High tensile and high toughness steels

Abstract

The present invention deals with alloyed steels having yield strength of at least 862 MPa (125 Ksi) and exhibiting outstanding hardness and toughness behavior, especially under stringent conditions which may be subjected to frost-heave and thaw settlement cycles, namely at subzero temperatures. The invention also relates to a seamless pipe comprising said steel and a method of production of said pipe thereof.

Claims

1. A seamless steel pipe having the following chemical composition consisting of in weight percent: C: from 0.27 to 0.30 wt %, Si: from 0.20 to 0.35 wt %, Mn: from 0.80 to 0.90 wt %, Cr: from 1.30 to 1.45 wt %, Mo: from 0.65 to 0.75 wt %, Ni: from 0.15 to 0.25 wt %, Cu: max 0.25 wt %, Al: from 0.015 to 0.035 wt %, Ti: from 0.031 to 0.038 wt %, N: max 0.012 wt %, V: max 0.05 wt % B: from 0.001 to 0.0025 wt %, Nb: from 0.02 to 0.03 wt %, wherein the balance of said steel being iron and unavoidable impurities from industrial processing including elements P, S, and H, the amounts of which, expressed in weight %, relative to the total weight of said chemical composition are as follows: P: ≤0.015%, S≤0.003%, and H≤0.003%, and having a yield strength (Ys) of at least 862 MPa and an ultimate tensile strength (UTs), wherein a ratio between the yield strength (Ys) and the ultimate tensile strength (UTs) is lower than 0.93, wherein its microstructure comprises at least 95% of martensite related to the entire microstructure, wherein the seamless steel pipe has a wall thickness which ranges from 38 to 78 millimeter.

2. The steel according to claim 1, wherein the chemical composition consisting of in weight percent: C: from 0.27 to 0.30 wt %, Si: from 0.22 to 0.30 wt %, Mn: from 0.80 to 0.85 wt %, Cr: from 1.30 to 1.40 wt %, Mo: from 0.65 to 0.70 wt %, Ni: from 0.15 to 0.20 wt %, Cu: from 0.10 to 0.20 wt %, Al: from 0.017 to 0.030 wt %, Ti: from 0.031 to 0.038 wt %, N: from 0.001 to 0.010 wt %, V: from 0.001 to 0.020 wt %, B: from 0.0010 and 0.0018%, Nb: from 0.020 to 0.025 wt %, wherein the balance of said steel being iron and unavoidable impurities from the industrial processing.

3. The steel according to claim 1, wherein the ratio between the yield strength (Ys) and the ultimate tensile strength (UTs) is lower than 0.9.

4. The steel according to claim 1, wherein the yield strength (Ys) is of at least 900 MPa.

5. The steel according to claim 1, wherein the ultimate tensile strength (UTs) is at least 950 MPa.

6. The steel according to claim 1, wherein the steel has a toughness value according to ASTM E23—Type A on a full size sample (10×10 mm) in the transverse direction at −40° C. which is at least: TABLE-US-00023 Yield strength Charpy test (kSi) energy (J) 125-135 100 (included) 135 (excluded)- 80 155

7. The steel according to claim 1, wherein the steel has a toughness value according to ASTM E23—Type A on a full size sample (10×10 mm) in the transverse direction at −60° C. which is at least: TABLE-US-00024 Yield strength Charpy test (kSi) energy (J) 125-135 80 (included) 135 (excluded)- 64 155

8. The steel according to claim 1, wherein the composition satisfies the relation below between the nickel, chromium and manganese contents:
Σ(Ni,Cr,Mn)≥2.36 wt %

9. The steel according to claim 1, wherein the composition satisfies the relation below between the nickel, chromium and manganese and silicon silicium contents:
Σ(Ni,Cr,Mn,Si)≥2.62 wt %

10. The steel according to claim 1, wherein its microstructure comprises at least 99% of martensite related to the entire microstructure.

