JP2010509494A - Steel sheet for ultra-high-strength line pipe excellent in low-temperature toughness and method for producing the same - Google Patents

Abstract

  The present invention relates to a steel sheet for ultra-high strength line pipe excellent in low-temperature toughness and a manufacturing method thereof. The present invention relates to a steel plate having a strength of 930 MPa or more and excellent in toughness even when a smaller amount of alloying elements is added, and a method for producing the same. The steel sheet is, by weight, C: 0.03-0.10%, Si: 0-0.6%, Mn: 1.6-2.1%, Cu: 0-1.0%, Ni: 0 to 1.0%, Nb: 0.02 to 0.06%, V: 0 to 0.1%, Mo: 0.1 to 0.5%, Cr: 0 to 1.0%, Ti: 0 0.005 to 0.03%, Al: 0.01 to 0.06%, B: 0.0005 to 0.0025%, N: 0.001 to 0.006%, Ca: 0 to 0.006%, P: 0.02% or less, S: 0.005% or less, balance Fe and other inevitable impurities are included, and the total of bainitic ferrite and acicular ferrite is 75% based on the area fraction as a microstructure. Included.

Description

  The present invention relates to a steel sheet for ultra-high-strength line pipe excellent in low-temperature toughness and a method for producing the same, and more specifically, it has a strength of 930 MPa or more even if a smaller amount of alloying element is added compared to a conventional steel sheet, The present invention also relates to a steel plate for line pipes excellent in toughness and a method for producing the same.

  Line pipes are mainly steel pipes buried underground for long-distance transportation of crude oil or natural gas. Since high-pressure gas or crude oil flows in the line pipes, high pressure is usually applied to the line pipes. Works.

  In order to increase the transportation efficiency of the line pipe, it is necessary to increase the amount of crude oil or gas (hereinafter simply referred to as “crude oil”) that can be transported per unit time. It is necessary to greatly increase the outer diameter of the line pipe.

  When the outer diameter of the line pipe is increased, the amount of crude oil and the like flowing through the inside of the line pipe can be increased, whereby the pressure acting on the line pipe by the crude oil and the like is similarly increased. In that case, the material for the line pipe needs to be developed as a material having higher strength, but from the viewpoint of the strength standard of the line pipe, it is a reality that still uses mainly steel of X70 grade. The X70 grade steel plate has a strength of 70 ksi, that is, about 480 MPa, and increasing the diameter of the line pipe using such a strength grade steel plate inevitably requires an increase in the thickness of the steel plate. It is uneconomical.

  Therefore, the demand for development of steel sheets with dramatically improved strength is gradually increasing compared to steel sheets for line pipes that have been used normally until now, but the development of steel sheets that satisfy all these requirements. Is a reality that has not yet been completed.

  The reason is that it becomes difficult to apply due to not only a barrier to the technology itself for increasing the strength of the steel sheet but also other problems that occur as the strength of the steel sheet is increased.

  That is, in order to increase the strength of the steel sheet, an alloy element effective for increasing the strength should be added. However, by adding such an alloy element, it becomes difficult to obtain a sufficiently high strength, and the alloy Increasing the amount of element added causes the problem that the low-temperature toughness of the weld and the low-temperature toughness of the base metal will be excessively inferior accordingly, so it is necessary to improve the low-temperature toughness at the same time to increase the strength of the steel sheet. It becomes.

  In order to improve the strength of the steel sheet, conventionally, the steel sheet is mainly quenched to form a low-temperature microstructure, particularly a structure such as lower bainite and martensite, thereby increasing the hardness and strength of the steel sheet. However, if a microstructure such as martensite is formed inside the steel sheet, the strength of the steel sheet is insufficient, or the toughness of the steel sheet due to residual stress inside the steel sheet. There is a problem that becomes extremely inferior.

  As described above, the strength and toughness of a steel sheet are conventionally two types of physical properties that are difficult to achieve, and it is generally recognized that as the strength of a steel sheet increases, the toughness decreases.

  Since then, efforts have been made to provide both high strength and high toughness steel materials while ensuring the strength and toughness of the steel sheet at the same time, but as one of them, a method called TMCP (Thermo Mechanical Controlling Process) has been proposed. It became so. The TMCP method is a generic term for a processing method that changes the physical properties of a steel sheet to a desired physical property by imparting a thermal history together with mechanical processing during rolling and cooling of the steel sheet, and is used after being changed to a very large number of forms. However, it mainly comprises a controlled rolling process in which rolling is performed under severe conditions at a predetermined temperature, and an accelerated cooling process in which the steel sheet is cooled at an appropriate range of cooling rate.

