KR101886171B1 - Process for making a high strength steel plate with high yield ratio - Google Patents

Abstract

The present invention relates to a method to manufacture a high strength steel plate with a high yield ratio, wherein tensile strength is 900 to 1,500 MPa, an elongation rate is 9% or more, a metal structure is a single phase of tempered martensite, and the yield ratio is 85% or more. According to the present invention, the method comprises: a step of quickly heating a base steel plate, which includes 0.20 to 0.50 weight percent of C, 0.01 to 2.0 weight percent of Si, 0.30 to 3.0 weight percent of Mn, 0.05 weight percent or less of P, 0.05 weight percent or less of S, and the remainder being Fe and unavoidable impurities, to an Ac3 transition point or higher for 3 to 60 sec by using an induction heating apparatus; a step of quenching the heated steel plate at 100°C/s or more by water or oil; a step of tempering the steel plate at 400°C to an A1 transition point for 3 to 60 sec inkling heating and holding times; a step of cold rolling the steel plate by a reduction rate of 5 to 30% after descaling; and a step of low temperature-annealing the rolled steel plate at 200 to 400°C for 3 to 60 sec by using the induction heating apparatus.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a method of manufacturing a high strength steel plate having a high porosity,

The present invention relates to a high strength steel sheet for automobiles, and more particularly, to a high strength steel sheet having a tensile strength of 900 to 1,500 MPa, an elongation of 9% or more, and a high specific gravity showing a yield ratio of 85% or more as a structure of tempered martensite single phase ≪ / RTI >

In recent steel sheets for automobiles, it is required to increase the weight of the vehicle body in accordance with the tendency of regulation of exhaust gas due to global warming, and also to strengthen the strength of the vehicle body structural member in order to secure the safety of the occupant in the crash.

Particularly, structural members such as bumper rails designed to absorb impacts in the event of a vehicle collision and various pillars, door impact beams and side seals supporting the passenger space minimize the deformation of the passenger space, shall. Accordingly, it is required to use a high strength composite material (YS / TS) having a high tensile strength (TS) and a high yield strength (YS) in the body structural member.

However, the development of materials for structural materials for body structure has been attempted with an emphasis on high strength, so that it is known to develop materials having high strength. However, these materials have low elongation ratio or low elongation required for workability or safety Therefore, there has been a real problem to be put to practical use as a material for a structural body part of a vehicle body.

The steel sheet disclosed in Japanese Patent Laid-Open No. 10-2014-0145107, which is a conventional material for a vehicle structural member, shows a relatively high strength in a tensile strength range of 998 to 1091 MPa, but a yield ratio of 70 to 80% The structure is composed of a mixed structure of martensite, bainite and ferrite.

The steel sheet disclosed in Japanese Patent Application Laid-open No. 10-2016-0047465 has a relatively high tensile strength of 998 to 1061 MPa, yield ratio of 81 to 88% and elongation of 18.3 to 20.5%, but the maximum tensile strength is 1061 MPa, In particular, in the case of a composite structure of ferrite, tempered martensite, bainite and retained austenite on the metal structure surface, there is a great variation in the physical properties depending on the phase fraction, and in order to maintain an accurate phase fraction of a predetermined value, And time must be strictly controlled. The problems associated with such a complex organization are also common to the above-described patent publication 10-2014-0145107.

As another prior patent document, Japanese Patent Registration No. 10-1543837 discloses a method of hot-rolling a slab having a composition of Ti, Nb and V added to a low carbon steel of 0.03 to 0.1% A high-strength hot-rolled steel sheet in which a nitride is deposited on a structure is disclosed. The steel sheet has a high yield ratio of 85 to 95% and a high elongation of 15 to 28%. However, the tensile strength is low at 548 to 911 MPa, which limits the tensile strength.

Japanese Patent No. 10-1406478 discloses a cold rolled steel sheet having a composite structure of self-tempered martensite, bainite, ferrite and martensite by controlled cooling of low carbon steel (0.05-0.15C) It can be said to be a high-strength steel having a relatively high strength in the range of 1027 to 1106 MPa, but there is a problem that the yield ratio is as low as 75 to 80%.

