementite uniform when annealed in a tissue including mainly pearlite tissues in which salt-bearing ferrite and bainite are suppressed, and it is possible to suppress formation of coarse spherical cementite. Found. Since coarse spherical cementite acts as a starting point of ductile fracture, it is effective to suppress this coarse spherical cementite in order to improve the workability of steel wire.

Further, the inventors found that by suppressing bainite structure and martensite structure in the steel wire, after preliminary drawing and low temperature annealing, the strength of the steel wire can be reduced to soften the steel wire, and the ductility can also be improved. It was. The bainite structure and martensite structure are effective for spheroidizing cementite, but have a high dislocation density. Therefore, in low temperature, such as low temperature annealing, it is inferred that soft nitriding of steel wire will be inadequate easily in short time annealing. MEANS TO SOLVE THE PROBLEM The present inventors made the examination repeatedly based on the said knowledge, and came to complete this invention. The present invention is as follows. In addition, below, content (mass%) of C is described as (C%).

(1) The steel wire for low temperature annealing according to one embodiment of the present invention is, by mass%, C: 0.10 to 0.60%, Si: 0.01 to 0.40%, Mn: 0.20 to 1.50%, P: 0 to 0.040%, S: 0 to 0.050%, N: 0.0005 to 0.0300%, Cr: 0.03 to 0.4%, V: 0.03 to 0.2%, Mo: 0.03 to 0.2%, and further, the remainder is Fe and inevitable It consists of impurities, and has a metal structure containing a saltpeter ferrite structure, a pearlite structure, and bainite structure, and the volume ratio of the said pearlite structure is 1.40 * (C%) * 100% or more and 100% or less, The said cornerstone The volume fraction of ferrite is 0% or more and [1-1.25 × (C%)] × 50% or less, the volume fraction of the bainite structure is 0% or more and 40% or less, and the volume fraction of the cornerstone ferrite structure The sum of the volume ratio of the bainite structure and the volumeite of the pearlite structure is 95% or more and 1 It is 00% or less, and tensile strength is 480 + 850xCeq.MPa or more and 580 + 1130xCeq.MPa or less.

However, Ceq. = (C%) + (Si%) / 7+ (Mn%) / 5+ (Cr%) / 9+ (Mo%) / 2 + 1.54 × (V%).

In addition, (C%), (Si%), (Mn%), (Cr%), (Mo%) and (V%) are the contents (mass%) of C, Si, Mn, Cr, Mo, and V, respectively. )to be.

(2) The low temperature annealing steel wire according to the above (1) is Al: 0.001 to 0.060%, Ti: 0.002 to 0.050%, Nb: 0.005 to 0.100%, B: 0.0001 to 0.0060%, Cu: 0.01 You may further contain 1 or more types from-0.3%, Ni: 0.01-0.7%, Ca: 0.0001-0.010%, Mg: 0.0001-0.010%, Zr: 0.0001-0.010%.

(3) In the steel wire for low temperature annealing as described in said (1) or (2), the average block size of the said pearlite structure may be 4 micrometers or more and 20 micrometers or less.

(4) In the steel wire for low temperature annealing according to one embodiment of the present invention, the steel strip having the composition as described in the above (1) or (2) is heated, hot rolled, then wound up, and thereafter, 400 ° C. or higher and 600 It is kept constant temperature for 30 seconds or more and 150 seconds or less in the molten salt bath below ° C, and then cooled and subjected to drawing processing with a cross-sectional reduction rate of 25 + 82 x F1% or more and less than 100%.

According to the present invention, it is possible to form a steel material into a component having a complicated shape by cold forging, so that the yield and productivity of the steel material can be improved, and the processing cost of the part can be reduced. Moreover, according to this invention, the temperature of soft-nitriding annealing can be reduced, heat processing cost can be reduced, and productivity can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows the relationship between the tensile strength TS of the steel wire before annealing, and the tensile strength TS of the steel wire after low temperature annealing.
Fig. 2 is a graph showing the relationship between the carbon equivalent (Ceq.) And the tensile strength (TS) after annealing.
3 is a diagram showing the relationship between the carbon equivalent (Ceq.) And the cross-sectional shrinkage ratio (RA) after annealing.
4 is a diagram illustrating a relationship between a F1 value and a fresh wire cross-section reduction rate.
It is a figure which shows the relationship between C content (C%) and the volume fraction of a pearlite structure.
It is a figure which shows the relationship between C content (C%) and the volume fraction of a cornerstone ferrite structure.
It is a figure which shows the relationship between the carbon equivalent (Ceq.) And the tensile strength (TS) of the steel wire before annealing.

