KR101091346B1 - High Tensile Steel with Excellent Low-Temperature Toughness, Manufacturing Method Thereof and Manufacturing Method of Deep Drawing Article - Google Patents

Abstract

The present invention, in weight%, C: 0.25-0.40%, Si: 0.15-0.40%, Mn: 0.4-1.0%, Al: 0.001-0.05%, Cr: 0.8-1.2%, Mo: 0.15-0.8%, P: 0.015% or less, S: 0.015% or less, Ca: 0.0005% to 0.002%, Nb: 0.05% or less, B: 0.0005% to 0.0025%, Ti: 0.005% to 0.025%, balance Fe and other unavoidable impurities, and include fine It provides a low-temperature pressure vessel steel sheet and a method for producing the same, comprising the Nb (C, N) precipitate in the tissue. The steel sheet of the present invention can secure both high strength and low temperature toughness by quenching / temperature holding treatment and tempering treatment for two times in order to facilitate the formation of Nb (C, N) precipitates.

The steel for deep drawing pressure vessel manufactured by the present invention can not only have a tensile strength of 1200MPa class, but also exhibit low temperature impact toughness of -50 ° C. of more than 100 Joules, and thus have wide application and very excellent physical properties.

Deep Drawing, Steel for Pressure Vessels, Low Temperature Impact Toughness, Tensile Strength, Precipitates

Description

High Tensile Steel with Excellent Low-Temperature Toughness, Manufacturing Method Thereof and Manufacturing Method of Deep Drawing Article}

The present invention relates to a steel material for a tensile strength 1200MPa low-temperature pressure vessel for deep drawing and a method for manufacturing the same, and more particularly, to deposit Nb carbonitride and add expensive Ni through two quenching treatments. The present invention relates to a low temperature pressure vessel steel material and a method of manufacturing the same, which ensure excellent low temperature toughness and are easy to use even in extreme places.

Conventionally, a method of manufacturing a cylinder for a pressure vessel by a spinning method using a seamless pipe to manufacture a steel plate for a low temperature pressure vessel having high tensile strength has been used. However, the cylinder produced in this way has a problem that the appearance of the joint is not only beautiful but also the physical properties of the joint portion may be degraded.

In particular, the conventional pressure vessel steel sheet manufactured to be used at low temperatures was often secured low temperature toughness by adding a large amount of Ni. However, the addition of Ni alone could not sufficiently secure the impact toughness in the extreme regions, and Ni is an expensive element, which is not preferable in terms of economical efficiency. Therefore, there is a need for a new concept of a low temperature pressure vessel steel plate that can replace the existing Ni-added steel.

The present invention is to solve the problems of the prior art described above by modifying the component system and heat treatment-cooling conditions without adding Ni to achieve a finer grain size and thereby improve the low temperature toughness tensile strength 1200MPa class low temperature pressure vessel To provide a molten steel, a manufacturing method and a deep drawing product.

The present invention, in weight%, C: 0.25-0.40%, Si: 0.15-0.40%, Mn: 0.4-1.0%, Al: 0.001-0.05%, Cr: 0.8-1.2%, Mo: 0.15-0.8%, P: 0.015% or less, S: 0.015% or less, Ca: 0.0005% to 0.002%, Nb: 0.05% or less, B: 0.0005% to 0.0025%, Ti: 0.005% to 0.025%, balance Fe and other unavoidable impurities, and include fine Provided is a steel sheet for low temperature pressure vessels containing Nb (C, N) precipitates in the tissue. In this case, the microstructure of the steel sheet may be formed of a composite structure of bainite and martensite.

Furthermore, the present invention provides a manufacturing method of manufacturing a steel sheet for low-temperature pressure vessel by reheating the steel slab of the composition at 1000 ~ 1250 ℃, the end of rolling at 750 ~ 1000 ℃, normalized heat treatment at a temperature of Ac1 ~ Ac3 after normalizing. . In this case, the spheroidization heat treatment of the heat treatment step may be performed for a time of 30 minutes or more.

