WO2006103991A1 - 穴拡げ加工性に優れた高強度熱延鋼板およびその製造方法 - Google Patents
穴拡げ加工性に優れた高強度熱延鋼板およびその製造方法 Download PDFInfo
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- WO2006103991A1 WO2006103991A1 PCT/JP2006/305700 JP2006305700W WO2006103991A1 WO 2006103991 A1 WO2006103991 A1 WO 2006103991A1 JP 2006305700 W JP2006305700 W JP 2006305700W WO 2006103991 A1 WO2006103991 A1 WO 2006103991A1
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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
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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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
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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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
-
- 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
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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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
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
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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
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with 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
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
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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
- C21D2211/002—Bainite
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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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- 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
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0226—Hot rolling
Definitions
- the present invention relates to a high-strength hot-rolled steel sheet used for automobiles such as passenger cars and trucks, industrial machines, and the like, and a method for producing the same.
- the present invention relates to a high-strength hot-rolled steel sheet that can be effectively used as a component material of the steel and a useful method for producing such a hot-rolled steel sheet.
- Patent Document 1 discloses a composite steel sheet having ferrite, bainite, retained austenite, and martensite microstructure
- Patent Document 2 describes a ferrite bainite structure mainly composed of ferrite, and the amount of non-fixed carbon that does not react with Ti and Nb in steel and precipitates at grain boundaries during aging treatment to increase strength.
- a high-strength steel sheet in which the amount of unprecipitated carbon is controlled has been proposed.
- Patent Document 3 improves the hole-expanding workability by using a high-strength hot-rolled steel sheet having a microstructure mainly composed of flite and composed of vinylite fligot and polygonal 'flight. Technology has been proposed.
- Patent Document 4 proposes a technique for improving hole expansibility by using a high-strength hot-rolled steel sheet having a microstructure composed of paytick 'ferrite and polygonal' flight. ing.
- a cooling condition and a method for controlling the cooling condition in the step of winding into the coil after completion of hot rolling are disclosed.
- Patent Document 1 Special Table 2004-536965 Publication, Claims, etc.
- Patent Document 2 Japanese Patent Laid-Open No. 2003-342684, claims, etc.
- Patent Document 3 Japanese Patent Laid-Open No. 2004-250749, claims, etc.
- Patent Document 4 Japanese Patent Application Laid-Open No. 2004-225109, Claims, etc.
- the present invention was made in order to solve the problems of the above-described conventional high-strength hot-rolled steel sheet, and its purpose is a high-strength hot-rolled steel sheet having a tensile strength of 780 MPa or more,
- An object of the present invention is to provide a high-strength hot-rolled steel sheet having excellent elongation and hole expansion workability, and a useful method for producing such a high-strength hot-rolled steel sheet.
- the hot-rolled steel sheet of the present invention that has achieved the above object is C: 0.05-0.15% (meaning mass%, the same shall apply hereinafter), Si: 1. 50% or less (0% ), Mn: 0.5-2. 5%, P: 0.035% or less (excluding 0%), S: 0.01% or less (including 0%), A1: 0.020 -0. 15%, Ti: 0.05-0. 2% steel, each containing 60% to 95% by volume of baitite, solid solution strengthened or precipitation strengthened ferrite or ferrite In addition, it has a gist in that the fracture surface transition temperature vTrs obtained by the impact test of the steel sheet is 0 ° C or less.
- the steel slab having the chemical component is heated to a temperature range of 1150 to 1300, and the heated steel slab is heated to Ar. Transformation point
- the process of hot rolling at a finishing temperature of 3 or more to form a steel sheet, and the steel sheet after hot rolling to a temperature range of 400 to 550 ° C is cooled to an average cooling rate of 30 ° CZ seconds or more to be turned into a coil. It is sufficient if the coil is manufactured to include a step of cooling and a step of cooling the coil after scraping to a temperature of 300 ° C or lower at an average cooling rate of 50 to 400 ° CZ.
- C 0.02 to 0.10%, Si: l.5% or less (not including 0%), Mn: 0.5 to 2.0%, P: 0.025% or less (not including 0%), S: 0.010 % Or less (including 0%), A1: 0.020 to 0.15%, Ni: 1% or less (not including 0%), Cr: 1% or less (not including 0%), Nb: 0.08% or less (including (0% not included), Ti: 0.05 to 0.2% of each steel sheet, where the metal structure is substantially a single-phase structure of ferrite, and the fracture surface transition temperature obtained in the impact test of the steel sheet The main point is that vTrs is 0 ° C or less.
- the steel slab having the chemical component is heated to a temperature range of 1150 to 1300, and the heated steel slab is heated to Ar. Transformation point
- a hot-rolled steel sheet is a high-strength hot-rolled steel sheet with a tensile strength of 780 MPa or more, an elongation of 20% or more, and a hole expansion ratio of 60% or more at a thickness of 2 mm.
