EP4640902A1

EP4640902A1 - Steel plate and method for manufacturing same

Info

Publication number: EP4640902A1
Application number: EP23907582.3A
Authority: EP
Inventors: Jun-Hak PARK; Han-Hwi KIM; Yun-Ik KWON; Jong-Won Park
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2022-12-19
Filing date: 2023-12-14
Publication date: 2025-10-29
Also published as: CN120202320A; JP2025534517A; WO2024136296A1; KR20240096056A; EP4640902A4

Abstract

The present invention relates to a steel sheet and a method for manufacturing same and, more specifically, to a high-strength steel sheet having excellent corrosion resistance and a method for manufacturing same.

Description

Technical Field

The present disclosure relates to a steel plate and a method for manufacturing the same, and more particularly, to a high-strength steel plate having excellent corrosion resistance and a method for manufacturing the same.

Background Art

A steel material used for a port structure or a pier of offshore bridge may need to use a material having excellent corrosion resistance in a seawater environment to ensure durability.
Generally, in order to increase seawater corrosion resistance, a method of increasing Cr and Cu contents in steel may be used. A steel type having high Cr and Cu contents may form a Cr oxide film and a Cu concentrated layer on a surface of a steel material, and both methods may have the effect of suppressing corrosion of the steel material in a seawater environment.
However, since the element is more expensive than general alloy elements, there may be a disadvantage of causing an increase in manufacturing cost of a steel material.
A high-strength steel material may need to be used to increase construction stability of a structure. In order to increase strength of a steel material, it may be effective to increase a C or Mn content in steel. This may be because a fraction of a carbon-concentrated region having high hardness may be increased in steel having a composite structure of a structure formed in ferrite and a carbon-concentrated region. Here, the carbon-concentrated region may be pearlite or bainite.
However, since corrosion usually occurs at a boundary between regions having different hardnesses, as described above, the method of increasing the fraction of pearlite or bainite may have the problem of increasing corrosion occurrence sites and inhibiting corrosion resistance.
Accordingly, in order to manufacture an economical and high-strength seawater-resistant steel material, a method of securing high strength while minimizing the use of Cu and Cr and reducing the fraction of pearlite or bainite.

Detailed description of present disclosure

Technical problems to solve

An aspect of the present disclosure is to provide a steel plate and a method for manufacturing the same.
An aspect of the present disclosure is to provide a high-strength steel plate having excellent corrosion resistance and a method for manufacturing the same.
The purpose of the present invention is not limited to the above-described features. Those skilled in the art will have no difficulty in understanding additional purpose of the present invention from the overall description of this specification.

Solution to Problem

An aspect of the present disclosure provides a steel plate including, by weight%, C: 0.030-0.070%, Si: 0.50-1.30%, Mn: 0.30-0.70%, Cr: 0.50-1.50%, Al: 0.05% or less, Cu: 0.25-0.50%, Ni: 0.05-0.50%, S: 0.0100% or less, Ti: 0.020-0.050%, Nb: 0.050-0.090%, and a balance of Fe and inevitable impurities,

wherein an R value defined in relational expression 1 as below is 0.75 or more,
wherein a microstructure at 1/4 point in a thickness direction from a surface includes 85% or more of ferrite and residual structure by area%,
wherein an average grain size of ferrite is 30 µm or less, and
wherein a solid-solution Cr content is 90% or more of a total Cr content:

$R = [Si] + ([Ni] / [Cu])$
where [Si], [Ni] and [Cu] are weight% of each element.

The residual structure includes one or more of pearlite and bainite.
A Q value of the steel plate, defined in relational expression 2 below, is 0.3-8.5:

$Q = ([Ti] + [Nb]) / [Cr])$
where [Ti], [Nb] and [Cr] are weight% of each element present in a form of particles such as a precipitate or an inclusion in steel.
The steel plate has tensile strength of 600 MPa or more and impact toughness of 100 J or more at -5°C.
Cr in an interfacial surface of a corrosion product of the steel plate, generated after a corrosion test in accordance with KS D ISO 14993, is 0.50% or more by weight%.
When conducting a corrosion test in accordance with KS D ISO 14993, a relative corrosion rate of the steel plate as compared to KS-SS275 is 60% or less.
An aspect of the present disclosure provides a method of manufacturing a steel plate including reheating a steel slab including, by weight%, C: 0.030-0.070%, Si: 0.50-1.30%, Mn: 0.30-0.70%, Cr: 0.50-1.50%, Al: 0.05% or less, Cu: 0.25-0.50%, Ni: 0.05-0.50%, S: 0.0100% or less, Ti: 0.020-0.050%, Nb: 0.050-0.090%, and a balance of Fe and inevitable impurities, and having R value of 0.75 or more, the R value defined in relational expression 1 as below;

hot-rolling the reheated steel slab at a finishing rolling temperature of 750-900°C;
primary cooling the hot-rolled steel plate to a temperature range of 500-650°C at a cooling rate of 5-20°C/s and winding the steel plate; and
secondary cooling the wound steel plate to 400°C at a cooling rate of 0.40°C/min or more:

$R = [Si] + ([Ni] / [Cu])$
where [Si], [Ni] and [Cu] are weight% of each element.

The reheating step is performed in a temperature range of 1100-1300°C.

Advantageous Effects of Invention

According to an aspect of the present disclosure, a steel plate and a method for manufacturing the same may be provided.
According to an aspect of the present disclosure, a high-strength steel plate having excellent corrosion resistance and a method for manufacturing the same may be provided.
According to an aspect of the present disclosure, a high-strength steel plate having excellent corrosion resistance in an environment in which the steel plate is in contact with seawater, such as a port structure, an estuary embankment sluice gate, or the like, and a method for manufacturing the same may be provided.

