SK882013U1 - Continuously annealed metal plates with two-phase structure and a method for production of the metal plates - Google Patents

Abstract

Continuously annealed sheets and bands with double phase structure ferrite and martensite with minimal fracture limit 450 MPa are characterized in that the steel is doped by carbon in the range 0,03 0,10 % by weight, manganese 1,20 2,00 % by weight, chromium in the range 0,40 0,70 % by weight in total amount and by admixtures of other elements silicon maximally 0,10 % by weight, phosphorus max. 0,25 % by weight, sulphur maximally 0,020 % by weight, aluminium minimally 0,020 % by weight, nitrogen maximally 0,012 % by weight, niobium maximally 0,050 % by weight, molybdenum maximally 0,060 % by weight, vanadium maximally 0,050 % by weight, nickel maximally 0,050 % by weight, copper maximally 0,50 % by weight in total amount and the rest is iron and accompanying impurities. The method for production of continuously annealed sheets and bands with double phase structure is characterizes in that, the steel is hot rolled with the finishing temperature 850 950 �C and the coiling temperature 500 750 �C. The hot rolled band is pickled and cold rolled by reduction 45 to 85 % to a final band with thickness 0,4 2,5 mm and is continuously annealed at 720 820 �C, where the annealed band is cleaned and flattened with the total prolongation 0,3 2 %.

Description

Continuously annealed two-phase structure sheets and strips and process for their production

Technical field

The invention relates to continuous annealed hot-dip galvanized or uncoated sheets and strips with a two-phase structure of ferrite amartensite and a minimum breaking strength of 450 MPa and a process for their production.

BACKGROUND OF THE INVENTION

Dual-phase steels are characterized by a defined distribution of martensite in a ferritic matrix. The proportion of martensite is approx. 20%. A standard procedure for the production of cold rolled DP steels is annealing to an intercritical temperature corresponding to 20% austenite and subsequent cooling at a supercritical rate to form martensite. During annealing, austenite is enriched with carbon. Dual-phase steels can be produced using chromium, molybdenum or chromium alloying along with molybdenum in combination with other elements such as carbon, manganese, silicon, phosphorus, vanadium, niobium, with the addition of niobium causing grain refinement and thereby contributing to an increase in strength of up to 10OMPa. The ferritic-martensitic structure can also be achieved by increasing the manganese content of conventional steels to 3%.

The rolling temperature of the cold-rolled two-phase steel is just above Ar3. Rolling at this temperature not only accelerates the ferritic transformation, but at the same time prevents the formation of deformation reinforced ferrite, which reduces the plastic properties of the material. The strip cooling rate after rolling and the coiling temperatures must be set so that cooling is terminated at a temperature above B _s (Bainite start, start of bainite transformation) and the resulting structure is formed by a ferritic matrix with uniformly distributed perlite, allowing subsequent cold rolling at lower deformation resistances as in case of deformation reinforced ferrite, resp. in combination with the presence of mixed structures of the ferritperlite-bainite-martensite type.

Cold rolled two-phase steels and cold rolled and coated two-phase steels are produced by a heat treatment consisting of intercritical annealing in the two-phase region austenite + ferrite. A controlled amount of austenite is formed in the intercritical region, the intercritical annealing temperature controlling the equilibrium volume of austenite and thus its carbon content. At higher temperatures, a greater proportion of austenite is formed, but its carbon content is lower, making it less stable. At lower temperatures, a smaller volume of more stable austenite with a higher carbon content is formed.

The cooling end temperature must be less than M _s (martensite start, martensitic start temperature) and low enough to prevent the austenite to ferrite from being terminated prematurely. The cooling rate from the intercritical temperature must be high enough to convert austenite to martensite in the case of coated two-phase steels before the strip enters the coating bath (hot-dip coating). The alloying elements influence the kinetics of transformations during continuous annealing.

In order to achieve a two-phase microstructure, the cooling rate must increase as the alloying element content decreases. 2.7 times higher than manganese. The critical cooling rate required to form a biphasic structure is not affected by the substitution of manganese, chromium or molybdenum. The strength limit R _m increases with increasing cooling rate. There is a certain cooling rate that minimizes the necessary alloying. The element content in masses, percentages and the type of alloying elements affects the cooling rate range, where optimum yield strengths Re and R _e / Rm are achieved. cooling rate, which requires a minimum of alloying. The reason for having an optimal cooling rate with minimum alloying is (1) the need for a high content of alloying elements to stabilize austenite in the transformation to martensite in the slow cooling region and (2) solidification of the ferritic matrix in the area of rapid cooling.

Existing concepts of cold-rolled and hot-dip galvanized steels produced on continuous lines use the carbon-manganese-chromium-molybdenum alloy listed in Table 1 according to EN 10346: 2009.

Tab. 1 Example of chemical composition of two-phase steels

Chemical element Chemical composition [wt%] C max. 0.14 Mn max. 2.0 Are you max. 0.8 P max. 0,080 WITH max.0,015 Al max. 2.0 Cr + Mo max. 1.0 Nb + Ti max.0,15 IN max.0,20 B Max. 0,005 The rest iron and accompanying impurities

In the production of molybdenum alloyed steel, FeMo alloy is added to the charge in oxygen converters. The FeMo alloy has a high melting point and therefore the total molybdenum content of the steel can no longer be controlled in further technological production steps. The FeCr alloy has a lower melting point than FeMo and can be added to the steel during ladle casting. Further alloying of FeCr is possible even in the process outside the steel furnace processing. The price of FeMo alloy is 6-7x higher compared to the FeCr price, which contributes to the production cost per ton of steel.

