KR102139255B1 - Steel wire with excellent delay resistance - Google Patents

Abstract

Chemical composition is by mass, C:0.60~1.1%, Si:0.05~1.5%, Mn:0.30~1.5%, P≤0.030%, S≤0.030%, Al:0.005~0.05%, N:0.001~ 0.006%, Cr: 0 to 1.5%, Ti: 0 to 0.02%, B: 0 to 0.005%, balance: Fe and impurities, metal structure made of pearlite, and cross section perpendicular to the longitudinal direction In, the steel wire having a bcc phase of {110} crystal plane with an orientation of 0.95 or more and a line diameter of 2.9 mm or more has a tensile strength of 2000 MPa or more, and is excellent in delayed fracture characteristics even in an environment where local corrosion occurs. For this reason, it is possible to cope with the enlargement of civil and architectural structures.

Description

Steel wire with excellent delay resistance

The present invention relates to a steel wire excellent in delayed fracture characteristics.

Steel wires freshly processed with pearlite are used in PC (prestressed concrete) steel wire, wire rope, PWS (parallel wire strand) for bridges, and the like. In recent years, the enlargement of civil engineering and building structures in which they are used has progressed, and the demand for cost reduction during construction is also increasing. In order to realize such a request, it is necessary to increase the strength of the steel wire.

Conventionally, it is known that high-carbon steel wires, such as PC steel wires, have superior delayed fracture characteristics compared to materials of tempered martensite structures. However, especially in a high-strength region of 2000 MPa or more, the delayed fracture characteristics are deteriorated even in high-carbon steel wires, and the risk of delayed fractures increases.

Therefore, conventionally, high-strength, freshly processed pearlite steel wires having been considered for delayed fracture have been studied. For example, in Patent Document 1, a PC steel wire excellent in delayed fracture characteristics with a limited amount of compressive residual stress applied to the surface layer portion, and Patent Document 2 has a high strength for a steel cord having a microstructure in which cementite is finely divided. In steel wire material and patent document 3, a bainite PC steel bar having a <110> aggregate structure is disclosed.

Japanese Patent Publication No. 2004-131797 Japanese Patent Publication No. Hei 11-269607 Japanese Patent Publication No. Hei 7-268545

The PC steel wire disclosed in Patent Literature 1 has excellent delay-destructive fracture characteristics. However, when local corrosion occurs and the inner side becomes a stress concentration portion than the surface layer having a compressive residual on the surface, it is assumed that sufficient delayed fracture characteristics cannot be obtained.

The high-strength steel wire material disclosed in Patent Document 2 is suitable for use in steel cords having a very fine diameter, since the strength after final drawing is high and does not cause longitudinal cracking in the salting test. However, it is difficult to use for large-sized civil and architectural structures.

Since the PC steel bar disclosed in Patent Document 3 has a small carbon content of 0.1 to 0.4% by mass, a high strength of 2000 MPa or more cannot be obtained with a tensile strength.

An object of the present invention is to provide a steel wire excellent in delayed fracture characteristics (particularly in an environment where local corrosion occurs).

This invention was made|formed in order to solve the said subject, and makes the steel wire excellent in the delayed fracture property shown below a summary.

(1) Chemical composition is in mass%,

C: 0.60-1.1%,

Si: 0.05-1.5%,

Mn: 0.30-1.5%,

P:0.030% or less,

S: 0.030% or less,

Al: 0.005 to 0.05%,

N:0.001~0.006%,

Cr: 0-1.5%,

Ti: 0~0.02%,

B: 0~0.005%,

Residue: made of Fe and impurities,

In the cross-section in which the metal structure is made of pearlite and perpendicular to the longitudinal direction, the orientation of the {110} crystal plane of the bcc phase is 0.95 or more,

Linear diameter, 2.9 mm or more,

Steel wire with excellent delay resistance.

(2) The chemical composition is in mass%,

Cr: containing 0.10 to 1.5%,

The steel wire excellent in the delayed-release fracture property described in (1) above.

(3) The chemical composition is in mass%,

Ti: 0.003 to 0.02%, and,

B: 0.0005~0.005%,

Containing one or more selected from,

The steel wire excellent in the delayed-destruction property described in (1) or (2) above.

