Micro alloyed steel and method for producing said steel

Abstract

The invention deals with Steel for seamless pipes comprising the following chemical composition elements in weight percent: 0.04≤C≤ to 0.18, 0.10≤Si≤0.60, 0.80≤Mn≤1.90, P≤0.020, S≤0.01, 0.01≤Al≤0.06, 0.50≤Cu≤1.20, 0.10≤Cr≤0.60, 0.60≤Ni≤1.20, 0.25≤Mo≤0.60, B≤0.005, V≤0.060, Ti≤0.050, 0.010≤Nb≤0.050, 0.10≤W≤0.50, N≤0.012, where the balance is Fe and inevitable impurities. The steel of the invention can be used in offshore applications, line process pipes, structural and mechanical applications, especially where harsh environmental conditions and service temperatures down to −80° C. occur.

Claims

1. Steel for seamless pipes, comprising, in weight percent: 0.04≤C≤ to 0.18, 0.10≤Si≤0.60, 0.80≤Mn≤1.90, P≤0.020, S≤0.01, 0.01≤Al≤0.06, 0.50≤Cu≤1.20, 0.10≤Cr ≤0.60, 0.60≤Ni≤1.20, 0.25≤Mo≤0.60, B≤0.005, V≤0.060, Ti≤0.010, 0.010≤Nb≤0.050, 0.10≤W≤0.50, N≤0.012, Fe and inevitable impurities.

2. The steel according to claim 1, wherein C is between 0.04% and 0.12%.

3. The steel according to claim 1, wherein C is between 0.05% and 0.08%.

4. The steel according to claim 1, wherein Mn is between 1.15% and 1.60%.

5. The steel according to claim 1, wherein Cu is between 0.60% and 1%.

6. The steel according to claim 1, wherein Mo is between 0.35% and 0.50%.

7. The steel according to claim 1, wherein W is between 0.10% and 0.30%.

8. The steel according to claim 1, wherein V is below 0.008%.

9. The steel according to claim 1, wherein a weight ratio of carbon to manganese satisfies: 0.031≤C/Mn≤0.070.

10. The steel according to claim 1, wherein in weight percent:
CE.sub.IIW≤0.65% and CE.sub.Pcm≤0.30%
where
CE.sub.IIW=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15 and applies when C>0.12%, and
CE.sub.Pcm=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10+5B and applies when C≤0.12%.

11. The steel according to claim 1, which has a microstructure comprising less than 15% of ferrite, the balance being bainite and martensite.

12. The steel according to claim 1, which has an average yield strength of 550 MPa to 890 MPa, and a toughness in joules at −60° C. of at least 10% of the yield strength.

13. The steel according to claim 1, which has an average yield strength of at least 690 MPa, and an average toughness at −80° C. of at least 69 J.

14. A method of producing a seamless steel pipe, the method comprising: hot forming the steel according to claim 1 at a temperature of 1100° C. to 1280° C. to obtain a pipe, then, heating the pipe to an austenitizing temperature AT above or equal to 890° C. and keeping the pipe at the austenitizing temperature AT for 5 to 30 minutes followed by cooling the pipe to an ambient temperature to obtain a quenched pipe, and then, heating the quenched pipe to a tempering temperature TT of 580° C. to 700° C. and keeping the pipe at the tempering temperature TT for a tempering time Tt of 20 to 60 minutes followed by cooling the pipe to the ambient temperature to obtain a quenched and tempered pipe.

15. A structural and/or mechanical component, made of the steel according to claim 1.

16. A line pipe component and/or an oil and gas accessory, made of the steel according to claim 1.

17. Steel for seamless pipes, comprising, in weight percent: 0.04≤C≤ to 0.18, 0.10≤Si≤0.60, 0.80≤Mn≤1.90, P≤0.020, S≤0.01, 0.01≤Al≤0.06, 0.50≤Cu≤1.20, 0.10≤Cr ≤0.60, 0.60≤Ni≤1.20, 0.25≤Mo≤0.60, B≤0.005, V≤0.060, Ti≤0.050, 0.010≤Nb≤0.050, 0.10≤W≤0.30, N≤0.012, Fe and inevitable impurities.