- hot-rolled steel sheets that were not conventionally applied from the viewpoint of formability can be applied to various parts such as automobiles and industrial machinery, which not only contributes to cost reduction of parts, but also to various parts. Therefore, it will be possible to reduce the plate thickness and improve the collision safety of automobiles, which will contribute to the improvement of automobile performance.
- FIG. 1 is a graph showing the relationship between fracture surface transition temperature vTrs and hole expansion rate in Example 1.
- FIG. 2 is a graph showing the relationship between the cooling rate after coil scraping and the fracture surface transition temperature vTrs in Example 1.
- FIG. 3 is a graph showing the relationship between the fracture surface transition temperature vTrs and the hole expansion rate ⁇ in Example 2.
- FIG. 4 is a graph showing the relationship between the cooling rate after coil scraping and the fracture surface transition temperature vTrs in Example 2.
- FIG. 5 is a graph showing the relationship between the fracture surface transition temperature vTrs and the hole expansion ratio ⁇ in Example 3.
- FIG. 6 is a graph showing the relationship between the cooling rate after coil scraping and the fracture surface transition temperature vTrs in Example 3.
- FIG. 7 is a graph showing the relationship between the fracture surface transition temperature vTrs and the hole expansion ratio ⁇ in Example 4.
- FIG. 8 is a graph showing the relationship between the cooling rate after coil scraping and the fracture surface transition temperature vTrs in Example 4.
- the present inventors studied from various angles to realize a high-strength hot-rolled steel sheet excellent in hole expansion workability.
- the chemical composition of the steel is adjusted appropriately, the production conditions are regulated, and the microstructure of the steel is fine, with a bainite volume fraction of 60-95% and the balance of TiC and Z or Nb and Mo carbides. Ferrite or ferrite and martenser deposited on It was found that a steel sheet with a tensile strength of 780 MPa or more can be realized if the structure includes a steel.
- the cooling condition of the coiling coil after coiling it is possible to control the fracture surface transition temperature vTrs required by the impact test. It was found that if it was within the range, the hole expansion caloric property of the hot-rolled steel sheet could be improved, and the present invention was completed. In the following, the effects of the present invention will be described along the process of completion of the present invention.
- the C is made as low as possible and the main phase is changed to a bainitic structure.
- it is effective to include a solid solution strengthened and precipitation strengthened ferrite structure at an appropriate volume fraction, and by reducing the C content, the hardness of bainite is reduced and the ductility of the bainite is improved.
- the difference in hardness from solid solution strengthened and precipitation strengthened ferrite can be reduced, so it is considered that high elongation and high hole expansibility can be secured.
- the hole expandability may change depending on the coil.
- the present inventors focused on the relationship between hole expansibility and toughness, and investigated the relationship between fracture surface transition temperature vTrs and hole expansibility required in impact tests, and found that there was a good correlation with these. There is a relationship, and in order to ensure a good hole expandability of 60% or more with a hole expansion rate (measurement method will be described later), the fracture surface transition temperature vTrs should be 0 ° C or lower. (See Figures 1 and 3 below).
- P that segregates at the ferrite grain boundaries as described above is that P is diffused and prayed at grain boundaries that are unstable compared to the inside of the grains due to the slow cooling of the coiling coil. I was able to think. The inventors have the viewpoint that the toughness can be improved by preventing the segregation of P as described above. As a result of further examination of the means, based on the idea that shortening the diffusion time is not effective, various angular forces were also examined for specific means.
- C is a basic component as a strength-enhancing element, and it is necessary to contain 0.05% or more in order to secure a tensile strength of 780 MPa or more of the steel sheet.
- the C content exceeds 0.15%, the second phase other than ferrite (for example, martensite, etc.) is generated and increased in the microstructure, and the hole expandability deteriorates.
- the preferable lower limit of the C content is 0.06%, and the preferable upper limit is 0.10%.
- Si l. 5% or less (excluding 0%)
- Si is an element that promotes the formation of polygonal ferrite and is effective in securing strength without reducing elongation and hole expansibility. These effects increase as the content increases, but if they are contained excessively, the surface properties will deteriorate significantly, the hot deformation resistance will increase, and it will become difficult to produce steel sheets.
- the amount should be 1.5% or less.
- the preferable lower limit of the Si content is 0.2%, and the preferable upper limit is 1.0%.
- Mn is an element useful for solid solution strengthening of steel, and in order to secure a bow strength of 780 MPa or more, it must be contained at least 0.5% or more. However, if Mn is contained excessively, the hardenability becomes too high and a large amount of transformation product is generated, making it difficult to secure a high hole expansion rate. Should.
- the preferable lower limit of the Mn content is 1.4%, and the preferable upper limit is 2.3%.
- P 0.035% or less (excluding 0%)
- P is an element effective for solid solution strengthening of steel without deteriorating ductility, and is an especially important element in the present invention. If the P content is excessive, it will be prayed to the grain boundaries during cooling after coil winding, causing toughness deterioration and raising the fracture surface transition temperature vTrs. For this reason, the P content is preferably 0.035% or less. A preferable upper limit of the P content is 0.025%.