ADVANTAGE OF THE INVENTION According to this invention, the steel wire excellent in the delayed fracture property with a tensile strength of 2000 MPa or more can be obtained.

Fig. 1 is a diagram showing the results of the tests of the examples taken together by taking a delayed breaking strength ratio and a tensile strength on the vertical axis and the horizontal axis, respectively.

In order to solve the above-mentioned problems, the present inventors have studied in detail the amount of strain in fresh processing and the delayed fracture characteristics. As a result, the following important knowledge was obtained.

(a) A steel wire made of pearlite with a metal structure has a bcc phase of the {110} crystal plane in the cross section perpendicular to the longitudinal direction (hereinafter sometimes simply referred to as "orientation of the {110} crystal plane"). When it is 0.95 or more, the delayed fracture property is remarkably improved.

(b) When the total strain in cold drawing processing of 2.3 or more is added to a steel wire whose metal structure is made of pearlite, the orientation degree of the {110} crystal plane of the bcc phase can be made 0.95 or more.

This invention was completed based on the above knowledge. Hereinafter, each requirement of this invention is explained in full detail.

(A) Chemical composition:

The reason for limiting the chemical composition of the steel wire according to the present invention is as follows. In the following description, "%" of the content of each element means "mass%".

C: 0.60 to 1.1%

C is an essential element in securing the strength of the freshly processed pearlite steel wire. When the content of C is less than 0.60%, even when the temperature is maintained at a suitable temperature range of 650 to 550°C, which will be described later, since the amount of ferrite is increased, the required strength (tensile strength of 2000 MPa or more) cannot be obtained. . On the other hand, when the content of C exceeds 1.1%, the amount of cementite cementite increases, and the drawing property is significantly deteriorated, and suitable cold drawing process of total deformation of 2.3 or more, which will be described later, cannot be performed. Therefore, the content of C is set to 0.60 to 1.1%. The preferable lower limit of the C content is 0.80%, and the preferred upper limit is 1.0%.

Si: 0.05-1.5%

Si has an effect of increasing strength by solid solution strengthening, and is an effective element to obtain strength. When the Si content is less than 0.05%, the above effects cannot be exhibited. On the other hand, if the Si content is too large, the precipitation of the cornerstone ferrite is promoted, the limit workability in the drawing process decreases, and a suitable cold drawing process with a total deformation of 2.3 or more described later cannot be performed. For this reason, the content of Si is set to 0.05 to 1.5%. The preferable lower limit of the Si content is 0.10%, and the preferred upper limit is 1.0%.

Mn: 0.30~1.5%

Mn is an element necessary not only for deoxidation and desulfurization, but also to form a lamella stably in a pearlite transformation process and to obtain a tensile strength of 2000 MPa or more. When the content of Mn is less than 0.30%, the above-mentioned effect cannot be obtained. On the other hand, even if it contains more than 1.5%, an effect suitable for the amount cannot be obtained. For this reason, the content of Mn is made 0.30 to 1.5%. The preferable lower limit of the Mn content is 0.40%, and the preferred upper limit is 0.90%.

P: 0.030% or less

P is contained as an impurity and segregates at grain boundaries, thereby deteriorating the delayed fracture characteristics. For this reason, the content of P is made 0.030% or less. It is preferable that the content of P is as low as possible.

S: 0.030% or less

S is contained as an impurity and segregates at grain boundaries, thereby deteriorating the delayed fracture characteristics. For this reason, the content of S is made 0.030% or less. It is preferable that the content of S is as low as possible.

Al: 0.005~0.05%

Al is an element effective as a deoxidizer, and also has an effect of refining austenite particles by generating nitride. However, when the content of Al is less than 0.005%, these effects are insufficient, and even if the content is more than 0.05%, the effect is saturated. For this reason, the content of Al is made 0.005 to 0.05%. The preferable lower limit of the Al content is 0.02%, and the preferred upper limit is 0.04%. In addition, the Al content of this invention refers to content in total Al.

N:0.001~0.006%

N has the effect of refining austenite particles by producing a nitride of Al. If the content of N is less than 0.001%, this effect is insufficient, while when it exceeds 0.006%, cold workability is reduced. For this reason, the N content is set to 0.001 to 0.006%. The preferable lower limit of the N content is 0.002%, and the preferred upper limit is 0.005%.

