JP5342912B2 - High strength cold-rolled steel sheet with excellent bending workability - Google Patents

Description

  The present invention relates to a high-strength cold-rolled steel sheet excellent in bending workability, and relates to a cold-rolled steel sheet having a high work strength (particularly, a tensile strength of 880 MPa or more) with a low work cracking defect rate in bending work.

  Considering the safety and environmental issues of automobiles, high strength steel sheets for automobiles are aimed at. In general, the workability decreases with increasing strength, but various steel plates having both strength and workability have been devised and put into practical use. For example, a composite steel sheet in which a ferrite phase and a low-temperature transformation phase such as martensite and bainite coexist is used as a high-strength steel sheet excellent in workability. A composite steel sheet is intended to simultaneously improve strength and workability by dispersing a hard low-temperature transformation phase in soft ferrite ground. However, in such a steel sheet for automobiles, work cracks starting from inclusions are a problem.

In view of such circumstances, there have been proposed techniques for improving the workability by controlling the inclusions. For example, Patent Document 1 shows that a cold-rolled steel sheet having excellent bending workability can be obtained by setting the number of inclusions having a diameter of 5 μm or more to 25 pieces / mm ² or less in terms of a circle. Patent Document 2 shows that, in Si deoxidized steel, a cold-rolled steel sheet having a high ductility can be obtained by reducing the number of oxide inclusions having a minor axis of 5 μm or more to 35 pieces / cm ² or less. Further, Patent Document 2 discloses that the inclusion composition is easy to be expanded and crushed to be refined.

  However, as in Patent Documents 1 and 2, even if the individual inclusions are fine and have a low density, cracks originating from the inclusions may occur depending on the distribution. Therefore, it is considered that further studies are necessary to reliably improve the workability (particularly the bending workability required for automotive steel sheets). In Patent Document 1, it is necessary to use low-sulfur steel, leading to an increase in cost. Further, Patent Document 2 does not describe bending workability particularly required for automobile steel sheets among workability.

Moreover, in Patent Document 3, observed in an arbitrary cross section parallel to the rolling surface of the cold-rolled steel sheet for cans, the point-like inclusions (three or more of the oxide-based inclusions are parallel to the rolling direction, and It is shown that the can-making failure can be reduced by setting the existence ratio of the ones arranged in a straight line at intervals of less than 200 μm to 6003 / m ² to 2 × 10 ⁴ / m ² . However, Patent Document 3 is limited to cans, and as its required characteristics, drawing is required, but bending workability required when used as the steel sheet for automobiles has not been studied.

Japanese Patent No. 3845554 JP 2005-272888 A Japanese Patent No. 3421194

  As described above, conventionally, a high-strength steel sheet with few inclusion property defects is realized mainly by strictly controlling the size and quantity of each inclusion. However, when the steel sheet is subjected to bending, cracks may be scattered even under extremely mild processing conditions, which raises the problem of productivity deterioration and cost increase due to inspection.

  The present invention has been made paying attention to the above-mentioned circumstances, and the purpose thereof is excellent in bending workability, which can sufficiently reduce the bending crack rate starting from inclusions in bending work. It is to obtain a high-strength cold-rolled steel sheet.

The high-strength cold-rolled steel sheet of the present invention that has solved the above problems is
The components of the steel plate
C: 0.12-0.3% (meaning mass%. The same applies to the components below),
Si: 0.5% or less (including 0%),
Mn: 1.5-3.0%
Al: 0.15% or less (excluding 0%),
N: 0.01% or less (excluding 0%),
P: 0.02% or less (not including 0%), and S: 0.01% or less (not including 0%)
The balance consists of iron and inevitable impurities,
The steel structure is a single martensite structure, and
In the surface layer region from the surface of the steel plate to the (plate thickness x 0.1) depth,
A n-th inclusions group determined by the n-th determination shown below, those outermost surface distance in the steel sheet rolling direction of the two outermost particles of the inclusion group is 100μm or more, the rolling surface 100 cm ² It is characterized in that it is 120 or less per hit.

(Nth judgment)
n-1 primary inclusion group (where n is an integer of 1 or more, and when n = 1, the 0th order inclusion group refers to inclusion particles) and one or more adjacent x order inclusion groups (x = 0 to 0). n-1, n is an integer of 1 or more, and the 0th order inclusion group means inclusion particles), and the minimum inter-surface distance between the nearest particles of the n-1st order inclusion group and the xth order inclusion group An inclusion group that satisfies the following formula (1) and is 60 μm or less is referred to as an “n-th order inclusion group”.

[In the formula (1),
λ: Minimum inter-surface distance (μm) between nearest particles of n−1 order inclusion group and x order inclusion group
σ _y : yield strength of steel sheet (MPa)
d ₁ : particle diameter in the steel sheet rolling direction of the n-1 primary inclusion group (when n = 1) or distance between the outermost surfaces of the two outermost particles in the steel sheet rolling direction (when n ≧ 2) (μm) )
d ₂ : Particle size in the steel plate rolling direction of the x-th order inclusion group (when x = 0) or the distance between the outermost surfaces of the two outermost particles in the steel plate rolling direction (when x ≧ 1) (μm)]

The steel sheet of the present invention is further as another element,
(A) Cr: 2.0% or less (not including 0%) and / or B: 0.01% or less (not including 0%);
(B) Cu: 0.5% or less (not including 0%), Ni: 0.5% or less (not including 0%), and Ti: 0.2% or less (not including 0%) At least one element selected from the group;
(C) V: 0.1% or less (not including 0%) and / or Nb: 0.1% or less (not including 0%);
May be included.

