KR101140941B1 - Steel sheet having excellent hydrogen delayed fracture resistance quality and ultra-high strength, and method for producing the same - Google Patents

Abstract

The present invention is C: 0.12 ~ 0.25 wt%, Si: 0.10 ~ 0.65wt%, Mn: 0.8 ~ 3.0wt%, Al: 0.01 ~ 1.00wt%, P: more than 0 0.02wt% or less, S: more than 0 0.001wt % Or less, Cu: 0.2 to 0.6 wt%, Ti: 0.01 to 0.10 wt%, Nb: 0.01 to 0.10 wt%, V: 0.01 to 0.10 wt% in steel having alloy composition of balance Fe and other unavoidable impurities Reheating the steel slab further comprising at least one of Mo: 0.1 to 0.5 wt% and Cr: 0.1 to 0.5 wt%, and hot rolling at a temperature below the austenite recrystallization temperature and above the austenite / ferrite transformation temperature (Ar3). Manufacturing a hot rolled steel sheet, cooling the hot rolled steel sheet to a predetermined winding temperature at a predetermined cooling rate, and winding the cooled hot rolled steel sheet at the predetermined winding temperature to produce a hot rolled coil.

According to the present invention, by minimizing the content of S in the steel sheet to reduce the brittleness of the material has an advantage of producing an ultra-high strength steel sheet having a tensile strength of 1300 Mpa or more and an elongation of 8% or more and improved hydrogen delayed fracture resistance There is this.

AOD, Hot Rolled

Description

Steel sheet having excellent hydrogen delayed fracture resistance quality and ultra-high strength, and method for producing the same

The present invention relates to an ultra-high strength steel sheet and a method of manufacturing the same, which are excellent in hydrogen delayed fracture resistance. It relates to a manufacturing method.

The bumper reinforcement or shock absorber in the door is a part directly related to passenger safety in the event of a vehicle collision. The super high strength hot rolled steel sheet with a tensile strength of 780 MPa or more is mainly used, and it has to have high elongation and high elongation. In addition, the use of high strength steels of higher strength parts is increasing in order to increase fuel efficiency in order to cope with increasingly severe environmental pollution regulations. Recently, research on commercialization of high strength steels of 780 MPa or more is increasing.

The manufacturing process of the super-high strength steel sheet is divided into reheating to re-use the components of the largely manufactured ingot, hot rolling rolling to the final thickness, cooling process to cool and wind the hot rolled sheet material, manufactured through the above process In the case of steel plates for automobile body collision members among the steel sheets, it is necessary to limit the content because the high S content shows brittleness in the structure and causes a defect of the material. In particular, it is composed of low temperature transformation structure such as bainite or martensite. In steel grades, it is necessary to restrict the content more strictly.

Meanwhile, steel sheets developed for automobiles require high strength, hydrogen delayed fracture resistance and low temperature toughness. Hydrogen delayed fracture is a form of hydrogen embrittlement and refers to a phenomenon in which fracture occurs at a lower strength than that at which fracture occurs. In other words, the material is destroyed when a long time stress is applied at room temperature even at a very small stress compared to the failure strength obtained by the load test.

In general, the average concentration of hydrogen present in the metal is not high enough to cause brittleness. However, in the case of steel sheet, hydrogen delayed destruction occurs frequently because water is used in the process of forming a press and quenching.

The principle of hydrogen delayed destruction is shown in FIG. 1.

That is, when the steel sheet is exposed to the moisture environment, the moisture generates a reduction reaction of 2H ⁺ + 2e ------ → H ₂ by the movement of electrons emitted from the base metal Fe. Hydrogen (H2) produced by the reduction reaction diffuses into the base metal at high speed through grain boundaries even at low temperatures, and weakens the grain boundary bonding force.

In addition, when hydrogen meets a sulfide interposed in the steel, the grain boundary bonding force is further weakened and becomes the starting point of the crack. As a result, sudden breakdown occurs over time.

The higher the strength of the steel, the more susceptible to embrittlement of hydrogen, the more frequent it is in high-strength steels, and it is almost unpredictable until the apparent signs of failure are reached.

Therefore, it is necessary to improve the chemical composition and microstructure to prevent hydrogen delayed destruction, which requires a separate manufacturing process, there is a problem that the manufacturing cost increases.