Cr: 0~1.5%

Cr is an element effective to refine the lamellar spacing of pearlite and improve strength. For this reason, you may contain Cr as needed. However, if the Cr content is too large, the transformation end time becomes long, and even if it is maintained in a suitable temperature range of 650 to 550° C. described later, pearlite transformation may not be completed and martensite may occur. Therefore, the upper limit of the Cr content when contained is set to 1.5%. The upper limit of the Cr content is preferably 0.60%. In addition, in order to obtain the above effects stably, the lower limit of the Cr content is preferably 0.10%.

Ti:0~0.02%

Ti is a deoxidizing element and has an effect of fixing solid solution N to improve the workability of drawing. For this reason, you may contain Ti as needed. However, when the content of Ti exceeds 0.02%, the effect may be saturated and coarse oxides may be formed, thereby deteriorating cold workability. Therefore, the upper limit of the Ti content when contained is set to 0.02%. In addition, in order to obtain the above effects stably, the lower limit of the Ti content is preferably 0.003%.

B: 0~0.005%

B has an effect of suppressing the formation of a cornerstone ferrite and increasing the tensile strength after pearlite transformation. For this reason, you may contain B as needed. However, even if the content of B exceeds 0.005%, the above effect is saturated. Therefore, the upper limit of the B content when contained is set to 0.005%. In addition, in order to obtain the above effects stably, the lower limit of the B content is preferably 0.0005%.

In the steel wire according to the present invention, the balance is Fe and impurities.

Here, "impurity" means a component that is incorporated by various factors in raw materials such as ores and scraps when industrially manufacturing a steel material, and various factors in the manufacturing process, and is allowed within a range that does not adversely affect the present invention. .

(B) Metal structure:

The metal structure of the steel wire according to the present invention is made of pearlite, and in the cross section perpendicular to the longitudinal direction, the orientation of the {110} crystal plane of the bcc phase is 0.95 or more. For this reason, as shown in the Examples described later, it is possible to achieve both high strength and excellent delayed fracture properties of 2000 MPa or more at a tensile strength. The preferable lower limit of the orientation degree is 0.97. On the other hand, in the case of a steel wire having a final linear diameter of 2.9 mm or more, about 0.99 is the upper limit of the orientation. In addition, the metal structure of the steel wire according to the present invention made of pearlite may contain, as an area ratio, 5% or less of either a cornerstone ferrite or a cornerstone cementite alone, or a combination of both cornerstone ferrite and a cornerstone cementite in a total amount of 5% or less.

The orientation degree of the {110} crystal plane on the bcc phase is X-ray diffracted in a cross section perpendicular to the longitudinal direction of the steel wire (cross section perpendicular to the fresh line processing direction), the integral intensity of each crystal plane is obtained, and calculated by the following equation.

F=(PP ₀ )/(1-P ₀ )

P=ΣI(110)/ΣI(hkl)

In the above two formulas, "F" is the orientation of the {110} crystal plane of the bcc phase, and "I(110)" and "I(hkl)" are the bcc phases in the cross section perpendicular to the drawing direction ( The integral strength of the 110) plane and the (hkl) plane, "P ₀ ", are values in an unoriented sample. In the examples described later, (110), (200), and (211) are used for the crystal plane, and the data of the non-alcoholic sample is the intensity described in the powder X-ray diffraction database (PDF (Powder Diffraction File)). The figures were used.

Further, the structure of the steel wire of the present invention is pearlite. The pearlite is one in which a ferrite phase and a cementite phase form a layered structure. Therefore, the orientation degree of the {110} crystal plane of the bcc phase is substantially the orientation degree of the {110} crystal plane of the ferrite constituting pearlite. However, as described above, there may be a case where a trace amount of gemstone ferrite of 5% or less is included. In this case, the orientation of the {110} crystal plane of the ferrite constituting pearlite and the orientation of the {110} crystal plane of the cornerstone ferrite cannot be determined separately. Therefore, the orientation degree of the {110} crystal plane of the ferrite constituting pearlite is not defined, but it is defined as the orientation degree of the {110} crystal plane of the bcc phase.

