Application in Production of Q345E Seamless Steel Pipe

On-line normalization process is on-line normalizing.A heat treatment process system that uses an online heating furnace to reheat the waste pipe to obtain fine grains and make the structure uniform, which can improve the impact toughness of the steel pipe. Q345E steel is the first high-strength low-alloy steel trial-produced in China. With Mn as the main element, its brittle transition temperature is about -40 ° C . , can be used as a variety of structural materials, but in actual production, it is rare to be used at low temperatures below - 40 C [ 1 ] . The specification of the steel pipe ordered by the user this time is ① 426 mm X 17 mm , which is mainly used for oil and gas pipelines in Siberia, and the service conditions are harsh. Therefore, users put forward new and higher requirements for the strength and low temperature toughness of steel. However, Baotou Steel Seamless Factory has never produced Q345E in ① 400 mm unit , so it organized scientific and technological research for this special contract, trying to adjust the process system, use the online normalization process, refine the grain, and produce a product that is stricter than the standard requirements. Low temperature and high toughness Q345E seamless steel pipe for conveying fluid.

1. Technical conditions
2. 1 for the comparison of Q345E performance requirements .

Table 1 Comparison of Q345E performance requirements

data request	Impact energy (longitudinal)		Yield Strength /MPa	tensile strength /MPa
	temperature /c	Impact value /J
Standard requirements [ 2 ]	-40	27.0	$ 315	490 ~ 665
user request	-46	29.4	$ 345	490 ~610
	-60	39.2

1. Production process

Converter smelting tLF out-of-furnace scouring — VD treatment T round billet continuous casting T tube billet sawing T heating T centering— N1 piercer T N2 piercer T pipe rolling mill T equalizing machine T air cooling T walking heating furnace T three rolls Calibration — N3 cooling bed tN4 or N5 cooling bed T straightening T cutting head and tail T inspection—spraying—storage .
Q345E in Baotou Steel Seamless Plant (① 180 mm unit) , an online normalization process is added.

1. Formulation of production process system

Baotou Steel Seamless Factory also has an example of producing Q345E , but the impact energy requirement of the product is -40 T , there is no impact energy value requirement of -46 °C , -60 °C , and the impact energy value requirement is higher than -40 C impact energy value. Focusing on this technical problem, the following research has been mainly carried out in cooperation with the Technology Center.
3 . 1 Formulation of normal process system and analysis of production results
Check the "Alloy Steel Handbook" [ 3 ] to get the physical properties of the steel: Ac3875 C , Ar3803 C , based on this and the field equipment process conditions, formulate the heating system of the tube billet and the final rolling temperature of the steel tube. The heating temperature control is shown in Table 2 :
Table 2 The actual heating temperature of the tube billet C

Preheating section	Heating section	Soaking stage I	Soak u section
700 ~1 100	1 250 ~ 1 290	1 250 ~ 1 290	1 230 ~ 1 270

The final rolling temperature is controlled at 860~ 900C .
August , 18 furnaces were first rolled , and their mechanical properties and -46C V -type impact and -60 CU -type impact were tested. The tensile strength, yield strength and elongation fully met the standards and user requirements.
In view of the above situation, in order to obtain a group with excellent comprehensive performance
Weaving, a reasonable online normalization process system has been formulated, see Figure 1 .

t 1 — rolling tube temperature; t 2 — reheating temperature; t 3 — Cooling temperature Figure 1 Schematic diagram of online normalization process

1. When entering the pipe rolling mill, the temperature should be controlled within the range of 950 ~ 980 C.

( 2 ) The temperature of the blank tube before entering the reheating furnace should be lower than 500° C . ( 3 ) The reheating temperature is 860~900° C .
3 . 2 . 2 Comparative analysis of steel pipe performance test results After discussion by the research group, it was decided to re-do online normalization treatment for 18 batches of steel pipes whose performance failed in the first ordinary production process . The performance after treatment is all qualified. See Table 3 and Table 4 for details .
Table 3 Comparison of steel pipe impact performance test results
A kv (- 46 C ) / J A ku (- 60 C ) /J
value
Ordinary process normalization process Ordinary process normalization process

maximum value	80	196	148	224
minimum value	6	twenty four	6	64
average value	29.96	109.54	93.00	143.12
user request	$ 29.4		$ 39.2

