KR20120063793A

KR20120063793A - Ferrite stainless-steel with good workability

Info

Publication number: KR20120063793A
Application number: KR1020100124927A
Authority: KR
Inventors: 김상석; 서보성
Original assignee: 주식회사 포스코
Priority date: 2010-12-08
Filing date: 2010-12-08
Publication date: 2012-06-18

Abstract

The present invention relates to a ferritic stainless steel and a method for manufacturing the same, specifically, the ferritic stainless steel according to the present invention in weight%, carbon (C): 0.02 or more and 0.05 or less, nitrogen (N): 0.01 or more and 0.03 or less, Silicon (Si): 0.01 or more and 0.5 or less, manganese (Mn): 0.01 or more and 0.70 or less, phosphorus (P): 0.001 or more and 0.035 or less, sulfur (S): 0.001 or more and 0.005 or less, chromium (Cr): 12.0 or more and 15.0 or less, Nickel (Ni): 0.001 or more and 0.50 or less, copper (Cu): 0.01 or more and 0.50 or less, titanium (Ti): 0.01 or more and 0.10 or less, aluminum (Al): 0.01 or more and 0.15 or less, tin (Sn): 0.001 or more and 0.30 or less, and It consists of the remaining iron (Fe) and unavoidable impurities, the austenite fraction γ (%) is defined by the following formula 1, and chromium (Cr) and tin (Sn) satisfy the following formula 2.
Equation 1: γ (%) = 420 * C + 470 * N + 23 * Ni + 9 * Cu + 10 * Mn + 180-11.5 * Cr-11.5 * Si-12.0 * Mo -52.0 * Al
Equation 2: 12.5 ≤ (Cr + 5 * Sn) (wt%) ≤ 15.5

Description

Ferritic stainless steel with good workability

The present invention relates to a ferritic stainless steel, and more particularly to a ferritic stainless steel having improved surface workability and corrosion resistance.

Ferritic stainless steel is cheaper than austenitic stainless steel, has low thermal expansion rate, good surface gloss, formability and oxidation resistance, and is widely used in heat-resistant appliances, sink tops, exterior materials, home appliances, and electronic components. Cold rolled sheet of ferritic stainless steel is manufactured through hot rolling process, annealing pickling process to remove surface scale of hot rolled coil and remove material internal stress, cold rolling and bright annealing process.

In addition, surface defects due to processing such as ridging and stretcher strain may occur depending on the plastic deformation region of the material when manufacturing thin sheet products using ferritic stainless steel. Machinability problems may occur. In addition, the problem of corrosion due to the inferior corrosion resistance of ferritic stainless steel containing less than 16Wt% Cr is the main issue of the product group.

The present invention adjusts the content of carbon (C), nitrogen (N), tin (Sn), chromium (Cr), aluminum (Al), titanium (Ti) and the like and the austenite fraction (γ (%)) to prevent roping. To provide a ferritic stainless steel that can improve workability such as elongation, but also has excellent aging and corrosion resistance.

Ferritic stainless steel according to the present invention is by weight%, carbon (C): 0.02 or more and 0.05 or less, nitrogen (N): 0.01 or more and 0.03 or less, silicon (Si): 0.01 or more and 0.5 or less, manganese (Mn): 0.01 or more 0.70 or less, phosphorus (P): 0.001 or more and 0.035 or less, sulfur (S): 0.001 or more and 0.005 or less, chromium (Cr): 12.0 or more and 15.0 or less, nickel (Ni): 0.001 or more and 0.50 or less, copper (Cu): 0.01 or more 0.50 or less, titanium (Ti): 0.01 or more and 0.10 or less, aluminum (Al): 0.01 or more and 0.15 or less, tin (Sn): 0.001 or more and 0.30 or less and the remaining iron (Fe) and inevitable impurities, and the austenite fraction γ ( %) Is defined by the following formula 1, and chromium (Cr) and tin (Sn) satisfy the following formula 2.

Equation 1: γ (%) = 420 * C + 470 * N + 23 * Ni + 9 * Cu + 10 * Mn + 180-11.5 * Cr-11.5 * Si-12.0 * Mo -52.0 * Al

Equation 2: 12.5 ≤ (Cr + 5 * Sn) (wt%) ≤ 15.5

The relationship between the carbon (C) and nitrogen (N) may satisfy the following Equation 3.