(C) Linear diameter:

The wire diameter of the steel wire according to the present invention (the final wire diameter of the steel wire) is 2.9 mm or more. This is because, in PC steel wires, etc., PC steel wires corrode due to the cracking of concrete, and in particular, in the case of small diameters with a line diameter of less than 2.9 mm, they are not delayed to break but shorten their life due to corrosion. Because there is. It is preferable that the said linear diameter is 3.0 mm or more. Although there is no particular limitation on the line diameter, an industrial upper limit of 7 mm is reasonable.

(D) Manufacturing method:

The steel wire of this invention can be suitably manufactured, for example by the method shown below. Moreover, it is not limited to this method.

After the low alloy network having the chemical composition described in the above (A) is melted, it is used as an ingot or cast by casting. Next, the cast ingot or cast piece is subjected to hot working such as hot rolling and hot forging to produce a steel piece, and the steel piece is rolled to finish with a steel or wire rod having a circular cross section. Thereafter, the steel bar or the wire rod may be freshly processed in an appropriate manner as necessary to form a steel wire. With respect to the steel bar, the wire rod, and the steel wire (hereinafter collectively referred to as "round steel") having a circular cross section, the steps from step (i) to step (iv) described below are sequentially performed, The steel wire excellent in the delayed fracture property of the present invention is produced. In addition, the aging treatment of step (v) may be performed after step (iv).

Process (i): Process of austenitizing by heating at 850~1050℃ for 5~30 minutes

When the austenitization temperature is less than 850°C, austenitization may be insufficient. On the other hand, when the austenitization temperature exceeds 1050°C, coarsening of the austenite particles occurs, and the fresh workability decreases, and cold drawing may not be performed in which the total deformation of step (iv) is 2.3 or more. For this reason, the austenitization temperature is set to 850 to 1050°C. The lower limit of the austenitization temperature is preferably 900°C. From the viewpoint of refinement of the austenite particles, the preferred upper limit of the austenitizing temperature is 1000°C, and the more preferred upper limit is 950°C. In addition, the said austenitization temperature points to the temperature in the surface of a round steel material.

Even in the above temperature range, when the austenitization time is less than 5 minutes, austenitization may be insufficient, and when it exceeds 30 minutes, the heating cost only increases. For this reason, the austenitization time is 5 to 30 minutes. The preferred lower limit of the austenitization time is 10 minutes, and the preferred upper limit is 20 minutes.

Process (ii): A process of cooling to a temperature range of 650 to 550°C at a cooling rate of at least 1°C/sec, and maintaining the temperature range for 1 to 30 minutes

The round steel material austenitized in step (i) is quenched to a temperature range of 650 to 550°C with a cooling rate of 1°C/sec or more, and maintained for 1 to 30 minutes in the temperature range to fine-tune the metal structure. Pearlite. When the cooling rate after austenitization is less than 1°C/sec, pearlite transformation starts before reaching the above-mentioned holding temperature range and becomes a coarse pearlite structure, so that cracking may occur during cold drawing. In addition, there may be cases where it is not possible to achieve a high strength with a tensile strength of 2000 MPa or more and excellent flame retardant fracture properties, such as by forming precipitated ferrite or precipitated by cementite cement before the initiation of pearlite transformation by maintenance in the above temperature range. In addition, the upper limit of the cooling rate after austenitization is industrially about 200°C/sec.

Even if the above cooling rate is 1°C/sec or more, when the cooling temperature exceeds 650°C, the pearlite block size becomes large, and cold drawing may not be carried out with a total deformation of 2.3 or more in step (iv). . On the other hand, when the cooling temperature is less than 550° C., the completion time of pearlite transformation may be long or martensite may be generated.

Moreover, when the holding time in the temperature range of 650 to 550°C is less than 1 minute, the pearlite transformation may not be completed due to the size of the round steel material and/or the influence of the containing element, and, on the other hand, exceeds 30 minutes. With long-term maintenance, the manufacturing cost increases. The preferable lower limit of the holding time is 3 minutes, and the preferred upper limit is 10 minutes.