  In the case of using such a TMCP method, there is an advantage that desired physical properties can be controlled smoothly to a certain degree by forming fine crystal grains inside a steel plate and appropriately controlling the structure to a desired form. .

  However, in order to produce a steel sheet having a desired strength through the TMCP accelerated cooling process as described above, it is necessary to form a hard structure as in the prior art. Accordingly, it is an unavoidable fact that even a steel plate manufactured by the TMCP method tends to decrease toughness as the strength increases.

  Therefore, in the field of high-strength steel materials, research and development have been continuously carried out in order to increase the strength level of steel materials, and efforts have been made continuously to secure means for improving low-temperature toughness.

  In order to solve such problems, tempering is most widely used.

For example, in Patent Document 1 to Patent Document 8, after performing TMCP treatment to perform rolling and cooling as illustrated in FIG. 1, tempering is performed at a temperature lower than _Ac1 transformation temperature (temperature at which ferrite transforms to austenite during heating). A manufacturing method including a step of additionally performing is described. However, since reheating is required to perform tempering after cooling the steel sheet, the energy consumption is large and a tempering process must be added separately, resulting in an increase in cost.

  In addition, many kinds of alloying elements are added to the steel material in order to increase the strength of the steel material, and one of the most effectively used elements is Mo. For example, Patent Documents 9 to 12 describe a technique in which lower bainite and lath martensite are included in the microstructure of a steel material by containing a large amount of Mo, particularly 0.2 wt% or more of Mo. In particular, as a similar technique, referring to Patent Document 13, a steel containing 0.35% by weight or more of Mo is adopted as a main invention example, and a comparative example showing a Mo content of 0.14% by weight. It can be seen that the tensile strength is lower than 930 MPa.

  However, since Mo itself is an expensive element, if Mo is used in an amount of 0.15% by weight or 0.2% by weight or more as described above, it causes an increase in the manufacturing cost of the steel material. Further, as is apparent from FIG. 2, the lower bainite structure shown in FIG. 3 has a very narrow temperature range in which phase transformation occurs, severe cooling conditions for producing steel, and an excessively high cooling rate. Therefore, the capacity requirements of the manufacturing equipment are very strict, and there is a possibility that the deformation of the plate may occur due to the high cooling rate. Therefore, after manufacturing the steel plate, additional processing to control the shape, etc. There is a problem that manufacturing conditions are complicated and difficult.

US Pat. No. 5,545,269 US Pat. No. 5,755,895 US Pat. No. 5,798,004 US Patent No. 5900075 US Pat. No. 6,045,630 US Pat. No. 6,183,573 US Pat. No. 6,245,290 US Pat. No. 6,532,995 US Pat. No. 6,224,689 US Pat. No. 6,228,183 US Pat. No. 6,248,191 US Pat. No. 6,264,760 Korean Patent Patent No. 2000-00533890

  An object of the present invention is to solve the above-mentioned problems, and to provide a steel sheet having high tensile strength and excellent low-temperature toughness without using a large amount of Mo, and a method for producing the same. And

  According to one aspect of the present invention, the steel sheet of the present invention is, by weight, C: 0.03-0.10%, Si: 0-0.6%, Mn: 1.6-2.1%, Cu: 0 to 1.0%, Ni: 0 to 1.0%, Nb: 0.02 to 0.06%, V: 0 to 0.1%, Mo: 0.1 to 0.5%, Cr : 0 to 1.0%, Ti: 0.005 to 0.03%, Al: 0.01 to 0.06%, B: 0.0005 to 0.0025%, N: 0.001 to 0.006 %, Ca: 0 to 0.006%, P: 0.02% or less, S: 0.005% or less, balance Fe and other inevitable impurities, and as a microstructure, bainitic ferrite (Bainitic Ferrite) and The total of acicular ferrite (also called “Acicular Ferrite”) is 7 based on the area fraction. It is characterized by containing 5% or more.

  According to another aspect of the present invention, the steel sheet of the present invention is, by weight, C: 0.03-0.10%, Si: 0-0.6%, Mn: 1.6-2.1% Cu: 0 to 1.0%, Ni: 0 to 1.0%, Nb: 0.02 to 0.06%, V: 0 to 0.1%, Mo: 0.1 to 0.5%, Cr: 0-1.0%, Ti: 0.005-0.03%, Al: 0.01-0.06%, B: 0.0005-0.0025%, N: 0.001-0. 006%, Ca: 0 to 0.006%, P: 0.02% or less, S: 0.005% or less, balance Fe and other unavoidable impurities, bainitic ferrite and acicular ferrite as microstructure Is 75% or more based on the area fraction, yield strength is 930 MPa or more, and Charpy impact absorption energy at −40 ° C. is 230 joules. Characterized in that at least.

