JP4646134B2 - Evaluation method of delayed fracture resistance of high strength steel sheet - Google Patents

Description

  The present invention relates to a method for evaluating delayed fracture resistance of a high-strength steel sheet, and relates to a method for evaluating delayed fracture resistance of a high-strength steel sheet having a tensile strength of 1180 MPa or more, which is used as a press part of an automobile.

  The present invention is a TRIP steel sheet applied to a pressed part that has been processed as a high-strength steel sheet, in particular, into a press part that has been processed in an unprecedented region of high elongation, and that has not been used from the processing limit so far. It is preferably applied to the evaluation of delayed fracture resistance of a transformation-induced plastic steel sheet comprising a mixed structure of ferrite, bainite and retained austenite.

  Demand for high-strength steel sheets is increasing more and more with the main background of ensuring safety in the event of a collision while reducing the fuel consumption associated with the reduction in weight of automobile steel sheets. The demand is further increasing.

  However, even for a high-strength steel sheet, there is a strong demand for formability, and it is required to have an appropriate formability according to each application. Especially for automotive panels and frames that are subjected to press molding with complex shapes, high strength that combines both stretch formability (ductility = elongation) and stretch flangeability [hole expandability (local ductility)]. There is a strong demand for steel sheets.

  As one of high-strength and high-ductility steel sheets developed for the purpose of reducing the impact safety and weight of automobiles while having the required properties that combine such excellent strength and ductility, TRIP steel sheets (Transformation Induced Plasticity; Transformation induced plastic steel sheet). This TRIP steel sheet is composed of a mixed structure of ferrite, bainite, and retained austenite in which retained austenite (γR) is generated in the structure. When the steel is deformed at a temperature equal to or higher than the martensite transformation start temperature (Ms point), the retained austenite (γR) is induced and transformed into martensite by stress, thereby obtaining a large elongation.

  For example, TRIP type composite structure steel (TPF steel) containing polygonal ferrite as a parent phase and containing retained austenite, TRIP type tempered martensite steel (TAM steel) containing tempered martensite as a parent phase and containing retained austenite, bay TRIP bainite steel (TBF steel) containing nitrite ferrite as a parent phase and containing retained austenite is known.

  On the other hand, there is a concern about delayed fracture due to hydrogen embrittlement as the strength of steel sheets increases. On the other hand, various hydrogen embrittlement evaluation methods exist in non-patent literature 1 and patent literature 1 in the field of members such as bolts, PC steel wires, and line pipes that have been improved in strength. .

  For example, in Patent Document 1, tension is applied to a tensile test piece, and the tensile test piece is arranged with a counter electrode in an aqueous solution containing an electrolyte, a negative potential is applied, and the tensile test is performed by hydrogen generated by electrolysis of the aqueous solution. The time until a piece becomes hydrogen embrittled, and the delayed fracture resistance of a steel material is evaluated.

  However, as described in Patent Document 2, the hydrogen embrittlement evaluation method in the field of thin steel sheets is different from the hydrogen embrittlement evaluation method for members such as bolts, PC steel wires, and line pipes. That is, since a member such as a bolt is used as it is, it is not necessary to consider the influence of the processing degree, the residual stress, the influence of the end face state, etc. in the evaluation. On the other hand, thin steel plates are used as members after processing such as pressing. Therefore, in the hydrogen embrittlement evaluation method for a thin steel sheet, an accurate evaluation is not possible unless an evaluation is performed in consideration of the influence of the degree of work and the influence of residual stress.

  Furthermore, since there is stress due to welding or assembly during use, or stress that is actually taken as a member during use, it is impossible to uniquely determine additional stress during use with thin steel sheets. It must be an evaluation method that can be changed freely.

  For this reason, various hydrogen embrittlement evaluation methods have been proposed in the field of thin steel sheets, though not yet established. For example, Patent Document 2 proposes an apparatus and method that can legitimately evaluate hydrogen embrittlement characteristics due to applied stress, residual stress, end face effect, bending R, and the like in consideration of the use environment in thin steel sheets. Specifically, in Patent Document 2, a thin steel plate test piece is previously bent into a U shape, stressed with a bolt, and subjected to electrolytic charging in an acidic solution, thereby evaluating hydrogen embrittlement resistance of a high strength steel plate. Is doing.

In Patent Document 3, as a method for evaluating delayed fracture characteristics of a high-tensile steel plate, a strip-shaped test piece having holes in the vicinity of both ends in the longitudinal direction is bent into a U shape, and a strain gauge is formed on the surface of the processed portion after this bending process. Then, a bolt is passed through the hole, and while observing the strain with the strain gauge, a desired stress is applied by tightening the bolt, and then a negative voltage is applied to the test piece in dilute sulfuric acid. It has been proposed to measure the time until a change appears in the value of the strain gauge.
"Delayed destruction" (Nippon Kogyo Shimbun, issued August 31, 1989) JP 2004-309197 A (full text) Japanese Patent Laying-Open No. 2005-134152 (full text) Japanese Patent Laid-Open No. 7-146225 (full text)

  These techniques can evaluate the delayed fracture characteristics of normal high-strength steel sheets and high-tensile steel sheets. However, it cannot be said to be sufficient as a method for evaluating delayed fracture resistance (hydrogen embrittlement characteristics) of high-strength steel sheets such as the TRIP steel sheets characterized by transformation-induced plasticity with a tensile strength of 1180 MPa or more.

