BR-112021001434-B1 - High manganese steel and its production method.

BR112021001434B1BR 112021001434 B1BR112021001434 B1BR 112021001434B1BR-112021001434-B1

Abstract

The present invention relates to a high manganese (Mn) steel possessing excellent low-temperature toughness and surface characteristics. The high Mn steel includes, in % by mass: C: 0.100% to 0.700%; Si: 0.05% to 1.00%; Mn: 20.0% to 35.0%; P: 0.030% or less; S: 0.0070% or less; Al: 0.010% to 0.070%; Cr: 0.50% to 5.00%; N: 0.0050% to 0.0500%; O: 0.0050% or less; Ti: 0.005% or less; and Nb: 0.005% or less. with the balance being Fe and the inevitable impurities, and has a microstructure with austenite as the matrix, in which the concentration of Mn in a concentrated portion of Mn in the microstructure is 38.0% or less, or the average value of the Kernel Mean Misorientation (KAM) is 0.3 or more; the upper yield strength is 400 MPa or more; the energy absorption vE-196 from a Charpy impact test at -196°C is 100 J or more; and the percentage of brittle fracture is less than 10%.

Inventors

DAICHI IZUMI
Shigeki Kitsuya
Keiji Ueda
Koichi Nakashima

Assignees

JFE STEEL CORPORATION

Dates

Publication Date: 20260317
Application Date: 20190731
Priority Date: 20180803

Claims (4)

1. High manganese (Mn) steel, characterized in that it has a chemical composition consisting of, in % by mass: C: 0.100% or more and 0.700% or less, Si: 0.05% or more and 1.00% or less, Mn: 20.0% or more and 35.0% or less, P: 0.030% or less, S: 0.0070% or less, Al: 0.010% or more and 0.070% or less, Cr: 0.50% or more and 5.00% or less, N: 0.0050% or more and 0.0500% or less, O: 0.0050% or less, Ti: 0.005% or less, Nb: 0.005% or less, and optionally one or more selected from: Cu: 0.01% or more and 0.50% or less, Mo: 2.00% or less, V: 2.00% or less, eW: 2.00% or less, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, eREM: 0.0010% or more and 0.0200% or less, with a balance consisting of Fe and unavoidable impurities; and a microstructure possessing austenite as a matrix with the austenite phase being 90% or more by area proportion, wherein, in the microstructure, a concentration of Mn of a concentrated portion of Mn is 38.0% or less, and an average Kernel Mean Misorientation (KAM) value is 0.3 or more and 1.3 or less, wherein the KAM value is obtained as follows: at each of the 1/4 and 1/2 depth positions in the thickness direction from the surface of the steel sheet after hot rolling, electron backscatter diffraction (EBSD) analysis is performed for any two observation fields of 500 μm x 200 μm; Based on the analysis results, the average value of the misorientation (orientation differences) between each pixel and its adjacent pixels within a crystal grain is calculated as the average KAM value; yield strength is 400 MPa or more according to JIS Z 2241 (1998); tensile strength is 800 MPa or more according to JIS Z 2241 (1998); absorbed energy vE-196 in a Charpy impact test at -196 °C is 100 J or more according to JIS Z 2242 (1998), measured with V-notch Charpy specimens collected at 1/4 the thickness or 1/2 the thickness of the surface of each steel plate, or measured with undersized V-notch Charpy specimens depending on the steel plate thickness; and brittle fracture percentage is less than 10% according to JIS Z 2242 (1998), in which high Mn content steel has a surface roughness Ra of 200 μm or less.
2. A method for producing a high-Mn steel, as defined in claim 1, characterized in that it comprises: heating a steel feedstock having the chemical composition as defined in claim 1 to a temperature range of 1100 °C or more and 1300 °C or less; subsequently subjecting the steel feedstock to a first hot rolling with a finishing rolling temperature of 1100 °C or more and a total rolling reduction of 20% or more; subsequently subjecting it to a second hot rolling with a finishing rolling temperature of 700 °C or more and less than 950 °C and a total rolling reduction of 50% or more, and performing a descaling treatment on the second hot rolling; Perform the cooling treatment after the final hot rolling, at an average cooling rate of 1.0 °C/s or higher in a temperature range of, or greater than, 100 °C below the rolling finish temperature down to a temperature of 300 °C or more and 650 °C or less.
3. A method for producing a high-Mn steel, as defined in claim 1, characterized in that it comprises: heating a steel feedstock having the chemical composition as defined in claim 1 to a temperature range of 1100 °C or more and 1300 °C or less; then subjecting the steel feedstock to a first hot rolling with a finishing temperature of 800 °C or more and less than 1100 °C and a total reduction in rolling of 20% or more; then reheating to 1100 °C or more and 1300 °C or less; then subjecting it to a second hot rolling with a finishing temperature of 700 °C or more and less than 950 °C and a total reduction in rolling of 50% or more, and performing a descaling treatment on the second hot rolling; Perform the cooling treatment after the final hot rolling, at an average cooling rate of 1.0 °C/s or higher in a temperature range of, or greater than, 100 °C below the rolling finish temperature down to a temperature of 300 °C or more and 650 °C or less.
4. Method for producing a high-Mn content steel according to claim 2 or 3, characterized in that the descaling treatment is carried out in the first hot rolling.

