JP2004332100A - High-strength thin steel sheet excellent in hydrogen embrittlement resistance, weldability and hole expandability, and method for producing the same - Google Patents

Abstract

An object of the present invention is to simultaneously improve hydrogen embrittlement resistance, weldability, hole expandability, and ductility of a high-strength steel sheet having a tensile strength of 800 MPa or more.
SOLUTION: In mass%, C, Si, Mn, P, S, Al, N are contained in predetermined amounts, Ni, Cu, Cr, Mo, Nb are contained in predetermined amounts, and the balance is iron. And 70% or more in total of one or both of bainite and bainitic ferrite in area ratio, less than 3% of retained austenite (Vγ), and a tensile strength (TS) of 800 MPa or more. , (1-1) to (1-3). (3.0Nb + 2.5Mo + 1 / 10Si + Mn) - (2C 0.5 +2)> 0 ... (1-1) 0 ≦ 0.8 × {2Cu + 20Mo + 3Ni + Cr + Vγ} - {0.1-3.5 × 10 7 × (TS) -3.1 … (1-2) 0> Si + Al + 7.67C-1.78 (1-3)
[Selection diagram] None

Description

  INDUSTRIAL APPLICABILITY The present invention has excellent weldability and hole expandability suitable for building materials, home appliances, automobiles, and the like, and in a high-strength steel sheet having a tensile strength of 800 MPa or more, particularly suppressed hydrogen embrittlement, placing cracks and delayed fracture, which are particularly problematic. The present invention relates to a high-strength thin steel sheet and a method for producing the same.

  Conventionally, high-strength steels are often used for applications such as bolts, PC steel wires and line pipes, and it is known that when the tensile strength exceeds 980 MPa, delayed fracture occurs due to intrusion of hydrogen into the steel. I have. On the other hand, since the thin steel sheet has a small thickness and is released in a short time even if hydrogen enters, it can be said that the awareness of the problem of so-called delayed fracture was low. However, recently, due to the need to reduce the weight of automobiles and improve collision safety, ultra-high-strength steel sheets of 980MPa or higher are subjected to press forming, pipe forming, bending, end face processing, hole expanding, etc. to make bumpers and impacts. The use of such materials as reinforcing materials such as beams, seat rails, and the like is rapidly increasing. Therefore, there is an urgent need to develop an ultra-high strength steel sheet having delayed fracture resistance.

  Heretofore, most techniques for improving delayed fracture resistance have been developed for steel materials, such as bolts, steel bars, and thick plates, which are often used as products and having a yield strength or yield stress or less. For example, in steels for steel bars and bolts, development has been conducted mainly on tempered martensite, and in Non-Patent Document 1 {"New development of delayed fracture elucidation" (Iron and Steel Institute of Japan, issued in January 1997)). It has been reported that an additive element exhibiting temper softening resistance, such as V or V, is effective in improving delayed fracture resistance. This is a technique in which an alloy carbide is precipitated and is used as a hydrogen trap site to shift a delayed fracture mode from a grain boundary to an intragranular fracture. However, since these steels have a C content of 0.4% or more and contain a lot of alloying elements, the workability and weldability required for thin steel sheets are inferior, and the precipitation heat treatment for alloy carbide precipitation is several hours or more. However, there is also a problem in manufacturability.

  Patent Document 1 (Japanese Patent Application Laid-Open No. 11-293383) states that an oxide mainly composed of Ti and Mg is effective in preventing hydrogen-induced defects. However, this is intended for thick steel plates, and in particular, delayed fracture after high heat input welding is considered. However, there is no sheet steel automobile that sufficiently considers the usage environment in parts. On the other hand, with respect to delayed fracture of a thin steel sheet, for example, Non-Patent Document 2 (CAMP-ISIJ, vol. 5, pp. 1839-1842, Yamazaki et al., October 1992, published by the Iron and Steel Institute of Japan) describes a work-induced transformation of the amount of retained austenite. It has been reported that the cause of delayed fracture caused by this is. This takes into account the forming of a thin steel sheet, but describes the regulation of the amount of retained austenite that does not degrade delayed fracture resistance. That is, it relates to a high-strength thin steel sheet having a specific structure, and cannot be said to be a fundamental measure for improving delayed fracture resistance.

  Furthermore, when assembling a member using such a high-strength material, ductility, bendability, hole-expandability, weldability, and the like are more serious problems than a high-strength steel plate having a tensile strength of up to about 590 MPa. Countermeasures are needed. The following measures are taken for each characteristic. For example, regarding the hole expandability, as described in Non-Patent Document 3 (CAMP-ISIJ, vol. 13 (2000) p. 395), the hole expandability is improved by using the main phase as bainite, and the overhanging formability is further improved. It is also disclosed that by forming retained austenite in the second phase, the second phase exhibits the same overhang property as the existing retained austenite steel. Furthermore, it is also shown that when austempering is performed at an Ms temperature or lower to generate retained austenite having an area ratio of 2 to 3%, tensile strength × hole expansion ratio becomes maximum. However, no consideration is given to the weldability that becomes apparent beyond 800 MPa and the softening behavior in the heat affected zone.

  Regarding the weldability, there are many cases where the softening behavior (HAZ softening behavior) in the heat affected zone is regarded as a problem. On the other hand, for example, as described in Patent Document 2 (Japanese Patent Application Laid-Open No. 2000-87175), it is shown that the HAZ softening behavior is suppressed by precipitation of carbides of Nb and Mo (Nb, Mo) C. However, although this technique is considered in terms of fatigue strength, it does not sufficiently consider workability such as hole expandability. Further, the effect of suppressing the HAZ softening behavior also has a low strength level, and it cannot be said that the weldability and workability of a very high-strength material of 800 MPa or more are sufficient. In particular, when the tensile strength is 800 MPa or more, the welding itself becomes difficult, and when the tensile strength is 980 MPa or more, it becomes more remarkable. For this reason, in some cases, laser welding or the like is partially applied in addition to conventional welding methods such as spot welding. However, the high-strength base metal has a remarkable variation in the material quality particularly in the welded portion and the heat-affected zone as compared with the high-strength material of the 590 MPa class. In addition, the use of martensite for increasing the strength promotes hole-expandability and ductility.