  At this time, it is more preferable that the Mo is added to the steel sheet in an amount of 0.015% or less.

  The fine structure of the steel sheet preferably has a granular bainite content of 5% or less based on the area fraction.

  The austenite grain size in the thickness direction is particularly preferably 15 μm or less.

According to still another aspect of the present invention, a preferable method for producing an ultra-high strength and high toughness steel material having a microstructure mainly including bainitic ferrite and acicular ferrite or a mixed structure thereof is substantially Heating the steel slab at a temperature sufficient to dissolve all vanadium and niobium carbides and carbonitrides, and one hot rolling or more in the range of austenite recrystallization temperatures a step of rolling the slab in step, is less than T _nr temperature (austenite temperature lower than the temperature at which no recrystallization), Ar ₃ transformation point (i.e., at the time of cooling, the transformation begins temperature from austenite to ferrite) exceeded A step of rolling the steel sheet in a hot rolling stage once or more in a temperature range, and the rolled steel sheet at 20 to 50 ° C. / Comprising the steps of cooling at a cooling rate, a step of stopping the cooling of the steel sheet at a temperature of 200 to 400 ° C., a step of cooling the steel sheet at ambient temperature, the.

  At this time, it is more preferable to add 0.015% or less of the Mo to the steel sheet.

  Moreover, after stopping the cooling of the steel sheet at a cooling rate of 20 to 50 ° C./sec, it is effective to air-cool the steel sheet to the atmospheric temperature.

  According to the present invention, it is possible to provide a steel sheet that can ensure high strength and is excellent in low-temperature toughness without adding a large amount of Mo.

It is the graph which compared the manufacturing method which temperes in order to ensure a physical property with respect to the steel plate produced through rolling and cooling, and the manufacturing method which ensures a physical property, without performing tempering. Other notations in the figure are Lower Bainite: lower bainite. Cooling conditions for steel materials that have lower bainite and lath martensite as main microstructures, and steel materials that have bainitic ferrite and acicular ferrite as main microstructures It is a TTT diagram shown in order to compare with cooling conditions. Other notations in the figure are Granular Bainite: Granular bainite and Ferrite: Ferrite. It is the photograph which observed the lower bainite with the transmission electron microscope. It is the photograph which observed bainitic ferrite with the transmission electron microscope. It is the photograph which observed the acicular ferrite with the transmission electron microscope. It is the photograph which observed the granular bainite with the transmission electron microscope.

  These and other aspects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings.

  Embodiments of the present invention will be described with reference to the accompanying drawings.

  As a result of diligent research to solve the above-mentioned problems of the prior art, the inventors of the present invention have found the following facts. That is, by adding less Mo than the conventional invention to increase the strength of the steel sheet, the steel sheet is a lower bainite formed from the conventional invention for producing ultra-high-strength steel or Sufficient strength can be obtained without forming lath martensite. At the same time, the steel sheet has good toughness by controlling the rolling conditions to finely adjust the size of austenite grains and forming a type of structure that is not an ultra-hard structure such as lower bainite or lath martensite. Can be obtained. The present invention is based on such a discovery.

  That is, the present invention reduces the Mo content and adjusts the addition amount of other alloy elements, while controlling with the bainitic ferrite and the acicular ferrite having fine crystal grains, the conventional hard structure. Compared to the steel sheet, the steel sheet has the same or higher strength, and has a low temperature toughness superior to that of conventional lower bainite and lath martensite by making the grain size of the above structure fine. And a method for producing the steel sheet.

  Hereinafter, it explains in detail in order of a composition of a steel plate of the present invention, a fine structure, and a manufacturing method.

(Composition of steel sheet)
In the present invention, the composition of the target steel sheet is selected as follows so as to have sufficient strength and toughness including the toughness of the welded portion.

Carbon (C): 0.03 to 0.10% by weight
C is the most effective element for strengthening the weld and its base material through solid solution strengthening, and is small in size cementite, V and Nb carbonitride [Nb (C, N)] and Mo carbide [Mo ₂ ]. The strengthening effect is obtained by precipitation hardening by forming C] on the steel material. Furthermore, Nb carbonitride can simultaneously improve strength and low-temperature toughness by refining crystal grains by suppressing recrystallization of austenite during hot rolling to prevent crystal grain growth. C also plays the role of improving the hardenability, which is the ability to form a strong microstructure inside the steel plate during cooling. Usually, when the amount is less than 0.03% by weight, such a strengthening effect cannot be obtained. When the amount exceeds 0.1% by weight, the steel sheet becomes sensitive to cold cracking after on-site welding. There is also a possibility that the toughness in the weld heat affected zone is lowered.