As a technique for improving cold forging of steel materials, various methods have been conventionally proposed. In the present invention, soft wire annealing can be carried out at a lower temperature than the conventional annealing temperature before cold forging, and in order to obtain a steel wire that is soft after the soft annealing and high in ductility and excellent in cold forging, the steel wire (wire) You need to control your organization to a specific organization.

Below, the steel wire for low temperature annealing which concerns on one Embodiment of this invention is demonstrated. First, the reason for limitation of an organization is demonstrated.

When the volume fraction of the cornerstone ferrite (the cornerstone ferrite structure) exceeds [1-1.25 × (C%)] × 50%, the cementite is unevenly distributed after annealing, thereby producing a portion having an uneven strength. If a portion having an uneven strength exists, cold forging cracks may occur due to the concentration of local deformation during forging. For this reason, the upper limit of the volume fraction of a saltpeter ferrite is [1-1.25 * (C%)] * 50%. In addition, since the cornerstone ferrite does not need to exist in the structure of the steel wire, the lower limit of the volume fraction of the cornerstone ferrite is 0%.

Although the bainite structure is effective for spheroidization of cementite and has an effect of improving the ductility of steel wire, the dislocation density is high, and thus the strength after low temperature annealing may increase. For this reason, the upper limit of the volume ratio of bainite structure is 40%. In addition, since bainite does not need to exist in the structure of a steel wire, the minimum of the volume ratio of bainite is 0%.

Since martensitic structure raises the intensity | strength after annealing, it is preferable to be suppressed to 5% or less.

The pearlite structure is effective for spheroidization of cementite after preliminary wire annealing, and has an effect of lowering the deformation resistance of steel wire. Moreover, when the volume ratio of a pearlite structure is large, the dispersion | variation in the magnitude | size of the spheroidized cementite after annealing becomes small, and ductility of a steel wire improves. When the volume ratio of the pearlite structure is less than 1.40 × (C%) × 100%, the effect of reducing the deformation resistance and improving the ductility is small, so the lower limit of the volume rate of the pearlite structure is 1.35 × (C%) × 100%. to be.

In addition, the metal structure of steel wire contains a cornerstone ferrite, bainite, and pearlite, and the sum of the volume ratio of the above-mentioned cornerstone ferrite structure, the volume rate of bainite structure, and the volumeite of pearlite structure is 95%. It is more than 100%.

The miniaturization of the average block size of the pearlite structure has the effect of reducing the ferrite grain size after annealing, and is effective for improving the ductility. Further, by miniaturization of the average block size, decomposition of lamellar pearlite and spheroidization of cementite are promoted, so that annealing time can be shortened. If the average block size of this pearlite structure is 20 micrometers or less, sufficient ductility can be ensured after annealing, shortening an annealing time. Therefore, it is preferable that the upper limit of the average block size of a pearlite structure is 20 micrometers. In addition, the minimum of the average block size of a pearlite structure may be 4 micrometers from the limitation on the measurement of an average block size.

In addition, in order to evaluate the volume ratios of the cornerstone ferrite structure, pearlite structure, and bainite structure, the C cross-section (cross section perpendicular to the longitudinal direction of the wire rod) of the wire rod was photographed at a magnification of 1000 times using a scanning electron microscope. The area ratio of each structure was determined by image analysis. Here, in the C cross section of the wire rod, the vicinity of the surface layer (surface) of the wire rod, 1 / 4D portion (part spaced 1/4 of the diameter of the wire rod in the direction of the center of the wire rod from the surface of the wire rod), and 1 / 2D portion ( The center part of the wire rod) was photographed, and the photographing regions at the respective positions were all 125 µm x 95 µm. In addition, since the area ratio of the tissue contained in the speculum plane (C cross section) is equivalent to the volume ratio of the tissue, the area ratio of each tissue obtained by image analysis was evaluated as the volume ratio of each tissue.