11. A method of production of steel seamless pipe according to claim 1 the method comprising: (i) providing a steel having the chemical composition as defined according to claim 1, (ii) hot forming the steel at a temperature ranging from 1100° C. to 1300° C. through a hot forming process to obtain a pipe, then (iii) heating up said pipe to an austenitizing temperature (AT) above or equal to 890° C. and keeping said pipe at the austenitizing temperature (AT) during a time comprised between 5 and 30 minutes, followed by cooling the pipe to a temperature of at most 100° C. in order to obtain a quenched pipe, and heating up and holding said quenched pipe at a tempering temperature (TT) ranging from 580° C. to 720° C. and keeping said pipe at the tempering temperature (TT) during a tempering time, and then cooling said pipe to a temperature of at most 20° C., in order to obtain a quenched and tempered pipe, (iv) measuring the yield strength to ultimate tensile strength ratio and controlling that said ratio is lower than 0.93.

12. An oil and gas accessory and/or mechanical component comprising at least a seamless pipe according to claim 1.

13. A method to manufacture an oil and gas accessory and/or a mechanical component, comprising employing the steel according to claim 1.

14. The seamless steel pipe according to claim 1, wherein the composition contains H: greater than 0% to 0.003%, expressed in weight %, relative to the total weight of said chemical composition.

15. A seamless steel pipe having the following chemical composition consisting of in weight percent: C: from 0.27 to 0.30 wt %, Si: from 0.20 to 0.35 wt %, Mn: from 0.80 to 0.90 wt %, Cr: from 1.30 to 1.45 wt %, Mo: from 0.65 to 0.75 wt %, Ni: from 0.15 to 0.25 wt %, Cu: max 0.25 wt %, Al: from 0.015 to 0.035 wt %, Ti: from 0.033 to 0.038 wt %, N: max 0.012 wt %, V: max 0.05 wt % B: from 0.001 to 0.0025 wt %, Nb: from 0.02 to 0.03 wt %, wherein the balance of said steel being iron and unavoidable impurities from industrial processing including elements P, S, and H, the amounts of which, expressed in weight %, relative to the total weight of said chemical composition are as follows: P: ≤0.015%, S≤0.003%, and H≤0.003%, and having a yield strength (Ys) of at least 862 MPa and an ultimate tensile strength (UTs), wherein a ratio between the yield strength (Ys) and the ultimate tensile strength (UTs) is lower than 0.93, wherein its microstructure comprises at least 95% of martensite related to the entire microstructure, wherein the seamless steel pipe has a wall thickness which ranges from 38 to 78 millimeter.

Description

EXAMPLES

(1) I. Steel-A (According to the Invention)

(2) The upstream process i.e. from melting to hot forming, is done with commonly-known manufacturing method for seamless steel pipes.

(3) For example, it is desirable that molten steel of the below constituent composition be melted by commonly-used melting practices. The common methods involved are the continuous or ingot casting process.

(4) Table 1 illustrates the chemical composition of a steel according to the present invention (the amounts indicated are calculated in weight percentage, the balance of said composition is made with iron).

(5) TABLE-US-00003 TABLE 1 Chemical composition of Steel-A Steel C Si Mn P S Cr Mo Ni A 0.29 0.26 0.81 0.007 0.001 1.38 0.66 0.17 Cu Al Ti Nb V B N 0.14 0.025 0.033 0.024 0.007 0.0014 0.008

(6) Next, these materials are heated at a temperature between 1100° C. and 1300° C., and then manufactured into pipe e.g. by hot working by forging, the plug or pilger mill process, which are commonly-known manufacturing methods, of the above constituent composition into the desired dimensions.

(7) The composition described in Table 1 then undergoes a production process that can be summarized in table 2 below with the step features disclosed below: the pipe is heated up to an austenitizing temperature (AT) of 910° C. and kept at this temperature for 10 minutes (At: austenitization time), then the pipe is cooled with water to a temperature of 100° C. or lower to obtain a quenched pipe and then said quenched pipe is heated up and held at a tempering temperature (TT) for 15 minutes, and then cooled to a temperature of 20° C. or lower in order to obtain a quenched and tempered pipe, the yield strength (Ys) to ultimate tensile strength (UTs) ratio is controlled after the tempering step.

(8) The above mentioned method has been carried out to obtain two seamless pipes (A-1.1 and A-1.2) each having a wall thickness of 38.1 mm (corresponding to 1.5 inch) and two seamless pipes (A-2.1 and A-2.2) each having a wall thickness of 76.2 mm (corresponding to 3 inches).

