JP4712839B2 - High strength cold-rolled steel sheet with excellent hydrogen embrittlement resistance and workability - Google Patents

Description

  The present invention relates to a high-strength cold-rolled steel sheet excellent in hydrogen embrittlement resistance and workability suitable for automobile parts and the like.

  For example, cold-rolled steel sheets used for automobile frame parts and the like are required to have high strength of 980 MPa or more for the purpose of achieving both collision safety and fuel efficiency reduction by reducing the weight of the vehicle body, and are processed into complex frame parts. Therefore, excellent moldability is also required.

  However, it is known that a new problem of delayed fracture due to hydrogen embrittlement newly occurs in a high strength region of 780 MPa or more, particularly 980 MPa or more. Delayed fracture is a high-strength steel in which hydrogen generated from a corrosive environment or atmosphere diffuses into defects such as dislocations, vacancies, and grain boundaries, embrittles the material, and breaks when stress is applied. This is a phenomenon, and as a result, it causes adverse effects such as a decrease in the ductility and toughness of the metal material.

  Conventionally, high strength steels often used for applications such as bolts, PC steel wires, and line pipes, when the tensile strength is 780 MPa or more, especially 980 MPa or more, hydrogen embrittlement (pickling brittleness, plating) due to hydrogen intrusion into the steel. It is widely known that brittleness, delayed fracture, etc.) occur. Therefore, most of the techniques for improving the hydrogen embrittlement resistance are directed to the steel materials for the bolts and the like. For example, Non-Patent Document 1 reports that if a metal structure is mainly tempered martensite and an element showing temper softening resistance such as Cr, Mo, V is added, it is effective in improving delayed fracture resistance. . This is a technology that suppresses fracture by shifting the delayed fracture mode from grain boundaries to intragranular fracture by depositing alloy carbides and utilizing them as hydrogen trap sites. However, these findings apply to medium carbon steel and cannot be used as it is for thin steel sheets with low carbon content that require weldability and workability.

  Therefore, the present applicants have a carbon structure satisfying C: more than 0.25 to 0.60%, the balance is made of iron and inevitable impurities, and the metal structure after tensile processing with a processing rate of 3% In the area ratio to the whole structure, the residual austenite structure: 1% or more, bainitic ferrite and martensite: 80% or more in total, the average axial ratio (major axis / minor axis) of the residual austenite crystal grains: 5 or more An ultra-high-strength thin steel sheet excellent in hydrogen embrittlement resistance characterized by satisfying the requirements has been developed and a patent application has already been filed (see Patent Document 1).

The above thin steel sheet exhibits excellent strength, elongation, and hydrogen embrittlement resistance. However, with regard to stretch flangeability, which has become increasingly important in recent years, retained austenite becomes the starting point of fracture, and the stretch flangeability is improved. Since it becomes a factor to reduce, it has been difficult to reliably achieve the desired level (at least 60000 / Ts% [unit of Ts: MPa], desirably 100%) for the stretch flangeability in recent years.
"New development of delayed fracture elucidation" (Iron Japan Institute, published in January 1997) p. 111-120 Japanese Patent Laid-Open No. 2006-207019

  Accordingly, an object of the present invention is to provide a high-strength cold-rolled steel sheet that has improved stretch embrittlement properties while ensuring excellent hydrogen embrittlement resistance.

The invention described in claim 1
% By mass (hereinafter the same for chemical components)
C: 0.05 to 0.30%
Si: 2.0% or less (including 0%),
Mn: 0.1 to 2.8%,
P: 0.1% or less,
S: 0.005% or less,
N: 0.01% or less,
Al: 0.01 to 1.00%
V: 0.01-1.00%
And the remainder has a component composition consisting of iron and inevitable impurities,
Tempered bainite contains 50% or more (including 100%) in area ratio, and the balance has a structure made of ferrite,
The distribution state of precipitates in the tempered bainite is
Precipitates having an equivalent circle diameter of 1 to 10 nm are 20 or more per 1 μm ^{2 of the} tempered bainite,
The precipitate containing V having an equivalent circle diameter of 20 nm or more is 10 or less per 1 μm ^{2 of the} tempered bainite, and is a high-strength cold-rolled steel sheet excellent in hydrogen embrittlement resistance and workability.

The invention described in claim 2
Ingredient composition further
Cr: 0.1 to 3.0%
The high-strength cold-rolled steel sheet having excellent hydrogen embrittlement resistance and workability according to claim 1.