Although the conventional steel sheets have a yield ratio of about 70 to 80%, they are low-level steel sheets having a low yield ratio and some show yield ratios as high as 81 to 88%. However, in the case of such steel sheets, the tensile strength is relatively low, Since the manufacturing process for obtaining the metal structure is complicated and strict control is required, it is practically difficult to obtain uniform physical properties.

The present invention has been made in view of the above-mentioned problems in conventional high strength steel sheets for automobiles, and it is an object of the present invention to provide a tempered martensite single phase having a tensile strength of 900 to 1,500 MPa, a elongation of 9% The present invention provides a method of manufacturing a high-strength steel sheet.

Another object of the present invention is to provide a method for manufacturing a high strength steel sheet which is simple and easy to control by simplifying a complex manufacturing process and a complicated metal structure obtained as a result in order to obtain a high tensile strength and a high yield ratio.

칭, 템퍼링이라는 프로세스를 사용하면 가능하다는 것은 잘 알려진 사실이지만, 목적하는 인장강도에서 높은 신율과 고 항복비를 얻는데는 용이하지만은 않은 것이 현실이다.As can be seen from the examples of various high-strength steel sheets related to the above, in order to obtain high tensile strength, various alloying elements and

It is a well-known fact that a process called quenching and tempering is possible, but it is not easy to obtain a high elongation and a high yield ratio at a desired tensile strength.

칭, 템퍼링을 행하여 최적의 금속조직을 형성한 후 특정 범위 내의 압하율로 냉간 압연을 행하고 적절한 저온소둔을 행하였을 때, 인장강도가 900 ∼ 1,500MPa이고, 신율이 9%이상이며, 항복비가 85% 이상인 템퍼드 마르텐사이트 단상의 조직으로 이루어진 초고강도 강판을 제조할 수 있음을 확인할 수 있었다. As a result of various attempts and numerous experiments to develop a steel sheet material having a high elongation and yield ratio while having a high tensile strength, the present inventors have found that the heating temperature, the heating time, and the cooling With proper speed control

Rolling and cold rolling at a reduction ratio within a specific range after forming an optimum metal structure by performing tempering and tempering at an appropriate low temperature annealing and a tensile strength of 900 to 1,500 MPa, an elongation of 9% or more, a yield ratio of 85 It is possible to produce an ultra-high strength steel sheet composed of a single-phase structure of tempered martensite.

Accordingly, the present invention provides a method for producing a steel sheet, which comprises 0.20 to 0.50 wt% of C, 0.01 to 2.0 wt% of Si, 0.30 to 3.0 wt% of Mn, 0.05 wt% or less of P and 0.05 wt% or less of S and the balance consisting of Fe and other unavoidable impurities Rapidly heating and holding the steel sheet to an Ac3 transformation point or more for 3 to 60 seconds using an induction heating apparatus, quenching the steel sheet in a heated state with water or oil at 100 DEG C / s or more, heating and holding time A step of cold rolling at a reduction rate of 5 to 30% after descaling, a step of cold rolling the steel sheet at a temperature of 200 to 400 DEG C for 3 to 60 seconds using an induction heating apparatus, Strength steel sheet having a metal structure of tempered martensite single phase and having a tensile strength of 900 to 1,500 MPa, a elongation of 9% or more, and a yield ratio of 85% or more .

Meanwhile, in the method of the present invention, the low temperature annealing step may be performed by using an electric furnace, a gas furnace or a heavy oil furnace instead of the induction heating apparatus.

The chemical composition of the steel sheet used in the method of the present invention contains 0.20 to 0.50 wt% of C, 0.01 to 2.0 wt% of Si, 0.30 to 3.0 wt% of Mn, 0.05 wt% or less of P and 0.05 wt% or less of S and other inevitable 0.05 to 2.0 wt% of Mo, 0.0003 to 0.0050 wt% of B, 0.01 to 0.20 wt% of Ti, 0.01 to 0.10% of Al, and the like, wt% can be additionally added, and the reason for limiting the range of each component is as follows.

