TPT-January-2015

Article

Shandong SiFang Technical Development Co Ltd

C

Si

Mn

P

S

Cr

Mo

V

1.50~ 1.60

0.10~ 0.40

0.15~ 0.45

≤ 0.030

≤ 0.030

11.0~ 12.0

0.60~ 0.80

0.90~ 1.10

Table 1: Chemical composition of German die steel (mass fraction, %)

pipe defect peaks from normal pipe contour effectively [1, 2] . The die-type crimping press has the upper die and the lower die of the certain working surface curve. As with producing, the upper die is fixed and the lower die is driven upwards by hydraulic cylinders, resulting in the margin crimping deformation of the plate between the upper die and the lower die. The crimping step number of the whole plate depends on the length of the dies and the plate. The crimping quality of the die-type process is superior compared with that of the roller-type press, especially for high-strength thick-wall pipes, also avoiding longitudinal extending of plate margins in roller- type press. 2 Baosteel crimping press and characteristic of its dies The crimping press of Baosteel UOE welded pipe production line consists of two oppositely arranged frameworks, which can move vis-à-vis depending on the width of the steel plate, and the upper die and the lower die are installed in the frameworks. Baosteel crimping press has the following characteristics: 1) high pressure; its maximum pressure reaches 40MN, making it possible to produce high steel grade size in the future; the capacity of great pressure can guarantee the production requirements of high steel grade and thick wall size, and also realise the large step operation of crimping press; as for its length, according to finite element analysis, the crimping force limit of 31.8mm X100 pipeline steel will reach 38MN 2) large length of single step; the effective length of its dies is about 4,900mm; there are only four crimping steps for completing the pipe of 18m long; the reduction of the step number means the improvement of production efficiency, and the reduction of the forming transition zone number so as to improve the product quality 3) few dies; as for 56" production line, there are only five sets of dies covering OD 508 ~ 1,422mm.

Forged alloy tool steel dies have been made in Germany for matching Baosteel 40MN crimping press before, and the chemical composition is shown in Table 1. The field test shows that the surface hardness of the die is 50-60 HRC. The German tool steel, as for the chemical composition, is equivalent to Chinese Cr12Mo1V1, American D2 and Japanese SKD11, which is cold working tool steel widely used internationally; it is ledeburite steel, and has high abrasion resistance and hardenability, which would be hardenable completely at less than 300-400mm in cross section. It is variously used as cold working die, cutting tools and measuring tools of high precision and long lifespan, such as punch dies, stretching dies, cut-off dies, cold extrusion dies, cold cutters, circular saw blades and standard measuring tools. The smelting of this kind of steel requires high purity by electric furnace and vacuum refining furnace, and generally forging stock of large dies are obtained through the electroslag remelting system. The forgeable temperature zone is narrow, exactly speaking, from the starting forging temperature 1000°C to the ending forging temperature 850°C. The upper die and the lower die are elongated parts, and should be completed after repeatedly heating and forging; meantime contour forging is difficult for the small production quantity. Generally speaking, elongated parts are easy to be stretched and hard to be upset, resulting in significant anisotropy, whose mechanical property can vary more than once. Manufacturing crimping dies of forged die steel brings on process complexity, high-energy consumption, low material utilisation and high cost. The heat treatment of this kind of steel is difficult and demanding, and the typical curve of heat treatment is shown in Figure 2. Original dies are all made in Germany, so if imported, there would be some trouble, such as long manufacturing cycle, high cost and fussy purchasing process. Some dies have been in service for too long, are badly worn, and there are no

Figure 2: Typical curve of heat treatment

1,010~1,040°C (object 1,025°C) soaking

heating

salt bath (air-cooled)

pre-heating 750~800°C

1 time high temperature 480~550°C low temperature 180~200°C

high temperature 480~550°C low temperature 180~200°C 2 times

heating

air-cooled

lower than 400°C

entrance

entrance

88

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J anuary 2015