KR20140091745A - Steel material with excellent crashworthiness and manufacturing process therefor - Google Patents
Steel material with excellent crashworthiness and manufacturing process therefor Download PDFInfo
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
- C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
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Abstract
A steel material excellent in impact resistance capable of increasing the energy absorbing ability at the time of impact, and a method of manufacturing the steel material.
Wherein the steel composition satisfies Ceq? 0.36% and the structure is composed of a ferrite phase and a hard phase, and the volume fraction of the ferrite phase is 75% or more in all of the plate thickness, Hv is 140 or more and 160 or less, Is not less than 2 占 퐉.
Description
The present invention relates to a steel material used for a large structure such as a ship and a manufacturing method thereof, and more particularly to a steel material having a high uniform elongation which is effective for suppressing damage such as a collision of a ship, ) Steel material excellent in collision energy absorbability and a method of manufacturing the same.
In recent years, environmental contamination due to outflow of oil due to stranding or collision of large tankers has become a problem. In order to prevent oil spills due to these accidents, measures have been taken in terms of the structure of the hull such as a double hull of a ship hull, but sufficient countermeasures have been considered for the steel for hull not. Among them, it has been proposed to absorb a large amount of energy at the time of collision in the steel itself as a countermeasure on the steel material surface for the ship, but a sufficient practical stage has not yet been reached.
As a method for improving the energy absorbing ability at the time of impact, a technique of reinforcing the ferrite phase by using the structure of the steel sheet as the main body of ferrite is proposed in Patent Document 1. This technique is characterized in that the volume fraction of ferrite F is 80% or more and the lower limit (H? 400-2.6 占 F) of hardness of ferrite H is defined have.
Further, a technique of including a retained gamma phase on the surface and back layers of a steel sheet is proposed in Patent Document 2. This technique is a method for producing a steel sheet which contains C, Si, Mn and Al, additionally contains reinforcing elements if necessary, and has a residual gamma of 1.0 to 20% .
In these techniques, the energy absorbability of a collision is defined as the ratio of the strength of the steel (mean of yield stress and rupture stress) to the total elongation As a product. Therefore, the absorbed energy is increased by the improvement of both the strength and the elongation percentage.
In addition, Patent Document 3 discloses that the volume fraction of ferrite phase in the steel sheet metal structure is 70% or more at the center of the plate thickness and 50% or more at the plate thickness portion, and the uniform elongation is increased Thereby improving the collision resistance.
Patent Document 4 discloses that the area fraction of the ferrite occupied in the whole structure in the entire metal structure of the steel sheet is 90% or more, the average ferrite grain size is 3 to 12 탆, It is possible to improve the collision absorbability by setting the maximum ferrite grain size to be not larger than 40 占 퐉 and the average diameter equivalent to a circle of the second phase not larger than 0.8 占 퐉 and by multiplying the uniform elongation and breaking stress Is proposed.
The evaluation of the absorbed energy by the elongation percentage used in the above Patent Documents 1 and 2 is not necessarily related to the evaluation of the safety of the hull structure and is not suitable when discussing the impact resistance. That is, in order to evaluate the elongation deformation of the shell plating supported on the stiffener by a large span which is not comparable to the point distance in the tensile test, Evaluation of the elongation including the local elongation is not appropriate. Therefore, when the absorbed energy at the time of impact is considered, it is necessary to evaluate the absorbed energy at a uniform elongation determined to be highly correlated with the elongation characteristics of the outer shell of the ship.
For example, in the technique of Patent Document 1, the ferrite grain size is 5 占 퐉 or less and the hardness of the ferrite is as high as Hv 160 to 190 in Examples (same document, Table 2). Therefore, it is estimated that the elongation percentage (EL in the same table) is 23 to 32%, and the uniform elongation can not be made higher than this, so it is estimated to be at most about half of the total elongation.
In addition, in the technique of Patent Document 2, a large amount of alloying element is added in order to contain residual? In the structure, and the steel of the embodiment has a high carbon equivalent (Ceq) or a high Si content.
For example, in Table 1 of this document, in steel type A, Ceq is about 0.38, and in steel types B to F, Si is 0.55 to 1.94%, which is all high. Therefore, even though the overall ductility is low and only the surface layer has a high degree of uniform elongation, it is presumed that it is difficult to improve the uniform elongation because the uniform elongation is determined in the low ductility portion.
For these steel types, no test results regarding toughness or weldability are disclosed at all. In addition, in this document, the term "shock absorbing energy" means EL × YP + TS / 2 in Table 2) and refers to the product of the total elongation and the strength. Therefore, it is presumed that the steel having a high Si content is low in toughness and the steel material having a high Ceq is inferior in weldability with respect to the materials of these steel types from the viewpoint of a normal steel material.
Generally, in a steel material for a hull, the required yield stress is determined from the design requirement, and the strength grade of the steel material is selected according to the part to be used, so that the required strength is not particularly required. In addition, in order to improve the strength, the increase of the cost due to the addition of the alloying element or the like and the deterioration of the weldability are caused, so that the improvement of the absorption energy by the increase of the strength is not preferable.