Equation 3: 0.025≤ (C + N) (wt%) ≤0.050

In addition, chromium (Cr), tin (Sn), carbon (C) and nitrogen (N) may satisfy the following Equation 4.

Equation 4: 0.35≤ (Cr + 5 * Sn) (C + N) (wt%) ≤0.80

In addition, the austenite fraction γ (%) may satisfy the following Equation 5.

Equation 5: 30≤γ≤40

In addition, the relationship between the ratio (Al / N) of the aluminum (Al) and the nitrogen (N) and the austenite fraction (γ) may satisfy Equation 6 below.

Equation 6: (Al / N) * γ ≤ 250

In addition, the elongation (%) in the direction perpendicular to the rolling after cold rolling annealing and the official potential of the cold rolling annealing material (mV vs AgCl) may satisfy the following Equation 7.

Equation 7: 3300 <(elongation (%)) * (formula potential (mV vs AgCl)).

In addition, the elongation (%) and chromium (Cr) and tin (Sn) in the perpendicular direction of rolling at room temperature after cold rolling annealing may satisfy the following Equation 8.

Equation 8: 1.9≤ (% Elongation) * (Cr + 5 * Sn (wt%)) ≤2.7.

According to the present invention, by controlling the components of chromium (Cr) and tin (Sn), it is possible to improve moldability such as elongation without reducing corrosion resistance.

In addition, according to the present invention by controlling the components of carbon (C) and nitrogen (N) has the effect of improving the leasing characteristics without reducing the elongation.

In addition, by controlling the amount of aluminum (Al), it is possible to prevent stretcher strain without adding a skin pass process, and to prevent the occurrence of sleeve defects by controlling the number and length of stretched inclusions.

That is, by controlling the carbon (C), nitrogen (N), chromium (Cr), tin (Sn), aluminum (Al), titanium (Ti) and austenite fractions (γ (%)) optimally without reducing corrosion resistance It is possible to improve moldability and unloading property.

1 is a carbon (C), nitrogen (N), tin (Sn), chromium (Cr), aluminum (Al), titanium (Ti) and the like content and austenitic fraction (γ (%)) of the ferritic stainless steel The table which shows the component system of the A-Q steel which melted and vacuum-melted.
2 shows the mechanical properties and the surface properties of the alloy steels of A to Q steels through the results of the strain resistance, the lowering resistance, the room temperature elongation, and the formula potential measurement results after the first pass of the skin pass rolling after the final annealing for the cold rolled annealing material. This table shows evaluation results on defective elements (rising, stretcher strain) and corrosion resistance.
Figure 3 is a graph schematically showing the relationship between the elongation of the cold-rolled product of the ferritic stainless steel according to an embodiment of the present invention and the small steel component.
4 is a graph showing the distribution range of the small steel component of ferritic stainless steel according to an embodiment of the present invention.
5 is a graph showing the official potential (mV vs AgCl), leasing grade, room temperature elongation (%) of the direction perpendicular to the rolling direction of the cold-rolled product of ferritic stainless steel according to an embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

Ferritic stainless steel according to the present invention is by weight%, carbon (C): 0.02 or more and 0.05 or less, nitrogen (N): 0.01 or more and 0.03 or less, silicon (Si): 0.01 or more and 0.5 or less, manganese (Mn): 0.01 or more 0.70 or less, phosphorus (P): 0.001 or more and 0.035 or less, sulfur (S): 0.001 or more and 0.005 or less, chromium (Cr): 12.0 or more and 15.0 or less, nickel (Ni): 0.001 or more and 0.50 or less, copper (Cu): 0.01 or more 0.50 or less, titanium (Ti): 0.01 or more and 0.10 or less, aluminum (Al): 0.01 or more and 0.15 or less, tin (Sn): 0.001 or more and 0.30 or less and the remaining iron (Fe) and unavoidable impurities. Hereinafter, each composition will be described in detail.

