JP2015212412A - Hot rolled wire - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot rolled wire having high strength and excellent in SSC resistance.SOLUTION: There is provided a hot rolled wire containing, by mass%, C:0.20 to 0.5%, Si:0.05 to 0.3%, Mn:0.3 to 1.5%, Al:0.001 to 0.1%, P:over 0% and 0.01% or less and S:over 0% and 0.01% or less and the balance iron with inevitable impurities, and having 30 or less of a segregation degree (S/S) which is a maximum value of S amount S(mass%) to an average value of S amount S(mass%) by measuring the S amount contained in the hot rolled wire with 200 μm interval at 300 or more points by using an electron beam microanalyzer.

Description

  The present invention relates to a hot rolled wire used for manufacturing a steel wire. More specifically, the present invention relates to a hot-rolled wire used for producing a steel wire used as a reinforcing material for a part used in a sour environment containing hydrogen sulfide or a flexible riser.

  The demand for oil has been increasing in recent years, and the development of subsea oil fields is being carried out. In oil field development, for example, a flexible riser is used to pump up crude oil. The flexible riser is manufactured using a resin pipe and a steel wire, and the steel wire is used as a reinforcing material for the resin pipe. Since the oil field is in a sour environment containing hydrogen sulfide, the steel wire has high strength and is capable of suppressing sulfide stress cracking (SSC) (hereinafter referred to as SSC resistance). Is required). Therefore, the hot-rolled wire used as the material for the steel wire is also required to have high strength and excellent SSC resistance.

  As a technique for providing a high-strength steel material having excellent SSC resistance, Patent Document 1 is known. The steel materials disclosed in this document are in mass%, C: 0.25 to 0.35%, Si: 0.10 to 0.30%, Mn: 0.8% or less, P: 0.010% Hereinafter, S: 0.003% or less, Al: 0.003-0.1%, N: 0.0040% or less, Cr: 0.5-0.7%, Mo: 0.5-1.0% Cu: 0.05-0.8%, Ti: 0.015-0.030%, Nb: 0.005-0.025%, V: 0.05-0.10%, B: 0.0005 Containing 0.005% and containing P, Ti, and N so as to satisfy P / effective Ti content <1.6, the balance consisting of Fe and inevitable impurities, and the prior austenite grains A tempered marte having an average particle diameter of 12 μm or less, a Mo segregation degree of 1.5 or less, and an average austenite grain size of 12 μm or less. Has an organization consisting of an insight phase.

  In the above document, as a method for producing the high-strength steel material, a high-temperature heat treatment is performed on a steel material satisfying the above component composition at a heating temperature in the range of more than 1200 ° C. to less than 1270 ° C. for 30 minutes. After that, the steel material is hot-worked to obtain a hot-rolled steel material, and then the hot-rolled steel material is subjected to a quenching treatment at least twice after being subjected to a tempering treatment. : Holding at a temperature in the range of 850 to 920 ° C. for 5 to 10 minutes, and then rapidly cooling to room temperature at an average cooling rate of 30 ° C./s or more, and the tempering treatment at a temperature in the range of 600 to 680 ° C. It is described that the process is held for 15 to 30 minutes.

JP 2013-227611 A

  The steel material disclosed in the above-mentioned Patent Document 1 is uniform in the distribution of alloy elements such as C, Cr, Mo, Nb, etc., reduces macro segregation, and further thoroughly suppresses coarse inclusions. The yield strength (hereinafter sometimes referred to as YS; YS is an abbreviation for Yield Strength) is 120 ksi (827 MPa) class or higher. However, the required characteristics in recent years have become stricter, and further enhancement of strength and improvement of SSC resistance are required.

  The present invention has been made paying attention to the above circumstances, and an object of the present invention is to provide a hot-rolled wire rod having high strength and excellent SSC resistance.

  The inventors of the present invention have made extensive studies in order to increase the strength of the hot rolled wire rod and to further improve the SSC resistance. As a result, it has been clarified that, by properly controlling the component composition of the hot-rolled wire rod and reducing the segregation of S generated inside the wire rod, the SSC resistance can be improved while securing the strength. That is, S is easily segregated at the crystal grain boundary, and the grain boundary strength is reduced by the segregation of S. Therefore, when the embrittlement by hydrogen proceeds, the grain boundary breaks easily. As a result, it is considered that the SSC resistance decreases.

As a result of further studies by the present inventors, the segregation degree (S _max / S _ave ) calculated based on the average value S _ave (mass%) of the S amount and the maximum value S _max (mass%) of the S amount. ) Of 30 or less, it was found that a hot-rolled wire rod having high strength and excellent SSC resistance can be provided, and the present invention was completed.

