X80 High Steel Grade Pipelines

This paper mainly summarizes the composition design ideas in the development process of X80 high-grade pipeline seamless steel pipes, and the properties of the sample pipes after heat treatment. After the development, it shows that C and Mn steel types are selected, V+B or Mo+V or added separately Micro-alloying of B elements can produce seamless steel pipes for pipelines with low carbon equivalent, good impact performance and strength meeting the standard requirements
Key words: high steel grade; pipeline; seamless pipe; microalloying; phase transition point; mechanical properties
oil and gas fields, seamless steel pipes for pipelines have changed from the initial X42 to the more commonly used X65 . A new version of the API Spec 5L standard also added the requirements for X90.X100 steel grade seamless steel pipes, which fully shows that high steel grades have become the future trend.
In the seamless steel pipe industry, pipeline pipes have been successfully developed to X100 in the world, and X80 has been developed in the actual engineering application . The domestic development has reached X90, and the engineering application is still at the X70 level. In order to catch up with the pace of industry development, It is necessary to develop seamless steel pipes for high-grade pipelines
Due to the limited data that can be used for reference, using the accumulated development experience in high-grade seamless line pipe for many years, with the help of the production capacity of multiple sets of smelting and rolling units, and on the basis of X65\X70 mass supply, actively carry out Focus on the development of seamless steel pipes for X80 high steel grade pipelines .

Ingredient group	C	mn	Ni	Cr	Know	V	Of	Nb	B	CEV	pcm	Nb+V+Ti
1	0.14	1.58	0.02	0.03	0.01	0.07	0.010	0.045	0.0015	0.429	0.249	0.12
	0.15	1.57	0.02	0.03	0.01	0.07	0.010	0.041	0.0005	0.438	0.254	0.12
	0.15	1.56	0.02	0.03	0.01	0.07	0.010	0.040	0.0005	0.437	0.254	0.12
2	0.14	1.513	0.016	0.028	0.121	0.084	0.020	0.034		0.444	0.247	0.138
	0.14	1.540	0.020	0.030	0.120	0.080	0.020	0.030	0.0005	0.447	0.250	0.13
	0.14	1.540	0.020	0.030	0.120	0.080	0.020	0.030	0.0005	0.447	0.250	0.13
3	0.12	1.49	0.04	0.1	0.02	0.004	0.02	0.038	0.0016	0.402	0.225	0.06
	0.13	1.48	0.04	0.1	0.03	0.010	0.02	0.033	0.0012	0.413	0.231	0.06

1 Difficulties in production
X80 steel grade seamless line pipe, due to its special requirements on composition and performance, not only has low carbon content, carbon equivalent ( CEV) and cold crack coefficient ( Pcm) , but also has high yield strength and impact toughness , which determines the high difficulty in the development of this steel type, and because its composition is highly sensitive to heat treatment, it is easy to cause poor performance stability-
2- ingredient design
Compared with sheet metal and welded pipe, seamless steel pipe has a relatively small amount of deformation due to its special forming process, so it cannot rely on the organizational requirements brought about by large deformation in the production process of sheet metal (the basic material of welded pipe is also sheet metal Then changes and optimizations must be made in terms of composition design-
At present, seamless steel pipes for pipelines basically use C and Mn steel types. With the development of micro-metallurgy technology, the production of group
Weaving is a uniform tempered sorbite structure, low carbon equivalent, good impact performance, and meets the standard requirements Carbon ( C) : As a traditional and widely used strengthening element, it has low cost and easy
It has become the industry consensus to use seamless steel pipes for strength-seeking pipelines. The characteristics of the control. The level of its content has a direct and significant relationship with the strength of the steel——
In the production process of seamless pipes for X80 high-grade steel pipelines, this idea is also adopted. The amount of knots is directly proportional to the strength, but when the content is high, its impact toughness will be significantly reduced; at the same time, Taiwan has many years of X65 . Summarize and conclude that the following main components are designed to ensure good welding performance, and the carbon content should be selected at a lower level. Therefore, it is more appropriate to control the carbon content at 0.08%-0.14% .

