KR20040054198A - Method for manufacturing high-tensile steel sheets having excellent low temperature toughness - Google Patents

Abstract

PURPOSE: A method for manufacturing high tensile strength steel sheets having excellent toughness at low temperature by properly controlling constituents and heat treatment conditions of steel, thereby controlling micro-structure of steel is provided. CONSTITUTION: The method comprises a step of hot rolling the reheated steel slab to a desired thickness after reheating a steel slab comprising 0.04 to 0.10 wt.% of C, 0.05 to 0.5 wt.% of Si, 0.4 to 1.0 wt.% of Mn, 0.1 to 0.5 wt.% of Mo, 4.0 to 6.0 wt.% of Ni, 0.005 to 0.1 wt.% of Al, 0.015 wt.% or less of P, 0.005 wt.% or less of S and a balance of Fe and other inevitable impurities, wherein contents of the C, Mn, Mo and Ni satisfy the following relational expression: 0.7<=£C(%)+0.4Mn(%)+0.8Mo(%)+0.1Ni(%)|x(A/24)<=1.5, where A is cooling rate (deg.C/sec); a step of reheating the hot rolled steel sheet to a temperature of Ac3 to 950 deg.C, maintaining the reheated hot rolled steel sheet for 1.9T(thickness of steel sheet, mm) to 1.9T+50 minutes, and water cooling the heated steel sheet in a cooling rate of 10 to 50 deg.C/sec; a step of reheating the water cooled steel sheet to a temperature of 700 to 750 deg.C, maintaining the reheated steel sheet for 1.9T to 1.9T+50 minutes, and water cooling the heated steel sheet in a cooling rate of 10 to 50 deg.C/sec; and a step of tempering the water cooled steel sheet at a temperature of 550 to 640 deg.C for 1.9T to 1.9T+50 minutes, and cooling the tempered steel sheet in a cooling rate of 10 to 50 deg.C/sec.

Description

Method for manufacturing high-tensile steel sheets having excellent low temperature toughness

The present invention relates to a method of manufacturing a high tensile strength steel sheet used for storage containers such as LNG, ethylene, LPG, and more particularly, by controlling steel microstructures by appropriately adjusting steel components and heat treatment conditions, thereby having excellent toughness at low temperatures. It relates to a method for producing high tensile steel sheet.

Since liquefied gas such as LNG, ethylene, LPG, and the like has a very low storage temperature, the material used for the storage container requires not only good toughness but also good strength at the liquefied temperature of the stored gas. This requirement has been satisfied by adding Ni to the steel, and according to the code of the International Marine Organization (IMO), 9% of Ni is used as the steel for LNG storage vessels with a liquefaction temperature of about -164 ° C. The steel for ethylene storage vessels with a liquefaction temperature of -105 ° C is added with 5% of Ni. The steel for LPG storage containers with a liquefaction temperature of -50 ° C is added with 1.5% of Ni. It is suggested to use. That is, by increasing the content of Ni as the use temperature is lowered, it is possible to obtain excellent toughness even at a lower temperature. However, since Ni is very expensive and raises the price of steel, there is a problem in economy. Therefore, it is desired to provide a steel material exhibiting excellent toughness while lowering the Ni content.

Conventional techniques for improving low temperature toughness and strength while lowering the content of Ni include lowering impurity elements such as P and S that are harmful to toughness. P is an element that causes embrittlement by segregation at the old austenite grain boundary by roughening in a steel having a martensite structure, and the toughness decreases as the content thereof increases. In the current refining technology it is possible to lower the P content to 20ppm level, but there is a problem of the cost increase due to the operation of the refining equipment.

Another conventional technique for improving the low temperature toughness and strength while lowering the content of Ni is a method using direct quenching, and examples thereof include JP-A-3-229818 and JP-A-61-17885. Published Patent Application Publication No. 6-192729. The prior art provides a steel having a material that can be used as a material for LNG storage vessels, even though the Ni content is reduced to about 5% by microstructure control during rolling and texture control by direct quenching. However, the conventional technologies have a problem that productivity is lowered because low-temperature rolling is required for tissue control as well as the error rate decreases due to defects caused by plate deformation generated during direct quenching. In addition, in the prior art, Nb is added to improve the controllability. However, the Nb causes embrittlement by secondary hardening, so that the toughness is considerably lower than that of the steel containing 9% Ni.

