BR112014025955B1 - high strength steel plate and method to manufacture the same - Google Patents

Abstract

Patent specification: "High strength steel plate and method of fabrication". Steel plate that exhibits crack resistance during press forming and has small planar anisotropy in ductility. The invention provides a steel plate comprising a chemical composition including by weight%: c: greater than 0.0005% and less than 0.10%; si: 1.5% or less; mn: 0.1% or less and 3.0% or less; p: 0.080% or less; s: 0.03% or less; Sun. al: 0.01% or less and 0.50% or less; n: 0.005% or less; an element selected from nb: 0.20% or less and ti: 0.20% or less; and the remainder as fe and incidental impurities, whose steel microstructure of the steel plate contains 60% ferrite phase per volume fraction, and as long as the 3d crystal orientation distribution function is represented by {? 1,?, ? 2}, odf intensity {0 ?, 0 ?, 45?} Where? = 0 ?,? 1 = 0? and? 2 = 45? is 3.0 or less; and intensity is odf {0 ?, 35 ?, 45?} where? = 35 ?,? 1 = 0? and? 2 = 45? is 2.5 or more and 4.5 or less.

Description

Descriptive Report of the Invention Patent for STEEL PLATE WITH HIGH RESISTANCE AND METHOD FOR MANUFACTURING THE SAME.

FIELD OF TECHNIQUE [001] The present invention relates to a steel plate with high resistance that must be formed by a press for use in automobiles, home applications and the like. The present invention also relates to a method for making the steel sheet with high strength.

BACKGROUND TECHNIQUE [002] There has been a great demand for improving the fuel efficiency of automobiles in recent years in order to reduce CO2 emissions to protect the global environment. In addition, there is a great demand for improving safety (improving safety properties for vehicle crashes, in particular) in order to ensure the safety of occupants in the event of a vehicle crash. Consequently, improving the strength of a motor vehicle body, as well as reducing its weight, are now being actively pursued.

[003] However, achieving both weight reduction and strength enhancement of a motor vehicle body simultaneously requires decreasing the thickness of the steel plate for weight reduction by significantly increasing the strength of a vehicle member material without adversely affect its rigidity. In view of this, a sheet of steel with high strength has been actively used for structural members of automobiles in recent years.

[004] The higher strength of a steel plate for use in an automobile structural member allows for the thinner plate thickness of the structural member, thus causing the best vehicle weight reduction effect. Automobile manufacturers therefore

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2/34 tend to use steel sheets with high strength, each of which has a tensile strength (TS) of at least 390 MPa for, for example, materials from the inner and outer plate panel of a vehicle body.

[005] However, the majority of such structural members of automobiles that use such steel sheet material as materials of the inner and outer plate panel as described above are subjected to press forming. A steel plate for an automobile structural member, therefore, needs to have good press formability. In this respect, most conventional high strength steel sheets exhibit significantly lower formability, ductility, deep stamping and the like than standard mild steel sheets, leaving room for improvement.

[006] Document JP S56-139654 A (PTL 1), for example, reveals, in order to address the conformability problem described above, a technique for obtaining a steel plate that has tensile strength (TS) of up to 440 MPa, from: adding to a steel sheet with ultra low carbon content that has good conformability sufficient quantities of Ti and Nb to fix solute carbon and solute nitrogen in the steel to obtain IF steel (free of interstitial elements); and further adding solute reinforcing elements such as Si, Mn and P to the IF steel thus obtained.

[007] Specifically, the document PTL 1 reveals a technique for obtaining a high-tensile cold-rolled steel sheet that has superior conformability, tensile strength of the order of 35 to 45 kg / mm ² (order of 340 to 440 MPa ) as anti-aging properties and a chemical composition that includes C: 0.002% to 0.015%, Nb: (C% x3) to (C% x8 + 0.020%), Si: 1.2%, Mn: 0.04% to 0 , 8% and P: 0.03% to 0.10%.

[008] Multiphase steel sheets have been produced commercially

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3/34 for a steel sheet that has a tensile strength (TS) of at least 590 MPa, of which the known examples include a DP (two-phase) steel sheet that has a two-phase structure, that is, a ferrite and martensite structure and a steel plate TRIP (three-phase) that additionally uses retained austenite (γ). A DP steel sheet typically exhibits a relatively high stress hardening capacity, despite the fact that it has a relatively low yield limit due to the residual stress around the martensite. A TRIP steel sheet characteristically exhibits relatively high uniform elongation due to plasticity-induced transformation of the martensite phase.

[009] The mechanical properties of a high tensile steel sheet are generally assessed by tensile properties in a specific direction (for example, in the direction orthogonal to the rolling direction) of the steel sheet. Such mechanical properties of a high tensile steel sheet, as described above, can occasionally be evaluated by Lankford values (r values) in the rolling direction, a 45 ° inclined direction in relation to the rolling direction and an inclined direction by 90 ° in relation to the lamination direction when planar anisotropy of the r (Δγ) value is investigated. However, as a result of the detailed analysis of steel sheets actually formed by press, it was revealed that the mechanical properties (elongation value, in particular) of a steel sheet in a direction that exhibits relatively low ductility, rather than in a direction in which the evaluation of mechanical properties is normally based, determines the press formability of the steel sheet.

[010] With respect to planar anisotropy, Document JP 2004197155 A (PTL 2) discloses a method for obtaining a cold rolled steel sheet that has good cooking hardening capacity

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4/34 to and relatively low planar anisotropy suitable for vehicle exterior panel members. According to the method of document PTL 2, it is presumably possible to make planar anisotropy and notch resistance of a steel sheet compatible by specifying the planar anisotropy of the r value, that is, Δγ, of the steel sheet according to carbon content and the rate of reduction of cold rolling of the same. However, this method requires conditions where cooling must be started within two seconds after hot rolling is complete and where cooling must be carried out at a cooling rate of at least 70 ° C / second (70 ° C / s) over a temperature range of 100 ° C or higher.

[011] Briefly, according to the PTL 2 technique, problems arise due to the fact that: low temperature transformed phases, such as bainite, must be generated by rapid cooling after hot rolling in order to suppress planar anisotropy the r-value of the steel sheet, whereby it is only possible to produce a cold-rolled steel sheet that has a somewhat limited range of strength; and planar anisotropy, in particular planar anisotropy in ductility, cannot be reliably suppressed when steel plate structures are exchanged.

[012] In relation to planar anisotropy, Document JP 2005256020 A (PTL 3) also reveals an excellent steel sheet in terms of shape fixability. Specifically, the document PTL 3 reveals a steel of composite structure containing 1% or more and 25% or less of martensite per fraction of volume, with ferrite or bainite as a phase of a maximum of fraction of volume, in which all ( i) to (iv), that is, (i) the average value (A) of the random X-ray intensity ratio of the orientation group {100} <011> to {223} <110> on the plate surface, being that at least in 1/2 to 1/4 the thickness of the plate is> 4.0, (ii) with the average value (B) of the intensity ratio

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5/34 X-ray randomization of three crystalline orientations of {554} <225>, {111} <112> and {111} <110> is <5.5, (iii) (A) / (B)> 1 , 5 and (iv) where the random reflective X-ray intensity ratio of {100} <011> is equal to or greater than {211} <011> the random X-ray intensity ratio, are satisfied; where at least one of the value r in the rolling direction and the value r in the direction at right angles to the rolling direction is <0.7 is satisfied; the uniform elongation AuEI anisotropy is £ 4%; the local elongation ALEI anisotropy is> 2%: and AuEI is <ALEI.