- S is an element that is inevitably mixed in the manufacturing process. It forms sulfide inclusions that adversely affect hole expansibility, so it is preferably reduced as much as possible. From this point of view, the S content should be suppressed to 0.01% or less.
- the preferable upper limit of the S content is 0.008%, more preferably 0.005% or less.
- A1 is added as a deoxidizing element during melting, it is an effective element for improving the cleanliness of steel. In order to exert such an effect, A1 needs to be contained in an amount of 0.02% or more. If the content is excessive, a large amount of alumina inclusions are formed and cause surface flaws. Must be 15% or less.
- the preferable lower limit of the A1 content is 0.025%, and the preferable upper limit is 0.06%.
- Ti is an effective element for strengthening the precipitation by using C and N in ferrite as precipitates, strengthening the ferrite, reducing the amount of solid solution C and cementite in ferrite, and improving the hole expandability. It is an important element in securing a tensile strength of 780 MPa or more. In order to exert these effects, the Ti content needs to be 0.05% or more. However, if the Ti content is excessive, the ductility deteriorates and the above effect is saturated, so it is necessary to make it 0.2% or less.
- the preferable lower limit of Ti content is 0.08%, and the preferable upper limit is 0.18%.
- the force is composed of Fe and inevitable impurities (for example, V, Sn, etc.). If necessary, Ni, Cr, Mo, Nb, B It is also effective to contain Ca, Cu and the like. The reason for specifying the range when these elements are contained is as follows. [0030] Ni: 1% or less (excluding 0%)
- Ni is an effective element for strengthening steel by solid solution strengthening. If its content is excessive, its effect is saturated and disadvantageous economically, so it should be 1% or less. The above-mentioned effect due to the Ni-added iron increases as its content increases. From the viewpoint of securing a tensile strength of 780 MPa or more in a ferrite single phase steel, Ni should be contained at least 0.1%. More preferably, the content is 0.2% or more. The upper limit of the Ni content is preferably 0.8%, more preferably 0.5% or less.
- Cr is an element effective for strengthening precipitation by strengthening ferrite using C as a precipitate. Even if its content is excessive, its effect is saturated and economically disadvantageous. It should be 0% or less.
- the above-mentioned effect due to Cr-added powder increases as its content increases.In order to exert the above-mentioned effect effectively, it is preferable to contain at least 0.1% of Cr. It is good to contain 2% or more. Further, the upper limit of the Cr content is preferably 0.8%, more preferably 0.5% or less.
- Nb 0.1% or less (excluding 0%)
- Nb is an element that contributes to improvement of hole expansibility by refining ferrite formed from austenite after hot completion. It is also effective for strengthening ferrite by precipitation of C and N in steel. This effect increases as the content increases. Even if the content is excessive, the effect is saturated and economically disadvantageous. Therefore, it should be 0.1% or less. In order to effectively exhibit the above-described effect due to Nb, it is preferable to contain 0.01% or more, more preferably 0.02% or more. The preferable upper limit of the Nb content is 0.08%, more preferably 0.07% or more. It is good to make it lower.
- B Q. 01% or less (excluding 0%)
- B is an element effective in reducing the grain boundary energy of steel and suppressing the grain boundary segregation of P. Such effects increase as the content increases, but even if contained excessively, the effects are saturated, so 0.01% or less is preferable.
- the preferable lower limit of the B content is 0.001%, and the more preferable upper limit is 0.005%.
- Ca is an element effective for improving the hole expansibility by spherically forming the sulfide in the steel sheet. However, if its content is excessive, its effect will be saturated, so it should be 0.01% or less. I like it. In order to effectively exhibit the effect of the Ca-added powder, Ca is preferably contained in an amount of 0.001% or more. A more preferable upper limit of Ca is 0.005%.
- Cu When Cu is added together with Ti and Nb, Cu promotes uniform fine precipitation of TiC and NbC, and increases strength and further expandability due to fine precipitation. Even if it is excessive, the effect is saturated and it is economically disadvantageous, so it should be 1.0% or less.
- the above effect due to the addition of Cu increases as the content increases. However, in order to effectively exhibit the above effect, it is preferable to contain at least 0.1% of Cu, more preferably 0.3%. It is good to contain above. The preferred upper limit for the Cu content is 0.8%.
- the structure of the metal structure is an important requirement in order to have high strength, high hole-expandability, and excellent ductility.
- it is necessary to use bainite as the main phase which is high in strength but has a smaller hardness difference from ferrite than martensite, and to contain ductility in order to ensure ductility. . From this point of view, by setting the bainite phase in the metal structure in the range of 60 to 95% by volume, a steel sheet having high strength and good workability can be obtained.
- the metal structure of the steel sheet of the present invention is basically (bainite + ferrite), but part of the ferrite may be martensite.
- the term “ferrite” includes polygonal ferrite and pseudopolygonal ferrite. High dislocation density and structure such as erite and paytic ferrite are included in the “bainite” in the present invention.
- the production method of the present invention In order to produce the high-strength steel sheet of the present invention, it is necessary to appropriately control at least the cooling rate after coil winding as described above, and other conditions (hot rolling conditions) may be in accordance with normal conditions.