(9) The parameters of the above method are summarized in Table 2 below:

(10) TABLE-US-00004 TABLE 2 process conditions of examples after hot rolling Wall Pipe AT At TT Tt thickness Steel no (° C.) (min) (° C.) (min) (mm) A A-1.1 910 10′ 650 15 38.1 A-1.2 910 10′ 650 15 38.1 A-2.1 910 10′ 620 15 76.2 A-2.2 910 10′ 620 15 76.2

(11) The process parameters disclosed in Table 2 are consistent with the present invention.

(12) This led to quenched and tempered steel pipes that, after final cooling from the tempering temperature, present a microstructure comprising at least 99% of martensite based on the microstructure.

(13) Furthermore, the quenched and tempered steel pipes obtained have an outer diameter of 304.8 mm.

(14) 1. Mechanical Properties

(15) 1.1. Hardness on the Quenched Seamless Pipe

(16) Hardness based on the Rockwell scale (HRC) is measured on the four quadrants (Q1, Q2, Q3 and Q4) of the quenched and tempered steel seamless pipe (specimen A-1.1; wall thickness corresponding to 38.1 mm) obtained from the composition disclosed in Table 1 (steel composition A). Each quadrant represents an angular orientation of 90°.

(17) For each quadrant, hardness has been measured three times on the external, inside and mid-wall of the steel seamless pipe.

(18) The results are summarized in Table 3:

(19) TABLE-US-00005 TABLE 3 Hardness (Rockwell scale HRC) Quad- rant External Mid-wall Internal Q1 49.5 49.3 48.5 51.3 52.0 51.5 50.3 48.8 49.6 Q2 48.7 48.6 48.8 52.3 51.8 50.5 49.8 48.8 49.3 Q3 48.7 49.3 48.7 51.6 50.8 51.3 49.6 49.3 50.2 Q4 49.3 48.5 48.1 51.0 51.1 52.0 49.8 49.3 49.8

(20) FIG. 1 illustrates the hardness values summarized in Table 3 for each quadrant as a function of the location where the hardness measurement has been determined on the pipe wall, i.e. external, internal and mid-wall.

(21) These results show that hardness is homogeneous throughout the seamless pipe.

(22) 1.2. Determination of Yield (Ys) and Tensile Strengths (UTs)

(23) 1.2.1. Wall Thickness: 38.1 mm (1.5 Inch)

(24) A set of two specimens has been taken, one at each end of the seamless pipe, from the seamless pipe A-1.1 (wall thickness: 38.1 mm) and the seamless pipe A-1.2 (wall thickness: 38.1 mm).

(25) On each specimen, yield strength (Ys in MPa), ultimate tensile strength (UTs in MPa), elongation at break (A %) and the reduction area (min %) have been assessed on two quadrants: 0° and 180° in the longitudinal direction.

(26) The results on the mechanical properties are summarized in Table 4:

(27) TABLE-US-00006 TABLE 4 Mechanical properties (Ys, UTs, A(%) and reduction area) Ys UTs Ratio Reduction Specimen (MPa) (MPa) Ys/UTs A % area min % A-1.1.a Q(0°) 911 1021 0.89 19.6 63.0 Q(180°) 907 1016 0.89 20.4 64.2 A-1.1.b Q(0°) 899 1002 0.90 21.7 64.1 Q(180°) 908 1018 0.89 20.2 63.8 A-1.2.a Q(0°) 912 1019 0.89 20.8 63.1 Q(180°) 908 1023 0.89 19.2 63.4 A-1.2.b Q(0°) 918 1026 0.89 19.4 63.3 Q(180°) 900 1009 0.89 20.7 63.7

(28) The entire specimens exhibit a ratio between yield strength and ultimate tensile strength lower than 0.93.

(29) From these results, one can see that each specimen has high yield and tensile strengths, a high elongation at break and a reduction area of at least 60% before breaking.

(30) Therefore, it means that the specimens made of the steel of the present invention can withstand a high strain deformation.

(31) 1.2.2. Wall Thickness: 76.2 mm (3 Inches)

(32) A set of two specimens has been taken, one at each end of the seamless pipe, from the seamless pipe A-2.1 (wall thickness: 76.2 mm) and the seamless pipe A-2.2 (wall thickness: 76.2 mm).

