KR20240097250A - Hot-rolled steel plate having exellent hydrogen induced crack resistance and good tensile property at high temperature and method for manufacturing - Google Patents

Abstract

The present invention relates to a hot-rolled steel sheet having excellent hydrogen-induced cracking resistance and high-temperature tensile strength and a method of manufacturing the same.

Description

Hot-rolled steel sheet with excellent hydrogen-induced cracking resistance and high-temperature tensile strength and its manufacturing method {HOT-ROLLED STEEL PLATE HAVING EXELLENT HYDROGEN INDUCED CRACK RESISTANCE AND GOOD TENSILE PROPERTY AT HIGH TEMPERATURE AND METHOD FOR MANUFACTURING}

The present invention relates to a hot-rolled steel sheet having excellent hydrogen-induced cracking resistance and high-temperature tensile strength and a method of manufacturing the same.

Recently, as energy demand increases, oil sands mining volume in Canada is increasing. Hydrogen-induced cracking resistance and high-temperature tensile properties are required for pipelines used in the process of mining and refining oil sands. Typically, the temperature during the oil sand mining and refining process is known to be 80~320℃, and oil sand mined in certain areas contains H ₂ S gas, so it is necessary to secure hydrogen-induced cracking resistance as well as excellent high-temperature tensile properties. It is known. Among high-temperature tensile properties, basically, high-temperature tensile strength decreases and then increases as temperature increases. This is a phenomenon caused by dynamic strain aging, and the critical temperature is around 150~160℃, which requires the greatest decrease in tensile strength. It is difficult to satisfy the strength specifications.

Accordingly, methods for effectively controlling hydrogen-induced cracking include a means of controlling the length of non-metallic inclusions and the hardness of the segregation zone, or a method of improving hydrogen-induced cracking resistance by controlling the composition of non-metallic inclusions. In addition, other than increasing the C content, little is known about suppressing the decrease in high-temperature tensile strength.

However, the above-described conventional technologies can guarantee each of the two physical properties, but cannot guarantee them at the same time, which is even more difficult in existing hydrogen-induced cracking-resistant steels in which the C content is minimized to suppress the occurrence of hydrogen-induced cracking. am. The reason is that when hydrogen-induced cracking resistance is secured by minimizing the C content, the high-temperature tensile strength decreases, and the two physical properties are in inverse proportion to each other.

Domizzi, G., et al., Corrosion Science, Vol. 43, 325-339(2001)

One aspect of the present invention is to provide a hot-rolled steel sheet with excellent hydrogen-induced cracking resistance and high-temperature tensile strength that can be preferably applied in a high-temperature environment, and a method for manufacturing the same.

The object of the present invention is not limited to the above-described content. Anyone skilled in the art to which the present invention pertains will have no difficulty in understanding the additional problems of the present invention from the content throughout the present invention specification.

One aspect of the present invention is,

In weight percent, C: 0.02-0.05%, Si: 0.05-0.3%, Mn: 0.5-1.4%, P: 0.01% or less (excluding 0%), S: 0.001% or less (excluding 0%), Al : 0.02~0.05%, Nb: 0.06~0.1%, Ti: 0.005% or less (excluding 0%), N: 0.004~0.01%, Cr: 0.1~0.5%, Mo: 0.03~0.2%, Ca: 0.0015~ 0.003%, including the balance Fe and other unavoidable impurities;

As the microstructure, the base structure is at least one selected from the group consisting of polygonal ferrite and acicular ferrite, and pearlite is included in an area percentage of 5% or less (excluding 0%),

The average grain size is 15㎛ or less (excluding 0㎛),

Provided is a hot rolled steel sheet that satisfies the following relational expression 1.

[Relationship 1]

35 ≤ (1000×[C]×[Nb]×[Mn] ² )/(3×[Cr]×[Mo]) ≤ 67

(In Equation 1 above, [C], [Nb], [Mn], [Cr], and [Mo] represent the weight percent content for each element in parentheses.)

