WO2012000071A1 - A cold-rolled annealed steel plate with twip effect and process for producing same - Google Patents

Abstract

The subject matter relates to cold-rolled annealed steels with TWIP effect and the manufacturing process thereof. Said steel contains a manganese level of 20% to 30%, a silicon level of 1% to 3%, an aluminium level of 2% to 6%, a nickel level of 1% to 3%, a carbon level of 0.03% to 0.4% and a nitrogen level of 0.001% to 0.008%, is ductile due to twinning during hardening (TWIP effect), can achieve a total elongation of up to 95%, and absorbs a huge amount of energy. Furthermore, said steel is less dense than carbon steel or stainless steel.

Description

 COLD LAMINATED STEEL SHEET WITH TWIP EFFECT

 OBTAINING PROCESS

 The treated material refers to the cold rolled and annealed TWIP steel and its manufacturing process. This steel has manganese levels between 20% and 30%, silicon between 1% and 3%, aluminum between 2% and 6%, nickel between 1% and 3%, carbon between 0.03% and 0, 4% and nitrogen between 0.001% and 0.008%, showing ductility due to the formation of soft during the hardening (TWIP effect), and can reach total elongations of up to 95%, absorbing huge amounts of energy. In addition, it has lower density when compared to carbon steels or stainless steels.

 When thinking of steels with a combination of mechanical strength and ductility so that they can be applied in various uses in mechanical, civil and marine construction, alloyed and heat treated steels are the most used options. In this context, TWIP (TWinning Induced Plasticity) steel - deformation-induced maclation - emerges as an alternative to meet this demand. As a non-limiting example, there is a growing need for the automotive industry for materials that combine high conformability, high mechanical strength and low specific weight to reduce fuel consumption and increase passenger safety.

Some prior art documents have been found in the prior art, for example: PI 0412867-2 which refers to the process of manufacturing high strength, excellent toughness and creep ability of austenitic ferro-carbon-manganese steel sheets cold, and the plates thus produced. Hot rolled ferro-carbon-manganese austenitic steel plate, with a strength exceeding 900 MPa; product: strength (MPa) x elongation at break (%) greater than 45000; chemical composition comprising: C, Mn, Si, Al, S, P, N, and optionally one or more elements such as: Cr, Mo, Ni, Ti, Nb, V and Cu (the remainder of the composition consisting of iron and impurities resulting from the preparation); the recrystallized fraction of steel exceeding 75%; the carbide surface fraction precipitated steel less than 1.5% and the average grain size of steel less than 18 micrometers.

 The document PI 0419185-4 relates to the superior strength steel plate or strip, having twip properties, and the process for its manufacture by direct strip casting, in uninterrupted work steps: a molten mass of the following composition: C, Mn, Ni, Si, Al, Cr, N, Cu, P, optionally one or more elements of the group Se, Te, V, Ti, Sn, B, REM, Mo, W, Co , Ca and Mg (containing iron as well as fusion-conditioned impurities, the contents of Sn, Sb, Zr, Ta and As not exceeding 0,30% being added to these impurities) are applied to a conveyor belt and cooled there until it compresses to a pre-tape. This is extracted from the conveyor belt, if required heat treated, hot rolled to a hot strip with a completely recrystallized texture, and the hot strip is wound.

 PI 0517890-8 relates to a method of producing austenitic iron / carbon / manganese steel sheets having very high strength and elongation characteristics and excellent homogeneity.

 US 6358338 relates to a process for manufacturing iron-carbon-manganese alloy straps and straps thus manufactured. A thin belt with a thickness of 1.5 to 10 mm is cast onto a casting machine directly from a liquid metal of composition: C, Mn, Ni, Al, Cr, P, Sn, Sb, As, S , If, Te, V, Ti, Nb, B, Zr, Mo, W, N, Cu, the rest being iron and impurities resulting from the elaboration; this belt is cold rolled at a reduction rate of between 10 and 90% in one or more steps; and an annealing of recrystallization of this belt is made. The invention also relates to a belt capable of being produced by this process.

 The commonly known process involves several hot work steps such as roughing, finishing and obtaining the hot rolled strip, and cold work to obtain the product.

On the other hand, the present invention describes strip solidification followed by cold rolling and annealing to obtain an austenitic grain size of the order of 2-10 micrometers. Unlike documents The aforementioned invention reduces the hot working steps of the alloy, such as roughing and reduction of 70 to 90% in the finishing step.

