Research and Development of Q460D Large -sized Heavy-wall High-strength Seamless Steel Tube

Research and Development of Q460D Large -sized Heavy-wall High-strength Seamless Steel Tube

Although the Q460D steel is widely used for plat e-making, it's very difficult to make the seamless tube with this type of steel. Nevertheless, a hi-strength and hi-toughness large-sized heavy-wall structure-purposed seamless tube in Q460D steel is successfully developed by means of optimizing the steel's chemical composition, and adopting proper heat treatment process. Relevant mechanical properties of the tube are as follows. After being normal- ized, its yield strength is A 440 MPa; tensile strength A 550 MPa, and longitudinal elongation A 17%. And the im - pact test as made at -20 t to the tube shows that the average longitudinal impact value is A 80 J, and the average transversal impact value A 60 J. All the above property test-resulting data are in compliance with relevant customer requirements.
Key words: Q460D steel ; structure-purposed seamless steel tube; chemical composition optimization ;
heat treatment; hot-rolling; hi- strength and hi- toughness

Q460 represents the yield strength of steel, which is divided into three grades C , D and E according to different requirements for impact performance, among which Q460C requires the impact performance at 0 t ; Q460D requires the impact performance at -20 t ; Q460E requires the impact performance at -20 t Impact performance E at -40 t .
Q460D belongs to low-alloy high-strength steel, which is widely used in vehicles , ships and pressure vessels . Q460D is mostly steel plate in the market, and it is easier to obtain high strength and good low-temperature impact toughness by using TMCP process in the preparation process ; but it is difficult to pass the traditional

  1. Difficulties in developing technology

Q460D large-diameter thick-walled high-strength seamless steel pipe needs to meet high strength and good low-temperature impact toughness at the same time, but the standard only limits the maximum value of the element mass fraction, and currently there is no optimized chemical composition and mature heat treatment process;
Therefore, the main technical difficulties that need to be solved in the production of Q460D large-diameter thick-walled high-strength seamless steel pipes include:
①Optimization of chemical composition; ②Guarantee high strength and good low temperature impact performance of structural pipes .

  1. Component Design

Studies have shown that the main reason for the poor low temperature impact toughness of steel pipes is that the ferrite structure is coarse, and the pearlite aggregates and grows into coarse clusters, and the grain refinement is the main measure to solve the problem [ 3 ] . The effect of different strengthening mechanisms on the ductile-brittle transition temperature of steel ] , it can be seen that grain refinement is the only method among various strengthening mechanisms that can not only improve the strength and toughness of steel, but also reduce the ductile-brittle transition temperature .

Grain refinement can not only increase the strength of steel, but also improve the toughness of steel. This is because the finer the grains of the material, the larger the grain boundary area, and the grain orientations on both sides of the grain boundary are completely different and irregular, and the arrangement of atoms at the grain boundary is quite disordered, so when plastic deformation and microcracks are formed by a When a grain crosses the grain boundary and expands to another grain, due to the existence of the grain boundary, the elastic deformation and plastic deformation at the grain boundary are uncoordinated, and stress concentration is induced at the grain boundary, making it difficult to pass through the grain boundary. ; In addition, the slip direction and crack propagation direction need to be changed after passing through the grain boundary. Therefore, the existence of grain boundaries can improve the strength and toughness of the material, and the finer the grains of the material, the higher the strength and toughness of the material [ 4-5 ].
At present, commonly used elements for grain refinement are Nb , V , Ti, etc., and their mass fractions are low, generally less than 0.1% , and they are called microalloying elements . Nb plays a significant role in steel: ①It can form fine strong carbon or nitride, and inhibit the growth of austenite grains; ②In the rolling process, it can increase the recrystallization temperature and inhibit the recrystallization of austenite. Maintain the deformation effect to refine the ferrite grains; ③ Nb precipitates in the ferrite to improve the strength of the steel and prevent the coarsening of the grains in the heat-affected zone during the welding process . In terms of the microalloying effect of Nb , V and Ti and the comprehensive influence on the structure and properties of steel,
Nb and Ti are superior to V , but Ti has a stronger affinity for 0 , N , S , and C elements, and the alloying of Nb does not need high-purity molten steel as a prerequisite . A large number of research results [ 4 ] have shown that: Nb , V and Ti in steel affect its strength and toughness through grain refinement, and the effect of Nb is the most significant .
Figure 2 shows the effect of alloying elements on the ductile-brittle transition temperature [ 9 ] , it can be seen that with the increase of carbon content, the ductile-brittle transition temperature rises rapidly, no matter in terms of low temperature toughness or weldability, w ( C must be limited below 02% . A small amount of P element can increase the ductile-brittle transition temperature, so it must be strictly limited . Mn and Ni elements are very effective in reducing the ductile-brittle transition temperature, the main reason is that The two elements lower the phase transition temperature of steel, and it is easy to obtain fine and tough ferrite grains [ 9 ] .
Based on the above situation, according to the requirements of Q460D structural seamless steel pipes for strength and low temperature impact toughness, refer to GB/T 1591-2008 " Low Alloy High Strength Structural Steel ", European Standard EN 10210-1 : 2006 ( non-alloy structural steel , The chemical composition requirements of the steel grade S460NH in the fine-grained structural steel hot-rolled steel pipes and the steel grade E460K2 in the European standard EN 10297-1 : 2003 ( Seamless circular steel pipes for mechanical and general engineering purposes ) are finally adopted . Optimize the chemical composition by increasing the carbon content , increasing the Mn and Ni content , adding Nb elements and limiting the P content . The optimized chemical composition of Q460D steel pipe and the chemical composition required by the standard are shown in Table 1 .
Process flow and test results
Process test
The tube blank is processed by the method of " electric furnace + refining outside the furnace + vacuum degassing "