(33) On each specimen, yield strength (Ys in MPa), ultimate tensile strength (UTs in MPa), elongation at break (A %) and the reduction area (min %) have been assessed on two quadrants: 0° and 180° in the longitudinal direction.

(34) The results on the mechanical properties are summarized in Table 5:

(35) TABLE-US-00007 TABLE 5 Mechanical properties (Ys, UTs, A(%) and reduction area) Ys UTs Ratio Reduction Specimen (MPa) (MPa) Ys/UTs A % area min % A-2.1.a Q(0°) 937 1031 0.91 16.8 58.4 Q(180°) 922 1018 0.91 19.4 60.4 A-2.1.b Q(0°) 917 1021 0.90 19.7 57.4 Q(180°) 930 1022 0.91 20.0 56.4 A-2.2.a Q(0°) 893 1002 0.89 19.1 56.8 Q(180°) 898 996 0.90 21.4 61.5 A-2.2.b Q(0°) 909 1007 0.90 19.7 62.4 Q(180°) 919 1017 0.90 18.2 59.1

(36) The entire specimens exhibit a ratio between yield strength and ultimate tensile strength lower than 0.93.

(37) From these results, one can see that each specimen has high yield and tensile strengths, a high elongation at break and a reduction area of about 60% before breaking.

(38) Therefore, it means that the specimens made of the steel of the present invention can sustain a high strain deformation.

(39) 2. Impact Energy Results (Wall Thickness: 38.1 mm)

(40) The toughness at low temperatures has been assessed for each previous specimen having a wall thickness of 38.1 mm.

(41) 2.2. Transverse Direction

(42) For each specimen, impact energy values in Joules (Kcv) have been determined in the transverse direction according to the Charpy impact tests ASTM E23—Type A on a full size sample (10×10 mm) at −20° C.

(43) For each specimen, those parameters have been determined three times. The average (Ave) is determined for the impact energy values. The results are summarized in Table 6:

(44) TABLE-US-00008 TABLE 6 Toughness at low temperatures (transverse) Temp Kcv1 Kcv2 Kcv3 Specimens Orientation (° C.) (J) (J) (J) Ave A-1.1.a transv −20° C. 134 131 133 134 A-1.1.b 139 136 129 135 A-1.2.a 136 136 135 136 A-1.2.b 139 139 139 139

(45) 2.3. Charpy Transition Values as a Function of Temperatures

(46) A specimen has been taken from the seamless pipe A-1.1 (wall thickness: 38.1 mm) in order to be standardized in dimension and shape for the Charpy tests.

(47) The impact energy values in Joules (Kcv) as a function of temperatures ranging from 0° C. to −60° C. have also been assessed for this specimen in the transversal direction. This parameter has been determined three times at each temperature. The results are summarized in Table 7:

(48) TABLE-US-00009 TABLE 7 Charpy transition values Temp Kcv1 Kcv2 Kcv3 Ave Specimen Orientation (° C.) (J) (J) (J) (J) A-1.2.c Transv 0 148 143 146 146 −20 135 142 146 141 −40 121 112 128 120 −60 88 94 91 91

(49) FIG. 2 illustrates the Charpy transition curves (Joules) as a function of temperatures in the transversal direction based on the values disclosed in Table 7 and representative of a steel seamless pipe according to the present invention with a wall thickness of 38.1 mm (1.5 inch).

(50) The results disclosed in Tables 7 clearly show that the steel has a ductile behavior at subzero temperatures. Especially, the specimen exhibits high impact energy values above 90 Joules at −60° C. and a steady behavior.

(51) 3. Impact Energy Results (Wall Thickness: 76.2 mm)

(52) The toughness at low temperatures has been assessed for the specimens A-2.1.a, A-2.1.b and A-2.2.a previously disclosed. For the purposes of this assessment, an additional specimen has also been taken out from the seamless pipe A-2 (specimen A-2.2.c).

(53) The measurements have been carried out in transverse directions.

(54) For each previous specimen, impact energy values in Joules (Kcv) have been determined in the transverse direction according to the Charpy impact tests ASTM E23—Type A on a full size sample (10×10 mm) performed at −20° C.