The EBSD apparatus was used for the measurement of the block size of a pearlite structure. The area | region of 275 micrometers x 165 micrometers was measured about the surface layer vicinity part, 1 / 4D part, and 1 / 2D part in C cross section of a wire rod. From the crystal orientation map of ferrite (ferrite in pearlite structure) measured by the EBSD apparatus, the boundary with an orientation difference of 15 degrees or more was determined as a block boundary.

In addition, in the steel wire of this embodiment, tensile strength TS is 480 + 850xCeq.MPa or more. In the case where the tensile strength TS is smaller than 480 + 850 × Ceq.MPa, soft nitriding of the steel wire after annealing is insufficient, and cold forging property is deteriorated. In addition, in order to ensure the deformation ability of a steel wire enough, tensile strength TS may be 580 + 1130 * Ceq.MPa or less. Here, carbon equivalent (Ceq.) Is represented by following formula (1).

In addition, (C%), (Si%), (Mn%), (Cr%), (Mo%) and (V%) are the contents (mass%) of C, Si, Mn, Cr, Mo, and V, respectively. )to be.

The steel wire of this embodiment is mass%, C: 0.10 to 0.60%, Si: 0.01 to 0.40%, Mn: 0.20 to 1.50%, P: 0 to 0.040%, S: 0 to 0.050%, N: 0.0005 to It contains 0.0300%, and further contains at least 1 type of Cr: 0.03-0.4%, V: 0.03-0.2%, and Mo: 0.03-0.2%. Below, the reason which limited the range of these elements is demonstrated. In addition,% described below is mass% with respect to content of each element.

C is added to steel in order to ensure the strength as a mechanical component. If C content is less than 0.10%, the strength required as a mechanical component cannot be ensured. Moreover, when C content exceeds 0.60%, cold forging property will deteriorate. Therefore, C content in steel shall be 0.10 to 0.60%. In order to ensure the strength of steel more reliably, it is preferable that content of C is 0.25 to 0.60%. Moreover, in order to ensure cold forging more reliably, it is more preferable that C content is 0.25 to 0.50%.

Si is an effective element for functioning as a deoxidation element, providing strength and hardenability necessary for steel, and improving tempering softening resistance. If the Si content is less than 0.01%, these effects are insufficient. Moreover, when Si content exceeds 0.40%, toughness and ductility will deteriorate, hardness will rise, and cold forging property will deteriorate. Therefore, Si content in steel is made into 0.01 to 0.40%. Moreover, in order to improve tempering softening resistance and cold forging more reliably, it is preferable that Si content is 0.05 to 0.30%.

Mn is an element necessary for providing strength and hardenability necessary for steel. If the Mn content is less than 0.20%, the effect of imparting strength and hardenability is insufficient. When Mn content exceeds 1.50%, hardness will rise and cold forging property will deteriorate. Therefore, Mn content is made into 0.20 to 1.50%. In addition, in order to ensure strength and cold forging more reliably, it is preferable that Mn content is 0.30 to 0.90%.

P raises the deformation resistance at the time of cold forging, and deteriorates workability. In addition, P segregates at grain boundaries to embrittle the grain boundaries after quenching and tempering, thereby degrading the toughness of the steel. Therefore, it is preferable to reduce P in steel as much as possible. Therefore, the upper limit of P content is limited to 0.040%. It is preferable that this P content is 0.020% or less. In addition, the minimum of P content is 0%.

S reacts with alloying elements, such as Mn, and exists as a sulfide. These sulfides improve the machinability of steel. However, when S is added in steel exceeding 0.050%, cold forging property will deteriorate, the grain boundary after hardening tempering will embrittle, and toughness will deteriorate. For this reason, the upper limit of S content is restrict | limited to 0.050%. It is preferable that this S content is 0.020% or less. In addition, the minimum of S content is 0%.