Another conventional technique related to improving the low temperature toughness and strength while lowering the content of Ni is a method of adding Mo, and examples thereof include Japanese Patent Application Laid-Open No. 53-97917 and Japanese Patent Laid-Open No. 4-9861. , JP-A 4-371520, JP-A 6-184630, and JP-A 9-302445. Japanese Laid-Open Patent Publication Nos. 53-97917, 4-49861, 4-43720 and 6-184630 disclose that Ni is at least 7.0%, substantially for LNG storage containers. As for the addition and addition of 9% or more of Ni, it cannot be said that the steel is provided economically as compared to the steel containing 9% of Ni. In addition, the Patent Publication No. Hei 9-302445 provides a method of increasing the strength and toughness of the base material and improving the weldability by adding 0.02 to 0.08% of Mo, but when Ni is as low as 5%, Ni is 9 Compared with the steel containing%, strength and toughness are much lower, and there is a problem that is not suitable for use as a material for LNG storage containers.

The present invention is to solve the above problems of the prior art, by controlling the microstructure by adjusting the composition and heat treatment conditions of the steel, to provide a method for producing a high-tensile strength steel sheet having excellent low-temperature toughness and economical, the object is .

1 is a graph showing the change in martensite fraction according to the change in H value

2 is a schematic diagram showing a heat treatment process of the present invention.

The present invention for achieving the above object is by weight, C: 0.04-0.10%, Si: 0.05-0.5%, Mn: 0.4-1.0%, Mo: 0.1-0.5%, Ni: 4.0-6.0%, Al : 0.005 ~ 0.1%, P: 0.015% or less, S: 0.005% or less, remaining Fe and other inevitable impurities, the content of the C, Mn, Mo, Ni is the following relationship,

0.7 ≤ [C (%) + 0.4Mn (%) + 0.8Mo (%) + 0.1Ni (%)] × (A / 24) ≤ 1.5

(Where A is the cooling rate (° C / sec))

To satisfy the river,

Reheat and then hot rolled to the desired thickness,

Reheating the hot rolled steel sheet to A _c3 ~ 950 ℃ and then maintained for 1.9T (thickness of the steel sheet, mm) ~ 1.9T + 50 minutes and cooling at a cooling rate of 10 ~ 50 ℃ / seconds,

Reheating the water-cooled steel sheet to 700 ~ 750 ℃ and then maintained for 1.9T ~ 1.9T + 50 minutes and cooling at a cooling rate of 10 ~ 50 ℃ / second and

The water-cooled steel sheet is subjected to annealing at 1.9T-1.9T + 50 minutes at 550-640 ° C. and then cooled at a cooling rate of 10-50 ° C./sec.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

As described above, the IMO code recommends that the Ni content of the steel for storage containers be increased as the liquefaction temperature of the liquefied gas is lowered because Ni has an effect of improving the low temperature toughness and strength of the steel. Therefore, it is very important to find a metallurgical method that can replace the effect of improving the low temperature toughness and strength by Ni in order to secure economical efficiency by reducing Ni content together with excellent materials.

The present inventors have conducted in-depth studies and experiments on the effect of microstructures on strength and toughness and metallurgical methods for controlling microstructures in martensitic steels containing Ni. It has been found to be very effective in improving low temperature toughness. That is, by controlling the alloy structure after quenching by appropriately adjusting the alloying component to refine the effective grains, by controlling the two-phase inverse heat treatment and the soaking temperature is to maximize the effective grain refinement.

First, in order to sufficiently fine martensite effective grains, the microstructure must be mostly martensite in the quenched state. The elements that greatly affect the metamorphic structure after quenching are C, Mn, Mo, and Ni. In the present invention, it was found that the metamorphic structure after quenching was determined by the formula combined by Equation 1 below, rather than the individual content of these elements.

