Abstract : The microstructure characteristics of FH690 ultra-high strength ship plate steel and its influence on low temperature toughness were studied by means of OM , SEM and TEM . The results show that the composite structure of lath bainite and a small amount of granular bainite is obtained by controlled rolling and controlled cooling . This structure has excellent mechanical properties and fully meets the standard of FH690 marine steel . The obtained fine lath-like bainite structure and its internal high-density dislocation substructure are the guarantee for improving the strength and low-temperature toughness of the experimental steel .
Key words : ship plate steel ; lath bainite ; low temperature toughness
In recent years , due to the rapid growth of world seaborne trade volume , the shipbuilding industry has experienced unprecedented prosperity . Ships are gradually developing in the direction of large-scale and light-weight ships . General-strength ship plate steel can no longer meet the requirements of hull structure . The application proportion of high-strength ship plate in the shipbuilding industry continues to increase . Plate steel is not only the need to improve economic efficiency , but also meets the direction requirements of the national development strategy .
With the maturity and improvement of controlled rolling and controlled cooling , microalloying and smelting technology , low carbon , high cleanliness , ultra-fine grain and microalloying have become the main development direction of high-strength ship hull steel in the new century . FH690 is a new generation of ultra-high-strength hull steel , which is still in the development stage in China . This paper discusses its chemical composition , structure type, rolling process and low-temperature toughness mechanism .
1 Experimental materials and methods Since ship plate steel requires good low-temperature impact toughness and low brittle transition temperature , high-strength ship plate has requirements for aluminum content and microalloying . The selection of microalloying elements should not only consider the strengthening effect of microalloying elements but also not have adverse effects on the structure of steel. The effect of microalloying elements on the brittle transition temperature of steel is also an important factor to consider . At the same time, in order to ensure good welding performance , it is necessary to control the upper limit of carbon equivalent . Finally, according to the comprehensive performance requirements of the high-strength ship plate , the chemical composition of the steel is determined as shown in Table 1 .
Table 1 Chemical composition of experimental steel ( mass fraction , % )
Tab.1 Chemical composition of testing steel(wt , % )
C |
Si |
mn |
al |
Cu |
Ni |
Mo |
Nb |
0.05 ~ 0.07 |
0.12 |
0.68 |
<0.005 |
1.4 ~ 1.6 |
1.6 ~ 1.8 |
0.6 |
0.02 ~ 0.04 |
The experimental steel was smelted in a vacuum induction furnace and cast into ingots, then forged into slabs with a thickness of 90 mm . The thermal simulation experiment sample is intercepted on the slab, and is prepared into a quasi 4mmx 10mm sample, and the continuous cooling curve of the experimental steel is measured with a thermal dilatometer. The experimental process is : at 10 °C/s
Heating at a rate of 1200 °C , holding for 300 s , then cooling at 3 °C/s for >20
Rapid cooling to 840°C for 60s , and then cooling at 1 , 3 , 5 , 8 , 10 , 15 and 20 °C/s respectively.
The controlled rolling and controlled cooling process is formulated as follows according to the test results measured : the slab after forging is heated to 1230 ° C , homogenized for 3 hours , rolled in the recrystallization temperature zone of 1150 ~ 1050 ° C , and the cumulative deformation is 56% ; 860 ~ 800 °C non-recrystallization temperature zone rolling , accumulative deformation 70% , rolling into 12mm plate , controlled cooling after rolling , cooling rate is 8 ~ 12 °C/s , final cooling temperature is 350 ~ 400 °C .
The metallographic sample of the steel plate is taken along the side of the plate to take a full-thickness sample . After grinding and polishing , it is etched with 4% nitric acid alcohol for metallographic observation and SEM observation . The transmission electron microscope sample uses 5% perchloric acid absolute ethanol solution as the electrolyte . Under -20 °C and 50V , the electrolytic double spray is thinned to perforation , the electron microscope used is Jtm-2010 , and the working voltage is 200kV .
2 Experimental results and discussion
2.1 Mechanical properties
Table 2 shows the tensile and impact properties of the tested steel (1 # and 2 # These are two samples taken at different positions after TMCP rolling ) : the yield strength reaches above 730MPa , and the elongation rate is above 21% ; Figure 1 is the temperature - zero impact energy-curve of the sample . It can be seen that the impact energy of the tested steel has not decreased significantly , and the impact energy at -80 °C has reached more than 80 J. It can be seen that all the mechanical indexes of the inspected steel meet the requirements of the classification society , and the advantages of the controlled rolling and controlled cooling process have been fully realized.