The present invention is to solve the conventional problems as described above, an object of the present invention is to control the content of S in the steel sheet to improve the tensile strength and elongation, ultra-high strength steel sheet excellent in hydrogen delayed fracture resistance and a method of manufacturing the same To provide.

According to the characteristics of the present invention for achieving the above object, the ultra-high strength steel sheet excellent in hydrogen delayed fracture resistance of the present invention is C: 0.12 ~ 0.25 wt%, Si: 0.10 ~ 0.65wt%, Mn: 0.8 ~ 3.0wt% , Al: 0.01 to 1.00 wt%, P: more than 0 and 0.02 wt% or less, S: more than 0 and 0.001 wt% or less, Cu: 0.2 to 0.6 wt%, and has an alloy composition of the balance Fe and other unavoidable impurities , Ti: 0.01 ~ 0.10wt%, Nb: 0.01 ~ 0.10wt%, V: 0.01 ~ 0.10wt%, Mo: 0.1 ~ 0.5wt%, Cr: at least one of 0.1 ~ 0.5wt%.

According to another aspect of the present invention, the method for producing an ultra high strength steel sheet having excellent hydrogen delaying resistance of the present invention is C: 0.12 to 0.25 wt%, Si: 0.10 to 0.65 wt%, Mn: 0.8 to 3.0 wt%, Al: 0.01 to 1.00 wt%, P: more than 0 and 0.02 wt% or less, S: more than 0 and 0.001 wt% or less, Cu: 0.2 to 0.6 wt% and having a alloy composition of the balance Fe and other unavoidable impurities, Ti: Reheat the steel slab further comprising at least one of 0.01 to 0.10 wt%, Nb: 0.01 to 0.10 wt%, V: 0.01 to 0.10 wt%, Mo: 0.1 to 0.5 wt%, and Cr: 0.1 to 0.5 wt%, Manufacturing a hot rolled steel sheet by hot rolling at an austenite recrystallization temperature of less than or equal to an austenite / ferrite transformation temperature (Ar ₃ ), cooling the hot rolled steel sheet to a predetermined winding temperature at a predetermined cooling rate, and cooling the And winding a hot rolled steel sheet at the predetermined winding temperature to produce a hot rolled coil.

In addition, the predetermined winding temperature is 200 ~ 400 ℃, the predetermined cooling rate may be 30 ~ 100 ℃ / sec.

The present invention has the advantage of producing an ultra-high strength steel sheet having a tensile strength of 1300 Mpa or more and an elongation of 8% or more by minimizing the content of S in the steel sheet to reduce brittleness of the material.

When the steel sheet is used for the body parts of an automobile, the rigidity of the parts is improved, and when the impact member is used for a collision member, the deformation of the body is greatly reduced, thereby securing a space for passengers, and the weight reduction can be achieved. .

In addition, the present invention can improve the hardenability of the steel sheet and stabilize the strength after quenching through the addition of inexpensive Cu and controlling the content of S, and also prevents the intrusion of hydrogen in the sulfide cathode reaction and grain boundary in the steel or the weld zone. It has a suppressing effect. Therefore, it is possible to prevent the hydrogen delayed fracture in the steel or welded portion, there is an advantage that can produce a steel sheet with improved resistance to hydrogen delayed fracture without increasing the manufacturing process and cost increase.

Hereinafter, the preferred embodiment of the ultra-high strength steel sheet excellent in the hydrogen delayed fracture resistance according to the present invention and a manufacturing method thereof will be described in detail.

The steel sheet of the present invention is C: 0.12 ~ 0.25 wt%, Si: 0.10 ~ 0.65wt%, Mn: 0.8 ~ 3.0wt%, Al: 0.01 ~ 1.00wt%, P: more than 0 0.02wt% or less, S: more than 0 Ti: 0.01 to 0.10 wt%, Nb: 0.01 to 0.10 wt%, V: 0.01 to 0.10 wt%, Mo: 0.1 to 0.5 in a steel containing 0.001 wt% or less and having an alloy composition of the balance Fe and other unavoidable impurities wt%, Cr: 0.1 to 0.5wt% at least one more.

In the present invention, by limiting the alloy composition as described above, the tensile strength and elongation of the steel sheet can be improved, and hydrogen delayed fracture of the steel sheet can be prevented.

The function and content of the alloying elements of the present invention are as follows.

