JP5363922B2 - High-strength cold-rolled steel sheet with an excellent balance between elongation and stretch flangeability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength cold-rolled steel sheet which secures a balance between elongation and formability for extension flange even in a higher strength class than in a conventional one and is excellent in formability. <P>SOLUTION: The cold-rolled steel sheet has a component composition comprising, by mass%, 0.03-0.30% C, 0.1-3.0% Si, 0.1-5.0% Mn, 0.1% or less P, 0.1% or less S, 0.01% or less N, 0.01-1.00% Al and the balance iron with unavoidable impurities; and has a structure which includes 70% or more (including 100%) by an area rate of tempered martensite having a hardness of more than 380 but 450 Hv or less and the balance ferrite. Cementite particles in the tempered martensite are distributed in such a state that there are 20 pieces or more of cementite particles having a circle-equivalent diameter of 0.02 &mu;m or more but less than 0.1 &mu;m in 1 &mu;m<SP>2</SP>of the tempered martensite and there are 1.5 pieces or less cementite particles having a circle-equivalent diameter of 0.1 &mu;m or more in 1 &mu;m<SP>2</SP>of the tempered martensite. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to a high-strength steel plate excellent in workability, and more particularly, to a high-strength steel plate excellent in the balance between elongation (total elongation) and stretch flangeability.

  For example, steel sheets used for automobile frame parts and the like are required to have high strength for the purpose of collision safety and fuel efficiency reduction by reducing the weight of the car body, and excellent forming process for processing into complex frame parts However, depending on the parts to be processed, whether the emphasis is more on strength or the emphasis on workability is different.

  For this reason, provision of the high strength steel plate provided with the balance of elongation and stretch flangeability according to a strength class is desired.

  The inventors of the present invention have a tensile strength of 780 to 980 MPa grade steel sheets based on a dual-phase structure steel composed of ferrite and tempered martensite (see Patent Documents 1 and 2), but with a lower proportion of ferrite than these conventional steel sheets. In addition to reducing the hardness of the tempered martensite, and by refining the cementite particles precipitated in the martensite during tempering, the stretch flangeability is improved while ensuring the tensile strength. We have succeeded in developing a high-strength cold-rolled steel sheet with a high balance with elongation, and have already filed a patent application (see Patent Document 1).

  However, even a high strength steel sheet having a tensile strength of 1180 MPa class or higher, which is a higher strength class than the high strength cold-rolled steel sheet (hereinafter referred to as “prior invention steel sheet”) described in Patent Document 1, is also stretched (total elongation). High strength steel sheet with an excellent balance between El) and stretch flangeability (hole expansion ratio; λ) has been eagerly desired, and El × λ can be secured at 1000 %% or more for a steel sheet having a tensile strength of 1180 MPa or more. Things are desired.

JP 2004-256872 A JP 2004-232022 A JP 2009-144239 A

  Accordingly, an object of the present invention is to provide a high-strength cold-rolled steel sheet having excellent formability and ensuring a balance between elongation and stretch flangeability even in a higher strength class than the above-described prior invention steel.

The invention described in claim 1
% By mass (hereinafter the same for chemical components)
C: 0.03 to 0.30%,
Si: 0.1 to 3.0%,
Mn: 0.1 to 5.0%,
P: 0.1% or less,
S: 0.1% or less,
N: 0.01% or less,
Al: 0.01 to 1.00%
And the remainder has a component composition consisting of iron and inevitable impurities,
A tempered martensite having a hardness of more than 380 and not more than 450 Hv includes an area ratio of 70% or more (including 100%), and the balance has a structure made of ferrite,
The distribution state of cementite particles in the tempered martensite is
The cementite particles having an equivalent circle diameter of 0.02 μm or more and less than 0.1 μm are 20 or more per 1 μm ^{2 of the} tempered martensite,
A cementite particle having an equivalent circle diameter of 0.1 μm or more is 1.5 or less per 1 μm ^{2 of the} tempered martensite, and is a high-strength cold-rolled steel sheet having an excellent balance between elongation and stretch flangeability.

The invention described in claim 2
Ingredient composition further
Cr: 0.01-1.0% and / or Mo: 0.01-1.0%
The high-strength cold-rolled steel sheet having an excellent balance between elongation and stretch flangeability according to claim 1.