Furthermore, the present invention, after re-heating the steel slab of the composition at 1000 ~ 1250 ℃, finish rolling at 750 ~ 1000 ℃, normalized heat treatment at the temperature of Ac1 ~ Ac3 after normalizing, after deep drawing treatment and temperature of 850 ~ 950 ℃ After maintaining the first quench, and then heated to 800 ~ 900 ℃ again, the temperature is maintained after the second quench, and finally provides a manufacturing method for manufacturing a deep drawing product by tempering at 550 ~ 625 ℃.

When the deep drawing container is manufactured from the steel of the present invention, it is possible to stably provide a steel sheet and a deep drawing product which can drastically improve low temperature toughness without adding expensive elements Ni or adding a separate facility.

In the component system of the present invention, the main content is not to add Ni, but instead to add Nb in an appropriate amount to induce precipitation of Nb carbonitride. Hereinafter, the component system of the present invention will be described in more detail.

C: 0.25-0.40%

In the present invention, C is limited to 0.25 to 0.40% as an element for securing strength. If the content of C is less than 0.25%, the strength on the matrix may be lowered, which is not preferable. However, if the content of C is more than 0.40%, there is a problem that the weldability is lowered and not suitable for use in a pressure vessel.

Si: 0.15 ~ 0.40%

Si adds 0.15% or more because it functions as a deoxidizer required in the steelmaking process and functions as a solid solution strengthening element. However, if the content exceeds 0.40%, the weldability is lowered and the oxide film may be severely formed on the surface of the steel sheet, so Si is added at 0.15 to 0.40%.

Mn: 0.4-1.0%

When Mn is excessively added as an element related to strength and toughness, MnS, which is a nonmetallic inclusion drawn with S, is formed to lower room temperature elongation and low temperature toughness, so Mn is controlled to 1.0% or less in the present invention. However, when Mn is less than 0.4% due to the component properties of the present invention, it is difficult to secure appropriate strength, so the amount of Mn added is limited to 0.4 to 1.0%.

Al: 0.001-0.05%

Al plays a deoxidation role in the steelmaking process like Si, so it adds 0.001% or more. However, if the content is added in excess of 0.05%, the effect is saturated, and the Al cost is limited to 0.001% to 0.05%, because the increase in manufacturing cost due to excessive addition is feared.

Cr: 0.8-1.2%

Cr is an alloying element for improving the hardenability and is added in the present invention at 0.8% or more. If the Cr content is less than 0.8%, the hardenability is difficult to secure the strength, whereas if the Cr content is more than 1.2%, the production cost is increased, so it is limited to 0.8 to 1.2%.

Mo: 0.15 ~ 0.8%

Mo is an alloying element effective for improving the hardenability and serves to prevent the occurrence of sulfide cracks. In addition, 0.15% or more is added since it is an element effective for securing strength by precipitation of fine carbide after hardening and annealing. However, Mo is also an expensive element, it is preferable to add in the range of 0.8% or less from the economical point of view.

P: 0.015% or less

Since P is an element that impairs low temperature toughness, it is better to manage it as low as possible, but to remove it excessively in the steelmaking process is expensive, so it is managed within 0.015% or less.

S: 0.015% or less

S is also an element that adversely affects low temperature toughness along with P, but like P, it may be excessively expensive to remove in steelmaking, so it is appropriate to manage it within 0.015%.

Ca: 0.0005-0.002%

Ca serves to suppress material anisotropy along the rolling direction even after rolling by spheroidizing the inclusions elongated in the rolling direction like MnS. To this end, Ca is added to 0.0005% or more. However, if the added amount is excessive, it reacts with O contained in the steel to generate CaO, which is a non-metallic inclusion, which is not good for physical properties, so the upper limit is set to 0.002%.

