Effect of normalizing on low temperature aging impact properties of EH36 ship plate steel

60 mm thick EH36 ship plate steel, samples before and after normalizing were taken for aging impact test .

The test results show that the normalized EH36 ship plate steel has the same structure from the surface to the core , the grains are fine and evenly distributed , the -40 mantra aging impact is qualified , and the low temperature aging performance has been significantly improved . The effects of interstitial atoms C and N distribution, dislocation changes and microstructure evolution on aging properties of EH36 grade ship plate steel after normalizing are analyzed and discussed . The analysis results show that the reduction of supersaturation of C and N atoms in ferrite after normalizing, the change of dislocation shape and the reduction of dislocation density are the main reasons for the improvement of aging impact performance .

In the manufacturing process of ships , steel plates have to go through various processes such as pretreatment , processing , cold forming , assembly , welding, etc. Due to the plastic deformation of cold working and the internal stress deformation of welding, the
The phenomenon that the steel strength and hardness increase while the plasticity and toughness decrease is called strain aging [ 1 ] . The strain aging test of ship plate steel is mainly artificial aging for simulating its strain aging effect in the shipbuilding process . The classification societies of various countries for marine structural steel plates clearly stipulate that a series of temperature strain aging impact tests must be carried out when the ship plate factory is approved , and the strain aging impact value must meet the requirements of the ship regulations. It can be seen that the strain of the ship plate steel Aging performance is a difficult point for steel mills to successfully pass the classification society certification .
The article introduces the 60 mm thick EH36 ship plate steel produced by Shaogang Hot Rolling Wide Plate Plant using TMCP process and TMCP process combined with normalizing heat treatment process , and analyzes the mechanical properties and microscopic properties of EH36 ship plate steel produced by TMCP process and normalizing process. Organizational conditions , and discuss the influence of normalizing process on the low temperature aging properties of thick gauge EH36 ship plate steel .

1 Design of composition of EH36 ship plate steel
According to the ship regulations GB 712 and the regulations of the classification societies of various countries , the 60 mm thick EH36 ship plate steel must not only meet a certain strength and plasticity , but also have a low temperature impact toughness of -40* , and must also ensure a low temperature of -40T in order to pass factory approval. For aging impact toughness, the specific mechanical property requirements are shown in Table 1 . Therefore, considering comprehensively , the composition of the 60 mm thick EH36 ship plate steel is designed according to the requirements in Table 2 .
( 1 ) Control of C content . The research shows that the essence of strain aging lies in the formation of Coriolis air mass . For low carbon microalloy steel , the main effect on strain aging is the Coriolis air mass formed around dislocations by the interstitial atoms C and N dissolved in a - Fe , the age hardening effect caused by the pinning effect on dislocations . Therefore , the key to improving the aging performance is how to control the content of free C and N and the way of existence of C and N. On the basis of meeting the strength of EH36 ship plate steel , it is necessary to reduce the C content in the steel as much as possible , and the reduction of C content is also conducive to improving the low temperature impact toughness and the welding performance of the steel plate .

Table 1 Mechanical property requirements of EH36 ship plate steel
Table 1 Mechanical property of EH36 grade shipbuilding steel

Yield strength /MPa	Tensile strength /MPa	Elongation /%	-40* shock value (longitudinal) /J	-40* aging impact value /J
$ 355	490 ~ 630	$ 21	$ 41	$ 41

Table 2 Chemical composition of EH36 ship plate steel
Table 2 Composition of EH36 grade shipbuilding steel %

project	w _	w _	w _	wxya _	w Si	w Als	w Nb+Ti	w _
standard	W 0. 18	W 0.035	W 0.035	0.90-1.60	W 0.35	$ 0.015	—	—
internal control	0.10-0.15	W 0.005	W 0.015	1.40-1.60	0.20-0.30	$ 0.020	Appropriate amount	W 40 x10 -4

( 2 ) Control of steel purity . The key to the purity of molten steel is to control the N content and the content of impurity elements that affect the low-temperature toughness of ship plate steel . When the N content is high, the amount of dissolved N in a - Fe will reach supersaturation , resulting in a stronger strain aging effect . According to the previous experience in the production test of ship plate steel, when the aging impact toughness is required , the N content is generally controlled below 40 x10 -4 % . The presence of a large number of inclusions or large inclusions can easily cause stress concentration, resulting in a decrease in toughness . S , P , As , Sn , etc. are easy to segregate elements in steel . Since they segregate at grain boundaries , they will reduce the surface energy of grain boundaries and lead to intergranular brittle fracture , thus significantly reducing the low temperature impact toughness of steel . Since S , P , As , Sn and other elements that are prone to segregation have great harmful effects , their content should be strictly controlled. Therefore, the content of N and inclusions should be strictly controlled during smelting , and the inclusions should be modified to Change the shape of inclusions and reduce the content of easily segregated elements .
( 3 ) Control of Nb and Ti microalloying elements . During the rolling process of ship plate steel, the precipitated Nb and Ti fine carbides and nitrides are pinned on the grain boundaries and dislocations through the strain-induced mechanism, which inhibits the progress of austenite recrystallization and the growth of grains. It plays the role of grain refinement ; the dispersion and precipitation during rolling and cooling also play the role of precipitation strengthening . Adding a certain amount of Ti element to steel is beneficial to form a large amount of stable TiN at high temperature , thereby reducing the existence of free N. Generally, it is more appropriate to add 0.010% - 0.020% Ti to low alloy steel .