  The present invention also includes a hot dip galvanized steel sheet that has been subjected to hot dip galvanizing and a galvannealed steel sheet that has been subjected to alloyed hot dip galvanizing.

  According to the present invention, a high-strength cold-rolled steel sheet excellent in bending workability can be obtained reliably, and for example, it can be used as a steel sheet for automobiles. Specifically, it is possible to provide a steel plate suitable for manufacturing vehicle components such as bumpers, collision parts such as front and rear side members, and pillars such as center pillar reinforcement.

FIG. 1 is a graph showing the relationship between the true inclusion particle diameter (d ^* ) and the void growth range (A) for each yield strength (YS) of the steel sheet. FIGS. 2A and 2B are diagrams illustrating an embodiment of the primary inclusion group. FIGS. 3A and 3B are diagrams illustrating an example of the secondary inclusion group. FIG. 4 is a graph showing the relationship between the major axis of the inclusion group and the cumulative probability of bending cracks caused by the specified inclusion group. FIG. 5 is a graph showing the relationship between the position from the surface (ratio to the plate thickness t) and the probability that a specified inclusion group causes a bending crack. FIG. 6 is a graph showing the relationship between the number density of the specified inclusion group and the bending crack rate due to the specified inclusion group.

As described above, the present inventors have intensively studied in view of the fact that cracking occurs in processing (particularly bending) even if the components and composition of individual inclusion particles are controlled. As a result, first, the following was found.
(1) The starting point of bending cracks is a group of inclusions distributed in a point sequence parallel to the rolling direction of the steel sheet.
(2) Even if the individual inclusion particles constituting the inclusion group are fine as defined in the prior art (for example, Patent Document 1), As a result, the voids generated around the individual inclusion particles at the time of processing coalesce, and a coarse and flat defect (void) is formed as compared with the void generated around the inclusion particles existing alone. about. Such coarse and flat defects (voids) are concentrated with a very large stress compared to the voids generated around the inclusion particles existing alone during bending, resulting in easy material handling. It is thought that it leads to breakage.

  Based on these findings, investigation was made as to the state in which the inclusion particle distribution was specifically formed to form the above-described coarse and flat defects (voids). As a result, it was found that when the distribution of the two inclusion particles satisfies the following formula (1), it behaves as an inclusion group that forms one long defect. The formula (1) is based on the idea that “the void generated in each inclusion particle needs to be plastically deformed in order for the voids to merge with the adjacent voids”. It was obtained experimentally in consideration of the plastic deformation range brought about by concentration.

[In the formula (1),
λ: Minimum surface distance (μm) between an arbitrary inclusion particle and an inclusion particle adjacent thereto
σ _y : yield strength of steel sheet (MPa)
d ₁ : Particle size (μm) of any inclusion particle in the rolling direction of the steel sheet
d ₂ : Particle diameter (μm) in the rolling direction of the steel sheet of inclusion particles adjacent to the arbitrary inclusions]
Here, in order to show the basic concept, λ, d ₁ and d _{2 in the} above equation (1) are defined as described above.

The method for deriving equation (1) is as follows. From the true inclusion particle diameter (d ^* ) and void diameter (D) formed around the inclusion particle observed on the fracture surface in the examples described later, the void growth range (A = (D−d) ^* ) / 2) and d ^* were obtained. A graph showing the relationship between the true inclusion particle diameter (d ^* ) and the void growth range (A) according to the yield strength of the steel sheet is shown in FIG. When the results obtained in FIG. 1 are arranged by the yield strength (YS = σ _y ) of the steel plate, the following equation (2) is obtained.

In general, the relationship between the inclusion particle diameter (d) observed on an arbitrary surface and the true inclusion particle diameter (d ^* ) is expressed by the following equation (3).
d * = 1.27d (3)

  From the above formulas (2) and (3), the void growth range (A) can be expressed as the following formula (4).

Therefore, if the particle diameters of adjacent inclusion particles are d ₁ and d ₂ , respectively, the sum of the respective void growth ranges (A ₁ + A ₂ ) is not less than the minimum inter-surface distance (λ) between the two inclusion particles. In some cases, the voids were considered to be combined, and the above formula (1) was obtained.

  Further, in the present invention, when the λ is larger than 60 μm, the correlation between the number density of the specified inclusion group described later and the bending cracking ratio due to the specified inclusion group is lowered, so that the λ is set to 60 μm or less. By defining λ as 60 μm or less in this way, even when the distance between inclusion particles is excessively large, it is possible to suppress an increase in cost as compared with the conventional technique requiring control.