Si: 0 to 0.6% by weight
Silicon (Si) plays the role of deoxidizing molten steel with the aid of Al, and is also effective as a solid solution strengthening element. If Si is excessively added in excess of 0.6% by weight, the on-site weldability and the toughness of the heat affected zone are greatly reduced. Since Al or Ti plays a role of deoxidation, it is not always necessary to add Si for deoxidation.

Mn: 1.6 to 2.1% by weight
Manganese (Mn) is an effective element for solid solution strengthening of steel materials, and 1.6% by weight or more is necessary for exhibiting high strength as well as an effect of increasing hardenability. However, if added in excess of 2.1% by weight, center segregation is promoted during continuous casting of the slab in the steel making process, and the toughness tends to be reduced. Furthermore, when Mn is added excessively, the hardenability is excessively improved, the on-site weldability is deteriorated, and the toughness of the weld heat affected zone is lowered.

Cu: 0 to 1.0% by weight
Copper (Cu) is an element that strengthens the base metal and the weld heat affected zone. However, when Cu is excessively added, the toughness of the heat affected zone and the field weldability tend to be lowered.

Ni: 0 to 1.0% by weight
Nickel (Ni) is an element that improves physical properties of low carbon steel without impairing on-site weldability and low temperature toughness. Compared to Mn and Mo, Ni forms less martensite-austentite constituents that lower the low-temperature toughness, improves the toughness of the heat affected zone, and adds Cu during continuous casting and hot rolling. Suppresses the occurrence of surface cracks in steel. However, Ni is an expensive element, and excessive addition of Ni lowers the toughness of the weld heat affected zone.

Nb: 0.02-0.06% by weight
Niobium (Nb) plays a role of simultaneously improving strength and toughness through refinement of the crystal grains of the rolled microstructure of the steel sheet. Nb carbonitride [Nb (C, N)] produced during hot rolling suppresses recrystallization of austenite, inhibits crystal grain growth, and refines austenite crystal grains. Further, when added together with Mo, recrystallization of austenite is suppressed, the effect of refining crystal grains is increased, and the strengthening effect by strengthening precipitation and improving hardening ability is further increased. When B is present, an effect of further increasing the curability is obtained. In order to acquire such an effect, it is preferable to contain 0.02 weight% or more. However, if it is added in excess of 0.06% by weight, it is difficult to expect a further increase in the effect, and the weldability and the toughness of the weld heat affected zone will be adversely affected.

V: 0 to 0.1% by weight
Vanadium (V) plays a role similar to Nb, but its effect is somewhat smaller than Nb. However, when both Nb and V are added, the effect is greatly expanded. However, considering the toughness and weldability of the weld heat affected zone, the upper limit is set to 0.1% by weight.

Mo: 0.1 to 0.5% by weight
Molybdenum (Mo) improves the hardenability, and particularly when added together with B, the effect of improving the hardenability appears very greatly. Further, when added together with Nb, recrystallization of austenite is suppressed, contributing to refinement of crystal grains. However, if Mo is added excessively, the toughness of the weld heat-affected zone is reduced during field welding, so it is preferable to maintain 0.5% or less, and more preferably 0.01 to 0.15%. .

Cr: 0 to 1.0% by weight
Chromium (Cr) plays a role of improving the hardenability. However, if Cr is added excessively, low temperature cracks are generated after on-site welding, and the toughness of the base material and the heat affected zone of the weld is reduced, so 1.0 wt% is made the upper limit.

Ti: 0.005 to 0.03% by weight
Ti forms fine Ti nitride (TiN) and contributes to refinement of crystal grains by suppressing coarsening of austenite crystal grains during slab heating. Furthermore, TiN prevents toughening of crystal grains in the weld heat affected zone and improves toughness by fixing free N in the molten steel. In order to sufficiently fix the free N, Ti is preferably 3.4 times (weight basis) or more of the N addition amount. Therefore, Ti is an element useful for increasing the strength of the base metal and the heat-affected zone of the weld and for making the crystal grains finer. Ti exists in steel as TiN and grows during the heating process for rolling. In addition, Ti remaining after reacting with nitrogen is dissolved in the steel, and is combined with carbon to form TiC precipitates. TiC formation is very fine and increases the strength of the steel. Greatly improve. If the amount of Al added is too small (less than 0.005% by weight), Ti oxide is formed and acts as a nucleation site for intragranular acicular ferrite in the weld heat affected zone. Accordingly, in order to obtain the effect of suppressing the growth of austenite crystal grains due to TiN precipitation and the increase in strength due to TiC formation, it is necessary to add at least 0.005% by weight of Ti. On the other hand, if Ti is added in excess of 0.03% by weight, Ti nitride coarsening and hardening by Ti carbide become excessive, the low temperature toughness is greatly reduced, and the steel plate is welded to produce a steel pipe. Since the toughness of the weld heat affected zone is lowered by rapid heating to the melting point and TiN being re-dissolved, the upper limit of Ti addition is 0.03% by weight.