The amount of carbon (C) is from 0.02 wt% to 0.04 wt% or less. Although carbon (C) in steel is an impurity that is inevitably included in ferritic stainless steel, it is an austenite stabilizing element of steel, and thus has an effect of maximizing austenite fraction (γ (%)) to suppress roping and ridging. However, when carbon is excessively included, the elongation is reduced to significantly reduce the workability of the product, so it is set to 0.1 wt% or less, but preferably limited to the above-mentioned range.

The amount of nitrogen (N) is 0.01 wt% to 0.03 wt% or less. Nitrogen (N) in steel exists as impurity element equally to carbon (C) and serves to increase the austenite fraction to precipitate austenite phase during hot rolling to promote recrystallization. It not only inhibits but also causes strainer strain of cold-rolled products, so the content is limited to the above-mentioned range.

Silicon (Si) contains 0.01 wt% to 0.5 wt% or less. Silicon is an unavoidable impurity contained in steel, but it is an element added as a deoxidizer during steelmaking and acts as a ferrite stabilizing element. However, if silicon is contained in a large amount in steel, hardening of the material causes ductility to be lowered, so the content thereof is limited to the aforementioned range.

The amount of manganese (Mn) is 0.01 wt% to 0.70 wt% or less. Manganese (Mn) is an impurity that is inevitably included in steel, but serves as austenite stabilizing element to suppress roping and ridging. However, when included in a large amount, manganese fume is generated during welding, which causes the MnS phase precipitation, thereby lowering the elongation. That is, when the manganese contains less than 0.01wt%, the roping and ridging inhibiting action is lowered, and when it contains more than 0.70wt%, the elongation is lowered.

The amount of phosphorus (P) is from 0.001 wt% to 0.035 wt% or less. Phosphorus (P) is an unavoidable impurity contained in steel, which causes grain boundary corrosion during pickling or impairs hot workability.

The amount of sulfur (S) is from 0.001 wt% to 0.005 wt% or less. Sulfur (S) is an unavoidable impurity contained in steel, and segregates at grain boundaries and impairs hot workability. Therefore, the content of sulfur (S) is limited to the aforementioned range.

The amount of chromium (Cr) is 13.0 wt% to 15.0 wt% or less. Chromium (Cr) is an alloying element added to improve the corrosion resistance of steel, the critical content of chromium is 12wt%. However, ferritic stainless steel containing carbon and nitrogen may cause intergranular corrosion and limit its content to the above-mentioned range in consideration of the possibility of intergranular corrosion and an increase in manufacturing cost.

The amount of nickel (Ni) is 0.001 wt% to 0.50 wt% or less. Nickel (S), together with copper (Cu) and manganese (Mn), is an austenite stabilizing element, which increases the austenite fraction to inhibit roping and leasing, and improves the corrosion resistance by adding a small amount. Deterioration and increase in manufacturing cost limit the content to the above-mentioned range.

The amount of copper (Cu) is 0.01 wt% to 0.50 wt% or less. Copper (Cu), together with nickel (Ni) and manganese (Mn), is an austenite stabilizing element, which increases the austenite fraction to inhibit roping and leasing, and improves the corrosion resistance by adding a small amount. It causes deterioration and increase in manufacturing cost. That is, when the amount of copper is included in 0.01wt% or less, roping and leasing is expressed, causing corrosion resistance is poor, and when it contains more than 0.50wt%, workability is inferior.

The amount of titanium (Ti) is 0.01wt% to 0.10wt% or less. Titanium (Ti) is an element that refines the equiaxed grain size of cast steel, and improves workability by fixing carbon and nitrogen, but increases the manufacturing cost and causes defects in the sleeve of the cold rolled strip when a large amount is added. In this case, the content is limited to the above-mentioned range.

The amount of aluminum (Al) is 0.01 wt% to 0.15 wt% or less. Aluminum (Al) is an alloy component added as a deoxidizer during steelmaking, but is present as a non-metallic inclusion when a large amount is added, which causes a defect in the sleeve of the cold rolled strip and causes a decrease in weldability, thereby limiting its content to the aforementioned range. The remainder of the ferritic stainless steel except for the above-mentioned elements consists of iron (Fe) and other unavoidable impurities.

