Influence of Sub-temperature Quenching and Tempering on Low Temperature Toughness of F550 Ship Plate Steel

Dual-phase steels consisting of ferrite and low-temperature transformation phases have good strength and toughness . The soft phase ferrite prevents cracks from expanding, and the low-temperature transformation phase is martensite or bainite hard phase structure, so that the strength of the material does not decrease significantly while the toughness is improved.

It can be obtained by two-phase zone controlled rolling and controlled cooling or two-phase zone heat treatment , but for medium and heavy plates with large thickness, it is difficult to achieve two-phase zone rolling due to the limitation of rolling force of the rolling mill . Therefore, adopting the heat treatment process in the two-phase region is an effective way to improve the low-temperature toughness of medium and heavy plates . This new heat treatment process has been applied to the research of many steel types, such as: 9Ni steel , 30CrMnSiA steel and so on .
In this paper, two heat treatment processes of sub-temperature quenching + tempering and complete quenching + tempering are compared to study the influence on the mechanical properties of F550 ship plate steel and explore the mechanism of improving low temperature toughness of high strength ship plate steel .

• Test materials and methods The materials used in the test are taken from 60 mm thick F550 high-strength ship plate steel produced by a steel factory, and its main chemical composition is shown in Table 1 .

Table 1 Chemical composition of test steel (mass fraction, % )
Table 1 Composition of tested steel ( mass fraction，% )

C	Si	Mn	Cu + Ni + Cr + M o	Nb V	Ti	P	S	Fe
<0.1	0. 18	1. 53	< 2. 1	0. 024 0. 05	0. 012	0. 008	0. 001	Bal．

A of the test steel was firstly measured with a thermal dilatometer according to the YB/T5128£3 standard . 】 is 690 °C , and c3 is 860 °C . Sub-temperature quenching is also called incomplete quenching, or critical zone quenching and two-phase zone heating quenching, which refers to heating hypoeutectoid steel to a certain temperature range in the ferrite + austenite dual- phase zone ( A ~ A. 3 ), the heat treatment process of quenching after holding for a certain period of time .
The selected sub - temperature quenching temperature is 820C , 50C , and the temperature in the complete quenching zone is 90C . Quenching heating adopts resistance heating furnace, after heating to the target temperature, it is loaded into the test steel, and then kept for 60 minutes, and immediately taken out of the furnace and put into the water tank for water quenching . The sample after quenching was tempered at 600 C , and the tempering holding time was 100 min .
Tensile and impact specimens were cut at the plate thickness]/ 4 along the direction perpendicular to the rolling direction . The tensile specimen size is ' 10mm X 140mm , and the gauge length is 50mm . The impact sample size is V -shaped 10 mm X 10 mm X 55 mm . The quenched sample was thermally etched by saturated picric acid aqueous solution in a constant temperature water bath, and its microstructure was observed under an optical microscope, and the volume fraction of ferrite in it was counted by Image-Pro Plus software . The metallographic sample was etched with 4% nitric acid alcohol solution for metallographic observation and SEM observation . The distribution of M/A islands under different heat treatment conditions was observed by color metallography , and quantitative statistics were made on the size distribution and volume fraction of M/A islands . The color metallographic display agent is etched by Lapara . The TEM sample uses 5% perchloric acid anhydrous ethanol solution as the electrolyte, and is electrolytically double-jet thinned to perforation at -20 C, 50 V. The electron microscope used is JEM - 2100FX, and the working voltage is 200 kV .
F550 ship plate steel test results
Mechanical Properties of F550 Ship Plate Steel
three quenching temperatures and mechanical properties of F550 ship plate steel test steel are shown in Table 2 . The tensile properties are the average value of two samples . It can be seen from the test results that with the increase of quenching temperature, the strength and yield ratio of the test steel increase, and the elongation decreases . The low-temperature impact values of No. 1 and No. 2 steels are about 200 J , and their impact toughness is about 60 J higher than that of the fully quenched No. 3 steel . Comparing the two quenching temperatures of 820 C and 850 C , it can be seen that: with the increase of quenching temperature, the strength and impact toughness of the test steel are greatly improved, and the yield strength and tensile strength are increased by 92 MPa and 47 MPa respectively. , the impact energy increased by about 20 J. Although the strength of the material after complete quenching is high, for F550 ship plate steel, No. 2 steel has better comprehensive mechanical properties .
Table 2 Mechanical properties of test steel
Table 2 Mechanical properties of tested steel

Simple	Quenching	R P0.2 /	R m /	A/	R P0.2	A KV (	- 80 C ) /
No. _	temperature / C	MPa	MPa	%	R m		J
1	820	563	678	24.4	0. 83	196	200	190
2	850	655	725	21. 2	0.90	216	233	210
3	910	798	825	19.7	0.97	160	164	173

F550 ship plate steel

• In the sub-temperature zone, quenching at 850 C and then tempering at 600 C can meet the performance requirements of F550 ship plate steel, and the low-temperature toughness ( -80 C ) of the test steel reaches 220 J ;
• volume fraction of ferrite in the structure quenched at 850 C and tempered at 600 C in the sub-temperature zone is 8% ; the volume fraction of M/A islands is 7.6 % , and the size is between 0.2 ~ 0.4 " m , and they are round dots shape diffuse distribution;
• Tempering after quenching in the sub-temperature zone, the polygonal ferrite grains are fine and interspersed with bainite, which can continuously change the crack propagation path, play a passivation effect on the crack growth, and effectively improve the F550 ship plate steel plate. Low temperature impact toughness .

---