H = [C (%) + 0.4Mn (%) + 0.8Mo (%) + 0.1Ni (%)] × (A / 24)

Equation 1 is an empirical formula obtained by the inventors during the study, each component means a mass percentage, A means a cooling rate (° C./sec) determined by the thickness of the steel sheet and the refrigerant.

In the above-mentioned method, the austenite is finely contained in the martensite structure by heat-treating at an appropriate two-phase temperature range of A _c3 or less in the quenched state, followed by annealing at a temperature range of 550 to 640 ° C. Precipitation can make the effective crystal grains very fine. At this time, if the precipitated austenite is thermally unstable, it is transformed to martensite when exposed to low temperature, which is the liquefied gas use temperature. Therefore, austenite precipitated by heat treatment should be thermally very stable even at service temperature (-165 ° C for LNG and -105 ° C for ethylene). According to the present invention, it is possible to make austenite very stable even at low temperatures of -196 ° C or lower by appropriately adjusting the final soaking temperature while quenching the intermediate heat treatment at an appropriate temperature of two phases in the middle of the hardening treatment and the soaking treatment. do.

In addition, by cooling at a high cooling rate after completion of the final soaking treatment, soaking embrittlement by P can be suppressed to improve low-temperature toughness. This results in improving the low temperature toughness as well as the effect of miniaturizing the effective grains by controlling the alloy composition and controlling the heat treatment conditions. In addition, this has the effect of obtaining excellent low temperature toughness without making the P content extremely low.

First, look at the reasons for limiting the components of the present invention.

C: 0.04-0.10 wt%

When the C content is less than 0.04% by weight, carbides hardly precipitate during the soaking treatment, so that not only the strength is too low but also the hardenability is greatly reduced, so that the effective martensite grains are not sufficiently refined. Even if the effective crystal grains are effectively miniaturized, the toughness deterioration due to carbides becomes very large, and the content thereof is preferably limited to 0.04 to 0.10% by weight.

Si: 0.05-0.5 wt%

The Si is a component added for the deoxidation and solid solution strengthening, if less than 0.05% by weight is less secured deoxidation and strength, when added in excess of 0.5% by weight not only the weldability is reduced, but also containing Si called iron olivine Since oxides are generated to cause surface defects, the content is preferably limited to 0.05 to 0.5% by weight.

Mn: 0.4-1.0 wt%

When the content of Mn is less than 0.4% by weight, not only the strength is lowered by the decrease of the solid-solution strengthening effect, but also the hardenability decreases, so that the effective grains are not sufficiently refined. As the brittle embrittlement is intensified, not only the toughness is lowered but also the weldability is deteriorated, so that the content is preferably limited to 0.4 to 1.0% by weight.

Mo: 0.1-0.5 weight%

The Mo is a very important component in miniaturizing the effective grains by securing martensite structure, when the addition of less than 0.1% by weight can not obtain the effect, the addition of more than 0.5% by weight is effective in reducing the effective grain size and manufacturing cost Since problems of the rise of and poor weldability occur, it is preferable to limit the content to 0.1 to 0.5% by weight.

Ni: 4.0-6.0 wt%

The Ni is a key component of the present invention that serves to refine the effective grain by improving the hardenability, and by lowering the A _e1 temperature to allow austenite to precipitate during heat treatment and preferentially solid solution in the austenite precipitated austenite It improves the thermal stability of the knight, making the effective grain very fine. When the content of Ni is less than 4.0% by weight, the effective grain refining effect cannot be obtained. When the content of Ni is more than 6.0% by weight, the production cost is increased. Therefore, the content is preferably limited to 4.0 to 6.0% by weight.

Al: 0.005 to 0.1 wt%

The Al is a component added for deoxidation. If less than 0.005% by weight is added, the deoxidation effect cannot be obtained. If Al is added in excess of 0.1% by weight, the Al content not only impairs weldability but also increases inclusion content, thereby deteriorating toughness. Is preferably limited to 0.005 to 0.1% by weight.