- the basic production conditions in the production method of the present invention are as follows.
- the steel sheet controlled to have a chemical composition as described above is used as a slab flake by a conventional method and subjected to hot rolling.
- the slab heating temperature at this time must be 1150 ° C or higher. This is the temperature at which TiC and Nb (C, N) begin to dissolve in austenite. Heating above this temperature can effectively dissolve the added Ti and Nb in steel. it can.
- the solid solution Ti and Nb react with the solid solution C and solid solution N in the flight during the formation of the flint after the hot rolling is completed, and precipitate as a composite, and the desired strength is achieved by precipitation strengthening the steel sheet. Tensile strength can be obtained. However, if this heating temperature becomes too high, the heating furnace will be damaged and the energy cost will increase, so it is necessary to keep it below 1300 ° C.
- the hot rolling finishing temperature is an Ar change in the austenite single-phase temperature range.
- the hot rolling temperature decreases and becomes less than the Ar transformation point.
- the average cooling rate from the cutting temperature to a temperature range of 300 ° C or lower was set to 50 ° CZhr. It is necessary to do this. If the cooling rate is lower than this average cooling rate, P precipitates at the grain boundaries during cooling, and the fracture surface transition temperature vTrs obtained by the impact test becomes high, so that good hole expandability cannot be obtained.
- the means for ensuring the cooling rate after coiling the coil as described above is not particularly limited.
- a method of blast cooling using a blower for the coil coil examples include a method in which mist is included in the blast (cool blast + mist), a method in which water is cooled by using a water spray nozzle in the scissor coil, and a method in which the scissor coil is immersed in a water tank.
- the present inventors studied from various angles to realize a high-strength hot-rolled steel sheet excellent in hole expansion workability. As a result, after properly adjusting the chemical composition of the steel, the production conditions are regulated, the microstructure of the steel is made into a ferrite single-phase structure, and TiC and Z, or Nb and Mo carbides are finely refined in this structure. It was found that steel sheets with a tensile strength of 780 MPa or more can be realized by precipitation. In addition, by controlling the cooling condition of the coiling coil after coiling, it is possible to control the fracture surface transition temperature vTr s required by the impact test. To be As a result, the present inventors have found that the hole expansion workability of the hot-rolled steel sheet can be improved. Hereinafter, along with the background of the completion of the present invention, the function and effect will be described.
- the present inventors paid attention to the relationship between hole expansibility and toughness, and investigated the relationship between fracture surface transition temperature vTrs and hole expansibility required in impact tests.
- the fracture surface transition temperature vTrs should be 0 ° C or lower. (See Figures 5 and 7 below.)
- a high-thickness (ie, low toughness) steel sheet with the above-mentioned fracture surface transition temperature vTrs was investigated in more detail! When the surface was analyzed using an Auger analyzer, it was observed that P grain boundary bias was occurring. In contrast, steel plates with good toughness (ie, low fracture surface transition temperature) can only be cleaved even when fractured at low temperatures, and the presence or absence of elements prejudiced at the grain boundaries has been confirmed. It turned out that it was not possible.
- P that segregates at the ferrite grain boundaries as described above is that P is diffused and prayed at grain boundaries that are unstable compared to the inside of the grains due to the slow cooling of the coiling coil. I was able to think. The inventors have the viewpoint that the toughness can be improved by preventing the segregation of P as described above.
- the basic mechanical properties yield strength YS, tensile strength TS, In order to provide the elongation EL, etc., it is necessary to adjust the chemical composition appropriately.
- the reason for limiting the range of the chemical composition defined in the present invention is as follows.
- C is a basic component as a strength-enhancing element, and in order to ensure a tensile strength of 780 MPa or more of the steel sheet, it is necessary to contain 0.02% or more. However, if the C content exceeds 0.10%, the second phase other than ferrite (for example, pearlite, bainite, martensite, etc.) is generated and increased in the microstructure, and the hole expandability is reduced. It will deteriorate.
- the preferable lower limit of the C content is 0.03%, and the preferable upper limit is 0.06%.
- Si l. 5% or less (excluding 0%)
- Si is an element that promotes the formation of polygonal ferrite and is effective in securing strength without reducing elongation and hole expansibility. These effects increase as the content increases, but if they are contained excessively, the surface properties will deteriorate significantly, the hot deformation resistance will increase, and it will become difficult to produce steel sheets.
- the amount should be 1.5% or less.
- the preferable lower limit of the Si content is 0.2%, and the preferable upper limit is 1.0%.
- Mn is an element useful for solid solution strengthening of steel, and in order to secure a bow strength of 780 MPa or more, it must be contained at least 0.5% or more. However, if Mn is contained excessively, the hardenability becomes too high and a large amount of transformation product is generated, making it difficult to secure a high hole expansion rate. Should.
- the preferable lower limit of the Mn content is 0.7%, and the preferable upper limit is 1.9%.