The invention according to claim 3
Ingredient composition further
Mo: 0.01 to 1.0%,
Cu: 0.05 to 1.0%,
Ni: 0.05 to 1.0%,
The high-strength cold-rolled steel sheet having excellent hydrogen embrittlement resistance and workability according to claim 1 or 2 , comprising one or more of the above.

The invention according to claim 4
Ingredient composition further
Ca: 0.0005 to 0.01%,
Mg: 0.0005 to 0.01%,
REM: 0.0004 to 0.01%
The high-strength cold-rolled steel sheet excellent in hydrogen embrittlement resistance and workability according to any one of claims 1 to 3 .

The invention according to claim 5
The organization
Tempered martensite and retained austenite are included in the total area ratio of less than 3%,
The hydrogen embrittlement resistance according to any one of claims 1 to 4 , wherein the number of cementite particles having an equivalent circle diameter of 0.1 µm or more present in the tempered bainite is 3 or less per 1 µm ^{2 of the} tempered bainite. It is a high-strength cold-rolled steel sheet with excellent properties and workability.

  According to the present invention, in the tempered bainite single-phase structure, or in the double-phase structure mainly composed of ferrite and tempered bainite, the ratio of tempered martensite and retained austenite was reduced as much as possible. By properly controlling the distribution of precipitates containing V precipitated in tempered bainite, it becomes possible to improve stretch flangeability while ensuring hydrogen embrittlement resistance, It has become possible to provide high-strength thin steel sheets that are both excellent in stretch flangeability.

  The present inventors have a high structure composed mainly of tempered bainite (hereinafter simply referred to as “bainite”), which has a higher deformability than tempered martensite (hereinafter sometimes simply referred to as “martensite”). Focusing on the strength steel plate, and adding V as an alloy element thereto, V carbides and carbonitrides (hereinafter collectively referred to as “V-containing precipitates”) that strongly act as hydrogen trap sites are sized. Investigating the influence of various factors on hydrogen embrittlement resistance and stretch flangeability, considering that it can improve stretch flangeability while ensuring hydrogen embrittlement resistance by introducing it into bainite with proper Etc. have been intensively studied.

  As a result of the above examination, the main structure is a tempered bainite structure having a higher deformability than tempered martensite, and if necessary, a certain degree of elongation is ensured by introducing a ferrite structure into the bainite structure. Hydrogen embrittlement resistance by reducing the proportion of tempered martensite and retained austenite (hereinafter referred to as “residual γ”) as much as possible and miniaturizing V-containing precipitates. The inventors have found that stretch flangeability can be improved while ensuring the characteristics, and have completed the present invention based on the findings.

  Hereinafter, the structure characterizing the steel sheet of the present invention will be described first.

[Structure of the steel sheet of the present invention]
As described above, the steel sheet of the present invention is based on a tempered bainite single phase or a multiphase structure mainly composed of ferrite and tempered bainite, and particularly reduces the tempered martensite structure and residual austenite structure as much as possible. At the same time, it is characterized in that the distribution state of the V-containing precipitates in the tempered bainite is controlled.

<Tempered bainite: 50% or more in area ratio (including 100%)>
Bainite is a homogeneous structure having high strength and excellent plasticity. By using a bainite-based structure having such properties, stretch flangeability can be improved while securing tensile strength and elongation.

  When the ratio of tempered bainite is reduced, the structure becomes non-uniform and stretch flangeability cannot be ensured. Therefore, the area ratio of tempered bainite is 50% or more, preferably 70% or more, more preferably, in order to effectively exhibit the above-described action. Is 90% or more (including 100%).

<Precipitates with equivalent circle diameter of 1 to 10 nm: 20 or more per 1 μm ^{2 of} tempered bainite>
By properly dispersing fine V carbides and carbonitrides that effectively act as hydrogen trap sites in the structure, hydrogen embrittlement resistance can be improved and delayed fracture resistance after processing can be ensured. it can. In other words, a large amount of fine V-containing precipitates having a large specific surface area are dispersed in a large amount to increase the number of hydrogen trap sites, and by making the V-containing precipitates fine, V-containing precipitates are produced with respect to the parent phase. A consistent strain field is provided around the object, and the capability as a trap site for hydrogen that tends to collect in the strain field can be enhanced, and the hydrogen embrittlement resistance is improved. In this particle size range (equivalent circle diameter of 1 to 10 nm), there are almost no precipitates containing no V. Therefore, in this rule, those containing V as in the case of precipitates having a circle equivalent diameter of 20 nm or more as described below. Without limiting to, all precipitates were targeted.