That is, the hot-rolled wire rod according to the present invention that has solved the above-mentioned problems is in mass%, C: 0.20 to 0.5%, Si: 0.05 to 0.3%, Mn: 0.00. 3 to 1.5%, Al: 0.001 to 0.1%, P: more than 0%, 0.01% or less, and S: more than 0%, 0.01% or less, with the balance being iron and Inevitable impurities. Then, the amount of S contained in the hot-rolled wire is measured at 300 or more locations at intervals of 200 μm using an electron beam microanalyzer, and the maximum amount S _max (mass) of the amount of S relative to the average value S _ave (mass%) of the amount of S. %) Is the degree of segregation (S _max / S _ave ), the gist is that the degree of segregation satisfies 30 or less.

The above hot-rolled wire material is further in mass% as another element,
(A) Cr: more than 0%, 1% or less, and B: more than 0%, 0.01% or less, (b) Ni: more than 0%, 0.5% or less, and Cu: 0% More than 0.5% or less,
(C) Ti: more than 0%, 0.1% or less and V: more than 0%, 0.5% or less,
(D) Mo: more than 0%, 1.5% or less,
(E) Nb: more than 0%, 0.1% or less,
Etc. may be included.

  According to the present invention, since the segregation of S generated inside the wire is reduced after appropriately controlling the component composition of the hot-rolled wire, the hot-rolled wire has high strength and excellent SSC resistance. Can provide.

  The hot rolled wire according to the present invention has a segregation degree of S of 30 or less, preferably 28 or less, more preferably 27 or less, and is preferably as small as possible.

The degree of segregation means that the amount of S contained in the hot-rolled wire is measured at 200 or more locations at intervals of 200 μm using an electron beam microanalyzer, the average value of S is S _ave (mass%), and the maximum amount of S When the value is S _max (mass%), it means the ratio of the maximum value S _max of the S amount to the average value S _ave of the S amount. Therefore, when segregation of S is not recognized, the average value S _ave (mass%) of the S amount is equal to the maximum value S _max (mass%) of the S amount, so the segregation degree (S _max / S _ave ) is 1

  The amount of S contained in the hot-rolled wire may be measured by performing element mapping toward the surface layer including the center of the hot-rolled wire, and may be measured so as not to be biased toward the center or the surface layer.

  The hot rolled wire rod of the present invention satisfies the segregation degree of 30 or less, and the component composition thereof needs to be appropriately controlled. That is, the hot-rolled wire rod of the present invention is mass%, C: 0.20 to 0.5%, Si: 0.05 to 0.3%, Mn: 0.3 to 1.5%, Al: 0.001 to 0.1%, P: more than 0%, 0.01% or less, and S: more than 0%, 0.01% or less.

  C is an element necessary for ensuring the strength of the wire, and is contained by 0.20% or more. The amount of C is preferably 0.22% or more, more preferably 0.23% or more. However, when the amount of C exceeds 0.5%, segregation of S is promoted, and the SSC resistance is lowered. Therefore, the C content is 0.5% or less, preferably 0.48% or less, more preferably 0.47% or less.

  Si is an element necessary for deoxidation and solid solution strengthening, and is 0.05% or more. The amount of Si is preferably 0.06% or more, more preferably 0.07% or more. However, as the amount of Si increases, S segregates and is liable to cause hydrogen embrittlement, and the SSC resistance decreases. Accordingly, the Si amount is set to 0.3% or less. The amount of Si is preferably 0.27% or less, more preferably 0.25% or less.

  Mn is an element that improves the hardenability and increases the strength of the wire, and it is necessary to contain 0.3% or more. The amount of Mn is preferably 0.4% or more, more preferably 0.45% or more. However, an excessive amount of Mn promotes segregation of impurity elements, particularly S. In addition, the strength becomes too high, the hardness becomes high, and the SSC resistance decreases. Therefore, the amount of Mn is 1.5% or less, preferably 1.40% or less, more preferably 1.30% or less.

  Al, like Si, is an element added for deoxidation, and is contained in an amount of 0.001% or more. The amount of Al is preferably 0.003% or more, and more preferably 0.005% or more. However, if the Al content exceeds 0.1%, the toughness of the wire decreases. Therefore, the Al content is 0.1% or less. The amount of Al is preferably 0.09% or less, more preferably 0.08% or less.

  P is an element that segregates at the crystal grain boundary, lowers the grain boundary strength, and easily causes grain boundary breakage due to hydrogen. Therefore, the P content is 0.01% or less. The amount of P is preferably 0.009% or less, more preferably 0.008% or less. The amount of P is preferably reduced as much as possible. However, since it is expensive to make the amount of P less than 0.0001%, it is preferably 0.0001% or more.