  And in this invention, what consists of the said 2 inclusion particle | grains which satisfy | fill said (1) Formula and (lambda) is 60 micrometers or less is an "inclusion group" which forms a coarse and flat defect (void) at the time of a bending process. It was supposed to be. This inclusion group is schematically illustrated in FIG. In FIG. 2 (a), the inclusion particle 3 at the right end does not satisfy the formula (1) as shown in FIG. 2 (a) in relation to the inclusion particle 2, or λ exceeds 60 μm. It is shown that the object particle 2 does not constitute an inclusion group.

In the above, the case where both of d ₁ and d ₂ are inclusion particles has been described. However, when the inclusion group consisting of the two inclusion particles is regarded as one inclusion particle, it is further closer to this. In some cases, the inclusion particles and other inclusion groups satisfy the formula (1) and λ satisfies 60 μm or less, thereby forming a coarser inclusion group. Therefore, in such a case, between the inclusion group composed of the above two inclusion particles and the inclusion particle or another inclusion group adjacent to the inclusion group, the expression (1) is further satisfied and λ is It is necessary to determine whether or not 60 μm or less is satisfied (the second and subsequent determinations).

  In this way, the determination of the inclusion group [whether the two inclusion particles or the inclusion group satisfies the above relationship (the expression (1) and λ is 60 μm or less) and constitutes a new inclusion group] The inclusion group of the present invention can be specified by repeatedly performing the determination of step 1 in a stepwise manner such as the first time, the second time, and so on.

  In addition, the above determination is performed until the inclusion particles or inclusion groups that satisfy the formula (1) with respect to the inclusion group and λ is 60 μm or less around the inclusion group do not exist. The inclusion group thus obtained is counted as one inclusion group.

  Therefore, for example, the inclusion group consisting of three inclusion particles (1 ″, 2 ″ and 3 ″) illustrated in FIG. 3A described later has an inclusion in the first determination. An inclusion group consisting of two inclusion particles 1 ″ and 2 ″ determined to be a group, and an inclusion particle 3 ″ determined to satisfy the above relationship with the inclusion group in the second determination; An inclusion group consisting of the two inclusion particles (1 ″ and 2 ″) and three inclusion particles (1 ″, 2 ″ and 3 ′) including the inclusion group. The inclusion particles 1 '', 2 '' and 3 'which are determined as inclusion groups in the second determination are not divided into the inclusion group consisting of') and counted as two inclusion groups. Count as one (secondary) inclusion group consisting of '.

  Specifically, for example, the inclusion group can be determined step by step as follows (in the following, the determination of the inclusion group up to the third time is specifically shown).

(I) First determination (determination of primary inclusion group)
When λ satisfies the above formula (1) and satisfies 60 μm or less between at least two inclusion particles, the inclusion group consisting of these is referred to as a “primary inclusion group” (FIG. 2A )).

  In addition, as illustrated in FIG. 2B, in the case where the inclusion particle 1 satisfies the above formula (1) and satisfies 60 μm or less in relation to the inclusion particle 2 ′ in addition to the inclusion particle 2 as well. The inclusion group consisting of these inclusion particles 1, 2, and 2 ′ is referred to as “primary inclusion group”.

(Ii) Second determination (determination of secondary inclusion group)
(Ii-1) When λ satisfies the formula (1) and satisfies 60 μm or less between the primary inclusion group and one or more adjacent inclusion particles, the inclusion group consisting of these Is a “secondary inclusion group”. This secondary inclusion group is schematically illustrated in FIG.

  (Ii-2) When λ satisfies the expression (1) and satisfies 60 μm or less between the primary inclusion group and one or more other adjacent primary inclusion groups, This inclusion group is referred to as a “secondary inclusion group”. This secondary inclusion group is schematically illustrated in FIG.

(Iii) Third determination (determination of tertiary inclusion group)
(Iii-1) When λ satisfies the formula (1) and satisfies 60 μm or less between the secondary inclusion group and one or more adjacent inclusion particles, the inclusion group consisting of these Is referred to as “tertiary inclusion group”.

  (Iii-2) In the case where λ satisfies the formula (1) and satisfies 60 μm or less between the secondary inclusion group and one or more adjacent primary inclusion groups, the inclusion consisting of these The object group is referred to as “tertiary inclusion group”.

  (Iii-3) When λ satisfies the formula (1) and satisfies 60 μm or less between the secondary inclusion group and one or more adjacent secondary inclusion groups, from these, This inclusion group is referred to as “tertiary inclusion group”.

  Thereafter, the fourth determination (determination of the quaternary inclusion group) is continued.

  From the above determination method, any inclusion group: n-th inclusion group determined by the nth determination (n is an integer of 1 or more) can be expressed as follows.

  That is, the n-th order inclusion group is an n-1 order inclusion group (where n is an integer of 1 or more, and when n = 1, the 0th-order inclusion group refers to inclusion particles) and one or more adjacent x. A secondary inclusion group (x = 0 to n-1, n is an integer of 1 or more, and a zero-order inclusion group refers to inclusion particles), and this n-1 order inclusion group and x order inclusion group. The inclusion group in which the minimum inter-surface distance (λ) of the closest particle satisfies the following formula (1) and is 60 μm or less.