Nb: 0.05% or less

Nb is dissolved in austenite to increase the hardenability of austenite, and precipitates as Nb (C, N), which is matched with matrix (Matrix) together with Ti, to act as an important element to improve the strength and toughness of the steel. However, when Nb is added in an excessively large amount, it may appear as coarse precipitate in the playing step and may act as a site of brittle cracking, so the content of Nb in the present invention is limited to 0.05% or less.

B: 0.0005 ~ 0.0025%

B is a very important element in the present invention, and when B is added at 0.0005% or more, B is easy to obtain high strength properties by improving the hardenability. However, if the content exceeds 0.0025%, the effect does not increase any more and is saturated, so the content of B is controlled to 0.0005 to 0.0025%.

Ti: 0.005-0.025%

Since the appropriate amount of Ti can maximize the effect of B, it is necessary to precisely control Ti in the present invention. If the added amount of Ti is less than 0.005%, such an effect is insignificant, whereas if it exceeds 0.025%, the manufacturing cost may be increased, so the range of Ti is limited to 0.005 to 0.025%.

Hereinafter, the microstructure constituting the present invention will be described in more detail.

Microstructure: Complex structure of bainite-martensite

In the present invention, the normalizing heat treatment is applied in the steel sheet manufacturing step to configure the steel sheet microstructure in the form of a composite structure of bainite and martensite. Such a composite structure is a structure structure that can shorten the spheroidizing heat treatment time that takes a long time while obtaining the effect of increasing the strength from the martensite and bainite structure. This is because in the low temperature transformation tissues such as martensite, bainite, and pearlite, the smaller the carbide, the faster the spheroidization, and the faster the spheroidization time in the order of martensite> bainite> perlite. .

Precipitate Conditions: Nb (C, N) Precipitates

In the present invention, Nb (C, N) carbonitride may be formed as a precipitate to improve the strength of the steel. In particular, Nb (C, N) carbonitrides can be sufficiently produced by the second quenching during the manufacturing process of the present invention, it is possible to secure the strength by the precipitation strengthening effect.

Hereinafter, a method of manufacturing the steel of the present invention and conditions thereof will be described in more detail.

Reheating Temperature: 1000 ~ 1250 ℃

In the present invention, the slab having the composition described above is reheated at 1000 to 1250 ° C. If the reheating temperature is lower than 1050 ℃, it is difficult to solute the solute atoms, while if the reheating temperature exceeds 1250 ℃ austenite grain size becomes too coarse to reduce the properties of the steel sheet.

Rolling finish temperature: 750 ~ 1000 ℃

End of rolling in this invention shall be made at 750-1000 degreeC. If the end of rolling is lower than 750 ℃, the anisotropy of the material may occur due to excessive amount of unrecrystallized zone rolling, and the deep drawing property is lowered. On the other hand, when the end of rolling temperature exceeds 1000 ℃, grain growth continues due to high temperature. This is because physical properties of the steel may be degraded because grains are coarsened.

Normalizing and nodular heat treatment: Ac _One ~ Ac ₃ Spheroidal heat treatment at temperature of 30 minutes and above

In the present invention, a normalized heat treatment is applied to the steel sheet manufactured by rolling to form a microstructure of the composite structure of bainite and martensite. Normalizing conditions are not particularly limited, and such bainite-martensite composite structure can be obtained if the component system of the present invention and other manufacturing conditions are satisfied. When martensite and bainite structures are formed, the synergistic effect of strength can be obtained and the spheroidization heat treatment time can be shortened.

Subsequently, a spheroidization heat treatment is performed to give the proper workability required for deep drawing. In the present invention, by maintaining at a temperature of Ac ₁ ~ Ac ₃ 30 minutes or more, preferably 30 to 90 minutes to exhibit a tensile strength of 700MPa or less prior to deep drawing to facilitate deep drawing molding.