In addition, another aspect of the present invention is,

In weight percent, C: 0.02-0.05%, Si: 0.05-0.3%, Mn: 0.5-1.4%, P: 0.01% or less (excluding 0%), S: 0.001% or less (excluding 0%), Al : 0.02~0.05%, Nb: 0.06~0.1%, Ti: 0.005% or less (excluding 0%), N: 0.004~0.01%, Cr: 0.1~0.5%, Mo: 0.03~0.2%, Ca: 0.0015~ Reheating the slab containing 0.003%, the balance Fe and other unavoidable impurities, and satisfying the following equation 1 at 1150-1350°C;

Obtaining a hot rolled steel sheet by performing final hot rolling on the reheated slab at an Ar3 temperature to a non-recrystallization temperature;

A cooling step of starting to cool the hot rolled steel sheet at a temperature of Ar3 to a non-recrystallization temperature and ending the cooling at 450 to 600°C; and

It provides a method of manufacturing a hot rolled steel sheet, including the step of winding the cooled steel sheet.

[Relationship 1]

35 ≤ (1000×[C]×[Nb]×[Mn] ² )/(3×[Cr]×[Mo]) ≤ 67

(In Equation 1 above, [C], [Nb], [Mn], [Cr], and [Mo] represent the weight percent content for each element in parentheses.)

According to one aspect of the present invention, it can be preferably used in a corrosive and high-temperature environment containing H ₂ S that has hydrogen-induced cracking resistance and satisfies a tensile strength of 570 MPa or more when tested at a high temperature of 160 ° C. We can provide hot rolled steel sheets that can be used.

The various and beneficial advantages and effects of the present invention are not limited to the above-described content, and may be more easily understood through description of specific embodiments of the present invention.

Figure 1a shows a photograph of the microstructure of a specimen obtained from Inventive Example 1 of the present invention using a scanning electron microscope (SEM), and Figure 1b shows a photograph of the specimen obtained from Comparative Example 1 of the present invention using a scanning electron microscope (SEM). Figure 1c shows a picture taken of the microstructure of the specimen obtained from Comparative Example 8 of the present invention using a scanning electron microscope (SEM), and Figure 1d shows a picture taken of the microstructure of the specimen obtained from Comparative Example 8 of the present invention. This shows a photograph of the microstructure of the obtained specimen using a scanning electron microscope (SEM).
Figure 2 shows a photograph of the microstructure of the specimen obtained in Comparative Example 4 using a scanning electron microscope (SEM).
Figure 3 is a graph showing the results of a high temperature tensile test at 160°C for invention steel 1 and comparative steel 3.

Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be modified into various other forms, and the scope of the present invention is not limited to the embodiments described below. Additionally, the embodiments of the present invention are provided to more completely explain the present invention to those with average knowledge in the relevant technical field.

Meanwhile, the terms used in this specification are for describing specific embodiments and are not intended to limit the present invention. For example, as used herein, singular forms include plural forms unless the relevant definition clearly indicates the contrary. Additionally, the meaning of “including” used in the specification is to specify a configuration and not to exclude the presence or addition of another configuration.

Hereinafter, a hot-rolled steel sheet with excellent hydrogen-induced cracking resistance and high-temperature tensile strength according to one aspect of the present invention will be described.

First, the reason for adding alloy components and limiting their content in the hot rolled steel sheet according to one aspect of the present invention will be described in detail. It is important to note that the content of each ingredient described below is based on weight percent unless specifically mentioned.

C: 0.02~0.05% by weight

The C is the most economical and effective alloy component for strengthening steel. However, if C is added in less than 0.02% by weight, the effect is small, making it difficult to secure yield strength and tensile strength, and if it exceeds 0.05% by weight, it reduces hydrogen organic cracking resistance (HIC resistance). There is a problem of increased segregation. Therefore, it is preferable that C is in the range of 0.02 to 0.05% by weight.

Si: 0.05~0.3% by weight

The Si is an effective component for deoxidation and solid solution strengthening, and when added in less than 0.05% by weight, the solid solution strengthening effect is small and the difference between yield strength and tensile strength is reduced, making it difficult to satisfy a low yield ratio. In addition, when the Si content exceeds 0.3% by weight, weldability and brittleness are reduced, so it is preferable that the Si ranges from 0.05 to 0.3% by weight.

Mn: 0.5~1.4% by weight

Mn is an essential ingredient for securing strength and toughness, but when added in less than 0.5% by weight, it is difficult to secure strength and toughness, and when added in excess of 1.4% by weight, it promotes center segregation during playing, resulting in hydrogen-induced cracking resistance. (HIC resistance) may be reduced. Therefore, it is preferable that the Mn ranges from 0.5 to 1.4% by weight.