Brief Description of Drawings

FIG. 1 is an image indicating results of measuring a Cr content at an interfacial surface of a corrosion product of inventive example 17.

Best Mode for Invention

Hereinafter, preferable embodiments of the present disclosure will be described with reference to the accompanying drawings. However, the embodiment of the present disclosure may be modified with various other forms, and the scope of the present disclosure is not limited to the embodiment described below. Also, embodiments of the present disclosure are provided to more fully describe the present disclosure to a person having average knowledge in the relevant technique field.
Hereinafter, the present disclosure may be described in detail.
In the description below, a steel composition of the present disclosure may be described in detail.
Unless otherwise indicated in the present disclosure, % indicating a content of each element may be based on weight.
A steel plate according to an embodiment of the present disclosure may include, by weight %, C: 0.030-0.070%, Si: 0.50-1.30%, Mn: 0.30-0.70%, Cr: 0.50-1.50%, Al: 0.05% or less, Cu: 0.25-0.50%, Ni: 0.05-0.50%, S: 0.0100% or less, Ti: 0.020-0.050%, Nb: 0.050-0.090%, a balance of Fe and inevitable impurities.

Carbon (C): 0.03 Carbon (C): 0.030-0.070%

Carbon (C) may be the most economical and effective element for strengthening steel, and may be added at 0.030% or more in the present disclosure. When the carbon (C) content is less than 0.030%, other alloying elements may need to be added to ensure sufficient strength, which may not be economical. In an embodiment of the present disclosure, 0.035% or more of carbon (C) may be included. When the content exceeds 0.070%, pearlite or bainite fraction in the structure may excessive increase such that corrosion resistance may decreases, and impact toughness may also decrease. In an embodiment of the present disclosure, an upper limit of carbon (C) content may be 0.065%.

Silicon (Si): 0.50-1.30%

Silicon (Si) may be generally added to deoxidize molten steel, and may also be effective in solid-solution strengthening. Also, silicon (Si) may form a protective film (H₄SiO₄) on the surface and may reduce the fraction of the carbon-concentrated region ultimately differentiated into pearlite or bainite, thereby improving corrosion resistance. When the content of silicon (Si) is less than 0.50%, the above-described effect and corrosion resistance improvement effect may be insufficient. In an embodiment of the present disclosure, a lower limit of silicon (Si) may be 0.55%. When the content exceeds 1.30%, weldability may be reduced, which may be problematic. In an embodiment of the present disclosure, an upper limit may be 1.25%.

Manganese (Mn): 0.30-0.70%

Manganese (Mn) may be effective in solid-solution strengthening of steel, but manganese (Mn) may cause embrittlement of steel and may reduce corrosion resistance by forming MnS. When the content of manganese (Mn) is less than 0.30%, the effect of strengthening steel may be insignificant. In an embodiment of the present disclosure, a lower limit may be 0.35%. When the content exceeds 0.70%, a large amount of MnS may be formed and the pearlite or bainite fraction in the structure may be excessively increased, which may cause a decrease in corrosion resistance. In an embodiment of the present disclosure, an upper limit may be 0.65%.

Chromium (Cr): 0.50-1.50%

Chromium (Cr) may be a basic alloying element improving seawater resistance. In the present disclosure, it was found that the seawater corrosion resistance was improved by action of chromium (Cr) being concentrated on an interfacial surface of the corrosion product and stabilizing the corrosion layer. When the chromium (Cr) content is lower than 0.50%, the above-mentioned effect may be insufficient. In an embodiment of the present disclosure, a lower limit may be 0.55%. When the content exceeds 1.50%, the seawater corrosion resistance effect may tend to converge. In an embodiment of the present disclosure, an upper limit may be 1.45%.

Aluminum (Al): 0.05% or less

Aluminum (Al) may be added to deoxidize molten steel and may improve corrosion resistance. However, when aluminum (Al) is added excessively, a great deal of oxide inclusions may be formed in steel, such that embrittlement of steel may deteriorate. Accordingly, in the present disclosure, the content of aluminum (Al) may be limited to 0.05% or less. In an embodiment of the present disclosure, an upper limit may be 0.045%. In an embodiment of the present disclosure, aluminum (Al) may be included in an amount of 0.001% or more.

Copper (Cu): 0.25-0.50%

Copper (Cu) may improve seawater resistance. In the present disclosure, copper (Cu) may be added to improve corrosion resistance, but when copper (Cu) is added excessively, economic efficiency may degrade and surface defects known as Cu shortness may be caused, and thus, the content of copper (Cu) may be limited to 0.50% or less in the present disclosure. In an embodiment of the present disclosure, the content may be 0.45% or less. When the content is less than 0.25%, the corrosion resistance improvement effect may be insufficient. In an embodiment of the present disclosure, a lower limit may be 0.27%.

Nickel (Ni): 0.05-0.50%

Nickel (Ni) may be essentially added to prevent surface defects known as Cu hot-shortness in general Cu-added steel. Nickel (Ni) may also be effective for improving seawater resistance together with Cu. However, since nickel (Ni) is an expensive element, excessive addition may not be preferable in terms of economic efficiency. When the content of nickel (Ni) is less than 0.05%, the effect of suppressing Cu shortness and improving corrosion resistance may be insufficient, whereas when the content exceeds 0.50%, the above effects may tend to converge. In an embodiment of the present disclosure, an upper limit may be 0.45%. In an embodiment of the present disclosure, a lower limit of nickel (Ni) may be 0.07%.

Sulfur (S): 0.0100% or less

Sulfur (S) may be detrimental to impact toughness and corrosion resistance. Sulfur (S) may be combined with manganese (Mn) in steel and may form non-metallic inclusions, MnS, and since the inclusion may act as a corrosion initiation site, it may be preferable to reduce the content as much as possible. Accordingly, the content of sulfur (S) may be limited to 0.0100% or less in the present disclosure. Meanwhile, 0% may be excluded in consideration of the case in which sulfur (S) is unavoidably included during the manufacturing process.