N is added for the purpose of miniaturizing austenite grains. N combines with an alloying element such as Al or Ti to form a nitride, and these nitrides function as pinning particles to refine the grains. If the N content is less than 0.0005%, the amount of deposition of nitride is insufficient, coarse grains and ductility deteriorate. Moreover, when N is added and N content exceeds 0.0300%, deformation resistance will increase by dynamic strain aging by solid solution N, and workability will deteriorate. Therefore, N content is made into 0.0005 to 0.0300%. In order to ensure ductility more reliably and to sufficiently lower strain resistance, the N content is preferably 0.0020 to 0.0150%.

Cr has the effect of improving hardenability and strength. If Cr content is less than 0.03%, there is no effect which raises hardenability and strength. When Cr is added and Cr content exceeds 0.4%, transformation time becomes long and productivity is impaired. Therefore, Cr content in steel is made into 0.03 to 0.4%. In order to raise productivity more, it is preferable that Cr content is 0.03 to 0.2%. Moreover, in order to raise hardenability and strength further, it is more preferable that Cr amount is 0.05 to 0.20%.

V has the effect of increasing the hardenability or depositing fine carbides to increase the strength. If the V content is less than 0.03%, there is no effect of increasing the hardenability and strength. When V is added and V content exceeds 0.2%, these effects will be saturated by formation of coarse carbide containing V. Therefore, V content in steel is made into 0.03 to 0.2%. In order to more effectively increase the hardenability and strength, the amount of V is preferably 0.05 to 0.15%.

Mo has the effect of improving hardenability and strength. If Mo content is less than 0.03%, there is no effect which raises hardenability and strength. When Mo is added and Mo content exceeds 0.2%, transformation time becomes long and productivity is inhibited. Therefore, Mo content is made into 0.03 to 0.2%. Moreover, in order to improve hardenability and intensity | strength while improving productivity, it is preferable that Mo content is 0.05 to 0.15%.

In addition, the steel wire of this embodiment is Al: 0.001-0.060%, Ti: 0.002-0.050%, Nb: 0.005-0.100%, B: 0.0001-0.0060% by mass% for the purpose of the improvement of the characteristic described below. , Cu: 0.01 to 0.3%, Ni: 0.01 to 0.7%, Ca: 0.0001 to 0.010%, Mg: 0.0001 to 0.010%, Zr: 0.0001 to 0.010%.

Al is added in steel for the purpose of deoxidation and refinement of austenite grains. Al functions as a deoxidation element and combines with N in steel to form AlN. This AlN functions as a pinning particle, refines a grain size, and improves workability. In addition, Al has the effect of fixing the solid solution N to suppress the dynamic strain aging and to reduce the strain resistance. If the Al content is less than 0.001%, these effects do not function. Moreover, when Al content exceeds 0.060%, the toughness of steel will deteriorate. Therefore, the upper limit of Al content is restrict | limited to 0.060%. Therefore, when Al is added to steel, Al content in steel is controlled to 0.001 to 0.060%. In consideration of the balance of the above effects and toughness, the Al content is more preferably 0.003 to 0.04%.

Ti and Nb both form carbonitrides. These carbonitrides are dispersed in steel and function as pinning particles to suppress coarsening of crystal grains, improve workability, and increase strength of steel.

Ti forms a compound with C or N and exists as TiC, TiN or Ti (CN). These carbonitrides are effective as pinning particles and have a function of increasing the strength of steel. In addition, Ti is added in order to fix N in steel and to effectively function the improvement effect of hardenability by B addition mentioned later. If the Ti content is less than 0.002%, these effects do not appear. When Ti is added and Ti content exceeds 0.050%, these effects will be saturated and hardness will rise and cold forging property will deteriorate. Therefore, when Ti is added to steel, Ti content in steel is controlled to 0.002 to 0.050%. Moreover, in order to improve the strength of steel and cold forging more, it is preferable that Ti content is 0.005 to 0.030%.

Nb combines with N or C to form NbN, NbC or Nb (CN) which is a composite inclusion thereof, and effectively functions to suppress coarsening of austenite grains. Therefore, Nb has a function of increasing the strength of the steel. If the Nb content is less than 0.005%, the effect of suppressing coarsening of the austenite crystal grains is insufficient. If Nb is added and Nb content exceeds 0.10%, this effect will be saturated. Therefore, when Nb is added to steel, Nb content in steel is controlled to 0.005 to 0.10%. Moreover, in order to raise the intensity | strength of steel more effectively, it is preferable that Nb content is 0.01 to 0.05%.