  The TRIP steel sheet is excellent in formability such as stretch flangeability (hole expanding property) while maintaining an excellent balance of strength and ductility by γR. For this reason, compared to ordinary high-strength steel sheets and high-tensile steel sheets, it is processed into a press part that has been processed in a region of high elongation that has never been seen before. Applied.

  Therefore, as described above, the difference between the hydrogen embrittlement evaluation method in the members such as bolts and the hydrogen embrittlement evaluation method in the field of thin steel plates, the TRIP steel plates are the above-described normal high-strength steel plates and high-tensile steel plates. In the hydrogen embrittlement evaluation method, accurate evaluation cannot be performed. In other words, in the hydrogen embrittlement evaluation method for TRIP steel sheets, the evaluation is not accurate unless evaluation is performed in consideration of the effects of the degree of processing of the TRIP steel sheets and the effects of residual stress.

  The present invention has been made in view of such circumstances, and the object thereof is processed into a press part by processing an unprecedented high elongation region such as a TRIP steel sheet, and has been processed so far. Therefore, an object of the present invention is to provide a delayed fracture resistance evaluation method (hydrogen embrittlement evaluation method) for high-strength steel sheets, which is applied to pressed parts that have not been used.

  The gist of the present invention for achieving the above object is a method for evaluating delayed fracture resistance of a high-strength steel sheet having a tensile strength of 1180 MPa or more, and the elongation of the high-strength steel sheet relative to a test piece of the high-strength steel sheet. After applying a tensile process with a plastic strain of 20 to 80% of the amount, a U-bending process in which the radius of the bent part is 5 to 30 mm, or an angle of the bent part is 30 to 90 degrees One of the V-bending processes is added, and further, a compressive stress of 500 to 2000 MPa is applied to both sides of the test piece to which the bending process is applied. This is to evaluate the delayed fracture resistance of the high-strength steel sheet by the time until the cathode test piece is cracked by energizing the current and charging the hydrogen.

  The present invention as described above is a high-strength steel plate having a tensile strength of 1180 MPa or more, particularly a TRIP steel plate with excellent workability (the transformation-induced plastic steel plate, wherein the high-strength steel plate is composed of a mixed structure of ferrite, bainite, and retained austenite. ) Is preferred.

In the evaluation method of the present invention, it is preferable that the time until a crack occurs in the cathode test piece satisfies the range of 500 to 10,000 in the following delayed fracture parameter formula. Delayed fracture parameter formula = [(tensile strength of the high-strength steel plate: MPa) / 1180] × (additional stress due to tightening) × [100 / (100−plastic strain:%)] × [(plate of the high-strength steel plate) (Thickness: mm) / (radius of the bent portion of the cathode test piece: mm) or (angle of the bent portion of the cathode test piece: degree) ^1/2 ] × [1 / (until the cathode test piece is cracked) Time: hr)] × 100

  In this invention evaluation method, after adding the said specific tension process with respect to the test piece of the high strength steel plate made into evaluation object, a specific bending process is further added previously. Due to this, in order to have excellent strength / ductility balance and stretch flangeability, such as TRIP steel sheet, it is processed into a press part that has never been used in the region of high elongation, and has not been used from the processing limit until now. It is possible to evaluate delayed fracture resistance of high-strength steel sheets applied to pressed parts, taking into account the effects of these press working degrees and the effects of residual stress.

  Therefore, the precise durability of high-strength steel sheets, such as TRIP steel sheets, applied to pressed parts that have been processed in a region of high elongation that has never been seen before, and that have been used so far due to processing limitations. A delayed fracture property evaluation method (hydrogen embrittlement evaluation method) can be provided.

The outline of the method for evaluating delayed fracture resistance of the present invention is as follows.
In the evaluation method of the present invention, as described above, in order to take into account the influence of the press working degree of the high-strength steel sheet and the residual stress on the test piece of the high-strength steel sheet to be evaluated, In addition, a specific bending process is added in advance.

(Tensile processing)
First, with respect to the test piece of the high-strength steel plate to be evaluated, a tensile process with a plastic strain of 20 to 80% is applied to the amount of elongation of the high-strength steel plate. The plastic strain of 20 to 80% with respect to the amount of elongation of the high-strength steel plate corresponds to the degree of processing in the actual drawing or overhang press forming of the high-strength steel plate.

  If the plastic strain at the time of tensile processing is less than 20% of the elongation of the high-strength steel sheet, it will be processed when it is processed into a press part by being processed in the above-described region of high elongation, or until now. When processed into a pressed part that has not been used from the limit, it is too small compared to the plastic strain applied to the high-strength steel sheet. For this reason, the evaluation conditions for delayed fracture resistance cannot correspond to the degree of processing in these press forming and overhang press forming, so the reliability and reproducibility of the delayed fracture resistance evaluation method, or the actual delayed fracture resistance The correlation with gender decreases.

  On the other hand, the plastic strain in the press forming of these drawing and overhanging does not increase beyond 80% with respect to the amount of elongation of the high-strength steel sheet. Therefore, it is not necessary to increase the plastic strain in the tensile processing of the test piece beyond 80% with respect to the elongation amount of the high-strength steel plate.

  FIG. 1 shows the tensile processing of the test piece 1. In FIG. 1, the test piece 1 is subjected to tensile processing F in the longitudinal direction (rolling direction) to give plastic strain. The shape of the test piece 1 is rectangular, and the size is not particularly limited. For example, the length in the longitudinal direction (rolling direction) is 100 mm and the width is 30 mm.

