KR100957973B1 - Hot Rolled Steel Sheet having Excellent Sour Resistance Properties in Cold Deformation - Google Patents

Abstract

It is an object of the present invention to provide a hot rolled steel having excellent sour resistance under cold deformation through controlling the size and rolling conditions of non-metallic inclusions.

Hot-rolled steel sheet having excellent sour resistance under cold deformation of the present invention for achieving the above object is a hot rolled steel sheet having excellent sour resistance under cold deformation of the present invention by weight%, C: 0.02 ~ 0.05%, Si: 0.05 ~ 0.5 %, Mn: 0.5-1.5%, P: 0.01% or less (without 0%), S: 0.001% or less (without 0%), Al: 0.02-0.05%, Cu: 0.25% or less ( Does not contain 0%), Ni: 0.25% or less (does not contain 0%), Nb: 0.04 to 0.06% (does not contain 0%), V: 0.04 to 0.06%, Ti: 0.005 to 0.02% , Cr: 0.1-0.5%, Ca: 0.0015-0.003% and the rest contains Fe and other unavoidable impurities, non-metallic inclusions are present, the area of the nonmetallic inclusions is 1000㎛ ² or less, and before and after cold deformation hardness difference And a hydrogen permeation amount, and the hardness difference and the hydrogen permeation amount before and after the cold deformation satisfy the following Equation 1.

[Equation 1] X / a + Y / b ≤ 2.0

(However, X: hardness difference before and after cold deformation, Y: hydrogen permeability, a and b: constant (a = 10, b = 25))

Sour, hydrogen organic crack, sulfide stress corrosion, non-metallic inclusions

Description

Hot Rolled Steel Sheet having Excellent Sour Resistance Properties in Cold Deformation

The present invention relates to a line pipe steel used for transporting high pressure natural gas, and more particularly, to a hot rolled steel having excellent sour resistance under cold deformation.

In recent years, as oil demand or oil fields in poor environments have been developed in accordance with the increase in energy demand, development of crude oil or natural gas with high H ₂ S gas content in the polar regions is in progress, and thus, steels having excellent breakage characteristics due to H ₂ S gas are required. It is becoming.

The mechanism of generating hydrogen organic cracks is caused by the hydrogen gas pressure generated by the infiltration and diffusion of hydrogen generated from the steel surface into the atomic state by the corrosion reaction between the steel material and the hydrogen sulfide atmosphere. It is known to become. In addition, as the demand for natural gas increases as a source of energy in recent years, high stress may be applied to line pipe steels according to high pressure natural gas transportation, and high strength steel having excellent sour resistance is required.

In the prior art, a method for finely dispersing carbonitrides, controlling rolling or reducing center segregation has been proposed. However, there is a problem that the characteristics obtained by these methods are deteriorated by the cold deformation generated in the pipe and pipeline construction.

In addition, there is a problem in that the hydrogen occlusion amount of the steel increases with increasing the dislocation density in the steel according to the deformation after cold deformation, thereby reducing the hydrogen embrittlement characteristics.

In addition, when stress concentration occurs during cold deformation, hydrogen is concentrated in the concentrated portion, which causes a problem of easy cracking and propagation.

As a result of examining the sour characteristics in the state in which the cold deformation was applied, the inventors found that the characteristic difference occurs depending on the hardness deviation and the hydrogen permeation amount.

Accordingly, an object of the present invention is to provide a hot rolled steel having excellent sour resistance under cold deformation through controlling the size and microstructure of the nonmetallic inclusion.

Hot-rolled steel sheet having excellent sour resistance under cold deformation of the present invention for achieving the above object is by weight, C: 0.02 to 0.05%, Si: 0.05 to 0.5%, Mn: 0.5 to 1.5%, P: 0.01% or less (Does not contain 0%), S: 0.001% or less (does not contain 0%), Al: 0.02 to 0.05%, Cu: 0.25% or less (does not contain 0%), Ni: 0.25% or less ( Does not contain 0%), Nb: 0.04 to 0.06% (does not contain 0%), V: 0.04 to 0.06%, Ti: 0.005 to 0.02%, Cr: 0.1 to 0.5%, Ca: 0.0015 to 0.003% And the remainder include Fe and other unavoidable impurities, the non-metallic inclusions are present, the area of the nonmetallic inclusions is 1000㎛ ² or less, the hardness difference before and after the cold deformation and the hydrogen permeability exists, and the hardness difference before and after the cold deformation Hydrogen permeation is characterized by satisfying the following formula (1).