The invention according to claim 3
Ingredient composition further
Cu: 0.05-1.0% and / or Ni: 0.05-1.0%
The high-strength cold-rolled steel sheet having an excellent balance between elongation and stretch flangeability according to claim 1 or 2.

The invention according to claim 4
Furthermore,
Ca: 0.0005 to 0.01% and / or Mg: 0.0005 to 0.01%
The high-strength cold-rolled steel sheet having an excellent balance between elongation and stretch flangeability according to any one of claims 1 to 3.

  According to the present invention, in a two-phase structure composed of ferrite and tempered martensite, by properly controlling the hardness of tempered martensite and its area ratio, and the distribution of cementite particles in the tempered martensite, The tensile strength can be further improved while ensuring the balance between the flangeability and the elongation, and a high-strength steel sheet excellent in formability can be provided even in a higher strength class than the above-mentioned prior invention steel.

  The present inventors are based on a high-strength steel sheet having a two-phase structure composed of ferrite and tempered martensite (hereinafter sometimes simply referred to as “martensite”) in the same manner as the above-described prior invention steel sheet. The ratio of ferrite is further reduced than that of the invention steel (that is, the ratio of martensite is further increased), and the cementite in the martensite is refined while maintaining the hardness of the martensite high by adjusting the tempering conditions. As a result of intensive investigations, it was concluded that the present invention could be completed as a result of intensive studies that increased martensite strength and improved stretch flangeability would provide a high-strength steel sheet that could satisfy the above-mentioned level of demand. It was.

  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 common to the steel sheet of the prior invention in that it has a two-phase structure composed of ferrite and tempered martensite, but is different in the following points.

  That is, the area ratio of the tempered martensite is 40% or more (including 100%) in the above-described steel sheet of the present invention, whereas the steel sheet of the present invention has 70% or more (100% of the martensite side). Including).

  Further, the hardness of the tempered martensite is 300 to 380 Hv in the above-described prior invention steel plate, whereas the hardness of the tempered martensite is controlled to be higher than 380 and 450 Hv or less on the higher hardness side in the steel plate of the present invention.

Furthermore, the distribution state of the cementite particles precipitated in the tempered martensite is 10 or more per 1 μm ^{2 of} tempered martensite, in the above-mentioned prior invention steel sheet, the cementite particles having an equivalent circle diameter of 0.02 μm or more and less than 0.1 μm. The number of cementite particles having an equivalent circle diameter of 0.1 μm or more is 3 or less per 1 μm ^{2 of} tempered martensite, whereas in the steel sheet of the present invention, the equivalent circle diameter of 0.02 μm or more and 0.0. The number of cementite particles less than 1 μm is 20 or more per 1 μm ^{2 of} tempered martensite, and the number of cementite particles having an equivalent circle diameter of 0.1 μm or more is 1.5 or less per 1 μm ^{2 of} tempered martensite.

<Tempered martensite with a hardness of more than 380 Hv and not more than 450 Hv: 70% or more (including 100%) in area ratio>
In order to secure a higher strength than the above-described prior invention steel plate, the tempered martensite has a hardness exceeding 380 Hv higher than that of the above prior invention steel plate (380 Hv or less), and the area ratio thereof is the above prior invention steel plate (40% or more). ) Greater than 70%. The balance is ferrite. However, if the hardness of the tempered martensite is too high, the elongation and stretch flangeability are deteriorated.

  A preferable range of the hardness of the tempered martensite is more than 380 Hv and not more than 420 Hv. Moreover, the preferable range of the area ratio of tempered martensite is 85 to 95% (the upper limit of the preferable range is 95% because ferrite is preferably present in order to ensure elongation).

<Cementite particles having an equivalent circle diameter of 0.02 μm or more and less than 0.1 μm: 20 or more per 1 μm ^{2 of} tempered martensite,
Cementite particles with an equivalent circle diameter of 0.1 μm or more: 1.5 particles or less per 1 μm ^{2 of} tempered martensite>
Both elongation and stretch flangeability can be improved by controlling the size and number of cementite particles precipitated in martensite during tempering. In other words, a large amount of moderately fine cementite particles are dispersed in martensite to increase the work hardening index by acting as a growth source of dislocations, contributing to improvement in elongation, and at the time of deformation of the stretch flange, Stretch flangeability can be improved by reducing the number of coarse cementite particles as starting points.