Titanium (Ti): 0.020-0.050%

Titanium (Ti) may be precipitated as TiC, or the like, in steel and may strengthen the steel. Meanwhile, when TiC is precipitated, C forming Cr carbide may be consumed, which ultimately increases solid-solution Cr. When the content of titanium (Ti) is less than 0.020%, the above effect may not be significant. In an embodiment of the present disclosure, a lower limit may be 0.025%. When the content exceeds 0.050%, it may form coarse carbonitride, which may reduce impact strength. In an embodiment of the present disclosure, an upper limit may be 0.045%.

Niobium (Nb): 0.050-0.090%

Niobium (Nb) may have a precipitation strengthening effect similarly to Ti, and may also induce non-recrystallization rolling such that a grain size may become fine, and consequently, niobium (Nb) may increase strength and impact toughness. When NbC is precipitated, C forming Cr carbide may be consumed, and consequently, solid-solution Cr may be increased. When the niobium (Nb) content is less than 0.050%, the above effect may not be significant. In an embodiment of the present disclosure, a lower limit may be 0.045%. When the content exceeds 0.090%, the above effect may tend to converge. In an embodiment of the present disclosure, niobium (Nb) may be included by 0.085% or less.
A remainder of the present disclosure is iron (Fe). However, in a general manufacturing process, inevitable impurities may be inevitably added from raw materials or an ambient environment, and thus, impurities may not be excluded. A person skilled in the art of a general manufacturing process may be aware of the impurities, and thus, the descriptions of the impurities may not be provided in the present disclosure.
As for the steel plate according to an embodiment of the present disclosure, an R value defined in relational expression 1 below may be 0.75 or more. $R = [Si] + ([Ni] / [Cu])$
(In the formula, [Si], [Ni] and [Cu] may be the weight% of each element)
The inventors of the present invention studied to simultaneously ensure mechanical properties and corrosion resistance, and confirmed that corrosion resistance may be deteriorated depending on a relationship between Si, Ni, and Cu content. When the R value defined in the relational expression 1 is less than 0.75, surface quality may be degraded, and in particular, surface deterioration due to Cu-hot shortness may cause a decrease in impact strength due to a phenomenon in which cracks of tens to hundreds of µm are left on the surface. An upper limit of the R value may not need to be specifically limited, but considering costs of alloy addition, the upper limit of the R value may be 3.00 in the present disclosure.
Hereinafter, a steel microstructure of the present disclosure will be described in detail.
Unless otherwise indicated in the present disclosure, % indicating the fraction of the microstructure may be based on an area.
In an embodiment of the present disclosure, a microstructure at 1/4 point in the thickness direction from the surface of the steel plate may include 85% or more of ferrite and residual structure by area%, and the residual structure may include one or more of pearlite and bainite.
In the present disclosure, the microstructure fraction may be observed using an optical microscope at 1/4 point in the thickness direction from the surface of the steel plate.
Ferrite may ensure corrosion resistance and impact toughness, and in the present disclosure, ferrite may include 85% or more. When the ferrite fraction is less than 85%, the desired level of corrosion resistance or impact toughness may not be ensured.
Pearlite and bainite may affect strength and impact value of steel, and may be factors affecting corrosion resistance in seawater in terms of uniformity of the structure in the surface layer. That is, as the pearlite and bainite fractions increase, strength may increase, and the impact value may decrease and seawater corrosion resistance may also decrease. When the area fraction of one or more of pearlite and bainite is 15% or less, target strength and impact value may be ensured, and also the pearlite fraction of the surface layer may be appropriately controlled such that corrosion resistance may be ensured. When the area fraction exceeds 15%, the above-mentioned effect may not be ensured, which may be problematic.
In an embodiment of the present disclosure, the average grain size of ferrite may be 30 µm or less.
In the present disclosure, the average grain size may be represented as an average value of five points by selecting five points arbitrarily, and measuring the ferrite grain size using a circular cross-section line method described in KS D 0205. Also, the average grain size of ferrite in the present disclosure may be measured at 1/4 points in the thickness direction from the surface, similarly to the microstructure fraction
Fine ferrite grains may be advantageous in ensuring strength and impact value of the steel material. In the present disclosure, a study was conducted on the assumption that the ferrite grain boundary may act as a passage for Cr to move to the corrosion product, and it was confirmed that, when the average grain size of ferrite at 1/4 in the thickness direction of the steel plate was 30 µm or less, the target strength and impact value was ensured, and also the ferrite grain size in the surface layer may be appropriately controlled, such that corrosion resistance was ensured. Meanwhile, the above-described effect may not be ensured when the average grain size of ferrite exceeds 30 µm. In an embodiment of the present disclosure, the average grain size of ferrite may be 8 µm or more.
As for the steel plate according to an embodiment of the present disclosure, a Q value defined in relational expression 2 as below may be 0.3-8.5. $Q = ([Ti] + [Nb]) / [Cr])$
(In the formula, [Ti], [Nb] and [Cr] may be weight% of each element present in the form of particles such as precipitates or inclusions in the steel)
When the Q value defined in relational expression 2 is less than 0.3, the amount of Cr precipitation may increase, and the solid-solution Cr content may decrease such that corrosion resistance may not be ensured. When the value exceeds 8.5, the Ti and Nb precipitates may become coarse and may act as corrosion sites, which may hinder corrosion resistance or may act as crack initiation points, such that impact strength may be deteriorated.
In the present disclosure, a method of extract precipitates and inclusions present in the form of particles in steel using an electrolytic extraction method, measuring the Ti, Nb, and Cr contents of the corresponding particles using induced-fault plasma optical emission spectroscopy was used.
As for the steel plate according to an embodiment of the present disclosure, a solid-solution Cr content may be 90% or more of the total Cr content.
In an embodiment of the present disclosure, the solid-solution Cr content may indicate the value obtained by subtracting the solid-solution Cr content from the total Cr content of the steel by measuring the content of Cr present in the form of particles such as precipitates or inclusions in steel. More specifically, in the present disclosure, a method of extracting precipitates and inclusions in the form of particles in steel using electrolytic extraction and measuring the Cr content of the particles using induced defect plasma optical emission spectroscopy was used. The solid-solution Cr content was calculated as the value obtained by subtracting the Cr content present in the form of particles from the steel Cr content.
In the present disclosure, it was confirmed that the seawater corrosion resistance was determined depending on the Cr content of the interfacial surface of the corrosion product, and that the seawater corrosion resistance was excellent when the Cr content of the interfacial surface of the corrosion product was 0.50% or more. Also, it was confirmed that in order for Cr to be concentrated at a specific level or more in the interfacial surface of the corrosion product, Cr may need to be sufficiently solid-solute in the steel material before the corrosion reaction. In the present disclosure, as a result of analyzing a steel material having excellent seawater corrosion resistance, it was confirmed that, when the solid-solution Cr content is 90% or more of the total Cr content, the Cr content of the interfacial surface of the corrosion product was 0.50% or more, and the seawater corrosion resistance was excellent. Accordingly, in the present disclosure, the solid-solution Cr content may be limited to 90% or more of the total Cr content.
In the present disclosure, the interfacial surface of the corrosion product may refer to a region present in the form of an interfacial surface within the corrosion layer formed on the surface of the steel plate after a corrosion test, and the Cr content may be represented as an average value of the measured values through EDS analysis in three positions.
In an embodiment of the present disclosure, tensile strength of the steel plate may be 600 MPa or more, impact toughness may be 100 J or more at -5°C, and after a corrosion test in accordance with KS D ISO 14993, the interfacial surface of the corrosion product Cr content generated on the surface may be 0.50% or more, and when the corrosion test in accordance with KS D ISO 14993 is performed, the relative corrosion rate as compared to KS-SS275 may be 60% or less, such that excellent corrosion resistance characteristics may be ensured.
In the present disclosure, when observing the interfacial surface of the corrosion product after a corrosion test, it was confirmed that the seawater corrosion resistance was improved when the Cr content was 0.50% or more, and the seawater corrosion resistance may be evaluated based on this. In the present disclosure, the interfacial surface of the corrosion product may indicate a region present in the form of an interfacial surface within the corrosion layer formed on the surface of the steel plate after a corrosion test, and the Cr content may be represented as an average value of the measured values through EDS analysis in three positions.
Hereinafter, the method for manufacturing steel of the present disclosure may be described in detail.
The steel plate according to an embodiment of the present disclosure may be manufactured by reheating, hot-rolling, primary cooling, secondary winding and cooling a steel slab satisfying the above-described alloy composition.