P: 0.01% by weight or less (excluding 0%)

If the P content exceeds 0.01% by weight, it promotes central segregation along with Mn during playing, thereby lowering hydrogen organic cracking resistance (HIC resistance), so it is preferable to control the P content to 0.01% by weight or less. do. However, considering unavoidable inclusion, 0% is excluded as the lower limit of the P content.

S: 0.001% by weight or less (excluding 0%)

The S is a component that greatly reduces brittleness by reacting with Mn in steel to form MnS, and when it exceeds 0.001% by weight, it significantly reduces hydrogen-induced cracking resistance. Therefore, it is desirable to control the S content to 0.001% by weight or less. However, considering unavoidable inclusion, 0% is excluded as the lower limit of the S content.

Al: 0.02~0.05% by weight

The Al is a component that acts as a deoxidizer together with Si, and when added in less than 0.02% by weight, it is difficult to obtain a deoxidizing effect. When added in amounts exceeding 0.05% by weight, it increases alumina aggregates and reduces hydrogen-induced cracking resistance. It is preferable to control the Al content to be in the range of 0.02 to 0.05% by weight.

Nb: 0.06~0.1% by weight

Nb is a component that exhibits a precipitation strengthening effect when added in a small amount, and in order to secure high strength through the above effect, it needs to be included in an amount of 0.06% by weight or more. In the carbon range of the present invention, if it exceeds 0.1% by weight, it is caused by a large amount of precipitates. Due to the decrease in weldability and hydrogen-induced cracking resistance (HIC resistance), it is desirable to control the content to 0.1% by weight or less. Therefore, it is preferable that the Nb content ranges from 0.06 to 0.1% by weight, respectively.

Ti: 0.005% by weight or less (excluding 0%)

The Ti is precipitated as TiN in the steel and plays a role in obtaining excellent impact toughness by suppressing the grain growth of austenite upon reheating. Therefore, in order to obtain the effect of adding Ti, 0% is excluded as the lower limit of the Ti content. However, in order to obtain the above effect and excellent high-temperature tensile strength in the carbon range of the present invention, the Ti content needs to be 0.005% by weight or less. On the other hand, if the Ti content exceeds 0.005% by weight, sufficient dissolved N is not secured and high-temperature tensile properties are impaired, so it is preferable to control the Ti content to 0.005% by weight or less.

Cr: 0.1~0.5% by weight

The Cr is added to increase strength and ensure corrosion resistance. However, if the Cr is added in less than 0.1% by weight, the effect is small, and if it exceeds 0.5% by weight, the risk of local corrosion increases, so it is preferable to control the content to 0.1 to 0.5% by weight.

Mo: 0.03~0.2% by weight

The Mo is added to increase strength and secure a needle-shaped ferrite structure, which is a low-temperature phase. However, when Mo is added in less than 0.03% by weight, the effect is small, and when Mo is added in amounts exceeding 0.2% by weight, a large amount of a light secondary phase such as MA (martensite austenite) is generated, which reduces the hydrogen-organic cracking resistance, so the content is reduced to 0.03%. It is desirable to control it to ~0.2% by weight.

N: 0.004~0.01% by weight

The N, together with C, forms TiN as a representative solid solution strengthening element, suppressing the growth of austenite grains when reheating the slab, and suppressing the decrease in tensile strength by promoting the dynamic aging phenomenon when tensed at high temperatures. If the N content is less than 0.004% by weight, the above effect is small, making it impossible to secure the resistance compound ratio at high temperatures, and if it exceeds 0.01% by weight, it is difficult to secure hydrogen-induced cracking resistance (HIC resistance) by forming TiNbN. Therefore, the N is preferably in the range of 0.004 to 0.01% by weight or less.

Ca: 0.0015~0.003% by weight

The Ca is a component that plays a role in suppressing the origin of hydrogen organic cracking by spheroidizing the shape of the emulsion-based inclusions. If the content is less than 0.0015% by weight, it is difficult to obtain the above effect, and if it exceeds 0.003% by weight, it is a non-metallic substance. The amount of inclusions may actually increase, reducing hydrogen-induced cracking resistance. Therefore, it is preferable that the Ca content ranges from 0.0015 to 0.003% by weight.

In addition to the above composition, the remainder is Fe. However, in the normal manufacturing process, unintended impurities from raw materials or the surrounding environment may inevitably be mixed, so this cannot be ruled out. Since these impurities are known to anyone skilled in the art, all of them are not specifically mentioned in this specification, but representative impurities are mentioned as follows.