  S is an element that segregates at the crystal grain boundary or the central part of the wire, lowers the grain boundary strength, and easily causes grain boundary breakage due to hydrogen. In particular, in a sour environment containing hydrogen sulfide, hydrogen easily penetrates into the wire, so that the SSC resistance decreases. Therefore, the S content is 0.01% or less. The amount of S is preferably 0.009% or less, more preferably 0.008% or less. The amount of S is preferably reduced as much as possible. However, since it is expensive to make the amount of S less than 0.0001%, it is preferably 0.0001% or more.

The component composition of the hot rolled wire according to the present invention is as described above, and the balance is iron and inevitable impurities. The above-mentioned wire rod is as another element,
(A) Cr: more than 0%, 1% or less, and B: more than 0%, 0.01% or less, (b) Ni: more than 0%, 0.5% or less, and Cu: 0% More than 0.5% or less,
(C) Ti: more than 0%, 0.1% or less and V: more than 0%, 0.5% or less,
(D) Mo: more than 0%, 1.5% or less,
(E) Nb: It may contain more than 0%, 0.1% or less.

  (A) Cr and B are elements that effectively act to increase the hardenability and increase the strength of the wire. In order to effectively exhibit such an effect, Cr is preferably contained in an amount of 0.05% or more, more preferably 0.1% or more, and further preferably 0.3% or more. However, when Cr is excessive, pits are formed on the surface of the wire, and breakage due to hydrogen embrittlement tends to occur, and the SSC resistance deteriorates. Therefore, Cr is preferably 1% or less, more preferably 0.95% or less, and still more preferably 0.9% or less.

  B is preferably contained in an amount of 0.0005% or more, more preferably 0.0007% or more, and further preferably 0.001% or more. However, if B is excessive, cracks are likely to occur during hot rolling. Therefore, B is preferably 0.01% or less, more preferably 0.008% or less, and still more preferably 0.007% or less. Cr and B may be used alone or in combination.

  (B) Ni and Cu are elements that form a film on the surface of the wire, prevent entry of hydrogen, and improve SSC resistance. In order to effectively exhibit such effects, Ni is preferably contained in an amount of 0.05% or more, more preferably 0.1% or more, and further preferably 0.12% or more. However, if Ni exceeds 0.5%, pits are formed on the surface of the wire, and breakage due to hydrogen embrittlement tends to occur, and the SSC resistance deteriorates. Therefore, Ni is preferably 0.5% or less, more preferably 0.47% or less, and still more preferably 0.45% or less.

  It is preferable to contain Cu 0.05% or more, More preferably, it is 0.1% or more, More preferably, it is 0.12% or more. However, even if Cu is contained in excess of 0.5%, the effect of addition is saturated, so Cu is preferably 0.5% or less. Cu is more preferably 0.47% or less, still more preferably 0.45% or less. Ni and Cu may be used alone or in combination.

  (C) Ti and V are elements that act effectively to form a hydrogen trap site and improve SSC resistance. That is, Ti is an element that combines with C or N in steel to form TiC or TiN serving as a hydrogen trap site, or a composite thereof, and improves SSC resistance. Ti is an element that refines crystal grains and improves toughness. In order to exhibit such an effect effectively, Ti is preferably contained in an amount of 0.005% or more, more preferably 0.01% or more, and further preferably 0.015% or more. However, when Ti is excessive, coarse TiN is generated and becomes a starting point of hydrogen embrittlement. Therefore, Ti is preferably 0.1% or less. Ti is more preferably 0.095% or less, still more preferably 0.09% or less.

  V is an element that combines with C in steel to form fine VC that becomes a hydrogen trap site and improves SSC resistance. If the amount of V is too small, it takes time to precipitate VC and the productivity is lowered. Therefore, V is preferably contained in an amount of 0.05% or more. V is more preferably 0.1% or more, and still more preferably 0.15% or more. However, when V is excessive, precipitated carbides increase and excess dislocations are likely to remain. Since dislocations have a hydrogen trapping effect, if excessive dislocations remain, hydrogen is excessively trapped and becomes the starting point of fracture. Therefore, V is preferably 0.5% or less, more preferably 0.45% or less, and still more preferably 0.4% or less. Ti and V may be used alone or in combination.