[In the formula (1),
λ: Minimum inter-surface distance (μm) between nearest particles of n−1 order inclusion group and x order inclusion group
σ _y : yield strength of steel sheet (MPa)
d ₁ : particle diameter in the steel sheet rolling direction of the n-1 primary inclusion group (when n = 1) or distance between the outermost surfaces of the two outermost particles in the steel sheet rolling direction (when n ≧ 2) (μm) )
d ₂ : Particle size in the steel plate rolling direction of the x-th order inclusion group (when x = 0) or the distance between the outermost surfaces of the two outermost particles in the steel plate rolling direction (when x ≧ 1) (μm)]

  As described above, “determined by the determination at the nth time” means inclusion particles or inclusions that satisfy the formula (1) and have λ of 60 μm or less around the inclusion group. This means that the above determination is repeated until there is no object group, and one inclusion group finally obtained is determined.

  In the determination, the lower limit of the particle diameter in the steel plate rolling direction of the inclusion particles as a target is about 0.5 μm.

[About major axis of inclusion group]
Even if it is the inclusion group calculated | required by the said determination, the influence which acts on bending workability changes with the magnitude | sizes. Therefore, the size of the inclusion group (the major axis of the inclusion group = the distance between the outermost surfaces in the rolling direction of the two outermost particles of the inclusion group) and the bending workability (bending cracks caused by the prescribed inclusion group) Rate). FIG. 4 shows an example in which the fracture surface of the sample cracked from the inclusion group is observed in the examples to be described later, and the major axis in the steel sheet rolling direction of the inclusion group at the fracture origin is determined. The major axis is, for example, 20 or more and less than 40 μm. Inclusion groups within the range of 40 to less than 60 μm, 60 to less than 80 μm,..., Are aggregated as inclusion groups of 20 μm, 40 μm, 60 μm, respectively, and bending cracks caused by the prescribed inclusion group for each major axis of 20 μm It represents the cumulative probability of.

  From FIG. 4, since the occurrence of cracks due to the inclusion group (cumulative probability> 0) was observed when the major axis of the inclusion group was 100 μm or more, the lower limit of the major axis of the inclusion group to be controlled was set to 100 μm. (Hereinafter, an inclusion group having a major axis of 100 μm or more may be referred to as a “specified inclusion group”).

[About the observation area]
Bending cracks become prominent due to the inclusion group defined above, particularly in the surface layer region of a steel sheet where a great amount of strain is introduced during bending, so the following measurements are taken and observed in the present invention. Identified the area. That is, the defect indication position (inclusion position) on the rolling surface was specified by the ultrasonic flaw detection method under the conditions of frequencies of 30 MHz and 50 MHz in advance using a steel plate of an example described later. And the bending process was implemented as shown in the Example mentioned later so that a bending ridgeline might become parallel to a rolling direction, and may correspond with the defect instruction | indication position (inclusion position) obtained by the said investigation.

  About the test piece which gave the bending process and was fractured, the fracture surface of a crack starting point was investigated. Then, the presence or absence of the specified inclusion group was confirmed, and the position (depth) from the surface was measured for the case where the specified inclusion group was present. In addition, for the test piece that did not break, it was ground from the defect indication position on the rolling surface to 0.5 t (t: plate thickness) in the plate thickness direction, and the specified inclusion within a range of 0.5 t depth from the surface The presence or absence of the group was confirmed.

  Then, at each measurement position from the surface, the probability (%) that the specified inclusion group causes bending cracking (distinguishable from “bending cracking ratio caused by the specified inclusion group” described later) is 100 ×. (Number of test pieces that were fractured by bending and had a specified inclusion group) / [(Number of test pieces that were broken by bending and had a specified inclusion group) + (Number of test pieces that did not break by bending and had the specified inclusion group)].

  A summary of the results is shown in FIG. In addition, the results of 0.02t (ratio to the plate thickness t of 0.02), 0.04t, 0.06t ... in FIG. 5 are the surface (depth 0 mm) to 0.02t, more than 0.02t to 0, respectively. .04t, more than 0.04t ~ 0.06t ... From FIG. 5, the inclusion group defined in the present invention causes bending cracks when it exists in the range from the surface of the steel plate to the thickness of the plate × 0.1 (0.1 t) depth. I understand. It can also be seen that the bending workability is strongly influenced by the surface layer region. Therefore, in the present invention, the observation region of the specified inclusion group is set to the depth of (plate thickness × 0.1) from the surface of the steel plate.

[Relationship between number density of specified inclusion group and bending workability]
Next, the present inventors investigated the relationship between the number density of the specified inclusion group and the bending workability (bending cracking ratio due to the specified inclusion group). FIG. 6 is a graph showing the relationship between the number density of the specified inclusion group and the bending crack rate caused by the specified inclusion group, which was obtained by the method shown in the examples described later. In addition, if the bending crack rate due to the specified inclusion group is 2.0% or less, it is separately confirmed that there is no problem in the actual product.

