Improving the Impact Toughness of A333Gr. 3 Seamless Steel Tube for Low Temperature

In order to improve the impact toughness of A333Gr. 3 seamless steel pipes for low temperature, the chemical composition, harmful elements, gas content, mechanical properties as rolled, metallographic structure and mechanical properties after heat treatment were analyzed and compared for three kinds of seamless steel pipes for low temperature .The carbon content has a significant impact on the low-temperature impact toughness of the steel pipe. The lower the carbon content, the higher the impact toughness. The phosphorus content in the steel can significantly increase the brittle transition temperature of the steel. The phosphorus content in the steel should be further reduced, and the rolling temperature of the steel pipe should be controlled at the same time. , reduce the final rolling temperature of the steel pipe, increase the cooling rate of the steel pipe, and refine the grain size. These measures are all helpful to improve the low-temperature impact toughness of the seamless steel pipe .
Key words seamless pipe; toughness; chemical composition; metallographic structure; mechanical properties

Usually, various liquefied petroleum gas, liquid ammonia, liquid oxygen, liquid nitrogen and other production, storage containers and pipelines, as well as equipment serving in cold regions, are called cryogenic containers, and the steel used to manufacture these containers is collectively referred to as low-temperature steel . Steel is generally divided into nickel-free steel and nickel-containing steel. Nickel-free steel generally refers to fine-grained steel and low-temperature high-strength steel, and its service temperature is above -60 °C; nickel-containing steel refers to the alloying element nickel added to the steel , so that it can be solid-dissolved in ferrite, so that the low-temperature toughness of the matrix is significantly improved, and the low-temperature brittleness common to metal materials with body-centered cubic lattices is changed, and its service temperature can reach below -196C. With the development of the petrochemical industry With the development of new technology and new equipment, the liquefaction, separation, storage, transportation and application of gas are very common in various countries. The development of these low-temperature technologies and equipment has promoted the development of steel for low-temperature pressure vessels .
As we all know, general structural steel will undergo a transition from toughness to brittleness with the decrease of temperature . If the structure or container is used at the transition temperature or brittle state, even at a lower stress level, it is inevitable that low stress failure will not occur . Therefore, the world For materials used at low temperatures, countries require not only a certain strength, but also sufficient toughness . Iron and steel materials, especially ferritic low-temperature steel, will show toughness when the temperature drops to a certain critical point or a certain area. Significant or sudden decrease, so this type of steel should not be used in this transition range, let alone at this temperature. This temperature is called the ductile-brittle transition temperature of steel, which is determined by Charpy V-notch impact test or notch drop weight test . sentence

A333Gr . 3 seamless steel for low temperature is used as the steel used at low temperature - 80 ~ - 101 °C , and its low temperature toughness is the most critical technical index in mechanical properties .
1 Standard requirements for A333Gr. 3 seamless steel pipes for low temperature
1.1 Chemical composition
The A333Gr. 3 seamless steel pipe for low temperature is according to the ASTM standard, and the chemical composition requirements are shown in Table 1 .
Table 1 chemical composition (mass fraction)
Table 1 Chemical composition %
C Si Mn PS Ni
W 0.19 0.18 ~ 0.37 0.31 ~ 0.64 W 0.025 W 0.025 3.18 ~ 3.82
1.2 Mechanical properties
2 for ASTM standard mechanical property requirements of A333Gr. 3 seamless steel pipes for low temperature .
Table 2 Mechanical properties
Table 2 Mechanical properties

level	Tensile Strength /MPa	yield Strength /MPa (	Impact energy -100C ), A kv /J	Elongation / %
A333Gr. 3	$ 450	$ 240	$ 18	$ 30

Note: The impact sample size is 10 mm X 10 mm X 55 mm.

2 Test and evaluation of low temperature steel
A333Gr.3 low-temperature seamless steel pipes produced by 3 companies (numbered 1 # , 2 # and 3 # ) were selected as samples, and the chemical composition, harmful elements in steel, gas content, metallographic structure, and mechanical properties were tested respectively. analysis .
The main element chemical composition and harmful element analysis results of A333Gr.3 seamless steel pipe for low temperature are shown in Table 3 and Table 4.

Table 3 Chemical composition of main elements (mass fraction)
Table 3 Chemical composition of main element %

serial number	C	S	And	min	p	-R	Mo	us	With	the
1 #	0.088	0.009	0. 266	0. 471	0.013	0.015	<0. 005	3. 52	<0. 005	<0. 005
2 #	0.069	0.002	0. 293	0. 486	0.006	0. 106	0.006	3. 52	0. 114	<0. 005
3 #	0. 118	0.001	0. 270	0. 488	0.004	0.021	0. 017	3. 40	0.070	0.022

Table 4 Harmful element content (mass fraction)					%
	Table 4 Harmful element content
serial number	Pb	sn	As	Sb	Bi
1 #	<0. 005	0.010	0.007	0. 000 6	<0. 00010
2#	<0. 005	0. 008	0. 009	0. 003 0	<0. 00001
3#	<0. 005	<0. 005	0. 006	0. 002 0	<0. 00001

From the comparison of chemical composition in Table 3 , it can be seen that 2 # has the lowest carbon content, 3 # has the highest carbon content, the content of silicon and manganese is basically the same among the three companies, the sulfur and phosphorus content of 1 # is relatively high, and the chromium content of 1 # and 3 # is almost the same, 2 # is higher, close to an order of magnitude, the nickel content of 1 # and 2 # is about 0.1% higher than that of 3 # , from table 4 it can be seen that the harmful elements of 1 # , 2 # and 3 # are basically the same .
the A333Gr. 3 low-temperature seamless steel pipe produced by 3 companies is shown in Table 5. From Table 5 , it can be seen that the hydrogen and oxygen content are basically the same, and the nitrogen content of 1 # is relatively low .
Table 5 Gas content (mass fraction)

	Table 5	gas content	10-6 _
serial number	h	o	N
1 #	0.4	31	54
2 #	0.3	29	88
3#	0. 3	30	68

The mechanical properties of A333Gr. 3 seamless steel pipes for low temperature produced by three companies are shown in Table 6. It can be seen from Table 6 that the yield strength of 2 # is higher than that of the other two, and the yield strength of 1 # is the lowest . # and 3 # have the same tensile strength, 1 # is lower, elongation is the highest in 2 # , followed by 1 # , 2 # has the highest impact toughness, and 1 # has the lowest impact toughness .