On the other hand, in the technique of Patent Document 3, the uniform elongation is improved by suppressing the addition amount of the alloying element to a low level and increasing the percentage of the ferrite phase having a low hardness and a high ductility. However, the development of a manufacturing method of increasing the ferrite phase fraction of the surface layer portion to the same level as that of the central portion of the plate thickness has not been developed yet. In addition, in the examples, only those having a sheet thickness of 25 mm or less, which is relatively small, are not disclosed. It is remarkably difficult to ensure the ferrite fraction of the surface layer portion because the amount of water and the amount of controlled cooling at the time of manufacture increase as the plate thickness increases.
In Patent Document 4, information on the chemical composition and the metal structure of the steel material is disclosed, but there are many practical points of uncertainty in the manufacturing method. That is, in the manufacturing method described in the detailed description, it is recommended to perform reheating after hot rolling and cooling. However, in a shipbuilding steel sheet which is inexpensive and also requires mass production, a process such as reheating is likely to be put to practical use from the viewpoint of production cost and manufacturing period. In addition, in cooling after rolling, it is suggested in Patent Document 3 that a characteristic difference in the plate thickness direction is likely to occur, but it is not considered in Patent Document 4, and the characteristic evaluation of the embodiment is only a plate thickness of 1/4, The characteristics of the surface layer portion of the thickness are not disclosed.
Considering the above, it is considered that the steel material having excellent energy absorption performance at the time of collision of a ship still needs to be improved in performance, and there is also a possibility of expanding the thickness of the plate that can be manufactured. In particular, it is necessary to establish an ideal metal microstructure in consideration of the entire plate thickness including the surface layer portion of the plate thickness and break throuhg of the manufacturing method thereof.
An object of the present invention is to provide a steel structure which is capable of increasing the energy absorbing ability at the time of collision as compared with the currently proposed steel material without increasing the cost due to the addition of alloying elements or the like, And an object of the present invention is to provide an excellent steel material and a manufacturing method thereof.
The features of the present invention to solve these problems are as follows.
In order to improve the uniform elongation, the steel material of the present invention may be a ferrite of a soft phase and a structure of two or more phases of a hard phase such as pearlite, bainite, or martensite It is made of steel. Further, the structure of this steel material was obtained while optimizing the mechanical properties of each phase and optimizing the combination thereof, and it was based on the following recognition.
Generally, in steels having two or more phases, the soft phase mainly plays a role of improving ductility and toughness, and the hard phase mainly plays a role of strength improvement. Therefore, in order to improve the uniform elongation, the properties of the ferrite phase as a soft phase were examined. It is clear that the uniform elongation is better as the soft material is. However, in the case where a hard phase exists in addition to the hard phase, the larger the difference between the two phases is, the larger the concentration of deformation into the soft phase becomes, and the soft phase contribution to the uniform elongation becomes large. In the case of a bainite phase having a comparatively low strength as a hard phase, in order to increase the concentration of deformation into a ferrite phase, the hardness of the ferrite phase has to be Hv 160 or less. In order to obtain a tensile strength of 490 MPa or more, Hv 140 or more is required.
Since the uniform elongation decreases as the grain size decreases, the influence of the ferrite crystal grain size of the dual-phase steel was investigated. As a result, it was confirmed that when the average crystal grain size was less than 2 탆, the uniform elongation rapidly decreased did. Here, since the local elongation ratio is relatively unaffected by the grain size, it is also confirmed that the decrease in the elongation percentage due to the decrease in the crystal grain size is relatively small as compared with the decrease in the uniform elongation. Therefore, also in this respect, when evaluating the ductility, it is necessary to distinguish between the uniform elongation and the total elongation.
As a result of examining the relationship between the ratio of the soft phase and the hard phase and the uniform elongation, it was found that the higher the volume fraction of the ferrite phase, the better the uniform elongation. In particular, it has been found that when the volume fraction of ferrite phase is 75% or more of the entire plate thickness, the uniform elongation is excellent. It has become clear that when the hardness of the ferrite phase is Hv 140 or more and 160 or less, it is particularly important to recognize that the influence of the surface layer portion is large, and that the increase of the volume fraction of ferrite phase as the whole plate thickness is important.
In order to secure a predetermined ratio of the ferrite phase volume fraction in this manner, the cooling conditions must be appropriately adjusted. That is, the cooling step is roughly divided into two stages, that is, a front stage focused on the transformation from the austenite structure to the ferrite phase at the end of rolling and a rear stage that causes the transformation to the hard phase.
In the cooling of the shear stage, the average temperature of the steel sheet is changed from a temperature (Ar 3 -50) 캜 or more at which ferrite transformation is difficult to proceed relatively easily, from the viewpoint of phase equilibrium and the ferrite transformation phase (from the viewpoint of ferrite phase transformation based on the phase equilibrium and kinetics) It is ideal to cool rapidly to an average steel sheet temperature of not less than (Ar 3 -150) ° C (Ar 3 -50) ° C or less. However, as the cooling rate is increased, the difference in cooling rate in the thickness direction of the steel sheet increases. For this reason, a transformation into a hard phase such as bainite or martensite takes place in place of the ferrite transformation in the plate thickness surface layer portion having a high cooling rate. Therefore, it is necessary to suppress the transformation into the hard phase. When the cooling rate of the surface of the steel sheet is controlled to be 100 deg. C / sec or more, the generation of the hard phase can be suppressed by controlling the temperature of the surface of the steel sheet not to be lower than 400 deg.