  FIG. 2 shows a mode in which bolt mounting holes 2 and 2 for applying a compressive stress to be described later to the test piece 1 are provided (perforated). The through-holes 2 and 2 are provided in a portion of 9 mm from both ends 1b and 1b of the test piece 1 with a diameter of 12 mm.

(Bending)
Next, after applying the above-mentioned specific tensile process to the test piece, the U-bending process in which the radius of the bent part becomes 5 to 30 mm, or the V-bend in which the angle of the bent part becomes 30 to 90 degrees. Add any of the processing. This bending process is also to cope with a bending process such as an actual hemming process after a press forming of a high strength steel sheet, for example, a drawing or an overhang press forming in an automobile outer panel.

  The aspect of the test piece 1 which carried out the V bending process in Fig.3 (a) and U-bending process in FIG.3 (b) is each shown. In FIG. 3A, θ is the angle of the V-bend portion, and in FIG. 3B, R is the radius of the U-bend portion.

  In this bending process, in the U bending process in which the radius R of the bent part exceeds 30 mm and the V bending process in which the angle θ of the bent part exceeds 90 degrees, the hem process or the like after the press forming of the drawing or the overhang is performed. In bending, it is too small as compared with bending applied to a high-strength steel plate. For this reason, the formed steel sheet is mildly resistant to hydrogen embrittlement, and the evaluation conditions for delayed fracture resistance cannot correspond to these actual press working degrees (bending degree). This reduces the reliability and reproducibility of the evaluation method, or the correlation with the actual delayed fracture resistance.

  On the other hand, the plastic strain of bending processing such as hem processing after the press forming of squeezing or overhanging has proven that U-bending processing where the radius of the bending portion is less than 5 mm or the angle of the bending portion is less than 30 degrees. There is no V bending. Therefore, it is not necessary to set the radius R of the U-bend portion to less than 5 mm and the angle θ of the V-bend portion to less than 30 degrees in each of the bending processes of the test piece.

(Compressive stress added)
Further, a compressive stress of 500 to 2000 MPa is applied (same meaning as a load) to the two side portions opposite to the bent portion of the test piece subjected to the tensile processing and bending processing. As a result, the influence of residual stress (such as the effect of spring back after bending) due to these tensile and bending processes can be taken into account. Moreover, the influence of the additional stress in the use state as a press-molded product can be taken into account (additional load, bolt tightening force, etc.).

  FIG. 4 (a) shows a state in which a bolt for applying compressive stress is attached to the test piece 1 subjected to U bending and FIG. 3 (b) is subjected to V bending. 4 (a) and 4 (b), reference numeral 3 denotes bolts 3 and 3 that are attached by penetrating through holes 2 and 2 provided in the sides 1a and 1a of the test piece 1. FIG.

  Here, the fastening force (compressive stress) with respect to each side 1a, 1a of the test piece 1 of the bolts 3 and 3 is set to a range of 500 to 2000 MPa. If the tightening force is less than 500 MPa, the influence of residual stress due to these tensile processing and bending processing cannot be taken into consideration. Moreover, the influence of the additional stress in the use condition as a press-molded product cannot be considered. On the other hand, since these residual stresses and additional stresses do not actually exceed 2000 MPa, it is not necessary to increase the sound tightening force for each side 1a, 1a of the test piece 1 of the bolts 3, 3 beyond 2000 MPa. .

(Test specimen immersion)
In this state, as shown in FIG. 5, the U-bending or V-bending test piece 1 is immersed in the electrolytic solution 19 in the test tank 13 as the cathode 11. The cathode 11 is connected to a lead wire 10 that is stretched over a support rod 17, is sandwiched and supported by a clip 18 that can apply a current to the cathode 11, and is immersed in an electrolytic solution 19. The cathode 11 and the anode 12 are charged with hydrogen by supplying a constant current from a power source (current control device) 14, and the delayed fracture resistance of the high-strength steel plate is the time until the cathode test piece 11 is cracked. To evaluate. The occurrence of cracks in the cathode test piece 11 may be detected by attaching a strain gauge (break detection device) 16 to the bent portion of the cathode test piece 11 and detecting the occurrence of cracks as a change in the amount of strain. 15 is a recorder of the strain gauge 16 and 10 is each lead wire.

  The power source 16 can use a constant current generator such as a potentiostat. The anode is platinum. The reason why the anode is preferably platinum is that the anode is made of platinum, so that corrosion from the electrolyte is suppressed and hydrogen is preferentially charged to the test piece.

(Current density)
The current density of the constant current charged to the cathode is preferably 0.1 to 0.01 mA / mm ² . When the current density exceeds 0.1 mA / mm ² , a large amount of hydrogen is occluded into the steel at one time. It becomes difficult. On the other hand, when the current density is less than 0.01 mA / mm ² , since much hydrogen is occluded in the steel, much time is required for evaluation.

(Electrolytic solution)
The electrolytic solution 13 is preferably an aqueous solution having a pH of 6 or less in order to effectively occlude hydrogen into the steel by applying a constant current. If the pH exceeds 6, hydrogen cannot efficiently enter the steel material, and proper evaluation cannot be performed. Therefore, the pH of the electrolytic solution 13 is set to 6 or less.