The cooling rate in the step (ii) indicates the average cooling rate on the surface of the round steel material. In addition, the temperature range to be cooled and maintained indicates, for example, a set temperature of an isothermal transformation treatment facility having good thermal conductivity such as a salt bath and a bath.

Process (iii): Process to cool to room temperature

After the process (ii) is completed, the round steel material is cooled to room temperature. The cooling rate at this time is not particularly limited.

Process (iv): Process of performing cold drawing of 2.3 or more by total deformation and making the final wire diameter into a steel wire of 2.9 mm or more

The round steel material having the chemical composition described in the above (A) and carrying out the steps from step (i) to step (iii) in order is cold drawn. In particular, by setting the total strain by cold drawing to 2.3 or more, a high strength of 2000 MPa or more can be provided with a tensile strength, and the orientation of the {110} crystal plane of the bcc phase can be made 0.95 or more. For this reason, the total deformation by cold drawing is set to 2.3 or more. The preferred lower limit of the total strain of cold drawn processing is 2.5, and the preferred upper limit is 3.0. If the total deformation is 2.3 or more, the number of cold drawn processing is not particularly limited, and may be one or more times. However, cold drawing in the step (iv) needs to be performed without softening the round steel material cooled to the room temperature in the step (iii). In addition, the total deformation ε is a value obtained using the following equation.

ε=ln(A ₀ /Af)

However, "A ₀ "and "A _f " respectively refer to the cross-sectional area of the round steel material before cold drawing and the cross-sectional area of the steel wire after final cold drawing.

Moreover, you may perform the descaling process by pickling etc. before cold drawing process to the round steel material cooled to room temperature as needed. In addition, it is preferable to perform lubrication by a suitable method during cold drawing of the round steel material.

Process (v): Process of aging by heating at 200~450℃ for 10 seconds~30 minutes

After the cold drawing, the aging treatment may be performed by heating the steel wire at 200 to 450° C. for 10 to 30 minutes to remove residual strain. This is because the effect cannot be sufficiently obtained when the heating temperature of the aging treatment is less than 200°C, and when the temperature exceeds 450°C, the tensile strength is significantly reduced. In addition, if the holding time in the temperature range of 200 to 450°C is less than 10 seconds, the effect cannot be sufficiently obtained, and even if it is maintained for more than 30 minutes, the effect is saturated and only increases the manufacturing cost. The aging treatment temperature indicates the temperature of the surface of the steel wire. In addition, cooling in the aging treatment is preferably cooled in the air.

Hereinafter, the present invention will be described in more detail by examples, but the present invention is not limited to these examples.

Example

Steels A to R having the chemical composition shown in Table 1 were melted, and the ingots poured into the mold were heated to 1250° C., and hot-forged steel was used as a round steel material (wire) having a diameter of 20 mm.

Steels A to L and steels N to R in Table 1 are steels whose chemical composition is within the range specified in the present invention. On the other hand, steel M is a steel whose chemical composition deviates from the conditions specified in the present invention.

The wire (round steel) having a diameter of 20 mm obtained as described above was pickled with hydrochloric acid at room temperature, subjected to a phosphate coating treatment, and preliminarily drawn to obtain a steel wire (round steel) having a diameter shown in Table 2.

Next, each steel wire obtained by preliminary drawing was heat-treated for 10 minutes at a temperature shown in Table 2, and then austenitized, followed by holding for 1 minute in a bath of the temperature shown in Table 2 to carry out transformation treatment. At that time, a thermocouple was installed on the steel wire and the cooling rate was measured. The cooling rate from the heating temperature of the steel wire to the transformation temperature (bath temperature) was 7 to 60°C/sec. Moreover, it cooled by water after a transformation process.

After cooling, the steel wire was pickled with hydrochloric acid at room temperature, subjected to a phosphate coating treatment, and was not subjected to a softening treatment in the middle, and cold drawn to the final linear diameter under the conditions shown in Table 2. For some of the steel wires, "aging treatment" was performed after drawing and heating for 5 minutes in the air at a temperature shown in Table 2 to cool.

Various investigations shown below were performed using each steel wire having the final line diameter.