  In addition, in order to increase the ductility of a high-strength material, it is common to actively use a composite structure. However, when martensite or retained austenite is used for the second phase, there is a problem that the hole expandability is significantly reduced. For example, Technical Document 4 (CAMP-ISIJ, vol. 13 (2000), p. 391) ). This document discloses that the main phase is ferrite, the second phase is martensite, and the hole expansion ratio is improved by reducing the hardness difference between the two, but the hole expansion ratio is 70%. Less than that is not a significant improvement.

"New developments in elucidating delayed fracture" (Iron and Steel Institute of Japan, published January 1997) CAMP-ISIJ, vol. 5 1839-1842, Yamazaki et al., October 1992, published by The Iron and Steel Institute of Japan CAMP-ISIJ, vol. 13 (2000) p. 395 CAMP-ISIJ, vol. 13 (2000), p. 391 JP-A-11-293383 JP 2000-87175 A

  As mentioned above, there are few development examples that take measures against hydrogen embrittlement-type delayed fracture in consideration of the working environment of automotive thin steel sheets in particular, and that fully consider the use properties such as weldability and hole expanding properties. . An object of the present invention is to provide a high-strength steel sheet which solves the above-mentioned problems of the prior art and simultaneously improves the weldability and hole expandability of a high-strength steel sheet having a tensile strength of 800 MPa or more, and a method for producing the same. And

  In view of the above background, the inventors have come to find a method of improving delayed fracture resistance while ensuring weldability and workability by sufficiently considering the use environment of a thin steel sheet. That is, it has been found that by controlling the trap site in the steel sheet and reducing the amount of hydrogen that can enter from the environment in addition to controlling the structure and precipitates of the steel sheet, the delayed fracture resistance due to hydrogen can be improved. Details are as follows.

(1) In mass%, C: 0.01% to 0.25%, Si: 0.01 to 3.0%, Mn: 0.01 to 4.0%, P: 0.0001 to 0. 0.05%, S: 0.0001-0.05%, Al: 0.01-3.0%, N: 0.0001-0.01%, Ni: 0.001-5.5%, Cu: 0.001 to 3.0%, Cr: 0.001 to 5.0%, Mo: 0.005 to 5%, Nb: 0.001 to 1.0%, and The balance consists of iron and unavoidable impurities, and the microstructure contains at least 70% of bainite or bainitic ferrite in total in area ratio, less than 3% of retained austenite (Vγ), and a tensile strength ( (TS) is 800 MPa or more and satisfy the following formulas (1-1) to (1-3). High-strength steel sheet having excellent sex and hole expandability.
(3.0Nb + 2.5Mo + 1 / 10Si + Mn) - (2C 0.5 +2)> 0 ... (1-1)
0 ≦ 0.8 × {2Cu + 20Mo + 3Ni + Cr + Vγ} − {0.1−3.5 × 10 ⁷ × (TS) ^−3.1 } −0.3Vγ (1-2)
0> Si + Al + 7.67C-1.78 (1-3)
Here, TS: tensile strength (MPa)
Element symbol is mass% of each element contained in steel

(2) In mass%, C: 0.01% to 0.25%, Si: 0.01 to 3.0%, Mn: 0.01 to 4.0%, P: 0.0001 to 0. 020%, S: 0.0001 to 0.020%, Al: 0.01 to 3.0%, N: 0.0001 to 0.01%, Ni: 0.001 to 5.5% , Cu: 0.001 to 3.0%, Cr: 0.001 to 5.0%, Mo: 0.005 to 5%, and Nb: 0.001 to 1.0%. And V: 0.005 to 1%, the balance being iron and unavoidable impurities, and the microstructure remaining at least 70% of bainite and / or bainitic ferrite in area ratio in total. It contains less than 3% austenite (Vγ), has a tensile strength (TS) of 800 MPa or more, and further has the following (2-1) to (2-3) Hydrogen embrittlement resistance characterized by satisfying the equation, weldability and hole expandability excellent high-strength steel sheet.
(3.0Nb + 2.5Mo + 1 / 10Si + Mn) - (2C 0.5 +2)> 0 ... (2-1)
0 ≦ 0.8 × {2Cu + 20Mo + 3Ni + Cr + 20V} − {0.1−V / 5−3.5 × 10 ⁷ × (TS) ^-3.1 } −0.3Vγ (2-2)
0> Si + Al + 7.67C-1.78 (2-3)
Here, TS: tensile strength (MPa),
Element symbols indicate mass% of each element contained in the steel.

(3) Further, in mass%, Se: 0.0002 to 0.05%, As: 0.0002 to 0.05%, Sb: 0.0002 to 0.05%, Sn: 0.0002 to 0%. 0.05%, Pb: 0.0002-0.05%, Bi: 0.0002-0.05%, and the total thereof satisfies 0.05% or less. A high-strength thin steel sheet excellent in hydrogen embrittlement resistance, weldability and hole expandability according to the above (1) or (4), characterized in that:
(4) Further, in mass%, REM: 0.0002 to 0.10%, Ca: 0.0002 to 0.10%, Y: 0.0002 to 0.10%, Mg: 0.0002 to 0%. High strength thinness excellent in hydrogen embrittlement resistance, weldability, and hole expandability according to any one of the above (1) to (3), characterized by containing one or more of 10%. steel sheet.