  (D) Mo is an element that suppresses segregation of S and effectively acts to improve SSC resistance. In order to exhibit such an effect effectively, it is preferable to make it contain 0.05% or more, More preferably, it is 0.1% or more, More preferably, it is 0.21% or more, Most preferably, it is 0.22% or more. However, when Mo is excessive, the hydrogen storage amount and the corrosion amount increase, so that the SSC resistance deteriorates. In addition, the cost of steel materials will rise. Mo is preferably 1.5% or less. Mo is more preferably 1.45% or less, still more preferably 1.4% or less, and particularly preferably 1.3% or less. Mo may be 1% or less, and further 0.98% or less. In particular, Mo may be 0.95% or less, or 0.7% or less.

  (E) Nb is an element that refines crystal grains and improves toughness. Nb is an element that improves the corrosion resistance. In order to exhibit such an effect effectively, it is preferable to make it contain 0.01% or more, More preferably, it is 0.03% or more, More preferably, it is 0.05% or more. However, when Nb is contained excessively, the toughness may be lowered. Nb is preferably 0.1% or less, more preferably 0.095% or less, and still more preferably 0.085% or less.

  Next, a method for producing a hot rolled wire according to the present invention will be described.

  The method for producing the hot-rolled wire rod of the present invention is not particularly limited, and a steel that satisfies the above component composition is melted by a conventional method, and a steel piece obtained by split rolling is heated and hot-rolled. Can be manufactured.

  The heating temperature of the steel slab may be 700 to 1000 ° C., for example, and hot rolling may be performed in this temperature range.

  The hot rolling may be performed using a rolling mill comprising a multi-stand rough rolling mill, an intermediate rolling mill, and a finishing rolling mill. In the present invention, the total rolling strain in the initial three passes in the rough rolling mill is calculated. It is recommended to be 0.3 or more. The initial three passes mean the first three rolling mills in the rough rolling mill.

  Dynamic recrystallization can be caused by setting the total rolling strain in the initial three passes to 0.3 or more. As a result, S can be diffused uniformly, segregation of S can be reduced, and SSC resistance can be improved. Moreover, since the number of passes will increase when the total of the rolling strain in the initial three passes is less than 0.3, the cost becomes high. The total rolling strain is preferably 0.4 or more, more preferably 0.5 or more. The upper limit of the total rolling strain is not particularly limited, but is usually 2.0 or less due to equipment restrictions.

The rolling strain, the cross-sectional area of the billet before hot rolling (mm ²⁾ and S _0, when the cross-sectional area of the billet in the initial 3-pass end during hot rolling (mm ²⁾ is S Can be calculated by the following formula (1).
Rolling strain = ln (S ₀ / S) (1)

  The wire obtained by hot rolling is preferably subjected to heat treatment such as quenching and tempering to make the metal structure martensite. Quenching may be performed, for example, after heating to 850 to 1000 ° C. and then cooling to room temperature at an average cooling rate of 30 ° C./second or more. The upper limit of the average cooling rate is, for example, 100 ° C./second. Tempering should just heat to 400-650 degreeC, for example.

  The heat treatment may be performed once, and by making the heat treatment one time, productivity can be improved as compared with Patent Document 1 in which the quenching process is performed twice or more.

  The hot-rolled wire obtained by heat treatment may be used as a material for producing a steel wire that requires SSC resistance, such as a part used in a sour environment containing hydrogen sulfide or a reinforcing material for a flexible riser. it can.

  EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples, and may be implemented with modifications within a range that can meet the above and the gist described below. Of course, these are all possible and are included in the technical scope of the present invention.

Steels having the component compositions shown in Table 1 below were melted, and the resulting molten steel was cast to produce steel pieces. The balance of the steel is iron and inevitable impurities. The obtained steel slab was subjected to block rolling, and the obtained billet was hot-rolled to produce a wire. The billet before hot rolling was a square of 155 mm × 155 mm, and was hot rolled to produce a wire having a diameter of 11 to 16 mm. In the hot rolling, the total rolling strain (ε) in the initial three passes was controlled as shown in Table 2 below. The rolling strain is expressed by the following formula, where S ₀ is the billet cross-sectional area (mm ² ) before hot rolling, and S is the billet cross-sectional area (mm ² ) at the end of the initial three passes during hot rolling. It can be calculated by (1).
Rolling strain = ln (S ₀ / S) (1)

  Next, the obtained wire was subjected to quenching and tempering treatment to obtain a test material. The quenching was performed by heating to 850 to 1000 ° C. and holding in this temperature range for 5 to 15 minutes, and then cooling to room temperature with an average cooling rate of 30 ° C./second or more. Tempering was carried out by holding at a temperature range of 400 to 650 ° C. for 50 to 70 minutes.

  About the obtained test material, observation of the metal structure and the segregation degree of S were measured.