[013] Under the above conditions, AuEI and ALEI are calculated according to the following formulas, respectively.

AuEI = {| uEI (L) - uEI (45 °) | + | uEI (C) - uEI (45 °) |} / 2 (3)

LAW = {| LAW (L) - LAW (45 °) | + | LAW (C) - LAW (45 °) |} / 2 (4) [014] In formulas, uEI (L), uEI (C), uEI (45 °) represent uniform elongation in parallel with the rolling direction (L direction), uniform elongation orthogonal to the rolling direction (C direction) and uniform elongation inclined by 45 ° in relation to the rolling direction, respectively, and LEI (L), LEI (C), LEI (45 °) represent elongation location in parallel with the lamination direction (L direction), local elongation orthogonal to the lamination direction (C direction) and local elongation inclined by 45 ° in relation to the lamination direction, respectively.

[015] In addition, the PTL 3 document essentially requires the optimization of the finishing conditions of the hot rolling and establishment of winding temperature at the critical or lower edge temperature according to equivalent Mn content as a means of achieving the conditions mentioned above. .

[016] However, problems arise in the PTL 3 document in which: suppressing uniform elongation anisotropy (AuEI) to <4% results in a significantly restricted range of resistance level

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6/34 due to the correlation between an absolute value of uniform elongation and resistance level; and texture growth {100} <011> deteriorates the stamping of a steel plate.

LIST OF QUOTES

PATENT LITERATURE [017] PTL 1: JP S56-139654 A [018] PTL 2: JP 2004-197155 A [019] PTL 3: JP 2005-256020 A

SUMMARY OF THE INVENTION

PROBLEM OF THE TECHNIQUE [020] The present invention aims to advantageously solve the aforementioned problems and an object of the same is to provide a steel sheet with high strength that has reduced ductility anisotropy to successfully suppress cracks in the press forming, as well as a advantageous production method.

SOLUTION TO THE PROBLEM [021] The inventor of the present invention was successful, as a result of a dedicated study to solve the aforementioned problems, in the suppression of planar anisotropy in ductility, in particular, planar anisotropy of uniform elongation of a steel sheet across specification of the reduction rate according to the Ti and Nb contents to develop a specific texture of the steel sheet.

[022] The present invention was completed on the basis of that successful disclosure.

[023] Specifically, primary resources of the present invention are as follows.

[024] A steel plate with high resistance that comprises a chemical composition that includes in% by mass:

[025] C: more than 0.0005% and less than 0.10%;

[026] Si: 1.5% or less;

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7/34 [027] Μη: 0.1% or more and 3.0% or less;

[028] P: 0.080% or less;

[029] S: 0.03% or less;

[030] sunshine. Al: 0.01% or more and 0.50% or less;

[031] N: 0.005% or less;

[032] at least one element selected from Nb: 0.20% or less and Ti: 0.20% or less; and [033] the remainder as Fe and incidental impurities, [034] where the steel microstructure of the steel plate contains 60% or more of the ferrite phase per fraction of volume, and [035] as long as the distribution function of 3D crystal orientation (orientation distribution function, ODF) is represented by {φ1, Φ, φ2}, ODF intensity {0 ^o , 0 ^o , 45 °} where Φ = 0 ^o , φ1 = 0 ^o and φ2 = 45 ° is 3.0 or less; and ODF intensity {0 ^o , 35 °, 45 °} where Φ = 35 °, φ1 = 0 ° and φ2 = 45 ° is 2.5 or more and 4.5 or less.

[036] The high strength steel sheet of (1) above, in which the chemical composition additionally includes in% by weight at least one element selected from V: 0.40% or less, Cr:

0.50% or less, Mo: 0.50% or less, W: 0.15% or less, Zr:

0.10% or less, Cu: 0.50% or less, Ni: 0.50% or less, B:

0.0050% or less, Sn: 0.20% or less, Sb: 0.20% or less, Ca:

0.010% or less, Ce: 0.01% or less and La: 0.01% or less.

[037] A method for manufacturing a sheet of steel with high resistance that comprises the steps of:

[038] subjecting a steel plate having the chemical composition of (1) or (2) above to hot rolling at a finishing delivery temperature of 820 ° C or higher and 950 ° C or lower to obtain a hot-rolled steel plate;

[039] subjecting the steel sheet to cold rolling at a rate of reduction (X%) so that X satisfies the formula (1) below;

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8/34 [040] subject the steel sheet to continuous annealing in a temperature range between the recrystallization temperature and 900 ° C; and [041] then, cool the steel sheet.

[042] 0.30 <{1.6- ([% Ti] + 2 [% Nb]) + 0.004X} <0.36 (1) [043] In formula (1), [% A] represents the steel element A content (% by mass).

ADVANTAGEOUS EFFECT OF THE INVENTION [044] According to the present invention, planar anisotropy of uniform elongation can be effectively suppressed, whereby it is possible to obtain a steel sheet with high strength that is equivalent in strength and ductility in the direction orthogonal (transverse) to the rolling direction, for conventional steel sheet and which exhibits significantly reduced crack generation in press forming compared to conventional steel sheet.

BRIEF DESCRIPTION OF THE DRAWINGS [045] The present invention will be further described below with reference to the accompanying drawings, in which:

[046] Figure 1A is a graph showing the relationship between (levels and reduction rate of Ti, Nb) and intensity f (Φ35 ^Ο ) of ODF {0 ^o , 35 °, 45 °}, where Φ = 35 °, φ1 = φ2 = 0 and ^the 45 °;

[047] Figure 1B is a graph showing the relationship between (levels and reduction rate of Ti, Nb) and planar anisotropy (AUEL) of uniform elongation; and [048] Figure 1C is a graph showing the relationship between f (Φ35 ^Ο ) and AUEL.

DESCRIPTION OF THE MODALITIES [049] The present invention will now be described in detail.

[050] The present invention resides in a new revelation that planar anisotropy in ductility, in particular, in stretching

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Uniform 9/34 of a steel plate can be reduced by specifying the reduction rate of cold rolling according to Ti and Nb contents to establish in a controlled way the intensity f (Φ35 ^Ο ) of ODF {0 ^o , 35 °, 45 °} described below to be 2.5 <f (Φ35 ^Ο ) <4.5.

