KR101902329B1 - Low Cost Alloy Steel for Oil Tools With High Hardenability and Low Temperature Toughness And Method for Manufacturing The Same - Google Patents

Abstract

The present invention relates to a low-cost oil-impregnated alloy excellent in incombustibility and impact resistance at low temperatures and a method for producing the same. Concretely, it is preferable to use a steel material containing 0.31 to 0.45 wt% of C, 0.15 to 0.50 wt% of Si, 0.40 to 1.00 wt% of Mn, 0.35 wt% or less of Cu (not including 0), 0.25 wt% 0.50 to 1.10% by weight of Cr, 0.15 to 0.25% by weight of Mo, 0.0010% by weight or less of B, 030 to 0.040% by weight of Al, 0.0080% by weight or less of N, % Or less (not including 0), S: not more than 0.030 wt% (not including 0), and the balance of Fe and unavoidable impurities.

Description

TECHNICAL FIELD [0001] The present invention relates to a low-cost oil steep alloy steel having excellent ductility and low-temperature impact toughness,

The present invention relates to a low cost alloy steel excellent in incombustibility and impact resistance at low temperatures and a method for producing the same.

Steels for oil drilling rigs are mainly used in underground or deep water so they should have impact toughness characteristics at low temperatures. Thus, in order to increase the impact toughness or to lower the transition temperature of the impact toughness to a low temperature, it is possible to suppress the bainite structure by suppressing the grain refinement or improving the incombustibility.

In particular, the steel specification includes the impact strength at low temperatures as well as at room temperature for oil steels in the United States Petroleum Association (API), which is a representative standard for steel pipes for oil rigs or oil drilling rigs. Typically, for API L80 grade steels, a grade with a tensile strength of 95ksi or more, a yield strength of 75ksi or more, and a low temperature (-60 ° C) impact toughness of 27J or more is required at 1.25 inches (surface) directly below the surface after QT (Quenching & Tempering) And also requires the Brinell hardness to be achieved from the surface of the material to the center in the range of 207-235 HB narrow. These properties should form a uniformly dense layer from the surface to the center and achieve high impact toughness performance at low temperatures by applying a high tempering temperature to reduce the strength. Above all, impact toughness requirements at low temperatures (-60 ° C) represent an increasingly poorer energy drilling environment, which has become a special feature of indispensable steels.

The present inventors have developed a steel for an oil fuel having excellent incombustibility and impact toughness and a manufacturing method thereof in Korean Patent No. 10-1554026. In the above patent, boron is added in ASTM standard 4140 or KS / JIS standard SCM440 to increase the durability after QT and to have a low temperature impact toughness of 47 J or higher at -29 캜.

In general, when the composition is controlled to improve the hardenability, the hardenability is improved through high cost ferroalloy such as Mn, Ni, Cr, Mo, However, unlike the conventional method, the present invention improves the hardenability by adding Al at least twice as much as the amount of the deoxidizing agent, and thus forms a high martensite fraction. It is another object of the present invention to provide a steel having a yield strength and an excellent impact toughness at room temperature and low temperature (-60 DEG C) by forming an AlN precipitate by bonding with N, which is a residual component in the steel, .

The above-mentioned problems are solved by a steel sheet having a composition of 0.31 to 0.45 wt% of C, 0.15 to 0.50 wt% of Si, 0.40 to 1.00 wt% of Mn, 0.35 wt% or less of Cu (not including 0), 0.25 wt% 0.50 to 1.10% by weight of Cr, 0.15 to 0.25% by weight of Mo, 0.0010% by weight or less of B, 030 to 0.040% by weight of Al, 0.0080% by weight or less of N, % Or less (not including 0), S: not more than 0.030 wt% (not including 0), and the balance of Fe and unavoidable impurities.

Preferably, the oil seamed steel has a tensile strength of 99 ksi or more, a yield strength of 74 ksi or more, a yield ratio of 0.75 or more, and a hardness of 45 HRC or more to 15 mm directly below the surface.

Further, preferably, the oil-seized steel has fine crystal grains of ASTM 10.3 or more.