Reheating

The steel slab satisfying the alloy composition in the present disclosure may be reheated.
In the present disclosure, the reheating temperature may not be limited to any particular example, but a reheating temperature commonly used in the same technical field may be applied. In an embodiment of the present disclosure, reheating may be performed in a temperature range of 1100-1300°C.

Hot-rolling

The reheated steel slab may be hot-rolled at a finishing rolling temperature of 750-900°C.
The finishing rolling temperature may be a factor affecting the grain size. As the finishing rolling temperature decreases, it may be advantageous for grain refinement, and it may be difficult to ensure the target fine grain at a temperature exceeding 900°C. A temperature less than 750°C may be difficult to be applied to the manufacturing process due to excessive rolling load.

Primary cooling and winding

The hot-rolled steel plate may be primary cooled to a temperature range of 500-650°C at an average cooling rate of 5-20°C/s and may be wound.
In the primary cooling, as the average cooling rate increases and the winding temperature decreases, the grain size may decrease. In the present disclosure, the average cooling rate may be controlled to be 5°C/s or more and the winding temperature to be 650°C or less to ensure the target grain size. When the average cooling rate exceeds 20°C/s or the winding temperature is less than 500°C, it may be difficult to implement the embodiment at a thickness of 10 mm or more, and the material may be locally overcooled, which may not be applicable.

Secondary cooling

The wound steel plate may be secondary cooled to 400°C at an average cooling rate of 0.40°C/min or more.
When the average cooling rate after winding is less than 0.40°C/min, Cr carbide formation may be promoted, such that the amount of solid-solution Cr may decrease. In the present disclosure, it was confirmed that, when the average cooling rate after winding to 400°C was 0.40°C/min or more, formation of Cr carbide was sufficiently suppressed, and the solid-solution Cr became 90% of the total addition amount. Although the maximum cooling rate after winding to 400°C may not be specifically limited, considering reproducibility of the ferrite grain size and the pearlite fraction suggested in the present disclosure, an upper limit may be 0.70°C/min. Mode for Invention
Hereinafter, the present disclosure may be described more specifically through embodiments. However, it should be noted that the embodiments below are merely intended to describe the present disclosure in greater detail based on embodiments, and are not intended to limit the scope of the rights of the present disclosure.

(Embodiment)

Steel slabs having the alloy compositions in Table 1 below were manufactured. Each steel slab was heated at 1250°C, and manufactured under the conditions in Table 2 below, thereby manufacturing a hot-rolled steel plate having a thickness of 16 mm. In this case, during secondary cooling, the cooling end temperature was the same, 400°C or less.