From FIG. 6, in order to achieve the bending cracking ratio due to the specified inclusion group: 2.0% or less, the number density of the specified inclusion group needs to be 120 or less per 100 cm ² of the rolling surface. I understand that. A preferred number density is 100 or less per 100 cm ² of the rolling surface.

  The measurement of the inclusion group defined above can be performed by visual observation with an optical microscope (magnification: 100 times), for example, as shown in Examples described later. In addition, after binarizing the observation result of the optical microscope, it can be automatically measured by an image analysis process in which the condition of the above equation (1) and the boundary value of λ (60 μm) are set in advance.

  The present invention requires that the form of the inclusion group satisfies the above definition, and the components of the individual inclusion particles constituting the inclusion group are not particularly specified. Inclusion particles such as Al, Si, Mn, Ca, Mg (Ca, Mg may be included from the furnace wall in the production process or by slag entrainment even when not added as a selective element) Oxide-based inclusions containing Mn, sulfide-type inclusions containing Mn, Ti and the like, or composite inclusions thereof. In addition, when Ca, Mg and rare earth elements are included as selective elements, there are oxide inclusions including these elements and sulfide inclusions (for example, sulfide inclusions including Ca and Mg). Yes.

In addition, as described above, the present invention has a point where it is controlled as an inclusion group, but the total number of inclusion particles in the steel sheet is preferably reduced as in the prior art. It is preferable that inclusion particles having a particle size in the direction of 5 μm or more are suppressed to 25 particles / mm ² or less.

[About steel structure]
When the cold-rolled steel sheet of the present invention is used as, for example, a steel sheet for automobiles, it is required to have higher strength (880 MPa or more, preferably 980 MPa or more) and workability as characteristics. When there are many ferrite structures, it is difficult to ensure higher strength. In addition, it is difficult to sufficiently improve the bending workability when the composite structure is used. Therefore, in the present invention, the bending workability is improved by adopting a single structure of a martensite structure (preferably, a martensite structure including tempered martensite).

  The single structure of the martensite structure means that the martensite structure is 95% by area or more (in particular, 97% by area or 100% by area may be used).

  In addition to the martensite structure, the steel sheet of the present invention may also contain a structure (ferrite structure, bainite structure, residual austenite structure, etc.) that can be inevitably included in the manufacturing process.

  In order to realize a steel sheet that fully exhibits the effect of the structure control including the inclusion form and surely increases the bending workability and has a balance between high strength and excellent workability, the following component composition It is necessary to satisfy. Moreover, it is recommended to manufacture on the manufacturing conditions mentioned later. Below, the component composition of a steel plate is first explained in full detail.

[About the component composition of steel sheet]
[C: 0.12-0.3%]
C is an element necessary for enhancing the hardenability and ensuring high strength, so is contained in an amount of 0.12% or more (preferably 0.15% or more). However, if the C content is excessive, spot weldability and toughness are lowered, and delayed fracture tends to occur in the quenched portion. Therefore, the C content is 0.3% or less, preferably 0.26% or less.

[Si: 0.5% or less (including 0%)]
Si is an element effective for tempering softening resistance, and is also an element effective for strength improvement by solid solution strengthening. From these viewpoints, it is preferable to contain Si by 0.02% or more. However, Si is a ferrite-forming element, and if it is contained in a large amount, the hardenability is impaired and it is difficult to ensure high strength, so the Si amount is set to 0.5% or less. Preferably it is 0.4% or less.

[Mn: 1.5 to 3.0%]
Mn is an element effective for improving the hardenability and increasing the strength. In order to ensure sufficient hardenability, the amount of Mn is 1.5% or more. Preferably it is 1.7% or more. However, if contained excessively, the strength of the steel sheet is increased more than necessary, and the toughness is deteriorated. Therefore, the Mn content is 3.0% or less. Preferably it is 2.8% or less.

[Al: 0.15% or less (excluding 0%)]
Al is an element added as a deoxidizer and also has an effect of improving the corrosion resistance of steel. In order to fully exhibit these effects, it is preferable to make it contain 0.05% or more. However, if it is excessively contained, a large amount of C-based inclusions are generated and cause surface defects, so the upper limit is made 0.15%. Preferably it is 0.10% or less, More preferably, it is 0.07% or less.

[N: 0.01% or less (excluding 0%)]
If the amount of N is excessive, the amount of precipitation of nitride increases and adversely affects toughness. Therefore, the amount of N is set to 0.01% or less, preferably 0.008% or less. In addition, considering the cost etc. on steelmaking, N amount is 0.001% or more normally.

[P: 0.02% or less (excluding 0%)]
Although P has the effect | action which strengthens steel, since ductility is reduced by brittleness, it is suppressed to 0.02% or less. Preferably it is 0.01% or less.

[S: 0.01% or less (excluding 0%)]
S produces sulfide-based inclusions and degrades workability and weldability. Therefore, the smaller the content, the better. In the present invention, S is suppressed to 0.01% or less. Preferably it is 0.005% or less, More preferably, it is 0.003% or less.