Further, the ferrite phase is formed during the process of recuperate of the surface temperature of the steel sheet after cooling by the heat at the central portion of the plate thickness. In addition, the steel plate average cooling temperature may not be (Ar 3 - 150) 캜 or higher (Ar 3 - 50) 캜 or lower due to the thickness of the plate being thick. In that case, cooling is repeated a plurality of times.
On the other hand, a method of suppressing generation of a hard phase in the surface layer portion of the steel sheet by slowing the cooling rate can also be considered. However, the cooling takes a long time to lower the production efficiency, and when the cooling rate is less than 100 ° C / second, the relationship between the cooling rate and the upper limit temperature of the generation of the hard phase also changes more complicatedly, which is difficult to control. If the cooling rate is 100 占 폚 / second or more, the transformation to the hard phase can be suppressed if the temperature is not lower than 400 占 폚, so control is easy.
By the cooling method described above, after cooling to a predetermined temperature, the ferrite transformation at the central portion of the plate thickness can be promptly advanced. It takes more than 10 seconds to make the volume fraction more than 75%.
Next, the cooling of the subsequent stage for generating the hard phase was examined from the viewpoint of the influence of the structure on the strength. The strength is greatly affected by the strength of the hard phase and the volume fraction. However, when the composition of the steel is constant, it is confirmed that even if the structure changes, it is possible to control to obtain an arbitrary strength according to the selection of the manufacturing conditions.
That is, when the volume fraction of the hard phase is relatively large, it is possible to obtain a predetermined strength by lowering the cooling stop temperature after rolling and lowering the hard phase strength by lowering the cooling rate.
On the other hand, when the volume fraction of the hard phase is relatively small, it is possible to obtain a predetermined strength by lowering the cooling stop temperature after rolling or increasing the strength of the hard phase by increasing the cooling rate.
Control of such a strength is relatively easily achieved from the principle that when the volume fraction of the hard phase is small, the carbon concentration to be concentrated from the ferrite phase to the hard phase at the time of transformation becomes high and the hard phase becomes harder to cure.
The method of controlling the cooling rate may be air cooling if the predetermined condition is satisfied. However, in the case of warming, a heat insulating cover is formed on the steel material, or in the case of increasing the cooling rate, water cooling is performed.
Finally, in steels used in ships and the like, toughness is also one of the important mechanical properties. In the steel of the structure of the ferrite main body to which the present invention is applied, the toughness is mainly influenced by the ferrite crystal grain size, and therefore it is preferable to set the grain size to 40 탆 or less. The control of the crystal grain size can be made by, for example, reducing the rolling reduction rate to a predetermined value or more in the rolling process.
The features of the present invention based on the above recognition are as follows.
The first aspect of the present invention is a first invention, wherein the steel composition satisfies Ceq? 0.36%, the structure consists of a ferrite phase and a hard phase, the volume fraction of the ferrite phase is 75% And a crystal grain size of 2 탆 or more.
However, Ceq is represented by the following formula (1).
Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (One)
However, the symbol of the element represents the mass% of each element.
The second invention is a steel material excellent in collision resistance according to the first invention, characterized in that the ratio of the volume fraction of the ferrite phase in the sheet thickness surface layer portion to the volume percentage of the ferrite phase in the plate thickness central portion is 0.925 or more and 1.000 or less .
A third aspect of the present invention is a ferritic stainless steel comprising, as the steel composition, 0.05-0.16% of C, 0.1-0.5% of Si, 0.8-1.6% of Mn and 0.002-0.07% of Sol.Al, Is a steel material excellent in collision resistance described in the first or second invention, characterized in that it is made of inevitable impurities.
The fourth invention is a steel material excellent in impact resistance according to the third invention, characterized by further containing, as a steel composition, Ti: 0.003 to 0.03% by mass%.
The fifth aspect of the present invention is a steel material excellent in collision resistance according to the third or fourth invention, characterized by further containing 0.005 to 0.05% Nb in terms of mass% as the steel composition.
A sixth aspect of the present invention is a steel ingot comprising, as the steel composition, one or two selected from the group consisting of 0.1 to 0.5% of Cr, 0.02 to 0.3% of Mo, 0.01 to 0.08% of V and 0.1 to 0.6% of Cu, Is a steel material excellent in collision resistance described in any one of the third to fifth inventions.
The seventh invention is a steel material excellent in collision resistance according to any one of the third to sixth inventions, characterized in that the steel composition further contains 0.1 to 0.5% of Ni by mass%.