[Table 1]

Sample No.	Alloy composition (weight%)									Relation al expressi on 1
Sample No.	C	Mn	Si	Cr	Ni	Ti	Nb	Cu	S	Relation al expressi on 1
1	0.030	0.30	1.10	0.70	0.06	0.020	0.050	0.25	0.0097	1.34
2	0.070	0.60	0.60	0.80	0.35	0.020	0.070	0.40	0.0037	1.48
3	0.040	0.50	1.20	1.30	0.38	0.020	0.090	0.40	0.0010	2.15
4	0.050	0.70	0.50	0.80	0.13	0.020	0.050	0.30	0.0063	0.93
5	0.050	0.30	0.90	0.90	0.14	0.040	0.080	0.40	0.0074	1.25
6	0.030	0.70	0.90	0.90	0.09	0.040	0.080	0.30	0.0094	1.20
7	0.060	0.30	1.20	0.70	0.27	0.020	0.060	0.40	0.0064	1. 88
8	0.040	0.40	1.10	0.60	0.19	0.050	0.050	0.30	0.0099	1.73
9	0.030	0.50	0.60	1.00	0.15	0.020	0.080	0.30	0.0079	1.10
10	0.030	0.40	0.60	0.60	0.42	0.040	0.090	0.40	0.0088	1.65
11	0.070	0.70	0.80	0.50	0.47	0.020	0.060	0.50	0.0055	1.74
12	0.060	0.40	0.61	1.20	0.07	0.050	0.060	0.50	0.0040	0.75
13	0.030	0.30	0.50	1.30	0.28	0.040	0.070	0.50	0.0062	1.06
14	0.040	0.50	1.20	0.90	0.05	0.030	0.050	0.50	0.0072	1.30
15	0.030	0.40	1.30	0.80	0.30	0.030	0.050	0.50	0.0097	1.90
16	0.040	0.60	0.90	0.60	0.26	0.030	0.050	0.40	0.0034	1.55
17	0.050	0.50	0.50	0.80	0.40	0.040	0.080	0.50	0.0011	1.30
18	0.060	0.40	1.20	0.90	0.42	0.040	0.070	0.25	0.0044	2.88
19	0.040	0.60	0.55	1.50	0.10	0.030	0.090	0.50	0.0017	0.75
20	0.060	0.40	1.00	1.20	0.47	0.030	0.090	0.30	0.0064	2.57
21	0.030	0.70	0.90	1.50	0.13	0.030	0.050	0.50	0.0047	1.16
22	0.040	0.50	0.50	1.50	0.43	0.050	0.080	0.40	0.0080	1.58
23	0.060	0.60	1.30	0.80	0.12	0.040	0.080	0.40	0.0053	1.60
24	0.070	0.70	1.20	1.30	0.08	0.040	0.050	0.50	0.0014	1.36
25	0.050	0.60	0.50	1.00	0.19	0.040	0.050	0.30	0.0028	1.13
26	0.071	0.60	0.60	0.80	0.35	0.020	0.070	0.40	0.0052	1.48
27	0.072	0.50	1.20	1.30	0.38	0.020	0.090	0.40	0.0050	2.15
28	0.060	0.40	0.49	1.20	0.47	0.030	0.090	0.30	0.0048	2.06
29	0.030	0.70	0.48	1.50	0.13	0.030	0.050	0.50	0.0033	0.74
30	0.040	0.71	0.90	0.60	0.26	0.030	0.050	0.40	0.0032	1.55
31	0.050	0.72	0.50	0.80	0.40	0.040	0.080	0.50	0.0092	1.30
32	0.060	0.40	0.60	1.20	0.07	0.050	0.060	0.50	0.0110	0.74
33	0.030	0.30	0.50	1.30	0.28	0.040	0.070	0.50	0.0120	1.06
34	0.040	0.40	1.10	0.49	0.19	0.050	0.050	0.30	0.0014	1.73
35	0.030	0.50	0.60	0.47	0.15	0.020	0.080	0.30	0.0094	1.10
36	0.030	0.40	1.30	0.80	0.30	0.019	0.050	0.50	0.0035	1.90
37	0.040	0.60	0.90	0.60	0.26	0.018	0.050	0.40	0.0054	1.55
38	0.050	0.50	0.50	0.80	0.40	0.051	0.080	0.50	0.0042	1.30
39	0.060	0.40	1.20	0.90	0.42	0.052	0.070	0.25	0.0027	2.88
40	0.030	0.40	0.60	0.60	0.42	0.040	0.049	0.40	0.0013	1.65
41	0.070	0.70	0.80	0.50	0.47	0.020	0.048	0.50	0.0045	1.74
42	0.050	0.50	0.50	0.80	0.04	0.040	0.080	0.50	0.0007	0.58
43	0.060	0.40	1.20	0.90	0.03	0.040	0.070	0.25	0.0034	1.32
44	0.060	0.40	1.00	1.20	0.47	0.030	0.090	0.24	0.0064	2.96
45	0.030	0.70	0.90	1.50	0.13	0.030	0.050	0.23	0.0071	1.47
46	0.030	0.70	0.90	0.90	0.09	0.040	0.080	0.30	0.0020	1. 20
47	0.030	0.40	0.60	0.60	0.42	0.040	0.090	0.40	0.0025	1.65
48	0.070	0.70	0.80	0.50	0.47	0.020	0.060	0.50	0.0044	1.74
49	0.040	0.60	0.50	1.50	0.10	0.030	0.090	0.50	0.0066	0.70
50	0.060	0.40	1.00	1.20	0.47	0.030	0.090	0.30	0.0027	2.57
51	0.050	0.50	0.50	0.80	0.40	0.040	0.080	0.50	0.0011	1.30
52	0.060	0.40	1.20	0.90	0.42	0.040	0.070	0.25	0.0044	2.88

R = [Si] + ([Ni] / [Cu])

(In the formula, [Si], [Ni] and [Cu] may be the weight% of each element.)