 In addition, the product generated by the treated steel process has excellent performance, with mechanical strength greater than 700 MPa and total elongation greater than 70%. A reduction of 70 to 90% in thickness applied for hot rolling is also achieved.

 By reducing the hot work steps of the alloy, industrial facilities can be reduced, reducing the number of equipment, energy consumed, inputs, labor and overall costs.

 Using the treated matter process you get an austenitic grain size of less than 5 micrometers.

LIST OF FIGURES

Figure 1 shows the X-ray diffraction of homogenized steel.

 Figure 2 shows the optical micrographs of annealed TWIP steel at different temperatures: (a) 750 ° C and (b) 850 ° C, corresponding to the points

(a) and (b) shown in Figure 4.

 Figure 3 shows the optical micrograph of hot rolled TWIP steel. Figure 4 shows the evolution of grain size as a function of annealing temperature.

 Figure 5 shows Engineering Stress versus Engineering Strain plots of cold rolled annealed TWIP steel at different temperatures: (a) 600 ° C; (b) 700 ° C; (c) 750 ° C; (d) 775 ° C; (e) 850 ° C and (f)

Cold rolled.

 Figure 6 shows the variation of the hardening exponent (a), the total elongation (b) and the strength limit (c) with the annealing temperature.

DETAILED DESCRIPTION OF THE INVENTION

 The process for preparing treated material steel comprises the following steps:

- melting of steel plates between 1550 ° C and 1650 ° C; - casting in molds with a temperature between 1500 ° C and 1550 ° C; - reheating for hot rolling at various passes with a temperature between 1 100 ° C and 1300 ° C for refining the structure and obtaining the hot rolled strip;

 - chemical pickling to remove surface oxides;

 - cold rolling in several passes with 50 to 90% thickness reduction to obtain cold rolled strip;

 - annealing the cold rolled strip at a temperature between 700 ° C and 1100 ° C for a minimum of 2 minutes and a maximum of 60 minutes.

 After this manufacturing process, the chemical composition of steel comprises, the contents being expressed by weight:

 Manganese: 20% <Mn <30%;

 Silicon: 1% <Si <3%;

 Aluminum: 2% <Al <6%;

 Nickel: 1% <Ni <3%;

 Carbon: 0.03% <C <0.4%;

 Nitrogen: 0.001% <N <0.008%;

 Optionally, one or more elements may be added, such as:

 Sulfur: S ≤ 0.05%; and

 Phosphorus: P <0.05%;

 The remainder of the composition of the treated material consists of iron and inevitable impurities resulting from the preparation.

 Cold rolled annealed steel sheet with TWIP effect is characterized by the average grain size of the steel being between 2 to 10 micrometers, having a mechanical strength of over 700 MPa, having a total elongation of over 70% and a achieve a 70 to 90% reduction in the thickness applied for hot rolling.

 Example 1)

As a non-limiting example of the treated material is the procedure for preparing the steel starting with the casting. Melting and casting were performed at 1558 ° C and 1510 ° C, respectively. At the end of the leak, specimens were removed for final chemical analysis. The result obtained is as follows.

Mn - 25%

Silicon - 2.0%

Al - 3.0%

Ni - 1.0%

Carbon - 0.06%

Nitrogen - 0.0050%

 The remainder of the composition of the treated matter consists of iron, as well as inevitable impurities, inherent and resulting from elaboration.

 The alloy was then cast in sheet metal, following the following dimensions: Length - 300 mm, Width - 200 mm and Thickness - 30 mm.

 The plates were then homogenized at 1,100 ° C for 3 hours, which allows the alloying elements to redistribute in the microstructure and it becomes evenly austenitic. This homogenization was proven from the x-ray diffraction analysis that indicated a completely austenitic structure, as shown in Figure 1. This is compatible with the microstructure observed in the optical microscope, as shown in Figure 2.

 From the cast plate a 30mm x 70mm x 100mm sample is taken for hot rolling with 40-60% reduction in thickness at temperature between 1000 and 1200 ° C and in four rolling passes.

 An image taken by light microscopy after the hot rolling process shows a structure with a smaller average grain size than the pre-laminated material, indicating the occurrence of static recrystallization of the material, as shown in Figure 3.