According to one aspect of the present invention, the hot rolled steel sheet may satisfy the following relational expression 1. As a result of extensive research, the present inventors found that there is a certain correlation between the C, Nb, Mn, Cr, and Mo contents, and by controlling the relationship between these contents to satisfy the following relational equation 1, hydrogen-induced cracking resistance was achieved. The present invention was completed after confirming that it was resistant and had a very high level of tensile strength during a tensile test even at a high temperature of 160°C, and that it could be preferably used even in a corrosive and high-temperature environment containing H ₂ S. Therefore, according to the present invention, it has hydrogen-induced cracking resistance, has excellent room temperature yield strength and room temperature tensile strength, and can secure high tensile strength even in the vicinity of 150 to 160 ° C., where the decline in tensile strength is particularly severe.

[Relationship 1]

35 ≤ (1000×[C]×[Nb]×[Mn] ² )/(3×[Cr]×[Mo]) ≤ 67

(In Equation 1 above, [C], [Nb], [Mn], [Cr], and [Mo] represent the weight percent content for each element in parentheses.)

According to one aspect of the present invention, the microstructure of the hot rolled steel sheet has at least one selected from the group consisting of polygonal ferrite and acicular ferrite as a base structure. In this specification, the above-mentioned matrix structure refers to a majority (i.e., more than 50% in area %) of one or more phases selected from the group consisting of polygonal ferrite and acicular ferrite in the microstructure of the hot rolled steel sheet. ) refers to an organization that includes

Meanwhile, although not particularly limited, according to one aspect of the present invention, the hot rolled steel sheet has, as a microstructure, at least 95% by area of at least one phase selected from the group consisting of polygonal ferrite and acicular ferrite ( (excluding 100%) can be included.

According to one aspect of the present invention, the hot-rolled steel sheet contains 5% or less (excluding 0%) of pearlite in area percent as a microstructure. If the pearlite area ratio exceeds 5%, it may be difficult to secure hydrogen-induced cracking resistance. Meanwhile, in terms of further improving the above-described effect, the lower limit of the pearlite area ratio may be 0.1%, or the upper limit of the pearlite area ratio may be 2.5%.

According to one aspect of the present invention, the hot rolled steel sheet may have an average grain size of 15 ㎛ or less (excluding 0 ㎛). If the average grain size of the hot rolled steel sheet exceeds 15㎛, it may be difficult to secure a very high tensile strength desired in the present invention due to a decrease in tensile strength when tensile at high temperature. Meanwhile, in terms of further improving the above-mentioned effect, the lower limit of the average grain size may be 5㎛, or the upper limit of the average grain size may be 13㎛.

Hereinafter, a method for manufacturing the hot rolled steel sheet of the present invention will be described.

First, prepare a slab with the above-described composition. Meanwhile, in the present invention, control of non-metallic inclusions can be obtained through control of process conditions in a typical secondary refining process. For example, the above-described secondary refining process can control inclusions by Ar bubbling in LF and Ar bubbling in degassing processes such as VTD or RH. Of course, the manufacturing method of the present invention is not necessarily limited to the above process conditions, and non-metallic inclusions can be controlled by various methods. After refining the molten steel, the molten steel can be continuously cast to produce a slab.

The aforementioned slabs are reheated at 1150-1350°C. The reheating temperature is determined by the solid solution temperature of the Nb-based precipitate, and in the composition range of the present invention, solid solution is possible above 1150°C. When the slab is heated above 1350°C, the crystal grain size of the steel sheet becomes very large, reducing toughness and high temperature tensile strength. Since the strength decreases, the reheating temperature range is preferably in the range of 1150 to 1350°C.

The reheated slab is subjected to final hot rolling at an Ar3 temperature to a non-recrystallization temperature to obtain a hot rolled steel sheet. The reduction amount below the non-recrystallization temperature has a very significant impact on the crystal grain size and uniformity of the microstructure of the hot rolled steel sheet. The crystal grain size and uniformity are highly correlated with hydrogen-induced cracking resistance, low-temperature toughness, and high-temperature tensile properties. Therefore, in order to control the crystal grain size and uniformity, it is desirable to ensure that the reduction ratio during rolling is 70% or more. However, if the reduction ratio is less than 70%, the uniformity of the crystal grain size may decrease, which may result in low-temperature toughness and high-temperature tensile properties. The reduction ratio may range from 70% to the maximum reduction ratio of the corresponding thickness. Meanwhile, finish hot rolling is performed in the temperature range from Ar3 to the non-recrystallization temperature. When finishing hot rolling is performed in a temperature range exceeding the non-recrystallization temperature, there is a high possibility that uneven and coarse grain growth may occur, which can reduce high-temperature tensile properties, and when finishing hot rolling is performed in a temperature range below Ar3, it is an ideal range. Rolling develops a (100) texture that allows large amounts of hydrogen to flow in, resulting in very low resistance to hydrogen-induced cracking. Meanwhile, the Ar3 temperature and non-recrystallization temperature mentioned in the present invention can be obtained through the following equation.