An eighth aspect of the present invention is a rolling method for rolling a steel material having a steel composition according to any one of the first aspect of the invention or the third to seventh inventions by rolling at a cumulative rolling reduction of 50% or more at a temperature range of Ar 3 point to 850 ° C I do. Then, the start of the shearing steels are cooled from an average temperature of (Ar 3 -50) ℃ or more and the cooling rate of the steel surface is 100 ℃ / sec or more, the steel material surface temperature is above (Ar 3 -50) ℃ below 400 ℃ Until the average temperature of the steel material becomes (Ar 3 - 150) 占 폚 or higher (Ar 3 - 50) 占 폚 or lower. Thereafter, cooling is performed for 10 seconds or more, and the rear end cooling is performed until the average steel temperature reaches 300 ° C or higher and 600 ° C or lower at an average steel cooling rate of 10 ° C / sec or higher from the average steel temperature (Ar 3 -150) Which is excellent in impact resistance.
However, the Ar 3 point is represented by the following formula (2).
Ar 3 = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo ... (2)
However, the symbol of the element represents the mass% of each element.
According to the present invention, by optimizing the mechanical properties of each phase by using ferrite as a soft phase and steel composed of two or more phases of a hard phase, which are almost the same as ordinary steels for general hull, It is possible to obtain a steel material having high strength and excellent impact resistance. In addition, the production method can be efficiently and stably produced because the efficiency is lowered and the controllability is not particularly difficult as compared with a conventional method for producing a steel material for a hull.
As a result, it is possible to provide a steel material excellent in energy absorption performance at the time of a collision of a ship, without increasing cost due to the addition of alloying elements or the like to currently used steel materials and without adding special manufacturing facilities, very big. In addition, from the viewpoint of preventing oil spill due to stranding or collision of a large tanker, the effect of environmental protection is also very great.
(Mode for carrying out the invention)
The reasons for limiting each constituent requirement of the present invention will be described below.
1. About metal structure
The steel material of the present invention is a steel material having substantially the same components as ordinary steels for hull, excellent in impact resistance, that is, excellent in uniform stretchability. In other words, to improve the uniform elongation without lowering the strength, the mechanical properties of each phase are optimized by using ferrites as soft phases and steels having two or more phases such as pearlite, bainite and martensite as hard phases Along with optimizing the combination.
The structure of the steel material of the present invention is composed of a ferrite phase and a hard phase. The hard phase is constituted by a structure having a hardness higher than that of a ferrite phase such as pearlite, bainite or martensite.
Percent volume fraction of ferrite: 75% or more in total plate thickness
The higher the volume fraction of the ferrite phase, the better the uniform elongation. The metal structure slightly changes in the thickness direction, but in order to obtain a sufficient uniform elongation, it is necessary that the volume fraction of the ferrite phase in the entire plate thickness is 75% or more.
Further, in the present invention, the surface layer portion of the plate thickness is a region from the surface of the plate to a depth of about 1/10 of the plate thickness. The plate thickness portion is a region where the cooling rate is relatively higher than that at the central portion of the plate thickness during cooling, a hard phase is likely to be generated, and a uniform elongation is likely to decrease. Considering the entire plate thickness, it is not so large in terms of fraction, and the influence can be imparted to a certain extent, but the influence can not be ignored if the characteristic difference from the central portion of the plate thickness is large. Therefore, it is also necessary to secure the ferrite phase volume fraction in the surface layer portion of the plate thickness.
In order to confirm whether the volume fraction of the ferrite phase in the entire plate thickness is within the range of the present invention as a main factor affecting the volume fraction of ferrite phase as described above, And the volume fraction of the ferrite phase with respect to the plate thickness surface layer portion having the greatest cooling rate in the plate thickness direction.
The ratio of the volume fraction of the ferrite phase in the surface layer portion to the volume fraction of the ferrite phase in the central portion of the plate thickness: 0.925 or more and 1.000 or less
The ratio of the volume fraction of the ferrite phase in the sheet thickness surface layer portion to the volume fraction of the ferrite phase in the central portion of the plate thickness (hereinafter also simply referred to as the volume fraction ratio) Quot;) is preferably 0.925 or more and 1.000 or less. When the volume fraction ratio is set to 0.925 or more, the difference in material between the plate thickness portion and the plate thickness central portion, particularly the difference in the uniform elongation, becomes sufficiently small and can be regarded as a substantially uniform structure in the plate thickness direction. It is preferable from the viewpoint of property. The volume fraction ratio is preferably 0.935 or more. In addition, the cooling rate of the sheet thickness surface layer portion is relatively higher than that at the center portion of the sheet thickness during cooling, and a hard phase is likely to be generated, so that the central portion of the sheet thickness is higher in ferrite volume fraction than the sheet thickness portion. For this reason, the volume fraction ratio is set to 1.000 as the upper limit.
Hardness in ferrite phase: Hv in the range of 140 to 160
The lower the hardness of the ferrite phase, the better the uniform elongation. Since the uniform elongation is excellent when the hardness of the ferrite phase is 160 or less in Hv, Hv is 160 or less. On the other hand, in order to obtain a strength of TS 490 MPa or more, Hv is 140 or more.
Average crystal grain size of ferrite phase: 2 탆 or more
The smaller the average crystal grain size of the ferrite phase, the lower the uniform elongation. Particularly, when the average crystal grain size is less than 2 占 퐉, the uniform elongation rate sharply deteriorates. By making the average crystal grain size of the ferrite phase 2 mu m or more, a high uniform elongation can be stably obtained. The average crystal grain size of the ferrite phase is preferably 4 mu m or more. In addition, when the ferrite structure is excessively large, the steel may be softened. Therefore, in order to stably obtain a tensile strength of 490 MPa or higher, the average crystal grain size of the ferrite phase is preferably 40 탆 or less.
