MXPA01000856A - Method and installation for producing dual-phase steel - Google Patents

Abstract

The invention relates to the production of dual-phase steel (2) which is obtained by targeted cooling after the final deformation step in a rolling stand (1), whereby a dual-phase structure of between 70 and 90%ferrite and between 30 and 10%martensite is adjusted. So as to beindependent of steel geometry and strip speed the invention provides for cooling to take the form of fluidized-bed cooling carried out in a cooling line (6) comprising water-cooling stages (7) arranged one behind the other.

Description

PROCEDURE AND APPARATUS FOR THE MANUFACTURE OF DUAL PHASE STEELS FIELD OF THE INVENTION The invention relates to a process for the manufacture of dual-phase steels from the state of hot rolling with a two phase composition of 70 to 90% ferrite and 30 to 10% martensite, by means of a temperature-controlled conduction and defined cooling strategy, during the cooling of the steel, between other cooling means by water after its finished rolling, where in a first cooling stage the cooling curve penetrates into the field of the ferrite and after The required fraction of ferrite is achieved by cooling in a second stage of cooling to temperatures below the temperature at which the martensite starts. BACKGROUND OF THE INVENTION The transformation of composition pursued by a corresponding cooling of the steel, is already known. For example, DE 44 16 752 Al discloses a method for generating a hot wide band in which, before the first transformation between the rope casting machine and an equalizing furnace, the surface temperature is lowered of plantain to a sufficient depth (at least 2 mm.), so much so that a transformation of the austenite composition into ferrite / perlite is adjusted. Here the cooling time is chosen in such a way that at least 70% of the austenite is transformed into ferrite / pearlite. In the equalization furnaces, a new transformation in austenite is then carried out, with a new orientation of the austenite grain boundaries. In this way it must be achieved that also the scrap of a second selection, especially scrap with copper fractions, can be used as a starting material without undesirable accumulations of copper in the grain boundaries of the primary austenite. In the manufacture of dual phase steels, a compositional transformation is also carried out which takes place with the aid of a quenched cooling but now, temporarily after the transformation carried out. The adjustment of a first dual-phase composition essentially depends here on the cooling speeds technically possible in the apparatus or system and the composition of the steel. It is important in the manufacture of dual-phase steels, a first sufficient ferrite formation in the first cooling stage. Technically, by the system or apparatus, sufficient ferrite formation is achieved, for example by cooling with water at a temperature of about 620-650 ° C with sufficient air cooling. The duration of the air cooling (approximately 8 seconds) is selected such that at least 70% of the austenite is transformed into ferrite before the second cooling step is applied. During the first stage of cooling, as well as during cooling with air, a transformation in the perlite stage must be avoided. In the second cooling stage, there must still be so many cooling capacities that the main temperatures are reached below the temperature at the start of the martensite. Only then is the formation of a dual phase composition with constitutive fractions of ferrite and martensite ensured. This known manufacture does not have problems for small band speeds, since after the first cooling stage there are sufficient cooling capacities for the martensite transformation. With very high web speeds, the principle of the second cooling stage can in any case be displaced both in the existing cooling sections, that the subsequent formation of martensite is only carried out incompletely or not at all, since then the Cooling capacity for setting the temperature sufficiently low (<220 ° C). A mixed composition of Bainite ferrite and martensite fractions is then produced which do not satisfy the desired mechanical properties of the pure two-phase composition. EP-A-0 747 495 discloses a method for the production of hot-rolled steel sheet, whose structure comprises at least 75% ferrite and at least 10% martensite. For the manufacture, the objective cooling of the steel after hot rolling is sought, and precisely in a first stage of cooling with a cooling speed of 2 to 15 ° C / s within a time of 8 to 40 seconds to a temperature between the point xi and 730 ° C and then in a second cooling stage with a cooling rate of 20 to 150 ° C per second up to a temperature of 300 ° C. Alternatively, the first cooling step is pre-adjusted by rapid cooling with a cooling rate of 20 to 150 ° C / sec which leads to below the temperature of point Ar3. According to a document of the Abtracts of Japan Vol. 006, No. 19KC-127) of September 30, 1982 and JP 57104650A (Kobe Steel Ltd) June 29, 1982 a method for the manufacture of a steel sheet is known. hot rolled consisting of ferrite and a fraction of 1 to 30% of martensite, which is also produced by a two-stage cooling. Firstly, in this known process, it is cooled slowly to a temperature between the Ari point and 550 ° C with a cooling rate of 5 to 30% C / s and then in a second cooling stage with a rapid cooling rate of more than 30 ° C / s up to a temperature in the range of 350 to 500 ° C. SUMMARY OF THE INVENTION Starting from this known state of the art it is the task of the present invention to provide a method and an apparatus or system for the manufacture of dual-phase steels, with which a sufficient quantitative transformation of the composition or content of the austenite to ferrite, also with high band speeds. The proposed task is solved according to the procedure with the characteristic measures of claim 1, so that during the first cooling stage the cooling curve of the steel is adjusted with a cooling rate as low as 20 K / s to 30 K / s, that the cooling curve runs or penetrates into the field of the ferrite with a temperature still so high that the ferrite formation can be carried out quickly and before the beginning of the second stage of cooling and when at least 70 has been transformed % of austenite in ferrite. By the slower cooling according to the invention, with a low cooling rate than in the known processes, but where the cooling curve with respect to time starts somewhat behind with respect to the known procedure in the field of ferrite, this is the transformation of austenite into ferrite, but at a higher temperature than in the known process, and develops at a higher temperature also faster. Favorably this works if the field of the ferrite with a simultaneously higher transformation temperature is reached as quickly as possible. In contrast to the known processes, a transformation speed of at least 70% is achieved as soon as sufficient cooling capacity is available in a given cooling section for the subsequent formation of martensite. That is, after the completion of the first cooling step, there is a sufficiently large amount of transformed ferrite austenite that the usual air cooling can disappear and the second cooling stage is immediately connected to the first cooling stage.