(5) Further, in mass%, one of Ti: 0.002 to 1%, Zr: 0.005 to 1%, Hf: 0.005 to 1%, Ta: 0.005 to 1%, or The high-strength steel sheet excellent in hydrogen embrittlement resistance, weldability and hole expandability according to any one of the above (1) to (4), comprising two or more kinds.
(6) The above-mentioned (1) to (1), further including one or two of W: 0.005 to 5% and Co: 0.005 to 2.0% by mass%. 5) A high-strength steel sheet excellent in hydrogen embrittlement resistance, weldability and hole expandability according to any one of 5).

(7) The hydrogen embrittlement resistance according to any one of the above (1) to (6), further comprising, by mass%, B: 0.0002 to 0.1%. High-strength steel sheet with excellent weldability and hole expandability.
(8) One phase of martensite, bainite, and bainitic ferrite having a microstructure having an area ratio of less than 0.1% of carbon or Vickers hardness of 450 or less is 70% or more in total of two or more phases. High-strength steel sheet excellent in hydrogen embrittlement resistance, weldability and hole expandability according to any one of the above (1) to (7), containing less than 3% of retained austenite (Vγ). .

(9) A slab having the composition described in any of (1) to (7) above is heated to 1100 ° C. or higher, and hot-rolled at a finishing temperature of ₃ points of Ar + 30 ° C. or higher, and 400 to 650 ° C. After cooling at a temperature of 30 ° C./s or more to a temperature range of not less than 30 ° C./s, then pickling, then cold rolling at a rolling reduction of 10 to 80%, and then the maximum temperature during annealing is 0.8 × (Ac ₃ −Ac ₁ ) + Ac ₁ (° C.) or more and after annealing at Ac ₃ +30 (° C.) or less, cooled to a temperature range of 350 to 500 ° C. at a cooling rate of 1 to 150 ° C./sec. A method for producing a high-strength thin steel sheet excellent in hydrogen embrittlement resistance, weldability, hole expandability, and ductility, wherein the steel sheet is held at the same temperature range for 1 second to 3000 seconds.

(10) After annealing, it is cooled to a temperature range of Mf + 10 ° C. or lower at a cooling rate of 1 to 150 ° C./sec, and subsequently kept at a temperature range of Mf to 450 ° C. for 1 second to 3000 seconds. (9) A method for producing a high-strength thin steel sheet excellent in hydrogen embrittlement resistance, weldability, and hole expandability described in (9). Here, Mf (° C.) = 361-474 × C (% by mass) -33 × Mn (% by mass) -17 × Ni (% by mass) -17 × Cr (% by mass) -21 × Mo (% by mass)

  According to the present invention, it is possible to obtain a high-strength high-ductility steel plate having improved weldability and hole expandability of a high-strength steel plate having a tensile strength of 800 MPa or more, and further improved delayed fracture resistance, and a method for producing the same.

  In conventional tempered martensitic steel, which is a high-strength steel material, delayed fracture caused by hydrogen causes voids to form due to the accumulation of hydrogen at the former austenite grain boundary, etc. It is believed to occur. Therefore, if the hydrogen trap sites are evenly and finely dispersed and hydrogen is trapped in that portion, the concentration of diffusible hydrogen decreases, and the sensitivity to delayed fracture decreases. As described in Patent Document 1 mentioned above, it has been found that by controlling the dispersion form of oxides in a thick steel sheet to which Mg and Ti are added in combination, delayed fracture resistance due to hydrogen is improved. However, considering the case where the amount of hydrogen coming from the environment is large even locally, no matter how many hydrogen trap sites are dispersed in the steel material, hydrogen-induced delayed fracture necessarily occurs. Therefore, first, (a) the trap sites are dispersed in the steel material to increase the allowable hydrogen amount of the steel material itself. In addition to suppressing the retained austenite, (b) the amount of hydrogen that can enter from the placed environment It is important to reduce

In view of the above-mentioned background, the present inventors, in the use environment of a thin steel sheet, in order to secure and improve delayed fracture resistance, various crystallized substances, dispersion of trap sites of precipitates and the influence of the strength of the steel sheet. In addition, the reduction of the amount of hydrogen that can enter from the environment was examined. As a result, the inventors have found a technique for improving and securing the delayed fracture resistance due to hydrogen under the use environment of a thin steel sheet (for example, under the application of design stress after press working). That is,
(A) Control of the amount of precipitates and retained austenite by the strength and composition of the steel sheet.
(B) Control of the resistance to intrusion hydrogen by the composition of the steel sheet.
Respectively, it is possible to improve the hydrogen embrittlement resistance under the usage environment of the automotive thin steel sheet. Equations (1-2) and (2-2) are defined as conditions for satisfying this.

By satisfying this expression, the delayed fracture resistance of the high-strength thin steel sheet can be secured.
Next, (b) the control of the hydrogen penetration characteristics by the components of the steel material will be described. In the process of hydrogen intrusion, when a reduction reaction of water molecules (in a neutral or alkaline environment) or hydrogen ions (in an acidic environment) occurs on the steel sheet surface due to corrosion or pickling, hydrogen atoms are generated on the steel sheet surface Adsorb. The adsorbed hydrogen atoms (1) are recombined and gasified as hydrogen molecules, or penetrate into the steel plate. The present inventors have conducted intensive studies on these processes. As a result, in order to reduce the hydrogen penetration rate, in addition to improving the corrosion resistance, (1) reducing the pH (hydrogen ion concentration) of the environment accompanying the progress of the corrosion reaction as much as possible. It has been found that it is effective to suppress the concentration and lower the concentration of adsorbed hydrogen atoms on the surface and (2) accelerate the recombination reaction (hydrogen generation reaction).