(55) For each specimen, this parameter has been determined three times. The average (Ave) is determined for the impact energy values. The results are summarized in Table 8:

(56) TABLE-US-00010 TABLE 8 Toughness at low temperatures (transverse) Temp Kcv1 Kcv2 Kcv3 Specimens Orientation (° C.) (J) (J) (J) Ave A-2.1.a transv −20° C. 106 104 103 104 A-2.1.b 121 125 124 123 A-2.2.a 119 105 121 115 A-2.2.c 117 124 125 122

(57) From these results, one can see that high values of the impact energy at −20° C. (higher than 100 Joules) are obtained which means that each specimen has a tough behavior at subzero temperatures.

(58) 3.3. Charpy Transition Values as a Function of Temperatures

(59) The impact energy values in Joules (Kcv) as a function of temperatures ranging from 0° C. to −60° C. have also been assessed for the specimen A-2.2.c in the transversal direction. This parameter has been determined three times at each temperature. The results are summarized in Table 9:

(60) TABLE-US-00011 TABLE 9 Charpy transition values Temp Kcv1 Kcv2 Kcv3 Ave Specimen Orientation (° C.) (J) (J) (J) (J) A-2.2.c Transv 0 127 133 138 133 −20 117 124 125 122 −40 107 106 111 108 −60 75 91 83 83

(61) FIG. 3 illustrates the Charpy transition curves (Joules) as a function of temperatures in the transversal direction based on the values disclosed in Table 9 and representative of a steel seamless pipe according to the present invention with a wall thickness of 76.2 mm (3 inches).

(62) From these results, one can see that high values of the impact energy at −60° C. (at least about 80 Joules in average) are obtained which means that each specimen has a tough behavior at subzero temperatures.

(63) Furthermore, the steel of the present invention displays excellent toughness behavior at subzero service temperatures, for example a toughness value in the longitudinal direction of at least 130 Joules at −40° C. and of at least about 100 Joules at −60° C. and a toughness value in the traverse direction of at least 100 Joules at −40° C. and of about 80 Joules at −60° C. according to the Charpy impact tests ASTM E23—Type A on a full size sample (10×10 mm) for a grade 150 ksi steel.

(64) As a consequence, specimens according to the present invention have a toughness and ductile behavior at subzero temperatures whether the wall thickness corresponds to 38.1 mm or 76.2 mm.

(65) 5. Impact Energy Results (Wall Thickness: 50.8 mm)

(66) The previously mentioned method has been carried out to obtain a seamless pipe (A-3) having a wall thickness of 50.8 mm (corresponding to 2 inches) from the chemical composition disclosed in Table 1 (steel-A according to the present invention).

(67) The parameters of the above method are summarized in Table 10 below:

(68) TABLE-US-00012 TABLE 10 Process parameters of the method Wall Pipe At At TT Tt thickness Steel no (° C.) (min) (° C.) (min) (mm) A A-3 910 10′ 650 15 50.8

(69) The impact energy values in Joules (Kcv) as a function of temperatures ranging from 0° C. to −60° C. has been assessed for this specimen.

(70) FIG. 4 illustrates the Charpy transition curves (Joules) in the transverse direction for this specimen.

(71) From these results, one can see that high impact energy values at −60° C. (at least about 90 Joules) are obtained which illustrates the toughness behavior of the tested specimen at subzero temperatures.

(72) II. Steel-B (Comparative Steel)

(73) Table 11 illustrates the chemical composition of a comparative steel (the amounts indicated are calculated in weight percentage, the balance of said composition is made with iron).

(74) TABLE-US-00013 TABLE 11 Chemical composition of Steel-B Steel C Si Mn P S Cr Mo Ni B 0.29 0.19 0.33 0.011 0.0014 0.95 0.8 0.04 Cu Al Ti Nb V B N 0.02 0.046 0.017 — 0.003 0.0012 0.0046

(75) The upstream process and the production process implemented for Steel-B are identical to those described for Steel-A.

(76) The implemented method has been carried out to obtain a seamless pipe (B-1) having a wall thickness of 76.2 mm (corresponding to 3 inches).