2. Composition
The reasons for defining the composition of the present steel material will be described. The term "%" means "% by mass".
Ceq: 0.36 or less
The higher the Ceq, the higher the strength and the higher the strength of the ferrite. Therefore, the uniform elongation decreases. When the Ceq exceeds 0.36, the uniform elongation decreases remarkably. In addition, Ceq is an index of toughness of the weld heat affected zone, and if it exceeds 0.36, the heat toughness of large-heat-input-welding decreases. For this reason, Ceq should be 0.36 or less. Here, Ceq is obtained by the following formula (1).
Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (One)
However, the symbol of the element represents the mass% of each element.
C: 0.05 to 0.16%
C is contained to secure strength. If the content is less than 0.05%, the effect is not sufficient. If it exceeds 0.16%, the structure of the ferrite main body can not be obtained and the uniform elongation is decreased. Therefore, the content of C is in the range of 0.05 to 0.16%.
Si: 0.1 to 0.5%
Si is contained as an element for de-oxidation and strength improvement in the steelmaking step. If the Si content is less than 0.1%, the effect is insufficient. If the Si content exceeds 0.5%, the ductility is deteriorated. Therefore, the Si content is in the range of 0.1 to 0.5%.
Mn: 0.8 to 1.6%
Mn is contained to secure strength. If the Mn content is less than 0.8%, the effect is insufficient. If the Mn content exceeds 1.6%, the structure of the ferrite main body can not be obtained. Therefore, the Mn content is within a range of 0.8-1.6%.
Sol.Al: 0.002-0.07%
Al is contained for deoxidation. When the amount of Sol.Al is less than 0.002%, the effect is not sufficient. When the amount of Sol.Al is more than 0.07%, surface scratches of the steel tend to occur, so the amount of Sol.Al is in the range of 0.002 to 0.07%. Preferably, it is in the range of 0.01 to 0.05%.
The above is the basic chemical constituent of the present invention, and the balance is Fe and inevitable impurities. Further, in order to improve the strength and toughness, Ti and Nb may be contained as selective elements.
Ti: 0.003 to 0.03%
In order to further improve toughness, Ti may be contained. Ti produces TiN during rolling heating or welding, and finely austenite grains improve the toughness of the base material and the toughness of the weld heat affected zone. If the content is less than 0.003%, the effect is not sufficient. If the content is more than 0.03%, the toughness of the weld heat affected zone is lowered. Therefore, when Ti is contained, the content is preferably in the range of 0.003 to 0.03% desirable. More preferably in the range of 0.005 to 0.02%.
Nb: 0.005 to 0.05%
In order to improve the strength, Nb may be contained. If the content is less than 0.005%, the effect is not sufficient. If the content is more than 0.05%, the toughness of the weld heat affected zone is lowered. Therefore, when Nb is contained, the content is preferably in the range of 0.005 to 0.05%. More preferably in the range of 0.005 to 0.03%.
In order to improve the strength, one or more of Cr, Mo, V, and Cu may be contained.
Cr: 0.1 to 0.5%
If the content of Cr is less than 0.1%, the effect is insufficient. If the Cr content exceeds 0.5%, the toughness of the weldability and the weld-affected area deteriorates. Therefore, when Cr is contained, the Cr content is preferably in the range of 0.1 to 0.5%.
Mo: 0.02 to 0.3%
If the content of Mo is less than 0.02%, the effect is insufficient. If the content of Mo exceeds 0.3%, the weldability and the toughness of the weld heat affected zone are remarkably deteriorated. Therefore, the content of Mo is preferably in the range of 0.02 to 0.3%.
V: 0.01 to 0.08%
If the content of V is less than 0.01%, the effect is insufficient. If the content of V exceeds 0.08%, the toughness deteriorates remarkably. Therefore, when V is contained, the content of V is preferably within a range of 0.01 to 0.08%.
Cu: 0.1 to 0.6%
If the content of Cu is less than 0.1%, the effect is not sufficient. If the content of Cu exceeds 0.6%, the possibility of cracking of the Cu increases. Therefore, in the case of containing Cu, the content of Cu is preferably in the range of 0.1 to 0.6%. More preferably in the range of 0.1 to 0.3%.
In order to improve toughness, Ni may also be contained.
Ni: 0.1 to 0.5%
If the Ni content is less than 0.1%, the effect is not sufficient. If the Ni content exceeds 0.5%, the increase in the cost of the steel is remarkable. Therefore, in the case of containing Ni, the Ni content is preferably in the range of 0.1 to 0.5%.
3. Manufacturing conditions
A steel material excellent in collision resistance according to the present invention can be produced under the following production conditions.
First, molten steel having the above composition is melted by a converter or the like, and made into a steel material (slab) by continuous casting or the like. Then, the steel material is heated to a temperature of 900 to 1150 占 폚, followed by hot rolling.