In order to effectively exhibit the above action, the number of fine precipitates having an equivalent circle diameter of 1 to 10 nm is 20 or more, preferably 50 or more, more preferably 100 or more, per 1 μm ^{2 of} tempered bainite. A preferable range of the size (equivalent circle diameter) of the fine precipitate is 1 to 8 nm, and a more preferable range is 1 to 6 nm.

  The reason why the lower limit of the equivalent circle diameter of the fine precipitates is set to 1 nm is that finer precipitates are less effective as hydrogen trap sites.

<Precipitates containing V having an equivalent circle diameter of 20 nm or more: 10 or less per 1 μm ^{2 of} tempered bainite>
Precipitates containing V, such as VC, have extremely high rigidity and critical shear stress compared to the parent phase, so even if the periphery of the precipitate is deformed, the precipitate itself is not easily deformed. A large strain is generated at the interface between the parent phase and the precipitate, and fracture occurs. For this reason, if there are a large amount of coarse precipitates containing V of 20 nm or more, stretch flangeability deteriorates. Therefore, stretch flangeability can be improved by restricting the density of coarse V-containing precipitates.

In order to effectively exhibit the above action, coarse precipitates containing V having an equivalent circle diameter of 20 nm or more are limited to 10 or less, preferably 5 or less, more preferably 3 or less, per 1 μm ^{2 of} tempered bainite.

  The structure of the steel sheet of the present invention is required to satisfy the above definition, but it is recommended that the following structure definition (a) be further satisfied in addition to the essential structure definition.

<(A) Total area ratio of tempered martensite and retained austenite: less than 3%,
Cementite particles with an equivalent circle diameter of 0.1 μm or more: 3 or less per 1 μm ^{2 of} tempered bainite>
When steel containing a large amount of alloying elements such as the high-strength cold-rolled steel sheet according to the present invention is bainite transformed, the transformation speed becomes slow, and not only bainite but also martensite is easily formed. Since martensite is hard, strain concentrates on the interface between ferrite and martensite and breakage easily occurs, and stretch flangeability deteriorates. In addition, residual austenite also transforms into hard martensite when strain is applied and becomes the starting point of fracture, so that stretch flangeability is also deteriorated. Therefore, stretch flangeability can be improved by reducing the ratio of these structures as much as possible. Therefore, it is recommended that the total area ratio of tempered martensite and retained austenite be limited to less than 3%, further 2% or less, particularly 1% or less.

  Also, in the case of steels mainly composed of bainite, such as conventional high-strength cold-rolled steel sheets, usually tempered martensite and retained austenite are likely to be formed, and these structures are the starting point of fracture, so during tempering The dispersion state of cementite particles precipitated in bainite does not significantly affect stretch flangeability. However, when tempered martensite and retained austenite are reduced, the starting point of fracture during deformation transitions to cementite particles precipitated in bainite, so that the presence state of cementite particles in bainite determines the stretch flangeability. . For this reason, in the case of a steel that suppresses the formation of tempered martensite and retained austenite as much as possible, such as the high-strength cold-rolled steel sheet according to the present invention, stretch flangeability is achieved by controlling the precipitation state of cementite particles that are the starting point of fracture. Can be adjusted. Of the cementite particles, only large particles serve as the starting point of fracture. Therefore, by reducing the number of coarse cementite particles, it can be improved to a predetermined stretch flangeability.

In order to effectively exhibit the above action, the cementite particles having an equivalent circle diameter of 0.1 μm or more present in the tempered bainite are 3 or less per 1 μm ^{2 of} the tempered bainite, further 2.5 or less, particularly 2 or less. It is recommended to limit.

  Hereinafter, each measuring method of each area ratio of tempered bainite, ferrite, tempered martensite and retained austenite, the size and the number of precipitates, and the size and the number of cementite particles will be described.

  First, regarding the area ratio of ferrite, each test steel sheet was mirror-polished, corroded with 3% nital solution to reveal the metal structure, and then 5 fields of view at a magnification of 2000 using a scanning electron microscope (SEM). The area ratio of the ferrite was calculated from the area ratio of the ferrite area to the whole structure by observing and using image analysis to make the equiaxed area not containing cementite the ferrite.

  Next, regarding the total area ratio of tempered martensite and retained austenite, each test steel sheet was mirror-polished and corroded with a repeller corrosive solution to reveal the metal structure, and then a scanning electron microscope (SEM) ), 5 areas are observed at a magnification of 2000 times, and the area that appears white from the image contrast by image analysis is defined as tempered martensite and retained austenite, and the total area of tempered martensite and retained austenite based on the area ratio of this area to the entire structure The rate was calculated.