  As an electrolytic solution, preferably, an aqueous solution of thiocyanate (potassium thiocyanate, ammonium thiocyanate, etc.) known as a solution that has a catalytic action of storing hydrogen in steel and easily charges hydrogen into steel. Is used. The pH of this aqueous solution is about 5.5. Therefore, sulfuric acid is added to this aqueous solution to obtain an acidic electrolytic solution in the above pH range. If the concentration of thiocyanate in the acidic electrolytic solution is too high, there is a possibility of elution due to corrosion of the cathode test piece 1 and an error occurs. Therefore, in order to have reproducibility of measurement, 1M or less, more preferably 0.5M or less.

  In addition, the measurement of the amount of hydrogen in steel when the cathode test piece 1 is broken can be measured by gas chromatography. By measuring the amount of hydrogen in the steel at the time of fracture in this way, it is possible to determine the critical hydrogen amount that leads to the fracture of the steel at a certain applied stress, and the delayed fracture resistance between the steel types due to hydrogen (hydrogen brittleness resistance). Can be compared.

(Delayed fracture parameter formula)
In the evaluation method of the present invention, the time until cracking occurs in the cathode test piece satisfies the range of 500 to 10,000 in the following delayed fracture parameter formula in order to optimize reproducibility and time required as an evaluation method. It is preferable. In other words, it is preferable to select each processing condition (tensile processing, bending processing, compressive stress) of the cathode test piece so that this delayed fracture parameter formula satisfies the range of 500 to 10,000.

The delayed fracture parameter formula is [(tensile strength of the high-strength steel plate: MPa) / 1180] × (additional stress due to tightening) × [100 / (100−plastic strain:%)] × [(the high-strength steel plate Plate thickness: mm) / (radius of the bent portion of the cathode test piece: mm) or (angle of the bent portion of the cathode test piece: degree) ^1/2 ] × [1 / (the crack in the cathode test piece Time until generation: hr)] × 100.
In this delayed fracture parameter equation, the term [(tensile strength of the high-strength steel plate: MPa) / 1180] represents a strength parameter.
Also, (additional stress due to tightening) × [100 / (100−plastic strain:%)] × [(plate thickness of the high-strength steel plate: mm) / (radius of the bent portion of the cathode test piece: mm) or The term “(angle of the bent portion of the cathode test piece: degree) ^1/2 ] represents a parameter of processing conditions of the cathode test piece.
Furthermore, the term [1 / (time until crack occurs in the cathode test piece: hr)] represents a parameter of crack time.

  When the time until the cathode test piece is cracked is less than 500 in the delayed fracture parameter formula, the cathode test piece may crack within 100 hours depending on the processing conditions of the cathode test piece. In some cases, it takes too much time and is not suitable as an evaluation method. In addition, when the time until the cathode test piece is cracked exceeds 10,000 in the delayed fracture parameter formula, the time until the cathode test piece is cracked due to excessive processing conditions of the cathode test piece. However, it is difficult to judge the superiority or inferiority of delayed destruction.

(TRIP steel sheet)
Here, the TRIP steel plate which is a high strength steel plate having a tensile strength of 1180 MPa or more and having an excellent strength / ductility balance and stretch flangeability, which is a main evaluation object of the present invention, will be described below.

The TRIP steel sheet (cold-rolled steel sheet), which is the main evaluation object of the present invention, is based on the premise that the steel structure is referred to as the TPF steel in order to ensure excellent elongation and stretch flangeability at a high strength of 1180 MPa or more. It is preferable to use a TRIP type composite structure with a steel structure space factor of 80% or more of polygonal ferrite, 1 to 15% of retained austenite, and the balance of bainite and / or martensite. Further, the steel structure is preferably a steel structure space factor referred to as the TAM steel, tempered martensite is 80% or more, retained austenite is 1 to 15%, and the balance is bainite and / or martensite. It is good also as a TRIP type composite organization.
Furthermore, the steel structure is preferably a steel structure space factor called the TBF steel, bainitic ferrite is 80% or more, residual austenite is 1 to 15%, and the balance is bainite and / or martensite. It is good also as a TRIP type composite organization which consists of.

  If the polygonal ferrite, which is the main phase in the TRIP steel sheet structure, has a space factor of less than 80%, the effect of securing elongation and stretch flangeability at a high strength of 1180 MPa or more by the polygonal ferrite is not exhibited. Therefore, in order to ensure elongation and stretch flangeability, the space factor of the polygonal ferrite with respect to the entire structure is preferably 80% or more. Similarly, in the TAM steel, the space factor for the entire structure of the annealed martensite, which is the main phase, is 80% or more, and in the TBF steel, for the reason of securing the elongation at a high strength of 1180 MPa or more and the stretch flangeability. However, it is preferable that the space factor of the main phase of bainitic ferrite with respect to the entire structure is 80% or more.

  Polygonal ferrite is a polyhedral massive ferrite, but has a substructure with little or no dislocation density, and a substructure with a high dislocation density (a lath structure does not have to have It is also different from bainitic ferrite, which is a plate-like ferrite with a fine structure, and quasi-polygonal ferrite structure with a substructure such as fine subgrains. See Photobook-1). Polygonal ferrite is clearly distinguished from bainitic ferrite and quasi-polygonal ferrite by scanning electron microscope (SEM) observation due to the above characteristics. That is, polygonal ferrite is black in the SEM structure photograph, has a polygonal shape, and does not contain retained austenite or martensite inside. On the other hand, bainitic ferrite shows a dark gray color in SEM structural photographs, and bainitic ferrite and bainite, retained austenite, and martensite cannot often be separated and distinguished. Tempered martensite has lath martensite, martensite lath having almost the same crystallographic orientation relationship, which gathers to form a block, and several blocks gather to form a packet. Bainitic ferrite is a plate-like ferrite and means a substructure having a high dislocation density, and is clearly distinguished from polygonal ferrite having no substructure or having a very small substructure.