In order to obtain good toughness, it is effective to lower the heating temperature and reduce the crystal grain size before rolling. If the heating temperature is lower than 900 ° C, the rolling load becomes excessive. If the heating temperature is higher than 1150 ° C, the austenite grains are coarsened to cause deterioration of toughness, oxidation loss becomes remarkable, . The heating temperature is preferably 900 to 1150 占 폚, because stable rolling is possible and satisfactory toughness can be obtained. A more preferable heating temperature range from the viewpoint of toughness is 1000 to 1100 deg.
Rolling condition: Cumulative reduction ratio of 50% or more at a temperature range from Ar 3 point to 850 ° C
The steel material is hot-rolled to produce a steel sheet having a desired sheet thickness. The starting temperature of the hot rolling is not particularly limited. Further, besides the rolling condition at the non-recrystallization temperature range of austenite to be described later, it is not necessary to form a constraint particularly as the rolling condition. Further, in order to make grain size reduction and grain size regulation of the austenite recrystallized structure prior to rolling at a non-recrystallization temperature range of the austenite described below, It is preferable to carry out rolling at a rate of 30% or more.
In rolling, processing strain is introduced in a temperature range of Ar 3 point to 850 ° C, which is the non-recrystallization temperature range of austenite, in order to improve toughness. With respect to the cumulative reduction ratio, at least 50%, the ferrite crystal grain size after transformation is sufficiently fine, and toughness can be improved. Therefore, the cumulative rolling reduction during rolling is made 50% or more at a temperature range of Ar 3 point or more and 850 ° C or less. It is preferably at least 55%. Although the upper limit of the cumulative reduction ratio does not need to be specially specified, it is preferably 80% or less. The Ar 3 point can be obtained by the following formula (2).
Ar 3 = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo ... (2)
However, the symbol of the element represents the mass% of each element.
The rolling finish temperature is preferably at least 3 points of Ar. If the rolling finish temperature is lower than the Ar 3 point, the processed ferrite structure may remain, which may lower the elongation of the finally obtained steel. Therefore, the rolling finish temperature is preferably 3 or more.
In the present invention, the steel sheet subjected to the hot rolling is subjected to the first stage cooling prior to the cooling, followed by the cooling, and then the second stage cooling.
The front end cooling of the first stage before cooling is focused on the transformation from the austenite phase structure to the ferrite phase at the end of rolling. By the subsequent cooling, the volume fraction, hardness and crystal grain size of the ferrite phase . For this reason, the shear cooling starts from a temperature at which the steel material average temperature is (Ar 3 -50) 占 폚 or more, and ferrite transformation during fermentation is easy to proceed from the viewpoint of phase equilibrium and kinetics, (Ar 3 -150) ° C or higher (Ar 3 -50) ° C or lower.
The front end of the cooling, the steel plate Because the average temperature of (Ar 3 -50) From the above ℃ temperature, (Ar 3 -150) ℃ than (Ar 3 -50) or more to rapidly cool the steel sheet to a temperature not higher than the average ℃, The cooling rate shall be not less than 100 ° C / s at the steel surface cooling rate. However, as the cooling rate is increased, the difference in the cooling rate in the thickness direction of the steel sheet becomes larger, so that the transformation from ferritic transformation to a hard phase such as bainite or martensite takes place . If the temperature of the surface of the steel sheet at the end of the shearing cooling is controlled so as not to become lower than 400 占 폚 when the cooling rate of the surface of the steel sheet is set to 100 占 폚 / The generation of the hard phase in the cooling process can be suppressed. If the cooling rate is less than 100 占 폚 / sec at the steel surface cooling rate, the ferrite transformation and the transformation of the hard phase are complicated and the transformation control during cold cooling becomes difficult. It is possible to increase the driving force of the ferrite transformation in the cold cooling step after the shearing cooling by securing a cooling rate of 100 deg. C / sec or more at the cooling rate of steel surface at one time to the predetermined temperature range, The volume fraction, the hardness, and the grain size of the crystal phase can be defined in the present invention.
The cooling method of the shearing cooling is performed once or twice more than the surface temperature of the steel sheet up to the temperature range of 400 ° C or higher (Ar 3 -50) or lower.
This is, when the surface of the steel sheet temperature is less than 400 ℃ discarded proceeds to the transformation of the hard phase sharply, and can not be obtained a desired ferrite phase volume fraction, while (Ar 3 -50) ℃ than the plate thickness of the entire The cooling effect is almost lost. Thus, the further, the steel sheet surface layer and as the front end of cooling the steel sheet surface temperature, when cooled to a temperature range of less than 400 ℃ (Ar 3 -50) ℃ to the steel sheet surface temperature, to secure the cooling effect of the entire thickness A ferrite phase of a predetermined volume fraction can be obtained. When the average temperature of the steel sheet does not reach the predetermined temperature by one cooling, the surface of the steel sheet can be cooled repeatedly under the same conditions after the surface of the steel sheet is reheated by the heat at the center of the plate thickness. Here, cooling the surface of the steel sheet after the second cooling is performed to prevent excessive cooling of only the surface layer portion of the steel sheet. By doing so, the cooling behavior of the entire steel sheet including the central portion of the steel sheet, Balance with the behavior can be obtained.