B is added for the purpose of improving hardenability. If the B content is less than 0.0001%, the effect of improving the hardenability is insufficient. When B is added and B content exceeds 0.0060%, the effect will be saturated. Therefore, when adding B in steel, B content in steel is controlled to 0.0001 to 0.0060%. Moreover, in order to improve hardenability more effectively, it is preferable that B is 0.0005 to 0.004%.

Cu increases the strength of the steel by precipitation strengthening. If the Cu content is less than 0.01%, there is no effect of increasing the strength of the steel. When Cu is added and Cu content exceeds 0.3%, hot rolling property deteriorates. Therefore, when adding Cu in steel, Cu content in steel is controlled to 0.01 to 0.3%. In addition, the Cu content is preferably 0.05 to 0.2% in order to sufficiently secure the hot rollability while effectively increasing the strength of the steel.

Ni has the effect of improving hardenability and ductility of steel. If Ni content is less than 0.01%, there is no effect which improves hardenability and ductility. When Ni is added and Ni content exceeds 0.7%, transformation time will become long and productivity will be inhibited. Therefore, when Ni is added to steel, Ni content in steel is controlled to 0.01 to 0.7%. Moreover, in order to acquire sufficient ductility improvement effect, it is preferable to add Ni and to contain Ni 0.02% or more. In addition, in order to further secure productivity, the Ni content is more preferably 0.02 to 0.5%.

O is inevitably contained in steel and exists as oxides, such as Al and Ti. If the O content is high, coarse oxides are formed and cause fatigue breakdown. Therefore, it is preferable to suppress O content to 0.01% or less. Moreover, as a deoxidation element, 1 or more types of Ca, Mg, Zr can be contained in steel. When Ca is added to steel, Ca content in steel is controlled to 0.0001 to 0.01%. When Mg is added to steel, Mg content in steel is controlled to 0.0001 to 0.01%. When Zr is added to steel, Zr content in steel is controlled to 0.0001 to 0.01%. Ca, Mg, and Zr are effective for deoxidation and have an effect of miniaturizing oxides to improve fatigue strength.

Moreover, the manufacturing method of the steel wire which concerns on one Embodiment of this invention is demonstrated below.

The steel piece which satisfy | fills the said composition required for the steel wire of embodiment mentioned above is heated, hot-rolled, and the steel wire of desired diameter is manufactured. The steel wire obtained after hot rolling is wound up and kept at constant temperature, and is cooled to room temperature. Although the winding temperature after hot rolling is not specifically limited, Usually, it is the range from 750 degreeC to 1000 degreeC.

The cooling rate after winding is not specifically limited, either. For example, when a wire rod having a wire diameter of 5 to 16 mm is immersed in a molten salt bath of 400 ° C. or more and 600 ° C. or less, the wire rod is usually cooled at a cooling rate of 10 ° C./sec or more. The cooling rate and the steel component affect the structure of the steel (steel wire). That is, when content of alloying elements, such as C, Si, Mn, Cr, Mo, V, B, Nb, is high, when the cooling rate is enlarged, the volume ratio of bainite structure will become high. In addition, when the content of such an alloying element is low, the volume ratio of the ferrite structure increases when the cooling rate is reduced. For this reason, what is necessary is just to select a steel component and a cooling rate so that a predetermined | prescribed structure | tissue may be obtained.

After winding the hot rolled steel wire, the wound steel wire is kept at a constant temperature for 30 seconds to 150 seconds in a molten salt bath of 400 ° C. or higher and 600 ° C. or lower, and then cooled. When the temperature of the molten salt bath is less than 400 ° C, the bainite structure fraction (volume ratio) in the steel wire is increased, the strength of the steel wire after annealing is increased, the transformation completion time is long, and productivity is inhibited. When the temperature of a molten salt bath exceeds 600 degreeC, a ferrite structure fraction (volume rate) will increase, a molten salt will decompose and productivity will be impaired. When the holding time of the steel wire in a molten salt bath is less than 30 second, since a constant temperature transformation is not completed but it cools, martensite structure is produced | generated. In this case, the softening time required for annealing is long, the strength is increased, and workability is deteriorated. Moreover, when the holding time of the steel wire in a molten salt bath is 150 second or more, productivity will be impaired. Therefore, this holding time is controlled to 30 seconds or more and 150 seconds or less.

