l555m steel pipeline (New technological inventions) with excellent low temperature toughness and manufacturing method thereof

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Technical field

The invention belongs to the field of high-strength low-alloy steel for metal materials, specifically relates to a manufacturing method of pipeline steel for oil and gas transmission pipelines, and in particular to a manufacturing method of l555m steel pipeline steel with excellent low-temperature toughness and its hot-rolled coils.

Background technique

In recent years, the construction of energy transmission pipelines has developed rapidly. In order to improve transmission efficiency and operation safety, the requirements for the strength and toughness of the selected steel raw materials are getting higher and higher. At present, major pipeline trunk lines at home and abroad basically use L485M and l555m steel pipeline grades.

When the pipeline area is in an extremely cold zone or the transportation medium is low-temperature liquefied natural gas, from the perspective of operational stability and safety, higher requirements are put forward for the low-temperature toughness index of the raw materials, such as low-temperature drop weight and ductile-brittle transition temperature requirement testing. The temperature is lower and Charpy impact energy requirements are higher. Therefore, these projects have put forward higher personalized requirements. It is no longer enough to just meet the API SPEC 5L specification.

Although there is currently a lot of information on hot-rolled coils of l555m steel pipeline at home and abroad, there are very few that can meet the requirements for low-temperature fracture toughness below -30°C. The following is a brief introduction to several similar documents:

A large-diameter, large-wall-thickness X80 spiral submerged arc welded pipe and its manufacturing method. The patented ingredients include C≤0.06%, Si 0.17%～0.25%, Mn≤1.6%, S≤0.004%, P≤0.012%, Cr 0.20%～0.30%, Mo 0.15%～0.30%, Ni 0.10%～0.20 %, Nb 0.07～0.10%, V≤0.05%, Ti≤0.03%, Cu 0.1%～0.2%, Al≤0.05%, etc. This patent adds more precious alloy elements to make up for the strength loss caused by low C and low Mn, which is costly.

The l555m steel pipeline steel disclosed in the above documents can all meet the strength index requirements, but most of them are difficult to meet the low temperature fracture toughness requirements below -30°C. Some alloy systems are complex, have many precious alloy elements, and are high in cost. Some processes adopt Conventional laminar cooling is not conducive to grain refinement and tissue uniformity, affects product performance and increases production costs.

Contents of the invention

The purpose of this invention is to design a simple and economical alloy system, match the precise two-stage controlled rolling and ultra-fast cooling + laminar cooling process, and provide an l555m steel pipeline hot-rolled coil with excellent low-temperature toughness and its Manufacturing method.

L555M-PSL2 chemical element content (%)

	Element	C ^*	And ^*	Mn ^*	P	S	Cr ^*	In ^*	with ^*	For ^*	B	More
welded pipe	minimum value	–	–	–	–	–	–	–	–	–	–	Nb+V+Ti≤0.15*
welded pipe	maximum value	0.12	0.45	1.85	0.025	0.015	0.5	1	0.5	0.5	0.001	Nb+V+Ti≤0.15*

L555M-PSL2 mechanical properties

condition	Test temperature	Tensile strength σ _b MPa	Yield point σ _s Mpa	Elongation after break or elongation δ %	Impact absorbed energy KV2 J
welded pipe		625~825	555~705	*
Outer diameter D≤48	0°C				≥40
Outer diameter D>48~56	0°C				≥54
Outer diameter D>56~84	0°C				≥68

The specific technical solutions are:

The present invention proposes a l555m steel pipeline with excellent low-temperature toughness. Its chemical composition weight percentage is: C0.045%~0.065%, Si 0.10%~0.25%, Mn 1.66%~1.76%, Nb 0.055%~0.075%, Ti0.010%～0.025%, Mo 0.21%～0.29%, Cr 0.11%～0.19%, Ni 0.05%～0.15%, Als0.015%～0.045%, P≤0.015%, S≤0.003%, N≤0.006 %, the balance is iron and inevitable impurities, and the cold crack sensitivity coefficient Pcm≤0.19%, where Pcm=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V /10+5B.

The composition design idea of l555m steel pipeline steel is to adopt C-Mn-Nb-Mo micro-alloy design, and add a small amount of Cr, Ni, Ti and other micro-alloy elements. The two-stage controlled rolling and ultra-fast cooling + laminar cooling process obtain fine particles. The acicular ferrite structure ensures that pipeline steel has high strength, high toughness and excellent low temperature fracture toughness. Its main basic elements and functions are as follows:

C: The C content in the present invention is 0.045% to 0.065%.
Si: The Si content in the present invention is 0.10% to 0.25%.
Mn: The Mn content of the present invention is 1.66% to 1.76%.
Nb: Nb is a precious element and the cost is high. Therefore, the Nb content in the present invention is 0.055% to 0.075%.
Mo: The Mo content in the present invention is 0.21% to 0.29%.
Ti: Therefore, the Ti content in the present invention is 0.010% to 0.025%.
Cr: The Cr content in the present invention is 0.11% to 0.19%.
Ni: The Ni content in the present invention is 0.05% to 0.15%.
Als: The Als content of the present invention is 0.015%-0.045%.

The invention also proposes a method for manufacturing hot-rolled coils of l555m steel pipeline steel with good low-temperature toughness by using thin slab continuous casting and rolling processes. Its production process involves: molten iron pretreatment – converter smelting – outside furnace refining (RH + LF + calcium treatment) – continuous casting – slab heating – rolling – ultra-fast cooling + laminar cooling – coiling.

The l555m steel pipeline grade hot-rolled coils of the present invention have excellent comprehensive properties: yield strength is 560-640MPa, tensile strength is 660-750MPa, elongation after break ≥30%, yield ratio ≤0.90; -40°C Charpy impact energy (3 samples) Akv≥300J, -40℃ average drop weight tear test shear area (2 samples) SA≥95%; hardness HV10≤240, cold bending qualified.

Realization effect:

Compared with the prior art, the present invention has the following beneficial effects:

(1) The alloy design is simple, using C-Mn-Nb-Mo micro-alloy design, and adding a small amount of Cr, Ni, Ti and other micro-alloy elements, without Cu, V, etc., and the alloy cost is low.
(2) The thickness of the continuous casting slab is ≤170mm, and the short-process continuous casting and rolling production process is adopted, which is highly efficient and saves energy.
(3) Ultra-fast cooling + laminar flow cooling are used to control cooling in two stages to ensure fine grains and uniform structure.
(4) The product has excellent comprehensive properties, especially low temperature toughness. At -40°C, the average shear area in the drop weight tear test is ≥95%.
(5) l555m steel pipeline steel produced using the above process can be widely used in cold areas near the poles or areas with harsh natural environments such as deserts and deep seas.