18. The steel according to claim 17, wherein Ti is below or equal to 0.010%.

19. The steel according to claim 17, wherein C is between 0.04% and 0.12%.

20. The steel according to claim 17, wherein C is between 0.05% and 0.08%.

Description

(1) In a preferred embodiment, such steel is used to obtain a seamless pipe with a wall thickness above 20 mm for structural, mechanical or line pipe applications either onshore or offshore.

(2) FIG. 1 illustrates the charpy transition curves (Joules) of steels 1 to 4.

(3) FIG. 2 illustrates the mechanical properties of steel 1 and 2 with tungsten, and 3 and 4 without tungsten.

(4) Also, within the framework of the present invention, the influence of chemical composition elements, preferable microstructural features and production process parameters will be further detailed below.

(5) It is reminded that the chemical composition ranges are expressed in weight percent and include upper and lower limits.

(6) Carbon: 0.04% to 0.18%

(7) Carbon is a strong austenite former that significantly increases the yield strength and the hardness of the steel according to the invention. Below 0.04% the yield strength and the tensile strength decrease significantly and there is a risk to have yield strength below expectations. Above 0.18%, properties such as weldability, ductility and toughness are negatively affected and a classical fully martensite microstructure is reached. Preferably the carbon content is between 0.04 to 0.12%. In an even preferred embodiment, the carbon content is between 0.05 and 0.08%, the limits being included.

(8) Silicon: 0.10% to 0.60%

(9) Silicon is an element which deoxidizes liquid steel. A content of at least 0.10% can produce such an effect. Silicon also increases strength and elongation at levels above 0.10% in the invention. Above 0.60% the toughness of the steel according to the invention is negatively affected, it decreases. To avoid such detrimental effect, the Si content is between 0.10 and 0.60%.

(10) Manganese: 0.80% to 1.90%

(11) Manganese is an element which improves the forgeability and hardenability of steel and it contributes to the steel quenchability. Furthermore, this element is also a strong austenite former which increases the strength of the steel. Consequently, its content should be at a minimum value of 0.80%. Above 1.90%, a decrease in weldability and toughness is expected in the steel according to the invention. Preferably, the Mn content is between 1.15% and 1.60%.

(12) Aluminium: 0.01% to 0.06%

(13) Aluminium is a powerful steel deoxidant and its presence also encourages the desulphurization of steel. It is added in an amount of at least 0.01% in order to have this effect.

(14) However, beyond 0.06%, there is saturation effect with regard to above mentioned effect. In addition, coarse and harmful to ductility Al nitrides tend to be formed. For these reasons, the Al content should be between 0.01 and 0.06%.

(15) Copper: 0.50% to 1.20%

(16) Copper is a very important for solution hardening but this element is known to generally be detrimental to toughness and weldability. In the steel according to the invention, Cu increases both yield strength and tensile strength. In combination with the Ni content of the invention, the loss of toughness and weldability attributed to the Cu presence is ineffective, Ni neutralizes the negative effect of Cu when combined with it in the steel. For this reason, the minimum Cu content should be 0.50%. Above 1.20% the surface quality of the steel according to the invention is negatively impacted by the hot rolling processes. Preferably, the copper content shall between 0.60 and 1%.

(17) Chromium: 0.10% to 0.60%

(18) The presence of Chromium in the steel according to the invention creates chromium precipitates that increase especially the yield strength. For this reason, a minimum Cr content of 0.10% is needed. Above 0.60% the precipitation density effects negatively the toughness and weldability of the steel according to the invention.

(19) Nickel: 0.60% to 1.20%

(20) Nickel is a very important element for solution hardening in the steel of the invention. Ni increases yield strength and tensile strength. In combination with the presence of Cu, it improves the toughness properties. For this reason, its minimum content is 0.60%. Above 1.20% the surface quality of the steel according to the invention is negatively impacted by the hot rolling processes.