- P is an element effective for solid solution strengthening of steel without deteriorating ductility, and is an especially important element in the present invention. If the P content is excessive, it will be prayed to the grain boundaries during cooling after coil winding, causing toughness deterioration and raising the fracture surface transition temperature vTrs. Therefore, the P content is preferably 0.025% or less. A preferable upper limit of the P content is 0.015%.
- A1 is added as a deoxidizing element during melting, it is an effective element for improving the cleanliness of steel. In order to exert such an effect, A1 needs to be contained in an amount of 0.02% or more. If the content is excessive, a large amount of alumina inclusions are formed and cause surface flaws. Must be 15% or less.
- the preferable lower limit of the A1 content is 0.03%, and the preferable upper limit is 0.06%.
- Ni 1% or less (excluding 0%)
- Ni is an effective element for strengthening steel by solid solution strengthening. If its content is excessive, its effect is saturated and disadvantageous economically, so it should be 1% or less. The above-mentioned effect due to the Ni-added iron increases as its content increases. From the viewpoint of securing a tensile strength of 780 MPa or more in a ferrite single phase steel, Ni should be contained at least 0.1%. More preferably, the content is 0.3% or more. The upper limit of the Ni content is preferably 0.8%, more preferably 0.6% or less.
- Cr is an element effective for strengthening precipitation by strengthening ferrite using C as a precipitate. Even if its content is excessive, its effect is saturated and economically disadvantageous. It should be less than%.
- the above effect due to the Cr-added soot increases as its content increases, but in order to effectively exhibit the above effect, it is preferable to contain at least 0.1% of Cr, more preferably 0. It is better to contain 3% or more. Further, the upper limit of the Cr content is preferably 0.8%, more preferably 0.5% or less.
- Nb Q. 08% or less (excluding 0%)
- Nb is an element that contributes to improvement of hole expansibility by refining ferrite formed from austenite after hot rolling. It is also effective in strengthening ferrite by making C and N in steel precipitates and strengthening precipitation. These effects increase as the content increases. Even if the content is excessive, the effect saturates and is not economical. Therefore, it should be 0.08% or less. In order to effectively exhibit the above-described effects due to Nb, it is preferable to contain 0.01% or more. The preferable upper limit of the Nb content is 0.06%, more preferably 0.05% or less.
- Ti is an effective element for strengthening the precipitation by using C and N in ferrite as precipitates, strengthening the ferrite, reducing the amount of solid solution C and cementite in ferrite, and improving the hole expandability. It is an important element in securing a tensile strength of 780 MPa or more. In order to exert these effects, the Ti content needs to be 0.05% or more. However, if the Ti content is excessive, the ductility deteriorates and the above effect is saturated, so it is necessary to make it 0.2% or less.
- the preferable lower limit of Ti content is 0.08%, and the preferable upper limit is 0.15%.
- the hot-rolled steel sheet of the present invention includes, in addition to the above components, Fe and inevitable impurities (for example, V and Sn) force. If necessary, Mo, Cu, B, Ca and the like are contained. It is also effective to do so. The reason for defining the range when these elements are contained is as follows.
- the amount of Mo necessary to exert these effects includes the force that changes depending on the P content.
- the amount of Mo and P is 1.0 or more (ie, the amount that satisfies the following formula (1)). It is good to let it go. However, since the effect is saturated when the Mo content becomes excessive, the Mo content is preferably 0.5% or less.
- Cu has the effect of increasing the mechanical strength of steel and improving the material. These effects increase as the Cu content increases, but if excessively contained, the workability deteriorates on the contrary, so 1.0% or less is preferable.
- Cu for exerting the above effect A preferable lower limit of the content is 0.05%, and a more preferable upper limit is 0.5%.
- B is an element effective in reducing the grain boundary energy of steel and suppressing the grain boundary segregation of P. Such effects increase as the content increases, but even if contained excessively, the effects are saturated, so 0.01% or less is preferable.
- the preferable lower limit of the B content is 0.001%, and the more preferable upper limit is 0.005%.
- Ca is an element effective for improving the hole expansibility by spherically forming the sulfide in the steel sheet, but if its content becomes excessive, its effect is saturated, so it is made 0.005% or less. It is preferable. In order to effectively exhibit the effect of the Ca-added powder, Ca is preferably contained in an amount of 0.001% or more. A more preferable upper limit of Ca is 0.004%.
- the microstructure is substantially a single-phase structure strength of frites.
- substantially ferrite single phase structure means that the ferrite phase is at least 90 area% or more. Therefore, the steel sheet of the present invention, is in its tissues, pearlite, bainite, martensite, organizations such as residual austenite is of even free base (10 area 0/0 or less).
- the term “ferrite” refers to the fact that force ferrite, ferrite, and baiterite ferrite, which include polygonal ferrite and pseudopolygonal ferrite, are not suitable for obtaining high ductility because of high dislocation density. Therefore, “ferrite” in the present invention is not included.
- the production method of the present invention In order to produce the high-strength steel sheet of the present invention, it is necessary to appropriately control at least the cooling rate after coil winding as described above, and other conditions (hot rolling conditions) may be in accordance with normal conditions.