(Mn, Ni, Cr, Mo, B, etc.), which are generally known, are added to improve the hardenability of the alloy steel. However, unlike the conventional method, The alloy composition was designed so as to improve the ingotability and grain refinement without increasing the grain size. This alloy design not only improves durability life but also reduces manufacturing costs in preparation for increasingly difficult oil and gas drilling environments.

FIG. 1 is a graph showing the Joe mini evaluation according to Al addition, that is, the difference in incombustibility
FIG. 2 is a photograph of the microstructure of inventive steel and comparative steel according to Al addition.
3 is a photograph of the grain diameters of inventive and comparative steels according to Al addition.
Fig. 4 is a photograph of a product defect that may occur when the N is not regulated.
Figure 5 TiN precipitates in the steel and acts as a starting point of the wavefront.

Unless defined otherwise, all technical terms used in the present invention have the following definitions and are consistent with the meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. In addition, preferred methods or samples are described in this specification, but similar or equivalent ones are also included in the scope of the present invention.

The term "about" is used herein to refer to a reference quantity, a level, a value, a number, a frequency, a percent, a dimension, a size, a quantity, a weight, or a length of 30, 25, 20, 25, 10, 9, 8, 7, Level, value, number, frequency, percent, dimension, size, quantity, weight or length of a variable, such as 4, 3, 2 or 1%.

Throughout this specification, the words "comprises" and "comprising ", unless the context requires otherwise, include the steps or components, or groups of steps or elements, Steps, or groups of elements are not excluded.

The present invention relates to a method for improving the strength and the excellent impact toughness at room temperature and low temperature (-60 ° C) by changing the alloy composition so as to increase the incombustibility without raising the cost in the conventional 4130 or 4140 (SCM430 or SCM440) .

Particularly, the present invention is characterized in that the hardening performance is improved by adding Al in a specific amount or more, rather than improving hardenability by Mn, Ni, Cr, Mo, B or the like, which has been made in the conventional 4130 or 4140 steel. The effect of this Al may be similar mechanism in suppressing the generation of ferrite (ferrite) like general boron effect. However, in the case of boron, the boron effect can be attained only by denitration by adding Ti It is different from the effect of Al. Also, in the case of TiN, non-metallic inclusions in micro size units are formed to form a starting point of cracks, which is a factor for inhibiting toughness. However, Al not only improves the hardenability but also forms a nano-sized AlN compound by binding with N, which is a residual component in the steel, and precipitates in the grain boundary at the nano-size to achieve a pinning effect, have.

Therefore, the present invention is characterized in that B is regulated to 0.0010% by weight or less in order to improve the hardenability by Al.

To explain the theoretical background in more detail, Al is known as an element acting as a deoxidizer or inhibiting coarse grains and grain growth. However, there is no theory that Al improves the ingotability. This is because, when the crystal grains become finer, the ingot property is inhibited. Therefore, the mechanism for improving the ingotability of Al, which is an element for grain refinement, has not yet been found.

However, the present invention confirms that the hardening performance is improved by steelmaking in a small lot (500 kg) of VIM (Vacuum Induction Melting), and then the possibility of actual mass production suitability is confirmed through mass production evaluation.

The steel of the present invention contains 0.31 to 0.45 wt% of C, 0.15 to 0.50 wt% of Si, 0.40 to 1.00 wt% of Mn, 0.030 wt% or less of P (not including 0) and 0.030 wt% or less of S (not including 0) , Cu: not more than 0.35 wt% (not included), Ni: not more than 0.25 wt% (not included), Cr: 0.50 to 1.10 wt%, Mo: 0.15 to 0.30 wt% , Al: 0.030 to 0.040% by weight N: 0.0080% by weight or less (not including 0) and the balance of Fe and unavoidable impurities.

Hereinafter, the alloy component of the present invention will be described in detail.

C: 0.31-0.45 weight%

C is an essential component for securing the strength of the steel. The higher the content, the higher the incombustibility and yield ratio. However, when the C content is 0.31 When the content is less than 1 wt%, it is difficult to form a martensite structure. When the content is more than 0.45 wt%, the material becomes brittle and toughness may be deteriorated. Therefore, in order to design appropriate strength and hardness in consideration of the use and grade of the steel, C is preferably 0.31 to 0.45 wt%.

Si : 0.15-0.50 weight%

The Si is added for the purpose of deoxidation at the time of dissolution, and the composition is preferably 0.15-0.50% by weight.