Table 6 Mechanical properties in as-rolled condition
Table 6		Mechanical property after hot-rolled
serial number	yield Strength /MPa	Tensile Strength /MPa	Elongation /%	Impact energy ( -100C ), A kv /J
1 #	335	475	33.5	4
2 #	425	525	40.0	61
3 #	380	525	32.5	33
standard	$ 240	$ 450	$ 30	—

Note: The impact test sample size of 1 # and 2 # is 10 mm X 7.5 mm X 55 mm , and the American standard impact value should be greater than or equal to 14J ; 3 #Steel impact test sample size is 10mmX 5 mm X 55 mm , and the American standard impact value should be greater than or equal to 9 J.
the A333Gr.3 low-temperature seamless steel pipe produced by the three companies after heat treatment is shown in Table 7. From Table 7 , it can be seen that the metallographic structure of the three companies is F + P , and the grain sizes of # and 3# are both Grade 11 , # is grade 9 , non-metallic inclusions are equivalent to 3 .
1 # , 2 # and 3 # steel after heat treatment is shown in Fig . 1 ~ 3. It can be seen from Fig. 1 ~ 3 that in 1 # metallographic structure, the pearlite is flaky, while 2 # steel and 3 # is granular pearlite .

Table 7 Heat treatment metallographic structure
Table 7 Metallurgical structure after heat treatment
Non-metallic inclusions (grade)

serial number	vulcanization thing	oxidation thing	Silicic acid Salt	spherical oxide	single particle Spheroids	organize	( class)
1 #	0.5	0.5	0.5	0.5	—	F+P	9
2 #	0.5	1. 0	1. 0	1. 0	0.5	F+P	11

3 # 0. 5 1. 0 0. 5 0. 5
1. 5 F + P 11

Fig． 3 Metallurgical structure of 3 # steel after heat treatment

Table 8 Mechanical properties after heat treatment
Table 8 Mechanical property after heat treatment

serial number	Yield strength / MPa	Tensile strength / MPa	Elongation/%	Impact energy (- 100 seven), A kv /J
1 #	390	505	39.0	18
2 #	395	530	41. 0	121
3 #	430	560	27.5	twenty three

Note: The heat treatment system is normalizing at 850 °C, the holding time for normalizing is 30 min , and air cooling .

Table 8 shows the mechanical properties of A333Gr. 3 low-temperature seamless steel pipes produced by three companies after heat treatment . It can be seen from Table 8 that 3 # The yield strength of steel is higher than the other two, the yield strength of 1 # is the lowest, the tensile strength of 3 # steel is the highest, the lowest of 1 # , the elongation of 2 # is the highest, the second is 1 # , the impact toughness of 2 # is the highest, 1# # The lowest impact toughness .
3 Analysis and Discussion
( 1 ) P is easy to produce grain boundary segregation, oxygen in the steel precipitates in the form of various oxides at the grain boundary, which significantly increases the brittle transition temperature of the steel, resulting in low stress brittle fracture . From the comparison of chemical composition, it can be seen that the P and S contents of 2 # and 3 # A333Gr . 3 # , so the impact performance is low . It can also be seen from the chemical composition of 1 # , 2 # and 3 # that the carbon content also affects the impact performance, and 2 # has a lower carbon content, so its impact performance is higher .

1. Grain size is an important factor affecting low stress brittle fracture of steel . Fine grains make the metal have higher fracture strength and lower the brittle transition temperature. The finer the grains, the higher the strength, the stronger the plasticity, and the higher the strength, the better the toughness . It can be seen from the metallographic examination that the grain size of 2 # and 3 # A333Gr. 3 low-temperature seamless steel pipes produced is finer than that of 1 # , and the pearlite in the formed structure is granular pearlite, while the produced 1 # is basically Flaky body, so the mechanical properties of 2 # and 3 # steel produced are better than 1 # , and the main factors affecting the structure are chemical composition and rolling temperature .
2. a of steel in the form of miscibility with iron, which reduces interstitial impurities, refines grains, and controls the size of the second phase in steel. shape distribution etc. It can also be seen from the chemical composition of 1 # , 2 # and 3 # steel that the main alloying element nickel in the steel is controlled at the standard center line .

 in conclusion
To produce A333Gr. 3 seamless steel pipes for low temperature that fully meet the standards and improve the low temperature impact toughness of A333Gr. 3 , the suggestions are as follows

three o'clock .

1. Reduce carbon content . Carbon content has a significant impact on low temperature impact toughness, the lower the carbon content, the higher the impact performance .
2. Control phosphorus and sulfur content, especially phosphorus content . Phosphorus is easy to produce grain boundary segregation, which significantly increases the brittle transition temperature of steel .
3. Control the rolling temperature, reduce the final rolling temperature of the steel pipe, increase the cooling speed of the steel pipe, and refine the grain .

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