C: 0.20 to 0.50 wt%

칭시 강재의 강도증가를 위하여 첨가되는 가장 중요한 원소로서, 0.20wt% 미만에서는 900MPa 이상의 인장강도를 얻는데 어려움이 있으며, 0.50wt%를 초과하게 되면 다량의 탄화물 석출로 인하여 연성이 저하되고 변형저항이 증대되어 강판을 자동차용 부재 등으로 가공할 때 균열의 발생 가능성이 높아지게 된다. C is

It is difficult to obtain a tensile strength of 900 MPa or more at less than 0.20 wt%, and when the amount exceeds 0.50 wt%, ductility is deteriorated due to deposition of a large amount of carbide, and deformation resistance is increased So that the possibility of occurrence of cracks becomes high when the steel sheet is processed into an automobile member or the like.

 Si: 0.01 to 2.0 wt%

Si is an element to be added to the steel for deoxidation. When the Si content exceeds 0.01 wt%, Si enhances the strength without inhibiting the ductility of the steel. When the Si content exceeds 2.0 wt%, the precipitated carbides increase, Cracks are generated and the tool life is deteriorated during processing of the steel sheet. This is because Si interferes with the movement of carbon solved in the precipitated carbide, thereby preventing sphericalization of the carbide.

Mn: 0.30 to 3.0 wt%

칭성을 높여 고강도를 확보하는데 유용한 원소로서, C 및 Si가 과잉으로 첨가될 경우 발생할 수 있는 변형저항의 증대를 피하기 위하여, 낮은 C, Si 첨가강에서 강도의 저하를 보완하기 위한 원소이다. 따라서 이와 같은 효과를 기대하기 위해서는 최소 0.30wt% 첨가가 필요하지만, 과잉 첨가되면 인성과 변형저항을 증대시키므로 그 첨가량은 3.0wt%를 초과하지 않도록 하여야 한다.Mn is steel

It is an element which is useful for securing high strength by increasing quenching. It is an element to compensate the decrease in strength in low C and Si added steels, in order to avoid an increase in deformation resistance which may occur when C and Si are added excessively. Therefore, it is necessary to add at least 0.30wt% in order to expect such an effect. However, if it is added excessively, the toughness and deformation resistance are increased. Therefore, the addition amount should not exceed 3.0wt%.

Cr: 0.05 to 2.0 wt%

칭 경도 및 인성의 향상을 위하여 첨가되는 원소로서, 0.05wt% 미만에서는 상기 특성들의 향상 효과가 미약하고, 비교적 고가인 까닭에 2.0wt%를 초과하게 되면 경제성이 떨어지게 된다.Cr has strength and

As the element added for the improvement of hardness and toughness, if the amount is less than 0.05 wt%, the effect of improving the characteristics is insignificant, and if it exceeds 2.0 wt%, the economical efficiency is lowered.

Mo: 0.05 to 2.0 wt%

The effect of adding Mo is almost the same as that of Cr. When the Mo content is less than 0.05wt%, the effect is weak. When the Mo content exceeds 2.0wt%, the deformation resistance for cold working is increased so that the addition amount is limited to not exceed 2.0wt% .

B: 0.0003 to 0.0050 wt%

칭성을 향상시키는 원소로서, 0.0003wt% 미만에서는 첨가효과가 불분명하고, 0.0050wt%를 초과하면 오히려

칭성을 저하시킨다.B is

When the content is less than 0.0003 wt%, the effect of addition is unclear. When the content is more than 0.0050 wt%

Thereby deteriorating quenching.

Ti: 0.01 to 0.20 wt%

칭 강도를 올리는 효과도 있지만, 오스테나이트의 결정립 미세화에 효과가 크다. 그러나 0.01wt% 미만에서는 그 효과가 미흡하고, 0.20wt%를 초과하면 개재물이 많게 되어 요구되는 각종 물성치를 저하시키게 된다.Ti coexists with B

Although the effect of increasing the quenching strength is also obtained, the effect of the austenitic grain grain is large. However, when the content is less than 0.01 wt%, the effect is insufficient. When the content exceeds 0.20 wt%, the amount of inclusions is increased, and various required physical properties are lowered.

