Normalizing Process on Microstructure and Properties of Class E Thick Ship Plate Steel

The effects of different normalizing temperatures and holding times on the microstructure and properties of 60mmE grade high-strength thick ship plate steel were studied by optical microscope and mechanical property testing equipment . The results show that : compared with the controlled rolling and controlled cooling (TMCP) steel plate , the steel normalized at 880 ~ 940°C has lower tensile strength , higher elongation , and greatly improved impact toughness at -40°C , which is mainly due to It is caused by normalizing to eliminate Widmanstatten structure and refine grains . With the extension of the holding time , the grain growth is not obvious . The best normalizing process of test steel is normalizing at 880-910C for 60min .
【Keywords】 ship plate steel normalizing treatment mechanical properties microstructure

In recent years , Due to the rapid growth of world seaborne trade volume , The shipbuilding industry has experienced unprecedented prosperity ［ 1 ］ . Ships are gradually developing in the direction of large-scale and light weight , and the general-strength ship plate steel can no longer meet the requirements of the hull structure . With the rapid development of China's shipbuilding industry , higher and higher requirements are put forward for the output , performance and quality of ship plate steel . The application proportion of high-strength ship plate in shipbuilding industry is increasing . Controlled rolling and controlled cooling (TMCP) process is an effective method to improve the structure and properties of steel ［ 8 ］ . However, for thicker ship plate steel, the structure of the product obtained by the controlled rolling and controlled cooling process is usually not uniform . For example, there are some pearlite band defects , causing performance fluctuations , Therefore, post-normalizing treatment is required to obtain the best comprehensive performance . At home and abroad , there are few studies on post-heat treatment of wide and thick plates in TMCP state . This paper mainly studies the effect of normalizing process on the structure and properties of 60mm thick high strength ship plate steel , in order to obtain a reasonable process system .
1 Test materials and methods
1 -1 Test material
The materials used in the test were taken from the high-strength ship plate steel trial-manufactured by the 60mm thick controlled rolling and controlled cooling process in the steel plant . The specific chemical composition of the test materials is shown in Table 1 .

Table 1 Chemical composition of Class E ship plate ( mass fraction , % )

C	Mn P	S	SiCr	NiCu	Nb Alt Ceq
0.115	1.140.007	0.0015	0.310.06	0.180.04	<0.05 0.034 0.34

12 Test method
Cut out a test block with a size of 200mm X 100mm X 60mm from a 60mm thick steel plate , and carry out normalizing treatment of a part of the test block at 880 ° C , 910 ° C and 940 ° C respectively ; 40min , 80min , 100min , 120min normalizing treatment at ℃ respectively .
After normalizing , 2 impact samples and 2 tensile samples were wire-cut from 14 places in the thickness direction of each test block , and 2 pieces were wire-cut from 1/2 ( core ) of each test block in the thickness direction. Impact specimen . The dimensions of the tensile and impact specimens are shown in Fig . 1 and Fig. 2 , respectively . Metallographic samples were obtained by wire cutting from the same part of the impacted sample . Grinding , polishing and corrosion of metallographic samples . The etchant was 4% nitric acid alcohol , and the microstructure was observed with an optical microscope . The mechanical performance test was carried out on a CMT5305 precision microcomputer-controlled universal testing machine , and the yield strength , tensile strength and elongation of the sample were measured . The impact toughness of the material was tested by a pendulum impact testing machine . Grain size ratings were performed using metallographic photographs of the X 500 against metallographic grading standards .