The amount of tin (Sn) is 0.001 wt% to 0.30 wt% or less. Tin (Sn) not only improves the stability of the passivation film by modifying the properties of the passivation film, but also improves the corrosion resistance by forming a Sn concentration layer directly under the passivation film. However, when a large amount is contained, a problem of inferior moldability occurs.

The austenitic fraction (γ (%)) of the ferritic stainless steel can be expressed by Equation 1 below.

Equation 1: γ (%) = 420 * C + 470 * N + 23 * Ni + 9 * Cu + 10 * Mn + 180-11.5 * Cr-11.5 * Si-12.0 * Mo-52.0 * Al

The austenite fraction refers to the fraction that can be transformed from delta ferrite to austenite during hot rolling.

On the other hand, the ferritic stainless steel of the present invention satisfies the following conditions.

First, the relationship between chromium (Cr) and tin (Sn) will be described. Chromium (Cr) and tin (Sn) are included to satisfy the following expression (2).

Equation 2: 12.5≤ (Cr + 5 * Sn) (wt%) ≤15.5

As the content of chromium (Cr) increases, the corrosion resistance increases. That is, in the case of ferritic stainless steel, chromium is included more than 16wt% for sufficient corrosion resistance. However, as the chromium content increases, the elongation is lowered, so that the elongation is adjusted to 15.5 wt% or less.

However, when the chromium content is reduced, there is a problem that the corrosion resistance is lowered. For this reason, tin was added for corrosion resistance. In the range where (Cr + 5 * Sn) (wt%) is 12.5 or less, the corrosion resistance is drastically reduced, while in the range of 15.5 or more, the moldability inferiority is brought about by the reduction of the elongation.

The relationship between carbon (C) and nitrogen (N) is demonstrated. Carbon (C) and nitrogen (N) should satisfy Equation 3 below, and satisfy Equation 4 under the conditions of Equations 2 and 3.

Equation 3: 0.025 ≤ (C + N) (wt%) ≤ 0.050

Equation 4: 30 ≤ γ ≤ 40

When the alloy design of chromium (Cr) content is less than 15.5wt% in STS430 steel, the austenite fraction (γ) is increased to 45% or more, resulting in moldability inferiority, so the γ balance is generally equivalent to that of STS430 steel. (C + N) content was limited as shown in Equation 3 to form. That is, in the range where (C + N) is 0.025 (wt%) or less, a lowering inferiority is obtained, and in the range of 0.050 or more, moldability is inferior. As a result, by controlling the chromium and nitrogen according to Equation 3, it is possible to improve the formability, and at the same time secure the lowering of the level of ordinary STS430 steel.

In addition, the relationship between chromium, tin, carbon and nitrogen should satisfy Equation 5 below. That is, (Cr + 5 * Sn) and (C + N),

Equation 5: 0.35≤ (Cr + 5 * Sn) (C + N) ≤0.80

The conditions of In the section where (Cr + 5 * Sn) (C + N) is 0.35 (wt%) or less, the corrosion resistance and the lagging resistance are poor. In the section above 0.8 (wt%), the moldability is inferior. That is, when the condition of Formula 5 is satisfied, corrosion resistance and elongation can be improved simultaneously.

In addition, the relationship between the ratio of aluminum (Al) to nitrogen (N) (Al / N) and the austenite fraction (γ) should satisfy Equation 6 below.

Equation 6: (Al / N) * γ ≤ 250

It is an alloy system designed to keep γ balance at the same level as STS430 steel while maintaining the ratio of aluminum and nitrogen (Al / N) of 3 or more, and surface defects after 1 pass pass pass after cold annealing. It is an alloy design concept that prevents stretcher strain.

On the other hand, after the cold rolling annealing, the relationship between the elongation (%) of the rolling direction at room temperature and the official potential of the cold rolling annealing material (mV vs AgCl) satisfies Equation 7 below.

Equation 7: 3300 <(elongation (%)) * (formula potential (mV vs AgCl)).

By limiting the contents of (Cr + 5 * Sn) and (C + N) within the ranges of Equation 2 and Equation 3 above, it is possible to maintain the value obtained by multiplying the elongation after cold rolling by the official potential at 3300 or more.

