MXPA97010229A - Multiphase steel, production of rolled products and the use of such ac - Google Patents

Abstract

The invention relates to a multiphase steel, to a process for producing roll products from this steel with a polygonal ferritic structure of up to 70% by volume, and its use. The steel must have high strength, good cold working properties and an improved surface quality after the final stage of hot work.

Description

STEEL OLTIFASES, PRODUCTION OF ROLLED PRODUCTS AND THE BEAR OF SAID STEEL The invention relates to a polyphase steel, a process for producing rolled products from this steel with up to 70% by volume of ferritic-polygonal structure, as well as the use of steel. The steel must be of high strength and must have a good capacity to be reduced in cold as well as it must also have a surface quality, improved after hot forming in the last stage of production. Double-phase steels are known to have a structure, for example, up to 80% by volume of relatively soft ferrite, polygonal with the remainder being carbon-rich martensite. The second phase, of smaller quantity, which is rich in carbon, gets into the ferritic, proeutectic phase, in the shape of an island. A steel of this kind has good mechanical properties and a good capacity to be cold reduced. Known steels with predominantly polygonal ferrite in the structure as well as martensite embedded herein, REF: 26570 comprises (in% by mass) 0.03 to 0.12% of C, up to 0.8% of Si and 0.8 to 1.7% of Mn (From 29 24 340 C2) or 0.02 to 0.2% of C, 0.05 to 2.0% of Si , 0.5 to 2% of Mn, 0.3 to 1.5% of Cr as well as up to 1% of Cu, Ni and Mo (EP 0 072 867 Bl). Both steels are quenched or deoxidized by aluminum and have soluble residual contents of less than 0.1% Al. The silicon in these steels promotes the transformation of the ferrite. In combination with manganese and if chromium is applicable, the formation of perlite is suppressed. In this way, adequate carbon enrichment in the second phase is ensured and polygonal ferrite formation is achieved in predominant relation to the second phase. However, these known alloys have the disadvantage that during hot rolling, an inhomogeneous surface structure is formed which becomes apparent from the red scale patterns. After cleaning with a chemical bath, the surface remains rough. For many applications, such material is not marketable. So far the improvement in surface quality of these hot-rolled steels has not been successful. Therefore these steels are not usable for certain purposes such as cold-reduced wheel discs for motor vehicles or other products made by cold reduction such as cold-reduced construction profiles and the like. In addition, steels of this type as a predominant amount of polygonal ferrite, relatively smooth in the structure only achieve tensile strength of up to 700 N / mm2. In this way, the reduction in weight that maintains a linear relationship to the resistance is severely limited. In this way, it is the object of the invention to develop a steel that maintains at least the remarkable spectrum of the mechanical properties of the known steels, has greater strength than the known double-phase steels, can be reduced in cold like these, but After production by hot forming it provides a better surface structure than these steels. To achieve this objective, a polyphase steel is proposed, which comprises (in% by mass) 0.12 to 0.3% of carbon 1.2 to 3.5% of manganese 1.1 to 2.2% of aluminum less than 0.2% of silicon the rest being iron, inclusive non-avoidable impurities such as phosphorus and sulfur with a perlite-free structure of less than 70% by volume of polygonal ferrite, soft and the remainder being bainitic ferrite and more than 4% by volume, preferably up to 20% by volume, of residual austenite, rich in carbon as well as also if applicable, smaller percentages of carbon-rich martensite containing aluminum in an amount in percent by mass of To < 7.6 • Cequ. - 0.36 with a carbon equivalent (Cequ.) Of 0.2 < Cequ. -% C + 1/20% Mn + 1/20% Cr + 1/15% Mo = 0.325.

Such a steel surpasses the performance Rm. A5 of the double phase steels alloyed with silicon, known and after the completion of hot forming, their surface quality is improved, as required, for example, for wheel discs for motor vehicles, discs which are produced by cold reduction of hot rolled steel. In addition, the following elements can be added to the steel in the percentages indicated, (in% by mass) by alloy t up to 0.05% titanium up to 0.8% chromium up to 0.5% molybdenum up to 0.8% copper up to 0.5% nickel.

Such steel, alloyed with aluminum instead of silicon, achieves a ductile performance Rm • A5 > 18,000 N / mm2 •%, ie a ductile yield of As > 18,000 / Rm in% at a tensile strength value of Rm up to 900 N / mm2. The steel according to the invention is characterized by an aluminum content of 1.1-2.2% which is significantly higher than that of the known steels. In contrast, according to the invention, the silicon content is limited to less than 0.2%. In contrast, known steels of this type usually contain silicon contents in excess of 0.5%. Aluminum alloyed steel, according to the invention, comprises the polyphasic microstructure with residual austenite, as described, and has excellent mechanical strength characteristics. Above all, the surface quality of the hot formed product after the last stage of hot forming is significantly improved when compared to the silicon alloyed steels hitherto known. There is a more pronounced delay in perlite formation when compared to known steels; Perlite formation can be safely avoided by observing the claimed process parameters. At 0.12 to 0.3%, the carbon content is in the usual range for steels of this type. In order to avoid the formation of perlite, manganese is added in percentages of 1.2 to 3.5%. Manganese has a hardening effect of solid solution and increases the level of resistance. In view of the avoidance of pearlite and the effect on ferrite formation, the carbon content and the manganese content are interchangeable within the limits of the carbon equivalent. The carbon equivalent is determined as follows: 0.2 = Cequ. =% C + 1/20% Mn + 1/20% Cr + 1/15% Mo < 0.325.

