Influence of normalizing process on microstructure and properties of FH36 high-strength ship plate

With the rapid development of the shipbuilding industry , the demand for structural steel plates for high-strength hulls has grown rapidly. At present , in order to improve the comprehensive mechanical properties of ship plates , steel mills generally adopt refining and micro-alloying processes in smelting, and use controlled rolling and controlled cooling processes in steel rolling to control the grain size of steel to obtain high toughness of materials .

However, for the 40 mm thick FH36 high-strength structural steel plate for hull , not only high yield strength and tensile strength are required , but also -60 low temperature impact toughness is required .

FH36 hull structural steel plate obtained by the controlled rolling and controlled cooling process is usually inhomogeneous , the banded structure is serious , and the ferrite and pearlite layers are distributed along the rolling direction . The banded structure makes the mechanical properties of the steel plate anisotropic and reduces its plasticity and toughness . In order to eliminate the banded structure and improve the comprehensive mechanical properties of FH36 high-strength ship hull structural steel plates, this paper studies the influence of normalizing process on the structure and properties of FH36 high-strength ship plates , and finds out the main factors that affect the mechanical properties during the normalizing process. Factors , and develop a reasonable normalizing process , improve the comprehensive mechanical properties .

1. Test method of FH36 high strength ship plate

specification of 270 mm x 2 100 mm cross -section is sent to the heating furnace for heating for 2.5 h , the temperature of the furnace is 1 150 T , the temperature of the rough rolling start is controlled at 1 050 ~ 1 100 °C , and the finish rolling is started The temperature is below 950 °C , and the finishing rolling temperature is below 850 °C . After heating, rough rolling, finishing rolling, straightening, cooling and other processes, 40 mm thick FH36 grade high-strength structural steel plates for hulls are rolled out .
Table 1 Chemical composition wt /% of FH36 high-strength hull structural steel plates

C	Si	mn	P	S	als	Ti + Nb	Cu	Cr	Mo
0-17	0-31	1-47	0-016	0-005	0-023	trace	0-014	0-120	0-045

1.2 Selection of test process
1.2.1 Normalizing process
Normalizing is one of the most basic heat treatment processes , which is to heat steel or steel parts to AC 3 (or AC cm ) above the appropriate temperature, heat for an appropriate time and then cool in the air to obtain the heat treatment process of pearlite structure . Normalizing can not only eliminate the process defects of metal materials , but also play an important role in improving the hardness of steel or steel parts , refining grains , and obtaining relatively uniform structures and properties .
Aiming at the problems of interlaminar tearing , low elongation and low impact energy when the 40 mm thick FH36 trial-manufactured ship plate was stretched , the influence of different normalizing process systems on its microstructure and properties was studied .

 Heating temperature Practice has proved that the normalizing heating temperature of steel is not a fixed parameter . Generally speaking , it needs to be determined according to the critical point of steel . According to the basic principle of normalizing heat treatment , the hypoeutectoid steel is heated to AC 3 + ( 30 ~50 ) °C , keep warm for a period of time, and air-cool to obtain a fine and uniform equiaxed ferrite + pearlite structure to improve the mechanical properties of the steel . In order to find the appropriate heating temperature , three heating temperatures ( 890 , 900 , 910 C ) were determined according to the chemical composition of the FH36 ship plate and the relevant theory of the Fe-C phase diagram for experiments .

The holding time refers to the time required for the steel plate to heat through or ensure that the transformation of the structure is basically completed during heat treatment . To determine the holding time at the normalizing temperature , it is necessary to consider the thickness of the steel plate, the arrangement in the heat treatment furnace, whether it is preheated, and the chemical composition .

According to the situation of FH36 grade high - strength ship plate, it is determined to use three holding times ( 1.3 , 1.5 , 1.7 min / mm ) to carry out the test respectively .

1 . 3 test method
The trial -manufactured FH36 grade high-strength ship plates were treated according to different normalizing processes , and the tensile specimens , Charpy V -notch longitudinal impact specimens and bending specimens were processed according to Chinese standards , and the corresponding mechanical properties were tested . Cut a small sample with a size of 1-5 cm X2. 0 cm , grind and polish the longitudinal section along the rolling direction , etch with 4% nitric acid alcohol solution to make a metallographic sample , and perform a Charpy V -notch impact test . The impact fracture microstructure of the samples was observed with a scanning electron microscope .

1. Test Results and Analysis Discussion

2- 1 Effect of normalizing process on performance of FH36 ship plate
FH36 ship plates under different normalizing processes are shown in Table 2 .

Table 2 Performance of FH36 ship plate under different normalizing process systems

sample serial number	Normalizing temperature / C	Holding time / (min • mm -1 )	Re/ MPa	Rm / MPa	A 50 / %	-60 C longitudinal KV 2 / J
						single value			average value
60#			417-70	534-46	22-77	82	86	85	84
61#	890	1-3	395-40	520-42	30-35	168	162	165	165
62#	900	1-3	393-23	518-79	29- 56	152	156	154	154
63#	910	1-3	392- 40	513- 67	27- 48	136	138	140	138
64#	890	1-5	397- 90	521- 62	31- 32	182	185	186	184
65#	890	1-7	391- 00	515- 23	27- 61	145	140	142	142

from Table 2 that after normalizing treatment, the yield strength and tensile strength of the FH36 high-strength ship plate samples decreased to varying degrees , compared with the yield strength of the unnormalized 60# sample, the drop range was 20-26 MPa, The decrease in tensile strength is between 14 and 21 MPa, but the elongation and low temperature impact toughness are both increased . Under the same holding time, as the normalizing temperature increases, the strength and toughness of the FH36 ship plate decrease. When the normalizing temperature is 890 C , its performance is better than other temperatures ; at 890 C normalizing temperature After prolonging the holding time , its performance showed a trend of first increasing and then decreasing . When the holding time is 1-5 min / mm , the elongation of the 64# sample is 37-5% higher than that of the 60# sample , indicating that the elongation at this holding time is better than that at other holding times .

According to the regulations of the relevant classification societies , the -60 C impact energy of the FH36 ship plate approved by the factory should be used as a reference value . With the increase of normalizing temperature , the longitudinal impact energy at -60 C decreases , but compared with that before normalizing , the impact energy values are greatly increased and are relatively stable . Under the same normalizing temperature of 890 C, the longitudinal impact energy first increases and then decreases with the extension of the holding time ; when the holding time is 1 . 5 min / mm , the maximum average impact energy is 184 J o It can be seen that at the normalizing temperature of 890 °C , the holding time is 1 . The mechanical properties of the FH36 ship plate are the best at 5 min / mm .

in conclusion
of different normalizing processes were carried out on FH36 ship plates. The results show that with the increase of normalizing temperature , the strength decreases , the elongation improves to a certain extent , and the low temperature impact toughness increases to a certain extent . At the normalizing temperature ( 890 ± 5 ) C and holding time 1.5 min/mm , the elongation of the 40 mm thick FH36 ship plate increased from 22.77% to 31.32% when it was not normalized, and the average impact energy at -60 C increased from 84 When J is increased to 184 J , the yield strength and tensile strength are only reduced by 10-20 MPa , which fully meets the mechanical properties requirements of FH36 ship plate .

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