Al: 0.01 to 0.10 wt%

Al has an effect of inhibiting the growth of austenite grains by bonding with nitrogen. However, if it is contained in a large amount, aluminum oxide-based inclusions are formed in a large amount, resulting in deterioration of ductility. Therefore, in order to achieve the object of the present invention, 0.01 to 0.10 wt% is preferable.

P, S: not more than 0.05 wt%

P and S are inevitable impurity elements of steel, which are segregated at crystal grain boundaries when kneaded to lower the impact toughness and lower the strain at the time of cold working. Therefore, it is necessary to limit the content so as not to exceed 0.05 wt%.

Hereinafter, each step of the method of the present invention will be described in detail.

First, as the first step, "the step of rapidly heating and holding the work steel sheet for 3 to 60 seconds to the Ac3 transformation point or higher by using the induction heating apparatus" finely grains the austenite crystal grains, so that even when the alloy element is small, If the time is less than 3 seconds, the heating is insufficient, and multiphase structures such as untransformed austenite, pearlite and austenite may occur. If the time exceeds 60 seconds, the grain size of the austenite becomes too large It affects brittleness. A more preferable heating time is 30 seconds or less.

In the present invention, the quenching step is followed by the step of "quenching at 100 ° C / s or more" following the rapid heating and holding step. The quenching step is performed at a cooling rate of 100 ° C / s or less, A complex structure of the site may occur, and the martensite single phase as intended in the present invention can not be obtained.

Next, in the manufacturing method of the present invention, after the quenching step, "a step of tempering from 400 ° C. to the A1 transformation point for 3 to 60 seconds" is carried out, and when it is heated to 400 ° C. or higher during tempering, the precipitated carbides are spheroidized, The occurrence rate of cracks during cold working such as annealing is remarkably reduced. However, when the tempering temperature is heated to exceed the A1 transformation point, there is a possibility that austenite is precipitated and a mixed structure of martensite and tempered martensite is formed after cooling, so that a uniform phase of tempered martensite can not be obtained , The tempering temperature is preferably in the temperature range between 400 ° C and the A1 transformation point.

On the other hand, the reason why the heating time including the holding time in the tempering step is limited to 3 to 60 seconds is because the tempering effect is insufficient when the heating time is less than 3 seconds and when the heating time exceeds 60, the deposited carbide coarsens and the strength is lowered Because. A more preferable tempering time is 3 to 30 seconds.

칭 후 곧바로 템퍼링하는 일련의 연속된 공정으로 이어지도록 한 이유는

칭 이후에 조속히 템퍼링을 함으로써

칭 크랙의 발생을 방지하고 작업효율을 높이기 위함이다.In the present invention

The reason for continuing to a series of sequential processes that are tempered immediately after quenching

By rapid tempering after quenching

This is to prevent generation of cracking and to increase work efficiency.

Next, in the method of the present invention, the step of "cold rolling at a reduction rate of 5 to 30% after the removal of the scale" is performed. If the reduction rate is less than 5%, the amount of dislocation is small and the effect of work hardening is insignificant, There is a problem in improving the yield strength by interfering with the displacement of the dislocation due to the formation of carbide during the low temperature annealing step. When it exceeds 30%, the increase of the tensile strength and thus the elongation decrease remarkably, Is preferably limited to within 30%.

As a step subsequent to the cold rolling step, "a step of low-temperature annealing the rolled steel sheet at 200 to 400 ° C. for 3 to 60 seconds using an induction heating apparatus" is a step of low-temperature annealing the potential generated during cold rolling, So that the displacement of the dislocations is disturbed by depositing carbide, thereby raising the yield strength. At this time, when the annealing temperature is less than 200 占 폚 or the annealing time is less than 3 seconds, the effect of increasing the yield strength by the low temperature annealing can not be expected. On the contrary, when the annealing temperature exceeds 400 占 폚 or the annealing time exceeds 60 seconds, The carbide coagulates and coarsens, resulting in a decrease in the yield strength.