Mn: 0.40 to 1.00 weight%

The Mn is an essential element for improving the strength of the steel together with C and is an important element for improving the incombustibility. The content is limited to 0.40 to 1.00% by weight.

Cu: 0.35 weight%  Less (0 not included)

Since Cu is an important element that uniformizes hardness without damaging machinability, it is limited to 0.35 wt% or less.

Ni : 0.25 weight%  Less (0 not included)

The Ni improves the incombustibility and impact toughness. Further, it is a component having a high temper softening resistance at a high temperature, but the present invention is limited to 0.25% by weight or less in the SCM system.

Cr : 0.50-1.10 weight%

Since Cr is an element that increases the incombustibility and improves the strength, it is limited to 0.50 to 1.10 wt%.

Mo : 0.15-0.25 weight%

The Mo content is limited to 0.15 to 0.25% by weight since it has an effect of improving the entrapment property and grain refinement effect by Mo carbide.

Al: 0.030 to 0.040 weight%

Al is an element added for deoxidation, but in the present invention, it is added for the purpose of improving hardenability. Al additionally reacts with N to form AlN, thereby exhibiting a pinning effect. In the present invention, the incombustibility is improved at 0.030 wt% or more of Al and the effect is not great at 0.040 wt% or more. On the other hand, if it exceeds 0.040% by weight, the possibility of causing surface defects increases during continuous casting, so that the content thereof is regulated to 0.040% by weight at maximum (see FIG. 4).

B: 0.0010 weight%  Less (0 not included)

B is an element which inhibits the generation of pro-eutectoid ferrite in the grain boundary and improves the incombustibility, and the effect is usually seen at 0.0020 to 0.0040 wt%. Since the boron effect is not used in the present invention, it is regulated to 0.0010% by weight or less.

N: 0.0080 wt%  Less (0 not included)

The N reacts with Al to precipitate on the grain boundaries to refine the grain. On the contrary, the grain boundary can be brittle to cause surface defects during the cooling process during continuous casting. Therefore, the content of N is 0.0080 wt% or less.

P, S: 0.030 weight%  Less (0 not included)

P and S were 0.030 wt% or less as impurities.

The steel of the present invention can be produced by the following production method.

The method is characterized in that it comprises the steps of: C: 0.31 to 0.45 wt%, Si: 0.15 to 0.50 wt%, Mn: 0.40 to 1.00 wt%, Cu: 0.35 wt% 0.001 wt% or less of Al, 0.030 wt% or less of Al, 0.0080 wt% or less of N, 0 wt% or less of P, Austenizing by heating continuously or in batches at 840 to 925 DEG C in the presence of Fe and unavoidable impurities in an amount of not more than 0.030 wt% The hardness of the martensite structure is obtained through quenching at ~ 150 ° C. and tempering at 500 to 700 ° C. to reduce the hardness variation in the material and improve the toughness and ductility Step < / RTI >

Hereinafter, the present invention will be described more specifically by way of examples. However, the scope of the present invention is not limited by the following examples.

Example

Tables 1 and 2 below show the chemical composition of the inventive and comparative steels. Inventive steels were evaluated for content by Al content of 0.030-0.040 wt.% Al, and the comparative steels were less than 0.030 wt.% And 0.040 wt.% Of steel and conventional mass steel.

(Unit: wt%) division C Si Mn Cu Ni Cr Mo B Al N P S Inventive Steel 1 0.33 0.20 0.57 0.04 0.01 1.09 0.24 0.0005 0.035 0.0077 0.010 0.007 Invention river 2 0.33 0.22 0.57 0.08 0.07 1.08 0.24 0.0007 0.038 0.0069 0.007 0.007 Comparative River 1 0.32 0.15 0.60 0.11 0.04 1.08 0.24 0.0006 0.025 0.0050 0.013 0.005 Comparative River 2 0.32 0.20 0.55 0.07 0.07 1.08 0.23 0.0005 0.045 0.0051 0.015 0.004 Comparative Steel 3 0.32 0.19 0.58 0.08 0.05 1.08 0.23 0.0005 0.055 0.0057 0.011 0.006 Comparative Steel 4 0.32 0.22 0.58 0.09 0.03 1.09 0.24 0.0008 0.058 0.0111 0.011 0.011

Table 1 shows invented steels and comparative steels in which Al was added to the 4130 series steel within the content range of the present invention. The comparative steel 1 is currently in mass production and is a steel grade that has been designed for hardenability by typical Mn, Cr, and Mo contents. However, in order to secure more improved hardenability and mechanical properties as compared with comparative steel 1, in the present invention, addition of Mn, Ni, Cr, Mo and B is restricted and Al is added twice or more, .

