JP5457840B2 - High strength cold-rolled steel sheet with excellent elongation and stretch flangeability - Google Patents

Description

  The present invention relates to a high-strength steel plate excellent in workability, and more particularly, to a high-strength steel plate with enhanced 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 Sex is also required.

  Therefore, it is possible to provide a high-strength steel sheet having both a tensile strength of 980 MPa class or higher (total elongation; referred to as El in this specification) and stretch flangeability evaluated by a hole expansion ratio (λ). Longed for.

  In response to the above needs, many high-strength steel sheets with improved balance between elongation and stretch flangeability have been proposed based on various structural control concepts, but in recent years, there has been a growing demand for the above balance improvement. The need for a high-strength steel sheet that has a tensile strength of 980 MPa or more, an elongation of 12% or more, and a hole expansion ratio of 100% or more is becoming apparent. However, the present situation is that the products satisfying this demand level have not yet been completed.

  For example, Patent Document 1 discloses a high-tensile cold-rolled steel sheet that contains at least one of Mn, Cr, and Mo in a total amount of 1.6 to 2.5% by mass and is substantially composed of a single-phase structure of martensite. Although the hole expansion ratio (stretch flangeability) is 100% or more, since the ferrite phase is almost absent, the elongation does not reach 10%.

  Patent Document 2 discloses a high-tensile steel sheet having a dual phase structure of ferrite with an area ratio of 65 to 85% and the balance being tempered martensite, and the elongation is about 10% or more. However, the hole expansion rate does not reach 100%.

  Patent Document 3 discloses a high-tensile steel plate having a two-phase structure in which the average crystal grain sizes of ferrite and martensite are both 2 μm or less and the volume ratio of martensite is 20% or more and less than 60%. . However, there is no example where the elongation shows about 10% or more and the hole expansion rate is 100% or more.

JP 2002-161336 A Japanese Patent Laid-Open No. 2004-256872 JP 2004-232022 A

  Accordingly, an object of the present invention is to provide a high-strength cold-rolled steel sheet having excellent formability that has a tensile strength of 980 MPa or more and satisfies both elongation of 12% or more and stretch flangeability of 100% or more.

The invention described in claim 1
% By mass (hereinafter the same for chemical components)
C: 0.03 to 0.30%,
Si: 3.0% or less (including 0%)
Mn: 0.5 to 5.0%,
P: 0.1% or less,
S: 0.005% 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,
In area ratio, ferrite has a structure consisting of tempered martensite of 10% or more and 80% or less, the total of martensite and residual austenite is less than 5% (including 0%), and the balance is hardness 320Hv or more and 450Hv or less,
The ratio of the total area of ferrite having a crystal orientation difference of 5 degrees or less with respect to the adjacent tempered martensite to the total area of all ferrite observed in a 50 μm × 50 μm visual field is 30% or more, It is a high-strength cold-rolled steel sheet excellent in elongation and stretch flangeability.

The invention described in claim 2
Ingredient composition further
Cr: 0.01 to 1.0%,
Mo: 0.01 to 1.0%,
Cu: 0.05 to 1.0%,
Ni: 0.05-1.0%
The high-strength cold-rolled steel sheet having excellent elongation and stretch flangeability according to claim 1, comprising one or more of the following.

The invention according to claim 3
Ingredient composition further
Ca: 0.0005 to 0.01% and / or Mg: 0.0005 to 0.01%
The high-strength cold-rolled steel sheet having excellent elongation and stretch flangeability according to claim 1 or 2.

  According to the present invention, in the multiphase structure mainly composed of ferrite and tempered martensite, by appropriately controlling the area ratio of ferrite, the hardness of tempered martensite, and the crystal orientation difference between ferrite and tempered martensite, It became possible to improve stretch flangeability while securing elongation, and to provide a high-strength steel sheet with better formability.

It is the schematic which shows the temperature pattern of annealing and tempering in the manufacturing method of this invention steel plate.