The steel wire is cooled after extraction from the molten salt bath, and the wire drawing of 25 + 82 * F1% or more of cross-sectional reduction rate is performed. Here, F1 value (F1 mentioned above) is represented by following General formula (2).

When the cross-sectional reduction rate of drawing is less than 25 + 82xF1%, soft nitriding of the steel wire after annealing is inadequate, and cold forging property deteriorates. Therefore, the lower limit of the cross-sectional reduction rate of the wire drawing was set to 25 + 82 × F1%. In addition, in order to soften the steel wire after low-temperature annealing, it is preferable that the cross-sectional reduction rate of wire drawing is 50% or more. Moreover, since it uses as a steel wire, the cross-sectional reduction rate of wire drawing is less than 100%.

In addition, the steel wire of the said embodiment is softened by low temperature annealing, and ductility improves. When the temperature of low-temperature annealing is less than 650 degreeC, intensity | strength is high and the effect of soft nitriding is small. When the temperature of low-temperature annealing is A _c1 or more, a pearlite structure mixes in the structure of the steel wire after annealing, and the strength and ductility of steel wire deteriorate. For this reason, it is preferable to control the temperature of low temperature annealing to 650 degreeC or more and less than A _c1 point. Although the holding time of low temperature annealing is not specifically limited, It is preferable that they are 30 minutes or more and 7 hours or less in order to improve the stability of a quality, and productivity. In addition, A _C1 (degreeC) is computed by following formula (3).

Example

Table 1 shows the components of the test steel, the carbon equivalent [Ceq. (%)] Calculated by the general formula (1), and A _c1 (° C) calculated by the general formula (3). In addition, steel grade L is a comparative example with many Cr contents.

The steel strips of these steel grades are heated to 950-1150 degreeC, the wire rod rolling is carried out to 5.5-14.5 mm of wire diameter hotly, and after this wire rod rolling, the constant temperature transformation process is carried out on the conditions shown in Table 2 using the molten salt bath on a rolling line. Was performed and cooled. The wire rod after this cooling was wire-drawn at the cross-sectional reduction rate shown in Table 2. Table 2 shows each manufacturing condition such as molten salt bath temperature, molten salt bath holding time, and fresh cross section reduction rate, the volume fraction of the pearlite structure, the cornerstone ferrite structure and the bainite structure of each wire rod after constant temperature transformation treatment, and the average block particle diameter of the pearlite structure. And the tensile strength TS of the steel wire. In addition, in Table 2, the lower limit of the volume fraction of the pearlite structure calculated as 1.40 × (C%) × 100%, and the volume fraction of the cornerstone ferrite structure calculated as [1-1.25 × (C%)] × 50% The upper limit, the lower limit of the wire cross-section reduction rate computed by 25 + 82xF1, and the lower limit of the tensile strength calculated by 480 + 850xCeq. Are also shown.

Level 13 and 15 of Table 2 are the conventional manufacturing methods which cooled the steel wire on the Stelmore, without performing constant temperature transformation process after winding up. Therefore, in these levels 13 and 15, the volume ratio of the pearlite structure was not enough, and the volume ratio of the cornerstone ferrite structure was excessive.

After heating the wire rod manufactured on each condition of Table 2 to 700 degreeC in the temperature increase time 4h, hold | maintaining for 5 h, the cold low temperature annealing process was performed, and the mechanical characteristic of the wire rod was evaluated.

In addition, Table 3 shows the mechanical properties of the steel wire of the comparative example manufactured by the following manufacturing method (conventional spheroidization annealing). First, the wire rod obtained by wire-rolling the steel piece of steel grades A-R on the same conditions as the above was cooled by the Stelmor after winding. Then, after carrying out the wire drawing of 25% of cross-sectional reduction rate, this wire rod was heated to 740 degreeC in the temperature increase time 4h, and it hold | maintained for 4h. Moreover, after cooling this heated wire rod to 15 degreeC / h to 650 degreeC, it cooled in air | atmosphere. In addition, in order to measure these mechanical characteristics, the tensile test based on the test method of JIS Z2241 was done using the 9A test piece of JIS Z2201, and the tensile strength (TS) and the cross-sectional shrinkage rate (RA) were evaluated.