  Finally, regarding the area ratio of tempered bainite, the area other than ferrite, martensite, and retained austenite is bainite, and the area ratio of ferrite calculated above from 100%, and the ratio of tempered martensite and retained austenite, respectively. The area ratio of tempered bainite was determined by subtracting the total area ratio.

[Method of measuring the size of precipitates and their number]
About the size and the number of the precipitates, a thin film sample was prepared by a thin film method or an extraction replica method, and this sample was 100,000 to 300,000 times using a field emission transmission electron microscope (FE-TEM). The area of 2 μm ² or more is observed, and the dark portion is marked as a precipitate from the contrast of the image. With the image analysis software, the equivalent circle diameter is calculated from the area of each marked precipitate, and per unit area. The number of precipitates of a predetermined size present was determined.

  However, for precipitates of 20 nm or more, only those that confirmed the presence of V using EDX or EELS attached to FE-TEM were counted.

[Method for measuring the size and number of cementite particles]
About the size of cementite particles and the number of them, each specimen steel plate was mirror-polished, corroded with picral, and after revealing the metal structure, the magnification in the field of view of 100 μm ² so that the area inside bainite could be analyzed. Observe a scanning electron microscope (SEM) image at a magnification of 10,000 times, mark the white part as cementite particles from the contrast of the image, and mark the equivalent circle diameter from the area of each marked cementite particle using image analysis software. While calculating, the number of cementite particles having a predetermined size per unit area was determined.

  Next, the component composition which comprises this invention steel plate is demonstrated. Hereinafter, all the units of chemical components are mass%.

[Component composition of the steel sheet of the present invention]
C: 0.03-0.30%
C is an important element that affects the area ratio of bainite and affects strength and stretch flangeability. Moreover, since it combines with V to form V carbide and V carbonitride, the balance between V content and C content changes, and precipitation and disappearance of V carbide and V carbonitride during heat treatment. It affects the coarsening behavior and affects the hydrogen embrittlement characteristics and stretch flangeability. If it is less than 0.03%, the bainite area ratio is insufficient, so that the strength cannot be secured. On the other hand, if it exceeds 0.30%, V carbide and V carbonitride become too stable during heating during annealing, so that fine precipitation occurs. As a result, no precipitate can be obtained, and the precipitates become coarse, so that the hydrogen embrittlement characteristics and stretch flangeability cannot be ensured. The range of C content is preferably 0.05 to 0.25%, more preferably 0.07 to 0.20%.

Si: 2.0% or less (including 0%)
Si is a useful element that can increase strength without deteriorating elongation as a solid solution strengthening element. If it exceeds 2.0%, the formation of austenite at the time of heating is inhibited, so the area ratio of bainite cannot be ensured and stretch flangeability cannot be ensured. Moreover, the formation of cementite is inhibited, and retained austenite (residual γ) and martensite are likely to remain. The range of Si content is preferably 1.8% or less, more preferably 1.5% or less (including 0%).

Mn: 0.1 to 2.8%
Mn is a useful element having an effect of enhancing the strength and stretch flangeability by increasing the hardenability and securing the bainite area ratio during rapid cooling after heating during annealing. If it is less than 0.1%, the bainite area ratio is insufficient, so that the strength and stretch flangeability cannot be ensured. On the other hand, if it exceeds 2.8%, bainite transformation is suppressed, martensite and austenite remain, and stretch flangeability is deteriorated. The range of Mn content is preferably 0.30 to 2.5%, more preferably 0.50 to 2.2%.

P: 0.1% or less P is inevitably present as an impurity element and contributes to an increase in strength by solid solution strengthening, but segregates at the prior austenite grain boundaries and embrittles the grain boundaries to increase stretch flangeability. Since it deteriorates, it is made 0.1% or less. Preferably it is 0.05% or less, More preferably, it is 0.03% or less.

S: 0.005% or less S is also unavoidably present as an impurity element, forms MnS inclusions, and becomes a starting point of cracks when expanding holes, thereby reducing stretch flangeability. . More preferably, it is 0.003% or less.

N: 0.01% or less N is also unavoidably present as an impurity element and lowers the elongation and stretch flangeability by strain aging, so the lower one is preferable, and the content is made 0.01% or less.

Al: 0.01 to 1.00%
Al combines with N to form AlN and reduces the solid solution N that contributes to the occurrence of strain aging, thereby preventing the stretch flangeability from deteriorating and contributing to the strength improvement by solid solution strengthening. If it is less than 0.01%, solute N remains in the steel, so strain aging occurs and elongation and stretch flangeability cannot be secured. On the other hand, if it exceeds 1.00%, austenite formation during heating is inhibited. The area ratio of bainite cannot be ensured, and stretch flangeability cannot be ensured.