(77) The parameters of the above method are summarized in Table 12 below:

(78) TABLE-US-00014 TABLE 12 process conditions of examples after hot rolling Wall Pipe At At TT Tt thickness Steel no (° C.) (min) (° C.) (min) (mm) B B-1 910 10′ 650 15 76.2

(79) 1. Mechanical Properties

(80) 1.1. Yield and Ultimate Tensile Strengths

(81) A set of three specimens has been taken from the seamless pipe B-1.

(82) On each specimen, yield strength (Ys in MPa), ultimate tensile strength (UTs in MPa) and elongation at break (A %) have been assessed in the longitudinal direction.

(83) In particular, the assessment of these properties has been made on the external wall of the specimens B-1.2 and B-1.3 and the internal wall of the specimen B-1.5.

(84) The results on the mechanical properties are summarized in Table 13:

(85) TABLE-US-00015 TABLE 13 Mechanical properties (Ys, UTs and A (%)) Ys UTs A Specimen (MPa) (MPa) (%) B-1.2 970 1046 18.7 B-1.3 987 1062 17.8 B-1.5 972 1049 16.3

(86) 2. Impact Energy Results

(87) A set of three specimens has been taken from the seamless pipe B-1 according to Charpy impact test ASTM E23—Type A on a full size sample (10×10 mm).

(88) The toughness for each specimen has been assessed by determining the impact energy values in the transverse direction at 0° C. For each specimen, impact energy values have been determined three times. The results are given below:

(89) TABLE-US-00016 TABLE 14 Impact energy values at 0° C. Kcv1 Kcv2 Kcv3 Orientation (J) (J) (J) B-1.6 transv 138 132 134 B-1.7 134 135 138

(90) For specimen B-1.8, measurements have been determined on the external, internal and mid-wall of the specimen.

(91) TABLE-US-00017 TABLE 15 Impact energy values at 0° C. B-1.8 Kcv1 (J) Kcv2 (J) Kcv3 (J) external-wall 131 130 138   mid-wall 121 126 112 internal-wall 137 146 152

(92) 3. Charpy Transition Values as a Function of Temperatures

(93) The impact energy values in Joules (Kcv) as a function of temperatures ranging from 20° C. to −40° C. have been assessed for the specimen B-1.6 in the transverse direction. This parameter has been determined three times at each temperature. The results are summarized in Table 16:

(94) TABLE-US-00018 TABLE 16 Charpy transition values Temp Kcv1 Kcv2 Kcv3 Ave Specimen Orientation (° C.) (J) (J) (J) (J) B-1.6 transv 20 114 123 119 119 0 138 132 134 135 −20 110 107 91 103 −40 79 64 82 75

(95) FIG. 5 illustrates the Charpy transition curves (Joules) in the transverse direction for this specimen.

(96) According to these results, one can see that the impact energy values are higher than 110 Joules at 20° C. but then significantly drop at subzero temperatures, especially at −40° C. Indeed, the impact energy is about 75 Joules at −40° C.

(97) Therefore the toughness of the tested specimen significantly decreases at very low temperatures.

(98) IV. Steel D According to the Invention

(99) Table 17 illustrates the chemical composition of a steel according to the present invention (the amounts indicated are calculated in weight percentage, the balance of said composition is made with iron).

(100) TABLE-US-00019 TABLE 17 Chemical composition of Steel-D Steel C Si Mn P S Cr Mo Ni D 0.28 0.32 0.87 0.011 0.001 1.45 0.71 0.18 Cu Al Ti Nb V B N 0.15 0.022 0.038 0.02 0.024 0.0017 0.005

(101) The upstream process and the production process implemented for Steel-D are identical to those described for Steel-A.

(102) In particular, the implemented method has been carried out to obtain a seamless pipe (D-1) having a wall thickness of 38.1 mm (corresponding to 1.5 inch).

(103) The parameters of the above method are summarized in Table 18 below:

(104) TABLE-US-00020 TABLE 18 process conditions of examples after hot rolling Wall Pipe At At TT Tt thickness Steel no (° C.) (min) (° C.) (min) (mm) D D-1 910 10′ 650 15 38.1

(105) The method led to a quenched and tempered steel pipe that, after final cooling from the tempering temperature, presents a microstructure comprising 99% of martensite, balance is ferrite and bainite.

(106) Furthermore, the quenched and tempered steel pipe obtained has an outer diameter of 374.65 mm.

