High-strength and impact-toughness steel material and method for manufacturing the same

Abstract

This invention provides a steel material with excellent strength and impact toughness, and a method for manufacturing it, even without the heat treatment process. [Solution] The composition is as follows (by weight%): C: 0.12-0.18%, Si: 0.2-0.5%, Mn: 1.0-1.7%, P: 0.012% or less, S: 0.003% or less, Al: 0.015-0.045%, Nb: 0.02-0.05%, V: 0.01-0.08%, Ti: 0.005-0.017%, N: 0.002-0.01%, with the remainder being Fe and unavoidable impurities. The carbon equivalent (Ceq) in the following [Relationship Formula 1] is 0.48 or less. The microstructure is characterized by containing 60-85% ferrite and the remainder being pearlite in terms of area fraction, and containing one or more precipitates of NbC and VC within the microstructure, with the size of the precipitates being 50 nm or less. [Relationship 1] Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15 (The element symbols represent the content (by weight) of each component.)

Claims

1. In weight percent, the composition is as follows: C: 0.12-0.18%, Si: 0.2-0.5%, Mn: 1.0-1.7%, P: 0.012% or less, S: 0.003% or less, Al: 0.015-0.045%, Nb: 0.02-0.05%, V: 0.01-0.08%, Ti: 0.005-0.017%, N: 0.002-0.01%, with the remainder being Fe and unavoidable impurities. The carbon equivalent (Ceq) in the following [Relationship Formula 1] is 0.48 or less, The microstructure consists of 60-85% ferrite by area fraction, with the remainder being pearlite. The microstructure contains one or more precipitates of NbC and VC, A steel material exhibiting high strength and impact toughness, characterized in that the size of the precipitates is 50 nm or less. [Relationship 1] Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15 (In the above relational equation 1, C, Mn, Cr, Mo, V, Cu, and Ni are the content (weight %) values ​​of each component.)

2. The steel material is characterized by further comprising one or more of Cu: 0.5% or less and Ni: 0.5% or less, as described in claim 1, which is a steel material with excellent high strength and impact toughness.

3. The steel material with high strength and impact toughness according to claim 1, characterized in that the ferrite crystal grain size is 30 μm or less.

4. The steel material according to claim 1, characterized in that one or more of the aforementioned NbC and VC precipitates are present within the ferrite crystal grains.

5. The steel material is characterized in that, at a point t / 4 in the thickness direction (where t means the thickness of the steel material (mm)), the yield strength evaluated perpendicular to the rolling direction is 370 MPa or more, the tensile strength is 520 MPa or more, and the Charpy impact absorption energy (CVN, -20°C) value at -20°C is an average of 40 J or more, making it a high-strength and impact-tough steel material as described in claim 1.

6. In weight percent, the composition is as follows: C: 0.12-0.18%, Si: 0.2-0.5%, Mn: 1.0-1.7%, P: 0.012% or less, S: 0.003% or less, Al: 0.015-0.045%, Nb: 0.02-0.05%, V: 0.01-0.08%, Ti: 0.005-0.017%, N: 0.002-0.01%, with the remainder being Fe and unavoidable impurities. The process involves heating a steel slab with a carbon equivalent (Ceq) of 0.48 or less, as shown in [Relationship Formula 1] below, under the conditions of [Relationship Formula 2] below, A method for producing a steel material with high strength and impact toughness, characterized by comprising the steps of: roughly rolling the heated steel slab at a temperature range of 900 to 1100°C, and then finishing hot rolling it to Ar3 or higher. [Relationship 1] Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15 (In the above relational equation 1, C, Mn, Cr, Mo, V, Cu, and Ni are the content (weight %) values ​​of each component.) [Relationship Equation 2] Slab extraction temperature (°C) > 10300 / {4.09 - log([Nb][C] ０．２４ [N] ０．６５ )}-273 (In the above relational equation 2, [Nb], [C], and [N] each represent the content (by weight) of the alloy composition.)

7. The method for producing a steel material with excellent high strength and impact toughness according to claim 6, characterized in that the steel slab further contains one or more of Cu: 0.5% or less and Ni: 0.5% or less.

8. The method for producing a steel material with high strength and impact toughness according to claim 6, characterized in that the slab extraction temperature is 1200°C or lower.

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