[051] The mechanism of the phenomenon described above is not clear. The present inventor believes that: the α fiber in which the <110> direction of the same is parallel to the RD direction and the γ fiber in which the <111> direction of the same is parallel to the ND direction presumably grows in texture from a steel plate cold rolled; the r-value presumably increases when the γ fiber, in particular, grows; and the present invention makes it possible to reduce the ductility anisotropy of a cold-rolled steel sheet, regardless of the accumulation of other orientations, for example, the γ fiber which presumably correlates to the r-value as an index of deep steel plate stamping by establishing in a controlled way the ODF intensity of a specific orientation in which the α fiber is found where {φ1, Φ, φ2} = {0 ^o , 35 °, 45 °}, that is, the intensity f (Φ35 °) ODF {0 °, 35 °, 45 °} where Φ = 35 °, φ1 = 0 ° and φ2 = 45 ° must be 2.5 <f (Φ35 °) <4.5.

[052] Furthermore, it was revealed that: Ti and / or Nb have to be added in a predetermined amount to adequately suppress the aforementioned ODF intensity; and, in addition to such a suitable addition of Ti and / or Nb to the steel, conducting adequate hot rolling in non-recrystallized austenite region, cold rolling and annealing subsequently results in the desired texture structure of the steel eventually obtained. Consequently, it is critically important to control the levels of Ti, Nb and the rate of reduction of cold rolling which should be predetermined ranges, respectively.

[053] In other words, the planar anisotropy of uniform elongation of a steel sheet is reduced by the satisfaction of the various

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10/34 conditions described above. As a result, for example, it is possible to manufacture a steel plate equivalent in strength and ductility in the direction transverse to the rolling direction of conventional steel plate and which exhibits significantly reduced crack generation in press forming compared to the steel plate. conventional.

[054] The following will describe the reasons why a chemical composition of a steel plate should be restricted to the aforementioned bands on the high strength steel plate of the present invention. % in the chemical composition in the present invention represents% by mass unless otherwise specified.

C: more than 0.0005% and less than 0.10% [055] Carbon is an element required to improve the strength of a steel sheet, although it suppresses the secondary phase area ratio. However, the carbon content can be reduced to as low as 0.0005% (0.0005% is generally the lowest carbon content limit according to standard ingot techniques) in the present invention due to the fact that planar anisotropy of uniform elongation can be adequately controlled, even in the case of single-phase ferrite as described below. Carbon content equal to or higher than 0.1% increases the ratio of the steel secondary phase area, whereby the ductility of the steel deteriorates and it becomes difficult to control planar anisotropy of uniform elongation in ferrite texture due to to the secondary phase form a network to surround the ferrite phase. Consequently, the carbon content in steel should be less than 0.10% and preferably less than 0.08%.

Si: 1.5% or less [056] Silicon causes several effects of: delaying the generation of scale in hot rolling to improve the quality of its

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11/34 surface of a steel plate; to properly delay an alloying reaction between zinc and base iron in galvanizing or galvanizing and annealing; improve the ability to harden ferrite tension; and the like, whereby the Si content is preferably at least 0.01% and more preferably at least 0.05%. However, the Si content that exceeds 1.5% not only deteriorates the appearance quality of a steel sheet, but also increases the α—> γ transformation point, thus making it impossible to perform hot rolling in the γ region to significantly change the texture of the steel. That is, in that case of very high Si content, the planar anisotropy of uniform elongation of the steel sheet can no longer be controlled. Consequently, the Si content should be 1.5% or less and, preferably, 1.2% or less.

Mn: 0.1% or more and 3.0% or less [057] Manganese not only suppresses the deterioration of hot ductility caused by FeS, but is also useful as a solute reinforcing element. Mn must be added to the steel by at least 0.1%. The Mn content of less than 0.1% facilitates excessive grain growth in steel, which is not preferred in terms of controlling planar anisotropy of steel.

[058] Furthermore, manganese improves the hardening capacity of steel tempering and is an effective element in terms of making martensite present in the secondary phase to increase the strength of the steel. Manganese must be added to the steel by at least 1.0% to make such a multiphase steel structure as described above. However, very high Mn content decreases the transformation temperature α- »γ in the annealing process to induce the generation of γ grains in fine ferrite grain boundaries immediately after recrystallization or at grain interfaces being recovered in recrystallization, extending , thus, ferrite grains that

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12/34 is non-uniform and makes the secondary phase instantaneous, eventually deteriorating the ductility of a steel plate and making it impossible to control the planar anisotropy of uniform elongation. Consequently, the Mn content must be 3.0% or less. The Mn content is preferably 2.5% or less to meticulously control the planar anisotropy of uniform elongation of a steel sheet.

P: 0.080% or less [059] Phosphorus is conventionally used as a solute reinforcing element. In addition, it has been revealed that the addition of phosphorus in a very small amount has a marked effect on improving the quenching capacity of the steel. The phosphorus content is preferably at least 0.005%, more preferably at least 0.010% and even more preferably at least 0.015% to sufficiently achieve such good effects by adding phosphorus as described above. However, phosphorus content in steel that exceeds 0.080% significantly hinders the reaction of making alloy between base iron and coating layer to deteriorate spray resistance and weldability of steel. Consequently, the phosphorus content should be 0.080% or less and preferably 0.050% or less.

S: 0.03% or less [060] Very high sulfur content in steel results in MnS too precipitated in the steel, thereby deteriorating ductility such as elongation and flangeability and, thus, the press formability of a steel plate. resulting steel. In addition, sulfur tends to deteriorate the hot ductility of slabs and facilitate the generation of surface defects in the slabs. Furthermore, sulfur gently deteriorates the corrosion resistance of steel. Consequently, the sulfur content in steel should be 0.03% or less. Sulfur content is preferred

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13/34 0.01% or less and more preferably 0.002% or less in terms of sufficiently improving ductility and corrosion resistance.

Sol. Al: 0.01% or more and 0.50% or less [061] Aluminum is useful as a steel deoxidizing element and causes a solute nitrogen fixing effect to improve resistance to aging at room temperature. Consequently, the aluminum content (in the form of sol. Al) in steel must be at least 0.01%. However, adding aluminum to steel for a content that exceeds 0.50% significantly increases the cost of production and induces surface defects on a resulting steel sheet. Consequently, the content of Al in steel should be 0.50% or less and preferably 0.08% or less.

N: 0.005% or less [062] The nitrogen content in steel is preferably decreased as much as possible, as the very high nitrogen content deteriorates the resistance to aging at room temperature of the steel and requires the addition of Al and Ti in large quantities to alleviate deterioration. Consequently, the upper limit of nitrogen content must be 0.005%.

At least one element selected from Nb: 0.20% or less and Ti: 0.20% or less;

Nb: 0.20% or less [063] Niobium is an important element in the present invention due to the fact that Nb makes fine steel structure grains and suppresses the recrystallization of austenite in the hot rolling process to make it possible to control the planar anisotropy of uniform elongation of a steel sheet after cold rolling and annealing. However, Nb content in steel that exceeds 0.20% not only significantly increases the cost of production, but also results

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14/34 in too much texture growth in hot rolling and too much increase in recrystallization temperature, thus making it impossible to control the planar anisotropy of uniform elongation of a resulting steel sheet. Consequently, the Nb content needs to be 0.20% or less and is preferably 0.12% or less. The Nb content in steel is preferably at least 0.005% to sufficiently obtain the good effects mentioned above.