  The space factor of transformation structure such as polygonal ferrite, other bainite, martensite, etc. is observed by SEM observation (magnification 4000 times) of 1/4 thickness part of the steel sheet, and then commercially available image software [general purpose The space factor is measured as an area ratio by image analysis using image processing software “Image-Pro Plus” (Media, manufactured by Cybernetics).

(Residual austenite)
Residual γ is an essential structure for exerting the TRIP (transformation-induced plasticity) effect, and is useful for improving elongation (ductility). In order to effectively exhibit such an action, the residual γ is set to 1% or more as a space factor with respect to the entire tissue. On the other hand, if it exceeds 15%, local deformability and stretch flangeability deteriorate. Therefore, the residual γ is a constant space factor at a relatively small level, which is 1 to 15%. This space factor (%) is determined by using a known saturation magnetization measuring apparatus and saturation magnetization measuring method as the volume fraction (volume fraction), the saturation magnetization amount (I) of the sample to be measured having a certain shape, and the measurement subject. The saturation magnetization amount (Is) when the component is substantially the same as the sample and γR is 0% in volume ratio is obtained by actual measurement or calculation, and γR (volume%) = (1−I / Is) × 100 It is calculated based on the formula.

  In the TRIP steel sheet, the bainite and / or martensite is included in the remaining structure as long as the steel structure satisfies the above-mentioned polygonal ferrite / or tempered martensite / or bainitic ferrite and retained austenite. It may be a complex organization. In order to improve stretch flangeability and elongation without fail, it is preferable to reduce the coarse massive second phase structure of the second phase structure of retained austenite and martensite in the composite structure.

(TRIP steel plate component)
Next, basic components constituting the TRIP steel plate will be described. Hereinafter, all the units of chemical components are mass%. In the TRIP steel sheet, in order to ensure the above-described structure and properties such as stretch flangeability and elongation, basically, C: 0.05 to 0.4%, Si: 1.0 to 3.0%, It is preferable to make a cold-rolled steel sheet containing Mn: 1.0 to 3.5% and comprising the balance Fe and inevitable impurities. In addition, these% all mean the mass%.

Moreover, on the assumption of the said component plasticity, the following allowable components can further be contained in the range which does not impair the characteristic of a TRIP steel plate.
Ti and / or V, Zr, W in total 0.003 to 1.0%, or Nb: 0.1% or less (not including 0%) (Aiming at high strength effect by precipitation strengthening and microstructure refining action) ).
One or more of Mo: 1.0% or less (excluding 0%), Ni: 0.003-1.5%, Cu: 0.003-1.0% (strengthening of steel, stable austenite And aim for effects such as contribution to generation of γR).
Ca: 0.003% or less (not including 0%), REM: 0.003% or less (not including 0%) or two types (aiming to improve workability by controlling sulfide morphology in steel).

  C ensures the strength and γR of the steel sheet. When the C content is small, γR present in the steel sheet is extremely small, and a space factor of 1% or more cannot be ensured with respect to the entire structure. For this reason, the desired TRIP effect by γR cannot be obtained sufficiently. On the other hand, when the content of C is too large, the generation of coarse massive second phase increases, and the starting point of fracture increases, so that elongation and stretch flangeability deteriorate. Therefore, the C content is in the range of 0.05 to 0.4%. If it is 0.05% or less, the carbon concentration in the retained austenite becomes low, the retained austenite becomes unstable, and good elongation cannot be secured.

  Si suppresses the formation of carbides by decomposition of γR. It is also useful as a solid solution strengthening element. If the Si content is too small, γR becomes extremely small, and a space factor of 1% or more cannot be ensured for the entire structure. For this reason, the desired TRIP effect by γR cannot be obtained sufficiently. On the other hand, if the Si content is too large, the effect is saturated, and on the other hand, hot brittleness is caused and the steel is easily cracked during rolling. Therefore, the Si content is in the range of 1.0 to 3.0%. If it is more than this, scale formation by hot rolling becomes remarkable, and cost is required for removing scratches.

  Mn stabilizes austenite and contributes to the generation of γR. If the Mn content is too small, γR present in the steel sheet becomes extremely small, and a space factor of 1% or more cannot be ensured with respect to the entire structure. For this reason, the desired TRIP effect by γR cannot be obtained sufficiently. On the other hand, when the content of Mn is too large, the above effect is saturated, and adverse effects such as cracking of the slab occur. Therefore, the Mn content is in the range of 1.0 to 3.5%.

Other elements are impurities, and it is preferable to reduce the content thereof.
For example, since P is an element that promotes grain boundary segregation, the content is preferably 0.15% or less. S is an element that promotes hydrogen storage in a corrosive environment, so 0.02% or less is preferable. N is preferably 0.02% or less.