Table 4 shows the tensile strength (TS) and cross-sectional shrinkage ratio (RA) after low temperature annealing. Table 4 also shows a comparison of the mechanical properties of the conventional spheroidized annealing material (usually annealing material) shown in Table 3. "Excellent" in Table 4 shows a characteristic superior to the conventional spheroidizing annealing material. Moreover, "possible" shows that the characteristic (tensile strength TS is less than +/- 10 Mpa, and cross-sectional shrinkage rate (RA) is less than +/- 2%) equivalent to the conventional spheroidizing annealing material. In addition, "impossible" has shown that the characteristic is inferior to the conventional spheroidizing annealing material.

Level 22 of Table 4 shows the characteristic of the steel wire of steel grade L with many Cr contents. At this level 22, even when the wire drawing of the cross-sectional reduction rate was 60%, the tensile strength TS after low temperature annealing was high, and the soft-nitriding characteristic of the steel wire was inferior to the conventional spheroidizing annealing material. On the other hand, as can be seen from the examples of levels 1, 2, 4, 6, 7, 11, 12, 14, 16, 17, 19, 21, 23 to 28 in Table 4, even at low temperature annealing, The mechanical properties of the steel wire produced by the same were equivalent or superior to those of the conventional spheroidized annealing material.

1 shows the relationship between the tensile strength TS of the steel wires of levels 8, 9, 10, 11, and 12 shown in Table 2, and the tensile strength TS after low-temperature annealing of these steel materials. Moreover, in the steel wires of these levels 8, 9, 10, 11, and 12, a steel component and a structure (a fraction of each structure) are equal, and tensile strength TS differs. As shown in FIG. 1, when tensile strength TS is 480 + 850xCeq.MPa or more (for example, 1064 degreeC or more), the tensile strength TS of the steel wire after low temperature annealing falls, and the steel wire softens It can be seen that.

2 shows the relationship between the carbon equivalents (Ceq.) Of the steel wires of the levels 1 to 28 shown in Table 4 and the levels 29 to 46 shown in Table 3, and the tensile strength (TS) after annealing. As shown in FIG. 2, in the Example of Table 4, compared with the conventional spheroidizing annealing material, it turns out that the tensile strength TS of a steel wire is low and a steel wire is softened.

3 shows the relationship between the carbon equivalent (Ceq.) Of the steel wires of the levels 1 to 28 shown in Table 4 and the levels 29 to 46 shown in Table 3, and the cross-sectional shrinkage ratio (RA) after annealing. In the Example of Table 4, compared with the conventional spheroidizing annealing material, it turns out that the cross-sectional shrinkage rate RA of a steel wire is high and it is excellent in ductility.

4 is a graph showing the relationship between the F1 value represented by the formula (2) of the steel wires of the levels 1 to 12, 14, 16 to 21, and 23 to 28 in Table 2, and the wire cross-sectional reduction rate. In addition, these steel wires satisfy the above-described structure (volume ratio of each tissue) and components. In the steel wire of "good soft-nitridation characteristic" in FIG. 4, the tensile strength TS of the steel wire after low temperature annealing shown in Table 4 is equal to or less than the conventional spheroidized annealing material. In the steel wire of "poor soft-nitridation characteristic", the tensile strength TS of the steel wire after low temperature annealing is higher than the tensile strength TS of the conventional spheroidizing annealing material. Thus, when the wire cross-section reduction rate is 25 + 82xF1% or more, it turns out that the soft-nitriding characteristic of the steel wire after low temperature annealing is excellent.

Fig. 5 shows the C content (C%) of the steel wires at levels 1, 2, 4, 6, 7, 11 to 17, 19, 21, 23 to 28 of Table 2 and the volume fraction (pearlite fraction) of the pearlite structure. The relationship is shown. When the volume ratio of the pearlite structure in the steel wire is smaller than 1.40 x (C%) x 100 (%) (levels 13 and 15), as shown in Table 4, the mechanical properties of the steel wire after low temperature annealing than conventional spheroidizing annealing materials You can see this falls.