  The present inventors pay attention to a high-strength steel sheet having a multiphase structure mainly composed of ferrite and tempered martensite (see Patent Documents 2 and 3 above). Considering that high-strength steel sheets that can satisfy the level can be obtained, we have conducted intensive studies such as investigating the effects of various factors on stretch flangeability. As a result, in addition to appropriately controlling the area ratio of ferrite in the matrix structure, the hardness of tempered martensite is reduced, and furthermore, the crystal orientation of ferrite and the surrounding tempered martensite is made closer, thereby extending the flange. The inventors have found that the properties can be improved, 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 multi-phase structure steel mainly composed of ferrite and tempered martensite, which is similar to the steel sheets of Patent Documents 2 and 3, and in particular, the hardened tempered martensite. Is controlled to 320 Hv or more and 450 Hv or less, and is different from the steel sheets of Patent Documents 2 and 3 in that the crystal orientation difference between ferrite and the surrounding tempered martensite is appropriately controlled.

<In area ratio, ferrite is 10% or more and 80% or less, the total of martensite and retained austenite is less than 5% (including 0%), and the balance is tempered martensite>
Ferrite is indispensable for securing the elongation, and it is possible to achieve both the tensile strength and the elongation by using a multiphase structure having an appropriate amount of ferrite and tempered martensite as main structures. On the other hand, an area ratio of ferrite that is too high is not preferable because it leads to a decrease in stretch flangeability.

  In order to effectively exhibit the above action, the area ratio of ferrite is 10% to 80%, preferably 20% to 70%, more preferably 30% to 60%. Although the balance is basically tempered martensite, it is acceptable to include martensite and / or retained austenite as long as the total area ratio is less than 5%. The total area ratio of the martensite and / or retained austenite is preferably less than 3%, more preferably less than 1%.

<Tempered martensite with a hardness of 320Hv to 450Hv>
While ensuring the tensile strength by making the tempered martensite more than a certain degree of hardness, limiting the hardness to a certain degree or less and enhancing the deformability of the tempered martensite, to the interface between ferrite and the tempered martensite This suppresses the stress concentration and makes it difficult for cracks to occur at the interface, thereby improving stretch flangeability.

  In order to effectively exhibit the above action, the tempered martensite has a hardness of 320 Hv to 450 Hv, preferably 320 Hv to 400 Hv, more preferably 320 Hv to 350 Hv.

<The ratio of the total area of ferrite having a crystal orientation difference of 5 degrees or less with respect to the adjacent tempered martensite to the total area of all ferrite observed in a 50 μm × 50 μm field of view is 30% or more>
By controlling the hardness of tempered martensite to a certain range below a certain level as described above, it becomes possible to suppress stress concentration at the interface between ferrite and tempered martensite, preventing cracking and stretch flangeability. It leads to improvement. However, this effect alone is insufficient to obtain the high stretch flangeability aimed at by the present invention, and the desired stretch flangeability can be obtained only by properly controlling the crystal orientation difference between ferrite and the tempered martensite adjacent thereto. can get.

  As the crystal orientation of ferrite and tempered martensite adjacent to it approaches (that is, the crystal orientation difference decreases), strain concentration and dislocation accumulation at the interface between ferrite and tempered martensite are reduced, and cracks are less likely to occur. Therefore, stretch flangeability is improved.

  In order to obtain this effect, the ratio of the total area of ferrite having a crystal orientation difference of 5 degrees or less with respect to the adjacent tempered martensite to the total area of all ferrite observed in a 50 μm × 50 μm visual field is 30% or more. Need to be. Preferably it is 40% or more, more preferably 50% or more. The “ferrite whose crystal orientation difference with adjacent tempered martensite is 5 degrees or less” in the present invention means that among tempered martensite grains adjacent to a certain ferrite, the crystal orientation difference with the ferrite is 5 degrees or less. Is a ferrite having at least one tempered martensite grain.

  Hereinafter, each measuring method of each area ratio of ferrite, tempered martensite, martensite and retained austenite, the hardness of tempered martensite, and the ratio of ferrite whose crystal orientation difference with the adjacent tempered martensite is 5 degrees or less explain.