In order to carry out the cooling with the desired low cooling rate, the principle of cooling by loosening is used according to the invention. This is a cooling with water in which with cooling stages with water arranged one behind the other at a certain distance, water is applied on the material to be cooled. By the influence intake in relation to the number of cooling stages with water, the effective length of the cooling stages with water is optimally adjusted and thus the cooling rate or the quantity of water applied to the cooling material (its mass of the material to be cooled and / or the surface of the material to be cooled). Cooling can also be carried out with quantities that can be changed step by step. By adjusting to the material to be cooled, the relaxed cooling can be extended with respect to time so that the desired degree of transformation is achieved without being endangered - as in the known procedures with faster cooling - than the cooling curve by too intensive cooling leave behind the ferrite field. In comparison with the cooling according to the state of the art, in a loosened cooling or with a variable amount of cooling medium without stages, less water is applied to reach the transformation temperature. That different amount of water can now be provided during the transformation, for the carbon mixture of the ferrite to be controlled in the remaining austenite, and thus accelerate the formation of ferrite. The austenite zones left behind are so enriched with carbon that they are transformed at cooling speeds of 20-30 K / s in martensite. Since a defined retention number for air cooling is no longer needed, in order to ensure sufficient ferrite formation, the manufacture of dual phase steels can be performed in a part of the cooling section. The used part of the cooling section is therefore much shorter than in known methods with air cooling. If the ductile components of the dual phase steels can be adjusted without cooling with air, then essential advantages are presented for operation. Component means of the apparatus or system for the manufacture of dual phase steels will be necessary. Simultaneously, the production spectrum can be broadened with various parameters of process and band (for example, higher band speed) than what has been practiced so far.