Meanwhile, in the method of the present invention, the metal structure of the steel sheet subjected to the final processing step is limited to the tempered martensite single phase because, as described above, the conventional high strength and high strength steel sheets are multi- It is difficult to obtain the expected yield ratio and elongation due to the increased brittleness due to the concentration of stress on the interface at the time of deformation, and it is difficult to obtain a uniform quality due to the difficulty in controlling the fraction of each phase upon heat treatment. Phase metal structure in order to avoid the problem.

However, even if microstructures other than tempered martensite occur in the heat treatment process, if the total amount is less than 10% by area ratio, it is regarded as a single-phase tempered martensite structure.

On the other hand, in the method of the present invention, an electric furnace, a gas furnace, or a heavy oil furnace may be used instead of the induction heating apparatus in heating and holding the steel sheet rolled at the low temperature annealing step at 200 to 400 ° C. The reason for this is that the low temperature annealing using an electric furnace or the like is not particularly limited in terms of the heating time in addition to the heating temperature since the cold-rolled steel sheet is continuously heated to 5 to 30% It is difficult to limit the heating time because the heating time is significantly different from that in the case of passing the furnace through the furnace and winding the furnace in the form of a coil and charging the furnace into a batch type furnace for heating.

Particularly, when it is wound into a coil form and put in a heating furnace, it is very difficult to specify the heating time because the heating time varies depending on the weight of the coil. However, since the heating temperature rather than the heating time is a factor that greatly affects the material properties of the steel sheet, the sample is taken in a state in which the heating temperature is kept within the above temperature range, and then subjected to a tensile test to determine the yield ratio And the elongation satisfies the property value range of the present invention, it is judged as a suitable time condition.

The high-strength steel sheet of the present invention has a tensile strength of 900 to 1,500 MPa, a elongation of 9% or more, and a yield ratio of 85% or more. As compared with the conventional steel sheet for automobiles, tensile strength and yield ratio are improved, It is possible to improve the fuel economy by reducing the weight of the vehicle body through the reduction of the weight of the vehicle and to increase the resistance against the collision on the basis of a high yield ratio so that the safety of passengers can be secured, .

In addition, since the high-strength steel sheet of the present invention is composed of only tempered martensite as a single phase rather than a composite structure formed by mixing a plurality of phases while maintaining a constant ratio, The heat treatment process is performed more easily than the severe control heat treatment process that must be performed, so that the manufacturing cost can be lowered.

1 is a schematic manufacturing process diagram of a high-strength steel sheet having a high porosity according to the method of the present invention.

The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

[Example]

The material steel sheet having the chemical composition as shown in Table 1 below was rolled to a thickness of 2 mm, and then, using a high frequency induction heating apparatus connected with a series of processes, the heating temperature, heating, holding time and cooling The steel plate specimens were obtained while varying the speed conditions. Tissue observation specimens were taken from steel specimens and observed by optical microscope after detaching corrosion. The tensile strength and elongation of mechanical properties were measured with a universal testing machine using JIS 5 specimen.

 division C Si Mn P S Cr Mo B Ti Al Fe Psalm 1 0.21 1.20 0.82 0.002 0.004 0.83 - 0.0021 - 0.04 honey. Psalm 2 0.32 0.22 1.33 0.004 0.005 0.68 0.20 - - - honey. Psalm 3 0.43 0.25 0.85 0.013 0.008 - - - 0.02 - honey.

division

칭조건

Naming condition      Tempering conditions
metal
group
The tensile strength
(MPa) Heating temperature
(° C) Heating, holding time (sec) Cooling rate
(° C / sec) Heating temperature
(° C) Heating, holding time (sec) Cooling method Psalm 1 A-1 930 8 300 680 8 Water cooling TM  920 A-2 930 8 80 650 8 Water cooling TM + F + B  910 Psalm 2
B-1 900 10 300 680 10 Water cooling TM  950 B-2 780 10 300 635 10 Water cooling TM + F  945 Psalm 3
C-1 880 15 250 650 15 Water cooling TM  1070 C-2 880 15 250 720 15 Water cooling TM + M  1170

   TM: tempered martensite, F: ferrite, M: martenside, B: bainite

칭, 템퍼링을 한 시편들의 경우에는 금속조직이 템퍼드 마르텐 사이트 단상을 띠고 있으며, 인장강도가 900MPa 이상을 나타내고 있음을 알 수 있다.As shown in Table 2, the steel sheet of the composition of the present invention was subjected to the heat treatment conditions defined in the present invention using an induction heating apparatus

In the case of the quenched and tempered specimens, the metal structure has a tempered martensite single phase and a tensile strength of 900 MPa or more.