Cooling condition: 1st quench after maintaining at 850 ~ 950 ℃, 2nd quench at 800 ~ 900 ℃

As described above, after the deep drawing is performed by manufacturing the steel having excellent moldability, the tensile strength of 1200 MPa should be ensured again. To this end, in order to transform the internal structure of the steel into an austenite structure, it is quenched firstly after maintaining at 850 to 950 ° C for a predetermined time, preferably 1.6t + (10 to 30) minutes (t: mm of steel thickness). Is carried out. If the holding temperature (cooling start temperature) is lower than 850 ° C, it is difficult to secure the strength because the solid solute elements are difficult to re-use, whereas when the holding temperature is higher than 950 ° C, grain growth occurs to lower the low-temperature toughness.

In addition, in the present invention, the quenching is performed twice, and after the cooling is completed, the steel is again heated to 800 to 900 ° C., and then quenched after maintaining it for a certain time. Second water cooling is also preferably quenched after maintaining at 800 ~ 900 ℃ preferably 1.6t + (10 ~ 30) minutes.

The intermediate temperature holding time in each quenching step is an austenizing step for the austenizing treatment, if the time is kept too short, the austenizing effect is less, while if it is kept too long, the strength and toughness may be degraded. The time is controlled in the preferred range.

Tempering Condition: 550 ~ 625 ℃

Water-cooled steel can be brittle, so tempering at 550 ~ 625 ℃ to give adequate low-temperature toughness. If the tempering temperature is lower than 550 ° C., the effect of tempering is low, making it difficult to secure toughness. If the tempering temperature is higher than 625 ° C., the strength is difficult to secure.

That is, the manufacturing process of the present invention is completed by a so-called QQ'T cooling process consisting of primary cooling-secondary cooling-tempering, and increase in strength and tempering by Nb (C, N) precipitates formed in this process. It is possible to manufacture excellent low-temperature pressure vessel through securing the toughness by.

The steel for deep drawing pressure vessel manufactured by the above-described process can not only have tensile strength of 1200MPa grade, but also exhibit low temperature impact toughness of -50 ° C over 100 Joules, and thus have wide application and very excellent physical properties.

(Example)

After casting the steel slab including the composition of Table 1, each steel slab was rolled, cooled and heat-treated under the conditions of Table 2 to prepare the invention steel and comparative steel. For reference, as can be seen in Table 1, the comparative material was added Ni, the manufacturing cost was higher than the invention material.

C Mn Si P S Ni Cr Mo Ca Ti Nb Al B Invention
(A)
0.35
0.85
0.25
0.011
0.002
-
0.92
0.44
0.0016
0.015
0.015
0.0033
0.0010 Comparative material
(B)
0.36
0.80
0.26
0.008
0.003
0.48
1.01
0.52
0.0012
0.012
-
0.0028
0.0020

Steel grade
division Rolling finish
Temperature
(℃) 1st quench
Temperature
(℃) 2nd quench
Temperature
(℃) Tempering
Temperature
(℃) Average
AGS **
(Μm) Inventive Steel 1 A 870 885 875 565 10.8 Inventive Steel 2 A 880 890 850 550 11.5 Inventive Steel 3 A 905 895 855 565 12.0 Comparative Steel 1 B 900 890 - 570 24.7 Comparative Steel 2 B 875 885 - 575 23.4 Comparative Steel 3 B 860 900 - 550 28.5

** AGS: Austenite Grain Size

Tensile strength, elongation, and impact toughness at −50 ° C. were measured for the invention thus prepared, and the results are shown in Table 3 below.