[Table 2]

Sample No.	Hot-rolling	Primary cooling and winding		Secondary cooling
Sample No.	Finishing rolling temperature (°C)	Cooling rate (°C/s)	Winding temperature (°C)	Cooling rate (°C/min)
1	883	12	561	0.58
2	795	11	642	0.69
3	779	9	639	0.47
4	824	19	548	0.62
5	783	9	570	0.60
6	887	11	637	0.45
7	841	11	501	0.54
8	890	5	554	0.43
9	801	12	610	0.51
10	767	19	528	0.49
11	879	12	549	0.40
12	892	14	576	0.70
13	896	13	513	0.50
14	897	5	558	0.58
15	799	19	546	0.69
16	833	6	549	0.70
17	900	15	638	0.66
18	860	5	623	0.57
19	834	14	565	0.47
20	869	16	630	0.70
21	892	19	544	0.47
22	795	11	636	0.62
23	837	9	565	0.68
24	802	17	638	0.61
25	789	18	610	0.56
26	844	17	569	0.67
27	841	16	537	0.64
28	899	17	502	0.67
29	882	15	610	0.51
30	783	10	508	0.47
31	775	16	523	0.70
32	837	17	528	0.63
33	832	9	643	0.51
34	819	10	566	0.64
35	821	10	639	0.51
36	804	14	609	0.61
37	766	11	547	0.51
38	871	7	589	0.54
39	895	19	648	0.52
40	787	9	545	0.62
41	870	19	596	0.62
42	760	18	605	0.42
43	891	18	545	0.63
44	834	7	544	0.51
45	899	19	504	0.47
46	901	15	518	0.54
47	839	4	540	0.41
48	834	3	627	0.57
49	789	11	651	0.42
50	875	15	652	0.70
51	900	15	638	0.39
52	860	5	623	0.38

As for each manufactured steel plate, the ferrite, pearlite and bainite fractions, and the grain size of ferrite were observed and listed. Also, the content of precipitates for Ti, Nb and Cr was measured, and relational expression 2 and the solid-solution Cr content fraction were calculated and listed in Table 3 below. The tensile strength, impact toughness at -5°C, Cr content in the interfacial surface of the corrosion product after corrosion, corrosion rate, and relative corrosion rate as compared to KS-SS275 were measured and listed.
The microstructure fraction was measured using an optical microscope after nital-etching the sample, and the ferrite grain size was measured at five points using the circular cross-section method described in KS D 0205 and the average value was listed. In this case, the microstructure characteristics were observed at 1/4 points in the thickness direction from the surface of the steel plate
When measuring relational expression 2 and the solid-solution Cr content, the precipitates and inclusions present in the form of particles in the steel were extracted using the electrolytic extraction method, and the Ti, Nb, and Cr contents of the corresponding particles were measured using the induced defect plasma optical emission spectroscopy. The solid-solution Cr content was calculated by subtracting the Cr content present in the form of particles from the Cr content in steel.
The corrosion test was carried out for 120 cycles (960 hours) as specified in the KS D ISO 14993 standard. The interfacial surface of the corrosion product may indicate the region present in the form of the interfacial surface within the corrosion layer formed on the surface of the steel plate after the corrosion test, and the Cr content of the interfacial surface was measured through EDS analysis at the corresponding position. Also, the corrosion products was removed by the KS D ISO 8407 method, and the corrosion rate was measured using the equation as below. Specifically, the conditions of 2 hours of 5% NaCl salt spray at 35°C, 4 hours of drying at 60°C, and 2 hours of wetting at 50°C were repeated for 960 hours.

Corrosion rate (mm/yr) = 87.6 W/DAT

(Here, W is the weight loss after corrosion (mg), D is the specific gravity of the metal (g/cm3), A is the area exposed to the corrosive environment (cm2), and T is the corrosion test time (hour). Here, 960 was used as a fixed value.)

Also, the relative corrosion rate was calculated by dividing the target material corrosion rate by the comparative material corrosion rate and representing the value as a percentage (%). The composition of KS-SS275 used as a comparative material included, by weight%, C: 0.13%, Si: 0.01%, Mn: 0.9%, Al: 0.02%, Cu: 0.01%, Ni: 0.01%, S: 0.005%, Nb: 0.013%, Ti: 0.005%, and a balance of Fe, and the material was manufactured by applying the manufacturing conditions suggested in the present disclosure.

[Table 3]