[Method for measuring area ratios of ferrite, tempered martensite, martensite and retained austenite]
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 with respect to the whole structure by observing and setting the equiaxed area not containing cementite as ferrite by image analysis.

  Next, regarding the total area ratio of martensite and retained austenite, each test steel plate was mirror-polished and corroded using a repelling corrosive solution to reveal the metal structure, and then subjected to a scanning electron microscope (SEM). Observe five fields of view at a magnification of 2,000 times, and the area that appears white from the image contrast by image analysis is martensite and / or retained austenite, and the total area ratio of martensite and retained austenite is calculated from the area ratio of this region to the entire structure. did.

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

[Measurement method of hardness of tempered martensite]
Next, regarding the hardness of the tempered martensite, the Vickers hardness (98.07 N) Hv of the surface of each test steel sheet is measured according to the test method of JIS Z 2244, and the hardness of the martensite is expressed using the following formula (1). Conversion to HvM was performed. The following formula (1) is derived on the assumption that the hardness of martensite and retained austenite is equal to the hardness of tempered martensite.

HvM = (100 × Hv−VF × HvF) / (100−VF) (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, [% X]: content (% by mass) of component element X.

[Method for measuring the proportion of ferrite with a crystal orientation difference of 5 degrees or less from adjacent tempered martensite]
The inside of a 50 μm × 50 μm field of view is observed with a 10,000 times SEM (scanning electron microscope), and for each of all ferrite grains observed in the field of view, the difference in crystal orientation with the adjacent tempered martensite grains is electron By dividing the total area of the ferrite grains measured by the line backscatter diffraction (EBSD) method and having a measured crystal orientation difference of 5 degrees or less by the total area of all the ferrite grains, the tempered martensite adjacent to each other is divided. The proportion of ferrite with a crystal orientation difference of 5 degrees or less was calculated.

  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 affecting the area ratio of martensite and its hardness, and affecting the tensile strength and stretch flangeability. If it is less than 0.03%, the area ratio of martensite is insufficient, so that the tensile strength cannot be secured. On the other hand, if it exceeds 0.30%, the hardness of martensite becomes too high and stretch flangeability cannot be secured. The range of C content is preferably 0.05 to 0.25%, more preferably 0.07 to 0.20%.

Si: 3.0% or less (including 0%)
Si is a useful element that can increase tensile strength without decreasing elongation and stretch flangeability by solid solution strengthening. If it exceeds 3.0%, 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 0.3-2.5%, More preferably, it is 0.5-2.0%.

Mn: 0.5 to 5.0%
Mn increases the tensile strength of the steel sheet by solid solution strengthening, has the effect of improving the hardenability of the steel sheet and promoting the generation of the low-temperature transformation phase, and is an element useful for securing the area ratio of martensite It is. If it is less than 0.5%, sufficient hardenability cannot be ensured, and a sufficient martensite area ratio cannot be ensured during rapid cooling, so that tensile strength cannot be obtained. On the other hand, if it exceeds 5.0%, a large amount of austenite remains and stretch flangeability is deteriorated. The range of the Mn content is preferably 0.7 to 4.0%, more preferably 1.0 to 3.0%.

P: 0.1% or less P is unavoidably present as an impurity element, and contributes to an increase in tensile strength by solid solution strengthening, but segregates at the prior austenite grain boundaries and embrittles the grain boundaries, thereby extending flangeability. 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 contributing to the occurrence of strain aging, thereby preventing the stretch flangeability from deteriorating and contributing to the improvement in tensile strength 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 to 1.0%,
Mo: 0.01 to 1.0%,
Cu: 0.05 to 1.0%,
Ni: 0.05-1.0%
These elements are useful elements that can increase tensile strength without lowering elongation and stretch flangeability by solid solution strengthening. When each element is added below the lower limit, the above-mentioned effects cannot be exhibited effectively. On the other hand, when each element exceeds 1.0%, a large amount of austenite remains during quenching, and stretch flangeability. Reduce.