Manganese (Nfa) : In steel, solid solution strengthening is the mainstay. Years of research and production have shown that in microalloying, its content is proportional to strength, has little effect on impact toughness, and is rich in resources. Within the scope of standard requirements It can be controlled at a slightly higher level, 1.4%~1.7% is suitable”
®g(Nb) : It is mainly used in steel for its fine-grain strengthening and precipitation strengthening mechanisms. Through the refinement of the grain, it is the most ideal strengthening method to increase the strength without reducing the low-temperature impact toughness of the steel, and it can also prevent the austenite rolling process through the pinning effect of the precipitates in the high-temperature section The growth of medium grains can also provide a good hot-rolled structure for subsequent heat treatment. However, the Nb element reduces the high-temperature thermoplasticity of the steel, thereby increasing the hot cracking tendency of the Nb- containing steel slab. ";Comprehensive consideration , Nb is suitable at 0.03%~0.05% ";
Starvation ( V) : Starvation induces the precipitation of VC\VN compounds during the rolling process, which can prevent the recrystallization of deformed austenite and prevent the coarsening of recrystallized austenite grains. Since the standard has a certain limit on the N content, the V microalloyed steel with ordinary N content will produce more obvious precipitation strengthening at temperatures below 800°C . Therefore, it is advisable to design the V content at 0.05%~0.08% ";
$&<&): It can delay the formation of pearlite during the cooling process of austenite, improve the hardenability of steel, and at the same time refine the grains, improve the strength and impact toughness of quenched and tempered steel. However, high Cr content will be bad for welding performance. This design content is advisable below 0 ";%.
Nipper ( Nfo) : The main alloying element of this steel, which delays the formation of ferrite during the transformation from austenite to ferrite, and promotes the formation of bainite, plays an important role in controlling the transformation structure, and It has a good effect on the hardenability of steel, so it is suitable to add about 0 ”% Mo to steel
Boron ( B) : The main function is to improve the hardenability of steel, and a very small amount of boron ( 0.0005 -0.0030%) can have a significant effect. When the boron content is less than 0.0005% , it has little effect on improving hardenability. When it is higher than 0.003% , the boron phase (Fe3(CB), Fe3(BC)6, Fe2B) produced in the steel will precipitate along the austenite grain boundary, resulting in hot embrittlement. Therefore, the B content is designed at 0.0005%~ 0.0020%, and the quenching heating temperature and time must be strictly controlled”
Other elements: copper, sickle, titanium, as residual elements, not intentionally added; phosphorus ( P) , sulfur ( S) non-metallic elements, controlled within the range of standard requirements P< 0.015%, S< 0.005%"; Nitrogen (N) < 0.01%,
three types of X80 steel grades with composition ratios were smelted . The ingredients are shown in Table 1";
references

1. ";High-strength and high-toughness X90 thick-wall seamless pipeline steel pipe and its manufacturing method";, Chinese National Patent CN102051527A.
2. Wang Xiaoxiang, Li Yanfeng. New progress in the development of high-strength pipeline steel pipes in China [J]. Steel Tube. 2011,2.40( 1) : 12-18.

3 heat treatment requirements
2 shows the transition point temperatures of the three steel grades that were sampled for inspection or calculated according to industrial formulas .
According to the transformation point temperature of each steel grade, a reasonable quenching and tempering temperature should be formulated.
Production mechanical properties are shown in Table 3";

Ingredient group	Yield Strength	tensile strength	Elongation	Yield ratio	Shock VH		10X10		(J)
	MPa	MPa	%	%	temperature	i	2	3	average
1	585	685	37.5	0.85	o°c	215	250	221	229
	565	685	38.5	0.79	o°c	242	234	221	232
	600	700	38	0.86	-10°C	96.7	102	104	101
	610	715	31	0.85	-10°C	107	75.9	94.2	92
	560	645	29	0.87	-20°C	73.4	75.7	88.1	79
	605	690	23	0.88	-20°C	59.5	101	78.7	80
2	620	725	40.5	0.86	o°c	168	161	124	151
	595	670	28	0.89	o°c	162	182	156	167
	615	705	28	0.87	-10°C	84	142	151	126
	630	705	27	0.89	-10°C	251	177	149	192
	635	715	30	0.89	-20°C	126	126	108	120
	610	685	27.5	0.89	-20°C	191	164	167	174
3	600	675	23.1	0.889	o°c	175	166	172	171
	605	680	21.3	0.89	o°c	159	155	168	160.7
	575	660	22.2	0.871	o°c	196	196	187	193
	605	680	22.2	0.89	o°c	194	213	199	202
API 5L requirements	555-705	625-825	20	W0.93	o°c	Single value three 20			227
Remarks: The above data are longitudinal tensile samples. No. 1 uses 25.4 as the lead sample,				2 uses 38.1 lead sample, No. 3 uses 12.7 round sample.

From the above results , the three kinds of composition design, the mechanical properties after quenching and tempering can all meet the requirements, indicating that the composition design, heat treatment method and process are reasonable.

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