Table 1 The optimized chemical composition of Q460D steel pipe and the chemical composition required by the standard (mass fraction)%


Require

C

Si

mn

P

S

Ni

Nb

Al

target ingredient

0.18

0.39

1.55

0.016

0.006

0.55

0.045

0.03

Smelting ingredients

0.16 - 0.20

0.30 - 0.53

1.45 - 1.65

= 0.025

= 0.020

0.40 - 0.60

0.030 - 0.050

M 0.02

GB/T1591

= 0.20

= 0.60

= 1.80

= 0.030

= 0.025

= 0.80

= 0.110

M 0.02

EN 10210-1

= 0.22

= 0.60

1.0 - 1.70

= 0.035

= 0.030

= 0.80

= 0.050

M 0.02

EN 10297-1

= 0.20

= 0.60

1.0 - 1.70

= 0.030

= 0.030

= 0.80

= 0.050

M 0.02

w ( Cd W 0.30% , w ( M0 W 0.10% , w ( V ) W 0.20% , wW 0.03% in the smelting composition .

For smelting, steel ingots are used for forging, and the forging ratio is not less than 3 . For ① 500 mm tube blanks , the low-magnification structure defects were evaluated according to the method A in GB/T 1979-2001 " Low-magnification structure defects of structural steel " and GB/T 10561-2005 ( Determination of the content of non-metallic inclusions in steel ) . And the rating of non-metallic inclusions, the shrinkage cavity , central porosity and segregation of the tube blank are not greater than 2.0 grades measured by the test, and the non-metallic inclusions are class A ,
Class B , Class C , Class D and Class Ds all meet the requirements . The chemical composition of the tube blank was determined by direct reading spectroscopy, and the results are shown in Table 2 .
Table 2 Chemical composition (mass fraction) of Q460D tube blank %


C

Si

mn

P

S

Ni

Nb

Al

0.16

0.35

1.59

0.014

0.006

0.42

0.05

0.027

Heat treatment tests were carried out in a laboratory box furnace . Its A c3 value is calculated according to Andrews empirical formula:
A c3 ten days =910-203C 1/2 -15.2Ni+44.7Si+104V+
31.5Mo+13.1W
The element symbol in formula ( 1 ) represents its mass fraction ( % ), and the composition range of applicable steel is: w ( C 5= 0.6% , w Mn 5= 4.9% , w ( Cd = 5% , w N ) = 5% , wMO = 5.4 % .
According to formula ( 1 ) , the A c3 value of Q460D is about 840 T. In general, the normalizing heating temperature is 30~50 T higher than that of A c3 . Considering the temperature deviation of the box furnace, the austenitizing temperature is 900 T , kept for 2 h , and air-cooled . The test results show that the yield strength of the Q460D steel pipe is lower than the customer's requirement and lower than the standard requirement .
For this reason, the normalizing heat treatment process was changed, using: austenitizing temperature of 900 T , heat preservation for 2 h , and rapid cooling . The test results are shown in Table 3 . Among them, the tensile sample is a transverse round bar sample with a size of ① 10 mmxl80 mmx70 mm (diameter X length X parallel length, gauge length 50 mm ± 0.13 mm ; the impact sample is a V -notch with a specification of 10 mm x 10 mm x 55 mm , test temperature -20 T .


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