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, a preferable production method for obtaining the steel sheet of the present invention will be described below. The effects of the steel sheet of the present invention can be obtained if the chemical components and the structure satisfy the above specified range. It is not limited to the manufactured steel plate.

[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, re-annealing, and further tempering are performed (see FIG. 1).

[Annealing conditions]
As annealing conditions, heating temperature (annealing heating temperature) T1: Ac _{3 to} 1000 ° C., holding time (annealing holding time) t1: 3600 s or less, and then from heating temperature T1 to Mf point or more and Ms point or less. Rapid cooling is performed at a cooling rate R1 of 50 ° C./s or higher up to the cooling stop temperature (annealing cooling stop temperature) T2.

<Annealing heating temperature T1: Ac _{3 to} 1000 ° C., annealing holding time t1: 3600 s or less>
Ferrite and martensite that have the desired crystal orientation in the subsequent cooling, reheating / holding, and recooling steps during subsequent annealing, reheating, holding, and recooling by completely transforming to austenite in the heating / holding step during annealing Can be obtained.

When the heating temperature T1 is less than Ac ₃ ° C., ferrite is generated in the heating / holding process. However, since this ferrite hardly has a specific crystal orientation relationship with the ferrite and martensite generated in the subsequent process, the final structure The orientation relationship between ferrite and tempered martensite is not desirable and is not preferable. On the other hand, when the heating temperature T1 exceeds 1000 ° C., the austenite structure becomes coarse, and the ferrite grains in the final structure become coarse, so that stretch flangeability cannot be obtained and the annealing equipment is deteriorated.

  Further, if the annealing holding time t1 exceeds 3600 s, productivity is extremely deteriorated, which is not preferable.

<Rapid cooling at a cooling rate R1 of 50 ° C./s or more to a cooling stop temperature T2 of Mf point or more and Ms point or less>
This is because the formation of a pearlite or bainite structure from austenite during cooling is suppressed, and a martensite structure having a desired crystal orientation relationship with austenite is obtained. Note that the desired structure at the end of the rapid cooling step is a two-phase structure of austenite and martensite.

   When quenching is terminated at a temperature higher than the Ms point, or when the cooling rate is less than 50 ° C./s, bainite is formed, crystal grains of the final structure become coarse, and stretch flangeability cannot be obtained. In addition, if the cooling is performed to a temperature lower than the Mf point, the entire martensite structure is formed, and the effects of the present invention are hardly obtained, and the stretch flangeability is deteriorated.

  At this time, the identification of the Ms point can be easily calculated by an expression as described later. However, for the Mf point, a general calculation expression or the like has not been proposed at present, and the identification is difficult. However, according to Steven et al., “The temperature difference between the Mf point and the Ms point is not much different depending on the steel and is almost 215 ° C.”, and the cooling is terminated at a temperature not lower than the Mf point and not higher than the Ms point in the above process. Considering that the purpose of this is to obtain a two-phase structure of austenite and martensite obtained by cooling from the austenite single-phase structure, the “temperature between the Mf point and the Ms point” is set to “(Ms point. The object can be achieved even if it is replaced by “temperature of −200 ° C. or more and Ms point or less”, preferably “(Ms point −150 ° C.) or more and Ms point or less”.

[Re-annealing conditions]
As re-annealing conditions, heating is performed to a heating temperature (re-annealing heating temperature) T3 of Ac ₁ or more and Ac ₃ or less, holding time (re-annealing holding temperature) t3: 600 s or less, and then Mf point from the heating temperature T3 Rapid cooling is performed at a cooling rate R2 of 50 ° C./s or higher to the following temperature. Here, when reheating to the heating temperature T4 during the re-annealing after the rapid cooling to the cooling stop temperature T2 during the annealing, reheating is performed immediately after the rapid cooling to the cooling stop temperature T2 (that is, t2 = 0). It may be started, or reheating may be started after holding for a certain time t2.

