Effect of Controlled Rolling and Controlled Cooling Process on Microstructure and Properties of AH32 Ship Plate Steel

Microstructure and properties of AH32 ship plate steel was studied by thermal simulation test .
The results show that the controlled compression deformation at low temperature can refine the ferrite grains of AH32 steel, and the rapid cooling after deformation can refine the pearlite lamellar. The combination of low temperature controlled rolling + rapid cooling can improve the strength and toughness of AH32 ship plate steel.

Key words: ship plate steel; rapid cooling; low temperature controlled rolling; microstructure and properties

AH32 ship plate steel test materials and methods :
The test steel is taken from AH32 ship plate steel, the chemical composition (mass fraction, % ) is: WO. 18C, 0. 1 ~ 0.5Si, 0.9 ~ 1.6Mn , 0.02 ~ 0.05Nb, 0. 02 ~ 0. 050V , W0. 02Ti , >0.015Als .
The test was carried out on the Gleeble3800 thermal simulation testing machine. The sample was a round bar with a middle size of 6 mm X 30 mm and two ends of 0.10 mm X 25 mm . Two compression tests were used, and each pass was deformed by 50% . Four processes were designed in the experiment : process 1 , one-stage continuous rolling + UFC ( rapid cooling) + ACC (layer cooling), the deformation temperature is above 950°C ; process 2, two-stage rolling + UFC + ACC , two The deformation temperatures of the stages are 1050 C and 910 C respectively ; process 3, two-stage low-temperature rolling + UFC + ACC ; process 4 , two-stage low-temperature rolling + ACC without UFC , and the two-stage deformation temperatures of process 3 and process 4 are 1050 C and 860C . _ In the test, the simulated UFC rapid cooling cooling rate is 80 C/s , and the ACC interlayer cooling cooling rate is 7 C/s . See Table 1 for temperature parameters .

Table 1 Thermal simulation test temperature parameters Table 1 Temperature parameters of the process simulation test
craft serial number	open cold temperature /c	Out UFC temperature /c	Out of ACC temperature /c	Final cold
1	950	680	610	Cool in air at 1.5 C/s to 300 C
2	900	680	610	Cool in air at 1.5 C/s to 300 C
3	850	650	580	Cool in air at 1.5 C/s to 300 C
4	850	none	610	Cool in air at 1.5 C/s to 300 C

The sample numbers obtained by simulating the above four processes are 1 , 2 , 3 , 4 in sequence . Samples were taken from the compression deformation part of each sample near 1/4 of the diameter of the galvanic couple side to make a metallographic sample. After grinding and polishing, it was corroded with 4% nitric acid alcohol, and the metallographic structure was observed with an Olympus ( PMG3 ) optical microscope . The metallographic sample was ground and polished again, sprayed with carbon in a vacuum state, extracted the carbon film, and observed the size and distribution of the two-phase particles under a Tecnai G2 20 transmission electron microscope. Then cut a 0.150 mm thick film sample by wire cutting here , grind it to a thickness of 0.030 mm , and observe the microstructure under the transmission electron microscope after double-jet shearing in the electrolyte .

Due to the small size of the thermal simulation sample, the strength cannot be tested by tensile test. The FV-300 Vickers hardness tester (load is 5 kg ) is used to test the Vickers hardness HV5 of the metallographic sample, and then use the "Conversion Value of Ferrous Metal Hardness and Strength" Table [ 3 ] is converted into intensity to estimate its intensity level.
AH32 ship plate steel
 Organization and Precipitated Phase
four samples are all ferrite + pearlite, and the morphology of ferrite is massive and needle-like. The ferrite and pearlite of sample 3 are the smallest, and the pearlite content is the least; the structure of sample 4 is slightly coarser than that of sample 3 , and there is no obvious difference between the structure of sample 1 and 2 , but it is coarser than that of samples 3 and 4 . The ferrite grain sizes of the four samples calculated by the intercept point method are sample 1 : & 5 ^m, sample 2 : 8. 9 p,m , sample 3 : 6. 6 ^m, sample 4 : 7. 6 fimo calculates the ratio of pearlite and ferrite in the microstructure photo with analysis software.

AH32 Ship Plate Steel
HV5 Vickers hardness was measured on the observation surfaces of the above four metallographic samples and converted into strength [ 3 ] . The results are shown in Table 2 . The maximum hardness difference of the four samples is 12 HV5, the hardness of sample 3 is the highest, and the hardness of sample 1 is the lowest.
Table 2 Mechanical properties of test steel under different processes
Table 2 Mechanical properties of the tested steels for
different processes

craft	1	2	3	4
Hardness /HV5	153.7	161. 3	165.3	157.3
Tensile strength /MPa	522	547	559	535

 The relationship between the microstructure and properties of AH32 steel The above test results show that the deformation temperature and cooling rate have a certain influence on the structure and properties of the ship plate steel. The grain size, pearlite content and hardness of the AH32 ship plate steel obtained by the four processes relation.