P: 0.015% by weight or less

P, as an impurity, impairs toughness by causing rough embrittlement in steel having a martensite structure. The lower the P content is, the better the toughness is, but it causes a load in the refining process. In the case of applying the heat treatment conditions proposed in the present invention, when P is 0.015% by weight or less, the brittle embrittlement is not a problem, but when it exceeds this, the toughness is lowered by the brittle embrittlement, so the content thereof is 0.015% by weight. It is preferable to limit to the following.

S: 0.005 wt% or less

S is also an impurity like P, and if its content exceeds 0.005% by weight, the toughness is impaired by the increase of MnS inclusions, so the content is preferably limited to 0.005% by weight or less.

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

In order to achieve the object of the present invention, the range of the above components is also important, but as described above, the range of the H value of Equation 1 combined by C, Mn, Mo, Ni, and A (cooling rate) is most important. Do. When the H value of the following Equation 1 is less than 0.7, the martensite transformation does not occur sufficiently, so that the effective grains cannot be effectively refined, so that the low temperature toughness decreases. When the H value exceeds 1.5, the effective grain refinement effect is saturated and rather excessive strength. Since the low temperature toughness decreases and the weldability decreases due to the increase, the H value of Equation 1 is preferably limited to the range of 0.7 to 1.5.

[Equation 1]

H = [C (%) + 0.4Mn (%) + 0.8Mo (%) + 0.1Ni (%)] × (A / 24)

(Where A is the cooling rate (° C / sec))

In other words, when the value of Equation 1 is less than 0.7, the effective grain size is not sufficiently achieved by affecting the structure in the hardened state. This is well illustrated in FIG. 1. That is, Figure 1 is a result of measuring the degree of change of the martensite fraction in accordance with the change in the H value of the following equation 1 in the quenched state, the martensite fraction greatly increases by H value change until H is less than 0.7, but in more than 0.7 It can be seen that more than 95% is martensite, there is no significant change above.

There is no particular limitation in hot rolling the steel slabs formed as described above, and a steel sheet having a desired thickness may be manufactured by a conventional manufacturing method through reheating and hot rolling.

More preferably sufficient rolling property and excessive high temperature slab reheating temperature range in order to prevent oxidation, it is necessary to a temperature of 1000 ~ 1350 ℃ and terminate the hot rolling at the above A _r3 to the rolling property to attract and to the air-cooling It is preferable.

Thereafter, the heat treatment process of the present invention is carried out in three stages of quenching heat treatment, two-phase reverse quenching heat treatment, and sorrow heat treatment as shown in FIG.

[Annealing Heat Treatment]

After performing the conventional hot rolling and air cooling, reheating to the A _c3 ~ 950 ℃ then maintained 1.9T (the thickness of the steel plate, mm) ~ 1.9T + for 50 minutes and cooled to 10 ~ 50 ℃ / sec cooling rate. The hardening heat treatment aims to form austenite at high temperature and then to martensite structure by hardening, and consists of a process of reheating and cooling at high temperature. If the reheating temperature is less than A _c3, sufficient austenitization is not achieved. If the reheating temperature is higher than 950 ° C., the austenite grains become coarse, so the toughness is lowered. Therefore, the reheating temperature is preferably limited to A _c3 to 950 ° C. In addition, if the holding time after reheating is less than 1.9T minutes, sufficient austenitization is not achieved. If the holding time is more than 1.9T + 50 minutes, the austenite grains become coarse and the toughness is lowered, so the holding time is 1.9T to 1.9. It is preferred to limit to T + 50 minutes. In addition, it is impossible to obtain sufficient martensite structure when the cooling rate is less than 10 ° C / sec in water cooling after the austenite heat treatment, and when the cooling rate exceeds 50 ° C / sec, there is a fear that severe deformation may occur. It is desirable to limit to ˜50 ^o C / sec.