In addition, the relationship between the elongation (%) and (Cr + 5 * Sn) (wt%) in the rolling direction at room temperature after cold rolling annealing satisfies Equation 8 below.

Equation 8: 1.9 ≤ (elongation (%)) / (Cr + 5 * Sn (wt%)) ≤ 2.7.

In the region where the ratio of percent elongation (%) to (Cr + 5 * Sn) (wt%) is 1.9 or less, the processability is inferior, and the ratio of percent elongation (%) to (Cr + 5 * Sn) (wt%) is obtained. In the region above 2.7, corrosion resistance inferiority is brought. That is, when controlling each component within the range of Formula 8, the effect which improves corrosion resistance, moldability, and repellency simultaneously can be acquired.

Hereinafter, the present invention will be described in more detail through each experimental example including a comparative example and an example.

A comparative example and an example will be described with reference to FIGS. 1 to 5. 1 is a carbon (C), nitrogen (N), tin (Sn), chromium (Cr), aluminum (Al), titanium (Ti) and the like content and austenitic fraction (γ (%)) of the ferritic stainless steel 2 is a table showing the component system of A to Q steels that have been vacuum-melted by adjusting, and FIG. Table shows the evaluation results on the mechanical properties and surface properties of the Q-alloy alloy components (rising, strain strain) and corrosion resistance. 3 is a graph schematically showing the relationship between the elongation of the cold-rolled product of the ferritic stainless steel according to an embodiment of the present invention and the small steel component, Figure 4 is a steel component of the ferritic stainless steel according to an embodiment of the present invention Figure 5 is a graph showing the distribution range of, Figure 5 is the official potential (mV vs AgCl), leasing grade of the cold-rolled product of ferritic stainless steel according to an embodiment of the present invention, room temperature elongation (%) in the direction perpendicular to the rolling direction The graph shown.

First, re-heat the A to Q test steels in which the content of each component is adjusted as shown in FIG. Was prepared. Next, batch annealing, cold rolling and cold annealing were performed in sequence.

Meanwhile, the leasing grade (based on R _t ) shown in FIG. 2 is 10 μm to 12 μm [1st grade], 12 μm to 14 μm [2nd grade], 14 μm to 16 μm [3 grade], 16 μm 18 micrometers were classified into [grade 4], and 18 micrometers-20 micrometers into [grade 5].

1 and 2, the M to Q experimental steel is a ferritic stainless steel according to an embodiment of the present invention, and the remaining A to L steel are experimental steels of a comparative example. 1 and 2 show that the embodiment of the present invention can provide a cold rolled product having a lowering property and excellent elongation and corrosion resistance.

M to Q steels are formed in the chromium content in the range of 14 to 14.5 wt%. As the content of chromium is lowered, an improvement in workability such as elongation can be expected. That is, as the content of chromium is lowered, the elongation is improved by moving from the region other than the present invention of FIG. 3 to the upper portion of the graph. Looking at the elongation of Figure 2, M ~ Q steel can be seen that the elongation of the cold annealed strip (cold annealed strip) is more than 29%.

However, as the chromium content decreases, the inferior corrosion resistance may be concerned. For this reason, as described above, the content of tin was increased to satisfy the condition of Equation 2 described above. That is, M-Q steel grades are formed in the range of 14.5-15.5 (Cr + 5 * Sn) as shown in FIG. In addition, M to Q steel grades are steel grades satisfying Equations 3 to 5, as shown in FIG. As a result, as shown in FIG. 5, M-Q steel grades have a lowering property within 1 to 2 grades, and have a formal potential (mV vs AgCl) of 120 or more and an elongation of 29% or more.

Hereinafter, each comparative example is demonstrated.

A, D, and K steels of Comparative Example 1 measured surface roughness after 15% elongation using JIS 13B tensile specimen of cold rolled annealing material, and as shown in FIG. Could know.

A, B, C, F, G, H, I, J, L steels of Comparative Example 2 are experimental steels having a high (C + N) content. In the case of the experimental steel of Comparative Example 2, the elongation is inferior because the content of (C + N) is high as described above. That is, as shown in Figure 2 it can be seen that the experimental steel of Comparative Example 2 has a low elongation compared to the embodiment having an elongation of up to 27 and the elongation of 29% or more.