In order to effectively exhibit the above action, 20 or more, preferably 25 or more, more preferably 30, suitably fine cementite particles having an equivalent circle diameter of 0.02 μm or more and less than 0.1 μm per 1 μm ^{2 of} tempered martensite. The number of coarse cementite particles having an equivalent circle diameter of 0.1 μm or more is limited to 1.5 or less, preferably 1.3 or less, more preferably 1.0 or less, per 1 μm ^{2 of} tempered martensite. To do.

  The reason why the lower limit of the equivalent circle diameter of the moderately fine cementite particles is set to 0.02 μm is that the finer cementite particles do not give sufficient strain to the martensite crystal structure, and the growth source of dislocations. It is because it is thought that it hardly contributes.

  Hereinafter, a method for measuring the hardness of tempered martensite and its area ratio, and the size and number of the cementite particles will be described. However, the method is the same as the method described in Patent Document 3 that discloses the above-described prior invention steel sheet. is there.

  First, regarding the area ratio of martensite, each test steel sheet was mirror-polished, corroded with a 3% nital solution to reveal the metal structure, and then a scanning type with a magnification of 20000 times for approximately 4 μm × 3 μm region 5 fields of view. An electron microscope (SEM) image was observed, and the area ratio of martensite was calculated from the area ratio of each area, with the area not containing cementite being ferrite and the remaining area being martensite by image analysis.

  Next, regarding the hardness of martensite, the Vickers hardness (98.07N) Hv of the surface of each test steel sheet is measured according to the test method of JIS Z 2244, and the hardness of martensite is expressed using the following formula (1). Conversion to HvM was performed.

HvM = (100 × Hv−VF × HvF) / VM (1)
However, HvF = 102 + 209 [% P] +27 [% Si] +10 [% Mn] +4 [% Mo] −10 [% Cr] +12 [% Cu] (Toshio Fujita et al .: “Design and Theory of Steel Materials” ( Maruzen Co., Ltd., published on September 30, 1981, p.10, Fig. 2.1, reads the degree of influence of each alloy element amount on the change in yield stress of low C ferritic steel (straight line) (Note that other elements such as Al and N do not affect the hardness of the ferrite.)
Here, HvF: hardness of ferrite, VF: area ratio (%) of ferrite, VM: area ratio (%) of martensite, [% X]: content (mass%) of component element X.

Regarding the size and the number of the cementite particles, each sample steel plate was mirror-polished and corroded with 3% nital to reveal the metal structure, and then the region inside the martensite was analyzed in a 100 μm ² region. A scanning electron microscope (SEM) image with a magnification of 10,000 times is observed for the field of view, and a white portion is marked as a cementite particle from the contrast of the image and marked, and the area of each marked cementite particle is circled by image analysis software. The equivalent diameter was calculated, and the number of cementite particles of a predetermined size existing 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 martensite and the amount of cementite precipitated in the martensite and affects the strength and stretch flangeability. If it is less than 0.03%, the strength cannot be ensured. On the other hand, if it exceeds 0.30%, the martensite hardness becomes too high, 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: 0.1-3.0%
Si has the effect of suppressing the coarsening of cementite particles during tempering, and prevents the formation of coarse cementite particles while increasing the number of moderately fine cementite particles. It is a useful element that contributes to compatibility. If it is less than 0.10%, the increase rate of coarse cementite particles is excessive with respect to the increase rate of moderately fine cementite particles at the time of tempering. If it is too high, the formation of austenite at the time of heating is inhibited, so the area ratio of martensite cannot be ensured and stretch flangeability cannot be ensured. The range of Si content becomes like this. Preferably it is 0.30-2.5%, More preferably, it is 0.50-2.0%.

Mn: 0.1 to 5.0%
Mn, like Si, has the effect of suppressing the cementite coarsening during tempering, and prevents the formation of coarse cementite particles, while increasing the number of moderately fine cementite particles, It is an element that contributes to both stretch flangeability and is useful for ensuring hardenability. If it is less than 0.1%, the increase rate of coarse cementite particles is excessive with respect to the increase rate of moderately fine cementite particles at the time of tempering. When it is too high, austenite remains at the time of quenching (at the time of cooling after annealing), and the stretch flangeability is deteriorated. The range of Mn content is preferably 0.30 to 2.5%, more preferably 0.50 to 2.0%.