The hot-rolled strip is machined to the thickness where no further traces of cold rolling scale are found. From then on, a sequence of nine passes is promoted so that a thickness reduction of 40-60% occurs. The result obtained is a plate of approximately 4 mm. To obtain a recrystallized microstructure, annealing is used in this cold-rolled strip, as follows:

 The cold rolled material is cut into several samples perpendicular to the rolling direction for recrystallization heat treatment at different temperatures from 400 ° C to 1100 ° C. The total time set for recrystallization was 300 seconds for each temperature, with 150 seconds being the soak time.

 The metallographic analysis of the polished section is also done at the perpendicular section to the lamination section.

 The micrographs of Figure 2 show the recovery of the material with subsequent disappearance of most of the masking systems that characterize the hardening of this alloy. However, for these temperature ranges there was no formation of new grains, therefore there was no recrystallization.

 For annealing temperatures from 700 ° C onwards, the formation of new grains in the material is observed, and the recrystallized fraction increased with increasing treatment temperature, until complete recrystallization at a temperature around 850 ° C. . The new grains have no preferential orientation according to the micrographs of figure 2. At treatment temperatures above 850 ° C there was also complete recrystallization followed by grain growth, as shown in Figure 4.

 Vickers microhardness tests were performed for all treatment temperatures in an annealing time of 300 seconds. These tests show that the microhardness of the annealed sample at 900 ° C was about 40% of the hardness of the hardened sample.

The microstructures of the annealed samples show a large concentration of deformation softwares arranged in two systems and with competing directions. The presence of these softeners after hardening characterizes the TWIP effect. To monitor sample temperature variation during annealing, an interface was connected to the samples. The thermal profiles obtained indicate a soaking time of 150 seconds.

 The percentage of recrystallized grain along the annealing temperatures was measured by the linear intercept method. With the values of the recrystallized fraction, it is observed that it grows with the growth of the annealing temperature.

 The mechanical characterization of the material was obtained through tensile tests. Engineering stress versus strain graphs for the 850 ° C sample showed elongation greater than 50% and a strength limit above 600 MPa, as shown in Figures 5 and 6.

 With the values of the hardening coefficient, n, of the samples, it can be concluded that with increasing annealing temperature, the hardening coefficient increases. The maximum value of the coefficient found n = 0.5 represents the uniqueness of this material.

 Very fine grain steels (in the range of 1-3 micrometers in diameter) can be produced by mechanical forming processes involving severe mechanical deformation of the alloy, such as extrusion in equal channels (ECAP) and angular or collamination. .

Claims

 1) Cold-rolled annealed steel sheet with twip effect, characterized in that it comprises the following chemical composition expressed by weight:

 - manganese: 20% ≤ Mn <30%;

 - silicon: 1% <Si <3%;

 - aluminum: 2% ≤ Al <6%;

 - nickel: 1% <Ni <3%;

 - carbon: 0.03% <C <0.4%;

 - nitrogen: 0,001% <N <0,008%;

 and optionally at least one added element selected from the group consisting of:

 - sulfur: S <0.05%;

 - phosphorus: P <0.05%;

 the remainder of the composition consisting of iron and impurities resulting from the preparation.

 Cold-rolled annealed twip steel sheet according to Claim 1, characterized in that the average grain size is less than 10 micrometers.

A cold-annealed, twip steel sheet according to claim 1, characterized in that it comprises a mechanical strength of greater than 600 MPa.

 A cold-rolled annealed steel sheet according to claim 1, characterized in that it comprises a total elongation of more than 70%.

 A cold-rolled annealed steel sheet according to claim 1, characterized in that it achieves a 70 to 90% reduction in the thickness applied for hot rolling.

 6) Process for obtaining cold rolled annealed steel sheet with twip effect, characterized in that it comprises the following steps:

- melting of steel plates between 1550 ° C and 1650 ° C; - casting in molds with a temperature between 1500 ° C and 1550 ° C;

 reheating for hot rolling at a temperature between 1100 ° C and 1300 ° C for refining the structure and obtaining the hot rolled strip;

 - chemical pickling to remove surface oxides;

 - cold rolling in several passes with 50 to 90% thickness reduction to obtain cold rolled strip;

 - Annealing the cold rolled strip at 700 ° C to 1100 ° C for a minimum of 2 minutes and a maximum of 60 minutes.