Steel grade
division The tensile strength
(MPa) Elongation
(%) -50 ℃
Impact Toughness (J) Inventive Steel 1 A 1212 16 115 Inventive Steel 2 A 1217 15 105 Inventive Steel 3 A 1215 16 117 Comparative Steel 1 B 1203 16 61 Comparative Steel 2 B 1205 15 65 Comparative Steel 3 B 1218 14 61

As can be seen in Table 3, Comparative steels 1 to 3 manufactured using Comparative Material B are relatively excellent in low temperature toughness of 60J or more because Ni contains, but Inventive Steel 1 to 1 using Inventive Material A of the present invention. In Fig. 3, the low-temperature toughness was significantly improved while the tensile strength and elongation were not inferior to those of the comparative steel. Through this, it is expected that the use of the steel in accordance with the conditions of the present invention can be used in a low temperature environment, including extreme cold and stable pressure vessels.

Claims (9)

By weight%, C: 0.25-0.40%, Si: 0.15-0.40%, Mn: 0.4-1.0%, Al: 0.001-0.05%, Cr: 0.8-1.2%, Mo: 0.15-0.8%, P: 0.015% S: 0.015% or less, Ca: 0.0005% to 0.002%, Nb: 0.05% or less (excluding 0%), B: 0.0005% to 0.0025%, Ti: 0.005% to 0.025%, balance Fe, and other unavoidable impurities , Steel sheet for low-temperature pressure vessel, characterized in that it comprises Nb (C, N) precipitate in the microstructure. The steel sheet for a low-temperature pressure vessel according to claim 1, wherein the microstructure is composed of a composite structure of bainite and martensite. By weight%, C: 0.25-0.40%, Si: 0.15-0.40%, Mn: 0.4-1.0%, Al: 0.001-0.05%, Cr: 0.8-1.2%, Mo: 0.15-0.8%, P: 0.015% S: 0.015% or less, Ca: 0.0005% to 0.002%, Nb: 0.05% or less (excluding 0%), B: 0.0005% to 0.0025%, Ti: 0.005% to 0.025%, balance Fe and other unavoidable impurities About the river slab, Reheating step to reheat at 1000 ~ 1250 ℃; A rolling step of finishing rolling at 750 to 1000 ° C; And Heat treatment step of spheroidizing heat treatment at the temperature of A _c1 ~ A _c3 after normalizing Method for manufacturing a steel sheet for low temperature pressure vessel comprising a. The method of claim 3, wherein the spheroidizing heat treatment of the heat treatment step is performed for at least 30 minutes. By weight%, C: 0.25-0.40%, Si: 0.15-0.40%, Mn: 0.4-1.0%, Al: 0.001-0.05%, Cr: 0.8-1.2%, Mo: 0.15-0.8%, P: 0.015% S: 0.015% or less, Ca: 0.0005-0.002%, B: 0.0005-0.0020%, Ti: 0.005-0.025%, Nb: 0.05% or less (excluding 0%), balance Fe and other unavoidable impurities About the river slab, Reheating step to reheat at 1000 ~ 1250 ℃; A rolling step of finishing rolling at 750 to 1000 ° C; A heat treatment step of spheroidizing heat treatment at a temperature of A _c1 to A _c3 after normalizing; A first quenching step of quenching first after maintaining a temperature of 850 to 950 ° C. after the deep drawing process; A second quenching step of quenching secondly after maintaining the temperature after the first cooling to 800 to 900 ° C; And Tempering step of tempering at 550 ~ 625 ℃ Method for producing a deep drawing product comprising a. The method of claim 5, wherein the spheroidizing heat treatment of the heat treatment step is performed for at least 30 minutes. The method of claim 5, wherein the first quenching step comprises an austenizing step that is maintained for 1.6t + (10-30 minutes) (t: mm in the thickness of the steel) at a temperature of 850 ~ 950 ℃ Method for producing a deep drawing product. The method of claim 5, wherein the second quenching step comprises an austenizing step maintained for 1.6t + (10-30 minutes) (t: mm in the thickness of the steel) at a temperature of 800 ~ 900 ℃ Method for producing a deep drawing product. The method of claim 5, wherein the deep drawing product has a tensile strength of at least 1200 MPa and a low temperature impact toughness of -50 ° C of at least 100 Joules.