  The basic components defined in the present invention are as described above, and the balance is iron and unavoidable impurities. As the unavoidable impurities, mixing of elements brought in depending on the situation of raw materials, materials, manufacturing facilities, etc. can be allowed. Furthermore, it is also possible to positively contain the following elements as long as the effects of the present invention are not adversely affected.

[Cr: 2.0% or less (not including 0%) and / or B: 0.01% or less (not including 0%)]
Both Cr and B are effective elements for increasing the strength by improving the hardenability. Cr is an element useful for increasing the temper softening resistance of martensitic steel. In order to fully exhibit these effects, in the case of Cr, 0.01% or more (more preferably 0.05% or more), and in the case of B, 0.0001% or more (more preferably 0.005% or more) is contained. Is preferred. However, when Cr is excessively contained, delayed fracture resistance is deteriorated. Moreover, when the amount of B becomes excessive, ductility will be reduced. Therefore, the Cr content is preferably 2.0% or less (more preferably 1.7% or less), and B is 0.01% or less (more preferably 0.008% or less).

[From the group consisting of Cu: 0.5% or less (not including 0%), Ni: 0.5% or less (not including 0%), and Ti: 0.2% or less (not including 0%) At least one element selected]
Cu, Ni, and Ti are effective elements for improving delayed fracture resistance by improving corrosion resistance. Such an effect is exhibited particularly effectively in a steel sheet having a tensile strength exceeding 980 MPa. Ti is also an element effective for increasing the temper softening resistance. In order to fully exhibit the effect, 0.01% or more (more preferably 0.05% or more) in the case of Cu, 0.01% or more (more preferably 0.05% or more) in the case of Ni, or Ti It is preferable to contain 0.01% or more (more preferably 0.05% or more). However, since the ductility and workability are reduced when excessively contained, it is preferable that the upper limits of Cu and Ni are both 0.5% and the upper limit of Ti is 0.2%. More preferably, Cu and Ni are each 0.4% or less, and Ti is 0.15% or less.

[V: 0.1% or less (not including 0%) and / or Nb: 0.1% or less (not including 0%)]
V and Nb are both effective elements for improving strength and improving toughness after quenching by γ grain refinement. In order to exhibit this effect sufficiently, it is preferable to contain 0.003% or more (more preferably 0.02% or more) in both V and Nb. However, when the above elements are excessively contained, precipitation of carbonitrides and the like increases, and workability and delayed fracture resistance decrease. Therefore, in both cases of V and Nb, the content is preferably 0.1% or less (more preferably 0.05% or less).

  Still other elements include, for example, Se, As, Sb, Pb, Sn, Bi, Mg, Zn, Zr, W, Cs, Rb, Co, La, Tl, Nd, Y, In, Be, Hf, Tc, Ta, O, Ca, or the like may be contained in a total of 0.01% or less for the purpose of improving corrosion resistance and delayed fracture resistance.

  The effect of the present invention is fully exhibited when the steel sheet is particularly a high-strength region (880 MPa or more, particularly 980 MPa or more).

  Although the present invention does not prescribe up to the method for producing the steel sheet, in order to realize the above-mentioned inclusion form, in particular, the rolling reduction during rolling in the temperature range of about 950 ° C. or less during hot rolling and It is recommended to control the total reduction ratio of the reduction ratio (cold rolling ratio) during cold rolling.

  As described above, the present invention does not define the components of inclusion particles, but in the component composition of the steel sheet of the present invention, the inclusions are often mainly composed of oxide inclusions, and are plastic. During rolling in a relatively low temperature range where the deformability is reduced, the oxide inclusions can be crushed and dispersed to form a defined inclusion group. As described above, the inclusions that are finely crushed and dispersed over a long distance are formed with long and flat defects (voids) during bending, and bending cracks occur due to large stress concentrations around these defects. . Therefore, it is recommended to reduce the degree of crushing by relatively reducing the rolling reduction in the temperature range.

  The oxide inclusions that may be present in the steel sheet satisfying the component composition of the present invention are Al, Si, Mn, Ti, Mg, Ca, rare earth element single oxides and / or composite oxides, and their softening points. Taking into account the deformability of the base metal, the reduction rate from about 950 ° C. to the room temperature region (specifically, the reduction rate in the temperature range of about 950 ° C. or less during hot rolling and the reduction during cold rolling) It is important to control the crushing / dispersing state by optimizing the total rate of reduction).

  Specifically, in the case of a steel sheet satisfying the component composition of the present invention, the total reduction ratio of the reduction ratio in the temperature range of about 950 ° C. or less during hot rolling and the reduction ratio during cold rolling is less than 97%. It is preferable that More preferably, it is 95% or less. On the other hand, when the total rolling reduction is too small, coarse inclusions are not refined, and bending workability may be deteriorated. Further, it becomes difficult to manufacture the thin steel plate itself. Therefore, the total rolling reduction should be at least about 90%.

  In order to suppress the total number of inclusion particles in the steel plate, killed steel deoxidized with Al is first refined in a converter or electric furnace, desulfurized in a ladle by the LF method, and then vacuum degassed. It is recommended to perform (for example, RH method).