[Equation 1] X / a + Y / b ≤ 2.0

(However, X: hardness difference before and after cold deformation, Y: hydrogen permeability, a and b: constant (a = 10, b = 25))

According to the present invention has the effect of providing a hot rolled steel sheet excellent in hydrogen organic crack resistance and sulfide stress corrosion characteristics through the control of the size and rolling conditions of the non-metallic inclusions of the steel.

Hereinafter, the component range of this invention is demonstrated concretely.

C: 0.02-0.05 wt%

The C is the most economical and effective alloying component to strengthen the steel, when less than 0.02% by weight of the combination of Nb, V or Ti is very effective to strengthen the steel, when added in excess of 0.05% by weight HIC resistance It is preferable to limit the content to 0.02 to 0.05% by weight since the central segregation to decrease the increase.

Si: 0.05-0.5 wt%

The Si is an effective component for deoxidation and solid solution strengthening, and when it is added less than 0.05% by weight, it is difficult to obtain a deoxidation effect, and when it is added in excess of 0.5% by weight, the weldability and brittleness are reduced. It is preferable.

Mn: 0.5-1.5 wt%

The Mn is an essential component for securing strength and toughness. If it is added less than 0.5% by weight, it is difficult to secure strength and toughness, and when it is added in excess of 1.5% by weight, Mn promotes central segregation during performance, thereby reducing impact toughness and HIC resistance. Therefore, it is preferable to limit the content to 0.5 to 1.5% by weight.

P: 0.01% by weight or less (does not include 0%)

When the content of P is added in excess of 0.01% by weight, it promotes central segregation with Mn when playing, thereby reducing impact toughness and emulsion stress cracking resistance as well as reducing weldability, and thus limiting the content to 0.01% by weight or less. .

S: 0.001% by weight or less (does not include 0%)

S is a component that greatly reduces brittleness by forming MnS together with Mn in steel. When S is contained in an amount exceeding 0.001% by weight, the hydrogen organic cracking resistance is greatly reduced, and the content thereof is preferably limited to 0.001% by weight or less. .

Al: 0.02-0.05 wt%

Al is a component that deoxidizes with Si, and when it is added less than 0.02% by weight, it is difficult to obtain a deoxidation effect, and when it is added in excess of 0.05% by weight, the alumina aggregate is increased to lower the hydrogen-organic crack resistance. It is recommended to limit it to 0.02 ~ 0.05% by weight.

Nb: 0.04-0.06 wt%, V: 0.04-0.06 wt%

The Nb and V is a component showing the precipitation strengthening effect by the addition of a small amount, in the carbon range of the present invention, since the strength increase due to precipitation strengthening is more than 0.06% by weight, respectively, the content is limited to 0.06% by weight or less, It is not effective at less than 0.04% by weight. Therefore, the content is preferably limited to 0.04 to 0.06% by weight.

Ti: 0.005-0.02 wt%

The Ti is precipitated with TiN in the steel to suppress the grain growth of austenite when reheated to obtain high strength and excellent impact toughness, and also precipitated by TiC, etc. to strengthen the steel. However, in the carbon range of the present invention, the content of Ti is 0.005% by weight, which is the minimum amount for obtaining the effect. If the content exceeds 0.02% by weight, the effect is not large. Therefore, the content of the Ti is limited to 0.005 to 0.02% by weight. desirable.

Cr: 0.1-0.5 wt%

The Cr is added to increase strength and ensure corrosion resistance. If the amount is less than 0.1% by weight, the above effect is less. If the amount is added more than 0.5% by weight, the risk of local corrosion is increased, so the content is preferably limited to 0.1 to 0.5% by weight.

Cu: 0.25% or less (does not contain 0%), Ni: 0.25% or less (does not contain 0%)

Cu and Ni have the effect of increasing the steel strength and miniaturizing grains. However, it was observed through the experiment that the role of reducing the corrosion resistance in NACE solution A conditions. When added in excess of 0.25%, the strength increases, but the corrosion resistance is markedly lowered, so it is limited to 0.25% or less. In this range, when the addition of Cr is added, the effect of reducing corrosion resistance is hardly observed.

Ca: 0.0015 to 0.003 wt%

The Ca is a component that suppresses the origin of hydrogen organic crack generation by spheroidizing the shape of the emulsion-based inclusions, it is difficult to obtain the effect when added less than 0.0015% by weight, the amount of non-metallic inclusions is added more than 0.003% by weight Rather, it increases and decreases the hydrogen organic crack resistance, it is preferable to limit the content to 0.0015 ~ 0.003% by weight.

In addition to the above compositions, the remainder is composed of Fe and other unavoidable impurities.