C: 0.12 ~ 0.25wt%

C is an indispensable element for imparting high strength to the steel sheet, and is a major element for increasing the hardenability of the steel sheet and determining the strength after the hardening.

Depending on the content of C and the method of manufacture, it may become a solid solution carbon in the material structure, and carbides may be formed by combining with elements having a very high property of binding to C. If the content of C is less than 0.12 wt%, there is a problem that the strength is lowered, and if the content of C is more than 0.25 wt%, there is a problem of deterioration of weldability, so the content is limited to the range of 0.12 to 0.25 wt%.

Si: 0.10 ~ 0.65wt%

Si is a substituted solid solution element. When the material is heated to a melting temperature or more and quenched, Si maintains the solid solution structure at room temperature as it is, thereby improving the strength of the material. If the content of Si is less than 0.10 wt%, the solid solution effect is reduced and the strength is lessened. If the content of Si is more than 0.65 wt%, defects are caused during welding, so the content is limited to the range of 0.10 to 0.65 wt%.

Mn: 0.8 ~ 3.0wt%

Mn is a substitutional solid solution, which has a very high solid solution strengthening effect, greatly improving the mechanical properties of the material. Mn in the material has an effect of increasing the strength of the steel sheet due to entering into the matrix of the structure to settle as a substituted solid solution. However, if the content is less than 0.8wt%, very fast cooling rate is required, so it is not industrially possible to prevent the production of pearlite, and if it exceeds 3.0wt%, the workability and weldability of steel are inhibited, so the content is limited to the range of 0.8 ~ 3.0wt%. .

Al: 0.01 ~ 1.00wt%

Al is added to the steelmaking process as a deoxidizer for removing oxygen in the steel.

Al has the effect of raising the hardenability of a steel plate and improving stabilization of the strength after hardening. However, if the content is less than 0.01wt%, the effect is insignificant, and if the content exceeds 1.00wt%, the cost increases and the surface defect occurrence rate is increased, so the content is limited to the range of 0.01 to 1.00wt%.

P: more than 0 and less than 0.02wt%

In the steel sheet of the present invention, P is an impurity, when the content exceeds 0.02 wt%, the weldability is lowered and work brittleness is caused, so the content is limited to 0.02 wt% or less.

S: more than 0 and less than 0.001wt%

S has the effect of increasing the hardenability of the steel sheet to increase the stabilization of strength after quenching, but exhibits brittleness in the structure of the material, causing defects in the material, and therefore the content is strictly limited to 0.001 wt% or less.

Cu: 0.2 ~ 0.6wt%

Cu is added to prevent cathodic reaction and hydrogen delayed destruction of sulfide inclusions. If Cu is added below 0.2wt%, it is difficult to expect the effect of improving hydrogen delayed breakdown. If it exceeds 0.6wt%, Cu penetrates into grain boundary when slab reheats and causes cracks during hot working. It is limited to wt%.

Figure 2 is a schematic diagram showing the principle of improving the hydrogen delayed fracture resistance Cu in the ultra-high strength steel sheet excellent in hydrogen delayed fracture resistance according to the present invention.

As shown in FIG. 2, it can be seen that Cu is distributed in non-uniform portions of tissues such as inclusions or grain boundaries to prevent diffusion of hydrogen through grain boundaries and prevent hydrogen from encountering sulfides.

Accordingly, the present invention is to limit the content of Cu as described above to increase the hardenability of the steel sheet and to stabilize the strength after quenching, and to suppress the intrusion of hydrogen in the sulfide cathode reaction and grain boundaries in the steel or welded portion Will have an effect.

Ti: 0.01 ~ 0.10wt%, Nb: 0.01 ~ 0.10wt%, V: 0.01 ~ 0.10wt%

Ti improves the toughness of the steel by miniaturizing austenite grain size by TiN precipitates, compensating for the drop in toughness by increasing the strength by V (C, N), and increasing the high temperature strength. Usually, it is known that content is sufficient as 0.10 wt% or less. If the Ti content is less than 0.01 wt%, it is difficult to obtain the austenite fine-graining effect and precipitation strengthening by TiN. Therefore, the Ti content is 0.01 to 0.10 wt%.