Regarding (1), it was found that REM, Ca, and Mg addition to steel was effective. Here, REM is an abbreviation of Rare Earth Metal and is a general term for lanthanoid elements starting with La. As industrial addition, it is often added in the form of misch metal, and in this case, the added amount of La or Ce increases. When REM, Ca, Y, and Mg are eluted in the corrosion reaction, the atmosphere is alkalized by the equilibrium reaction of the hydroxide, that is, a decrease in pH due to the corrosion reaction is suppressed.

Regarding (2), two methods were found. The first method is to increase the exchange current density of the reduction reaction of hydrogen ions or water. Cu, Ni, Cr, and Mo are effective, and when 0.1 ≦ 2Cu + 20Mo + 3Ni + Cr + 20V is satisfied, the hydrogen permeation rate is significantly suppressed. The second method reduces the exchange current density described above. Alternatively, this is a method of restricting an impurity element that significantly increases the hydrogen generation overvoltage. By limiting Se, As, Sb, Pb, and Bi as the corresponding impurity elements, an increase in the hydrogen permeation rate can be suppressed.

  In the use of automotive steel sheets, hydrogen intrusion occurs in the following process. First, processing steps such as press working, second, anticorrosion coating steps such as pickling, degreasing, water washing, and painting, and third, corrosion in the use environment. In any environment, the control of the hydrogen penetration characteristics by the components of the steel material described above is effective. It is necessary to add a large amount of expensive elements in order to improve the bare corrosion resistance of steel sheets for automobiles and suppress hydrogen intrusion. However, in these methods (1) and (2), the addition of a very small amount is significant. There is an advantage that a special effect can be obtained.

  Furthermore, in order to ensure the weldability, hole expansion and ductility, the microstructure and the component range and the limits according to the formulas (1-1) and (2-1) are limited, so that the welding heat can be maintained at a high strength of 800 MPa or more. Suppressing the softening behavior of the affected part and further ensuring a hole expandability of 70% or more with a hole expansion ratio: (inner diameter of hole before hole expansion test / hole diameter before hole expansion test-1) × 100. Was found. In order to ensure sufficient hole expandability, it is effective to use one or both of bainite and bainitic ferrite. The area ratio is set to 70% or more. In order to form such a structure, it is necessary to satisfy the expressions (1-3) and (2-3).

  In addition, the bainite referred to here includes both upper bainite in which carbide is generated at the lath boundary and lower bainite in which fine carbide is generated in the lath. Bainitic ferrite means bainite having no carbide, and for example, acicular ferrite is one example. In order to improve the hole expandability, it is preferable that the lower bainite in which carbides are finely dispersed or the bainitic ferrite without carbides is the main phase, and the area ratio exceeds 97%. On the other hand, prevention of softening in the heat affected zone becomes a problem. In order to cope with this, the weldability of a high-strength material having a tensile strength of 800 MPa or more is satisfied by satisfying formulas (1-1) and (2-1) in which the components are specified as described later.

  Further, if hard martensite coexists, it is difficult to ensure softening resistance during welding, etc., but the amount of carbon in the martensite is less than 0.1% by mass% or Vickers hardness is 450 or less. In such a case, the degree of decrease in weldability is not large (however, from the viewpoint of weldability, lower carbon and lower hardness are desirable). Therefore, low carbon or relatively low hardness martensite functions sufficiently as a main phase. Generally, it is difficult to distinguish this type of low-temperature transformation product. However, they can be distinguished by etching with a repeller solution or collecting expansion / contraction curves. For example, it can be distinguished by the difference of the inflection point observed in the contraction curve at the time of cooling.

  Specifically, Ms (° C.) = 561-474 × C (mass%) − 33 × Mn (mass%) − 17 × Ni (mass%) − 17 × Cr (mass%) − 21 × Mo (mass%) )), The transformation is observed at a temperature lower than or equal to the temperature represented by (B) (C) = 830 to 270 × C (mass%)-90 × Mn (mass%). ) The transformation product observed in the temperature range of -37 x Ni (mass%)-70 x Cr (mass%)-83 x Mo (mass%) or less is bainite. Further, the hardness can be determined by measuring the micro Vickers hardness under a load of 100 g or less. Normally, martensite is composed of very fine structural units, but here it is desirable to determine the hardness of the martensite phase as the main phase and determine the size of the indentation under measurement conditions of about several tens of microns or less. .

From the viewpoint of ensuring ductility and increasing the strength, ferrite having an area ratio of less than 30% may be included. On the other hand, it is not desirable to include austenite and / or hard martensite from the viewpoint of hole expanding workability and softening behavior of the heat affected zone by welding, but when the area ratio is less than about 3%, remarkable property deterioration is recognized. Therefore, the area ratio may be less than 3%. Further, inclusions such as oxides and sulfides may be inevitably contained.
When the formulas (1-1) and (1-2) are not satisfied, the tensile strength cannot be 800 MPa or more, the softening of the weld heat-affected zone cannot be suppressed, and the hole expandability cannot be reduced. It is also difficult to secure.
(3.0Nb + 2.5Mo + 1 / 10Si + Mn) - (2C 0.5 +2)> 0 ... (1-1) (2-1)

  In addition to the above, a case where one or two or more of carbide, nitride, sulfide and oxide are contained at an area ratio of 1% or less as the remaining structure of the microstructure can be used in the present invention. It was included in the area ratio of the phase. The identification of each phase of the microstructure, ferrite (bainitic ferrite), bainite, austenite, martensite, interfacial oxide phase and the remaining structure, observation of the existing position, and measurement of the area ratio were carried out using the Nital reagent and the method disclosed in Corrosion of the cross section in the rolling direction or the direction perpendicular to the rolling direction by the reagent disclosed in JP-A-59-219473, and quantification by 500 to 1000 times optical microscope observation and 1000 to 100,000 times electron microscope (scanning type and transmission type). Is possible. By observing at least 20 visual fields, the area ratio of each tissue can be determined by the point count method or image analysis.