2 Experimental results and discussion
2.1 The effect of normalizing temperature on steel microstructure and properties The samples normalized at different temperatures were subjected to longitudinal stretching and
-40°C longitudinal impact test , mechanical performance test results are shown in Table 2
shown .
Table 2 Effect of normalizing temperature on mechanical properties of E -class ship plate steel

Normalizing temperature	H/ MPa	Rm/ MPa	A/%	-40C longitudinal impact /
				1/4 thickness	1/2 thick
Hot rolled	355.0	511.5	29.78	5.0	4.5
880C	357.5	471.7	38.55	207.0	212.0
910C	352.5	466.0	38.50	224.0	169.5
940C	375.0	463.0	39.15	201.0	165.5

Note : The samples are kept warm for 60min at the normalizing temperature

From the data analysis in Table 2 , it can be seen that : Compared with the hot-rolled steel plate with controlled rolling and controlled cooling , the normalized steel has lower tensile strength , higher elongation , and greatly improved low-temperature impact toughness . The -40°C impact toughness value of the normalized steel reaches more than 150J , and the comprehensive mechanical properties are good .

structure and harmful black structure , and refine grains , thereby improving the mechanical properties of high-strength steel plates .
The grain ratings after normalizing at different temperatures are shown in Table 3 .
2.2 Effect of holding time on tissue performance
The test results of mechanical properties at different holding times are shown in Table 4 , and the ratings of grains at different holding times are shown in Table 5 .

Table 3 Grain rating at different normalizing temperatures

Normalizing temperature	Grain Rating
	1/2 thickness	1/4 thickness
Hot rolled	6.5	( Assessment of ferrite only ) 10.5
880C	9	9
910C	8	8.5
940C	8	8

From the comprehensive analysis of table 2 , 3 and figure 4 , the grain refinement after normalizing , The tensile strength decreased , The reason is that normalizing refines the grains to increase the strength , At the same time, normalizing is a process of recovery . It will significantly reduce the density of defects such as dislocations in the sample , As a result, the strength is reduced , and the overall strength is reduced due to comprehensive factors . pass the comprehensive exam
Table 4 The holding time at 90°C for normalizing to E class
Influence of mechanical properties of ship plate steel

Holding time / min	H/ MPa	Rm/ MPa	A/ %	—40°C Longitudinal impact /
				1/4 thickness	1/2 thick
40	347.5	465.0	38.05	185.5	185.5
60	352.5	466.0	38.50	224.0	169.5
80	362.5	466.5	39.15	194.0	171.0
100	342.5	465.1	38.35	216.0	171.0
120	377.5	463.1	37.80	198.0	153.0

It can be seen from Table 4 that the holding time has no obvious effect on the mechanical properties of E -class ship plate steel during normalizing at 90°C . Good stability. from table 5

Because of the precipitation of dispersed fine carbonitride particles in the process of cooling to room temperature , they are pinned at the subgrain boundary or grain boundary , and the grain growth is hindered .
Table 5 Grain ratings of normalizing at 910°C with different holding times

Holding time / min	Grain Rating
	1/2 thickness	1/4 thickness
40	9	9
60	85	9
80	85	8.5
100	85	8.5
120	85	8.5

The body , the black tissue is pearlite . It is found that no matter in the center or at the thickness of 14 , there are obviously pearlite bands . The reason for the formation of the banded structure is generally the dendrite segregation formed during the solidification process of the continuous casting slab , which extends into alternating bands of pearlite and ferrite during the hot deformation process . The existence of banded structure makes the structure of steel uneven and affects the performance of steel .

From the analysis in table 4 , it can be seen that the samples with a normalizing holding time of about 60 minutes have diffused completely , and their mechanical properties are similar to those of holding time of 80 , 100 and 120 minutes , indicating that better performance can be achieved without a longer holding time . It can be seen from Table 5 and Figure 5 that the grains of the heat preservation for 60 minutes are smaller than those of the heat preservation for a longer time . By comprehensively considering tables 4 , 5 and figure 5 , It can be considered that the normalizing holding time of about 60 minutes is the most appropriate .

 

in conclusion

1. with controlled rolling and controlled cooling (TMCP) steel plate , The normalized steel has lower tensile strength , higher elongation , significantly improved impact toughness at -40°C , and good stability . The best normalizing process for Class E ship plate steel can be considered as 880°C ~ 910°C X 60min , At this time, the performance is better , The production cycle is short .
2. Through normalizing treatment , the coarse Widmanstatten structure and black unbalanced structure existing in the original TMCP state can be eliminated . And refine the grain .
3. With the extension of the holding time , The grain growth of normalized steel is not obvious.

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