(TRIP steel sheet manufacturing method)
A preferable method for producing a TRIP steel sheet is that the hot rolling process is performed at a temperature of about 500 to 600 ° C. after cooling at an average cooling rate of about 30 ° C./s after completion of hot rolling at an Ar 3 point or higher. It is preferable to adopt. Further, it is recommended that the cold rolling is performed at a cold rolling rate of about 30 to 70%. The condition for continuous annealing of the cold-rolled steel sheet is that after the cold-rolled steel sheet is heated to an austenite (γ) temperature range of A3 or higher in order to make the steel structure a composite-structure steel sheet having the above-mentioned structure space factor, to the bainite transformation range. Rapid cooling is performed at a cooling rate as fast as possible with an average cooling rate of 30 ° C./s or more. This supercooling from the γ region increases the number of ferrite transformation nuclei. Compared to heating in the normal two-phase region (A1 to A3 points) and cooling from the two-phase region, ferrite grain growth Can easily occur uniformly, and the second phase can be made fine.

  Cold-rolled steel sheet that has been continuously annealed remains as a cold-rolled steel sheet that is not surface-treated, or if necessary, electroplating, hot-dip plating, or chemical surface treatment or surface coating, as well as various coating treatments, paint substrate treatments, organic coating treatments, etc. The product is made into a product cold-rolled steel sheet.

  Examples of the present invention and comparative examples will be described below. The present invention is not limited to this example, and can be implemented with appropriate modifications within a range that can be adapted to the gist of the present invention, all of which are within the technical scope of the present invention. include.

  With respect to each steel plate having the chemical composition shown in Table 1 and the structure and mechanical properties shown in Table 2, delayed fracture resistance was evaluated by the evaluation method of the present invention. Mechanical properties are measured using a JIS No. 5 tensile test piece for tensile strength (TS: MPa) and total elongation (T-EL:%).

  The delayed fracture resistance of each of these steel plates was evaluated in advance and given the order shown in Table 2. That is, the delayed fracture resistance of D is 990 MPa high-strength steel plate (1st), and below, B 1191 MPa TRIP steel plate (2nd), A 1479 MPa TRIP steel plate (3rd), C 1510 MPa high-strength steel plate (4) Order).

  The test of delayed fracture resistance of each steel sheet simulates actual press parts, narrows each steel sheet into a cup shape having a flange part at a drawing ratio of 2.0, and part of the flange part is 180 degrees. Bending was performed. The salt spray was continuously applied to the entire cup, and the rank was evaluated based on the length of time until a crack occurred in the bent portion bent 180 degrees.

  In addition, the high-strength steel plate described in Table 1 is manufactured by a conventional method. In addition, TRIP steel sheets are obtained by heating a slab obtained by converter melting and continuous casting at 1200 ° C., finish rolling at 900 ° C., cooling, winding at 500 ° C., and hot rolling with a thickness of 3.2 mm. A steel plate was obtained. And after pickling this hot-rolled steel sheet, after obtaining a cold-rolled steel sheet having a thickness of 1.2 mm by cold rolling, it is heated to 930 ° C. in the austenite (γ) temperature range in a continuous annealing line (CAL). After recrystallization annealing, after annealing, it was rapidly cooled to the bainite transformation region at an average cooling rate of 30 ° C./s or more as fast as possible.

(Evaluation method of the present invention)
In the evaluation method of the present invention, the delayed fracture resistance was evaluated by test pieces collected from the respective steel plates. That is, under each condition shown in Tables 3 to 5, the test piece to be evaluated is subjected to the tensile work shown in FIG. 1, the U-bending work shown in FIGS. 3 (a) and 3 (b), or the V-bending. Processing, stress application processing shown in FIGS. 4A and 4B was performed in advance. Tables 3 to 5 also show the delayed fracture parameter formula values of these machining conditions in each example.

In this state, as shown in FIG. 5, each test piece was immersed in the electrolytic solution in the electrolytic cell as a cathode, and platinum was used as an anode, and a constant current with a current density of 0.1 mA / mm ² was applied. Hydrogen charging was performed, and the time until the cathode specimen was cracked was investigated. These results are also shown in Tables 3-5. The electrolytic solution used in common with each example was an aqueous solution with pH 1, sulfuric acid concentration: 0.5M, and potassium thiocyanate concentration: 0.01M.

  In the time display until cracks occur in the cathode test pieces in Tables 3 to 5, those times described as “100” are not cracked even after 100 hours (hr) have elapsed since the start of the test. The delayed fracture resistance evaluation test (immersion test) was interrupted. In addition, what is described as “−” is an example in which the immersion test was not possible because the fracture occurred during the stress-added processing shown in FIGS. 4 (a) and 4 (b). These examples are not good as an evaluation (test) method unless there is a difference between the examples compared with each other (individual evaluation as a test method is indicated as “X” in each table), but there is a difference between the examples compared with each other. Can be used as a relative comparison.

(Table 3)
Table 3 shows the influence of the amount of plastic strain in the tensile processing shown in FIG. That is, 80%, 40%, 20% of the amount of elongation of the steel sheet, and the invention examples 1 to 12 to which a tensile process with a plastic strain of 20 to 80% was added are the delayed fracture parameters of these processing conditions. It exists in the range of 500-10000 by a formula value. For this reason, each time until a crack occurs in the cathode test piece shown in Table 3 is appropriately short and optimal (individual evaluation as a test method is described as “◯” in each table), and there is a difference in time between each other. Yes. As a result, the order of delayed fracture resistance according to the evaluation method of the present invention (time until cracks occur in the cathode test piece) was compared with the delay resistance by the salt spray test simulating the actual press parts in the comparison of the same processing conditions. The order is consistent with (corresponding to) the order of Table 2 of the destructive evaluation test. In Comparative Examples 4, 8, and 12, cracks do not occur even after 100 hours, and it is not good as an individual evaluation method. Can be used.