• experiment method

2 . 1 Production process route
60 mm thick EH36 ship plate steel is: hot metal desulfurization treatment — 120 t converter smelting — LR refining T RH vacuum degassing — slab continuous casting ( 250 mm thick slab ) —cutting and storage — continuous casting slab heating — Descaling — Rolling in 3 450 mm rolling mill — ACC cooling — Straightening — Finishing ( or surface shot blasting — Normalizing — Air cooling ) — Sampling — Inspection and storage .
2 . 2 Production process of test steel
The test steel is 250 mm thick continuous casting slab produced by the steelmaking department of Shaoguan Iron and Steel Co., Ltd. , which is produced by two-stage controlled rolling ( that is, austenite recrystallization zone + non-recrystallization zone controlled rolling ), combined with appropriate ACC controlled cooling process. 60 mm thick EH36 ship plate steel . for comparison

The effect of different processes on the low temperature aging performance , some ship plate steels were taken for normalizing treatment . According to the Andrews temperature empirical formula 2 , the A c3 temperature is estimated to be 845* , combined with the experience of the previous production test , the heating temperature of the EH36 ship plate steel is controlled according to A c3 + (50 - 100 ) * , that is, set at 920 ±10* , the heating coefficient is set to 2. 0 ~ 2. 1 min/mm , and air-cooled to room temperature after normalizing .
2 . 3 Sample preparation and test
In order to analyze the effect of normalizing process on the low-temperature aging properties of EH36 ship plate steel, the samples of TMCP process (ie before normalizing ) and normalizing process ( ie after normalizing ) were respectively taken for a series of temperature strain aging impact tests . The samples before and after normalizing were processed into 12 mm X 12 mm X 400 mm splines, pre-stretched at 5% strain, and then processed into V -notch impact samples, and then the samples before and after normalizing were placed in the Heating in a box-type resistance furnace at 250* , holding for 1 h, air cooling to room temperature, and finally performing a series of temperature impact tests , and taking samples before and after normalizing and processing them into metallographic samples for microstructure analysis .

1. Test results and analysis and discussion

3 . 1 Performance before and after normalizing and before and after aging
Table 3 is the mechanical properties of 60 mm thick EH36 ship plate before and after normalizing, Table 4 is the aging impact performance of a series of temperatures before and after normalizing at the 1/4 thickness of 60 mm thick EH36 ship plate steel , and Table 5 is 60 Impact properties of mm thick EH36 grade normalized ship plate steel at 1/2 thickness before and after aging . It can be seen from Table 3 that after normalizing 1/

1. thickness and 1/2 thickness of the yield strength change is small , the tensile strength is slightly reduced , but the elongation after fracture is increased , and the mechanical properties after normalizing meet the requirements of ship regulations ; it can be seen from Table 4 that the 60 The 0* and -20 * aging impact properties of the mm thick EH36 ship plate are both qualified (the standard requires an average impact value of M 41 J, and a single value of M 29 J ) , but the impact value margin is not large, and the -40* aging impact The performance is unqualified ; the 0* ~ -40* aging impact performance of the EH36 ship plate after normalizing is all qualified , the fluctuation is small and rich

Table 3 Mechanical properties of EH36 ship plate before and after normalizing
Table 3 Mechanical properties of EH36 grade shipbuilding steel before and after normalizing

Steel plate sampling position	state	Yield strength / MPa	Tensile strength / MPa	Elongation after break /%
1/4 thickness	Before normalizing	395	555	28
1/2 thickness	Before normalizing	365	540	29
1/4 thickness	After normalizing	385	525	35
1/2 thickness	After normalizing	370	515	30

margin is large , and it can be seen that the low-temperature aging performance after normalizing is improved by at least one level compared with that before normalizing ; from Table 5 , it can be seen that the impact value of the normalized sample after aging is slightly lower than that before aging , but its impact value margin is still large , meeting the requirements of ship regulations .
Table 4 Aging impact performance of EH36 ship plate before and after normalizing
Table 4 Aging impact property of EH36 grade shipbuilding steel before and after normalizing

Steel plate sampling position	Aging impact value before normalizing /J	Aging impact value after normalizing /J	T /*
1/4 thickness	160,170,176	213,235,200	0
	175,78,33	182,180,240	-20
	7,86,19	136,172,178	-40

Table 5 Impact properties of EH36 grade normalized ship plate steel before and after aging
Table 5 Impact property of normalized EH36 grade shipbuilding steel before and after aging

Steel plate sampling position	Impact value before aging /J	Impact value after aging /J	T /*
1/2 thickness	268,254,273	170,213,184	0
	206,202,248	160,148,156	-20
	178,173,172	134,130,114	-40

in conclusion
(1 ) The -40* aging impact toughness of the 60 mm thick EH36 ship plate steel produced by the TMCP process combined with the normalizing heat treatment process meets the requirements of the classification societies of various countries .
( 2) Normalizing heat treatment makes C and N atoms in the supersaturated solid solution a - Continuous diffusion and redistribution in Fe reduces the supersaturation of C and N atoms in a-Fe, weakening the aging strengthening effect ; normalizing heat treatment significantly reduces the dislocation density , and the dislocation shape changes , resulting in dislocation The aggregation effect of solute atoms , the trend of aging strengthening is weakened .
( 3 ) After the normalizing treatment, the grains are refined and homogenized, which increases the total area of the grain boundary, reduces the segregation of impurity elements on the grain boundary , and avoids the aging precipitation and aggregation of C and N , thereby reducing the ductile-brittle transition temperature ; The phenomenon of stress concentration is reduced , so as to obtain higher low temperature impact toughness and low temperature aging impact toughness .

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