Generally, the boron effect is used to improve the hardenability at low cost. In fact, boron is also known to lower costs and improve its hardenability. However, for the boron effect, soluble boron should be precipitated, and a denitration element (Ti) must be added to the boron. However, the nonmetallic inclusion in the micro unit due to the combination of Ti and N acts as a starting point of the crack, which hinders impact toughness. Fig. 5 is a photograph showing TiN precipitated in the steel and serving as a starting point of the wave front.

5 is a photograph of a steel in which TiN is generated when 0.035% by weight of Ti is added to 4130-type steel material and boron is added in an amount exceeding 0.0010% by weight in order to improve the hardenability. A micro-sized TiN inclusion is formed, and it can be seen that a cleavage wave front (brittle fracture) occurs from this point. In these specimens, impact toughness is typically only 5 ft-lbs even at room temperature.

However, Al does not need to be supplied with additional elements, and when the target content of 0.030 to 0.040 wt% is met, a cost of about 35,000 won is added to the production of 100 ton of steel, which is a significant cost advantage .

The comparative steels 2 to 4 and the comparative steels 6 and 7 were fed with the content of Al exceeding 0.40 wt%, and the effect was confirmed.

(Unit: wt%) division C Si Mn Cu Ni Cr Mo B Al N P S Invention steel 3 0.40 0.24 0.80 0.08 0.06 0.95 0.16 0.0007 0.035 0.0080 0.012 0.005 Inventive Steel 4 0.41 0.20 0.75 0.07 0.04 0.98 0.15 0.0006 0.038 0.0051 0.014 0.007 Comparative Steel 5 0.41 0.22 0.78 0.13 0.05 0.95 0.16 0.0006 0.025 0.0071 0.011 0.008 Comparative Steel 6 0.40 0.20 0.76 0.04 0.04 0.95 0.16 0.0004 0.043 0.0068 0.013 0.010 Comparative Steel 7 0.40 0.19 0.81 0.09 0.05 0.96 0.15 0.0005 0.051 0.0153 0.008 0.009

Table 2 shows the composition of the 4140 series steel. The basic concept is the same as in Table 1, and it was tried to verify whether the effect of Al content control improves the hardenability in general alloy steel, that is, a general applicable technique, not a phenomenon specialized in 4130 steel.

The steel of the composition shown in Table 1 and the unavoidable impurities was continuously rolled into a 190.00 mm diameter round bar and then subjected to osteogenization by heating at 840 to 925 ° C in a continuous furnace and quenched at 100 to 150 ° C , And tempered at 500 to 720 캜 to produce a steel. Their tensile strength, yield strength, impact toughness and yield ratio were measured and are shown in Table 3 below (4130 L80 grade). The L80 grade is a steel with a yield strength of 80 ksi after the QT heat treatment specified by API (American Petroleum Institute).

A steel having the composition shown in Table 2 and inevitable impurities is continuously cast and rolled in a round bar size of 170.00 mm and heated in a continuous furnace at 840 to 925 ° C for osteonization and quenched at 100 to 150 ° C, The steel was tempered at 500 to 720 캜. Their tensile strength, yield strength, impact toughness and yield ratio were measured and shown in Table 4 below (4140 series P110 grade). The P110 grade is a steel with a yield strength of 110 ksi or higher after QT heat treatment as specified by API (American Petroleum Institute).