- the basic production conditions in the production method of the present invention are as follows.
- the steel sheet controlled to have a chemical composition as described above is used as a slab flake by a conventional method and subjected to hot rolling.
- the slab heating temperature at this time must be 1150 ° C or higher. This is the temperature at which TiC and Nb (C, N) begin to dissolve in austenite. Heating above this temperature can effectively dissolve the added Ti and Nb in steel. it can. Ti and Nb in solid solution
- the desired tensile strength can be obtained by precipitating solute C or solute N in the ferrite during the formation of ferrite after hot rolling, and strengthening the steel sheet by precipitation strengthening.
- this heating temperature force S becomes too high, it is not preferable because it causes damage to the heating furnace and an increase in energy costs.
- the hot rolling finishing temperature is an Ar change in the austenite single-phase temperature range.
- the hot rolling temperature decreases and becomes less than the Ar transformation point.
- the reason why the staking temperature is in the temperature range of 500 to 650 ° C is that the microstructure of the steel is a ferrite single phase structure. In other words, when the staking temperature is lower than 500 ° C, the bainite structure is mixed and elongation is lowered. Further, the precipitation strengthening amount of carbonitride is insufficient, and the desired strength cannot be obtained. In order to ensure better elongation, it is preferable to set the staking temperature to 550 ° C or higher.
- the precipitation size of (carbides, nitrides and carbonitrides) becomes coarse and the strength decreases instead. For these reasons, it is necessary to set the staking temperature in a temperature range of 500 to 650 ° C, preferably in a temperature range of 550 to 650 ° C. [0075]
- the average cooling rate from the cutting temperature to a temperature range of 300 ° C or lower is set to 50 ° CZhr. It is necessary to do this. If the cooling rate is lower than this average cooling rate, P precipitates at the grain boundaries during cooling, and the fracture surface transition temperature vTrs obtained by the impact test becomes high, so that good hole expandability cannot be obtained.
- Means for ensuring the cooling rate after coiling the coil as described above is not particularly limited.
- a method of blast cooling using a blower for the coil coil examples include a method in which mist is included in the blast (cool blast + mist), a method in which water is cooled by using a water spray nozzle in the scissor coil, and a method in which the scissor coil is immersed in a water tank.
- Examples 1 and 2 are related to the above-described first embodiment, and Examples 3 and 4 are related to the above-described second embodiment.
- the hot-rolled steel sheet thus obtained was subjected to an impact test in the direction perpendicular to the rolling direction (C direction) using a JIS No. 5 test piece and subjected to mechanical properties (yield strength YS, tensile strength TS, elongation EL Etc.) and the hole expandability is evaluated by the hole expansion ratio ⁇ measured by the following method.
- the fracture surface transition temperature vTrs was measured by the following method.
- ferrite, bainite and martensite were identified with a scanning electron microscope, and the bainite area ratio was measured with an image analyzer.
- both sides of the obtained hot-rolled steel sheet were ground, and the test was performed with a sub-size test piece having a thickness of 2.5 mm.
- Initial hole diameter 10mm (d) punch hole is pushed from the punching side with a 60 ° conical punch.
- brittle fracture surface ratio (or ⁇ ductile fracture surface ratio '' The transition temperature vTrs at which the brittle fracture surface ratio is 50% was determined from the curve of (test temperature vs. brittle fracture surface ratio).
- test temperature was changed at 10 ° C or 20 ° C intervals.
- test temperature (test piece temperature) was controlled according to the conditions defined in JIS Z2242. Then, an impact test is performed, and then the fracture surface of the specimen is observed to distinguish between the area showing the brittle fracture surface and the area showing the ductile fracture surface. The surface area was calculated.
- B brittle fracture surface ratio (%)
- C brittle fracture surface area
- A total area of fracture surface
- test temperature and the brittle fracture surface ratio were plotted to obtain an approximate curve, and the test temperature at which the approximate curve showed a brittle fracture surface ratio of 50% was defined as the transition temperature vTrs.
- Fig. 1 shows the relationship between fracture surface transition temperature vTrs and hole expansion rate
- Fig. 2 shows the relationship between cooling rate after coil scraping and fracture surface transition temperature vTrs.
- the hot-rolled steel sheet thus obtained was subjected to a tensile test in a direction perpendicular to the rolling direction using a JIS No. 5 test piece and mechanical properties (yield strength YS, tensile strength TS, elongation EL, etc.) ) And hole expansibility and fracture surface transition temperature were measured by the same method as in Example 1.
- the results are shown in Table 4 below together with the manufacturing conditions (rolling finishing temperature, milling temperature, cooling rate after milling). Based on these results, the relationship between the fracture surface transition temperature vTrs and the hole expansion rate is shown in Fig. 3, and the relationship between the average cooling rate after coil scraping and the fracture surface transition temperature vTrs is shown in Fig. 4, respectively.