Sam ple No.	Microstructure					Properties		Corrosion results			Classi ficati on
Sam ple No.	P and B fracti on (area% )	F fracti on (area% )	F avera ge grain size ( µm)	Relat ional expre ssion 2	Solid solut ion Cr conte nt (%)	Tensil e streng th (MPa)	-5°C impact toughn ess (J/cm²)	Cr in inter facia l surfa ce in Corro sion produ ct (wt%)	Corros ion rate (mm/yr )	Relat ive corro sion rate (%)	Classi ficati on
1	13	87	12	0.6	93	604	153	0.70	0.86	57%	Invent ive exampl e 1
2	13	87	11	6.4	92	658	106	0.85	0.82	55%	Invent ive exampl e 2
3	12	88	10	6.3	96	655	134	1. 65	0.67	45%	Invent ive exampl e 3
4	11	89	8	0.8	91	639	114	0.75	0.90	60%	Invent ive exampl e 4
5	8	92	8	0.4	92	652	118	1.05	0.76	51%	Invent ive exampl e 5
6	15	85	21	5.9	93	667	120	0.95	0.82	54%	Invent ive exampl e 6
7	8	92	6	0.3	92	649	131	1. 05	0.73	48%	Invent ive exampl e 7
8	11	89	19	2.5	93	642	122	0.55	0.80	54%	Invent ive exampl e 8
9	15	85	10	0.3	95	623	132	1.10	0.85	57%	Invent ive exampl e 9
10	11	89	6	3.8	92	622	114	0.65	0.82	54%	Invent ive exampl e 10
11	12	88	11	0.5	90	665	106	0.50	0.81	54%	Invent ive exampl e 11
12	12	88	13	4.0	94	660	100	1.30	0.76	51%	Invent ive exampl e 12
13	8	92	7	0.4	96	625	117	1.45	0.78	52%	Invent ive exampl e 13
14	13	87	21	8.5	94	666	125	0.95	0.70	47%	Invent ive exampl e 14
15	9	91	7	3.2	94	653	131	1.25	0.70	47%	Invent ive exampl e 15
16	12	88	12	3.4	91	652	112	0.55	0.78	52%	Invent ive exampl e 16
17	9	91	18	0.3	92	646	105	1.15	0.84	56%	Invent ive exampl e 17
18	12	88	30	0.6	93	654	130	1.35	0.78	52%	Invent ive exampl e 18
19	11	89	9	1.5	97	657	115	1.85	0.74	49%	Invent ive exampl e 19
20	14	86	15	1.9	97	655	128	1. 90	0.75	50%	Invent ive exampl e 20
21	10	90	9	5.6	96	677	113	2.00	0.67	45%	Invent ive exampl e 21
22	11	89	11	2.5	98	659	101	1.75	0.78	52%	Invent ive exampl e 22
23	12	88	12	7.6	92	688	117	0.95	0.71	47%	Invent ive exampl e 23
24	13	87	10	8.5	98	710	110	1.55	0.65	43%	Invent ive exampl e 24
25	13	87	8	0.7	93	641	105	1.10	0.87	58%	Invent ive exampl e 25
26	16	84	9	8.6	94	659	98	0.85	0.93	62%	Compar ative exampl e 1
27	17	83	8	8.7	96	670	96	1. 95	0.94	63%	Compar ative exampl e 2
28	16	84	6	0.29	96	618	117	1.25	0.92	61%	Compar ative exampl e 3
29	16	84	15	0.32	95	641	45	1. 85	0.93	62%	Compar ative exampl e 4
30	17	83	6	0.31	91	660	110	0.80	0.91	61%	Compar ative exampl e 5
31	18	82	6	0.40	93	668	96	0.95	0.91	61%	Compar ative exampl e 6
32	9	91	7	0.90	91	660	33	1.30	0.96	64%	Compar ative exampl e 7
33	10	90	18	0.70	94	625	93	1.45	0.94	63%	Compar ative exampl e 8
34	8	92	10	0.29	89	639	123	0.46	0.92	61%	Compar ative exampl e 9
35	7	93	15	0.28	88	610	132	0.49	0.95	63%	Compar ative exampl e 10
36	8	92	10	0.28	89	646	137	0.49	0.93	62%	Compar ative exampl e 11
37	12	88	7	0.27	89	645	120	0.49	0.91	61%	Compar ative exampl e 12
38	9	91	20	0.28	88	652	98	0.49	0.91	61%	Compar ative exampl e 13
39	13	87	16	0.40	91	662	99	0.95	0.88	59%	Compar ative exampl e 14
40	11	89	31	0.50	95	614	99	0.80	0.89	59%	Compar ative exampl e 15
41	8	92	32	0.40	89	661	97	0.49	0.91	61%	Compar ative exampl e 16
42	14	86	6	3.30	94	649	52	0.70	0.91	61%	Compar ative exampl e 17
43	11	89	9	5.20	91	657	133	1.45	0.91	61%	Compar ative exampl e 18
44	13	87	11	0.80	98	647	133	1.25	0.93	62%	Compar ative exampl e 19
45	13	87	6	0.70	93	663	132	1. 95	0.95	63%	Compar ative exampl e 20
46	1	99	32	0.40	94	667	99	0.85	0.92	61%	Compar ative exampl e 21
47	16	84	32	1.50	95	622	114	0.55	0.92	61%	Compar ative exampl e 22
48	17	83	33	2.60	93	665	98	0.50	0.94	63%	Compar ative exampl e 23
49	9	91	32	4.50	94	657	36	2.00	0.91	61%	Compar ative exampl e 24
50	8	92	31	0.90	96	655	128	1. 65	0.92	61%	Compar ative exampl e 25
51	10	90	22	0.29	89	646	102	0.46	0.93	62%	Compar ative exampl e 26
52	14	86	28	0.28	88	654	125	0.49	0.95	63%	Compar ative exampl e 27

* P: pearlite, B: bainite, F: ferrite

Q = ([Ti] + [Nb]) / [Cr])

(In the formula, [Ti], [Nb] and [Cr] may be the weight% of each element present in the form of particles such as precipitates or inclusions in steel.)