<1> area ratio of pearlite:

For each steel wire of the final linear diameter, the cross section perpendicular to the longitudinal direction is mirror-polished, then etched with a picral solution, and (1/4)D of the cross section with a scanning microscope (where "D" is the diameter of the steel wire. In the position of ), an arbitrary 8 field of view was observed at 5000 times, a picture was taken, the pearlite portion was visually determined, and image analysis was performed to obtain the area percentage of pearlite in the metal structure.

<2> tensile characteristics:

Tensile test pieces of No. 9B were taken from each steel wire of the final linear diameter according to JISZ2241 (2011), and subjected to tensile testing in an atmosphere at room temperature to obtain tensile strength.

<3> Delay resistance characteristics:

For the test number obtained by the tensile strength of 1700 MPa or more in the irradiation of <2>, using the test piece provided with a cut having a depth of 0.5 mm, an angle of 60° and a cut bottom radius of 0.1 mm in each steel wire of the final line diameter, the following method The delayed delay characteristics were investigated.

At room temperature, a positive load stress was applied to the test piece in a polarized environment of -1.2 V with respect to the Ag/AgCl electrode in 3% by mass saline, hydrogen charge was started immediately, and a test of up to 200 hours was performed. In addition, various static load load stresses in the above environment were varied to determine the maximum load stress (T1) that did not break. Similarly, in the air at room temperature, a tensile test was performed using the test piece provided with the cut, the fracture stress (T2) in the air was determined, and the value obtained by dividing T1 by T2 was used as the delayed breaking strength ratio. In addition, the delayed fracture strength ratio is closer to 1, the better the delayed fracture property.

Orientation of the {110} crystal plane on the <4> bcc phase:

For the test number obtained by tensile strength of 1700 MPa or more in the irradiation of <2> above, for each steel wire of the final line diameter, by the method described in (B) above, the {110} crystal plane of the bcc phase in the metal structure The orientation degree (F) was calculated.

Table 2 shows the results of each survey. In Fig. 1, delayed fracture strength ratio and tensile strength are respectively taken on the vertical axis and the horizontal axis, and the delayed fracture characteristics of each steel wire are compared and shown.

From Table 2 and FIG. 1, it is clear that Test Nos. 1 to 25 of the present invention examples are superior in both tensile strength and delayed fracture properties, as compared to Test Nos. 26 to 29 in Comparative Examples.

In the case of Test Nos. 26 to 28 in Comparative Examples, the chemical compositions of the steel A and the steel B used were all within the ranges specified in the present invention, but the orientation of the {110} crystal plane of the bcc phase was small, from 0.76 to 0.92, and was defined in the present invention. Since it deviates from the conditions described above, it is separated from both the tensile strength and the delayed fracture property compared to the present invention.

In the test number 29 of the comparative example, since the C content of the steel M used was as low as 0.38%, and deviated from the conditions specified in the present invention, the tensile strength was only 1458 MPa, and it was very far from the inventive example.

Industrial availability

The steel wire excellent in the delayed fracture property of the present invention has a tensile strength of 2000 MPa or more, and is excellent in the delayed fracture property even in an environment where local corrosion occurs, and thus can cope with the enlargement of civil and architectural structures. For this reason, the industrial contribution of the present invention is very remarkable.

Claims (5)

Chemical composition, in mass%,
C: 0.60-1.1%,
Si: 0.05-1.5%,
Mn: 0.30-1.5%,
P:0.030% or less,
S: 0.030% or less,
Al: 0.019-0.05%,
N:0.001~0.006%,
Cr: 0-1.5%,
Ti: 0~0.02%,
B: 0~0.005%,
Residue: made of Fe and impurities,
The metal structure is made of pearlite, and in the cross section perpendicular to the longitudinal direction, the orientation of the {110} crystal plane of the bcc phase is 0.95 or more,
A steel wire having a line diameter of 2.9 mm or more and excellent in delayed fracture characteristics. The method according to claim 1,
The chemical composition, in mass%,
A steel wire having Cr:0.10 to 1.5% and having excellent delayed fracture properties. The method according to claim 1 or claim 2,
The chemical composition, in mass%,
Ti: 0.003 to 0.02%, and,
B: 0.0005~0.005%,
Steel wire excellent in delayed-breaking properties, containing one or more selected from. delete delete

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