The six specimens obtained in Table 2 were subjected to pickling and neutralization to remove the surface scale and then subjected to cold rolling and cold annealing at a reduction rate of 5 to 40% under the conditions shown in Table 3 below, ≪ / RTI > The tensile strength, elongation and yield ratio for these final specimens were measured and the results are shown in Table 3.

division
minuteness
group Cold rolling
Reduction rate
(%)     Low temperature annealing Seal
burglar
(MPa) Elongation
(%) Yield ratio
(%)

Remarks Heating temperature
(° C) Heating, holding time (sec)



Psalm 1






A-1



TM

5 300 5 963 16.9 91.8 Example 1 10 300 5 1012 13.4 92.2 Example 2 20 300 10 1087 11.2 92.1 Example 3 30 300 10 1163 9.7 91.7 Example 4 40 300 20 1219 8.6 91.2 Comparative Example 1

A-2



TM + F + B

5 300 5 943 11.1 78.3 Comparative Example 2 10 300 5 967 9.8 80.2 Comparative Example 3 20 300 10 1007 8.1 81.3 Comparative Example 4 30 300 10 1038 7.3 82.0 Comparative Example 5 40 300 20 1072 6.9 82.6 Comparative Example 6



Psalm 2






B-1



TM

5 300 5 994 14.6 92.3 Example 5 10 300 5 1063 13.8 92.9 Example 6 20 300 10 1197 12.3 93.1 Example 7 30 300 10 1273 10.1 92.7 Example 8 40 300 20 1350 8.1 92.5 Comparative Example 7

B-2



TM + F

5 300 5 963 10.7 76.3 Comparative Example 8 10 300 5 971 9.1 75.8 Comparative Example 9 20 300 10 995 8.3 75.6 Comparative Example 10 30 300 10 1017 7.5 76.1 Comparative Example 11 40 300 20 1036 6.4 75.2 Comparative Example 12



Psalm 3



C-1



TM

5 300 5 1121 13.1 91.7 Example 9 10 300 5 1193 12.9 92.6 Example 10 20 300 10 1307 11.2 93.2 Example 11 30 300 10 1442 9.7 93.1 Example 12 40 300 20 1560 7.1 91.9 Comparative Example 13
C-2

TM + M
5 300 5 1135 8.3 91.5 Comparative Example 14 10 300 5 1174 7.6 91.3 Comparative Example 15 20 Cracking during rolling

As shown in Table 3, the test specimens in which the metal structure was a single phase of tempered martensite were subjected to cold rolling at a reduction ratio (5 to 30%) determined in the present invention and low temperature annealing using a high frequency induction heating apparatus. In the case of the specimens of the invention, the single-phase tempered martensite structure exhibits a elongation of 9% or more and a yield ratio of 85% or more, even though the tensile strength is 900 to 1,500 MPa in high strength.

On the other hand, even when the low-temperature annealing condition defined in the present invention is applied to the single-phase tempered martensitic metal structure specimen, when the reduction ratio exceeds 30%, the elongation is less than 9% (Comparative Example 1, 7 and 13).