The tensile strength
(ksi) Yield strength
(ksi) -60 ° C IV
(ft-lbs) Yield ratio
(YS / TS) Inventive Steel 1 105 78 51 0.74 Invention river 2 104 78 63 0.75 Comparative River 1 101 71 22 0.70 Comparative River 2 99 74 57 0.75 Comparative Steel 3 101 75 49 0.74 Comparative Steel 4 108 80 59 0.74

The tensile strength
(ksi) Yield strength
(ksi) Room temperature IV
(ft-lbs) Yield ratio
(YS / TS) Invention steel 3 149 129 101 0.86 Inventive Steel 4 146 126 121 0.87 Comparative Steel 5 141 109 74 0.77 Comparative Steel 6 144 127 99 0.88 Comparative Steel 7 146 126 101 0.86

First of all, before discussing the mechanical properties, The hardenability was measured and shown in Figs. 1A and 1B. In FIG. 1A, when the comparison depths 1 and 2 of inventive steels 1 and 2 containing 0.35 wt.% And 0.38 wt.% Al added with comparative steel 1 containing only 0.025 wt.% Of Al are compared, It can be observed that the curing is well performed to a depth of 10 to 15 mm. 1B shows that the comparative steel 5 in which Al is added in an amount of 0.025 wt% and the steel 4 in which Al is added in an amount of 0.38 wt% are hardened to a depth of 30 mm from the surface in comparison with the comparative steel 6 in which Al is added in an amount of 0.043 wt% It can be observed that it is well done.

Further, as shown in Tables 3 and 4, when the Al content was controlled to 0.03 to 0.04% by weight, it was observed that the strength and toughness were improved. Of course, an increase of more than 0.040 wt% can be seen, but in this case, it is highly likely to cause serious defects in the manufacturing process (continuous casting).

As shown in Table 3, the yield ratio of 4130 steels was not significantly improved compared with the 4140 steels of Table 4. This is due to the difference in carbon content, because the martensite fraction of 4130 steels is low. However, when the yield ratio is slightly improved and the impact toughness is improved, it can be seen that the martensite fraction is improved in comparison with the comparative steel 1, which is a conventional mass production steel, and the effect of grain refinement is also mixed with the additional effect (See FIG. 2).

Fig. 2 is a photograph (x500) of the microstructure at a point 30 mm directly below the surface of the 4130 inventive steel 1 (Fig. 2a) and the comparative steel 1 (Fig. 2b). The cooling rate is slowed down to the deep part of the lecture? Qing Si, resulting in incomplete texture or bainite texture. However, a hardened steel has a hardened martensite structure even if the cooling rate is slowed down. 2A and 2B, it can be visually confirmed that the inventive steel 1 (FIG. 2A) has a large martensite structure fraction, which is the hardened structure, in comparison with the comparative steel 1 (FIG.

As shown in Table 4, when the Al content is controlled to 0.03 to 0.04% by weight, the yield ratio is remarkably improved and the impact toughness is also improved. It can be said that the addition of Al improves the hardenability and inhibits the formation of bainite structure, facilitates the formation of the martensite structure, and additionally, is a composite result of grain refinement.

FIG. 3 is a photograph showing the graininess of the invention steel 3 (FIG. 3A) and the comparative steel 5 (FIG. 3B), which are 4140 series steels according to the present invention. 3, it can be seen that Invention Steel 3 (ASTM No. 10.3) having an Al content of 0.035% by weight is finer than that of Comparative Steel 5 (ASTM No. 8.6) containing only 0.025% by weight of Al .

4 is a cross-sectional photograph of a comparative steel 4 having an N content exceeding 0.008 wt%. Referring to FIG. 4, when the content of N is not regulated, it is seen that excessive AlN is generated and intergranular breakdown occurs and cracks are generated on the surface of the steel material.

Claims (3)

(Ni): 0.25 wt% or less (inclusive), Cr: 0.50 to 0.5 wt%, C: 0.31 to 0.45 wt%, Si: 0.15 to 0.50 wt%, Mn: 0.40 to 1.00 wt% (B): 0.0010 wt% or less (not including 0), Al: 0.030 to 0.040 wt%, N: 0.0080 wt% or less (not including 0), P: 0.030 wt% or less Of not less than 0.030% by weight of S and the balance of Fe and unavoidable impurities and having a tensile strength of 99 ksi or more, a yield strength of 74 ksi or more, a yield ratio of 0.75 or more, Wherein the steel has a hardness. delete The oil casting steel according to claim 1, wherein the oil casting steel has fine grain grains of ASTM 10.3 or more.

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