Ti: 0.20% or less [064] Titanium is an important element in the present invention due to the fact that Ti makes the steel structure grains thin and suppresses the recrystallization of austenite in the hot rolling process to make it possible control the planar anisotropy of uniform elongation of a steel sheet after cold rolling and annealing. However, Ti content in steel that exceeds 0.20% not only significantly increases the cost of production, but also results in too much texture growth in hot rolling and too much increase in recrystallization temperature, thus making it impossible to control the planar anisotropy of uniform elongation of a resulting steel sheet. Consequently, the Ti content needs to be 0.20% or less and is preferably 0.12% or less. The Ti content in steel is preferably at least 0.005% to sufficiently obtain the good effects mentioned above.

[065] In addition to the basic components described above, the chemical composition of the high-strength steel plate of the present invention may contain other elements such as V, Cr, Mo, W, Zr, Cu, Ni, B, Sn, Sb, Ca , Ce and La in contents shown below.

V: 0.40% or less [066] Vanadium is an element that improves the quench hardening capacity without greatly deteriorating the coating or plating quality and corrosion resistance of a steel sheet.

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15/34

It is thus possible to use vanadium instead of Mn and / or Cr. However, the vanadium content in steel is preferably 0.40% or less due to the fact that the addition of vanadium in a content that exceeds 0.40% significantly increases the cost of production.

Cr: 0.50% or less [067] Chromium is, like Mn, an element that produces a composite or multiphase structure of a steel plate to help improve the strength of the steel plate. The Cr content in steel is preferably at least 0.10% to achieve this effect sufficiently. However, adding too much Cr to steel is not preferred, as the effect reaches a level and the production cost increases significantly due to the costly alloy. Consequently, the upper limit of Cr content in steel should be 0.50%.

Mo: 0.50% or less [068] Molybdenum is an element that improves the quench hardening capacity to suppress the generation of perlite and contribute to improving the strength of steel. However, unnecessarily high content of Mo in steel increases the cost of production due to the fact that Mo is a very expensive element. Consequently, the Mo content in steel is preferably 0.50% or less.

W: 0.15% or less [069] Tungsten can be used as an element to improve quench hardening and precipitation reinforcement. However, very high tungsten content in steel deteriorates the ductility of a resulting steel sheet. Consequently, the W content in steel is preferably 0.15% or less.

Zr: 0.10% or less [070] Zirconium can be used as an element to improve quench hardening and precipitation reinforcement. However, the very high zirconium content in steel deteriorates

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16/34 the ductility of a resulting steel sheet. Consequently, the Zr content in steel is preferably 0.10% or less.

Cu: 0.50% or less [071] Recycled materials can be used as raw materials and, thus, the production cost can be reduced by allowing copper to be mixed in the chemical composition of the steel sheet of the present invention. In one case, Cu is actively added to steel, and the Cu content in steel is preferably at least 0.03% also in view of its corrosion resistance enhancing effect. However, the very high Cu content in steel causes surface defects to occur in a resulting steel plate. Consequently, the upper limit of steel content is preferably 0.50%.

Ni: 0.50% or less [072] Nickel is an element that improves the corrosion resistance of steel and causes a reduction effect on surface defects that possibly occur in steel when steel contains Cu. Consequently, the Ni content in steel is preferably at least 0.02% in terms of improving corrosion resistance and thus the surface quality of a steel plate. However, the very high Ni content in steel not only results in uneven scale generation in a heating furnace causing surface defects in a resulting steel plate, but also significantly increases the cost of production. Consequently, the upper limit of Ni content in steel is preferably 0.50%.

B: 0.0050% or less [073] Boron is an element that improves the hardening capacity of steel quenching. In addition, in particular, boron successfully suppresses the brittle fracture after forming by press on a single-phase ferrite microstructure. These superior effects cause

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17/34 by boron reach levels when the boron content exceeds 0.0050%. Consequently, in a case where boron is added, the boron content in steel is preferably 0.0050% or less.

Sn: 0.20% or less [074] Tin is preferably added to steel to suppress nitriding and oxidation of an outer surface of a steel sheet and / or suppress decarbonisation and deboronization of a surface layer of the steel sheet caused by oxidation. The Sn content in steel is preferably at least 0.005% to sufficiently suppress such nitriding and oxidation from an outer surface of a steel sheet as described above. However, the Sn content in steel that exceeds 0.20% results in increased production resistance (YP) and deterioration of the steel's hardness. Consequently, the Sn content in steel is preferably 0.20% or less.

Sb: 0.20% or less [075] Antimony, like tin, is preferably added to steel to suppress nitriding and oxidation of an outer surface of a steel sheet and / or suppress decarbonization and deboronization of a surface layer of the steel sheet caused by oxidation. Antimony, by such suppression of nitriding and oxidation of an outer surface of a steel sheet as described above, prevents the amount of martensite generated in a surface layer of a steel sheet from decreasing. In addition, the antimony prevents the quenching ability from deteriorating by suppressing deboronization of a steel sheet surface layer as described above. Furthermore, the antimony improves the wettability of hot dip galvanizing to enhance the appearance of coating quality. The steel Sb content is preferably at least 0.005% to sufficiently suppress such nitriding and oxidation from an outer surface of a

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18/34 steel plate as described above. However, the Sb content in steel that exceeds 0.20% results in an increase in the production resistance (YP) and deterioration of the steel hardness. Consequently, the Sb content in steel is preferably 0.20% or less.

Ca: 0.010% or less [076] Calcium causes sulfur-fixing effects on steel like CaS, increasing the pH in corrosion products and improving corrosion resistance in the vicinity of a portion processed by edge forming and / or the portion welded in points. In addition, calcium causes an effect of forming CaS to suppress the generation of MnS which would otherwise deteriorate the elasticity flangeability of a steel plate, eventually improving, in this way, the elasticity flangeability of the steel plate. The Ca content in steel is preferably at least 0.0005% to sufficiently achieve these effects. However, calcium tends to float and be separated as an oxide in molten steel and therefore cannot be contained in a stable manner by a large amount of steel. Consequently, the Ca content in steel is preferably 0.010% or less.

Ce: 0.01% or less [077] Cerium can be added to steel to fix sulfur in steel. However, cerium is a costly element and adding too much of it significantly increases the cost of production. Consequently, the Ce content in steel is preferably 0.01% or less.

La: 0.01% or less [078] Lanthanum can be added to steel to fix sulfur in steel. However, lanthanum is a costly element and adding it too significantly increases the cost of production. Consequently, the La content in steel is preferably 0.01% or less.

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19/34 [079] The rest, in addition to the aforementioned elements, of the chemical composition is Fe and incidental impurities.

[080] In the following, the steel microstructure of the high strength steel plate of the present invention will be described.

60% or more of ferrite phase in fraction of volume as steel microstructure [081] The ferrite texture must be controlled in the present invention and X-ray diffraction is generally employed when such steel textures, such as ferrite texture, are examined . However, the primary ferrite phase and the secondary martensite and / or bainite phase cannot be clearly distinguished by X-ray diffraction, whereby a problem arises in which uniform elongation anisotropy is not controllable by texture control. of ferrite, which is the key resource of the present invention, when the secondary phase fraction is relatively high. In addition, another problem arises when the fraction of the secondary phase is relatively high, in which the secondary phase forms a network to surround ferrite, whereby the microscopic plastic behavior of a steel plate no longer depends on the orientation of the ferrite crystal.