- the fracture surface transition temperature vTrs which affects the hole expansion rate, changes depending on the cooling rate that simulates the cooling of the winding coil. At this time, in order to ensure the target fracture surface transition temperature vTrs below 0 ° C, it is necessary to cool at an average cooling rate of 50 ° CZhr or higher.
- the portion surrounded by the broken line in FIG. 4 is that the fracture surface transition temperature vTrs is increased by the chemical composition composition being outside the range defined in the present invention.
- Test Nos. 1—12 to 15, 1 17, 1 18, 1—20 to 25, 1—27, 1—28, 1—30, 1—31 satisfy all the requirements defined in the present invention. It is satisfactory that a hot-rolled steel sheet with satisfactory mechanical properties and hole expansion ratio, high strength and good workability can be realized.
- test Nos. 1-16, 1-19, 1-26, 1-29, 1-32-39 lack any of the requirements defined in the present invention. At least the mechanical characteristics and hole expansibility are degraded.
- test Nos. 1-16, 1 19, 1 26, 1-29 had a low average cooling rate after coil scraping, and the fracture surface transition temperature vTrs was high. Good hole expandability is not obtained.
- Test Nos. 1-32 and 1 33 are steel plates with excessive Si content (steel grade 1 J in Table 3), and the fracture surface transition temperature vTrs is high, resulting in good hole expansibility. It is not done.
- Test Nos. 1-34 and 1 35 are steel sheets with excessive Mn content (Steel grade 1-K in Table 3), and the ductility (elongation) decreases and the fracture surface transition temperature vTrs is As a result, the hole expandability is not good.
- Test No. 1-36 is a steel sheet with an excessive P content (Steel grade 1 L in Table 3), and the fracture surface transition temperature vTrs is too high to achieve good hole expansibility.
- Test Nos. 1-37 and 1 38 are steel plates with excessive Ti and C contents (steel types 1 M and 1 N in Table 3), respectively, and the ductility (elongation) decreased. Yes. In Test No. 1-39, the C content was insufficient (steel grade 10 in Table 3), and the tensile strength was reduced. The
- Each steel slab having the chemical composition shown in Table 5 below is held at a slab heating temperature of 1250 ° C for 30 minutes, and then the final rolling temperature is 900 ° C by a normal hot rolling process. Thickness: 4mm A hot rolled steel sheet was obtained. After that, cooling was performed at an average cooling rate of 30 ° CZs, and after 30 minutes of scraping treatment at a scraping temperature of 600 ° C using an electric carousel furnace, the cooling rate was controlled in order to change the subsequent cooling rate. Cooling by furnace cooling, cooling after taking out from the furnace, air blast cooling, (air blast + mist) cooling, shower cooling, immersion in a water bath, etc., gave various hot rolled steel sheets.
- the hot-rolled steel sheet thus obtained was subjected to an impact test in the direction perpendicular to the rolling direction (C direction) with a JIS No. 5 test piece and subjected to mechanical properties (yield strength YS, tensile strength TS, elongation E L) and the like, and the hole expandability was evaluated by the hole expansion ratio ⁇ measured by the following method, and the fracture surface transition temperature vTrs was measured by the following method.
- the mouth structure of each steel plate was observed with an optical microscope.
- both sides of the obtained hot-rolled steel plate were ground, and the test was performed with a sub-size test piece having a thickness of 2.5 mm.
- Initial hole diameter 10mm (d) punch hole is pushed from the punching side with a 60 ° conical punch.
- brittle fracture surface ratio (or ⁇ ductile fracture surface ratio '' The transition temperature vTrs at which the brittle fracture surface ratio is 50% was determined from the curve of (test temperature vs. brittle fracture surface ratio). Details are as described in the first embodiment.
- the fracture surface transition temperature vTrs should be 0 ° C or lower in order to ensure the.
- the fracture surface transition temperature vTrs which affects the hole expansion ratio I, varies depending on the cooling rate that simulates the cooling of the winding coil.
- the fracture surface transition temperature vTrs It can be seen that cooling at an average cooling rate of 50 ° CZhr or higher is necessary to secure the target of 0 ° C or lower. Note that the portion surrounded by the broken line in FIG. 8 is that the fracture surface transition temperature vTrs is increased due to the chemical composition being out of the range defined in the present invention.
- Test Nos. 2—12 to 15, 2—17, 2—18, 2—20 to 25, 2—27, 2—28, 2—30, 2—31 are the requirements specified in the present invention. It satisfies all of the requirements, and both the mechanical properties and the hole expansion ratio are good, and it can be said that a hot-rolled steel sheet with high strength and good workability can be realized.
- test Nos. 2-16, 2-19, 2-26, 2-29, 2-32-39 lack any of the requirements defined in the present invention. At least the mechanical characteristics and hole expansibility are degraded.
- Test Nos. 2-16, 2-19, 2-26, and 2-29 the average cooling rate after coil scraping is low, and the fracture surface transition temperature vTrs is high. As a result, good hole expansibility is not obtained.
- Test Nos. 2-32 and 2-33 are steel plates with excessive Si content (Table 7—J grade in Table 7), high fracture surface transition temperature vTrs, and good hole expansibility. Is not obtained.