Nb is an element that does not need to be added, but may be added to obtain better high temperature strength, but when added, 0.01 to 0.10 wt% is added. In other words, Nb is an element that improves high-temperature strength and toughness by inhibiting austenite grain growth and increasing quenchability during hot rolling, and is present as fine carbide in steel to improve high-temperature strength. If the content exceeds 0.10wt%, the weldability is impaired, so the content is 0.01 to 0.10wt%.

V is an element that precipitates as fine carbonitrides in steel to increase room temperature strength and high temperature strength. V is mostly dissolved at less than 0.01wt% and does not contribute to increase of room temperature and high temperature strength. Since it falls remarkably, content is made into 0.01 to 0.10 wt%.

Mo: 0.1-0.5wt%, Cr: 0.1-0.5wt%

Mo is present in steel in the form of fine carbides, which has the effect of increasing the high temperature strength, and is effective in improving the room temperature strength by improving the hardenability. If the added amount is less than 0.1wt%, it is difficult to obtain the required high temperature strength, and if it exceeds 0.5wt%, the weldability is deteriorated, so that the content is limited to 0.1 to 0.5wt% in consideration of securing the appropriate high temperature strength and weldability.

Cr is an effective element for improving the hardenability and being present in the form of fine carbides to increase room temperature and high temperature strength. If the addition amount is less than 0.1wt%, the effect is insignificant, and if the addition amount is more than 0.5wt%, the weldability is impaired, so the addition amount is limited to 0.1 to 0.5wt%.

In the present invention, in consideration of similar effects of Ti, Nb, V, Mo, and Cr as described above, at least one of Ti, Nb, V, Mo, and Cr having the above contents is included.

The steel sheet of the present invention contains the above components, and the rest are substantially Fe and unavoidable elements, and the incorporation of minute amounts of unavoidable impurities is also allowed as elements contained depending on the situation of raw materials, materials, manufacturing facilities, and the like.

Figure 3 is a schematic diagram showing a desulfurization process in the manufacturing process of the ultra-high strength steel sheet according to an embodiment of the present invention.

Referring to FIG. 3, the control to reduce the content of S to 0.001 wt% or less is performed through decarburization, deoxidation, and desulfurization in an AOD (Argon Oxygen Decarburization) refining furnace after electric tapping. It consists of removing S in molten steel by the reduction reaction using slag.

Of the impure elements, S is inevitably mixed from raw materials and can be removed in an AOD (Argon Oxygen Decarburization) or VOD (Vacuum Oxygen Decarburization) process, but AOD having excellent slag / molten steel stirring power is more effective for removing S, Remove S from AOD.

Referring to FIG. 3, S contained in the molten stainless steel melted and tapped in an electric furnace is removed by CaO-SiO ₂ -CaF ₂ -MgO-based slag formed by adding auxiliary materials such as quicklime and fluorite in an AOD refining furnace. . This is also called desulfurization operation by single slag method. At this time, in order to obtain molten steel containing an extremely low level of S, which is 0.001 wt% or less, a large slag volume is maintained while maintaining a high ratio of CaO and SiO _{2 in} the slag [(% CaO) / (% SiO ₂ )], that is, slag basicity. You can use the maintenance method.

In addition, the amount of slag that can absorb S from molten steel, that is, the amount of sulfur (S) absorbed into the slag (saturation solubility) is limited. by calcium oxide, calcium fluoride to be removed by tilting the furnace body portion after the molten steel temperature rises purpose blown oxygen gas and newly added with SiO ₂ formed by the oxidation of [Si] in the molten steel with the slag to the CaO-SiO ₂ -CaF ₂ - MgO slag is also formed.

The concentration of S in the newly formed refinery slag is lower than the concentration of S in the slag before the furnace is inclined to be removed, so that S in the molten steel can be additionally absorbed. This is called desulfurization operation by double slag method.

The desulfurization operation such as the single slag method or the double slag method as described above is merely an example for desulfurization, but the desulfurization operation of the present invention is not limited thereto.

Through the above-mentioned desulfurization process, the content of S can be minimized. In addition, inclusions of molten steel transferred to the casting ladles by controlling the temperature after the desulfurization process and the molten steel temperature during refining of the casting ladle and controlling the casting temperature. It is also possible to minimize the content of S by minimizing the number.

Through the above process, it is possible to minimize the content of S and improve the cleanliness of the steel, thereby minimizing the content of S in the steel sheet to minimize the occurrence of non-metallic inclusions, and can significantly improve the brittleness of the steel sheet. .