[082] The volume fraction of ferrite needs to be at least 60%, preferably at least 75%, in the present invention for the reasons described above.

[083] The volume fraction of the ferrite phase can be determined: first, by obtaining the secondary phase area ratio according to the procedures described below; as to the ratio of the secondary phase area as the volume fraction of the secondary phase; and the performance of necessary calculations based on the volume fraction of the secondary phase.

[084] The secondary phase area ratio is determined by: polishing an L cross section (a vertical cross section in

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20/34 parallel to the rolling direction) of a steel plate and subject the transversal cut to corrosion by nital; observation and photography of 10 fields of the cross section in magnification x 4,000 by using a scanning electron microscope (SEM); and analysis of the photographs of steel microstructures thus obtained. Ferrite is seen as a region with a matte black contrast, pearlite and bainite are seen as regions that have carbides generated in a lamellar manner or a sequence of points and retained martensite or γ is seen as a region that has grains with white contrast in such photographs of steel microstructures as described above. Each of the fine dot-type particles has a diameter no greater than 0.4 pm observed in these SEM photos are predominantly carbides and the ratio of their area is very small, whereby it is reasonably assumed that these fine dot-type particles hardly affect the quality of steel. Consequently, particles in which each has a particle diameter not greater than 0.4 m are excluded from the area ratio assessment in the present invention. The secondary phase area ratio is determined by using a square mesh and by measuring a secondary phase ratio present in the square mesh grid (the method of counting points). The area ratio (%) of the secondary phase thus determined is considered, as it is, to be the volume fraction (%) of the secondary phase. The volume fraction (%) of the ferrite phase is obtained by subtracting the volume fraction (%) of the secondary phase from 100%.

As long as the 3D crystal orientation distribution function (i.e., the orientation distribution function, ODF) is represented by {φ1, Φ, φ2}, the ODF intensity {0 ^o , 0 ^o , 45 °} in that Φ = 0 ^o , φ1 = 0 ^o and φ2 = 45 ° is 3.0 or less.

[085] Although the ODF intensity f (Φ35 ^Ο ) where Φ = 35 °, φ1 = 0 ^o and φ2 = 45 ° is the most important among the requirements for

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21/34 to control uniform elongation anisotropy, the ODF intensity {0 ^o , 0 °, 45 °} where Φ = 0 °, φ1 = 0 ° and φ2 = 45 ° still need to be controlled due to the fact that the intensity too high ODF {0 °, 0 °, 45 °} results in poor deep stamping and therefore poor press formability. Consequently, the ODF intensity {0 °, 0 °, 45 °} must be 3.0 or less.

As long as the 3D crystal orientation distribution function (that is, the orientation distribution function, ODF) is represented by {φ1, Φ, φ2}, the ODF intensity {0 °, 35 °, 45 °} in that Φ = 35 °, φ1 = 0 ° and φ2 = 45 ° is 2.5 or more and 4.5 or less.

[086] It is essential to establish that the ODF intensity {0 °, 35 °, 45 °} where Φ = 35 °, φ1 = 0 ° and φ2 = 45 ° must be in the range of 2.5 or more and 4, 5 or less as described above in order to control uniform elongation anisotropy. In a case where the ODF intensity {0 °, 35 °, 45 °} is less than 2.5, the uniform elongation in the lamination direction and the uniform elongation in the orthogonal or transverse direction to the lamination direction (whose direction will be referred to as the transverse direction from now on), in particular, the uniform elongation in the rolling direction is relatively low, thus facilitating the generation of cracking in conformation by press.

[087] In a case where the ODF intensity {0 °, 35 °, 45 °} is greater than 4.5, the uniform elongation in the D direction (a direction inclined by 45 ° to the lamination direction) it is relatively low presumably due to the fact that the texture of a steel sheet affects the production resistance anisotropy, to bring the steel sheet's yield and ductility limit to an exchange ratio. That is, the uniform elongation in a high strength direction is relatively low. In this respect, it is additionally assumed that Ti and Nb, which extend crystal grains in the rolling direction, also affect anisotropy in ductility of steel microstructure.

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22/34 [088] Intensities of crystal orientations different from those described above, for example, γ fiber intensity, are not particularly restricted due to the fact that these intensities do not affect the uniform elongation anisotropy.

[089] The intensity of OFD {φ1, Φ, φ2} in the present invention is determined: by measuring three-sided pole figures (200), (211) and (110) by the reflection method to obtain three pole figures incomplete; by converting these three incomplete pole figures into 3D crystal orientation distribution (ODF) using the series expansion method; and by determining the intensities of the respective desired crystal orientations.

[090] In the following, a method for making the hot-rolled high-strength steel sheet of the present invention will be described.

[091] Although a steel plate for use in the manufacturing method of the present invention is preferably produced by continuous casting to prevent macrosegregation of components from occurring, the steel plate production method is not particularly restricted and the plate can be produced by casting by casting or thin plate casting process. The steel plate thus produced can be cooled to room temperature and then heated again according to the conventional method. Alternatively, what is called an energy saving process such as direct hot rolling can be used without problem in which either a hot steel plate without being completely cooled is loaded into a heating and hot rolled oven or a hot plate. steel, which is kept warm for a short period, is quickly hot rolled.

[092] The heating temperature of the plate, although no particular restrictions are required on it, is preferably set to be relatively low to make the precipitates

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23/34 thick produce sufficient recrystallization texture {111} and enhance deep stamping. However, the heating temperature of the plate below 1000 ° C increases the lamination load and thus the risk of setbacks occurring during hot rolling, whereby the heating temperature of the plate is preferably not lower than than 1,000 ° C. The upper limit of the plate heating temperature is preferably 1,300 ° C to suppress the increase in scale loss caused by the increased weight of oxidized steel.

[093] The steel plate heated under the conditions described above is subjected to hot rolling which includes rough rolling and finishing rolling. Specifically, the steel plate is rolled into a plate bar by rough rolling. The conditions of rough rolling are not particularly restricted and can be carried out according to the conventional method. The use of what is called a plate bar heater to heat a plate bar is effective in terms of keeping the plate heating temperature relatively low and preventing setbacks from occurring during hot rolling.

Finishing delivery temperature: 820 ° C or higher and 950 ° C or lower [094] The sheet bar thus obtained is laminated by finalization on a hot-rolled steel sheet. The finishing delivery temperature (which will hereinafter be referred to as FT) needs to be set to be in the range of 820 ° C or higher and 950 ° C or lower, so that the texture that is preferred in terms of anisotropy planar of uniform elongation is obtained after cold rolling and recrystallization annealing. The FT lower than 820 ° C not only increases the rolling load, but also results in ferrite region rolling and thus significant steel textures

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24/34 changed in some component systems. The FT greater than 950 ° C makes the steel microstructure thick and also makes it impossible to conduct hot rolling in a state of non-crystallized austenite in a satisfactory manner, whereby uniform elongation in the D direction decreases cold annealing.