Ar3 temperature (℃) (Ouchi formula): 910 - 310×C - 80×Mn - 20×Cu - 15×Cr - 55×Ni -80×Mo + 0.35×(steel plate thickness (mm)-8)

Non-recrystallization temperature (℃) (Boratto formula): 887 + 464×C + 6445×Nb -644×sqrt(Nb) + 732×V - 230Хsqrt(V) + 890ХTi + 363ХAl - 357ХSi

Next, the hot rolled steel sheet is cooled starting from the Ar3 temperature to the non-recrystallization temperature and ending at 450 to 600°C.

Cooling of the hot rolled steel sheet obtained through the hot rolling process is preferably initiated at Ar3 temperature or higher. If the cooling is initiated at a temperature below Ar3, coarse ferrite may be formed before cooling, which may reduce high-temperature tensile properties and develop a brittle fracture texture that reduces hydrogen-induced cracking resistance.

At this time, the cooling is preferably completed at 450 to 600° C., and then the hot-rolled steel sheet is preferably wound in the above temperature range. If the cooling end temperature exceeds 600°C, the transformation may be unstable and a coarse pearlite structure may be formed, which may reduce hydrogen-induced cracking resistance. When the cooling end temperature is less than 450°C, normal winding is very difficult due to the high rigidity of the steel sheet. Therefore, the cooling end temperature is preferably in the temperature range of 450 to 600°C.

Meanwhile, although not particularly limited, according to one aspect of the present invention, during the cooling, the average cooling rate may be 20°C/s or more. If the average cooling rate is less than 20°C/s, there may be a problem in that a coarse pearlite structure that reduces hydrogen-induced cracking resistance may be easily formed. During the cooling, the upper limit of the average cooling rate is not particularly limited, but may be 60°C/s.

Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are only for illustrating the present invention by way of example and are not intended to limit the scope of the present invention. This is because the scope of rights of the present invention is determined by matters stated in the patent claims and matters reasonably inferred therefrom.

(Example)

Molten steel having the composition shown in Table 1 below was refined to control non-metallic inclusions, and then reheated, finish rolled, and coiled under the manufacturing conditions shown in Table 2 below, and then cooled at an average cooling rate of 20 to 60°C/s. After cooling, hot rolled steel sheets with a thickness of 7 to 15 mm were manufactured. For the steel sheets manufactured in this way, the tensile strength was measured after a high-temperature tensile test at 160℃, where the tensile strength is typically the lowest in the high temperature range of 80~320℃, and the pearlite fraction, grain size, and cracks in the steel sheet specimens caused by hydrogen were measured. The CLR (Crack Thickness Ratio), which is the total length divided by the total length of the specimen, was measured, and the results are shown in Table 3 below. The high-temperature tensile test was conducted based on ASTM standards, and the pearlite fraction was measured using an optical microscope and an image analyzer at 500x magnification. At this time, the microstructure was confirmed to be ferrite, and the remainder other than pearlite was confirmed to be ferrite. It was confirmed that the area ratio of ferrite to the ferrite was more than 95% (excluding 100%) and that it had a ferrite base structure. In addition, the grain size was calculated as the effective grain size using EBSD.