Other than the above, it is not particularly limited. For example, after obtaining the steel slab such as slab by continuous casting after the above-described melting, it is heated to about 1100 to 1250 ° C. and then hot-rolled. (Preferably, finishing temperature: hot rolling is completed at ₃ points or more of Ar), pickling, pickling and cold rolling (cold rolling rate is about 30 to 70%) to obtain a steel sheet. Next, an annealing process is performed. In this annealing treatment, for example, after holding at 800 to 1000 ° C. for 5 to 300 seconds, cooling from 600 to 1000 ° C. (quenching start temperature) to room temperature by rapid cooling (for example, 20 ° C./s or more), and further 100 to 600 ° C. It is good to obtain a martensite single structure by re-heating to tempering and holding in the temperature range for 0 to 1200 seconds.

  The annealing treatment can be performed, for example, in a hot dip galvanizing line when obtaining the following hot dip galvanized steel sheet or alloyed hot dip galvanized steel sheet.

  In the present invention, not only a cold-rolled steel sheet, but also a hot-dip galvanized steel sheet (GI steel sheet) obtained by subjecting a cold-rolled steel sheet to hot dip galvanization, An alloyed hot-dip galvanized steel sheet (GA steel sheet) obtained by the chemical treatment is also included. By applying these plating treatments, corrosion resistance is improved. In addition, what is necessary is just to employ | adopt the conditions currently performed about these plating processing methods and alloying processing methods.

  The high-strength cold-rolled steel sheet of the present invention includes automotive parts, such as bumpers, front and rear side members and crash parts such as crash boxes, pillars such as center pillar reinforcement, roof rail reinforcement, side sill, It can be used for manufacturing body components such as floor members and kick parts.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

  Steels having the chemical composition shown in Table 1 were melted. Specifically, desulfurization was performed in a ladle after primary refining in a converter or electric furnace. Moreover, the vacuum degassing (for example, RH method) process was implemented after ladle scouring as needed. Thereafter, continuous casting was performed by a conventional method to obtain a slab. And hot rolling, pickling by the usual method, and cold rolling were performed in order, and the steel plate with a plate thickness of 1.6 mm was obtained. Next, continuous annealing was performed. In continuous annealing, after holding at the annealing temperature shown in Table 2 for 180 seconds, it is cooled to the quenching start temperature shown in Table 2 at a cooling rate of 10 ° C / s, and then rapidly cooled from the quenching start temperature to room temperature (cooling rate of 20 ° C / s or more). The mixture was further reheated to the tempering temperature shown in Table 2, and held at that temperature for 100 seconds to obtain a martensite single structure. In addition, the total reduction ratio of the reduction ratio in the temperature range of about 950 ° C. or less in the hot rolling and the reduction ratio in the cold rolling is as shown in Table 2. Moreover, the conditions of hot rolling are as follows.

(Hot rolling)
Heating temperature: 1250 ° C
Finishing temperature: 880 ° C
Winding temperature: 550 ° C
Finish thickness: 2.6-3.2mm

  And various test pieces were produced from the steel plate (steel strip) obtained as mentioned above, and the structure | tissue observation shown below and evaluation of the characteristic were performed.

(Measurement of inclusion group)
As for the sampling position of the test piece, the rolling direction of the steel strip is arbitrary, and the sheet width direction is each position of w / 8, w / 4, w / 2, 3w / 4, 7w / 8 (w: sheet width). Three test pieces each having a rolled surface size of 30 mm square were collected, and the rolled surface (ND surface) was ground from the surface to 0.1 t (t: plate thickness) at a pitch of 10 μm, each time (every 10 μm ground) And) by visually observing with an optical microscope (magnification 100 times), confirming the position of the inclusions, and defining the inclusion group (the n-th inclusion group described above, the two most The number of outer particles having a distance between outermost surfaces in the steel plate rolling direction of 100 μm or more was measured, and the number per observation area was calculated and converted to 100 cm ^{2 of} the rolling surface. The results (number density of specified inclusion groups) are shown in Table 2.

(Observation of microstructure)
A test piece of 1.6 mm × 20 mm × 20 mm was cut out, a cross section parallel to the rolling direction was polished, and repeller corrosion was performed, and then a t (plate thickness) / 4 position was measured. Then, an image analysis was performed by observing a measurement region of about 80 μm × 60 μm at a magnification of 1000 times with an optical microscope. In addition, the measurement was performed about arbitrary 5 visual fields. As a result, all examples had a martensite single structure with a martensite structure of 95 area% or more.

(Evaluation of tensile properties)
The tensile strength (TS) was measured according to JIS Z 2241 by taking a JIS No. 5 tensile test piece from the steel plate so that the direction perpendicular to the rolling direction of the steel plate and the longitudinal direction of the test piece were parallel. In this example, a material having a tensile strength of 880 MPa or more was evaluated as having high strength. The results are shown in Table 2. For reference, the yield strength (YP) and elongation (EL) of the steel sheet are also shown in Table 2.

