Development of EEH550 seamless steel pipe for offshore platform

Abstract: DNV Classification Society EEH550 material is mostly used for steel plates. Currently, only BBN Steel Co., Ltd. has successfully developed and passed the DNV certification for this steel grade seamless steel pipe. Through theoretical analysis and design of the composition of EEH550 , combined with the advantages of our heat treatment equipment, we have successfully developed and mass-produced high-strength, high-toughness and thick-walled EEH550 offshore platform pipes. The test results show that: EEH550 steel pipes with a wall thickness below 40mm use low-carbon Mn -Mo , Cr~Mo materials, after quenching and tempering, the performance can meet the requirements of EEH550 , the carbon equivalent is below 0.50 % , the yield strength is as high as 550MPa , and the impact energy at -40°C is as high as 112J . The material has high strength and high toughness , Uniform performance and good weldability.

Keywords: seamless steel pipes for offshore platforms; thick-walled pipes; high strength and high toughness; weldability; rich oil and gas resources in the ocean.

With the shortage of oil resources and the rise of oil prices, the development of marine resources is becoming more and more important. The demand for platform steel is gradually increasing. Since 2004 , China's offshore engineering industry has entered a period of vigorous development. Jack-up drilling platforms are currently the offshore oil production platforms with the largest demand in the world, and have broad market prospects. The development of jack-up drilling platforms has effectively driven the demand for structural steel pipes. Take the 400 - foot jack- up drilling platform as an example . With the increase of working water depth, the requirement of lightweight hull, the change of working environment, and the increase of structural strength, structural steel pipes (especially steel pipes used in pile leg structures) are trending towards large diameter, large wall thickness, high grade, high strength and high precision. develop.
EEH550 is a product of high-strength quenched and tempered steel for welded structures in the Det Norske Veritas specification. It has good mechanical properties and welding properties and is widely used in the construction of offshore platforms. In the specification, mechanical properties such as yield strength, tensile strength and impact Requirements such as toughness are higher.
In order to prevent the pile leg branch from breaking, the steel pipe must meet the following requirements: ( 1) Stability and consistency of mechanical properties; ( 2) Low carbon equivalent can effectively improve welding quality; ( 3) High-precision seamless steel pipe to prevent pipe end necking ;
At present, for steel plates and bars, micro-alloying control combined with controlled rolling and controlled cooling technology is often used. The hot-rolled steel plates and bars produced have high strength and low-temperature impact toughness. For seamless steel pipes, especially thick-walled seamless For steel pipes, since the amount of reduction in each pass is uncontrollable, and the reducing/sizing process of the final forming requires the temperature of the steel pipe to be above 900°C and the amount of deformation to be below 5% , it is impossible to carry out controlled rolling; while the hollow section of the steel pipe Therefore, the controlled rolling and controlled cooling cannot be used in the manufacture of seamless steel pipes to improve the strength and toughness of the material]. Since the final use of the material needs to be welded, in order to ensure the welding performance, it is necessary to control the lower carbon equivalent, but in the heat treatment process, the lower carbon equivalent makes it difficult for the material to be hardened, especially in the production of thick-walled pipes. This problem is particularly prominent.
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Through repeated explorations and attempts, we have overcome many difficulties through reasonable composition design and heat treatment design, and successfully produced high-strength, high-toughness and thick-walled EEH550 offshore platform pipes, which have passed the certification of NDV classification society.
EEH550 seamless steel pipes for offshore platforms with good mechanical properties and weldability by optimizing the design composition and heat treatment process system .
1. Composition and performance requirements of EEH550
1.1 Composition requirements
According to the regulations of DNV-0S-B101 , the composition of EEH55O should meet the requirements of Table 1 .
Table 1 Chemical composition requirements of EEH550 (mass fraction%)

c	Si	mn	P	S Cr	Ni	Cu	Mo	Nb	V	Al	CEV
< 0.20	0.10 0.55	< 1.70	< 0.030	<< 0.030 0.20	< 0.40	< 0.35	< 0.08	0.02 0.05	0.05 0.10	> 0.020	< 0.50

Remarks: If Cr is intentionally added to the material, there is no requirement for Cr content.