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: 0.1% or less S is also unavoidably present as an impurity element, forms MnS inclusions, and becomes a starting point of a crack when expanding a hole, thereby reducing stretch flangeability. . More preferably, it is 0.01% 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 martensite cannot be secured, and stretch flangeability cannot be secured.

  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: 0.01-1.0% and / or Mo: 0.01-1.0%
These elements are useful elements for increasing the precipitation strengthening amount while suppressing deterioration of stretch flangeability by precipitating as fine carbides instead of cementite. Addition of less than 0.01% of each element cannot effectively exert the above-described effect, while addition of more than 1.0% of each element results in excessive precipitation strengthening and high martensite hardness. It will become too long and flangeability will fall.

Cu: 0.05 to 1.0% and / or Ni: 0.05 to 1.0%
These elements are elements useful for improving the balance between elongation and stretch flangeability because it becomes easy to obtain moderately fine cementite by suppressing the growth of cementite. When less than 0.05% of each element is added, the above-described effects cannot be exhibited effectively. On the other hand, when more than 1.0% of each element is added, austenite remains at the time of quenching, and stretch flangeability is deteriorated. .

Ca: 0.0005 to 0.01% and / or Mg: 0.0005 to 0.01%
These elements are useful elements for improving stretch flangeability by miniaturizing 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. To do.

  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 composition is melted and hot rolled after being formed into a slab by ingot forming or continuous casting. As hot rolling conditions, the finishing temperature of finish rolling is set to Ar ₃ point or higher, and after appropriate cooling, winding is performed in a range of 450 to 700 ° C. 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: [(Ac1 + Ac3) / 2] to 1000 ° C., annealing holding time: 3600 s or less, and then from annealing heating temperature to directly below Ms point 50 ° C./s or more It is better to quench at a cooling rate.

<Annealing heating temperature: [(Ac1 + Ac3) / 2] to 1000 ° C., annealing holding time: 3600 s or less>
This is because the area ratio of martensite that is sufficiently transformed into austenite during annealing and is transformed from austenite during subsequent cooling is ensured to be 70% or more.
If the annealing heating temperature is less than [(Ac1 + Ac3) / 2] ° C., the amount of transformation to austenite is insufficient during annealing heating, so the amount of martensite that is transformed from austenite during subsequent cooling decreases, and the area ratio is 70%. On the other hand, when 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 exceeds 3600 s, productivity is extremely deteriorated, which is not preferable.

<Rapid cooling at a cooling rate of 50 ° C./s or higher to a temperature below Ms>
This is because a martensite structure is obtained by suppressing the formation of a ferrite or bainite structure from austenite during cooling.

  When the rapid cooling is finished at a temperature higher than the Ms point or when the cooling rate is less than 50 ° C./s, bainite is formed, and the strength of the steel sheet cannot be secured.

[Tempering conditions]
As tempering conditions, from the temperature after the annealing cooling to the tempering heating temperature: 500 to 600 ° C., heated at an average heating rate of 10 ° C./s or more, tempering holding time: kept within 30 s, then 10 ° C./s or more. What is necessary is just to cool at an average cooling rate.

  This is because, by tempering at a high temperature for a short time, both the reduction of the martensite hardness due to the reduction of the dislocation density in the martensite and the coarsening of the cementite particles in the martensite are prevented. In order to realize tempering for a short time at a high temperature, it is necessary to raise and lower the temperature at a certain rate.

  If the tempering heating temperature is less than 500 ° C, the tempering is insufficient and the strength is high, but the elongation and stretch flangeability are deteriorated. Elongation and stretch flangeability are reduced. In addition, when the average heating rate and the average cooling rate are less than 10 ° C./s, the cementite particles are coarsened. However, a speed exceeding 20 ° C./s is not necessary because the above effect is saturated.

Steels having the components shown in Table 1 below were melted to produce 120 mm thick ingots.
This was hot rolled to a thickness of 25 mm, and then hot rolled again to a thickness of 3.2 mm. After pickling this, it cold-rolled to 1.6 mm in thickness to make a test material, and heat-treated on the conditions shown in Table 2.

  About each steel plate after heat processing, the area ratio of martensite and its hardness, and the size and the number of the cementite particles were measured by the measurement method described in the above-mentioned section [Mode for Carrying Out the Invention].

  Moreover, about each said steel plate, tensile strength TS, elongation El, and stretch flangeability (lambda) were measured. 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.