(1) Area limitation of nonmetallic inclusions

Non-metallic inclusions, which are indispensable in steel materials, are crushed or stretched in the rolling direction in the hot rolling step, and play a role of hydrogen organic crack or sulfide stress corrosion. Therefore, by limiting the non-metallic inclusions of the steel before rolling can improve the sour resistance. When it has 1000 micrometer <^2> or less, sour resistance and low temperature toughness are very excellent.

(2) Limiting the relationship between hardness difference and hydrogen permeation amount before and after deformation

Deformed steel exhibits a work hardening phenomenon, which is due to an increase in dislocation density in the material. This increase in potential increases the hydrogen occlusion in the steel and also acts as a cause of increasing the hydrogen permeation of the steel. The increase in the amount of hydrogen occlusion and hydrogen permeation easily exceeds the amount of hydrogen required to initiate the hydrogen organic crack or sulphide stress corrosion, thereby causing a decrease in the sour properties. Therefore, when satisfactory relationship between hardness change and hydrogen permeation is satisfied as follows, sour resistance deterioration is reduced even after cold deformation.

[Equation 1] X / a + Y / b ≤ 2.0

(However, X: hardness difference before and after cold deformation, Y: hydrogen permeability, a and b: constant (a = 10, b = 25))

Hereinafter, the manufacturing method of this invention is demonstrated.

After reheating the steel slab having the above composition in the range of 1150 to 1250 ° C., the steel slab is rolled under ordinary hot rolling conditions and then wound.

Hereinafter, the present invention will be described in detail through examples.

Example 1

After reheating the steel composition as shown in Table 1 in the laboratory for 2 to 3 hours in the range of 1150 ~ 1250 ℃, rolled to a thickness of 12mm thick under ordinary hot rolling conditions, was wound at 550 ℃. Corrosion rate after 100 hours deposition in 5% NaCl + 0.5% CH ₃ COOH solution saturated with 1 atm H ₂ S gas through surface polishing and degreasing process by processing specimens of diameter 50mm and thickness 10mm from the steel produced as described above Was measured. The measurement results are shown in Table 1.

division C Si Mn P S Al Nb V Ti Cr Ca Cu Ni Corrosion rate (mpy) Steel 1 0.030 0.23 1.27 0.0065 0.008 0.03 0.042 0.032 0.015 - 0.002 0.17 0.20 11.2 Steel 2 0.045 0.22 1.30 0.0089 0.001 0.03 0.051 0.048 0.010 - 0.002 0.28 - 8.9 Steel 3 0.038 0.22 1.30 0.0055 0.0015 0.03 0.05 0.050 0.011 - 0.0015 - 0.21 12.1 Steel 4 0.035 0.23 1.28 0.010 0.001 0.03 0.053 0.051 0.015 0.15 0.0025 0.22 0.2 5.1 Steel 5 0.041 0.21 1.31 0.009 0.001 0.03 0.052 0.042 0.011 0.21 0.0021 0.25 0.02 4.3 Steel 0.041 0.21 1.31 0.009 0.001 0.03 0.052 0.042 0.011 0.13 0.0021 0.02 0.02 4.8

As can be seen in Table 1, it can be seen that the corrosion rate of Cu-Ni or Cu, Ni-added steels (steels 1 to 3) is much higher than that of Cr-added steels (steels 4 to 5).

Example 2

After cold deformation, after re-heating the steel composition as shown in Table 2 in the range of 1150 ~ 1250 ℃ in order to investigate the hydrogen organic cracking and sulfide stress corrosion characteristics, rolling finish at 850 ℃ or higher and start cooling at above Ar3 temperature to 500 ~ Winding was carried out in 600 degreeC range. As such hydrogen induced crack resistance resulting from the steel material was a row from one atmosphere saturated with H ₂ S gas _{5% NaCl + 0.5% CH 3} COOH solution according to NACE TM0284, a crack was observed extent by the ultrasonic flaw detection method. In addition, sulfide stress corrosion characteristics were performed according to NACE TM0177 Method A, and full size round bar specimens of NACE TM0177 Method A were used. In the sulfide stress corrosion test, 80% of the steel yield stress was applied to the specimen. Before the test, the cold deformation was performed using a plate test specimen and subjected to a hydrogen organic crack test or a sulfide stress corrosion test after adding a strain of 3% using a general tensile tester. Vickers hardness was measured on the specimen before and after deformation under 5kg load before the test.