(21) Molybdenum: 0.25% to 0.60%

(22) Molybdenum increases both yield and tensile strength and supports the homogeneity of the mechanical properties, the microstructure and the toughness in the base material through the length and thickness of the pipe. Below 0.25% the above described effects are not effective enough. Above 0.60% the steel behavior when it comes to weldability and toughness is negatively impacted. Preferably the Mo content is between 0.35 and 0.50%, limits being included.

(23) Niobium: 0.010% to 0.050%

(24) Niobium presence leads to carbide and/or nitride precipitates leading to a fine grain size microstructure by grain boundary pinning effects. Therefore increase in yield strength is obtained by Hall Petch effect. The homogeneity of grain sizes improves the toughness behavior. For all these effects, a minimum of 0.010% of Nb is needed. Above 0.050%, a strict control of the nitrogen content is needed so as to avoid a brittle effect of NbC. In addition above 0.050%, a decrease of the toughness behavior is expected for the steel according to the invention.

(25) Tungsten: 0.10% to 0.50%

(26) The addition of tungsten is intended to provide to the produced tubes with a stable yield strength i.e. low variation of yield strength up to an operational temperature of 200° C. The addition of tungsten brings also a steady stress-strain relation. Above 0.10%, tungsten also additionally supports the positive effects of molybdenum alloying mentioned above. For this reason a minimum content of 0.10% of tungsten is needed in the steel according to the invention. Above 0.50% of tungsten, the toughness and weldability of the steel according to the invention start to decrease. Preferably, the tungsten content is between 0.10% and 0.30%.

(27) Boron: ≤0.005%

(28) Boron is an impurity in the steel according to the invention. This element is not voluntarily added. Above 0.005% it impacts negatively the weldability because after welding it is expected to create hard spots in the heat infected zone, thus decreasing the weldability of the steel according to the invention.

(29) Vanadium: ≤0.060%

(30) Above 0.060% vanadium precipitates increase the risk of having a scatter in toughness values at low temperatures and/or a shift of transition temperatures to higher temperatures. Consequently, the toughness properties are negatively impacted by vanadium contents above 0.060%. Preferably, the vanadium content is strictly below 0.008%.

(31) Titanium: ≤0.050%

(32) This is an impurity element. It is not voluntarily added in the steel according to the invention. Above 0.050%, carbon and nitrogen precipitates with Ti such as TiN and TiC change the balance of carbide and nitride precipitation with niobium and in consequence the beneficial effects of niobium will be hindered. The yield strength of the steel will be negatively affected, it will decrease. Preferably, the Ti content is below or equal 0.010%.

(33) Nitrogen: ≤0.012%

(34) Above 0.012% big sized nitride precipitations are expected and these precipitates will negatively affect the toughness behavior by changing the transition temperature in the upper range.

(35) Residual Elements

(36) The balance is made of Fe and inevitable impurities resulting from the steel production and casting processes. The contents of main impurity elements are limited as below defined for phosphorus and sulfur:
P≤0.020%
S≤0.005%

(37) Other elements such as Ca and REM (rare earth minerals) can also be present as unavoidable impurities.

(38) The sum of impurity element contents is lower than 0.1%.

(39) It should be noted that in a preferred embodiment, 0.031≤C/Mn≤0.070. This range allows the steel of the invention to be less sensitive to cooling rates most importantly for thick products where the cooling rate modifies significantly the microstructural features. The stability of properties such as toughness and yield strength is better in this range of chemical composition in weight percent.

(40) Method of Production

(41) The method claimed by the invention comprises at least the following successive steps listed below. In this best embodiment, a steel pipe is produced.