Hereinafter, the present invention will be described in more detail. First, the reasons for limiting the chemical components of steel in the present invention will be described.
C is an element added for the purpose of controlling the ratio of the main phase and the second phase for securing a good strength-hole expanding property balance. It also affects the fine uniformity of the substrate. The lower limit is set to 0.01% in order to secure the strength and the area ratio of each second phase. Conversely, if the content is large, the amount of cementite, which is the starting point of brittle fracture, is increased, which causes hydrogen embrittlement and deterioration of hole expandability. Make it easier. Therefore, the upper limit is set to 0.25%. Further, the ranges of the expressions (1-3) and (2-3) are satisfied from the viewpoint of suppressing the generation of retained austenite and martensite and ensuring excellent hole expandability.

Si is a substitutional solid solution strengthening element that greatly hardens the material, and is effective in increasing the strength of the steel sheet by containing 0.01% or more, and is an element that suppresses cementite precipitation. If it exceeds 0%, the toughness itself of the steel material is adversely affected, so the upper limit is 3.0%.
Mn is an element effective for increasing the strength of the steel sheet. Further, it is effective to suppress the ferrite transformation and make the main phase bainite or bainitic ferrite or both. Furthermore, it is added for the purpose of suppressing carbide precipitation and pearlite generation, which are one of the causes of a decrease in strength and a deterioration in hole expandability. However, if the content is less than 0.01%, this effect cannot be obtained, so the lower limit is set to 0.01%. Conversely, if the content is large, segregation becomes remarkable, and workability may be deteriorated. Therefore, the upper limit is set to 4.0%.

P is an element that promotes grain boundary destruction due to grain boundary segregation, and is preferably as low as possible. However, since the extreme decrease is not preferable in terms of manufacturing cost, the lower limit is set to 0.0001%. Further, since it is an element that deteriorates corrosion resistance, the upper limit is set to 0.05%.
S is an element that promotes hydrogen absorption in a corrosive environment, and the lower the better, the upper limit is made 0.05%. On the other hand, the extremely lowering is not preferable in terms of manufacturing cost, so the lower limit was made 0.0001%.

Al is added in an amount of 0.01% or more for deoxidation. Increasing the amount of Al increases inclusions such as alumina and deteriorates workability, so the upper limit is 3.0%.
N should be small as it contributes to deterioration of workability and generation of blowholes during welding. If it exceeds 0.01%, workability deteriorates, so the upper limit is made 0.01%. In addition, since the extreme decrease is economically disadvantageous, the lower limit is set to 0.0001%.
Ni has the effect of suppressing hydrogen intrusion and improving delayed fracture characteristics, and the effect of securing the strength of the steel sheet by increasing the hardenability of the steel sheet. However, if the content is less than 0.001%, these effects cannot be obtained, so the lower limit is set to 0.001%. Conversely, if the content exceeds 5.5%, the workability deteriorates, so the upper limit is set to 5.5%.

Cu is effective in strengthening delayed fracture characteristics by suppressing hydrogen penetration and strengthening, and is effective for strengthening. In addition, since fine precipitation of self contributes to improvement in delayed fracture, Cu is added in an amount of 0.001% or more. Moreover, since excessive addition causes deterioration of workability, the upper limit is set to 3.0%.
Cr is an element that suppresses hydrogen intrusion and improves delayed fracture characteristics, and is effective for increasing the strength of steel sheets. However, if the content is less than 0.001%, these effects cannot be obtained, so the lower limit is set to 0.001%. Conversely, if the content exceeds 5%, the workability decreases, so the upper limit is set to 5%.

  Mo is an element that suppresses hydrogen intrusion and improves delayed fracture characteristics, and is an effective element for improving the hardenability of steel sheets and obtaining martensite stably with continuous annealing equipment, as well as strengthening grain boundaries. This has the effect of suppressing the occurrence of hydrogen embrittlement. In addition, the generation of carbides and pearlite that degrade the strength-hole expandability balance is suppressed. Furthermore, it is effective for suppressing the ferrite transformation and making the main phase bainite or bainitic ferrite, and is an important additive element for obtaining a very good balance of good strength-hole expanding property-weldability. It is. However, if the content is less than 0.005%, these effects cannot be obtained, so the lower limit is set to 0.005%. If the content exceeds 5%, these effects are saturated, so the upper limit is set to 5%.

  Nb is an element effective for increasing the strength and reducing the grain size of the steel sheet. By forming fine carbides, nitrides or carbonitrides, it is extremely effective in strengthening steel sheets. It also slows down ferrite transformation and promotes the formation of bainite and bainitic ferrite. Furthermore, it is also effective in suppressing softening of the heat affected zone. However, if the content is less than 0.001%, these effects cannot be obtained, so the lower limit is set to 0.001%. Conversely, if the content exceeds 1%, the precipitation of carbonitrides increases and the workability and the delayed fracture resistance decrease, so the upper limit was made 1%.

  V can be used as a trap site for hydrogen by controlling the carbonitride shape, in addition to the effect of suppressing hydrogen intrusion and improving delayed fracture characteristics, increasing the strength and reducing the grain size of the steel sheet. It is an important additive element for improving hydrogen embrittlement. However, since the effect cannot be obtained with less than 0.005%, the lower limit is set to 0.005%. Conversely, when the content exceeds 1%, precipitation of carbonitride becomes remarkable, and ductility decreases remarkably. Therefore, the upper limit is set to 1%.