Table 4 Comparison of test results of mechanical properties of steel pipes

value	Ordinary craft R el /MPa R m /MPa		A/%	R el /MPa	Normalization process R m /MPa	A/%
maximum value	375	585	36.5	415	590	35
minimum value	335	535	27.5	345	535	25
average value	355	559	32	372	564	30
Standard [ 2 ]	$ 325	470 ~ 630	$ 21	$ 325	470 ~ 630	$ 21
user request	$ 315	490 ~ 665	$ 21	$ 315	490 ~ 665	$ 21

Demand, but the impact energy value can not meet the user's requirements.
3 . 2 Formulation of online normalization process system and analysis of production results

from table 3 and table 4 that after adopting the online normalization process, Craft system A kv (_ 46 C ) increased by 79. 58 J , and kU ( - 60 C ) increased by 50. 12 J. The normalization process has little change in the mechanical properties of the steel pipe.
3 . 2 . 3 Promotion and application of online normalization process system
With the successful experience of the first 18 batches of online normalization process, we have subsequently promoted the production of ① 426 mm X 17 mm ,
① 426 mm X 40 mm , ① 325 mm X 12 mm , ① 273 mm X 9/11 mm and other specifications of Q345E seamless steel pipe with low temperature impact toughness. The performance is all in line with the standard and the special requirements of users.
Here, only the tensile properties and impact energy values of ① 426 mm X 17 mm steel pipes are statistically analyzed, with a total of 111 batches. The test results fully meet the requirements of the standard and users, and provide data for the production of Q345E steel in the future. We have also carried out corresponding tests on the impact energy value required by the standard. See Table 5 and Table 6 for specific data .
Table 5 Tensile properties of actual production steel pipes

project	Yield Strength /MPa	tensile strength /MPa	Elongation / %
range value	345 ~ 415	530 ~ 590	28. 5 ~38. 5
average value	381	566	31. 2
user request	$ 315	490 ~ 665	$ 21.0
	Table 6 The impact energy of the actual production steel pipe		J
project	Impact energy A kV	Impact energy A kV	Impact energy A k
	( -40C )	( -46C )	( -60C ) _
Range Wei	50 ~192	52 ~196	94 ~218
average value	141	135	164
user request	$ 29.4	$ 29.4	$ 39.2

3 . 3 Comparison of tissues after online normalization
2 for the organization after online normalization . In the figure, the normalization process is used for the fine grains, and the normal process is used for the coarse grains.
It can be seen that through online normalization, the dispersion of pearlite is increased and the structure is refined. The grain size has also changed: the as-rolled grain size is 5.5 to 6.0 , and the normalized grain size is 8.0 to 8.5 .
4 Conclusion
( 1 ) Through the online normalization process design, the low temperature impact energy value of Q345E can be significantly improved, which can meet the user's requirements for impact energy values at -46 T and -60 , and the distribution is relatively concentrated. Therefore, it is entirely possible for Q345E seamless pipes to be used at low temperatures of -46 °C and -60 °C .

1. The on-site actual production shows that the low-temperature impact toughness can be improved by using the on-line normalization process, and the process system is suitable for mass production on site, and the qualified rate of performance reaches 100% . This batch of steel pipes has been sold for more than half a year and is in good condition.
2. Using the on-line normalization process, we have produced Q345E seamless steel pipes with specifications of $ 426 X 40 mm , ① 325 X 12 mm , ① 273 X9/11 mm, etc., whose low-temperature impact toughness meets the standard requirements .

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