Hydrogen-induced cracking resistance of steel sheets was tested according to NACE TM0284 by immersing the specimen in a 5% NaCl + 0.5% CH ₃ COOH solution saturated with 1 atm H ₂ S gas for 96 hours, then observing the degree of cracking by ultrasonic inspection. , The total length of cracks in the steel sheet specimen generated by hydrogen was then evaluated as the value divided by the total length of the specimen (CLR).

division C Si Mn P S Al Ti Nb Cr Mo N Ca Invention lecture 1 0.035 0.2 1.2 0.0076 0.0007 0.025 0.002 0.085 0.31 0.12 0.0083 0.0024 Invention lecture 2 0.043 0.17 0.8 0.0062 0.0009 0.03 0.003 0.068 0.22 0.08 0.0046 0.0026 Invention lecture 3 0.04 0.25 1.0 0.0051 0.0008 0.032 0.004 0.075 0.15 0.1 0.0063 0.0019 Comparative lecture 1 0.062 0.18 1.3 0.007 0.0009 0.03 0.004 0.065 0.21 0.08 0.0065 0.0024 Comparison lecture 2 0.045 0.22 1.5 0.006 0.0008 0.031 0.003 0.072 0.26 0.1 0.0085 0.0022 Comparison lecture 3 0.039 0.21 1.2 0.008 0.0008 0.025 0.008 0.065 0.18 0.07 0.0092 0.0018 Comparison lecture 4 0.041 0.25 1.0 0.009 0.0007 0.031 0.003 0.12 0.27 0.13 0.0073 0.0023 Comparison lecture 5 0.043 0.19 1.2 0.007 0.0008 0.027 0.004 0.071 0.17 0.12 0.003 0.0026 Comparative lecture 6 0.042 0.26 0.9 0.007 0.0006 0.036 0.002 0.062 0.21 0.26 0.0091 0.0027 Comparison lecture 7 0.044 0.24 0.8 0.008 0.0007 0.026 0.001 0.063 0.15 0.1 0.0083 0.0037 Comparative lecture 8 0.035 0.2 1.2 0.0076 0.0007 0.025 0.002 0.04 0.31 0.12 0.0083 0.0024

division Steel grade No. reheat
temperature
(℃) finish
hot rolling
Temperature (℃) winding
temperature
(℃) Invention Example 1 Invention lecture 1 1284 853 543 Invention Example 2 Invention lecture 2 1277 847 516 Invention Example 3 Invention lecture 3 1318 838 567 Comparative Example 1 Comparative lecture 1 1291 859 536 Comparative Example 2 Comparison lecture 2 1287 839 573 Comparative Example 3 Comparison lecture 3 1278 868 564 Comparative Example 4 Comparison lecture 4 1236 839 556 Comparative Example 5 Comparison lecture 5 1258 848 543 Comparative Example 6 Comparative lecture 6 1269 816 551 Comparative Example 7 Comparison lecture 7 1279 854 539 Comparative Example 8 Invention lecture 1 1303 836 641 Comparative Example 9 Invention lecture 2 1369 892 574 Comparative Example 10 Comparative lecture 8 1284 853 543

division The value of relation 1 Room temperature yield strength (MPa) Room temperature tensile strength
(MPa) Average pearlite fraction (%) Average grain size (㎛) CLR
(%) 160℃
High temperature
Tensile strength (MPa) Invention Example 1 38.39 547 606 0.2 12.1 0 585 Invention Example 2 35.44 537 617 0.6 7.8 0.12 576 Invention Example 3 66.67 541 611 2.4 5.7 0 592 Comparative Example 1 135.13 543 641 2.7 13.2 32.9 598 Comparative Example 2 93.46 553 636 1.3 8.7 19.8 588 Comparative Example 3 96.57 539 604 0.8 7.9 0 546 Comparative Example 4 46.72 548 618 1.9 6.6 24.3 591 Comparative Example 5 71.84 543 614 0.6 9.2 0.17 558 Comparative Example 6 12.88 554 629 0.3 9.1 17.6 597 Comparative Example 7 39.42 531 613 0.4 8.3 28.9 579 Comparative Example 8 38.39 528 598 7.6 11.6 26.6 588 Comparative Example 9 35.44 542 618 2.3 16.7 4.6 562 Comparative Example 10 18.06 474 554 0.9 9.6 0.43 506

As can be seen from the experimental results in Table 3, in the case of Invention Examples 1 to 3 that meet the alloy composition and manufacturing conditions of the present invention, not only are the room temperature yield strength and room temperature tensile strength excellent, but the CLR characteristics are also excellent, It was confirmed that hydrogen-induced cracking resistance was also excellent. In addition, it was confirmed that it was possible to secure a very high tensile strength of 570 MPa or more even in the high temperature range of 160°C, where the decline in tensile strength was particularly high.

On the other hand, in the case of Comparative Examples 1 to 10 that did not meet one or more of the alloy composition and manufacturing conditions of the present invention, it was confirmed that one or more properties among room temperature yield strength, room temperature tensile strength, CLR, and high temperature tensile strength at 160°C were inferior. did.