V: 0.001 to 1.00%
V promotes the production of α-FeOOH, which is iron oxide, which is said to be thermodynamically stable and protective among rust produced in the atmosphere, and also contains fine carbides and carbonitrides in steel. Since it exists as a hydrogen trap site, it is an important element for improving hydrogen embrittlement resistance. If it is less than 0.01%, the effect of improving the hydrogen embrittlement resistance cannot be sufficiently obtained. On the other hand, if it exceeds 1.00%, the V flanges or V carbonitrides which are present in an undissolved state in the steel during heating during annealing and grow coarsely will increase, and the stretch flangeability will deteriorate. The range of V content is preferably 0.01% or more and less than 0.50%, more preferably 0.02% or more and less than 0.30%.

  The steel of the present invention basically contains the above components, and the balance is substantially iron and impurities. In addition, the following allowable components can be added as long as the effects of the present invention are not impaired.

Cr: 0.1-3.0%
Among the bainite, the upper bainite, which is mainly targeted by the steel of the present invention, is (1) formation of bainitic ferrite → (2) spout of carbon from bainitic ferrite to austenite → (3) cementite from austenite It is formed by a transformation phenomenon that proceeds in the flow of formation. During this flow, the formation of cementite from austenite is delayed by the addition of an alloy element such as Si, so that residual austenite and tempered martensite are easily formed. On the other hand, Cr is an element that increases the driving force for nucleation of cementite, and promotes the formation of cementite, thereby suppressing the formation of retained austenite and tempered martensite. The formed cementite is usually coarsened due to the diffusion rate-determining of carbon having a high diffusion rate, and thus is likely to be coarsened. Since it progresses, cementite coarsening can be suppressed. As a result, it is possible to finely disperse cementite that is the starting point of fracture, and to improve stretch flangeability. If it is less than 0.1%, the above effect cannot be exhibited effectively, the ratio of tempered martensite and retained austenite increases, and cementite coarsens, so that stretch flangeability deteriorates. On the other hand, if it exceeds 3.0%, retained austenite is formed at the time of quenching, the bainite area ratio cannot be ensured, and the strength and stretch flangeability deteriorate. The range of Cr content becomes like this. Preferably it is 0.3-2.5%, More preferably, it is 0.6-2.0%.

B: 0.0002 to 0.0050%
B is an element useful for improving the hardenability and increasing the area ratio of bainite by being present in the austenite grain boundary in a solid solution state in the steel. If the addition is less than 0.0002%, the above-described effects cannot be exhibited effectively. On the other hand, if the addition exceeds 0.0050%, Fe ₂₃ (CB) ₆ is formed and the solid solution B decreases. Therefore, the hardenability improving effect is reduced.

Nb and / or Ti: ([N] −0.003) / 12 ≦ [Nb] / 96 + [Ti] / 48 ≦ ([N] +0.01) / 12 ([] is the content (mass of each element) %).)
N forms BN, consumes B, and diminishes the hardenability improving effect of solid solution B. On the other hand, Ti and Nb are elements useful for exerting a hardenability improving effect by B because N is strongly fixed as TiN or Nb (CN) and the formation of BN is suppressed. If the addition amount of these elements is insufficient, the above-mentioned BN formation suppressing action is not effectively exhibited, while if excessive, the formation of cementite is inhibited, the ratio of tempered martensite and retained austenite increases, and stretch flangeability deteriorates. To do.

Mo: 0.01 to 1.0%,
Cu: 0.05 to 1.0%,
Ni: 0.05-1.0%
These elements are useful elements for increasing the strength and stretch flangeability by increasing the hardenability and contributing to securing the area ratio of bainite. Of these elements, Mo forms alloy carbides and carbonitrides that can serve as hydrogen trap sites during tempering, and Cu and Ni, like V, promote the formation of α-FeOOH. , Both have the effect of improving the hydrogen embrittlement resistance. For each element, when the amount is less than the above lower limit, the above-described effects cannot be exhibited effectively. On the other hand, when the amount exceeds 1.0%, austenite remains at the time of quenching, and stretch flangeability is deteriorated.

Ca: 0.0005 to 0.01%,
Mg: 0.0005 to 0.01%,
REM: 0.0005 to 0.01%
These elements are useful elements for improving stretch flangeability by refining inclusions and reducing the starting point of fracture. If less than 0.0005% of each element is added, the above effect cannot be exhibited effectively. On the other hand, if more than 0.01% of each element is added, inclusions are coarsened and stretch flangeability is lowered. Resulting in.