Similarly, FIG. 6 has shown the relationship of C content of the steel wire of the level similar to the level used in FIG. 5, and the volume ratio (stone-stone ferrite fraction) of a cornerstone ferrite structure. When the volume fraction of the cornerstone ferrite structure is larger than [1-1.25 x (C%)] x 50 (%) (levels 13 and 15), as shown in Table 4, the steel wire after low temperature annealing than the conventional spheroidizing annealing material It can be seen that the mechanical properties of are inferior.

7 shows the relationship between the carbon equivalents (Ceq.) Of the steel wires at levels 1 to 21 and 23 to 28 in Table 2, and the tensile strength (TS) before annealing. When the tensile strength TS is 480 + 850 × Ceq.MPa or more (levels 1, 2, 4, 6, 7, 11, 12, 14, 16, 17, 19, 21, 23 to 28), it is shown in Table 4 Similarly, after low-temperature annealing, it turns out that the characteristic more than equivalent to the conventional spheroidizing annealing material is acquired.

[Industrial Availability]

As described above, according to the present invention, it is possible to form the steel material into parts having a complicated shape by cold forging, to improve the yield and productivity of the steel material, and to reduce the machining cost of the parts. Moreover, according to this invention, the temperature of soft-nitriding annealing can be reduced, heat processing cost can be reduced, and productivity can be improved. For this reason, the steel wire which concerns on this invention is suitable for using as a raw material of mechanical components, such as a bolt, a screw, and a nut shape | molded by cold forging.

Claims (4)

In mass%,
C: 0.10 to 0.60%,
Si: 0.01 to 0.40%,
Mn: 0.20 to 1.50%,
P: 0% to 0.040%,
S: 0% to 0.050%,
N: 0.0005 to 0.0300%,
Cr: 0.03 to 0.4%,
V: 0.03 to 0.2%,
Mo: 0.03 to 0.2%
It further contains one or more of these, the remainder is composed of Fe and unavoidable impurities, has a cornerstone ferrite structure, a pearlite structure and a metal structure including bainite structure, the volume ratio of the pearlite structure is 1.40 × ( C%) × 100% or more and 100% or less, the volume fraction of the cornerstone ferrite is 0% or more and [1-1.25 × (C%)] × 50% or less, and the volume ratio of the bainite structure is 0%. It is 40% or less, the sum of the volume fraction of the saltpeter ferrite structure, the volume fraction of the bainite structure, and the volume fraction of the pearlite structure is 95% or more and 100% or less, and the tensile strength is 480 + 850 × Ceq.MPa or more. Moreover, it is 580 + 1130 * Ceq.MPa or less, The steel wire for low temperature annealing characterized by the above-mentioned.
However, Ceq. = (C%) + (Si%) / 7+ (Mn%) / 5+ (Cr%) / 9+ (Mo%) / 2 + 1.54 × (V%). The method according to claim 1, wherein in mass%,
Al: 0.001-0.060%,
Ti: 0.002 to 0.050%,
Nb: 0.005 to 0.100%,
B: 0.0001 to 0.0060%,
Cu: 0.01-0.3%,
Ni: 0.01 to 0.7%,
Ca: 0.0001 to 0.010%,
Mg: 0.0001 to 0.010%,
Zr: 0.0001 to 0.010%
The steel wire for low temperature annealing which further contains 1 or more types of them. The steel wire for low temperature annealing according to claim 1 or 2, wherein the average block size of the pearlite structure is 4 µm or more and 20 µm or less. The steel piece which has a composition of Claim 1 or 2 is heated,
Hot rolled,
After that, winding up,
Thereafter, the mixture was kept at a constant temperature for 30 seconds to 150 seconds in a molten salt bath of 400 ° C or more and 600 ° C or less,
After that, cool down,
The manufacturing method of the low-temperature annealing steel wire characterized by performing the wire drawing whose cross-sectional reduction rate is 25 + 82 * F1% or more and less than 100%.
Provided that F1 = (Cr%) + (Mo%) / 4+ (V%) / 3.

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