Meanwhile, Figures 1 and 2 of the present application show photographs of the microstructure of the above-described invention examples and comparative examples obtained from the present invention. Specifically, Figure 1a shows a photograph of the microstructure of a specimen obtained in Inventive Example 1 of the present invention using a scanning electron microscope (SEM). In addition, Figure 1b shows a photograph of the microstructure of a specimen obtained from Comparative Example 1 of the present invention using a scanning electron microscope (SEM), and it can be confirmed that central segregation was found in Comparative Example 1. In addition, Figure 1c shows a photograph of the microstructure of the specimen obtained in Comparative Example 8 of the present invention using a scanning electron microscope (SEM), and it was confirmed that the pearlite area ratio in Comparative Example 8 was 7.6%. Figure 1d shows a photograph of the microstructure of a specimen obtained from Comparative Example 9 of the present invention using a scanning electron microscope (SEM), and it was confirmed that the average grain size in Comparative Example 9 was 16.7㎛.

Figure 2 shows a photograph of the microstructure of the specimen obtained in Comparative Example 4 using a scanning electron microscope (SEM), and it was confirmed that coarse Nb precipitates were generated in Comparative Example 4.

In addition, Figure 3 shows the results of a high temperature tensile test at 160°C for invention steel 1 and comparative steel 3.

As described above, the detailed description of the present invention has described preferred embodiments of the present invention, but those skilled in the art can make various modifications without departing from the scope of the present invention. Of course this is possible. Therefore, the scope of rights of the present invention should not be limited to the described embodiments, but should be determined not only by the claims described later, but also by their equivalents.

Claims (5)

In weight percent, C: 0.02-0.05%, Si: 0.05-0.3%, Mn: 0.5-1.4%, P: 0.01% or less (excluding 0%), S: 0.001% or less (excluding 0%), Al : 0.02~0.05%, Nb: 0.06~0.1%, Ti: 0.005% or less (excluding 0%), N: 0.004~0.01%, Cr: 0.1~0.5%, Mo: 0.03~0.2%, Ca: 0.0015~ 0.003%, including the balance Fe and other unavoidable impurities;
As the microstructure, the base structure is one or more types selected from the group consisting of polygonal ferrite and acicular ferrite, and contains pearlite in an area percentage of 5% or less (excluding 0%),
The average grain size is 15㎛ or less (excluding 0㎛),
A hot rolled steel sheet that satisfies the following relational expression 1.
[Relationship 1]
35 ≤ (1000×[C]×[Nb]×[Mn] ² )/(3×[Cr]×[Mo]) ≤ 67
(In Equation 1 above, [C], [Nb], [Mn], [Cr], and [Mo] represent the weight percent content for each element in parentheses.)
According to claim 1,
A hot-rolled steel sheet containing 0.1 to 2.5% pearlite by area as a microstructure.
According to claim 1,
The average grain size is 5 to 13㎛, a hot rolled steel sheet.
In weight percent, C: 0.02-0.05%, Si: 0.05-0.3%, Mn: 0.5-1.4%, P: 0.01% or less (excluding 0%), S: 0.001% or less (excluding 0%), Al : 0.02~0.05%, Nb: 0.06~0.1%, Ti: 0.005% or less (excluding 0%), N: 0.004~0.01%, Cr: 0.1~0.5%, Mo: 0.03~0.2%, Ca: 0.0015~ Reheating the slab containing 0.003%, the balance Fe and other unavoidable impurities, and satisfying the following equation 1 at 1150-1350°C;
Obtaining a hot rolled steel sheet by performing final hot rolling on the reheated slab at an Ar3 temperature to a non-recrystallization temperature;
A cooling step of starting to cool the hot rolled steel sheet at a temperature of Ar3 to a non-recrystallization temperature and ending the cooling at 450 to 600°C; and
A method of manufacturing a hot rolled steel sheet, comprising: winding the cooled steel sheet.
[Relationship 1]
35 ≤ (1000×[C]×[Nb]×[Mn] ² )/(3×[Cr]×[Mo]) ≤ 67
(In Equation 1 above, [C], [Nb], [Mn], [Cr], and [Mo] represent the weight percent content for each element in parentheses.)
According to claim 4,
During the cooling, the average cooling rate is 20 to 60°C/s.