(42) A steel having the composition claimed by the invention is obtained according to casting methods known in the art. Then the steel is heated at a temperature between 1100° C. and 1280° C., so that at all points the temperature reached is favorable to the high rates of deformation the steel will undergo during hot forming. This temperature range is needed to be in the austenitic range. Preferably the maximum temperature is lower than 1280° C. The ingot or billet is then hot formed in at least one step with the common worldwide used hot forming processes e.g. forging, pilger process, conti mandrel, premium quality finishing process to a pipe with the desired dimensions.

(43) The minimum deformation ratio shall be at least 3.

(44) The pipe is then austenitized i.e. heated up to a temperature AT where the microstructure is austenitic. The austenitization temperature AT is above Ac3, preferably above 890° C. The pipe made of steel according to the invention is then kept at the austenitization temperature AT for an austenitization time At of at least 5 minutes, the objective being that at all points of the pipe, the temperature reached is at least equal to the austenitization temperature. So as to make sure that the temperature is homogeneous throughout the pipe. The austenitization time At shall not be above 30 minutes because above such duration, the austenite grains grow undesirably large and lead to a coarser final structure. This would be detrimental to toughness.

(45) Then, the pipe made of steel according to the invention is cooled to the ambient temperature, preferably using water quenching. Then, the quenched pipe made of steel according to the invention is preferably tempered i.e. heated and held at a tempering temperature TT comprised between 580° C. and 700° C. Such tempering is done during a tempering time Tt between 20 and 60 minutes. This leads to a quenched and tempered steel pipe.

(46) Finally, the quenched and tempered steel pipe according to the invention is cooled to the ambient temperature using air cooling.

(47) In this manner, a quenched and tempered pipe made of steel is obtained which contains in area less than 15% percentage of polygonal ferrite, the balance is bainitic structure and martensite. The sum of polygonal ferrite, bainite and martensite is 100%.

(48) Microstructural Features

(49) Martensite

(50) The martensite content in the steel according to the invention depends on cooling speed during quenching operation. In combination with the chemical composition it depends on wall thickness and the martensite content is between 5% and 100%. The balance to 100% is polygonal ferrite and bainite.

(51) Polygonal Ferrite

(52) In a preferred embodiment, the quenched and tempered steel pipe according to the invention, after final cooling, presents a microstructure with less than 15% of polygonal ferrite in volume fraction. Ideally, there is no ferrite in the steel since it would impact negatively the YS and UTS of the steel according to the invention.

(53) Bainite

(54) The bainite content in the steel according to the invention depends on cooling speed during quenching operation. In combination with the chemical composition it is limited to a maximum of 80%. The balance to 100% is polygonal ferrite and martensite. A bainite content above 80% leads to low yield strength and tensile strength as well as inhomogeneous properties though the wall thickness.

(55) The invention will be illustrated below on the basis of the following non-limiting examples:

(56) Steels have been prepared and their compositions are presented in the following table 1, expressed in weight percent.

(57) The compositions of steels 1 and 2 are according to the invention.

(58) For the purpose of comparison the composition 3 and 4 are used for the fabrication of the reference steel and are therefore not according to the invention.

(59) TABLE-US-00001 TABLE 1 Chemical compositions of examples Steel No C Si Mn P S Al Cu Cr 1 0.06 0.41 1.53 0.013 0.002 0.03 0.83 0.25 2 0.07 0.37 1.42 0.012 0.003 0.03 0.67 0.23 3 0.06 0.40 1.48 0.013 0.002 0.03 0.17 0.24 4 0.06 0.40 1.49 0.013 0.002 0.03 0.42 0.24 Steel No Ni Mo B V Ti Nb W N 1 0.87 0.48 <0.002 0.002 0.006 0.015 0.27 0.007 2 0.80 0.46 <0.002 0.003 0.004 0.020 0.19 0.009 3 0.18 0.48 <0.002 <0.005 0.010 0.014 <0.01  0.007 4 0.50 0.48 <0.002 0.06 0.010 0.014 <0.01  0.007

(60) Underlined values are not in conformity with the invention.