The C, D, and E steels of Comparative Example 3 generally represent steels having a low chromium (Cr) content in STS430 steel. In the present invention, the elongation is increased while maintaining the corrosion resistance by adding tin while lowering the content of chromium for workability such as the elongation rate. However, in the case of Comparative Example 3, tin (Sn) was not added in spite of the reduced chromium content, indicating inferior in corrosion resistance. As shown in FIG. 2, the C, D, and E steels of Comparative Example 3 typically have an official potential of 100 (mV vs AgCl) or less, which is the official potential level of the STS430 steel.

In conclusion, the ferritic stainless steel according to the present invention maintains the corrosion resistance at a level equal to or higher than that of the STS 430 steel, further including tin, while increasing the elongation by reducing the amount of chromium compared to the conventional STS 430 steel.

Although preferred embodiments of the present invention have been described above, the technical spirit of the present invention is not limited to the above-described preferred embodiments, and various ferritic stainless steels in a range not departing from the technical spirit of the present invention specified in the claims. It can be implemented by the manufacturing method.

Claims

in wt%,
Carbon (C): 0.02 or more and 0.05 or less,
Nitrogen (N): 0.01 or more and 0.03 or less,
Silicon (Si): 0.01 or more and 0.5 or less,
Manganese (Mn): 0.01 or more and 0.70 or less,
Phosphorus (P): 0.001 or more and 0.035 or less,
Sulfur (S): 0.001 or more and 0.005 or less,
Chromium (Cr): 12.0 or more and 15.0 or less,
Nickel (Ni): 0.001 or more and 0.50 or less,
Copper (Cu): 0.01 or more and 0.50 or less,
Titanium (Ti): 0.01 or more and 0.10 or less,
Aluminum (Al): 0.01 or more and 0.15 or less,
Tin (Sn): 0.001 or more and 0.30 or less and
Consisting of the remaining iron (Fe) and unavoidable impurities,
Austenitic fraction γ (%) is a ferritic stainless steel excellent in workability as defined by the following equation (1) and satisfying the following equation (2).
Equation 1: γ (%) = 420 * C + 470 * N + 23 * Ni + 9 * Cu + 10 * Mn + 180-11.5 * Cr-11.5 * Si-12.0 * Mo-52.0 * Al
Equation 2: 12.5 ≤ (Cr + 5 * Sn) (wt%) ≤ 15.5

The method of claim 1,
A ferritic stainless steel having excellent workability in which the relationship between carbon (C) and nitrogen (N) satisfies Equation 3 below.
Equation 3: 0.025≤ (C + N) (wt%) ≤0.050

The method according to claim 1 or 2,
A ferritic stainless steel having excellent workability in which chromium (Cr), tin (Sn), carbon (C), and nitrogen (N) satisfy the following formula 4.
Equation 4: 0.35≤ (Cr + 5 * Sn) (C + N) (wt%) ≤0.80

The method according to claim 1 or 2,
The austenitic fraction γ (%) is a ferritic stainless steel having excellent workability that satisfies Equation 5 below.
Equation 5: 30≤γ≤40

The method according to claim 1 or 2,
A ferritic stainless steel having excellent workability in which the relationship between the ratio (Al / N) of the aluminum (Al) and the nitrogen (N) and the austenite fraction (γ) satisfies Equation 6 below.
Equation 6: (Al / N) * γ ≤ 250

The method according to claim 1 or 2,
Elongation (%) in the direction perpendicular to the rolling after cold rolling annealing and the official potential of the cold rolling annealing material (mV vs AgCl) is a ferritic stainless steel excellent in workability satisfying the following formula (7).
Equation 7: 3300 <(elongation (%)) * (formula potential (mV vs AgCl)).

The method according to claim 1 or 2,
Elongation (%) and chromium (Cr) and tin (Sn) in the rolling direction at room temperature after cold rolling annealing are ferritic stainless steels excellent in workability satisfying Equation 8 below.
Equation 8: 1.9≤ (% Elongation) * (Cr + 5 * Sn (wt%)) ≤2.7.