Hydrogen permeation was also measured using the electrochemical method shown in ASTM G148. Specimens were used with 30mm diameter and 1mm thick specimens, and the surface was ground to 1200 times abrasive paper. The hydrogen intrusion side used 5% NaCl + 0.5% CH ₃ COOH solution saturated with 1 atm H ₂ S gas, and the hydrogen measurement side used 0.1N NaOH solution. In order to measure the amount of permeated hydrogen, a hydrogen permeation current value was measured by applying a saturated red electrode reference + 0.15V to the test piece. The permeation current of hydrogen transmitted in the thickness direction of the specimen by exposing the specimen to the solution gradually increases with time to reach the final steady state. At this time, the normal current was defined as the hydrogen permeation amount. The results are shown in Table 3 below.

division C Si Mn P S Al Nb V Ti Cr Ca Cu Ni A 0.030 0.23 1.27 0.0065 0.0008 0.03 0.042 0.032 0.015 0.21 0.002 0.17 0.20 B 0.035 0.23 1.28 0.010 0.001 0.03 0.053 0.051 0.015 0.18 0.0025 0.22 0.2 C 0.045 0.22 1.30 0.0089 0.001 0.03 0.051 0.048 0.010 0.25 0.002 0.28 0.02 D 0.038 0.22 1.30 0.0055 0.00015 0.03 0.05 0.050 0.011 0.20 0.0015 0.02 0.21

division Placement area (㎛ ² ) Strain (%) Vickers hardness before deformation (Hv) Vickers hardness after deformation (Hv) △ Hv Hydrogen permeation amount (μA / cm ² ) Equation 1 Hydrogen organic crack area ratio (CAR,%) Sulfide Stress Corrosion (SSCC) Pass / NonPass A 754 3 182 194 12 22 2.08 0 F X 857 3 193 202 9 19 1.66 0 NF O 736 3 176 186 10 18 1.72 0.1 NF O 852 3 195 203 8 19 1.56 0.1 NF O 953 3 201 222 21 32 3.38 16.2 F X 930 3 186 209 23 31 3.54 2.4 F X 754 3 176 195 19 28 3.02 6.8 F X B 1118 3 185 197 12 22 2.08 2.5 NF X 1119 3 194 203 9 26 1.94 0.8 F X 1199 3 185 191 6 16 1.24 1.2 NF X 1074 3 175 196 21 32 3.38 21.3 F X C 865 3 182 190 8 19 1.56 0.1 NF O 932 3 203 215 12 21 2.04 3.4 F X 945 3 176 187 11 18 1.82 0.2 NF O 963 3 195 210 15 25 2.5 2.1 F X 897 3 201 214 13 19 2.06 5.1 NF X 898 3 184 201 17 18 2.42 5.4 NF X 865 3 176 197 21 32 3.38 12.1 F X 978 3 185 210 25 29 3.66 8.2 F X 853 3 194 220 26 38 4.12 11.2 F X D 882 3 183 188 5 14 1.06 0 NF O 996 3 175 186 11 19 1.86 0.1 NF O 807 3 175 194 19 29 3.06 14.9 F X 834 3 205 227 22 34 3.56 16.2 F X 943 3 184 212 28 32 4.08 10.2 F X

[Equation 1] X / a + Y / b ≤ 2.0

(However, X: hardness difference before and after cold deformation, Y: hydrogen permeability, a and b: constant (a = 10, b = 25))

SSCC: Non-Failure (NF), F (Failure)

Pass / NonPass Standard: CAR 0.5% or less and Non-Failure

As shown in Table 3, in the case of steel type B, a steel material having a nonmetallic inclusion area in the steel having a diameter of 1000 μm ² or more may satisfy the condition of Equation 1, but the hydrogen organic cracking property is poor, or the sulfide stress corrosion property is poor. Inferiority can be seen.

Steels A, C, and D are steels with a nonmetallic inclusion area of 1000 µm ² or less, and have excellent hydrogen organic cracking and sulfide stress corrosion characteristics when having a value of 2 or less (Equation 1).

Claims (1)

By weight, C: 0.02 to 0.05%, Si: 0.05 to 0.5%, Mn: 0.5 to 1.5%, P: 0.01% or less (not including 0%), S: 0.001% or less (not including 0%) Al: 0.02 to 0.05%, Cu: 0.25% or less (without 0%), Ni: 0.25% or less (without 0%), Nb: 0.04 to 0.06% (without 0%) ), V: 0.04 to 0.06%, Ti: 0.005 to 0.02%, Cr: 0.1 to 0.5%, Ca: 0.0015 to 0.003% and the remainder contain Fe and other unavoidable impurities, and nonmetallic inclusions are present The inclusions have an area of 1000 μm ² or less, the hardness difference before and after cold deformation and hydrogen permeation are present, and the sour resistance under cold deformation is characterized in that the hardness difference before and after cold deformation and hydrogen permeation satisfy the following formula (1). Excellent hot rolled steel sheet. [Equation 1] X / a + Y / b ≤ 2.0 (However, X: hardness difference before and after cold deformation, Y: hydrogen permeability, a and b: constant (a = 10, b = 25))