Al: 0.01 to 0.06% by weight
Aluminum (Al) is usually added for the purpose of deoxidizing steel. Also, not only by refining the microstructure, but also by removing free N in the coarse grain region of the weld heat affected zone, where TiN is partially dissolved by welding heat and thereby nitrogen is dissolved. Improves the toughness of the heat affected zone. However, if it exceeds 0.06% by weight, Al oxide (Al ₂ O ₃ ) is formed, and the toughness of the base metal and the heat affected zone is lowered. Al is not an important element since it can be deoxidized by the addition of Ti and Si.

B: 0.0005 to 0.0025% by weight
Boron (B) greatly improves the hardenability of the low carbon steel and increases the weldability and the resistance to cold cracking. In particular, it plays a role of increasing the effect of improving the hardenability of Mo and Nb, increases the strength of the crystal grain boundary, and suppresses intragranular cracking generated by hydrogen. However, excessive addition of B causes embrittlement due to precipitation of Fe ₂₃ (C, B) ₆ embrittled particles. Therefore, the content of B must be determined in consideration of the content of other curable elements. In the present invention, the B content is preferably in the range of 0.0005 to 0.0025% by weight as described above.

N: 0.001 to 0.006% by weight
Nitrogen (N) suppresses the growth of austenite crystal grains during slab heating, forms TiN precipitates, and suppresses the growth of austenite crystal grains in the weld heat affected zone. However, when N is added excessively, the surface defects of the slab are promoted to reduce the effect of the hardening ability of B, and when there is solute nitrogen, the toughness of the base material and the weld heat affected zone is lowered.

Ca: 0 to 0.006% by weight
Ca is used as an element that controls the shape of MnS inclusions and improves low-temperature toughness. However, when Ca is added excessively, a large amount of CaO—CaS is formed and bonded to form large clusters and coarse inclusions, so that the weldability of the steel is deteriorated as well as the cleanliness of the steel is lowered.

P: 0.02% or less Phosphorus (P) forms a non-metallic inclusion by bonding with Mn and the like to cause a problem of embrittlement of steel. Therefore, it is necessary to actively reduce it, but P is reduced to the limit. In order to do this, the process load of steelmaking is deepened, and since the above problem does not occur greatly at 0 to 0.02%, the upper limit is made 0.02% by weight.

S: 0.005% or less Sulfur (S) is an element that binds with Mn or the like, embrittles the steel, and causes red heat embrittlement. Limited to 0.005% by weight.

(Fine structure)
In addition to having the above-described component system, in order to produce a steel sheet excellent in strength and toughness, the steel sheet preferably has the following microstructure.

  That is, the microstructure inside the steel sheet provided by the present invention includes 75% or more in total of the bainitic ferrite having the shape shown in FIG. 4 and the acicular ferrite having the shape shown in FIG. Here, the ratio of the structure means an area fraction.

  In addition to the fine textured phase as described above, granular bainite can be formed in part. Since the granular bainite becomes a cause of inhibiting low temperature toughness, the content thereof must be limited to 5% or less based on the area fraction.

  The steel sheet of the present invention has a very fine microstructure. This is because the smaller the microstructure, the more the crack propagation can be prevented and the brittle fracture can be prevented. According to the inventors of the present invention, the preferred crystal grain size is austenite. It is 15 μm or less on the basis of crystal grain size.

  As described above, a steel sheet having a component system and satisfying the conditions of the microstructure has a yield strength of 930 MPa or more and an impact toughness of −40 ° C. of 230 joules or more. It is a steel plate that fills.

(Production method)
In order to produce a steel material that satisfies the object of the present invention as described above, the most preferable method derived by the present inventors will be described below.

In the production method of the present invention, generally, a steel slab is heated, and the steel slab is hot-rolled once in the austenite recrystallization temperature region or two or more multi-stage hot-rolled, and is below the T _nr temperature. Yes, further rolling is performed by one hot rolling or two or more multi-stage hot rolling in a temperature range exceeding the Ar ₃ transformation point, and the rolled steel sheet is cooled at a cooling rate of 20 to 50 ° C./second, Including the step of stopping cooling of the steel sheet at 400 ° C. It is preferable that the cooled steel sheet is cooled at room temperature or at room temperature.