[095] At least a part of the finishing lamination can be conducted as a lubrication lamination to decrease the lamination load in hot rolling. Conducting lubrication rolling in such a way is effective in terms of making the shape and material quality of a steel sheet uniform. In this respect, the coefficient of friction between rolling mills and a steel plate are preferably established to be in the range of 0.10 to 0.25. In addition, it is preferable to employ the process of continuous lamination of finishing lamination in a continuous manner of welded sheet bars in line. The application of a continuous lamination process to the method of the present invention is preferred in view also of the stable operation of hot rolling.

[096] The winding temperature (CT) in the present invention is, although there is no particular restriction to it, preferably in the range of 400 ° C or higher and 720 ° C or lower. The winding temperature that exceeds the upper limit not only results in coarse crystal grains and decreased strength, but also obstructs reaching a sufficiently high r value after cold annealing.

[097] Then, the hot rolled steel sheet is subjected to pickling and cold rolling to obtain a cold rolled steel sheet. Stripping can be carried out according to the conventional method.

[098] Cold rolling in the present invention needs to be carried out so that the reduction rate (X%) of the same satisfies the form 870180072929, of 20/08/2018, pg. 31/48

25/34 mule (1) below.

[099] 0.30 <{1.6 ([% Ti] + 2 [% Nb]) + 0.004X} <0.36 (1) [0100] Ti and Nb are important elements in terms of conducting adequate hot rolling in the non-recrystallization temperature region of austenite. Due to the γ texture growth caused by hot rolling in the austenite non-recrystallization temperature region and the variant restriction during subsequent transformation, Ti and Nb are reasonably said to greatly affect the textures eventually obtained by rolling. Furthermore, the reduction rate is a critically important condition in terms of the growth of textures eventually obtained by lamination.

[0101] In view of the facts described above and based on a thought that the texture derived from thickened ferrite particles and the texture derived from hot rolling in the austenite non-recrystallization temperature region cause opposite effects in relation to anisotropy planar elongation and thus these two textures and effects need to be balanced with each other, the inventor of the present invention investigated the relationship between uniformly elongated planar anisotropy and intensity f (Φ35 ^Ο ) of ODF {0 ^o , 35 °, 45 °} where Φ = 35 ° in steel samples that have varying Ti, Nb contents and reduction rates. The inventor of the present invention: assumed that the content of Nb ([% Nb]) affects approximately twice as much as the content of Ti ([% Ti]) due to the difference between them in atomic weight and suppression effect of recrystallization caused by Nb, Ti precipitates and Ti solute, Nb solute, respectively; and considered the difference in effect between Ti and Nb in the analysis. It was evaluated from the results thus obtained as Ti, the Nb contents and the reduction rate affect the intensity f (Φ35 ^Ο ) and planar anisotropy (AUEL) of uniform elongation, respectively. The results of these assessments are shown in Figure 1A and Figure 1B,

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26/34 respectively.

[0102] Furthermore, Figure 1C shows the relationship between intensity f (Φ35 ^Ο ) and planar isotropy (AUEL) of uniform elongation.

[0103] It can be understood from Figure 1A, in particular, that {1.6 ([% Ti] + 2 [% Nb]) + 0.004X} shows a good relationship with f (Φ35 ^Ο ). Furthermore, it is understood from Figures 1B and 1C that the value of f (Φ35 ^Ο ) can be reliably established to be 2.5 <f (Φ35 ^Ο ) <4.5 and, thus, planar anisotropy (AUEL ) of uniform elongation can be sufficiently suppressed when formula (1) above is satisfied.

[0104] As for planar anisotropy, anisotropy was evaluated as a uniform AUEL elongation standardized by UELl using an index formula (2) shown below due to the fact that the absolute ductility value of a steel plate will boo according to the level strength of the steel sheet.

AUEL = {(UELl / UELl) + (UELc / UELl) - 2 (UEL _d / UEL _l )} / 2 = {UELl + UELc - 2 UEL _d } / (2 UELl) (2) [0105] In the formulas ( 2), UELl, UELd, UELc represent uniform elongation in the L direction, uniform elongation in the D direction and uniform elongation in the C direction, respectively.

[0106] The steel sheet is then subjected to annealing at an annealing temperature between recrystallization temperature and 900 ° C and then cooled.

[0107] The annealing temperature should be the recrystallization temperature or higher, to suppress residual stress by hot rolling and prevent the ductility of a steel sheet from deteriorating. However, the annealing temperature must be 900 ° C or lower due to the fact that the annealing temperature exceeds 900 ° C not only shortens the life of an annealing furnace, but also results in abnormal grain growth, fraction γ very high and similarPetition 870180072929, of 20/08/2018, p. 33/48

27/34 res, which can drastically change the texture of the steel after reverse transformation. In the present invention, recrystallization temperature can be determined by: subjecting a cold rolled steel sheet to a short-time annealing process of heating the steel sheet to a predetermined annealing temperature and then immediate cooling (within 1 hour). second of retention time) of the steel plate; then, temper the steel sheet by immersing it in water; observe the steel microstructure; and repeating the above steps at various annealing temperatures with a gradual increase in temperature, to find the temperature at which no more crystallization is observed. In this observation, the annealing temperature can be changed, for example, from 650 ° C in increments of 10 ° C.

[0108] Although the cooling rate after annealing described above is not particularly restricted, the average cooling rate in the temperature range from the annealing temperature to 500 ° C is preferably in the range of 5 ° C / s or more high to 15 ° C / s or lower in a case where the martensite must be conformed as a secondary phase. The average cooling rate in the above temperature range lower than 5 ° C / s can hinder the formation of martensite, which may result in a single-phase ferrite microstructure that has insufficient resistance by microstructure control.

[0109] In the present invention that accepts the presence of a secondary phase that includes martensite, the average cooling rate from the annealing temperature to 500 ° C is preferably set to be equal to or higher than the frontier cooling rate. critical, ie approximately 5 ° C / s or higher. However, the same average cooling rate that exceeds 15 ° C / s results in a very high fraction of the secondary phase, which is an unfavorable distribution

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28/34 in terms of the ductility of a steel sheet, although the steel sheet has somehow multiphase structure. Consequently, the average cooling rate from the annealing temperature to 500 ° C is preferably in the range of 5 ° C / s or higher to 15 ° C / s or lower.

[0110] Cooling in a temperature range of 500 ° C or lower, where the γ phase is relatively stable due to the preceding cooling, does not need to be particularly restricted. However, the average cooling rate over a temperature range from 500 ° C to 300 ° C is preferably at least 5 ° C / s. In a case where excessive aging treatment is done, the average cooling rate for the excessive aging treatment temperature is preferably at least 5 ° C / s.