(61) The upstream process i.e. from melting to hot forming, is done with commonly-known manufacturing method for seamless steel pipes after heating at a temperature between 1150° C. and 1260° C. for hot forming. For example, it is desirable that molten steel of the above constituent composition be melted by commonly-used melting practices. The common methods involved are the continuous or ingot casting process. Next, these materials are heated, 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.

(62) The compositions of table 1 have undergone a production process that can be summarized in the table 2 below with:

(63) AT (° C.): Austenitization temperature in ° C.

(64) At: Austenitization time in minutes

(65) The cooling after austenitization is done with water quenching.

(66) TT: Tempering temperature in ° C.

(67) Tt: Tempering time in minutes

(68) The cooling after tempering is an air cooling.

(69) TABLE-US-00002 TABLE 2 process conditions of examples after hot rolling Heat Treatment Wall AT At TT Tt thickness No [° C.] [min] [° C.] [min] [mm] 1 930 10′ 630 60′ 30 2 a 930 630 45′ 40 b 920 640 20′ 27.8 3 930 630 60′ 30 4 930 630 60′ 30

(70) The steel references 1 and 2 are according to the invention while reference 3 and 4 are not, in terms of chemical composition. The process parameters are all according to the invention. This led to quenched and tempered steel tubes that, after final cooling from the tempering temperature, present a microstructure comprising less than 15% of ferrite, the balance being bainite and martensite.

(71) The process of table 2 applied to the chemical compositions of table 1 led also to specific mechanical behavior, and toughness values that are summarized in table 3 and 4. YS, in MPa and ksi, is the yield strength obtained in tensile test as defined in standards ASTM A370 and ASTM E8. UTS, in MPa and ksi, is the tensile strength obtained in tensile test as defined in standards ASTM A370 and ASTM E8.

(72) TABLE-US-00003 TABLE 3 Impact energy results thickness avg [J] [mm] T [° C.] transverse min [J] max [J] Steel N.sup.o1 30 0 186 170 200 −20 175 173 177 −40 154 144 165 −60 134 120 146 −80 100 97 105 Steel N.sup.o2-a 40 0 225 210 247 −20 202 197 208 −40 182 178 184 −60 134 130 143 −80 103 101 106 Steel N.sup.o2-b 27.8 0 272 262 282 −20 246 220 279 −40 225 220 227 −60 192 181 194 −80 155 150 159 Steel N.sup.o3 30 0 227 222 231 −20 220 212 228 −40 222 212 234 −60 176 148 196 −80 122 116 130 Steel N.sup.o4 30 0 178 160 190 −20 117 92 114 −40 101 18 144 −60 53 22 102 −80 19 12 26

(73) The mean impact energy values of the steels according to the invention is equal or above 100 J at −80° C. Steel No. 3 has good charpy values as well but the mechanical properties are too low. Steel 4 has sufficient mechanical properties but the charpy values start to scatter already at −40° C.

(74) TABLE-US-00004 TABLE 4 Mechanical properties YS [MPa] UTS [MPa] elongation A.sub.min [%] Steel N.sup.o1 776 820 22.0 Steel N.sup.o2-a 740 806 19.9 Steel N.sup.o2-b 707 786 20.8 Steel N.sup.o3 667 728 26.0 Steel N.sup.o4 747 821 25.0

(75) The steel according to the invention has preferably more than 690 MPa of yield strength and an impact energy average value of at least 100 J at −80° C.

(76) Welding tests have been performed on steel No. 2 by using FCAW process. The results of the charpy tests at −60° C. in the fusion line and heat effected zone are shown in table 5.

(77) TABLE-US-00005 TABLE 5 Impact energy at −60° C. for steel N.sup.o2-b location Kcv 1 [J] Kcv 2 [J] Kcv 3 [J] Kcv avg [J] FL 265 273 269 269 FL + 2 198 198 172 189 FL + 5 224 211 235 223

(78) Where FL is the fusion line and FL+X represents distance X in mm away from the fusion line. The impact energy values for the steels with tungsten are even in the welded condition very good and suitable for arctic applications.