2.2 Continuous cooling transformation curve and microstructure type of experimental steel
Figure 2 is the continuous cooling transformation curve of the experimental steel . It can be seen that granular bainite and ferrite are mainly obtained at a cooling rate of 1 to 5 °C/s , and lath bainite and granular ferrite are mainly obtained at a cooling rate of 8 to 20 °C/s . Mixed structure of bainite . The temperature at which the phase transformation ends gradually decreases from about 550 °C to about 430 °C with the increase of cooling rate .
0 000
1 00 0 10 0 0
Time S s Figure 2 Static CCT curve of test steel
Fig.2 Con ii nous coo l ing t ran sftnm ia i io n curv es of he t es t in g s t eel
>()
o
sample |
Tensile strength /MPa |
Yield strength /MPa |
Elongation (%) |
1 # |
970 |
733 |
21.5 |
2 # |
980 |
741 |
22.4 |
Table 2 Mechanical properties of experimental steel
Tab.2 Mechanical properties of the testing steel
Fig. 3 is the tissue morphology of different cooling rates, when the cooling rate is low (1°C/s)
Fig.3 Microstructure of experimental steel under different cooling rates
|
|
|
|
* |
|
|
|
Li is |
5X?/s |
ior/s |
20*C/s |
Fig.3 Microstructure of the testing steel at different cooling rates
It is mainly ferrite and granular bainite structure based on ferrite, and the grains are relatively coarse due to the low cooling rate ; when the cooling rate increases to 5 ~ 8°C/s , the granular bainite gradually decreases , the lath bainite gradually increases ; as the cooling rate further increases to 20°C/s , it is mainly lath bainite , and only a small amount of granular bainite can be seen , and the grains gradually become tiny .
2.3 Microstructure analysis
Figure 4 is the metallographic and scanning structure of the experimental steel after TMCP rolling , which is mainly a mixed structure of lath bainite and granular bainite , of which lath bainite accounts for the main part . The lath bainite structure of the experimental steel can be further observed through the scanning electron microscope , and it can be found that the bainite laths are roughly parallel to each other , the lath bundles are interlaced , and the laths are small . In terms of morphology , it is similar to the general upper bainite , but unlike upper bainite , it has less carbon-rich phase and does not contain carbide .
Fig.4 Metallographic and SEM structure of experimental steel
Fig-4 OM and SEM micro ^ phs o f the testing steel
Lath bainite is formed at the lowermost end of the bainite transformation temperature zone of continuously cooled low-carbon and low-alloy steel . Since the amount of martensite in bainite steel is very small , lath bainite can be regarded as bainite It is the structure with the lowest transformation temperature and the highest microhardness among all-tensitic steels . In the experiment, when the cooling rate was 8-12 °C/s and the final cooling temperature was 350-400 °C , a mixed structure of lath bainite and a small amount of granular bainite was obtained .
In order to obtain fine bainite slabs , the austenite grains must be refined by rolling in the recrystallization zone ( rolling in the recrystallization temperature zone of the experimental steel , the cumulative deformation is 56%) , and then rolled in the non-crystallization zone to make Austenite grains elongated and broken ( the experimental steel was rolled in the non-recrystallization temperature zone , and the cumulative deformation was 70%) . This can increase the nucleation sites during cooling , thereby achieving the effect of refining grains . At the same time, rolling in the non-recrystallization zone can also cause work hardening of austenite grains , which can not only increase the nucleation position and nucleation rate of ferrite , but also refine the structure after phase transformation.
For low-carbon steel, the solid solution strengthening effect of carbon is weakened, and the proportion of grain refinement strengthening and dislocation strengthening is relatively increased , so the importance of controlling dislocation structure increases [5] . Figure 5 is the TEM photo of the experimental steel. The dislocation density of lath bainite is relatively high , and there are a large number of network substructures inside . These dislocations play the role of carbide precipitation nucleation sites during tempering . And contribute to precipitation strengthening , so the density of dislocations in bainitic laths is very important .