  Table 3 shows the measurement results.

  As shown in the table, Steel No. 1, 5, and 10 to 20 all have a tensile strength TS of 1180 MPa or more, and El × λ satisfies 1000% ·% or more, and satisfies the desired level described in the above [Background Art] section. Even in a higher strength class than the above-described prior invention steel sheet, a high-strength cold-rolled steel sheet having an excellent balance between elongation and stretch flangeability was obtained.

  On the other hand, steel No. which is a comparative example. 2-4, 6-9, 21-23 are inferior in any one characteristic.

  Among these comparative examples, Steel No. 2 to 4 and 6 to 9 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, the tensile strength [TS], and the elongation. And / or the balance of stretch flangeability [E1 × λ] is inferior.

  That is, Steel No. In No. 2, since the annealing heating temperature is too low, the martensite area ratio is insufficient (that is, the ferrite area ratio becomes excessive), and the tensile strength is insufficient.

  Steel No. No. 3 has a structure in which bainite is mixed because the annealing cooling rate is too low, and the balance between elongation and stretch flangeability is inferior.

  Steel No. In No. 4, since the tempering heating rate is too low, the cementite particles are coarsened, the stretch flangeability is lowered, and the balance between elongation and stretch flangeability is inferior.

  Steel No. No. 6, because the tempering holding time is too long, the martensite hardness is too low and the tensile strength is insufficient, the cementite particles are coarsened and the stretch flangeability is lowered, and the balance between stretch and stretch flangeability is also poor. Yes.

  Steel No. In No. 7, since the tempering cooling rate is too low, the cementite particles are coarsened, the stretch flangeability is lowered, and the balance between elongation and stretch flangeability is inferior.

  Steel No. In No. 8, since the tempering heating temperature is too low, the martensite hardness cannot be sufficiently lowered, the stretch flangeability is lowered, and the balance between stretch and stretch flangeability is inferior.

  Steel No. Nos. 21 to 23 do not satisfy at least one of the requirements for defining the structure of the present invention because the component composition of the steel is out of the specified range of the present invention, and the tensile strength [TS] and the elongation and elongation. At least one of the balance of flangeability [E1 × λ] is inferior.

  That is, Steel No. No. 21 satisfies all the requirements for defining the structure of the present invention, but the tensile strength is inferior because the C content is too low.

  Steel No. In No. 22, when the Si content is too low, the cementite particles are coarsened, the stretch flangeability is lowered, and the balance between elongation and stretch flangeability is inferior.

Steel No. No. 23 cannot sufficiently reduce the martensite hardness because the C content is too high, the elongation is lowered, and the balance between elongation and stretch flangeability is inferior.

Claims (4)

% By mass (hereinafter the same for chemical components)
C: 0.03 to 0.30%,
Si: 0.1 to 3.0%,
Mn: 0.1 to 5.0%,
P: 0.1% or less,
S: 0.1% or less,
N: 0.01% or less,
Al: 0.01 to 1.00%
And the remainder has a component composition consisting of iron and inevitable impurities,
A tempered martensite having a hardness of more than 380 and not more than 450 Hv includes an area ratio of 70% or more (including 100%), and the balance has a structure made of ferrite,
The distribution state of cementite particles in the tempered martensite is
The cementite particles having an equivalent circle diameter of 0.02 μm or more and less than 0.1 μm are 20 or more per 1 μm ^{2 of the} tempered martensite,
A cementite particle having an equivalent circle diameter of 0.1 μm or more is 1.5 or less per 1 μm ^{2 of the} tempered martensite. A high-strength cold-rolled steel sheet having an excellent balance between elongation and stretch flangeability. Ingredient composition further
Cr: 0.01-1.0% and / or Mo: 0.01-1.0%
The high-strength cold-rolled steel sheet having an excellent balance between elongation and stretch flangeability according to claim 1. Ingredient composition further
Cu: 0.05-1.0% and / or Ni: 0.05-1.0%
The high-strength cold-rolled steel sheet having an excellent balance between elongation and stretch flangeability according to claim 1 or 2. Furthermore,
Ca: 0.0005 to 0.01% and / or Mg: 0.0005 to 0.01%
The high-strength cold-rolled steel sheet excellent in the balance of elongation and stretch flangeability according to any one of claims 1 to 3.