- Test Nos. 2-34 and 2-35 are steel sheets with excessive Mn content (Steel 7-K in Table 7), and the ductility (elongation) decreases and the fracture surface transition temperature vTrs However, good hole expansibility is not obtained.
- Test No. 2-36 is a steel plate with excessive P content (Steel type 2—L in Table 7), and the fracture surface transition temperature vTrs is high, so that good hole expandability is not obtained. .
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Abstract
Description
Claims
Priority Applications (4)
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US11/908,423 US8038809B2 (en) | 2005-03-28 | 2006-03-22 | High strength hot rolled steel sheet excellent in bore expanding workability and method for production thereof |
EP06729667A EP1865083B1 (en) | 2005-03-28 | 2006-03-22 | High strength hot rolled steel sheet excellent in bore expanding workability and method for production thereof |
CN2006800046812A CN101120113B (zh) | 2005-03-28 | 2006-03-22 | 扩孔加工性优异的高强度热轧钢板及其制造方法 |
US13/210,807 US8486205B2 (en) | 2005-03-28 | 2011-08-16 | High strength hot rolled steel sheet excellent in bore expanding workability and method for production thereof |
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JP2005-092610 | 2005-03-28 | ||
JP2005092611A JP3889766B2 (ja) | 2005-03-28 | 2005-03-28 | 穴拡げ加工性に優れた高強度熱延鋼板およびその製造方法 |
JP2005-092611 | 2005-03-28 | ||
JP2005092610A JP3889765B2 (ja) | 2005-03-28 | 2005-03-28 | 穴拡げ加工性に優れた高強度熱延鋼板およびその製造方法 |
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US11/908,423 A-371-Of-International US8038809B2 (en) | 2005-03-28 | 2006-03-22 | High strength hot rolled steel sheet excellent in bore expanding workability and method for production thereof |
US13/210,807 Division US8486205B2 (en) | 2005-03-28 | 2011-08-16 | High strength hot rolled steel sheet excellent in bore expanding workability and method for production thereof |
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US (2) | US8038809B2 (ja) |
EP (2) | EP2351867A1 (ja) |
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GB2448114A (en) * | 2007-03-15 | 2008-10-08 | Kobe Steel Ltd | Hot rolled bainitic-ferrite steel sheet |
GB2448114B (en) * | 2007-03-15 | 2010-05-12 | Kobe Steel Ltd | High strength hot rolled steel sheet with excellent press workability and method of manufacturing the same |
US8052808B2 (en) | 2007-03-15 | 2011-11-08 | Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.) | High strength hot rolled steel sheet with excellent press workability and method of manufacturing the same |
EP2152451A1 (en) * | 2007-05-06 | 2010-02-17 | Nucor Corporation | A thin cast strip product with microalloy additions, and method for making the same |
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EP2162252A1 (en) * | 2007-05-06 | 2010-03-17 | Nucor Corporation | A thin cast strip product with microalloy additions, and method for making the same |
EP2152451A4 (en) * | 2007-05-06 | 2014-08-20 | Nucor Corp | THIN CASTED STEEL BANDING PRODUCT WITH MICRO-ALLOYING EQUIPMENT AND METHOD OF MANUFACTURING THEREOF |
EP2162251A4 (en) * | 2007-05-06 | 2014-08-27 | Nucor Corp | THIN CASTED STEEL BANDING PRODUCT WITH MICRO-ALLOYING EQUIPMENT AND METHOD OF MANUFACTURING THEREOF |
EP2162252A4 (en) * | 2007-05-06 | 2014-09-03 | Nucor Corp | THIN CASTED STEEL BANDING PRODUCT WITH MICRO-ALLOYING EQUIPMENT AND METHOD OF MANUFACTURING THEREOF |
JP2021505759A (ja) * | 2017-12-04 | 2021-02-18 | エスエスアーベー テクノロジー アーベー | 高強度熱間圧延鋼および高強度熱間圧延鋼の製造方法 |
US11655528B2 (en) | 2017-12-04 | 2023-05-23 | Ssab Technology Ab | High strength hot-rolled steel and method for manufacturing high strength hot-rolled steel |
Also Published As
Publication number | Publication date |
---|---|
KR20090050105A (ko) | 2009-05-19 |
KR100942088B1 (ko) | 2010-02-12 |
EP1865083A4 (en) | 2009-02-25 |
KR100942087B1 (ko) | 2010-02-12 |
KR20070107167A (ko) | 2007-11-06 |
US20110297281A1 (en) | 2011-12-08 |
EP1865083A1 (en) | 2007-12-12 |
CN101906567A (zh) | 2010-12-08 |
CN101906567B (zh) | 2014-07-02 |
US20090050243A1 (en) | 2009-02-26 |
EP1865083B1 (en) | 2011-08-17 |
US8486205B2 (en) | 2013-07-16 |
US8038809B2 (en) | 2011-10-18 |
EP2351867A1 (en) | 2011-08-03 |
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