Reheating the Mf point above Ms point or less of the cooling stop temperature T2 to Ac ₁ or Ac ₃ below heating temperature T3, the two-phase structure of austenite and martensite with a specific crystal orientation relationship, a specific crystal orientation relationship It changes into a two-phase structure of austenite and ferrite. At this time, the distribution of the area ratio of austenite and ferrite is determined depending on the heating temperature T3 of Ac ₁ or more and Ac ₃ or less, and the desired distribution of the area ratio can be obtained by controlling this. Then, the desired structure | tissue which makes a ferrite and a martensite the main structure | tissue can be obtained by rapidly cooling by the cooling rate R2 of 50 degrees C / s or more from this heating temperature T3 to the temperature below Mf point.

[Tempering conditions]
As-annealed martensite is very hard and stretch flangeability decreases. In order to ensure stretch flangeability while ensuring tensile strength, it is necessary to make the martensite hardness 320Hv or more and 450Hv or less, and for that purpose, heating temperature (tempering heating temperature) T4: in the temperature range of 300 to 550 ° C. Holding time (tempering holding time) t4: It is necessary to perform tempering (reheating treatment) so as to hold 60 s or more and 1200 s or less.

  If the holding temperature T4 in this tempering process is less than 300 ° C., the martensite is not sufficiently softened, so that the stretch flangeability is deteriorated. On the other hand, when the holding temperature T4 is higher than 550 ° C., the martensite hardness is excessively lowered and the tensile strength cannot be obtained.

  Further, if the holding time t4 in the tempering step is less than 60 s, the martensite is not sufficiently softened, so that the elongation of the steel sheet and the stretch flangeability are deteriorated. On the other hand, if the holding time t4 is longer than 1200 s, the martensite becomes too soft and it becomes difficult to ensure the tensile strength.

  This holding time t4 is preferably 90 s or more and 900 s or less, more preferably 120 s or more and 600 s or less.

  Steels having the components shown in Table 1 below were melted to produce 120 mm thick ingots.

Table 1 also shows the Ac ₁ point, Ac ₃ point, and Ms point of each steel type. Ac ₁ point, Ac ₃ point and Ms point were determined by the following formulas (2) to (4).

Ac ₁ (° C.) = 723 + 29.1 · [Si] −10.7 · [Mn] + 16.9 · [Cr] −16.9 [Ni] (2)
Ac ₃ (° C.) = 910−203 · √ [C] −15.2 · [Ni] + 44.7 · [Si] + 31.5 · [Mo] −30 · [Mn] −11 · [Cr] −20 [Cu] +700 [P] +400 [Al] Formula (3)
Ms (° C.) = 550−361 · [C] −39 · [Mn] −20 · [Cr] −17 · [Ni] −10 · [Cu] −5 · [Mo] + 30 · [Al] 4)
However, [C], [Ni], [Si], [Mo], [Mn], [Cr], [Cu], [P], and [Al] are C, Ni, Si, Mo, Mn, Content (mass%) of Cr, Cu, P, and Al is shown.

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.

  For each steel plate after the heat treatment, the structure was quantified by the measurement method described in the above [Mode for Carrying Out the Invention]. Specifically, for all steel sheets heat-treated under each heat treatment condition shown in Table 2, each area ratio of ferrite, tempered martensite, martensite and retained austenite, hardness of tempered martensite, and adjacent tempered martensite and The proportion of ferrite with a crystal orientation difference of 5 degrees or less was measured.

  Moreover, in order to evaluate mechanical characteristics 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.

  The determination of the test result is the desired level described in the above [Background Art] section, that is, the tensile strength TS is 980 MPa or more, the elongation El is 12% or more, and the stretch flangeability (hole expansion ratio) λ is 100%. Those satisfying all the above conditions were regarded as acceptable.

  Table 3 shows the measurement results.

  As shown in the table, Steel No. 1, 2, 4, 5, 7, 8, 11, 12, 14-20, 22, 23, 25, 26, 29, 30, 32, 33, 36 are all tensile strength TS, elongation El, elongation Both the flangeability (hole expansion ratio) λ satisfied the above conditions and were judged as acceptable, and a high-strength cold-rolled steel sheet having both elongation and stretch flangeability was obtained.

