TPT July 2011

A rticle

2. Researching and developing of new pipe straightening roll, HFW/ERW welding pipe roll and cold roll forming steel roll 2.1 Design Any material is not universal, the key is whether the material corresponds to use in the conditions of a working environment. Any development of an advice with new material must correspond to the requirement of working environment and to reach as possible as ideal condition. At the same time, the ability of the user to afford the cost must be considered. If the manufacturing cost is too high to reach the best material property, maybe this product will be difficult to make popular. First, the company systematically analyses the working features and service condition of steel pipe straightening roll, HFW/ERW welding pipe roll and cold roll forming steel roll, they then make up the design plan of new roll which is different from the design of forging steel roll and plate, strip roll at home and abroad. 2.1.1 Material design of roll In the past ten years the roll production technology has developed at a pace, the working layer of roll is developed in a direction of high alloying and high resistance to wear, the technology of high speed steel cast roll is constantly being perfected, and new production methods are continuously appearing. Because of the limit of manufacture and equipment some of these technologies are not suitable for our research. If using D2, X155CrVMo121, SKD11 and Cr12MoV material to cast shapes, the microstructure is poorer than forging microstructure and defects of segregation and uneven carbide can be present and the utilisation property is certainly poorer than that of forging steel roll, so it cannot satisfy all the requirements. Before the 1990s high chromium alloy cast iron roll was extensively used in the hot mills for plate and strip steel. Because of certain limits in the resistance to heat, cracking and resistance to wear, the used quantity of high chromium alloy cast iron roll is continuously reduced [2] , so it cannot also be used to satisfy the requirement to copy high chromium cast iron roll. Considering the above mentioned factors the only way to optimise material design and to invent new high chromium alloy material is by raising quality and extending utilisation potential. At the present, the boundary line between high chromium steel and high chromium iron is not clear in the international market. A number of countries in the world are in habit of dividing steel and iron by 2% carbon content, but the carbon content of high carbon high chromium steel is limited to 2.20-2.30%. For example China’s Cr12, Germany’s X210Cr12, America’s D3, Britain’s BD3 and Japan’s SKD1 [3] , and the carbon content of high chromium iron чX22C, чX28 are separately 0.6-1.0% and 0.5-1.6% in Russia. [4] The Association of Iron and Steel Engineers state that the content of 0.3-2.5% carbon is called a steel matrix roll and the content of 2.5-3.5% carbon is called as iron matrix roll. [5] The carbon content limit of high chromium material researched by the project consists of high chromium alloy steel and high chromium alloy iron as a traditional idea, so we call this material high chromium alloy.

2.1.2 Design of manufacturing process The cast process is a shaping process and it is optimised and innovative. At the same time the heat treatment process and machining process are also optimised and innovative. 2.2 Researching and developing 2.2.1 Material researching Through researching the difference of working environment of rolls, the company has researched and developed a new series of high chromium alloy material with different chemical compositions and different heat treatment processes. Because of the strengthening and use of fine crystal and multi-alloy deterioration, changing the growth condition of eutectic structure, improving the kind, shape and distribution of carbide, the utilisation quality is raised, the cast property is improved and the defect of the cast microstructure is also improved. The carbide is mainly M7C3 and the micro-hardness up to 1800Hm, microstructure appeared as no continuous strip state, block state, particle state and chrysanthemum state. The matrix of alloy is austenite or martensite, the hardness and toughness have fine combination, macro-hardness is up to HRc50-65, so the excellent wear resistance of the product is apparent during utilisation. Tempered chromium austenite eliminates casting stress and has a special strong mounting ability and good support ability. It can be used to prevent and passivate the development of brittle cracks from a carbide crystal to another carbide crystal and can be used to produce work hardening on the face layer, and the hardness can be up to 1000HV [6] after work hardening. High chromium alloy with austenite is suitable for working condition of high wear resistance, high stress and low energy impact. The matrix of high chromium alloy quenching microstructure is martensite – adding a little bit of residual austenite, first carbide is still isolated. Second, carbide is precipitated from the original austenite and appeared small grain, the uneven degree of carbide is reduced and appears even in distribution. The macrohardness is raised and further homogeneity is possible. At present, high chromium alloy for pipe straightening roll manufactured by Shandong Province Sifang Technical Development Co Ltd has been first selected material and this material has also got good approval in the way of selected material for rolls of big HFW/ ERW welding pipe and cold roll forming steel at home and abroad. This material system has achieved proprietary intellectual property rights and two patents in China. 2.2.2 Researching and developing of casting process Smelting is used in the degassing process and purifying process for raising purity and avoiding the production of cast defects, at the same time reducing influence of harmful trace elements. Through setting the controlling temperature device and machinery power plant in cast mould system during the casting process and achieving effective control to solidification process, the ideal casting can be completed. Controlling the solidification process can be fine dendritic crystal and eutectic structure, effectively controlling and reducing segregation, lowering unevenness of carbide. Adjusting and controlling cooling speed can be used to get the required matrix structure. Achieving “controlling order solidification” can effectively avoid some cast defects such as cracks and shrinkage.

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J uly 2011

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