STEEL PROCESSING AND METALLURGY

Transcription

1 91 STEEL PROCESSING AND METALLURGY TERÄKSEN VALMISTUS html.stx Opiskelija perehtyy teräksen valmistukseen ja ominaisuuksiin. ks ESDEP oppimisympäristö: suomenkielinen versio

2 92 Course Contents APPLIED METALLURGY Lecture 2.1 : Characteristics of Iron-Carbon Alloys Lecture 2.2 : Manufacturing and Forming Processes Lecture : Introduction to the Engineering Properties of Steels Lecture : Advanced Engineering Properties of Steels Lecture 2.4 : Steel Grades and Qualities Lecture 2.5 : Selection of Steel Quality Lecture 2.6 : Weldability of Structural Steels

3 1. STEELMAKING TECHNOLOGY 1.1 Introduction Steelmaking technology has greatly changed during the last two decades under the pressure of increased demand, new specifications and the need to reduce energy and material consumption. Production efficiency has been improved by increasing the melt capacity of furnaces, implementing on-line computer control modules, and introducing new technologies, such as the combined blowing process for LD (Linz Donawitz) converters, the Ultra High Power (UHP) electric furnace, the ladle steelmaking processes and continuous casting. Steel is produced by two process routes (Figure 1): 93 The Blast Furnace-Basic Oxygen Converter (BOF) The Electrical Arc Furnace (EAF) In both routes the process consists of producing refined iron to which is added the required alloying elements to produce the finished steel specification. Their respective shares in crude steel production are 70% (BOF) and 30% (EAF). High production rates and low impurity steel production give a dominant role to the first process route. Low energy costs and an ample supply of recycled scrap ensure a competitive market share for the second process route, especially when using the UHP furnace. Before casting, the steel can be refined in the ladle by various processes according to the specification with respect to its deoxidation state, inclusion content and level of phosphorus, sulphur, nitrogen and hydrogen. At the same time, its content of carbon, manganese and microalloying elements such as niobium, vanadium and titanium can be adjusted. This process step is generally referred to as Secondary or Ladle steelmaking.

4 During the last step of steelmaking, the steel is cast either into slabs, blooms or billets on a continuous casting machine or into ingots, depending on the final product. Flat products and light shapes are normally produced from continuous cast feedstock; whereas heavy beams and plates are more likely to follow the ingot route. 94 The blast-furnace is a shaft type furnace operating by the counterflow technique: the descending burden of sinter and coke, charged from the top of the furnace, is heated and reduced by the combustion gases ascending from the tuyere zone where a hot air blast is injected to burn C to CO. The air blast is compressed by a blower and heated in special stoves to 1100 C by combustion of the cleaned furnace exhaust gases. The iron oxides (FeO, Fe 2 O 3 ) and some of the elements present in the gangue of the sinter are reduced by CO gases to produce hot metal. The continuous casting process has become the major casting technology for steel plants. The reasons are:

5 yield improvement energy conservation (direct production of semi-finished products) savings in manpower. 95

6 96 3. CONCLUDING SUMMARY Steel production involves the refining of molten iron, the removal of impurities and the addition of alloying elements. The process may take place in an oxygen converter, yielding low impurity steel from molten iron (derived mainly from iron ore) at high rates of productivity. Alternatively, electric arc furnaces may be used to process scrap steel. Further refinement of the steel to achieve the required composition is carried out in the secondary or ladle steelmaking unit by the addition of appropriate elements and removal of unwanted products.

7 Molten steel is solidified using either continuous casting into semi-finished products or by casting ingots. Structural steel products are most commonly manufactured by hot rolling - squeezing the steel between rollers to achieve the required cross-section shape. Cold rolling produces a wide range of thin steel products, often with surface coatings, that have good surface quality and forming qualities. Special techniques are required for the manufacture of structural hollow sections. By controlling the temperature regime during rolling, improved steel characteristics can be obtained. 4. ADDITIONAL READING 1. The Making, Shaping and Treating of Steel Edited by Harold E McGannon USS (United States Steel) 10th Edition Published Brockenbrough, R. L., Metallurgy Chapter 1.1 Constructional Steel Design, an International Guide, Alexander, W., Metals in the Service of Man, Penguin Books, London, Tamura, I, Theromechanical Processing of High Strength Low Alloy Steels, Butterworths,