If Se, As, Sb, Sn, Pb, and Bi are contained alone in an amount exceeding 0.05% or in a total amount exceeding 0.05%, the delayed fracture resistance is significantly impaired. The upper limit was set to 0.05%, and the upper limit was set to 0.05% for the total of the elements. On the other hand, in order to reduce restrictions on recycling, the lower limit was set to 0.0002% for each.
REM, Ca, and Mg are effective elements for suppressing an increase in the hydrogen ion concentration in the interface atmosphere due to corrosion of the steel sheet surface, that is, for suppressing a decrease in pH. However, these effects cannot be obtained if each is less than 0.0002%, so the lower limit was made 0.0002%. Conversely, if the content exceeds 0.1%, the workability deteriorates, so the upper limit was made 0.1%.

Y is effective for controlling the form of inclusions and contributes to delayed fracture resistance. Therefore, Y was added in an amount of 0.0002% or more. On the other hand, excessive addition deteriorates hot workability, so the addition was made 0.1% or less.
Ti is an element necessary to generate precipitates and inclusions. However, if the content is less than 0.002%, the precipitate cannot be used, so the lower limit is set to 0.002%. Conversely, if it exceeds 1%, coarse precipitates or exudates are formed, so that workability and delayed fracture resistance are reduced. Therefore, the upper limit is set to 1%.

Zr is an element effective for increasing the strength and reducing the grain size of the steel sheet. However, if the content is less than 0.005%, these effects cannot be obtained, so the lower limit is set to 0.005%. Conversely, if the content exceeds 1%, the precipitation of carbonitrides increases and the workability and the delayed fracture resistance decrease, so the upper limit was made 1%.
Hf is an element effective for increasing the strength and reducing the grain size of the steel sheet. However, if the content is less than 0.005%, these effects cannot be obtained, so the lower limit is set to 0.005%. Conversely, if the content exceeds 1%, the precipitation of carbonitrides increases and the workability and the delayed fracture resistance decrease, so the upper limit was made 1%.

Ta is an element effective for increasing the strength and reducing the grain size of the steel sheet. However, if the content is less than 0.005%, these effects cannot be obtained, so the lower limit is set to 0.005%. Conversely, if the content exceeds 1%, the precipitation of carbonitrides increases and the workability and the delayed fracture resistance decrease, so the upper limit was made 1%.
W is an element effective for increasing the strength of the steel sheet. However, if the content is less than 0.005%, these effects cannot be obtained, so the lower limit is set to 0.005%. Conversely, if the content exceeds 5%, the workability decreases, so the upper limit is set to 5%.

Since Co is effective for strengthening, 0.005% or more is added. Moreover, since excessive addition causes deterioration of workability, the upper limit is set to 2.0%.
B is an element effective for increasing the strength of the steel sheet. However, if these effects are not obtained at less than 0.0002%, the lower limit is set to 0.0002%. Conversely, if the content exceeds 0.1%, the hot workability deteriorates, so the upper limit was set to 0.1%.

Next, a manufacturing method will be described. In particular, in order to ensure the surface condition of the product plate, it is desirable to adopt the following manufacturing method from the viewpoint of sufficiently forming oxide scale and deskewing in the manufacturing process.
First, the heating temperature at the time of hot rolling is set to 1100 ° C. or higher from the viewpoint of deformation resistance. If the temperature is too high, there are problems such as grain coarsening and increase in scale formation. In hot rolling, hot rolling is performed at Ar ₃ + 30 ° C. or higher to prevent excessive reduction of workability due to excessive strain applied to ferrite grains, and even if the temperature is too high, the recrystallized grain size after annealing becomes unnecessarily large. The finishing temperature is desirably 940 ° C. or less for coarsening.

  From the viewpoint of preventing scale formation as much as possible after finishing, the cooling rate is set to 30 ° C./s, and the winding temperature is set to a high temperature to promote recrystallization and grain growth, and it is desired to improve workability. Since the formation of scale generated during hot rolling is promoted and the pickling property is reduced, the temperature is set to 650 ° C. or lower. On the other hand, if the temperature is too low, the material is hardened, so that the load during cold rolling increases. Therefore, the temperature is set to 400 ° C. or higher. Here, a winding process at 550 to 650 ° C. is desirable in order to positively precipitate fine precipitates as trap sites during winding.

In cold rolling after pickling, if the rolling reduction is low, the shape correction of the steel sheet is difficult, so the lower limit is set to 10%. Further, when rolling is performed at a rolling reduction of more than 80%, the upper limit is set to 80% due to the occurrence of cracks at the edges of the steel sheet and the disorder of the shape.
When performing annealing after cold rolling, the annealing temperature is expressed by the temperatures Ac ₁ and Ac ₃ determined by the chemical composition of the steel (for example, “Steel and Materials Science”: WC Leslie, translated by Shigeyasu Koda, Maruzen P273). If less than 0.8 × (Ac ₃ −Ac ₁ ) + Ac ₁ (° C.), the amount of austenite obtained at the annealing temperature is small, so that one of bainite and bainitic ferrite is mainly contained in the final steel sheet. Or neither can be generated. Further, as the annealing temperature increases, the coarsening of crystal grains and the surface oxidation are promoted, and the upper limit of the annealing temperature is set to Ac ₃ +30 (° C.) in order to increase the manufacturing cost. The annealing time in this temperature range is preferably 10 seconds or more for uniformizing the temperature of the steel sheet and securing austenite. However, if the time exceeds 30 minutes, the formation of the grain boundary oxidized phase is promoted and the cost is increased.