(Observation of metal structure)
A test piece for observing the metal structure collected from the test material was embedded in the mount, and the metal structure was observed with an optical microscope at an observation magnification of 400 times. As a result, the metal structure of all the test materials was martensite.

(Segregation degree of S)
An electron probe microanalyzer (EPMA) was used, and the element to be measured was set to S, and segregation was likely to occur in the central portion, so element mapping was performed toward the surface including the center of the specimen. The measurement includes the center and the surface layer, and measurement is performed at 300 or more positions at intervals of 200 μm. The amount of S (mass%) was calculated based on the spectral intensity of characteristic X-rays, and the average value S _ave (mass%) and the maximum value S _max (mass%) were determined. The maximum value S _max (mass%) of the S amount relative to the average value S _ave (mass%) of the S amount is defined as the degree of segregation (S _max / S _ave ), and the results are shown in Table 2 below.

  Next, a JIS No. 14A test piece was sampled from the obtained specimen, and a tensile test was performed based on JIS Z2241 (2011) to measure the yield strength (YS). The unit of yield strength is MPa. The measurement results are shown in Table 2 below. In the present invention, a yield strength of 900 MPa or more was determined as high strength, and was determined as acceptable.

Next, the SSC resistance of the obtained specimen was evaluated by the following procedure. A test piece for Method A method defined by NACE TM0177 was collected from the obtained test material, and the SSC resistance of the test material was evaluated by Method A method. The SSC resistance was evaluated by immersing the obtained test piece in Solution A containing 5.0% by mass of NaCl and 0.5% by mass of CH ₃ COOH, saturating H ₂ S gas in the solution, A stress of 80% of the yield strength measured in (1) was applied, and the time until fracture was measured. The measurement results are shown in Table 2 below. In the present invention, it was evaluated that the break time was 720 hours or more and the SSC resistance was excellent.

  From Tables 1 and 2, it can be considered as follows. No. 2 to 4, 11 to 15, 17, and 19 to 23 are examples that satisfy the requirements defined in the present invention. Since the component composition and the segregation degree of S were appropriately controlled, the yield strength was high strength of 900 MPa or more, and the SSC resistance could be improved.

  On the other hand, no. 1, 5 to 10, 16, and 18 are examples that do not satisfy the requirements defined in the present invention. Of these, No. In No. 1, since C was too small, the yield strength was less than 900 MPa. No. 5 and 16 are examples in which S segregates because the rolling strain introduced in the initial three passes was less than 0.3. Since the segregation degree of S exceeded 30, SSC resistance could not be improved. No. 6 contained excessive Si, and the segregation degree of S exceeded 30, so SSC resistance could not be improved. No. No. 7 contained excessive Mn, and the segregation degree of S exceeded 30, so SSC resistance could not be improved. No. No. 8 contained S in excess, and the segregation degree of S exceeded 30, so SSC resistance could not be improved. No. In No. 9, since there was too little Si, solid solution strengthening became inadequate and yield strength became less than 900 MPa. No. In No. 10, since there was too little Mn, hardenability became inadequate and yield strength became less than 900 MPa. No. No. 18 contained C excessively, and the segregation degree of S exceeded 30, so SSC resistance could not be improved.

Claims (6)

% By mass
C: 0.20 to 0.5%,
Si: 0.05-0.3%
Mn: 0.3 to 1.5%,
Al: 0.001 to 0.1%,
P: more than 0%, 0.01% or less, and S: more than 0%, 0.01% or less,
The balance is a hot-rolled wire consisting of iron and inevitable impurities,
The amount of S contained in the hot-rolled wire is measured at 300 points or more at intervals of 200 μm using an electron beam microanalyzer, and the maximum value S _max (mass%) of the S amount relative to the average value S _ave (mass%) of the S amount. Is the degree of segregation (S _max / S _ave ),
The hot-rolled wire, wherein the degree of segregation is 30 or less. Furthermore, as other elements,
The hot-rolled wire rod according to claim 1, comprising at least one of Cr: more than 0%, 1% or less, and B: more than 0%, 0.01% or less. Furthermore, as other elements,
The hot-rolled wire rod according to claim 1 or 2, comprising at least one of Ni: more than 0%, 0.5% or less, and Cu: more than 0%, 0.5% or less. Furthermore, as other elements,
The hot-rolled wire according to any one of claims 1 to 3, comprising at least one of Ti: more than 0%, 0.1% or less, and V: more than 0%, 0.5% or less. Furthermore, as other elements,
Mo: Hot rolled wire rod according to any one of claims 1 to 4, comprising more than 0% and not more than 1.5%. Furthermore, as other elements,
Nb: The hot-rolled wire rod according to any one of claims 1 to 5 containing more than 0% and 0.1% or less.