8 5. STRUCTURAL STEEL GRADES In this section the following structural steel grades are described: Hot-rolled products in non-alloy steels for general structural applications to EN [4]. Hot-rolled products in weldable fine grain structural steels to EN10113 [5]. Structural steels for offshore applications. Weathering steels to Euronorm 155 [6]. Anti-lamellar steel grades. Steel grades for hot dip galvanizing. 5.1 Hot-Rolled Products in Non-Alloy Steels for General Structural Applications to EN [4] General Description This standard specifies the requirements for long products (such as sections and bars) and flat products (such as plate, sheet and strip) of hot-rolled non-alloy general purpose (base) and quality steels. These steels are intended for use in welded, bolted and riveted structures for service at ambient temperature Designation of the Steels The designation consists of: The number of the European standard (EN 10025). The symbol FS. The indication of the minimum specified yield strength for thicknesses 16mm expressed in N/mm 2. The quality designation in respect of weldability and resistance to brittle fracture JR, J0, J2 and K2. If applicable, an indication of the deoxidation method (G1 or G2). If applicable, the letter symbolic for the suitability for cold flanging, cold rolling or cold drawing. If applicable the indication + N when the products have normalizing rolling. Example: Steel with a specified minimum tensile strength at ambient temperature of 510 N/mm 2, quality grade J0 and with no requirements for deoxidation and suitable for cold flanging (designation C) is given by: 98 Steel EN S355 JO C Grade 0 C Suitable for Cold forming

9 99 RAKENNUSTEKNIIKKA Steel Grades There are three standard grades of structural steel. These are summarized in the following table: Steel grade Yield strength min. [N/mm 2 ] 1) Quality Impact test temperature ( C) Type of deoxidation 2) S JR +20 optional FU,FN J0 0 FN J2-20 FF S JR +20 FN J0 0 FN J2-20 FF S JR +20 FN J0 0 FN J2-20 FF K2-20 FF 1) based on material thickness 16mm 2) FU = rimming steel FN = rimming steel not permitted FF = fully killed All these grades are C-Mn steels, which can be supplied in the as-rolled, thermomechanically treated or normalised condition. Steel grade S355 has the highest manganese content and can also be microalloyed.

10 5.2 Hot-Rolled Products in Weldable Fine Grain Structural Steels to EN [5] General Description This standard applies to hot rolled, weldable structural steels of special quality, which are delivered in the form of flat and long products. The steels are used in heavily loaded parts of welded structures such as bridges, storage tanks, etc. The minimum yield strength of these steel grades lies between 275 and 460N/mm 2 and the chemical composition is chosen in such a way that good weldability is guaranteed. The steels are fully killed and contain nitrogen binding elements in amounts sufficient to bind the available nitrogen. The steels have a fine grain structure Delivery Conditions The supply condition for all products is normalized or normalizing formed (N) or thermomechanically formed (M) as defined in Appendix Classification of Qualities All grades can be delivered in the following qualities: KG: for qualities with specified minimum values of impact energy at temperatures not lower than -20 C. KT: extra low temperature with specified minimum values of impact energy at extra low temperatures not lower than -50 C Designation The designation of the steels consists of the following: The number of the standard EN The symbol S. The indication of the minimum specified yield strength for thicknesses 16mm expressed in N/mm 2, preceded by S. The delivery condition N or M. The capital letter for the quality with specified minimum values of impact energy at temperatures not lower than -50 C. Example: Steel with a specified minimum yield strength at ambient temperature of 355 N/mm 2, thermo-mechanically formed, which is appropriate for the application at -50 C: 100

11 EN S355 M L Standard Grade Supply condition Impact test at -50 C Steel Grades and Qualities The steel grades and qualities of this standard are summarized as follows: Steel grade Quality Yield strength minimum [N/mm 2 ] 1) Impact test temperature [ C] S275 M or N ML or NL -50 S355 M or N ML or NL -50 S420 M or N ML or NL -50 S460 M or N ML or NL -50 1) for thickness 16mm It should be noted that for the impact test, values are specified for the longitudinal and for the transverse direction, whereas for EN [4] only values in the longitudinal direction are required. Minimum values are also quoted for higher test temperatures but, unless specified at the time of the enquiry and order, the impact value shall be verified with longitudinal test pieces tested at either -20 C or -50 C according to quality.