1.2 Mechanical property requirements
According to the stipulations of DNV-0S-B101 , the mechanical performance samples of steel pipes are taken at a position 1/4T away from the outer surface in the delivery state , and their performance should meet the requirements in Table 2 .
Table 2 Mechanical property requirements

Yield strength Rp0.2 (Mpa)	Tensile strength Rm (Mpa)	A (%)	KV2 Longitudinal -40°C (J)
>550	670-830	>16	>37

1.3 Grain size requirements: The actual grain size should not be less than 6 grades in the delivery state .

2. Composition design
2.1 Effect of various components on performance

1. C : form pearlite or dispersed precipitated alloy carbides in the steel to strengthen the steel. With the increase of carbon content, the strength increases and the plasticity, toughness and weldability decrease. In order to ensure the toughness and weldability of the steel, reduce c is an inevitable trend;
2. Mn : A high Mn/C ratio is good for improving the yield strength and impact toughness of steel.
3. Mo : Aluminum-containing steel has higher strength and higher toughness than traditional ferrite-pearlite steel. Clamps have an inhibitory effect on the pearlite transformation of steel during cooling. The aluminum content in acicular ferritic steel and ultra-low carbon bainite steel is generally 0. 2-0. 4% .

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1. Nb ; Adding 0.02-0.5 % Nb to low-carbon manganese steel or manganese-aluminum steel has obvious grain refinement and precipitation hardening effects.
2. Ni : Improves the strength of steel while maintaining good plasticity and toughness. But due to its scarcity, the price is higher;
3. Cr : Improve material hardenability, increase strength, hardness and wear resistance, while reducing plasticity and toughness;
4. P : increase the cold brittleness of steel, make the welding performance worse, reduce the plasticity, and make the cold bending performance worse;
5. S : It causes hot brittleness in steel, reduces the ductility and toughness of steel, and sulfur is also detrimental to welding performance;

It can be seen that in order to ensure that the material has high strength, high elongation, high and low temperature impact toughness and good welding performance, C , P and S should be reduced as much as possible during composition design, and Mn , Mo , V , Nb , Cr and Ni should be added in appropriate amounts and other alloying elements. At the same time, in order to reduce production cost and carbon equivalent, alloy elements such as Mo and Ni should be reduced as much as possible.
2.2 Chemical composition requirements of high-strength steel at home and abroad
In order to facilitate intensive production, reduce production costs, and simplify the process route, its chemical composition should meet the chemical composition of high-strength steel at home and abroad. The specific chemical composition requirements are shown in Table 3 .
Table 3 Chemical composition requirements of high-strength steel at home and abroad

Grade	C	Si	mn	P	S	Cr	Ni	Cu	Mo	Nb	V
S550-	<	<	<	<	<	<	<	<	<	<	<
S690	0.20	0.80	1.70	0.020	0.010	1.50	2.00	0.50	0.70	0.11	0.12
Q550	<	<	<	<	<	<	<	<	<	<	<
	0.18	0.60	2.00	0.025	0.020	0.80	0.80	0.80	0.30	0.11	0.12
Q690	<	<	<	<	<	<	<	<	<	<	<
	0.18	0.60	2.00	0.025	0.020	1.00	0.80	0.80	0.30	0.11	0.12

2.3 Composition of domestic and foreign steel mills
The current chemical composition design of Tenaris is shown in Table 4 .
Table 4 Chemical composition of Tenaris and a domestic factory

Element	C	Si	mn	P	S	Cr	Mo	Nb	Ni	Ti	V	CEV
Tenaris	0.14	0.50	1.36	0.013	0.002	0.26	0.34	0.027	0.32	0.004	0.05	0.52

There is following defective in above-mentioned composition:

1. The material alloy content is high, the production cost is high, and it is difficult to promote;
2. Due to the addition of more Ni , V , Mo and other alloys to the steel types, the production cost is relatively high;
3. The two types of blanks can be used for foreign Q550-S690Q, but the carbon equivalent does not meet the requirements of EEH585 0 ;

2. 4- Component Design
Mn-Mo , Mn-V , and Cr-Mo , were used in the test . The chemical composition of the specific test steel is shown in Table 5 .
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Table 5 Chemical Composition of Test Steel

Element	C	Si	mn	P	S	Cr	Mo	Nb	Ni	Ti	V	CEV
Mn-Mo	0.16	0.22	1.26	0.009	0.006	0.23	0.20	0.03	/	/	0.07	0.50
Mn-V	0.17	0.42	1.50	0.012	0.005	0.20	/	0.03	/	/	0.08	0.47
Cr-Mo	0.14	0.23	0.48	0.010	0.006	0.90	0.44	0.03	/	/	/	0.49

3. Process flow and heat treatment
3.1 Process flow
Electric furnace + refining outside the furnace + vacuum degassing f continuous casting round billet ( 460mm) ~ steel tube into pipe ( 508X41mm) - quenching and tempering treatment ~ flaw detection, finishing.
3.2 Heat treatment
Due to the low carbon content of the material, the hardenability of the steel pipe is poor. In order to obtain a uniform structure after quenching and tempering, the cooling method of external spraying + internal spraying is adopted while the steel pipe is rotating. Specifically, the heat treatment scheme in Table 6 is adopted .

Table 6 heat treatment process
plan	Quenching temperature	Quenching heating time	steel pipe temperature after cooling	tempering temperature	Tempering heating time
Mn-Mo	950°C	185min	room temperature	580°C	420min
Mn-V	950 °C	185min	room temperature	570°C	420min
Cr-Mo	950°C	185min	room temperature	590°C	420min

4. Test results
Mechanical properties of Mn-Mo material
the Mn-Mo material after quenching and tempering are shown in Table 7, and the impact test results are shown in Table 8 .
Table 7 tensile test results

Location	Rp0.2	R m	A
head	625	755	21.0
tail	630	755	21.5

Table 8 Impact test results

direction	o°c				-20°C		-40 e C			-60°C
portrait	215	235	224	200	196	185	155	153	168	93	101	87
horizontal	198	195	180	175	166	180	112	127	141	42	42	46

From the test data in Table 7 and Table 8 , it can be seen that the material has stable high strength and relatively high high and low temperature impact, and the difference in mechanical properties between the head and the tail is small, and the difference in low temperature impact between the transverse and longitudinal directions is small.
Mechanical properties of Mn-V materials
Mn-V material after quenching and tempering are shown in Table 9, and the impact test results are shown in Table 10 .
Table 9 tensile test results

	Location			Rp0.2		R m				A
	head			581		709				21.5
	tail			566		697				21.5
Table 10 Impact test results
direction		0°C			-20°C		-40°C			・60°C
portrait	140	138	177	70	74 100	78	76	82	42	44	74
horizontal	132	130	120	90	104 108	37	32	26	32	10	26

From the test data in Table 9 and Table 10 , the impact energy of the material is unstable, and the impact energy in the horizontal and vertical directions is quite different, which cannot meet the low temperature impact requirements of EEH550 .
4.3 Mechanical properties of Cr-Mo material
the Cr-Mo material after quenching and tempering > see Table 12 for the impact test results-
Table 11 Tensile test results

	Location			Rp0.2 Rm					A
	head			608	733				20.3
	tail			607	734				22.1
Table 12 Impact test results
direction		0°C			・20°C	-40°C			・60°C
portrait	210	229	221	190	197 199 162	196	203	128	81	133
horizontal	198	187	200	169	170 168 67	160	196	162	61	62

From the test data in Table 11 and Table 12 , it can be seen that the material has stable high strength and relatively high high and low temperature impact, and the difference in mechanical properties between the head and the tail is small, and the difference in transverse and longitudinal low temperature impact is small.

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