[2-phase quenching heat treatment]

The quenched heat-treated steel sheet is reheated to 700 to 750 ° C., which is a two-phase zone of A _c3 or less, and then maintained for 1.9T to 1.9 T + 50 minutes and cooled at a cooling rate of 10 to 50 ° C./sec. The purpose of the two-phase heat treatment is to make the austenite precipitated in the subsequent soaking process very thermally stable. For this purpose, after the appropriate amount of austenite is formed in advance during the two-phase heat treatment, it is transformed back into martensite. It is important. If the reheating temperature is less than 700 ° C., there is no two-phase reheating effect, and if the reheating temperature is more than 750 ° C., grains become coarse. Therefore, the reheating temperature is preferably limited to 700 to 750 ° C. In addition, if the holding time after reheating is less than 1.9T minutes, austenite is not sufficiently formed, and if the holding time exceeds 1.9T + 50 minutes, grains become coarse, and thus the holding time is limited to 1.9T to 1.9T + 50 minutes. It is preferable. In addition, if the cooling rate is less than 10 ° C / sec in water cooling after the two-phase heat treatment, austenite produced in the two-phase heat treatment is impossible to sufficiently convert to martensite, and if it exceeds 50 ° C / second, severe deformation may occur. Therefore, the cooling rate after the two-phase reverse heat treatment is preferably limited to 10 ~ 50 ℃ / sec.

[Sour heat treatment]

The steel sheet subjected to the two-phase quenching heat treatment was subjected to annealing at 550 to 640 ° C for 1.9T to 1.9T + 50 minutes and then cooled at a cooling rate of 10 to 50 ° C / sec. The heat treatment is a very important process of finally determining the material of the steel provided by the present invention. In general steels, the temperature range of heat treatment is preferred in terms of the combination of strength and toughness. However, in the present invention, the precipitation of stable austenite for effective grain refinement should be considered first with the viewpoint of such a combination of strength and toughness. In order to satisfy both of these conditions, it is preferable to limit the sour heat treatment temperature to 550 to 640 ° C. This is because when the sorption heat treatment temperature is less than 550 ° C., austenite is very hard to precipitate, and when the sorption heat treatment temperature exceeds 640 ° C., the austenite precipitated is not only thermally unstable, but also causes a great decrease in yield strength. In addition, when the soaking treatment time is less than 1.9T minutes, the soaking treatment is not sufficiently performed. If the soaking treatment time is more than 1.9T + 50 minutes, the soaking treatment may be excessively excessive and the strength may be reduced. It is desirable to limit to +50 minutes. The reason for the water cooling after the soaking treatment is to suppress the soaking embrittlement. The blurring embrittlement is a phenomenon in which toughness is lowered as the impurity element P moves to the old austenite grain boundary or martensite packet boundary and segregates during cooling after the soaking treatment. Rapid cooling after the soaking treatment prevents P from segregating to the former austenite grain boundary or martensite packet boundary, thereby suppressing soot embrittlement and obtaining excellent low temperature toughness even when the P content is high. When the cooling rate after the soaking treatment is less than 10 ℃ / sec, the fragility embrittlement due to the segregation of P is a problem, if exceeding 50 ℃ / second plate deformation may occur, the cooling rate is limited to 10 ~ 50 ℃ / second It is desirable to.

Hereinafter, the present invention will be described in more detail with reference to Examples.

Example 1

This embodiment is to find the tensile properties and low temperature toughness of the specimen according to the steel component.

The steel slab, as shown in Table 1 below, was heated to 1200 ° C. and then hot rolled into a steel plate having a thickness of 20 mm. After the rolled steel sheet was subjected to austenitization heat treatment at 900 ° C. for 58 minutes, water cooling was performed at the cooling rate of Table 1 below. Thereafter, two-phase reverse heat treatment was performed at 720 ° C. for 58 minutes, and then water cooling was performed at the cooling rate of Table 1 below. Then, after finally performing a heat treatment for 68 minutes at 580 ℃, it was water-cooled at the cooling rate of Table 1 below. Tensile tests and Charpy impact tests were performed on the steel sheets prepared as described above, and the results are shown in Table 2 below.