  Hereinafter, details of each method will be described.

Slab heating: 1050-1150 ° C
The heating process of the slab is a process of heating the steel so that the subsequent rolling process can be performed smoothly and sufficient physical properties of the target steel sheet can be obtained, and therefore within a temperature range suitable for the purpose. A heating step must be performed. What is important in the above heating process is not only uniform heating to the extent that precipitation elements can be sufficiently dissolved in the slab, but also to prevent the crystal grains from excessively growing due to the heating temperature. That's what it means. If the heating temperature of the slab is less than 1050 ° C., Nb and V are not re-dissolved in the slab, it is difficult to increase the strength of the steel sheet, and austenite crystal grains are formed unevenly due to partial recrystallization. Therefore, it is difficult to increase toughness. When the temperature exceeds 1150 ° C., the austenite crystal grains are excessively coarsened to provide a cause for increasing the crystal grain size of the steel sheet, and as a result, the toughness of the steel sheet is extremely lowered. Therefore, the range of the heating temperature is preferably 1050 to 1150 ° C.

Rolling conditions In order for the steel sheet to have low temperature toughness, the austenite crystal grains should be present in a fine size, which is made possible by controlling the rolling temperature and the rolling reduction. The rolling in the present invention is preferably carried out in two temperature regions, but since the recrystallization behavior differs in the two temperature regions, it is preferable to set the rolling conditions at the respective rolling temperatures. First, in the recrystallization temperature region of austenite, one hot rolling or two or more multi-stage hot rolling is performed at a total rolling reduction of 20 to 80% with respect to the thickness of the initial slab. Rolling in the recrystallization temperature region of austenite as described above has the effect of reducing crystal grains through recrystallization of austenite. However, when performing multi-stage rolling, each rolling is performed so that the crystal grains do not grow after recrystallization of austenite. The stage reduction rate and time must be properly controlled. The fine austenite crystal grains formed by the above-described process serve to improve the low temperature toughness of the final steel plate material. Thereafter, rolling is performed once in the non-recrystallization region of austenite between T _nr (temperature at which austenite does not recrystallize) and Ar ₃ temperature (temperature at which austenite is transformed into ferrite), or two or more multi-stage rolling Is performed again to produce a steel plate. At this time, rolling is performed at a total rolling reduction of 40 to 80% with respect to the thickness of the slab that has been rolled in the austenite recrystallization temperature region. Such rolling in the temperature region between T _nr (temperature at which austenite does not recrystallize) and Ar ₃ (temperature at which austenite transforms to ferrite) crushes the grains and dislocations due to deformation inside the grains. It develops to act as a nucleation site that forms a low-temperature transformation phase during cooling after rolling.

Cooling rate: 20 to 50 ° C./second The cooling rate is one of the important factors for improving the toughness and strength of the steel sheet. The cooling rate is for controlling the structure of the steel sheet with bainitic ferrite or acicular ferrite, as described above. When the cooling rate is slow, polygonal ferrite (Polygonal Ferrite) or FIG. An unfavorable structure such as granular bainite in the form shown in FIG. 5 is formed with a coarse crystal grain size, and the strength and toughness may be greatly reduced. However, in the case of cooling at a high cooling rate exceeding 50 ° C./second, a hard phase such as martensite is formed due to an excessive amount of cooling water, or a distortion phenomenon of the steel sheet occurs. The shape of the steel sheet becomes defective.

Cooling stop temperature: 200-400 ° C
In order to control the microstructure of the steel sheet, it is necessary to cool to a temperature at which the effect of the cooling rate is sufficiently exhibited. If the cooling stop temperature, which is the temperature at which cooling stops, exceeds 400 ° C, it is difficult to sufficiently form bainitic ferrite and acicular ferrite with fine crystal grains inside the steel plate, thus improving yield strength. The effect to make becomes insufficient. Therefore, the upper limit of the cooling stop temperature is limited to 400 ° C. However, if the cooling stop temperature is less than 200 ° C., not only the effect is saturated, but also a problem of distortion of the plate due to excessive cooling may occur.