  On the other hand, Examples 13 to 16 in which the amount of plastic strain in tensile processing is less than 20% with respect to the amount of elongation of the steel sheet, or Examples 17 to 20 in which tensile processing is not performed are tested in comparison between the same processing conditions. Cracks did not occur even after 100 hours from the start, and there was no difference in delayed fracture resistance. For this reason, it does not correspond (correspond) with the order of Table 2 of the delayed fracture resistance evaluation test by the salt spray test simulating the actual press parts.

  Further, in a 990 MPa high-strength steel sheet having a tensile strength of less than 1180 MPa, the amount of plastic strain in tensile processing is within the range compared to high-strength steel sheets A, B, and C having a tensile strength of 1180 MPa or more. Even if it is in the range of 500 to 10000 in the delayed fracture parameter formula value, cracks do not occur even after 100 hours from the start of the test. For this reason, the method of the present invention is an effective evaluation method for a high-strength steel sheet having a tensile strength of 1180 MPa or more, and an accurate delayed fracture resistance evaluation cannot be performed for a high-strength steel sheet having a tensile strength of less than 1180 MPa. I understand that.

(Table 4)
Table 4 shows the influence of the bending radius amount of the U-bending process shown in FIG. That is, the invention examples from 21 to 28 to which a U-bending process with a bending radius in the range of 30 mm and 5 mm is added are in the range of 500 to 10,000 in terms of the delayed fracture parameter formula values of these processing conditions. For this reason, each time until a cathode test piece cracks is suitably short and optimal (individual evaluation as a test method is described as ◯ in each table), and there is a difference in each other's time. As a result, according to the evaluation method of the present invention, in the comparison of the same processing conditions (time until the cathode test piece is cracked), the rank of the delayed fracture resistance evaluation test by the salt spray test simulating the actual press parts and The order of matching (corresponding). In Comparative Examples 24 and 28, cracks do not occur even after 100 hours have elapsed, and it is not good as an individual evaluation method, but there is a relative difference from the above invention examples, and it can be used as a relative comparison. .

  On the other hand, Examples 29 to 32 having a bending radius of 3 mm and a lower limit of less than 5 mm were broken during the stress-added processing shown in FIGS. 4A and 4B, and an immersion test was possible. There wasn't. Further, in Examples 33 to 36 in which bending work and stress application were not performed, cracks did not occur even after 100 hours had elapsed from the start of the test in comparison between the same working conditions, and no difference in delayed fracture resistance was observed. . For this reason, none of them agrees (corresponds) with the rank of the delayed fracture resistance evaluation test by the salt spray test simulating the actual press parts.

  Moreover, in the 990 MPa high strength steel sheet of D with a tensile strength of less than 1180 MPa, the bending radius is within a range as compared with the high strength steel sheets A, B and C having a tensile strength of 1180 MPa or more, and the delayed fracture of these processing conditions Even if it is in the range of 500 to 10000 in the parameter formula value, cracking does not occur even after 100 hours from the start of the test. For this reason, the method of the present invention is an effective evaluation method for a high-strength steel sheet having a tensile strength of 1180 MPa or more, and accurate delayed fracture resistance cannot be evaluated for a high-strength steel sheet having a tensile strength of less than 1180 MPa. I understand that.

(Table 5)
Table 5 shows the influence of the bending angle of the V-bending process shown in FIG. That is, the invention examples 37 to 48 to which the V-bending process in which the bending angle is in the range of 90 degrees, 60 degrees, and 30 degrees are added are in the range of 500 to 10000 in the delayed fracture parameter formula values of these processing conditions. Is in. For this reason, each time until a cathode test piece cracks is suitably short and optimal (individual evaluation as a test method is described as ◯ in each table), and there is a difference in each other's time. As a result, according to the evaluation method of the present invention, in the comparison of the same processing conditions (time until the cathode test piece is cracked), the rank of the delayed fracture resistance evaluation test by the salt spray test simulating the actual press parts and The order of matching (corresponding). In Comparative Examples 40, 44, and 48, cracks do not occur even after 100 hours, and it is not good as an individual evaluation method, but there is a relative difference from the above invention example, and as a relative comparison, Can be used.

  On the other hand, Examples 49 to 52 having a bending angle of 20 degrees and less than the lower limit of 30 degrees were broken during the process of applying stress shown in FIGS. 4A and 4B, and the immersion test was performed. I could not.

  Moreover, in the 990 MPa high strength steel sheet of D with a tensile strength of less than 1180 MPa, the bending angle is within the range compared to the high strength steel sheets A, B, and C with a tensile strength of 1180 MPa or more, and the delayed fracture of these processing conditions Even if it is in the range of 500 to 10000 in the parameter formula value, cracking does not occur even after 100 hours from the start of the test. For this reason, the method of the present invention is an effective evaluation method for a high-strength steel sheet having a tensile strength of 1180 MPa or more, and accurate delayed fracture resistance cannot be evaluated for a high-strength steel sheet having a tensile strength of less than 1180 MPa. I understand that.