As listed in Table 3, inventive example s 1 to 25 satisfying the alloy composition and the manufacturing conditions of the present disclosure satisfied the microstructure characteristics suggested in the present disclosure and ensured the properties targeted in the present disclosure.
FIG. 1 is an image indicating results of measuring a Cr content at an interfacial surface of a corrosion product of inventive example 17. FIG. 1 is an image of the corrosion product, and it may be confirmed that the average Cr content of the interfacial surface of the corrosion product was 1.15%, satisfying the conditions of the present disclosure, and that the corrosion reduction was excellent at 60% or less as compared to KS-SS275.
In comparative example s 1 and 2, carbon content exceeded the range suggested in the present disclosure, such that ferrite formation was insufficient, and as a result, the targeted impact toughness was not ensured, and the corrosion rate was also high.
In comparative example s 3 and 4, the silicon content was below the range suggested in the present disclosure, and accordingly, the formation of ferrite was insufficient. In particular, in comparative example 4, relational expression 1 was not satisfied, and accordingly, impact toughness and corrosion rate were below the suggested level.
In comparative example s 5 and 6, the manganese content exceeds the range suggested in the present disclosure. Accordingly, pearlite was excessively formed, such that ferrite was insufficiently formed, and accordingly, corrosion occurred excessively and rapidly.
In comparative example s 7 and 8, the sulfur content exceeded the range suggested in the present disclosure, and due to the influence of MnS inclusions, impact toughness was reduced, and corrosion was also carried out at a high rate.
In comparative example s 9 and 10, the chromium content was below the range suggested in the present disclosure. Accordingly, the solid-solution Cr content was less than the value suggested in the present disclosure, the Cr content of the interfacial surface in the corrosion product was insufficient, and the corrosion resistance was reduced.
In comparative example s 11 to 14, the titanium content was beyond the range suggested in the present disclosure. In comparative example s 11 and 12, the titanium content was insufficient, such that the formation of TiC was suppressed and the formation of Cr carbides was relatively promoted, and accordingly, the solid-solution Cr content was insufficient, and accordingly, the corrosion resistance was degraded. In comparative example s 13 and 14, titanium was excessive, such that impact toughness was degraded.
In comparative example s 15 and 16, the niobium content was less than the range suggested in the present disclosure, and as the NbC formation was suppressed, the formation of Cr carbide was relatively promoted, and accordingly, the solid-solution Cr content was insufficient. Accordingly, corrosion resistance was degraded, and also the grain size was coarse due to the failure of non-recrystallization rolling, such that the impact toughness was degraded.
In comparative example s 17 and 18, the nickel content was insufficient, such that the corrosion resistance was degraded. In particular, comparative example 17 also did not satisfy relational expression 1, and accordingly, the impact toughness was also degraded.
In comparative example s 19 and 20, the copper content was insufficient, such that the corrosion resistance was degraded.
Comparative example 21 satisfied the alloy composition condition suggested in the present disclosure, but when hot-rolling, the finishing rolling temperature was excessively high, and the ferrite average grain size exceeded the range suggested in the present disclosure, and accordingly, the impact toughness and corrosion resistance was degraded.
In comparative example s 22 and 23, the cooling rate was excessively slow during cooling. Accordingly, ferrite was insufficiently formed, and the ferrite average grain size was also coarse, such that corrosion resistance was degraded.
In comparative example s 24 and 25, the winding temperature exceeded the suggested temperature range, and the ferrite grain size was coarse, and the corrosion resistance was degraded.
In comparative example s 26 and 27, the cooling rate during secondary cooling was less than the range suggested in the present disclosure, and the solid-solution Cr content was insufficient, and the corrosion resistance was degraded.
Although the present disclosure has been described in detail through embodiments above, other forms of embodiments may also be possible. Therefore, the technical spirit and scope of the claims described below are not limited to the embodiments.

Claims

A steel plate, comprising:
by weight%, C: 0.030-0.070%, Si: 0.50-1.30%, Mn: 0.30-0.70%, Cr: 0.50-1.50%, Al: 0.05% or less, Cu: 0.25-0.50%, Ni: 0.05-0.50%, S: 0.0100% or less, Ti: 0.020-0.050%, Nb: 0.050-0.090%, and a balance of Fe and inevitable impurities,

wherein an R value defined in relational expression 1 as below is 0.75 or more,

wherein a microstructure at 1/4 point in a thickness direction from a surface includes 85% or more of ferrite and residual structure by area%,

wherein an average grain size of ferrite is 30 µm or less, and

wherein a solid-solution Cr content is 90% or more of a total Cr content: $R = [Si] + ([Ni] / [Cu])$

where [Si], [Ni] and [Cu] are weight% of each element.
The steel plate of claim 1, wherein the residual structure includes one or more of pearlite and bainite.
The steel plate of claim 1, wherein a Q value of the steel plate, defined in relational expression 2 below, is 0.3-8.5: $Q = ([Ti] + [Nb]) / [Cr])$ where [Ti], [Nb] and [Cr] are weight% of each element present in a form of particles such as a precipitate or an inclusion in steel.
The steel plate of claim 1, wherein the steel plate has tensile strength of 600 MPa or more and impact toughness of 100 J or more at -5°C.
The steel plate of claim 1, wherein Cr in an interfacial surface of a corrosion product of the steel plate, generated after a corrosion test in accordance with KS D ISO 14993, is 0.50% or more by weight%.
The steel plate of claim 1, wherein, when conducting a corrosion test in accordance with KS D ISO 14993, a relative corrosion rate of the steel plate as compared to KS-SS275 is 60% or less.
A method of manufacturing a steel plate, the method comprising:
reheating a steel slab including, by weight%, C: 0.030-0.070%, Si: 0.50-1.30%, Mn: 0.30-0.70%, Cr: 0.50-1.50%, Al: 0.05% or less, Cu: 0.25-0.50%, Ni: 0.05-0.50%, S: 0.0100% or less, Ti: 0.020-0.050%, Nb: 0.050-0.090%, and a balance of Fe and inevitable impurities, and having R value of 0.75 or more, the R value defined in relational expression 1 as below;

hot-rolling the reheated steel slab at a finishing rolling temperature of 750-900°C;

primary cooling the hot-rolled steel plate to a temperature range of 500-650°C at a cooling rate of 5-20°C/s and winding the steel plate; and

secondary cooling the wound steel plate to 400°C at a cooling rate of 0.40°C/min or more: $R = [Si] + ([Ni] / [Cu])$

where [Si], [Ni] and [Cu] are weight% of each element.
The method of claim 7, wherein the reheating step is performed in a temperature range of 1100-1300°C.