The cooling after the shearing cooling is carried out for 10 seconds or longer in the temperature range of (Ar 3 -150) - (Ar 3 -50) ° C at the average temperature of the steel material.
The cooling after the shearing cooling is carried out in order to make the volume fraction of the ferrite phase, the hardness and the grain size to be predetermined. With respect to the cooling temperature range, when the average steel temperature is lower than (Ar 3 -150) ° C, it takes a long time to proceed the ferrite transformation, and when the transformation temperature exceeds (Ar 3 -50) Lt; / RTI > Therefore, the cooling temperature range is set to (Ar 3 - 150) ° C or higher (Ar 3 - 50) ° C or lower as the steel average temperature. With respect to the cooling time, if it is less than 10 seconds, the desired ferrite phase dispersion control (ferrite volume fraction: 75% or more, average crystal grain size: 2 탆 or more) can not be achieved because ferrite transformation does not proceed sufficiently, The diffusion from the phase to the austenite phase does not proceed sufficiently and the hardness of the ferrite phase does not become Hv 160 or lower. Therefore, the cooling time is set to 10 seconds or more. As described above, the ferrite phase volume fraction, hardness, and crystal grain size can be set to predetermined values by performing cooling for 10 seconds or more in a temperature range of (Ar 3 -150) to (Ar 3 -50) ° C at an average steel temperature.
The average temperature of the steel material can be obtained by simulation calculation or the like when the shape of the steel material, the surface temperature, the cooling conditions, and the like are given.
In the second stage cooling, the steel sheet is cooled from 300 ° C to 600 ° C at a cooling rate of 10 ° C / sec or more from a temperature of (Ar 3 -150) ° C or more at an average temperature of steel.
The second stage of cooling, which is the cooling of the second stage, controls the cooling start temperature, the cooling rate, and the cooling termination temperature in order to ensure the predetermined strength by causing transformation from the austenite phase structure to the hard phase. As the cooling start temperature is lower, the strength is lowered. When the average steel temperature is lower than (Ar 3 -150) ° C, a predetermined strength can not be obtained. Therefore, for the purpose of securing a predetermined strength, 3 -150) ℃ or higher.
Since the steel material average cooling rate is higher than the average cooling rate, the strength is improved. However, since the predetermined strength can not be obtained at an average cooling rate of the steel material of less than 10 DEG C / second, Or more.
The lower the cooling end temperature is, the more the strength is improved. However, when the temperature is lowered to less than 300 ° C, the softening is deteriorated. On the other hand, since cooling can be stopped at a temperature exceeding 600 占 폚, a predetermined strength can not be obtained. Therefore, the cooling termination temperature is set to 300 占 폚 or more and 600 占 폚 or less in terms of the steel average temperature.
Example 1
Hereinafter, embodiments will be described. Table 1 shows the components of the sample steel used in the examples. The remainder that is not marked consists of iron and inevitable impurities. The steel types A to H in Table 1 are steels having the composition satisfying the present invention, and the steel type I has Ceq outside the scope of the invention (upper limit: 0.36%).
The cast slab having these steel compositions was heated, rolled into a steel sheet having a thickness of 12 to 50 mm, and cooled in various cooling patterns. Table 2 shows the production conditions. Steel Nos. 1 to 10 are the inventions satisfying the composition and manufacturing conditions of the present invention, and Steel Nos. 11 to 16 are comparative examples in which the manufacturing conditions or the composition of the components are out of the scope of the present invention.
The microstructures of these steel sheets were observed by an optical microscope, and the central portion of the plate thickness, the volume fraction of the ferrite phase in the plate thickness portion, and the crystal grain size (average crystal grain size) of the ferrite were measured. The hardness of the ferrite phase was measured by a micro Vickers hardness meter (load: 25 gf) with respect to the plate thickness central portion and the plate thickness surface portion portion, and the average value thereof was determined.
Further, strength, uniform elongation and toughness were obtained as mechanical properties. The tensile test was carried out by collecting JIS 1B test pieces of full thickness in the direction perpendicular to the rolling direction of the steel sheet. The uniform elongation was evaluated as elongation at the time of maximum stress. The impact test was conducted by taking a JIS No. 4 standard test piece in parallel with the rolling direction and near the surface layer (the distance between the surface of the steel and the cross section of the test piece was 2 mm or less). Toughness was evaluated by vTrs (brittle ductility transition temperature).
Table 3 shows test results such as microstructure and mechanical properties of the steel sheet.
As shown in Table 3, the steel casings 1 to 10 of Inventive Examples all have excellent properties such that TS (tensile strength) is 520 MPa or more and uniform elongation is 22% or more. YS (yield strength) of the steel No. 1 to 10 is 390 MPa or more and vTrs is lower than -40 ° C, and YS? 355 MPa, TS? 490 MPa, uniform elongation? 20% and vTrs? Respectively.