It can be seen that the elongation and yield ratio of cold rolled specimens are not less than 9% and less than 85%, respectively, even when cold rolling is performed in the range of 5 ~ 30% in the case of specimen in which the metal structure before cold rolling is not single phase of tempered martensite . (Comparative Examples 2 to 5, Comparative Examples 8 to 11 and Comparative Examples 14 to 15)

Table 4 below shows the results of measurement of mechanical properties of the specimens obtained by performing the low temperature annealing in Table 3 above at a temperature of 300 DEG C for 30 to 50 minutes using a continuous electric furnace instead of the high frequency induction heating apparatus.

division
minuteness
group Cold rolling
Reduction rate
(%)      Low temperature annealing Seal
burglar
(MPa) Elongation
(%) Yield ratio
(%)

Remarks Heating temperature
(° C) Heating, holding time (minutes)



Psalm 1






A-1



TM

5 300 30 937 16.8 92.1 Example 13 10 300 40 986 13.9 92.7 Example 14 20 300 40 1061 11.5 93.1 Example 15 30 300 50 1145 9.6 93.0 Example 16 40 300 50 1196 8.8 92.6 Comparative Example 16

A-2



TM + F + B

5 300 30 915 11.5 77.1 Comparative Example 17 10 300 40 944 10.2 79.5 Comparative Example 18 20 300 40 989 8.2 79.5 Comparative Example 19 30 300 50 1029 7.4 80.1 Comparative Example 20 40 300 50 1051 7.0 79.8 Comparative Example 21



Psalm 2






B-1



TM

5 300 30 975 14.9 91.6 Example 17 10 300 40 1040 14.2 92.5 Example 18 20 300 40 1163 12.3 92.4 Example 19 30 300 50 1231 10.2 92.3 Example 20 40 300 50 1291 8.2 91.9 Comparative Example 22

B-2



TM + F

5 300 30 937 10.6 77.1 Comparative Example 23 10 300 40 957 9.3 76.5 Comparative Example 24 20 300 40 981 8.4 74.9 Comparative Example 25 30 300 50 1004 7.3 75.6 Comparative Example 26 40 300 50 1033 6.7 75.3 Comparative Example 27


Psalm 3



C-1



TM

5 300 30 1109 13.4 92.0 Example 21 10 300 40 1160 12.7 92.4 Example 22 20 300 40 1275 11.0 93.3 Example 23 30 300 50 1388 9.5 92.9 Example 24 40 300 50 1460 7.3 91.6 Comparative Example 28
C-2

TM + M
5 300 30 1116 8.1 90.7 Comparative Example 29 10 300 40 1196 7.5 90.8 Comparative Example 30 20 Cracking during rolling

As shown in Table 4, in the example of the present invention (Example 13) in which the annealing was performed at a relatively longer heating and holding time than the heating and holding time at annealing using the induction heating apparatus shown in Table 3 using an electric furnace, To 24) also show tensile strengths of more than 9% and yield ratios of 85% or more at 900 to 1,500 MPa.

Claims (5)

0.20 to 0.50 wt% of C, 0.01 to 2.0 wt% of Si, 0.30 to 3.0 wt% of Mn, 0.05 wt% or less of P and 0.05 wt% or less of S and the balance of Fe and other unavoidable impurities, Rapidly heating the steel sheet in a heated state to 100 ° C / s or more with water or oil, heating and holding the steel sheet at 400 ° C to A1 Tempering for 3 to 60 seconds to a transformation point, cold rolling at a reduction rate of 5 to 30% after descaling, low temperature annealing at 200 to 400 캜 for a rolled steel sheet, and a metal structure of tempered martensite single phase Wherein the steel sheet has a tensile strength of 900 to 1,500 MPa, a elongation of 9% or more, and a yield ratio of 85% or more. The method of manufacturing a high strength steel sheet according to claim 1, wherein the low temperature annealing step is performed for 3 to 60 seconds using an induction heating apparatus. The method of manufacturing a high strength steel sheet according to claim 1, wherein the low temperature annealing step is performed using an electric furnace, a gas furnace, or a heavy oil furnace. The method of manufacturing a high strength steel sheet according to claim 3, wherein the low temperature annealing step is performed for 20 to 60 minutes using an electric furnace or the like The steel sheet as set forth in claim 1, wherein said steel material comprises at least one of 0.05 to 2.0 wt% of Cr, 0.05 to 2.0 wt% of Mo, 0.0003 to 0.0050 wt% of B, 0.01 to 0.20 wt% of Ti and 0.01 to 0.10 wt% Wherein the high-strength steel sheet has a high porosity.

Images