[0111] In the present invention, the steel sheet can be provided with a zinc coating on it according to need. As for the hot dip galvanizing line, the average cooling rate from the annealing temperature or sink temperature to the galvanizing bath temperature (which is generally maintained in the temperature range of 450 ° C to 500 ° C) it is preferably in the range of 2 ° C / s to 30 ° C / s in a case where martensite must be conformed as a secondary phase. The cooling rate lower than 2 ° C / s results in too much perlite conformation in the temperature range of 500 ° C to 650 ° C, making it impossible to obtain the second hard phase. The cooling rate that exceeds 30 ° C / s significantly facilitates the transformation γ- »α in a temperature range around 500 ° C approximately when the steel sheet is immersed and a galvanizing bath, thus making the fine secondary phase and deteriorating the ductility of the steel sheet.

[0112] In a case where the zinc-coated steel sheet is then galvanized and annealed, the galvanized and annealed steel sheet

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29/34 is cooled to 100 ° C or lower at the average cooling rate in the range of 5 ° C / s to 100 ° C / s. The cooling rate lower than 5 ° C / s above results in the generation of perlite around 550 ° C and bainite with carbide precipitation around 400 ° C to 450 ° C, which increases YP and deteriorates the balance between strength and ductility of the steel sheet. The cooling rate that exceeds 100 ° C / s above results in insufficient self-curing of martensite generated during continuous cooling, thereby excessively hardening the martensite, increasing YP and deteriorating the ductility of the steel sheet.

[0113] In addition, the cold-rolled and tempered steel sheet and the cold-rolled and coated tempered steel sheet of the present invention can be subjected to tempering lamination or leveling process for shape correction, surface roughness adjustment It's similar. The total elongation rate of lamination by tempering or leveling process is preferably within the range of 0.2% to 15%. The total elongation rate less than 0.2% cannot achieve the desired objective such as shape correction and surface roughness adjustment. The total elongation rate that exceeds 15% significantly deteriorates the ductility of the steel sheet. It has been confirmed that tempering lamination and leveling process do not make much difference in effect, although tempering lamination and leveling process are significantly different from one another in the type of processing. Tempering lamination and leveling process after galvanizing still have good effects, respectively.

EXAMPLES [0114] Plate samples (steel material) were prepared by submitting corresponding steel samples that have chemical compositions shown in Table 1 to ingot techniques that use a

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30/34 converter and subsequent continuous casting, respectively. Each plate sample was heated to 1,250 ° C and subjected to rough lamination to obtain a plate bar. The sheet bar thus obtained was then hot rolled with finishing lamination performed under conditions shown in Table 2, to obtain a hot rolled steel sheet. The hot rolled steel plate was subjected to pickling and then the hot rolling process at the corresponding reduction rate (CR) shown in Table 2, to obtain a sample of cold rolled steel plate. The cold rolled steel sheet sample was then fed to a continuous annealing line and subjected to continuous annealing at the corresponding annealing temperature (AnnT) shown in Table 2. The cold tempered steel sheet sample thus tempered was , then, subjected to tempering lamination at the elongation rate: 0.5%. The recrystallization temperatures of the steel plate samples, determined by observing the steel microstructure of the samples after short-time heating and quenching as described above, were unanimously in the range of 700 ° C to 760 ° C and equal to or higher than than the required recrystallization temperature regardless of changes in other conditions.

[0115] Steel plate sample n ² 5 was prepared as a hot-dip galvanized steel sheet by submitting the sample to the cold-rolled steel sheet annealing process and subsequent in-line hot-dip galvanizing (temperature plating bath: 480 ° C) in a continuous hot dip galvanizing line.

[0116] Test pieces were collected from each of the cold-rolled and tempered sheet steel samples and the sample and microstructure of the hot-dip galvanized steel sheet and tensile properties and orientation distribution function of

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31/34 3D crystal (ODF) of the test pieces were analyzed by the methods described below, respectively.

TRACTION PROPERTIES [0117] JIS n ² 5 tensile test pieces for the L direction (the direction inclined by 0 ^o in relation to the rolling direction), for the D direction (the direction inclined by 45 ° in relation to the direction of rolling) and for the C direction (the direction inclined by 90 ° in relation to the rolling direction) were collected, respectively, from each of the cold rolled and tempered steel plate samples and the immersion galvanized steel plate sample the heat thus obtained. The JIS n ² 5 tensile test pieces were subjected to tensile tests according to Document n ² JIS Z 2241 at a crosshead rate of 10mm / m, to determine stress produced (YS), tensile strength (TS) and uniform elongations (UEL) of the respective directions of the same.

3D CRYSTAL ORIENTATION DISTRIBUTION FUNCTION, IE, ODF [0118] As long as ODF is represented by {φ1, Φ, φ2}, the ODF intensity {0 ^o , 0 °, 45 °} where Φ = 0 ° , φ1 = 0 ° and φ2 = 45 ° and the ODF intensity {0 °, 35 °, 45 °} where Φ = 35 °, φ1 = 0 ° and φ2 = 45 ° were determined according to the method described above , respectively.

UNIFORM STRETCH PLANAR ANISOTROPY [0119] Uniformly stretched planar anisotropy was evaluated by obtaining the AUEL value according to formula (2) below. It was evaluated that planar anisotropy of uniform elongation is excellent when the AUEL value obtained by formula (2) is in the range of -0.020 to 0.020 in the present invention.

[0120] The results obtained are shown in Table 2.

AUEL = {UELl + UELc - 2 UELd} / (2 UELl) (2)

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32/34 [0121] The microstructure (ferrite volume fraction) was analyzed based on the secondary phase area ratio (volume ratio) obtained by the point counting method using SEM photos described above.

TABLE 1

Steel sample type Chemical composition (% by mass) Ç Si Mn P s Sol .Al N You Nb Others THE 0.0022 0.01 0.18 0.013 0.009 0.043 0.0020 0.041 0.001 B: 0.0006 B 0.093 0.12 1.55 0.008 0.005 0.035 0.0015 0.006 0.001 - Ç 0.064 0.48 1.45 0.012 0.004 0.037 0.0029 0.001 0.006 - D 0.085 1.43 1.58 0.013 0.006 0.038 0.0024 0.003 0.042 - AND 0.025 0.01 0.22 0.012 0.008 0.040 0.0022 0.007 0.001 - F 0.081 0.03 2.16 0.017 0.002 0.031 0.0039 0.018 0.048 Mo: 0.1 G 0.078 0.03 3.10 0.015 0.003 0.040 0.0025 0.016 0.045 - H 0.126 1.29 1.93 0.023 0.002 0.045 0.0034 0.006 0.004 - 1 0.026 0.01 1.81 0.025 0.006 0.055 0.0025 0.006 0.001 Cr: 0.15 J 0.025 0.01 1.79 0.023 0.003 0.045 0.0015 0.006 0.001 V: 0.03 K 0.023 0.01 1.85 0.020 0.002 0.043 0.0017 0.006 0.001 W: 0.01 L 0.025 0.02 1.82 0.025 0.001 0.058 0.0018 0.006 0.001 Zr: 0.02 M 0.025 0.01 1.78 0.021 0.005 0.035 0.0016 0.006 0.001 Cu: 0.1, Ni: 0.04 N 0.026 0.01 1.76 0.026 0.005 0.035 0.0014 0.006 0.001 Sn: 0.010 O 0.023 0.02 1.70 0.021 0.006 0.036 0.0020 0.006 0.001 Sb: 0.008 P 0.022 0.01 1.72 0.022 0.007 0.034 0.0021 0.006 0.001 Ca: 0.008 Q 0.022 0.01 1.82 0.025 0.007 0.030 0.0022 0.006 0.001 Ce: 0.005, La: 0.006