The slabs having the composition as described above are obtained through ingot or continuous casting process after obtaining molten steel through steelmaking process, where reheating, hot rolling to final thickness, and cooling / winding of hot rolled sheet are performed. Going through.

Detailed description of each process is as follows.

The slab having the above composition is reheated at 1100 to 1300 ° C. in order to reclaim segregated components during casting, and rough rolling is carried out above the austenite recrystallization temperature.

If the homogenization treatment temperature is less than 1100 ° C, segregated components are not reusable. If the homogenization temperature is higher than 1300 ° C, the austenite grain size increases and the ferrite grain size is coarsened, resulting in a decrease in strength.

Thereafter, the hot rolling is finished at a temperature below the austenite recrystallization temperature and above the austenite / ferrite transformation temperature (Ar ₃ ).

After finishing hot rolling, it cools to predetermined winding temperature by predetermined cooling rate, ie, 30-100 degreeC / sec.

If the cooling rate after the hot rolling is less than 30 ℃ / sec is a large amount of precipitation of ferrite and pearlite is made it becomes impossible to obtain a mixed structure of hot rolled bainite, bainite and martensite or martensite structure. In addition, in order to obtain a cooling rate of more than 100 ℃ / sec requires a new equipment, such as a pressurized rapid cooling equipment rather than the conventional method is a cause of cost increase.

The predetermined winding temperature is 200 to 400 ° C. If the coiling temperature is less than 200 ℃ may cause cracks due to the transformation of martensite during the winding, there is a problem that the shape of the hot-rolled steel sheet is bad. If it exceeds 400 ℃ coarse pearlite is formed on the hot-rolled steel sheet to obtain a low-temperature structure, it is difficult to secure high strength. In practice, the winding temperature largely depends on the performance of the winder.

The microstructure of the applied steel sheet of the present invention is composed of martensite or bainite single phase, or has two-phase martensite and bainite or three-phase structure of martensite, bainite and ferrite.

That is, in the steel sheet of the present invention, martensite having excellent strength or bainite single phase having a little lower strength may exist in the metal structure, or martensite and bainite phase may coexist at the same time. In this case, a mechanism having excellent mechanical properties and molding processability to some extent is achieved by maintaining an appropriate fraction. In addition, the three-phase structure of martensite, bainite, and ferrite may be used to maintain an appropriate fraction of each tissue.

For example, in the case of having a three-phase structure of martensite, bainite, and ferrite, the phase fraction of the martensite structure is 50 to 70 vol%, the phase fraction of the bainite is 15 to 40 vol%, and the phase ratio of the ferrite. It can be made of 5 ~ 15Vol%, this structure is determined by the alloying composition and the cooling pattern. Here, the above-described phase fraction is merely an example and may have other phase fractions in order to satisfy mechanical properties and molding processability.

The steel sheet wound as described above is then manufactured in various component shapes through a process such as roll forming or hot pressing.

Table 1 shows the component ratios of the Examples and Comparative Examples of the present invention, Table 2 shows the specimen of the steel sheet wound by winding the steel slab having the composition of Table 1 after the hot rolling to the winding temperature of 200 ℃ or 350 ℃ The mechanical properties of the specimens were taken from the sample, showing TS (MPa) for tensile strength, YS (MPa) for yield strength, and EL (%) for elongation.

(Alloy component [wt%], balance Fe) division C Mn Si P S Al N Cu Ti Nb Mo Cr Example 1 0.18 1.80 0.50 0.015 0.001 or less 0.040 0.006 0.2 - - - 0.5 Example 2 0.15 1.80 0.50 0.015 0.001 or less 0.040 0.006 0.2 0.02 0.03 - - Example 3 0.18 1.80 0.50 0.015 0.001 or less 0.040 0.006 0.2 - - 0.5 - Comparative Example 1 0.18 1.80 0.50 0.015 0.005 0.040 0.006 0.2 - - - 0.5 Comparative Example 2 0.15 1.80 0.50 0.015 0.005 0.040 0.006 0.2 0.02 0.03 - - Comparative Example 3 0.18 1.80 0.50 0.015 0.005 0.040 0.006 0.2 - - 0.5 -

division
Mechanical property (when coiling temperature is 200 ℃)
Mechanical property (when winding temperature is 350 ℃)
TS (Mpa) YP (Mpa) EI (%) TS (Mpa) YP (Mpa) EI (%) Example 1 1385 901 10.2 1285 986 8.7 Example 2 1377 935 8.4 1322 1005 8.2 Example 3 1454 945 9.7 1349 1017 8.3 Comparative Example 1 1220 825 9.3 987 724 9.6 Comparative Example 2 1189 805 8.1 1016 649 10.8 Comparative Example 3 1361 909 8.7 1285 986 9.7

Referring to Table 1, Example 1 and Comparative Example 1, Example 2 and Comparative Example 2, Example 3 and Comparative Example 3 were set to the same content of the other elements and only the content of S was different.