  Note that REM refers to a rare earth element, that is, a group 3A element in the periodic table.

  Next, the preferable manufacturing method for obtaining this invention steel plate is demonstrated below.

[Preferred production method of the steel sheet of the present invention]
In order to manufacture the cold-rolled steel sheet as described above, first, steel having the above-described composition is melted and formed into a slab by ingot forming or continuous casting, followed by hot rolling (hot rolling).

[Hot rolling conditions]
As hot rolling conditions, it is recommended that the hot rolling heating temperature is set to 900 ° C. or higher, the hot rolling finish rolling temperature is set to 800 ° C. or higher, and after appropriate cooling, winding is performed at a temperature of 450 ° C. or lower.

  By performing hot rolling under such temperature conditions, V is completely dissolved in the heating stage, suppressing precipitation during hot rolling and precipitation of V carbide and carbonitride during winding, and annealing. During heating, coarse V carbides and carbonitrides can be prevented from remaining.

  After hot rolling is completed, pickling is performed and then cold rolling is performed. The cold rolling rate is preferably about 30% or more.

  Then, after the cold rolling, annealing and further tempering are performed.

[Annealing conditions]
As annealing conditions, annealing heating temperature Ta (° C.): [−9500 / {log ([% C] · [% V]) − 6.72} −273] ° C. or more and heating to Ac _{3 to} 1000 ° C. Annealing holding time: After holding for 20 to 3600 s, after quenching from the annealing heating temperature to 300 to [Bs-100] ° C. (cooling end temperature) at a cooling rate of 10 to 200 ° C./s, at the cooling end temperature It is good to hold for 10 to 600 seconds.

<Annealing heating temperature Ta (° C.): [−9500 / {log ([% C] · [% V]) − 6.72} −273] ° C. or higher and Ac _{3 to} 1000 ° C., annealing holding time: 20 to 3600s>
Ta (° C.) ≧ [−9500 / {log ([% C] · [% V]) − 6.72} −273] ° C. is achieved by completely dissolving V carbide and the like during annealing heating. In order to reduce the abundance of coarse V-containing precipitates of 20 nm or more and to sufficiently transform to austenite at the time of annealing and heating, to ensure an area ratio of bainite that is transformed from austenite at the time of subsequent cooling by 50% or more. It is.

  Annealing heating temperature Ta (° C.) <[− 9500 / {log ([% C] · [% V]) − 6.72} −273] ° C., ie, log [% V] <[− 9500 / (Ta + 273) ] -Log [% C] is not preferable because undissolved V carbide or the like remains during annealing and is coarsened, and the starting point of fracture increases when the stretch flange is deformed. The relational expression Ta (° C.) ≧ [−9500 / {log ([% C] · [% V]) − 6.72} −273] ° C. is edited by the Japan Iron and Steel Institute: 3rd edition Steel Handbook, Volume I Fundamentals, p. The linear plot showing the temperature dependence of the solubility product of [V] · [C] shown in FIG. 743 of 412 is read and transformed so that the temperature at which V is completely dissolved can be calculated. Asked.

Also, if the annealing heating temperature is less than Ac ₃ , the amount of transformation to austenite is insufficient during annealing heating, so that the amount of bainite that is transformed from austenite during subsequent cooling decreases, and an area ratio of 50% or more cannot be secured. On the other hand, if the temperature exceeds 1000 ° C., the austenite structure becomes coarse and the bendability and toughness of the steel sheet deteriorate, and the annealing equipment deteriorates.

  Further, if the annealing holding time is less than 20 s, V carbide and the like cannot be completely dissolved, while if it exceeds 3600 s, productivity is extremely deteriorated, which is not preferable.

<Cooling end temperature: 300 to [Bs-100] ° C., cooling rate: 10 to 200 ° C./s, holding time at cooling end temperature: 10 to 600 s>
This is because the bainite transformation from the austenite transformed during the annealing heating is sufficiently advanced to secure a bainite area ratio of 50% or more.

  When the cooling end temperature is less than 300 ° C., martensite is formed and stretch flangeability is deteriorated. On the other hand, when it exceeds [Bs-100] ° C., it takes too much time to complete the bainite transformation, and martensite and residual austenite are generated. As a result, stretch flangeability cannot be secured.

  If the holding time at the cooling end temperature is less than 10 s, the bainite transformation does not proceed sufficiently and the elongation and stretch flangeability cannot be ensured. On the other hand, if it exceeds 600 s, the productivity is extremely deteriorated.

