TPT May 2021
AR T I C L E
Shandong Province Sifang Technical Development Group
the value of σb is around 300MP and the value of αk is between 2.5 – 3 J/cm 2 . Lower mechanical property causes a break especially when sizing rolls serves at a later stage. The mechanical property and wear resistance of nodular cast iron decreases as the temperature rises. Rough surfaces, surface pockmarks or surface spots are inclined to come into being in the use of nodular cast iron rolls. The carbide of nodular cast iron rolls is M3C type, which occupies 15-20 per cent with microhardness 900-1100HV. Its matrix is bainite or pearlite with micro hardness of 500-300HV. Due to low carbide hardness the surface of nodular cast iron roll is rough and the iron scale with high hardness is easy to intrude in.
According to China industry standard ‘Roll for hot roll of seamless-tube rolling mill Stretch-reducing mill roll and Sizing mill roll’ (YB/T4124.1-2016), regarding the chemical composition of nodular cast iron the addition of Ni, Mo, Cr is 1.0-3.5 per cent, 0.6-1.0 per cent, 0.2-1.0 per cent respectively contrast to the conventional addition of Ni, Mo, Cr as 2.8-3.2 per cent, 0.8-1.2 per cent, 0.5 per cent respectively. Normally the valid wall thickness of sizing rolls is large, and the nodular cast iron workpieces with wall thickness exceeding 100mm are considered large thick workpieces, which are more difficult for spheroidising, so it is still a global challenge to realise high quality spheroidising of large thick nodular cast iron workpieces. And low spheroidising is one of the factors that leads to inferior mechanical properties. Then engineers found that when the elements such as Ni, Cr and Mo were added, to a certain extent the bainite structure could be obtained via normalising and tempering. So the centrifugal casting and tempering process are adopted in the production of nodular cast iron sizing rolls, and the chemical content of Ni, Cr, Mo is 3 per cent, 0.5 per cent, 1 per cent respectively. Through physical analysis we can summarise that normally
Shandong Province Sifang Technical Development Group
Furthermore it causes pipe surface scratches. Since the Ni content of nodular cast iron is comparatively high the price cost is not competitive.
Common rough working surface of nodular cast iron roll after use
C
Si
Mn
P
S
Ni
Cr
Mo
Chemical composition Mass fraction %
3.2 ～ 3.5
1.2 ～ 1.4
1.0 ～ 1.4
0.06 ～ 0.08
0.015 ～ 0.024
1.8 ～ 2.2
0.3 ～ 0.4
0.2 ～ 0.5
Ф 550 × 280
Size
hardness 380HB
Table 1: Chemical composition and hardness of sizing rolls imported by TPCO
C
Si
Mn
P
S
Ni
Cr
Mo
Mg
Chemical composition Mass fraction %
3.54
1.34
0.81
0.048
0.008
2.09
0.38
0.26
0.046
Metallurgical structure
pearlite + spheroidal graphite + a small amount of bainite + 35% cementite
Table 2: Chemical composition and metallurgical structure of stretch reducing roll, which is made in Germany, imported by Hengyang Steel Tube Co. Ltd
C
Si
Mn
P
S
Ni
Cr
Mo
Mg
Chemical composition Mass fraction %
＜ 0.01
3.17
1.49
0.6
0.036
2.5
0.75
0.27
0.04
Metallurgical structure
Carbide 20% + spheroidal graphite 5.37% + pearlite matrix
Carbide 824 ～ 1035HV, matrix 330 ～ 360HV
Micro hardness
σ b (MPa) ： tangential direction 441 ～ 490 axial direction 392 ～ 441 α k (J/cm 2 ) ： tangential direction 3.924 axial direction 2.453 ～ 2.943 HRC ： 40 (R ＝ 20mm area) 35.5 (R ＝ 65mm area)
Mechanical property
Table 3: The physical analysis results of stretch reducing roll (A roll), which is made in Germany, imported by BAOSTEEL
Chemical composition Mass fraction %
C
Si
Mn
P
S
Ni
Cr
Mo
Mg
＜ 0.01
2.19 1.41
0.5
0.039
3.51
0.46
0.54
0.038
Metallurgical structure
Carbide 14.37% + spheroidal graphite 4.01% + bainite matrix
Carbide: 915 ～ 1150HV , matrix : 420 ～ 455HV
Micro hardness
σ b (MPa) : 392 ～ 441 α k (J/cm 2 ) : 3.924 ～ 4.905 HRC : 47.4 (R ＝ 20mm area) 45 (R ＝ 65mm area)
Mechanical property
Table 4: The physical analysis results of stretch reducing roll (B roll), which is made in Germany, imported by BAOSTEEL
69
www.read-tpt.com
MAY 2021
Made with FlippingBook - Online catalogs