Description

Technical field [001] The present invention relates to a high manganese (Mn) content steel possessing excellent toughness, particularly at low temperatures, and suitable for structural steel used in very low temperature environments such as liquefied gas storage tanks, and to a method for producing the same. Background of the technique [002] Operating environments for structures such as liquefied gas storage tanks reach very low temperatures, and therefore the hot-rolled steel plates used for such structures need to have excellent low-temperature toughness as well as excellent strength. For example, a hot-rolled steel plate used for liquefied natural gas storage needs to have excellent toughness in a temperature range lower than -164°C, which is the boiling point of liquefied natural gas. If the low-temperature toughness of the steel plate used for the very low-temperature storage structure is insufficient, the safety of the very low-temperature storage structure is liable to be undermined. There is, therefore, a strong need to improve the low-temperature toughness of the steel plate used. [003] In response to this need, an austenitic stainless steel is conventionally used, having, as the microstructure of the steel sheet, austenite, which is not embrittled at very low temperatures, 9% Ni steel, and 5000 series aluminum alloys. However, due to the high cost of alloys or the production costs of these materials, there is a demand for a steel material that is not expensive and has excellent low-temperature toughness. [004] A structure such as a liquefied gas storage tank needs to be coated to prevent rust and corrosion on the steel sheet. It is important to achieve an aesthetic appearance after coating, for environmental harmony. Therefore, the hot-rolled steel sheet used for liquefied gas storage still needs to have excellent surface characteristics of the steel sheet that is the basis of the coating. That is, the surface roughness of the steel sheet needs to be low. [005] In view of this, for example, JP 2017-507249 A (Patent Literature 1) proposes the use of, as a new steel material to replace conventional steels for use in very low temperatures, a high-Mn steel containing a large amount of Mn, which is a relatively inexpensive element and austenite stabilizer, for structural steel in very low temperature environments. The technique proposed in Patent Literature 1 involves controlling the stacking failure energy to achieve excellent low-temperature toughness without surface irregularities. List of citations Patent Literature [006] PTL 1: JP 2017-507249 A Summary Technical problem [007] With the technique described in Patent Literature 1, a high-Mn steel with excellent surface quality can be supplied without surface irregularities after work such as tensile work. However, Patent Literature 1 does not address the surface roughness of the hot-rolled steel sheet produced. The hot-rolled steel sheet produced is generally shipped after its surface has been made uniform by abrasive blasting treatment. In the case where the surface of the steel sheet after abrasive blasting treatment is rough, local rust occurs. To avoid this, the surface characteristics need to be adjusted by a grinder or similar. This causes a decrease in productivity. [008] It should therefore be useful to provide a high Mn content steel that has excellent low-temperature toughness and excellent surface characteristics. It should also be useful to provide an advantageous method of producing high-Mn content steel. Here, "excellent low-temperature toughness" means that the absorbed energy vE-196 in the Charpy impact test at -196°C is 100 J or more and the percentage of brittle fracture is less than 10%, and "excellent surface characteristics" means that the surface roughness Ra after typical abrasive blasting treatment is 200 μm or less. Solution to the problem [009] Intensive studies were conducted on various factors that determine the chemical composition and microstructure of a steel sheet for high-Mn steel, and the following were found: a. If a concentrated Mn portion with a Mn concentration of more than 38.0% by mass forms a high-Mn austenitic steel, the percentage of brittle fracture reaches 10% or more at low temperatures, and the low-temperature toughness decreases. Consequently, an effective way to improve the low-temperature toughness of high-Mn steel is to limit the Mn concentration of the concentrated Mn portion to 38.0% by mass or less. b. If a high-Mn austenitic steel contains Cr in an amount greater than 5.00% by mass, descaling during hot rolling is insufficient. This causes the hot-rolled steel sheet after abrasive blasting treatment to have a rough surface Ra of more than 200 μm. Therefore, the Cr content needs to be 5.00% by mass or less to improve the surface characteristics of high-Mn steel. c. By performing hot rolling and descaling under suitable conditions, the previous items a and b can be achieved without an increase in p