[Tempering conditions]
As the tempering conditions, the temperature after the annealing cooling to the tempering heating temperature Tt (° C.): 480 to Ac ₁ ° C. and the tempering holding time t (s) is Pg = exp [−13123 / (Tt + 273)] × t After holding under the condition of <1.21 × 10 ⁻⁵ , it may be cooled to room temperature.

  In order to precipitate V carbide or the like during tempering, it is necessary to heat to 480 ° C. or higher, and in order to control the size of the precipitate, it is necessary to appropriately control the relationship between the heating temperature and the holding time.

  Here, Pg = exp [−13123 / (Tt + 273)] × t is based on the grain growth model of precipitates described in the formula (4.18) of Koichi Sugimoto et al .: Material Histology [Asakura Shoten Publishing], p106. These are parameters that define the size of precipitates, with variables set and simplified.

Under the condition of Pg = exp [−13123 / (Tt + 273)] × t ≧ 1.21 × 10 ⁻⁵ , the coarsening of the precipitate proceeds and the number of coarse precipitates of 20 nm or more becomes too large. The stretch flangeability cannot be secured.

It is recommended that the tempering holding time t (s) be Pg <0.20 × 10 ⁻⁵ , whereby the stretch flangeability can be further improved by preventing the growth of cementite.

Steels having the components shown in Table 1 below were melted to produce 120 mm thick ingots.
After this was hot rolled to a thickness of 25 mm, it was again hot rolled to a thickness of 3 mm. After pickling this, it cold-rolled to thickness 1.2mm to make a test material, and heat-treated on the conditions shown in Table 2.

  About each steel plate after the heat treatment, by the measurement method described in the above section [Best Mode for Carrying Out the Invention], each area ratio of tempered bainite, ferrite, tempered martensite and retained austenite, the size of precipitates, and The existence number (existence density), the size of cementite particles and the existence number (existence density) were measured.

  For each steel sheet, tensile strength TS, elongation El, stretch flangeability λ are measured to evaluate mechanical properties, and hydrogen embrittlement risk index is measured to evaluate hydrogen embrittlement resistance. did.

  The tensile strength TS and elongation El were measured in accordance with JIS Z 2241 by preparing a No. 5 test piece described in JIS Z 2201 with the long axis perpendicular to the rolling direction.

  Moreover, stretch flangeability (lambda) performed the hole expansion test according to the iron continuous standard JFST1001, and measured the hole expansion rate, and made this the stretch flangeability.

The hydrogen embrittlement risk index is as follows. A low strain rate tensile test (SSRT) with a strain rate of 1 × 10 ⁻⁴ / s was performed using a 2 mm flat plate test piece, and a hydrogen embrittlement risk index was calculated according to the following definition formula.

Hydrogen embrittlement risk index (%) = 100 × (1−E ₁ / E ₀ )

Here, E ₀ indicates the elongation at break of a test piece substantially free of hydrogen in steel, and E ₁ indicates a steel material (test piece) electrochemically charged with hydrogen in sulfuric acid. Elongation at break is shown. The hydrogen charge is performed by immersing a steel material (test piece) in a mixed solution of H ₂ SO ₄ (0.5 mol / L) and KSCN (0.01 mol / L) at room temperature and a constant current (100 A / m ^2). ).

  When the hydrogen embrittlement risk index exceeds 15%, there is a risk of causing hydrogen embrittlement during use. Therefore, in the present invention, 15% or less was evaluated as having excellent hydrogen embrittlement resistance.

  The measurement results of the mechanical characteristics and hydrogen embrittlement resistance are shown in Table 3.

  As shown in the table, invention steels (steel Nos. 2, 3, 10, 11, 14, 16, 17, 20, 21 that satisfy the essential constituent requirements of the present invention (the above-mentioned component composition rules and the above-mentioned essential structure rules)). , 26, 28, 32 to 37; those marked with a circle ○) are tensile strength TS of 780 MPa or more, index TS × λ for evaluating the balance between tensile strength TS and stretch flangeability (hole expansion ratio) λ. A high-strength cold-rolled steel sheet having both workability and resistance to hydrogen embrittlement with a hydrogen embrittlement risk index of 15% or less was obtained.

In contrast, comparative steels (steel Nos . 1, 4-9, 12 , 15, 18 , 19, 19) that lack at least one of the essential constituent requirements of the present invention (the above-mentioned compositional composition rules and the above-mentioned essential structure rules). 23 , 24 , 27; those marked with x) are inferior in any of the mechanical characteristics and the hydrogen embrittlement resistance.