(107) 1. Determination of Yield (Ys) and Tensile Strengths (UTs)

(108) A specimen has been taken from the seamless pipe D-1. Yield strength (Ys in MPa), ultimate tensile strength (UTs in MPa) and elongation at break (A in %) have been assessed in the longitudinal direction.

(109) The results on the mechanical properties are summarized in Table 19:

(110) TABLE-US-00021 TABLE 19 Mechanical properties (Ys, UTs and A (%)) Ys UTs Ratio A Specimen (MPa) (MPa) Ys/UTs (%) D-1.1 996 1134 0.88 17.6

(111) 2. Hardenability According to Jominy Tests

(112) Hardenability (based on the Rockwell scale) of a specimen obtained from the composition disclosed in Table 17 has been studied according to the Jominy tests.

(113) 2.1. Procedure

(114) The shape and dimension of the specimen have been standardized according to the requirements of the Jominy test (ASTM A255).

(115) The Jominy testing was performed after austenization at an austenitizing temperature (AT) of 910° C. and kept at this temperature for 10 minutes (At: austenitization time).

(116) These tests were performed by quenching one end of the specimen with a water quench, measuring the hardness of the specimen at 1.5 mm (approximately one-sixteenth inch) increments from the quenched end and then preparing a plot of the hardness measurements versus distance from the quenched end.

(117) A rapid drop-off in hardness with increasing distance from the quenched end is indicative of low hardenability (hardness). Hence the closer the Jominy curve is to a horizontal line, the greater is the hardenability (hardness).

(118) Generally, the distance from the water quenched end at which the hardness becomes less than Rockwell 50 HRC is referred to herein as the Jominy depth.

(119) 2.2. Results

(120) FIG. 6 illustrates the Jominy curve (hardness based on the Rockwell scale) wherein hardness measurements versus distance from the water quenched end are plotted.

(121) The results on this figure show that the Jominy curve remains flat, approximately around 50 HRC, up to a distance of 40 mm from the quenched end of the specimen.

(122) These results demonstrate that hardness remain stable throughout the length of the tested specimen shows a high hardenability.

(123) It is estimated that such quenchability could enable to obtain an entirely martensitic structure (99.9%) for a pipe of 40 mm wall thickness quenched with water.

(124) In other words, the production of a purely martensitic structure for the specimen made with the steel of the present invention was further corroborated by its hardenability Jominy curve.

(125) 3. Hardenability Comparison with Comparative Steels

(126) 3.1. Steel Composition

(127) Table 20 illustrates the chemical composition of a comparative steel (the amounts indicated are calculated in weight percentage, the balance of said composition is made with iron).

(128) TABLE-US-00022 TABLE 20 Chemical composition of Steel-F Steel C Si Mn P S Cr Mo Ni F 0.29 0.19 0.33 0.011 0.0014 0.95 0.8 0.04 Cu Al Ti Nb V B N 0.02 0.046 0.017 — 0.003 0.0012 0.0046

(129) 3.2. Procedure

(130) Specimen issued from steel compositions F has been standardized according to the requirements of the Jominy test.

(131) The Jominy testing was performed after austenization at an austenitizing temperature (AT) of 910° C. and kept at this temperature for 10 minutes (At: austenitization time).

(132) 3.3. Results

(133) FIG. 7 illustrates the Jominy curves (hardness based on the Rockwell scale) of specimen from steel composition F wherein hardness measurements versus distance from the water quenched end are plotted.

(134) The results on this figure show that the Jominy curve of this specimen is not flat and significantly drops-off with increasing distance from the quenched end.

(135) In particular, the curve of the specimen obtained from steel composition F has an inflexion point around 15 mm before significantly dipping.

(136) These results clearly show that hardness is not stable throughout the length of the tested specimens.

(137) These results also corroborate the fact that the performed quenchability is not capable of leading to an entirely martensitic structure. Indeed, the structure of this specimen is composed of less than 90% of martensite at a distance of 40 mm from the quenched end.

(138) In particular, it means that such quenchability will not enable to obtain an entirely martensitic structure (99.9%) for a pipe of 40 mm wall thickness quenched with water (whether measured with external quench or external and internal quench) but rather a structure having less than 90% of martensite.