In this case, referring to Table 2, it can be seen that the tensile strength and elongation characteristics of the embodiments of the present invention in which the content of S is 0.001 wt% or less are significantly improved compared to the comparative examples in which the content of S is 0.005 wt%.

Therefore, as shown in Table 2, by limiting the content of S to 0.001wt% or less, excellent tensile strength and elongation characteristics can be secured.

From the above results, it can be confirmed that the ultrahigh strength steel sheet manufactured according to the present invention has improved tensile strength and elongation. The mechanical properties are very good and the elongation is more than 8%. When used in the body parts of automobiles, the parts stiffness is improved, and when used in collision members, the body deformation due to external shocks can be greatly reduced, thereby ensuring the passenger's survival space. In addition, weight reduction can be achieved.

Although the present invention has been described with reference to the embodiments shown in the drawings, this is merely exemplary, and it will be understood by those skilled in the art that various modifications and equivalent other embodiments are possible. Therefore, the true technical protection scope of the present invention will be defined by the technical spirit of the appended claims.

1 is a schematic diagram showing the appearance of hydrogen delayed fracture by the moisture attached to the steel sheet surface in the Cu-free steel.

Figure 2 is a schematic diagram showing the principle of improving the hydrogen delayed fracture resistance Cu in the ultra-high strength steel sheet excellent in hydrogen delayed fracture resistance according to the present invention.

Figure 3 is a schematic diagram showing a desulfurization process in the manufacturing process of the ultra-high strength steel sheet according to an embodiment of the present invention.

Claims (3)

C: 0.12 to 0.25 wt%, Si: 0.10 to 0.65 wt%, Mn: 0.8 to 3.0 wt%, Al: 0.01 to 1.00 wt%, P: more than 0 and 0.02 wt% or less, S: more than 0 and 0.001 wt% or less, Cu: 0.2 to 0.6 wt%, Ti: 0.01 to 0.10 wt%, Nb: 0.01 to 0.10 wt%, V: 0.01 to 0.10 wt%, Mo: to steel having alloy composition of balance Fe and other unavoidable impurities 0.1-0.5wt%, Cr: further comprises at least one of 0.1-0.5wt%, Ultra high strength steel sheet having excellent hydrogen delayed fracture resistance, characterized in that the tensile strength is 1300Mpa or more. C: 0.12 to 0.25 wt%, Si: 0.10 to 0.65 wt%, Mn: 0.8 to 3.0 wt%, Al: 0.01 to 1.00 wt%, P: more than 0 and 0.02 wt% or less, S: more than 0 and 0.001 wt% or less, Cu: 0.2 to 0.6 wt%, Ti: 0.01 to 0.10 wt%, Nb: 0.01 to 0.10 wt%, V: 0.01 to 0.10 wt%, Mo: to steel having alloy composition of balance Fe and other unavoidable impurities Steel slab further comprises at least one of 0.1 ~ 0.5wt%, Cr: 0.1 ~ 0.5wt% Reheating and hot rolling at a temperature below the austenite recrystallization temperature to austenite / ferrite transformation temperature (Ar ₃ ) to produce a hot rolled steel sheet; Cooling the hot rolled steel sheet to a predetermined winding temperature at a predetermined cooling rate; And Winding the cooled hot rolled steel sheet at the predetermined winding temperature to produce a hot rolled coil; Tensile strength of the steel sheet produced is 1300Mpa or more, characterized in that the production method of ultra-high strength steel sheet excellent in hydrogen delayed fracture resistance. The method according to claim 2, The predetermined winding temperature is 200 ~ 400 ℃, the predetermined cooling rate is 30 ~ 100 ℃ / sec method of producing a super high strength steel sheet excellent in hydrogen delayed fracture resistance, characterized in that.

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