  For example, steel no. 1 has an excessively low V content, so that the number (existence density) of fine precipitates having a circle-equivalent diameter of 1 to 6 nm is insufficient. Therefore, although it has excellent tensile strength and stretch flangeability, The brittleness and brittleness characteristics are poor.

  Steel No. No. 15 is excellent in tensile strength and hydrogenation embrittlement resistance because the existence number (existence density) of coarse precipitates having an equivalent circle diameter of 20 nm or more becomes excessive because the V content is too high. The stretch flangeability is inferior.

  Steel No. No. 18, because the total area ratio of martensite and retained austenite (residual γ) becomes excessive because the Si content is too high, it has excellent tensile strength and resistance to hydrogenation embrittlement, but stretch flangeability Is inferior.

  Steel No. No. 19 has an inferior tensile strength although it has excellent stretch flangeability and hydrogenation embrittlement resistance because the bainite area ratio is insufficient because the C content is too low.

  Steel No. 23, although the C content is too high, the residual amount of martensite and residual austenite (residual γ) increases, and the number of coarse precipitates of 20 nm or more becomes excessive, so the tensile strength is excellent. The stretch flangeability and hydrogenation embrittlement resistance are inferior.

  Steel No. No. 24 has an inferior tensile strength although it has excellent stretch flangeability and hydrogenation embrittlement resistance because the bainite area ratio is insufficient because the Mn content is too low.

  Steel No. In No. 27, when the Mn content is too high, the residual amount of martensite and retained austenite increases, and thus the tensile strength is excellent, but the stretch flangeability and hydrogenation embrittlement resistance are inferior.

Steel No. Nos. 4 to 9 and 12 do not satisfy at least one of the requirements for defining the structure of the present invention because the annealing condition or the tempering condition is out of the recommended range, and any of the characteristics is inferior.

Moreover, as shown in the same table, in addition to the essential constituent requirements of the present invention (the above-mentioned compositional composition rule and the above-mentioned essential structure rule), the recommended steel (steel No. 29 to 31; which satisfies the above recommended structure rule (a); All of the items marked with ◎ satisfy the tensile strength TS of 980 MPa or more, the stretch flangeability (hole expansion ratio) λ of 100% or more, and the hydrogen embrittlement risk index of 15% or less. It was found that a high-strength cold-rolled steel sheet having even higher strength and workability than steel can be obtained.

Claims (5)

% By mass (hereinafter the same for chemical components)
C: 0.05 to 0.30%
Si: 2.0% or less (including 0%),
Mn: 0.1 to 2.8%,
P: 0.1% or less,
S: 0.005% or less,
N: 0.01% or less,
Al: 0.01 to 1.00%
V: 0.001 to 1.00%
And the remainder has a component composition consisting of iron and inevitable impurities,
Tempered bainite contains 50% or more (including 100%) in area ratio, and the balance has a structure made of ferrite,
The distribution state of precipitates in the tempered bainite is
Precipitates having an equivalent circle diameter of 1 to 10 nm are 20 or more per 1 μm ^{2 of the} tempered bainite,
A high-strength cold-rolled steel sheet excellent in hydrogen embrittlement resistance and workability, wherein the number of precipitates containing V having an equivalent circle diameter of 20 nm or more is 10 or less per 1 μm ^{2 of the} tempered bainite. Ingredient composition further
Cr: 0.1 to 3.0%
The high-strength cold-rolled steel sheet having excellent hydrogen embrittlement resistance and workability according to claim 1. Ingredient composition further
Mo: 0.01 to 1.0%,
Cu: 0.05 to 1.0%,
Ni: 0.05 to 1.0%,
The high-strength cold-rolled steel sheet excellent in hydrogen embrittlement resistance and workability according to claim 1 or 2 , comprising one or more of the following. Ingredient composition further
Ca: 0.0005 to 0.01%,
Mg: 0.0005 to 0.01%,
REM: 0.0004 to 0.01%
The high-strength cold-rolled steel sheet excellent in hydrogen embrittlement resistance and workability according to any one of claims 1 to 3 . The organization
Tempered martensite and retained austenite are included in the total area ratio of less than 3%,
The hydrogen embrittlement resistance according to any one of claims 1 to 4 , wherein the number of cementite particles having an equivalent circle diameter of 0.1 µm or more present in the tempered bainite is 3 or less per 1 µm ^{2 of the} tempered bainite. High-strength cold-rolled steel sheet with excellent properties and workability.