  A slab having the composition shown in Table 1 below was heated, rolled, and cooled to produce a steel plate having a thickness of 16 mm. The manufacturing conditions for each steel plate were set to be the same. That is, regardless of the type of steel material, the heating temperature for the slab is 1120 ° C, and thereafter, multistage rolling of 9 to 11 stages is performed at a total reduction of 73% at 1050 to 1100 ° C (recrystallization temperature of austenite), Then, the second multi-stage rolling of 9 to 11 stages was performed at 750 to 950 ° C. (non-recrystallization temperature of austenite) at a total rolling reduction of 76%, and steel sheets were manufactured. Immediately after rolling, cooling was performed at a cooling rate of 25 to 35 ° C./second, the cooling was stopped at 250 to 350 ° C., and then left in the atmosphere to be air cooled.

  However, the content unit of the element indicated by * in Table 1 is ppm, and the content unit of the remaining element is wt%.

  As apparent from Table 1 above, Invention Steel 1 to Invention Steel 4 satisfy the conditions of the present invention. However, the comparative steel 1 is applicable when C is excessively low, and the comparative steel 2 is applicable when C is excessively high. Moreover, the comparative steel 3 is a case where Mn is excessively high, and the comparative steel 4 is a case where Ti is excessively high. Comparative steels 5 and 6 correspond to the case where B is excessively high.

  A steel plate sample using a slab having the composition shown in Table 1 was manufactured, and a tensile test, an impact test and a ductile-brittle transition temperature were measured. The results are shown in Table 2.

  In Table 2, vE-40 is impact toughness at -40 ° C, vTrs is ductile-brittle transition temperature, BF is bainitic ferrite, and AF is acicular ferrite.

  As is apparent from the results in Table 2 above, all the inventive steels having the composition of the present invention have a tensile strength of 930 MPa or more, an impact toughness at −40 ° C. of 230 Joules or more, and a ductile-brittle transition temperature also. It is 70 degrees C or less, and has shown the favorable physical property. On the other hand, in the case of the comparative steel 1 with a low C content, the impact toughness is good, but the tensile strength shows a very low value at about 1/2 level of the inventive steel in the present invention, and the comparative steel with an excessively high C content. In the case of No. 2, the tensile strength was 1000 MPa or more, which was very high, but the impact toughness at −40 ° C. was 102 joules, the ductile-brittle transition temperature was −48 ° C., and the strength and toughness were not compatible. The problem was shown as it was. In addition, even in the case of the comparative steel 3 to which Mn was excessively added, it was confirmed that the behavior similar to that of the comparative steel 2 was exhibited. In Comparative Steel 4, when Ti was added excessively, it was confirmed that the impact toughness at −40 ° C. and the ductile-brittle transition temperature were not sufficient. Moreover, although it was excellent in the intensity | strength also in the case of the comparative steel 5 and the comparative steel 6 with excessively high B content, it has confirmed that it did not satisfy | fill impact toughness and a ductile-brittle transition temperature.

  Therefore, it was possible to confirm the influence of the composition of the steel sheet targeted in the present invention.

  A slab having the composition of Invention Steel 1 was selected and rolled under the conditions shown in Table 3 below.

  As apparent from Table 3 above, Invention Examples 1 to 4 are cases where all the conditions of the present invention are satisfied, and Comparative Example 1 is a case where the cooling rate is excessively high. Comparative Example 2 and Comparative Example 3 are cases where the reheating temperature of the slab is excessively high, and in particular, Comparative Example 3 is a case where not only the slab heating temperature is high but also the cooling stop temperature is excessively high. Comparative Example 4 and Comparative Example 5 are cases where the cooling rate is excessively low, and in particular, Comparative Example 5 is a case where the cooling rate is excessively low and the cooling stop temperature is excessively high. Comparative Example 6 is a case where the rolling reduction of the non-recrystallized region is excessively low.

  Steel sheets were produced under the conditions described in Table 3 above, tensile tests, impact tests, and ductile-brittle transition temperatures were measured. The results are shown in Table 4 below.

  In Table 4, vE-40 is impact toughness at -40 ° C, vTrs is ductile-brittle transition temperature, BF is bainitic ferrite, and AF is acicular ferrite.