Effect of Controlled Rolling on Microstructure and Properties of AH32 Steel
Samples 1 , 2 , and 3 have the same cooling path and different deformation temperatures, resulting in different microstructures and properties.
Compared with sample 1 with two-stage deformation , sample 1 with one-stage continuous deformation has a lower deformation temperature and slightly higher hardness than sample 1 , but there is no significant difference in ferrite grain size and pearlite content. This may be related to the small size of the thermal simulation sample, the few deformation passes and the small amount of deformation. In actual production, the two-stage controlled rolling has a significant effect on grain refinement. Multi-pass continuous rolling in the high-temperature austenite recrystallization zone makes the austenite structure undergo repeated strong deformation. The recrystallization strengthening mechanism forms uniform and fine austenite recrystallized grains; then, multi-pass large deformation rolling is carried out in the non-recrystallized area to form deformation bands inside the grains, providing ferrite nucleation points, and can be further refined ferrite grains.
Samples 2 and 3 are two-stage deformation, sample 3 has a low final deformation temperature, the precipitated two-phase particles are the smallest, the average size of ferrite grains is the smallest, and its toughness is the highest. This is because lowering the final rolling temperature is conducive to the accumulation of deformation, which slows down the recovery of austenite structure to a certain extent, makes more structural defects and deformation energy remain until phase transformation occurs, promotes the nucleation of ferrite grains, and is easy to obtain Fine-grain structure improves the strength level of the material. On the other hand, under the same deformation conditions, the size of the deformed austenite unrecrystallized and recrystallized grains will increase with the increase of temperature, so reducing the finish rolling temperature will make the microstructure after phase transformation The growth trend of the steel becomes slow [ 6 ] , therefore, the ferrite and pearlite formed by low-temperature deformation are finer, thereby improving its strength and toughness.

Effect of Cooling Rate on Microstructure and Properties of AH32 Steel
The cooling rate also has a great influence on the microstructure and properties of the steel. Different cooling conditions and cooling rates can obtain different microstructures at room temperature, thereby affecting the properties. The microstructure of AH32 ship plate steel is mainly ferrite and pearlite, and the cooling rate will affect its phase transition temperature and the size of ferrite and pearlite after phase transition. Compared with samples 3 and 4 , both simulate two-stage controlled rolling with the same finish rolling temperature, but different cooling paths. The ferrite grains and pearlite lamellar spacing of sample 3 which adopts rapid cooling are finer than that of sample 4 , and correspondingly, its toughness is also better than that of sample 4 . This is because increasing the cooling rate can inhibit the growth of austenite and ferrite grains after phase transformation, and further refine the ferrite grains; on the other hand, the interlamellar spacing of pearlite mainly depends on its formation temperature , the cooling rate of sample 3 after rolling is fast, the undercooling degree increases, and the phase transition temperature decreases, so the pearlite lamellar spacing is finer. The fine spacing of pearlite sheets is beneficial to improve the toughness of steel, and the refinement of ferrite grains is conducive to improving the strength and toughness of steel. Therefore, sample 3 has the best toughness.
AH32 ship plate mainly depends on the ferrite grain size and pearlite lamellar spacing, and the pearlite content has little effect on the toughness .

The two-stage deformation in the recrystallization zone and the non-recrystallization zone can refine the ferrite grains of AH32 ship plate steel, and the low temperature deformation can promote the precipitation of fine and dispersed niobium and titanium carbonitrides in the steel, which is beneficial to the ferrite grains. Nucleate and inhibit the growth of ferrite grains, obtain fine ferrite and pearlite structures, and improve the toughness of AH32 ship plate steel.

Rapid cooling can refine the pearlite lamellar spacing of AH32 steel, improve its hardness, and can also inhibit the growth of austenite and phase-transformed ferrite grains, and further refine the ferrite grains.
From the above studies, it can be seen that low temperature deformation has a significant effect on refining the ferrite grains of AH32 ship plate steel, and rapid cooling after rolling can further refine the interlamellar spacing of pearlite and improve the toughness. Therefore, the strength and toughness of ship plate steel can be improved by the process of low temperature controlled rolling + rapid cooling.
references:

1. Imai S . Recent progress and future trends for shipbuilding steel [ J ] . Welding International , 2008 , 22 ( 11 ): 755-761 .
2. Wu Di, Wang Guodong, Zhao Xianming, et al . Rapid cooling production process of high-strength ribbed steel bars after rolling P . State Intellectual Property Office of the People's Republic of China: CN 1718770a,2006.

3 GB/T 1172-1999 , ferrous metal hardness and strength conversion value S.
[ 4 ] Liu Yunxu, Zhu Qihui, Liu Ping . Pearlite phase transformation and mechanical properties of high carbon non-quenched and tempered steel during continuous cooling J. Metal Heat Treatment , 2000,25(4 ) : 9-11.
5 Weimin Mao, Jingchuan Zhu, Jian Li , et al . Structure and Properties of Metallic Materials [ M ] . Beijing: Tsinghua University Press, 2008 : 217-218 .
[ 6 Tang Di, Gao Yuanjun, Xu Hongqing . Effects of controlled rolling and controlled cooling on microstructure of DH36 steel and optimization of process parameters J. Steel Rolling , 2000,17(4 ) : 7-11.

The statistical results of the two-phase particle distribution of the four samples are shown in Fig. 3 . It can be seen from Fig. 3 that the number of two-phase particles in the four samples is not much different. The particle size range of sample 1 is 30–100 nm ; the particle size of sample 2 is mainly about 50 nm ; the particle size of sample 3 is smaller, Mainly 20 - 40 nm ; sample 4 is dominated by 30 - 80 nm particles. Two-phase particle morphology

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