An apparatus or system for carrying out the process according to the invention is characterized by a cooling section arranged behind the last section to complete the rolling, cooling section consisting of several stages of cooling with water or cooling systems placed at some distance from each other with a variable amount of cooling medium without stages. The number of cooling stages with water, their effective length and their distance from each other are variable according to the invention, so that these cooling sections can be adjusted in a simple manner to the modified geometries of the material to be cooled, as well as to different band speeds. DESCRIPTION OF THE DRAWINGS Other advantages particularities and characteristics of the invention are produced by the following explanation of an embodiment represented schematically in the drawing. Where it shows: Figure 1, a schematic representation of the rapid cooling and the loosened cooling thus coiriq its coordination to a laminate track; Figure 2, an image of the time-temperature transformation; Figure 3, the degree of the transformation of austenite in the rapid transformation; Figure 4, the degree of the transformation of austenite in the loosened cooling. DESCRIPTION OF THE INVENTION In figure 1, it is represented schematically the end of a rolling track consisting of the last rolling finishing equipment 1 to the rolling material or material to be cooled 2 and a lathe 3 with deflection rollers or impeller 4. Above this part of a rolling track are indicated two different cooling sections. With the cooling section 5 according to the state of the art, a water-dependent feed is provided by an early and rapid cooling of the material to be cooled 2. In the cooling section 6, cooling stages with water placed one after the other are arranged. from the other at a distance, with which in cooling it "loosens". The different transformation results produced by the different cooling processes 5, 6 are repeated again by way of example in the following schematic representations. Figure 2 shows a time-temperature transformation image of the course of the cooling curve 9 in a cooling according to the known method and the cooling curve 10 with a relaxed cooling where it occurs in the line of the abscissa, the time (Z) in seconds and in the line of the ordinate the temperature (T) in degrees Celsius. The cooling curve 9 shows the course of cooling in the strategy usually used at present, cooling (early and fast at a given maintenance temperature with subsequent cooling by air, then cooling to lower temperatures at a lower starting temperature of martensite). The cooling curve reaches its first cooling stage 11 relatively early or soon at point 8 in the transformation field for the formation of ferrite (ferrite field) and remains due to the residence time "air cooling also a time relatively long in this field (F) before by the second cooling step 13 from point 17 more cooling takes place at a temperature below a martensite temperature (M = martensite, B = Bainite, P = Perlite). on the contrary, it reaches the cooling curve 10 with its first cooling stage 14 in a relaxed cooling with respect to the cooling curve 9, the ferrite field (F) in point 16 only afterwards, since after reaching the field of ferrite (F) are maintained firstly the loose cooling does not need any time of maintenance costly in time with cooling with air, and the cooling curve 10 abandons more p silly with respect to time, again the ferrite field (F). The loosened cooling is maintained with this as much as is convenient within the ferrite field (F) until the desired degree of transformation has been achieved. The next cooling is then carried out directly with the second cooling stage 16. The austenite transformation rates achievable with the different cooling strategies shown, the known rapid cooling and the loose cooling, are taken from the following two representations in the figures 3 and 4; here each time the cooling time (Z) is represented in the abscissa line in seconds and in the line of the ordinates the degree of transformation (U) of the transformation of austenite into ferrite. In the rapid cooling (Fig. 3) during the first cooling stage 11 of the cooling curve 9, first a strong ferrite formation takes place up to about 53% which then in the next air cooling 12 increases to approximately 62%. However, this is still not enough for the manufacture of dual-phase steels. In the case of the loosened cooling, FIG. 4, according to the cooling curve 10, on the contrary, higher ferrite contents are formed in the same time in the first cooling stage 14, and approximately 82% of the austenite is already converted into ferrite, before the second cooling stage 16 is applied (the dual phase steels currently produced generally have a ferrite fraction greater than 80%). The invention is not limited to the curves given as an example and described in the drawings, but other cooling curves are also possible, for example in cooling systems with a variable amount of cooling medium without stages, which can lead in the direction of the invention at higher transformation temperatures. Also, the invention is not limited to cooling with water, but other cooling systems can be used which lead to an earlier range of the ferrite field at elevated temperatures.

Claims (4)

1. NOVELTY OF THE INVENTION Having described the invention as above, the content of the following CLAIMS is claimed as property 1. Procedure for the manufacture of dual-phase steels from a state of hot rolling with a content in two phases of 70 to 90% Ferrite and 30% to 10% of martensite, by means of a controlled temperature conduction and cooling strategy defined during the cooling of the steel, and also by means of water cooling, after its completion, where in a first stage of cooling With a cooling speed of less than 30 K / s, the cooling curve in the ferrite field penetrates or penetrates and after reaching the required ferrite fraction, it is continued cooling in a second cooling stage with a higher cooling rate of 30 K / s at temperatures below the martensite start temperature, procedure characterized in that a) the first cooling stage is carried out by adjusting in a cooling section composed of cooling tops with water arranged one behind the other, at a certain distance or in a cooling system where the continuous cooling means are varied with a cooling speed of 30 K / s; b) the cooling curve runs at such a high temperature in the field of the ferrite that, the formation of ferrite can be carried out rapidly; c) before the beginning of the second cooling stage, which takes place without an intermediate air cooling and with a holding time of the first cooling stage, at least 70% of the austenite in ferrite has already been transformed.
2. 2. Method according to claim 1, characterized in that during the transformation of the austenite into ferrite the cooling is continued in the first stage of cooling of the desired ferrite content of the apparatus
3. 3. Apparatus or system for carrying out the process according to one or more of the preceding claims for the manufacture of dual phase steels from the hot rolling state, characterized by a cooling section disposed after the last rolling finishing equipment with several water cooling stages arranged one
4. 4. The apparatus according to claim 3, characterized in that the number of cooling with water, its effective length and the distance of the other at a distance or with cooling systems with a variable or variable amount of cooling medium. each other is variable or is continuously adjustable in a quantity regulation.