(Evaluation of bending workability (measurement of bending crack rate due to inclusions))
Folding style bending is performed on the total number of test pieces: 1000 under the following conditions, and for the test pieces with cracks, the cross-sectional part (thickness direction) of the crack starting point is observed with SEM and EDX. The presence or absence of inclusion groups was confirmed. In addition, all of the specified inclusion group that became the crack starting point existed within 0.1 t.

  Then, the bending cracking rate (%) due to the specified inclusion group is 100 × (number of test pieces that are fractured by bending and have the specified inclusion group) / (total number of test pieces = 1000). The results are shown in Table 2.

<Folding style bending conditions>
Processing equipment: Aida Engineering Co., Ltd. NC1-80 (2) -B
Processing speed: 40 times per minute Clearance: Plate thickness + 0.1mm
Die punch radius: Limit bending radius of material (R / t) + 0.5 / t
(R: mold radius (mm), t: specimen thickness (mm))
Punch angle: 90 °
Test piece size: t × 80 mm or more (W) × 30 mm (L)
(L and steel strip rolling direction are parallel)
Bending direction: Specimen rolling direction and bending ridge line are parallel Test number and test position: Longitudinal direction of steel strip: Arbitrary position, w / 8, w / 4, w / 2, 3w / 4, 7w / in the plate width direction 200 sheets each at a position of 8 (w: board width), a total of 1000 sheets

<Derivation of the above limit bending radius>
For example, bending was performed according to the following procedure with different bending radii such as 2.0 mm, 1.5 mm, and 1.0 mm, and the minimum bending radius at which no bending cracking occurred was defined as the critical bending radius.
・ Folding style bending ・ Measurement position and number of tests: w / 4 position, 2 at each bending radius ・ Other conditions are the same as above.

  Tables 1 and 2 can be considered as follows. No. 1, 3, 5-9, 11, 13, 15, 18-24, 26, 27 satisfy the requirements of the present invention, so the bending cracking rate due to inclusions is small and the bending workability is excellent. I understand that. In contrast, no. 2, 4, 10, 12, 14, 16, 17, and 25 have a high density of inclusions and are inferior in bending workability. This is presumably because the reduction rate during the reduction from about 950 ° C. to room temperature in the production process was not within the recommended range.

Claims (6)

The components of the steel plate
C: 0.12-0.3% (meaning mass%. The same applies to the components below),
Si: 0.5% or less (including 0%),
Mn: 1.5-3.0%
Al: 0.15% or less (excluding 0%),
N: 0.01% or less (excluding 0%),
P: 0.02% or less (not including 0%), and S: 0.01% or less (not including 0%)
The balance consists of iron and inevitable impurities,
The steel structure is a single martensite structure, and
In the surface layer region from the surface of the steel plate to the (plate thickness x 0.1) depth,
A n-th inclusions group determined by the n-th determination shown below, those outermost surface distance in the steel sheet rolling direction of the two outermost particles of the inclusion group is 100μm or more, the rolling surface 100 cm ² A high-strength cold-rolled steel sheet excellent in bending workability, characterized by being 120 or less per piece.
(Nth judgment)
n-1 primary inclusion group (where n is an integer of 1 or more, and when n = 1, the 0th order inclusion group refers to inclusion particles) and one or more adjacent x order inclusion groups (x = 0 to 0). n-1, n is an integer of 1 or more, and the 0th order inclusion group means inclusion particles), and the minimum inter-surface distance between the nearest particles of the n-1st order inclusion group and the xth order inclusion group An inclusion group that satisfies the following formula (1) and is 60 μm or less is referred to as an “n-th order inclusion group”.
[In the formula (1),
λ: Minimum inter-surface distance (μm) between nearest particles of n−1 order inclusion group and x order inclusion group
σ _y : yield strength of steel sheet (MPa)
d ₁ : particle diameter in the steel sheet rolling direction of the n-1 primary inclusion group (when n = 1) or distance between the outermost surfaces of the two outermost particles in the steel sheet rolling direction (when n ≧ 2) (μm) )
d ₂ : Particle size in the steel plate rolling direction of the x-th order inclusion group (when x = 0) or the distance between the outermost surfaces of the two outermost particles in the steel plate rolling direction (when x ≧ 1) (μm)]   The high-strength cold-rolled steel sheet according to claim 1, further comprising, as other elements, Cr: 2.0% or less (not including 0%) and / or B: 0.01% or less (not including 0%). . As other elements,
Cu: 0.5% or less (excluding 0%),
Ni: 0.5% or less (not including 0%), and Ti: 0.2% or less (not including 0%)
The high-strength cold-rolled steel sheet according to claim 1 or 2, comprising at least one element selected from the group consisting of:   The element according to any one of claims 1 to 3, further comprising V: 0.1% or less (not including 0%) and / or Nb: 0.1% or less (not including 0%) as other elements. High strength cold rolled steel sheet.   A hot-dip galvanized steel sheet obtained by applying hot-dip galvanizing to the high-strength cold-rolled steel sheet according to any one of claims 1 to 4.   An alloyed hot-dip galvanized steel sheet obtained by applying alloying hot-dip galvanizing to the high-strength cold-rolled steel sheet according to claim 1.