(Table 6)
Table 6 shows the influence of the additional compressive stress shown in FIG. 4 among the processing conditions preliminarily performed on the test piece. That is, the invention examples from 57 to 64 to which additional stresses of 2000 MPa and 500 MPa are applied are in the range of 500 to 10000 in terms of the delayed fracture parameter formula values of these processing conditions. For this reason, each time until a cathode test piece cracks is suitably short and optimal (individual evaluation as a test method is described as ◯ in each table), and there is a difference in each other's time. As a result, according to the evaluation method of the present invention, in the comparison of the same processing conditions (time until the cathode test piece is cracked), the rank of the delayed fracture resistance evaluation test by the salt spray test simulating the actual press parts and The order of matching (corresponding). In Comparative Examples 59 and 60, the value of the delayed fracture parameter expression exceeds 10,000, and in Comparative Example 59, the time until cracking occurs in the cathode test piece is too short, and Comparative Example 60 breaks during stress application processing. Therefore, the immersion test could not be performed. In Comparative Example 64, the delayed fracture parameter formula value was less than 500, and no cracks occurred even after 100 hours. Although these examples are not good as individual evaluation methods, they have a relative difference from the above-described invention examples, and can be used as a relative comparison except for Comparative Example 60 (Comparative Examples in which an immersion test could not be performed). For 60, the delayed fracture resistance is ranked for convenience according to the results in Table 2).

  On the other hand, Examples 53 to 56 having an applied stress of 2200 MPa and exceeding the upper limit of 2000 MPa were broken during the stress-added processing shown in FIGS. 4A and 4B, and the immersion test could not be performed. . Further, in Examples 65 to 68 in which no additional stress was applied, cracks did not occur even after 100 hours had elapsed from the start of the test in comparison between the same processing conditions, and there was no difference in delayed fracture resistance. For this reason, none of them agrees (corresponds) with the rank of the delayed fracture resistance evaluation test by the salt spray test simulating the actual press parts.

  In addition, in a 990 MPa high strength steel sheet of D with a tensile strength of less than 1180 MPa, the applied stress is within the range compared to the high strength steel sheets A, B, and C with a tensile strength of 1180 MPa or more, and the delayed fracture of these processing conditions. Even if it is in the range of 500 to 10000 in the parameter formula value, cracking does not occur even after 100 hours from the start of the test. For this reason, the method of the present invention is an effective evaluation method for a high-strength steel sheet having a tensile strength of 1180 MPa or more, and an accurate delayed fracture resistance evaluation cannot be performed for a high-strength steel sheet having a tensile strength of less than 1180 MPa. I understand that.

  From the above results, the requirements of the present invention as a method for evaluating delayed fracture resistance of high-strength steel sheets having a tensile strength of 1180 MPa or more, the provision of the steel sheet tensile strength, tensile processing, bending processing, and additional stress processing on the test piece in advance. The critical significance of the regulations such as the amount of plastic strain, the radius or angle of the bent part, and the applied stress during the tensile process is supported.

  The present invention is particularly suitable for high strength steel sheets, such as TRIP steel sheets, which are applied to press parts that have been processed in a region of high elongation unprecedented and formed into press parts and have not been used so far due to processing limitations. A delayed fracture property evaluation method (hydrogen embrittlement evaluation method) can be provided. As a result, it is possible to expand the application of high-strength steel sheets having a tensile strength of 1180 MPa or more used as press parts for automobiles.

It is a perspective view which shows the aspect of the tension processing of the test piece in this invention delayed fracture resistance evaluation method. It is a perspective view which shows the aspect of the drilling process of the test piece in this invention delayed fracture resistance evaluation method. It is a perspective view which shows the aspect of the bending process of the test piece in the delayed fracture resistance evaluation method of this invention. It is a perspective view which shows the aspect of the stress addition to the test piece in the delayed fracture resistance evaluation method of this invention. It is a perspective view which shows the aspect of the immersion test of the test piece in this invention delayed fracture resistance evaluation method.

Claims (3)

  A method for evaluating delayed fracture resistance of a high-strength steel sheet having a tensile strength of 1180 MPa or more, wherein the test piece of the high-strength steel sheet is accompanied by a plastic strain of 20 to 80% with respect to the elongation amount of the high-strength steel sheet. After applying the tensile process, either a U-bending process in which the radius of the bending part is 5 to 30 mm or a V-bending process in which the angle of the bending part is 30 to 90 degrees is added. In a state where a compressive stress of 500 to 2000 MPa is applied to both sides of the processed test piece, it is immersed in an electrolytic solution as a cathode, and a constant current is applied to the cathode and the anode to perform hydrogen charging. A method for evaluating delayed fracture resistance of a high-strength steel sheet, wherein the delayed fracture resistance of a high-strength steel sheet is evaluated by the time until cracking occurs.   The method for evaluating delayed fracture resistance of a high-strength steel sheet according to claim 1, wherein the high-strength steel sheet is a transformation-induced plastic steel sheet composed of a mixed structure of ferrite, bainite, and retained austenite. 3. The method for evaluating delayed fracture resistance of a high-strength steel sheet according to claim 1, wherein a time until cracking occurs in the cathode test piece satisfies a range of 500 to 10,000 in the following delayed fracture parameter formula.
Delayed fracture parameter formula = [(tensile strength of the high-strength steel plate: MPa) / 1180] × (additional stress due to tightening) × [100 / (100−plastic strain:%)] × [(plate of the high-strength steel plate) (Thickness: mm) / (radius of the bent portion of the cathode test piece: mm) or (angle of the bent portion of the cathode test piece: degree) ^1/2 ] × [1 / (until the cathode test piece is cracked) Time: hr)] × 100

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