On the other hand, the steel numbers 11 to 16 are comparative examples, the steel No. 11 has a high Ceq, and even if the manufacturing conditions are sought, the desired characteristics can not be obtained, the volume fraction of the ferrite phase in the sheet thickness surface layer portion is small and the uniform elongation is inferior have. In the steel No. 12, since the cooling start temperature of the front end is excessively low, the volume fraction of the ferrite phase in the plate thickness central portion and the plate thickness portion is reduced and the uniform elongation is inferior. Steel coil 13 has a slow cooling rate with respect to the prescribed cooling rate (100 DEG C / s or more) of the front end cooling, so that the volume fraction of the ferrite phase is small and the uniform elongation is inferior.
Steel No. 14 has a too low termination temperature for shear cooling, and thus has a low volume fraction of ferrite phase and poor uniform elongation. In the steel No. 15, since the end temperature of the shearing cooling is too low, the volume fraction of the ferrite phase is small and the uniform elongation is inferior. Since the cooling time between the front end cooling and the rear end cooling was short in the steel No. 16, the volume fraction of the ferrite phase was low and the uniform elongation was inferior.
Claims (8)
However, Ceq is represented by the following formula (1).
Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (One)
However, the symbol of the element represents the mass% of each element.
Wherein the ratio of the volume fraction of the ferrite phase in the surface layer portion to the volume fraction of the ferrite phase in the central portion of the plate thickness is 0.925 or more and 1.000 or less.
A steel composition comprising 0.05 to 0.16% of C, 0.1 to 0.5% of Si, 0.8 to 1.6% of Mn and 0.002 to 0.07% of Sol.Al as the steel composition in mass%, the balance being iron and inevitable impurities Features excellent impact resistance.
A steel material excellent in collision resistance, characterized by further comprising, by mass%, Ti: 0.003 to 0.03%.
The steel material according to any one of claims 1 to 3, further comprising 0.005 to 0.05% of Nb in terms of mass%.
The steel composition according to any one of claims 1 to 3, further comprising one or more selected from the group consisting of 0.1 to 0.5% of Cr, 0.02 to 0.3% of Mo, 0.01 to 0.08% of V and 0.1 to 0.6% of Cu, Wherein the steel sheet has excellent impact resistance.
The steel material according to any one of claims 1 to 3, further comprising, by mass%, Ni: 0.1 to 0.5%.
However, the Ar 3 point is represented by the following formula (2).
Ar 3 = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo ... (2)
However, the symbol of the element represents the mass% of each element.
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| JP6007968B2 (en) * | 2014-12-26 | 2016-10-19 | 新日鐵住金株式会社 | High-strength and highly ductile steel plate and its manufacturing method |
| JP6477020B2 (en) * | 2015-02-27 | 2019-03-06 | 新日鐵住金株式会社 | Hot rolled steel sheet and manufacturing method thereof |
| KR102164112B1 (en) | 2018-11-29 | 2020-10-12 | 주식회사 포스코 | High-strength steel sheet having excellent ductility and low-temperature toughness and method for manufacturing thereof |
| KR102296840B1 (en) | 2019-12-16 | 2021-09-01 | 주식회사 포스코 | High strength steel material having excellent ductility and manufacturing method for the same |
| KR102236853B1 (en) | 2019-12-18 | 2021-04-06 | 주식회사 포스코 | Steel material having excellent strength and ductility and manufacturing method for the same |
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| JP3434431B2 (en) | 1997-04-28 | 2003-08-11 | 新日本製鐵株式会社 | Steel plate excellent in impact energy absorbing ability and method for producing the same |
| JP3572894B2 (en) * | 1997-09-29 | 2004-10-06 | Ｊｆｅスチール株式会社 | Composite structure hot rolled steel sheet excellent in impact resistance and formability and method for producing the same |
| JP3434444B2 (en) * | 1997-12-26 | 2003-08-11 | 新日本製鐵株式会社 | Steel plate for hull with excellent shock absorption capacity |
| JP3499126B2 (en) | 1998-03-03 | 2004-02-23 | 新日本製鐵株式会社 | Steel plate for hull excellent in impact energy absorption capacity and method of manufacturing the same |
| JP2000319752A (en) * | 1999-05-07 | 2000-11-21 | Nippon Steel Corp | Steel for structural purpose excellent in corrosion resistance and its production |
| JP4291480B2 (en) * | 1999-12-03 | 2009-07-08 | 新日本製鐵株式会社 | Structural steel with excellent corrosion resistance and corrosion fatigue resistance |
| JP3525849B2 (en) * | 2000-03-17 | 2004-05-10 | Ｊｆｅスチール株式会社 | Steel material excellent in collision resistance and method for producing the same |
| JP2002105586A (en) * | 2000-09-29 | 2002-04-10 | Nkk Corp | Shape steel having excellent collision resistance and its production method |
| JP3578126B2 (en) * | 2001-09-14 | 2004-10-20 | Ｊｆｅスチール株式会社 | Steel material excellent in collision resistance and method for producing the same |
| JP4470701B2 (en) * | 2004-01-29 | 2010-06-02 | Ｊｆｅスチール株式会社 | High-strength thin steel sheet with excellent workability and surface properties and method for producing the same |
| JP4476923B2 (en) * | 2005-12-15 | 2010-06-09 | 株式会社神戸製鋼所 | Steel sheet with excellent impact absorption and base metal toughness |
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