As shown in Table 2, the invention material (1 ~ 12) manufactured using the invention steel (A ~ L) satisfying the scope of the present invention is yield strength 590MPa, tensile strength 690MPa required for steel for LNG storage vessels And it can be seen that it has excellent properties compared to the impact toughness 100J at -196 ℃.

However, the comparative material (1) having a lower C content than the range of the present invention has a low strength, which is less than the yield strength of 590 Mpa and the tensile strength of 690 Mpa required for the LNG storage vessel steel, and the C content is higher than the present range. The ash (2) exhibits not only low toughness but also high strength compared to 100J, which is the impact toughness at minus 196 ^° C required by LNG storage vessel steels.

In addition, the comparative material (3) having a Mn content lower than the range of the present invention has both low strength and toughness, and the comparative material (4) having a Mn content higher than the range of the present invention has very high strength and toughness of 100 J or less. low.

In addition, the comparative material 5 having a Mo content lower than the range of the present invention had both low strength and toughness, and no more material improvement was observed for the comparative material 6 exceeding 0.5 wt%.

In addition, the comparative material (7) having a Ni content lower than the range of the present invention has a strength that is almost similar to that required by LNG storage vessel steels, but has a very low toughness.

In addition, the comparative material 8 having an impurity P content higher than the range of the present invention exhibits very low toughness of 100 J or less.

Finally, the comparative material 9 having a H value lower than the range of the present invention has a low toughness of 100 J or less, and the comparative material 10 having a H value exceeding the range of the present invention has a large increase in strength but rather toughness. It is greatly reduced to show 100 J or less.

Example 2

This embodiment is to determine the tensile properties and low temperature toughness of the specimen according to the heat treatment conditions.

In the above Table 1, the invention steel (A-B) corresponding to the component range of the present invention was heated to 1200 ° C., and then hot rolled into a steel plate having a thickness of 20 mm. Tensile test and Charpy impact test were performed after the rolled steel sheet was heat treated under the heat treatment conditions shown in Table 3 below, and the results are shown in Table 4 below.

As shown in Table 4, the invention materials 13 to 24 satisfying the heat treatment conditions of the present invention compared to the yield strength of 590MPa, tensile strength 690MPa and impact toughness 100J at -196 ℃ required for steel for LNG storage vessels It can be seen that it has excellent characteristics.

However, it can be seen that the comparative materials 11 to 30 in which the present invention does not satisfy the heat treatment conditions do not have the tensile properties and low temperature impact toughness required in the steel for LNG storage containers.

As described above, the present invention controls the microstructure by controlling the steel components and heat treatment conditions, thereby providing a high tensile strength steel sheet having excellent low temperature toughness, as well as reducing the manufacturing cost due to the reduction of the Ni content.

Claims (1)

By weight%, C: 0.04-0.10%, Si: 0.05-0.5%, Mn: 0.4-1.0%, Mo: 0.1-0.5%, Ni: 4.0-6.0%, Al: 0.005-0.1%, P: 0.015% Or less, S: 0.005% or less, remaining Fe and other inevitable impurities, the content of the C, Mn, Mo, Ni is the following relationship, 0.7 ≤ [C (%) + 0.4Mn (%) + 0.8Mo (%) + 0.1Ni (%)] × (A / 24) ≤ 1.5 (Where A is the cooling rate (° C / sec)) To satisfy the river, Reheat and then hot rolled to the desired thickness, Reheating the hot rolled steel sheet to A _c3 ~ 950 ℃ and then maintained for 1.9T (thickness of the steel sheet, mm) ~ 1.9T + 50 minutes and cooling at a cooling rate of 10 ~ 50 ℃ / seconds, Reheating the water-cooled steel sheet to 700 ~ 750 ℃ and then maintained for 1.9T ~ 1.9T + 50 minutes and cooling at a cooling rate of 10 ~ 50 ℃ / second and A method of producing a high tensile strength steel sheet having excellent low temperature toughness, which comprises cooling the water-cooled steel sheet at 550 to 640 ° C. for 1.9T to 1.9 T + 50 minutes and then cooling it at a cooling rate of 10 to 50 ° C./sec.