  Subsequent primary cooling is important for forming one or both of bainite and bainitic ferrite while suppressing the transformation from the austenite phase to the ferrite phase to some extent. Setting the cooling rate to less than 1 ° C./sec promotes the formation of ferrite and pearlite, which may cause a decrease in strength. Therefore, the lower limit of the cooling rate is set to 1 ° C./sec. On the other hand, when the cooling rate is more than 150 ° C./sec, the hard steel phase such as the martensite phase in the final steel sheet becomes large and the operation is difficult, so the upper limit is set.

  When the primary cooling is performed to less than 350 ° C., a large amount of martensite is generated during the cooling, which promotes hole expandability and delayed fracture. Therefore, the cooling stop temperature was set to 350 to 500 ° C. Further, if the cooling stop temperature exceeds 500 ° C., carbides are generated in a short time at the time of subsequent holding, which causes a decrease in strength. Next, in order to promote the progress of bainite transformation, the temperature is maintained in this temperature range. If the dwell time is long, it is not preferable in terms of productivity, and carbide is generated. In order to advance the bainite transformation, it is desirable to hold for 1 second or more, preferably for 15 seconds to 20 minutes. If the temperature is lower than 200 ° C., bainite transformation hardly occurs. If the temperature exceeds 500 ° C., carbides are generated, and it becomes difficult to leave a sufficient residual austenite phase.

  When a relatively low carbon or low hardness martensite phase is used as a main phase or is used as a part of the main phase, it is not particularly necessary to promote bainite transformation. Therefore, the upper limit of the primary cooling stop temperature is set to Mf + 10 ° C., and a low carbon or low hardness martensite phase is secured. Thereafter, for adjusting the hardness, holding at a temperature of not less than Mf and not more than 450 ° C. can be performed. When the holding temperature exceeds 450 ° C., precipitation of carbides and the like occurs, which leads to deterioration of hole expandability and a decrease in strength. Here, the Mf temperature is Mf (° C.) = 361-474 × C (mass%) − 33 × Mn (mass%) − 17 × Ni (mass%) − 17 × Cr (mass%) − 21 × Mo ( mass%). In addition, the present invention is also within the scope of the present invention, even if welding methods such as arc, TIG, MIG, mash, and laser are performed.

Hereinafter, the present invention will be described in more detail with reference to examples.
A steel sheet having a composition as shown in Table 1 was heated to 1150 to 1250 ° C., hot rolling was completed at an Ar ₃ + 30 ° C. transformation temperature or higher, and a steel strip wound up after cooling was pickled and then cold rolled to 1 .2 mm thick. After the temperature is raised and maintained in an atmosphere of 10% H ₂ —N ₂ at an annealing temperature calculated from the Ac ₁ and Ac ₃ transformation temperatures, it is cooled to 200 to 450 ° C. at a cooling rate of 3 to 150 ° C./sec. Then, after holding for 1 to 3000 seconds, it was cooled. Steel type No. After annealing, the samples Nos. 18 to 21 were cooled to a temperature range of Mf + 10 ° C. or lower at a cooling rate of 1 to 150 ° C./sec, and subsequently kept at a temperature range of Mf to 450 ° C. for 1 second to 3000 seconds. JIS No. 5 tensile test pieces were collected from these steel sheets, and their mechanical properties were measured. Further, a hole expansion test was performed in accordance with the standards of the Iron and Steel Federation to determine the hole expansion ratio. Regarding the weldability, various weldings were performed with the steel plates joined together, a ball head overhang test was performed using Teflon (registered trademark) lubrication, and the overhang height and the fracture position with respect to the base metal were measured.

The details of the method for evaluating the delayed fracture resistance of a steel sheet are as follows.
(1) After temper rolling, a 2% strain is applied to the steel sheet for the purpose of simulating the strain during pressing.
(2) A notched plate-shaped tensile test piece having a stress concentration rate of 3.2 is sampled from a steel sheet.
(3) A constant current cathode charge is performed at 0.01 to 0.025 mA / cm ² in a 3% NaCl-3 g / 1 NH ₄ SCN aqueous solution.
(4) Cd plating is performed.
(5) Apply a constant load of 0.8 times the tensile strength.
(6) The test is performed up to 100 hours, and it is determined whether the sample is broken or not broken.

Table 2 shows the microstructure and each material of each steel, and Tables 3 and 4 show each manufacturing condition and material. It is understood that the inventive steel satisfying the requirements of the present invention is excellent in weldability, strength (800 MPa or more in tensile strength), and hole expandability. In particular, the hole expanding property shows an excellent value of 80%. On the other hand, in the comparative examples that deviate from the conditions of the present invention, any of the overhang height, the tensile strength, the hole expandability, and the delayed fracture evaluation of the welded portion are inferior.
In addition, a steel type No. having a low carbon / low hardness martensite phase as a main phase. It can be seen that invention steels satisfying the requirements of the present invention of Nos. 18 to 21 have excellent weldability, strength (800 MPa or more in tensile strength), and hole expandability. On the other hand, steel types No. 6-1 and 17-2, which do not satisfy the requirements even when the main phase is martensite, have high strength, but are inferior in all of delayed fracture, weldability and hole expandability. .