According to the invention, the intersection of the value of the carbon equivalent and the coupling of the aluminum value should be within the shaded area shown in Figure 1 in order to ensure a ferrite content below 70% by volume and a content of residual austenite that exceeds 4% by volume. In addition, 0.05% titanium ensures the adjustment of nitrogen and prevents the formation of elongated manganese sulphides. Up to 0.8% by mass of chromium can be added to improve the tempering properties of martensite and prevent the formation of perlite. Molybdenum, up to 0.5% by mass, increases the range of successful cooling speeds. Copper and nickel, up to 0.5% by mass of each, can contribute to the reduction of the transformation temperature and avoid perlite. In order to influence the coalescence of the sulfides, the treatment of the bath in fusion with calcium-silicon is advantageous.

The final temperature of the ET hot rolled should be in the range of Ar3 - 50 ° C < ET < Ar3 + 100 ° C.

The temperature Ar3 that should be in the range of 750 to 950 ° C is calculated as follows 750 ° C = Ar3 = 900 + 100% Al - 60% Mn - 300% C = 950 ° C The cooling down of the final temperature of hot rolling at the winding temperature which is between 200 and 500 ° C takes place in an accelerated manner at a cooling rate of 15 to 70 K / s. When the cooling down of the final hot-rolled temperature, in the process according to the invention, it is possible in the range of Ar3 to Ar3 equal to 200 ° C, to further improve the polygonal ferrite formation by observing the Cooling pause from 2 to 30 s, during which the cooling rate is below 15 K / s.

Figure 2 shows a diagrammatic representation of hot strip production coupled with the cooling progression of the steel according to the invention, in and after hot rolling. It is shown that any undesirable entry of the perlite region can be safely avoided if the conditions established for the final hot-rolled temperature, the cooling rate and the winding temperature are observed. A steel A according to the invention, a composition according to Table 1 was hot rolled to a final strip thickness of 3.7 mm at a final hot rolling temperature of 855 ° C. The cooling from this temperature was at 30 K / s at the winding temperature (RT) of 415 ° C. The characteristics of this steel A according to the invention were determined according to DIN EN 10002 in flat-stretched specimens. Table 2 shows the values for the apparent yield point, the tensile strength, elongation and the yield point relationship to the tensile strength for the length and width layers of the laminate direction. By way of comparison, Table 2 also shows the respective strength properties of a steel B, known from EP 0 586 704 Al, with a composition as shown in Table 1. Due to its spectrum of characteristics, the steel according to with the invention it is particularly suitable for the production of cold-reduced structural elements for motor vehicles, such as floor reinforcement elements, transverse connections or links or for wheel discs.

Table 1 % C% Mn% Al% Si% Cu% Ni% Cr% P% S% N A 0.216 1.38 1.83 0.06 0.57 0.27 0.52 0.010 < 0.001 0.0024 B * 0.21 1.50 0.057 1.48 < 0.01 0.01 0.02 < 0.005 0.004 0.006 * according to EP 0 586 704 Al Table 2 Steel HT HT Ratio to Reh / Rp02 Rm A5 Rm - A5 ° C ° C direction of N / mm2 N / mm2% N / mm2. % Laminate structure A 855 415 L 610 725 32.5 23562.5 Ferrite Q 657 726 27.4 19892.4 Polygonal / Bainite / Austenite to 50/40/10 B '860 350 570 742 27.0 20034.0 Polygonal Ferrite / Perlite / Bainite / 6% Austenite L - along Reh / Rp02 - Product resistance Q - transverse Rm - Resistance to tension A5 - elongation after fracture It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Having described the invention as above, the content of the following claims is claimed as property.

Claims (6)

1. A polyphase steel, characterized in that it comprises (in% by mass) 0.12 to 0.3% of carbon 1.2 to 3.5% of manganese 1.1 to 2.2% of aluminum less than 0.2% of silicon the rest being iron, including non-avoidable impurities, including phosphorus and sulfur, with a perlite-free structure comprising up to 70% by volume of polygonal ferrite, soft and the remainder being bainitic ferrite and more than 4% by volume, of residual austenite, rich in carbon as well as if applicable, smaller percentages of carbon-rich martensite containing aluminum in an amount by mass percent of To < 7.6 • Cequ. - 0.36 with a carbon equivalent (CßqU.) Of 0.2 = Cequ. -% C + 1/20% Mn + 1/20% Cr + 1/15% Mo = 0.325.

2. A polyphase steel according to claim 1, characterized by a residual austenite content of up to 20% by volume.

3. A process for the production of laminated products of a polyphase steel composite according to claim 1 or 2, characterized in that it is of high strength, high tenacity, good surface quality in the hot rolling condition and good capacity to be laminated cold, with a perlite-free structure comprising up to 70% by volume of polygonal ferrite with the remainder being bainitic ferrite and more than 4% by volume, residual austenite, rich in carbon as well as if applicable, in addition, smaller percentages of carbon-rich martensite; steel that is continuously cast and hot-rolled at an initial hot-rolling temperature exceeding 1000 ° C and a final hot-rolled temperature (ET) in the range of Ar3 = 50 ° C < ET < Ar3 + 100 ° C, and subsequently cooled down to the final temperature of hot rolling (ET) at a speed of 15 to 70 K / s at a winding temperature in the range of 200 to 500 ° C and winding.

4. A process according to claim 3, characterized in that one or more of the following are added to the steel by means of the alloy (in% by mass): up to 0.05% titanium up to 0.8% chromium up to 0.5% molybdenum up to 0.8% copper up to 0.5% nickel.

5. A process according to claim 3, characterized in that in the temperature range between Ar3 to Ar3 equal to 200 ° C, a cooling pause of 2 to 30 s is observed, during which the cooling rate is below 15 sec. K / s.

6. The use of a steel with a composition according to claim 1, 2 or 4 as a material for the production of cold-reduced structural elements for motor vehicles, such as floor reinforcement elements, connections or transverse links or for discs of wheel.