The essence of the technical solution

These drawbacks largely eliminate continuously annealed sheets and strips with a two-phase structure and a process for their manufacture from low-carbon manganese and chromium alloyed steels which provide the desired structure with a martensite content of 10 - 30% and achieve the desired mechanical properties according to the following chemical composition of the steel : 0.03-0.10 wt.% carbon, manganese 1.20-2.00 wt.%, chromium 0.40-0.70 wt.%, and additives of other elements in the silicon range not more than 0.10 wt.% , phosphorus maximum 0.025%, sulfur maximum 0.020%, aluminum minimum 0.020%, nitrogen maximum 0.012%, niobium maximum 0.050%, molybdenum maximum 0.060%, vanadium maximum 0.050%, nickel not more than 0,50% by weight, copper not more than 0,50% by weight and the remainder being iron and accompanying impurities, Table 2:

Tab. 2 Chemical composition

Chemical element Chemical composition [wt%] C 0.03 to 0.10 Mn 1.20-2.00 Are you max.0,10 P Max.0,025 WITH Max.0,020 Al Min. 0,020 N Max. 0,012 nb Max. 0,050 Cr 0.40 - 0.70 Mo Max. 0,060 IN Max. 0,050 Ni Max. 0,500 Cu Max. 0,500 The rest iron and accompanying impurities

The process for producing continuously annealed sheets and strips with a two-phase structure according to the invention starts with the step when the steel is cast on a continuous casting machine. In the next step, it is hot rolled with a rolling temperature of 850-950 ° C and a coiling temperature of 500-750 ° C to provide a soft ferritic-pearlitic structure for further cold rolling. Subsequently, the warm strip is pickled and cold rolled by reducing 45 to 85% to a final strip having a thickness of 0.4-2.5mm. The final treatment on a continuous hot-dip galvanizing line consists of annealing at a temperature of 720-820 ° C and subsequent cooling of the strip to a temperature of 465-520 ° C before entering the zinc melt. This is followed by hot-dip galvanizing in a zinc melt with a temperature of 455-475 ° C and a smoothing and straightening of the strip with a total elongation of 0.3-2.0%.

The values of mechanical properties in the transverse direction to the rolling direction of the produced strip by this technology reach the following values R _p o, 2 = 260-340MPa; Rm = min. 450Mpa; Ago = min. 27%; πιο-ue ~ min. 0.16; BH2 = min. 30MPa.

Example

An exemplary embodiment of continuous annealed sheets and strips with a two-phase structure and a method for their production is the technology of melting in an oxygen converter and out-of-furnace steel processing, which leads to the production of steel of this chemical composition in weight percent: 0.052% C; 1.531% Mn; 0.025% Si; 0.014% P; 0.004% S; 0.053% Al; 0.002% Mo; 0.001% Ti; 0.002% V; 0.002% Nb; 0.018% Cu; 0.009% Ni; 0.584% Cr; 0.008% N; rest iron and accompanying elements. The steel is continuously cast into the gates eg. with a thickness of 220 mm and a length of 8 m. The frames are heated to a picking temperature of e.g. 1232 ° C with temperatures in each zone and length of residence in the furnace so as to achieve a homogeneous temperature throughout the volume of the frame. After pre-rolling in the preparatory order, the preform thickness of e.g. 41 mm enters the first stool of the finished order of a warm broadband track at an average temperature of e.g. 1053 ° C and it is rolled in the last mill at an average temperature of e.g. 892 ° C for a warm strip of thickness e.g. 3.0 mm. The web is coiled at a temperature of e.g. 584 ° C. After pickling in hydrochloric acid, the coil is cold rolled to a thickness of e.g. 1.2 mm with a total reduction of 60.0%. The final treatment on a continuous hot-dip galvanizing line consists of annealing in the intercritical region at a temperature of e.g. 775 ° C and a web speed of approximately 90 m / min and subsequent cooling of the web to 485 ° C at a rate of 38 ° C / s before it enters the zinc melt. This is followed by hot-dip galvanizing in the zinc melt with a temperature of 462 ° C, material smoothing and strip straightening with a total elongation of e.g. 0.6%.

Claims (2)

PROTECTION REQUIREMENTS Continuously annealed sheets and strips with a two-phase structure of ferrite amartensite and a minimum breaking strength of 450 MPa, characterized in that the steel is alloyed with carbon in the range of 0.03-0.10 wt.%, Manganese 1.20-2.00 wt. %, chromium in the range of 0.40-0.70 wt.% in total and additives of other elements in the silicon range of not more than 0.10 wt.%, phosphorus max. 0.025 wt%, sulfur maximum 0.020 wt%, aluminum minimum 0.020 wt%, nitrogen maximum 0.012 wt%, niobium maximum 0.050 wt%, molybdenum maximum 0.060 wt%, vanadium maximum 0.050 wt%, nickel maximum 0 % By weight, copper at most 0.50% by weight in total, and the remainder being iron and accompanying impurities. Method for producing continuously annealed sheets and strips with a two-phase structure according to claim 1, characterized in that the steel is hot-rolled with a rolling temperature of 850-950 ° C and a coiling temperature of 500-750 ° C, the hot-rolled strip being pickled and rolled cold by reducing 45 to 85% to a final strip of 0.4-2.5 mm thickness and continuously annealed at a temperature of 720-820 ° C, the annealed strip being superfinished and equal to an overall elongation of 0.3-2.0%.