Patent applicant Nippon Steel Corporation
Attorney Patent Attorney Shiina Jin and others 1

Claims (10)

In mass%,
C: 0.01-0.25%,
Si: 0.01 to 3.0%,
Mn: 0.01 to 4.0%,
P: 0.0001-0.05%,
S: 0.0001-0.05%,
Al: 0.01 to 3.0%,
N: 0.0001 to 0.01%,
Containing
Ni: 0.001 to 5.5%,
Cu: 0.001 to 3.0%,
Cr: 0.001 to 5.0%,
Mo: 0.005 to 5%,
Nb: 0.001 to 1.0%
And the balance consists of iron and unavoidable impurities, and the microstructure is bainite and / or bainitic ferrite in an area ratio of 70% or more in total and 3% of retained austenite (Vγ). %, And has a tensile strength (TS) of 800 MPa or more, and further satisfies the following formulas (1-1) to (1-3). Excellent high strength steel sheet.
(3.0Nb + 2.5Mo + 1 / 10Si + Mn) - (2C 0.5 +2)> 0 ... (1-1)
0 ≦ 0.8 × {2Cu + 20Mo + 3Ni + Cr + Vγ} − {0.1−3.5 × 10 ⁷ × (TS) ^−3.1 } −0.3Vγ (1-2)
0> Si + Al + 7.67C-1.78 (1-3)
Here, TS: tensile strength (MPa),
Element symbols indicate mass% of each element contained in the steel. In mass%,
C: 0.01% to 0.25%,
Si: 0.01 to 3.0%,
Mn: 0.01 to 4.0%,
P: 0.0001 to 0.020%,
S: 0.0001 to 0.020%,
Al: 0.01 to 3.0%,
N: 0.0001 to 0.01%
Containing
Ni: 0.001 to 5.5%,
Cu: 0.001 to 3.0%,
Cr: 0.001 to 5.0%,
Mo: 0.005 to 5%,
Nb: 0.001 to 1.0%,
And V contains 0.005 to 1%, and the balance consists of iron and unavoidable impurities, and the microstructure is bainite or bainitic ferrite in terms of area ratio. Contains 70% or more in total, less than 3% of retained austenite (Vγ), has a tensile strength (TS) of 800 MPa or more, and further satisfies the following formulas (2-1) to (2-3). High-strength steel sheet with excellent hydrogen embrittlement resistance, weldability, and hole expandability.
(3.0Nb + 2.5Mo + 1 / 10Si + Mn) - (2C 0.5 +2)> 0 ... (2-1)
0 ≦ 0.8 × {2Cu + 20Mo + 3Ni + Cr + 20V} − {0.1−V / 5−3.5 × 10 ⁷ × (TS) ^−3.1 } −0.3Vγ (2-2)
0> Si + Al + 7.67C-1.78 (2-3)
Here, TS: tensile strength (MPa),
Element symbols indicate mass% of each element contained in the steel. Furthermore, in mass%,
Se: 0.0002-0.05%,
As: 0.0002-0.05%,
Sb: 0.0002-0.05%,
Sn: 0.0002-0.05%,
Pb: 0.0002-0.05%,
Bi: 0.0002-0.05%,
3. Hydrogen embrittlement resistance, weldability and hole expandability according to claim 1 or 2, wherein one or more of the following are contained, and the total of them satisfies 0.05% or less. High strength steel sheet. Furthermore, in mass%,
REM: 0.0002-0.10%,
Ca: 0.0002 to 0.10%,
Y: 0.0002 to 0.10%,
Mg: 0.0002 to 0.10%
The high-strength steel sheet excellent in hydrogen embrittlement resistance, weldability and hole expandability according to any one of claims 1 to 3, which comprises one or more of the following. Further, in mass% Ti: 0.002 to 1%,
Zr: 0.005 to 1%,
Hf: 0.005 to 1%,
Ta: 0.005 to 1%,
The high-strength thin steel sheet excellent in hydrogen embrittlement resistance, weldability and hole expandability according to any one of claims 1 to 4, comprising one or more of the following. Further, in mass% W: 0.005 to 5%,
Co: 0.005 to 2.0%,
The high-strength thin steel sheet excellent in hydrogen embrittlement resistance, weldability and hole expandability according to any one of claims 1 to 5, comprising one or two of the following. Furthermore, in mass%,
B: 0.0002 to 0.1%,
The high-strength thin steel sheet excellent in hydrogen embrittlement resistance, weldability and hole expandability according to any one of claims 1 to 6, characterized by comprising: One phase of martensite, bainite, and bainitic ferrite having a microstructure having an area ratio of less than 0.1% of carbon or a Vickers hardness of 450 or less is 70% or more in total of two or more phases, and retained austenite. The high-strength steel sheet excellent in hydrogen embrittlement resistance, weldability and hole expandability according to any one of claims 1 to 7, wherein the steel sheet contains (Vγ) less than 3%. A slab made of the composition according to any one of claims 1 to 7 is heated to 1100 ° C or higher, hot-rolled at a finishing temperature of Ar ₃ points + 30 ° C or higher, and heated to a temperature range of 400 to 650 ° C at 30 ° C. / S or more, coiling at the same temperature range, then pickling, cold rolling at a rolling reduction of 10 to 80%, and then the maximum temperature during annealing is 0.8 × (Ac ₃ −Ac ₁ ) After annealing at + Ac ₁ (° C.) or more and Ac ₃ +30 (° C.) or less, the sample is cooled to a temperature range of 350 to 500 ° C. at a cooling rate of 1 to 150 ° C./sec, and subsequently for 1 second at the same temperature range. A method for producing a high-strength thin steel sheet excellent in hydrogen embrittlement resistance, weldability, and hole expandability, which is held for up to 3000 seconds. 10. Annealing, cooling at a cooling rate of 1 to 150 [deg.] C./sec to a temperature range of Mf + 10 [deg.] C. or less, and subsequently maintaining the temperature range of Mf to 450 [deg.] C. for 1 to 3000 seconds. For producing high-strength thin steel sheets with excellent hydrogen embrittlement resistance, weldability and hole expandability.
Here, Mf (° C.) = 361-474 × C (% by mass) -33 × Mn (% by mass) -17 × Ni (% by mass) -17 × Cr (% by mass) -21 × Mo (% by mass)