  As is apparent from Table 4 above, in all of Invention Examples 1 to 4 that satisfy the conditions of the present invention, the tensile strength is 930 MPa or more and the impact toughness (−40 ° C.) is 230 joules or more. It was confirmed that it had. However, Comparative Example 1 was a case where the cooling stop temperature was excessively high, a fine low-temperature phase was not properly formed, and the tensile strength was low. Comparative Example 2 resulted in low low temperature toughness due to excessively high slab reheating temperature and coarse austenite grains. Comparative Example 3 is a case where the reheating temperature and cooling stop temperature of the slab are excessively high, and low temperature toughness is low for the same reason as in Comparative Example 2, and tensile strength is the same as in Comparative Example 1. Showed low results. In Comparative Example 4, when the cooling rate was excessively low, the intended microstructure was not properly formed, a mixture of polygonal ferrite or granular bainite was formed, and both the tensile strength and the low temperature toughness were low. Since Comparative Example 5 has an excessively low cooling rate and an excessively high cooling stop temperature, both the tensile strength and the low temperature toughness were low for the reasons described above. Comparative Example 6 is a case where the reduction rate of the non-recrystallized region is excessively low, the austenite crystal grains cannot be properly stretched, dislocations cannot be accumulated inside the crystal grains, and the low-temperature transformation phase is appropriately It was not formed, and the low temperature toughness was too low.

  Thereby, the effect of the manufacturing method by this invention was able to be confirmed.

Claims (8)

  C: 0.03-0.10%, Si: 0-0.6%, Mn: 1.6-2.1%, Cu: 0-1.0%, Ni: 0-1. 0%, Nb: 0.02 to 0.06%, V: 0 to 0.1%, Mo: 0.1 to 0.5%, Cr: 0 to 1.0%, Ti: 0.005 to 0 0.03%, Al: 0.01 to 0.06%, B: 0.0005 to 0.0025%, N: 0.001 to 0.006%, Ca: 0 to 0.006%, P: 0.00. 02% or less, S: 0.005% or less, balance Fe and other inevitable impurities are included, and the sum of bainitic ferrite and acicular ferrite as a fine structure is contained by 75% or more based on the area fraction, Super high strength steel plate with excellent low temperature toughness.   C: 0.03-0.10%, Si: 0-0.6%, Mn: 1.6-2.1%, Cu: 0-1.0%, Ni: 0-1. 0%, Nb: 0.02 to 0.06%, V: 0 to 0.1%, Mo: 0.1 to 0.5%, Cr: 0 to 1.0%, Ti: 0.005 to 0 0.03%, Al: 0.01 to 0.06%, B: 0.0005 to 0.0025%, N: 0.001 to 0.006%, Ca: 0 to 0.006%, P: 0.00. 02% or less, S: 0.005% or less, balance Fe and other inevitable impurities, the total of bainitic ferrite and acicular ferrite as a microstructure is 75% or more based on the area fraction, Super high strength steel plate with excellent low temperature toughness with yield strength of 930 MPa or more and Charpy impact absorption energy at −40 ° C. of 230 joules or more.   The ultra high strength steel plate excellent in low temperature toughness according to claim 1 or 2, wherein the Mo is added to the steel plate in an amount of 0.015% or less.   The ultra-high-strength steel sheet with excellent low-temperature toughness according to claim 1 or 2, wherein the fine structure of the steel sheet has a granular bainite content of 5% or less based on the area fraction.   The ultra high strength steel sheet excellent in low temperature toughness according to claim 1 or 2, wherein the size of the austenite crystal grains in the thickness direction is 15 µm or less. C: 0.03-0.10%, Si: 0-0.6%, Mn: 1.6-2.1%, Cu: 0-1.0%, Ni: 0-1. 0%, Nb: 0.02 to 0.06%, V: 0 to 0.1%, Mo: 0.1 to 0.5%, Cr: 0 to 1.0%, Ti: 0.005 to 0 0.03%, Al: 0.01 to 0.06%, B: 0.0005 to 0.0025%, N: 0.001 to 0.006%, Ca: 0 to 0.006%, P: 0.00. Heating a steel slab containing 02% or less, S: 0.005% or less, the balance Fe and other inevitable impurities at 1050 to 1150 ° C .;
Rolling the heated steel slab once in a temperature range of 20-80% in a temperature range above the austenite recrystallization temperature or two or more multi-stage hot rolling;
The rolled steel slab is manufactured as a steel sheet by hot rolling once or twice or more multi-stage hot rolling at a rolling reduction of 40 to 80% in a temperature range from below the austenite recrystallization temperature to Ar ₃ or more. And steps to
Cooling the rolled steel sheet at a cooling rate of 20 to 50 ° C./second;
And a step of stopping cooling of the steel sheet at a temperature of 200 to 400 ° C., and a method for producing an ultra-high strength steel sheet having excellent low-temperature toughness.   The said Mo is added to the said steel slab 0.015% or less, The manufacturing method of the ultra high strength steel plate excellent in the low temperature toughness of Claim 6.   The manufacturing method of the ultra high strength steel plate excellent in the low temperature toughness of Claim 6 or Claim 7 which cools the said steel plate by air cooling or room temperature after the cooling of the said steel plate is stopped.

Images