669.14.018.8:620.18 A. V. Knyazev, and S. V. StukalinCorrosion tests are performed on developed chromium corrosion-resistant steels alloyed with nitrogen. A set of tests shows that depending on chemical composition and production parameters the corrosion resistance indices for steels of one class may differ by an order of magnitude. Revelation of a dependence makes it possible to conclude that an increase within steel of nitrogen and molybdenum content combined with quenching and low-temperature tempering at 400°C makes it possible to prepare steel with good pitting corrosion resistance in aqueous media, particularly, under sea water conditions. An important requirement for steels used in the manufacture of equipment, creation of buildings and structures for various purposes in the Arctic and Antarctic regions is a combination of high strength with resistance to corrosion and corrosion-mechanical breakdown. Under Arctic and Antarctic marine conditions, stainless steels are most subject to such forms of corrosion and breakdown as pitting and intercrystalline corrosion (ICC), stress corrosion cracking, knife-line, contact, and exfoliation corrosion. One of the most critical forms of corrosion for stainless steels under the conditions in question is pitting corrosion [1,2]. Chromium martensitic stainless steels used previously had high strength but exhibited markedly lower pitting corrosion resistance than austenitic and austenitic-ferritic steels [3]. Contemporary martensitic steels, including the so-called supermartensitic stainless steels [4], have a considerably better set of properties, which points to the possibility of using them under Arctic and Antarctic marine conditions. In order to improve steel corrosion resistance and weldability, the carbon content within it is reduced to 0.02% or less. Alloying with nickel and nitrogen provides a martensitic structure without δ-ferrite. In addition, introduction of molybdenum and nitrogen improves the local corrosion resistance, primarily pitting.The aim of this work is to study the effect of chemical composition on indices for martensitic chromium-steel pitting corrosion resistance.The test steel chemical composition is provided in Table 1. With a chromium content from 14.2 to 16.7%, primarily the nickel, molybdenum, niobium, vanadium, and nitrogen contents were varied.Steel was melted in a Vacuum Industries vacuum induction furnace with a crucible capacity up to 7 kg for liquid steel in a nitrogen atmosphere. After cleaning ingots and removing the head and riser sections, the workpieces obtained were forged into sheet billets with a size of 55 × 55 × 300 mm. Then the sheet billets were rolled in a DUO 300 laboratory mill to a strip thickness of 4 mm by a regime: heating to 1200°C, soaking for 30 min, temperature for the start of rolling 1100°C; temperature for the end of rolling 900-930°C, accelerated cooling to 600°C, then strip was placed in a furnace heated to this temperature and cooled in the furnace.
The effect of process parameters in the production of progressive multiphase automobile steels on the assimilation of the elements used for alloying and microalloying is examined in order to develop an effi cient steelmaking technology for this purpose. Data from the production and ladle treatment of 96 experimental heats of steels that are similar to these steels in chemical composition is statistically analyzed to improve these elements' assimilation by optimizing the regimes used for the addition of ferroalloys and other materials at different stages of the steels' treatment. It is established that the assimilation coeffi cients for Mn, Cr, Nb, V, and Si (to a lesser extent) are nearly independent of the timing and volume of the in-treatment additions of materials that contain these elements. On the other hand, the effi ciency of the assimilation of Al and Ti depend to an appreciable extent on the type of material used and the treatment stage in which it is added. The results that are obtained are used to formulate a new approach to optimizing the addition of materials during the production of two-phase ferritic/martensitic steels of the HCT980X type. The adequacy of this approach was demonstrated by a trial heat that was made of this type of steel. Keywords: multiphase, two-phase ferritic/martensitic automotive sheet steels, steelmaking, ladle treatment, assimilation of components, regime for the addition of materials, manufacturing technology.Promising high-strength multiphase automotive steels with a yield point above 1000 MPa are being used with increasing frequency in automobile construction abroad. Among the most promising of these materials are two-phase ferritic/ martensitic steels (TFMSs) and steels whose ductility is formed by the γ→α transition (TRIP-steels). They have a complex composition and high contents of chemically active alloying and microalloying elements. Russia has mastered the industrial production of almost none of these progressive grades of steel, mainly due to problems in adequately assimilating these elements and obtaining the prescribed chemical composition in steels that have high contents of chemically active components. The goal of this investigation is to search for effective methods of making steels of this type and ways of putting those methods to use on an industrial scale.Studies were performed on the two-phase ferritic/martensitic steels that are currently in the greatest demand -steels of type HCT980X. The requirements on the chemical composition of these steels are set forth by the standard PREN103338:2007 and are as follows (wt.%, max): 0.23 C, 0.8 Si, 2.5 Mn, 0.08 P, 0.015 S, ≤2.00 Al, 1.00 Cr+Mo, 0.15 Nb+Ti, 0.2 V, 0.005 B.It is apparent from these data that these steels have high contents of Al, Si, Mn, Cr, and Mo and also contain as much as 0.15-0.20% microalloying elements (Ti, Nb, V). The laws that determine how processes in the steelmaking operation and in the treatment of the metallic melt affect the degree of assimilation of the alloying and microalloying elem...
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