Fig.5 Transmission morphology of bainite lath bundles ( a ) and dislocations in the lath bundles (b )
Fig.5 TEM micrographs showing tathbainite (a) ndd
dislccatinns in the lath Q)
A higher dislocation density can increase the strength of the steel , and at the same time, the dislocations are pinned by fine precipitates in the matrix during the movement in the plastic zone of the crack tip . This structure results in a regular arrangement of dislocation entanglements . As the degree of dislocation crowding increases , the dislocation entanglements arranged in parallel gradually connect into a dislocation wall , forming a structure similar to grain boundaries . After the movement of the crack is severely hindered by the dislocation wall , under the joint action of the applied load , the direction of the slip system and the dislocation wall , the crack tip is passivated , thereby slowing down the crack propagation and improving the impact toughness.
3 Conclusion
- of lath bainite and a small amount of granular bainite is obtained by controlled rolling and controlled cooling . This structure has excellent mechanical properties and fully meets the standards of FH690 marine steel .
- The obtained fine lath bainite structure and its internal high-density dislocation substructure can improve the strength of the experimental steel ( continued to page 72 ). According to the
observation results , the cracks in the sample also exist in these brittle hard in the organization . Multiple isolated cracks are interconnected and aggregated , eventually leading to the cleavage fracture morphology in Fig. 2(()) .
The methods to eliminate the banded structure include : improving the dendrite segregation of the continuous casting slab , increasing the reduction and so on . However, in the choice of finish rolling temperature , various viewpoints are not unified , and opposite conclusions often appear . Literature [4] proposed the concept of relative grain size parameter :
8 d = ( dS ) / S⑶
where 8 d is the relative grain size , d is the average grain diameter of ferrite after phase transformation , and S is the interband spacing of manganese-rich bands . When &<-0.4 , the finish rolling temperature decreases and the band structure lightens ; when &>-0.4 , the finish rolling temperature increases and the band structure lightens .
4 Influence of Widmanstatten structure on transverse elongation of steel plate For hypoeutectoid steel , when coarse austenite passes through the phase transformation region at a rapid cooling rate , ferrite not only precipitates and grows along the austenite grain boundary , but also forms Many ferrite sheets are inserted into the interior of the austenite grains, and the austenite between the ferrite sheets finally transforms into pearlite . These pro-eutectoid ferrites distributed in the original austenite grains are called ferrite. Body Wei's organization .
The formation of Widmanstatten structure in steel mainly depends on the chemical composition of steel , the size of austenite grains and the cooling rate . The carbon content range in which the Widmanstatten structure is most likely to form in hypoeutectoid steel is 0.15% to 0.5% . When the composition is constant , the size of austenite grains and the cooling rate determine the formation of Widmanstatten structure . Usually when the austenite grains are smaller than grade 5 , it is easy to form Widmanstatten structure , and increasing the cooling rate will promote the formation of Widmanstatten structure .
Regarding the influence of Widmanstatten structure on the mechanical properties of steel plates , the current views are not unified . Some people believe that the existence of Widmanstatten structure will greatly reduce the plasticity and impact toughness of steel . Other views believe that when the grain size of austenite is the same , the mechanical properties of the Widmanstatten structure obtained by different cooling rates are not low compared with the granular structure , so the Widmanstatten structure cannot be simply regarded as Coarse austenite before phase transformation is an important factor that cannot be ignored to reduce the plasticity of steel .
Although the two views are not unified , for actual production , no matter which view is adopted , the refinement of the austenite grain size before transformation is beneficial to the improvement of the plasticity of the steel plate . Therefore , when the Widmanstatten structure is found in the steel plate with unqualified elongation , the heating temperature should be appropriately reduced , the reduction amount of the rough rolling pass should be increased , and the means of refining austenite grains such as promoting austenite dynamic recrystallization , which is beneficial for improving product quality .
5 Conclusion
Based on the above analysis, it can be seen that the brittle and hard structure in the steel plate, the manganese sulfide contained therein , and the bad structure are the main reasons for the unqualified transverse elongation of the steel plate . Therefore , try to reduce the sulfur content in the steel , reduce the macro center segregation and manganese element dendrite segregation during the solidification process of the slab , appropriately reduce the heating temperature , increase the reduction in the rough rolling pass , and refine the austenite before phase transformation. The process system such as grain size is an effective measure to improve the transverse elongation of the steel plate . Practice has proved that the transverse elongation of the steel plate has been greatly improved by adopting the optimized process system .
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