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33/34

Table 1 (Continued)

Steel sample type Ti + 2 * Nb (% by mass) Note THE 0.043 Within the scope of the present invention B 0.008 Within the scope of the present invention Ç 0.013 Within the scope of the present invention D 0.087 Within the scope of the present invention AND 0.009 Within the scope of the present invention F 0.114 Within the scope of the present invention G 0.106 Beyond the scope of the present invention H 0.014 Beyond the scope of the present invention 1 0.008 Within the scope of the present invention J 0.008 Within the scope of the present invention K 0.008 Within the scope of the present invention L 0.008 Within the scope of the present invention M 0.008 Within the scope of the present invention N 0.008 Within the scope of the present invention O 0.008 Within the scope of the present invention P 0.008 Within the scope of the present invention Q 0.008 Within the scope of the present invention

The remainder of the aforementioned composition is Fe and additional impurities.

TABLE 2

No. Steel sample type FT CT CR AnnT {1.6 * ([% Ti] + 2 * [% Nb]) + 0.004X} Ferrite fraction ° C ° C % ° C % % 1 THE 900 630 72 820 0.36 100 2 THE 800 620 72 815 0.36 100 3 B 880 550 68 800 0.28 80 4 Ç 850 500 60 800 0.26 82 5 D 860 530 50 800 0.34 75 6 D 860 530 70 800 0.42 74 7 AND 880 560 70 800 0.29 98 8 AND 880 560 77 800 0.32 98 9 F 880 580 60 800 0.42 85 10 G 885 585 60 800 0.41 78 11 H 880 520 50 810 0.22 55 12 I 870 500 76 790 0.32 92 13 J 870 500 76 790 0.32 93 14 K 880 500 76 790 0.32 93 15 L 870 500 76 800 0.32 93 16 M 870 520 76 800 0.32 92 17 N 880 500 77 790 0.32 93 18 O 870 520 76 800 0.32 93 19 P 870 520 76 800 0.32 93 20 Q 870 520 77 800 0.32 93

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34/34

Table 2 (Continued)

At the. TS in direction C UEL {Φ1, Φ2} = {0 °, 45 °} Note MPa L (%) D (%) Ç (%) Δ Φ = 0 ° Φ = 35 ° 1 305 25.8 26.4 25.7 -0.02 1.2 4.0 Example 2 297 22.5 21.5 22.5 0.04 £ 6 L9 Comparative Example 3 466 16.9 16.1 16.7 0.04 2.0 2.4 Comparative Example 4 639 16.2 14.7 15.8 0.08 1.9 1.8 Comparative Example 5 828 9.7 9.4 9.1 0.00 2.5 3.3 Example 6 835 8.7 9.5 9.2 -0.06 £ 5 £ 8 Comparative Example 7 317 21.3 19.9 21.0 0.06 2.0 14 Comparative Example 8 319 21.0 20.5 20.7 0.02 1.4 2.7 Example 9 640 18.5 20.0 17.9 -0.10 9.5 5.2 Comparative Example 10 720 12.1 14.1 12.5 -0.15 10.5 5.3 Comparative Example 11 999 9.1 8.8 8.6 0.01 15 12 Comparative Example 12 460 21.2 21.5 22.2 0.01 2.3 3.1 Example 13 458 22.1 22.0 22.5 0.01 2.2 3.1 Example 14 457 21.8 21.7 22.3 0.02 2.3 3.1 Example 15 456 20.8 21.0 21.8 0.01 2.2 3.2 Example 16 463 21.9 21.7 22.1 0.01 2.2 3.3 Example 17 457 21.4 21.5 22.1 0.01 2.2 3.2 Example 18 456 21.3 21.5 22.3 0.01 2.2 3.1 Example 19 456 22.0 22.0 22.7 0.02 2.2 3.1 Example 20 457 21.8 21.9 22.8 0.02 2.3 3.2 Example

[0122] As shown in Table 2, each of the steel plate samples of the Examples according to the present invention exhibited: ODF intensity {0 ^o , 0 ^o , 45 °} is 3.0 or less; ODF intensity {0 ^o , 35 °, 45 °} is 2.5 or more and 4.5 or less; and AUEL value in the range of -0.020 to 0.020, thus providing a sample of steel plate with sufficiently high resistance and satisfactorily small planar anisotropy of uniform elongation.

[0123] In contrast, each of the steel plate samples of Comparative Examples in which at least one of the steel components or the condition of production is beyond the scope of the present invention had texture beyond the scope of the present invention, thereby exhibiting way, significantly large planar anisotropy of uniform elongation.

Claims (2)

1. High strength steel plate, characterized by the fact that it comprises a chemical composition consisting of% by mass: C: more than 0.0005% and less than 0.10%; Si: 1.5% or less; Mn: 0.1% or more and 3.0% or less; P: 0.080% or less; S: 0.03% or less; Sun. Al: 0.01% or more and 0.50% or less; N: 0.005% or less; at least one element selected from Nb: 0.20% or less and Ti: 0.20% or less; optionally at least one element selected from V: 0.40% or less, Cr: 0.50% or less, Mo: 0.50% or less, W: 0.15% or less, Zr: 0.10 % or less, Cu: 0.50% or less, Ni: 0.50% or less, B: 0.0050% or less, Sn: 0.20% or less, Sb: 0.20% or less, Ca : 0.010% or less, Ce: 0.01% or less and La: 0.01% or less; and the remainder as Fe and incidental impurities, where the steel microstructure of the steel plate contains 60% or more of the ferrite phase in fraction of volume, and as long as the 3D crystal orientation distribution function ( orientation, ODF) is represented by {φ1, Φ, φ2}, the intensity of ODF {0 ^o , 0 ^o , 45 °} where Φ = 0 ^o , φ1 = 0 ^o and φ2 = 45 ° is 3.0 or any less; and the intensity of ODF {0 ^o , 35 °, 45 °} where Φ = 35 °, φ1 = 0 ° and φ2 = 45 ° is 2.5 or more and 4.5 or less. 2. Method for making a sheet of steel with high resistance characterized by the fact that it comprises the steps of: submit a steel plate that has the chemical composition Petition 870180072929, of 20/08/2018, p. 42/48 2/2 ca, as defined in claim 1, to hot rolling at a finishing delivery temperature of 820 ° C or higher and 950 ° C or lower to obtain a hot rolled steel sheet; subjecting the steel sheet to cold rolling at a rate of reduction (X%) so that X satisfies the formula (1) below; subject the steel sheet to continuous annealing in a temperature range between the recrystallization temperature and 900 ° C; and then cool the steel sheet. 0.30 <{1.6 ([% Ti] + 2 [% Nb]) + 0.004X} <0.36 (1) In formula (1), [% A] represents the content of element A in steel (% by mass).