  On the other hand, in comparative steels other than those described above, either the steel plate component or the structure did not satisfy the specified range of the present invention, and the above-mentioned desired level could not be satisfied, resulting in a failure determination.

  Steel No. In No. 3, since the hardness of the tempered martensite was too low due to the C content being too low, the tensile strength TS was insufficient and the test was rejected.

  On the other hand, Steel No. No. 6 is that the C content is too high, the area ratio of ferrite becomes insufficient and the area ratio of other structures becomes excessive, and the tempered martensite hardness becomes too high. Was insufficient and was rejected.

  Steel No. 9 has an area ratio of tempered martensite that is too high due to the Si content being too high, and the area ratio of ferrite is excessive, and the heating temperature T1 during annealing is too high and the ferrite grains are coarsened. The strength TS and stretch flangeability λ were insufficient and the test was rejected.

  Steel No. No. 10 was rejected because the Mn content was too low and the area ratio of tempered martensite was insufficient and the area ratio of ferrite was excessive, resulting in a decrease in tensile strength TS and stretch flangeability λ.

  On the other hand, Steel No. In No. 13, since the area ratio of martensite and / or retained austenite (hereinafter referred to as “other structure”) is excessive because the Mn content is too high, the elongation El and the stretch flangeability λ are insufficient. , Failed.

  Steel No. No. 21 is a ferrite whose area ratio of ferrite becomes excessive due to the heating temperature T1 during annealing being too low, and the crystal orientation difference between adjacent tempered martensites is 5 degrees or less (hereinafter referred to as “tempered martensite”). Since the ratio of “ferrite having a specific crystal orientation relationship”) was insufficient, the tensile strength TS and the stretch flangeability λ were insufficient, resulting in failure.

  On the other hand, Steel No. In No. 24, since the ferrite grains were coarsened due to the heating temperature T1 during annealing being too high, the elongation El and the stretch flangeability λ were lowered, and it was rejected.

  Steel No. No. 27, because the cooling rate R1 during annealing was too low, the area ratio of other structures became excessive, and the ratio of ferrite having a specific crystal orientation relationship with tempered martensite decreased, so that the tensile strength TS Further, the stretch flangeability λ was insufficient, and it was rejected.

  Steel No. No. 28, because the cooling stop temperature T2 at the time of annealing was too high, the proportion of ferrite having a specific crystal orientation relationship with tempered martensite was reduced, so that the stretch flangeability λ was lowered and failed. .

  On the other hand, Steel No. No. 31, because the cooling stop temperature T2 at the time of annealing was too low, the proportion of ferrite having a specific crystal orientation relationship with the tempered martensite was reduced, so the stretch flangeability λ was insufficient and was rejected. .

Steel No. No. 34 was rejected due to insufficient tensile strength TS and stretch flangeability λ because the area ratio of other structures was excessive because the cooling rate R2 during re-annealing was too low.
Steel No. No. 35 was rejected because the tempered martensite hardness was too high because the heating temperature T4 during tempering was too low, and the elongation El and stretch flangeability λ were insufficient.

On the other hand, Steel No. No. 37 was unsuccessful because the tempered martensite hardness was too low because the heating temperature T4 during tempering was too high, resulting in insufficient tensile strength TS.
Steel No. No. 38 was rejected because the tempered martensite hardness was too high because the holding time t4 during tempering was too short, and the elongation El and the stretch flangeability λ were insufficient.

Steel No. No. 39, which is a conventional process, was immediately heated to and maintained at a two-phase region temperature during annealing, and then tempered without re-annealing, and a specific crystal orientation relationship with tempered martensite. Since the proportion of ferrite contained was insufficient, the elongation El and the stretch flangeability λ were insufficient, resulting in failure.

Claims (3)

% By mass (hereinafter the same for chemical components)
C: 0.03 to 0.30%,
Si: 3.0% or less (including 0%)
Mn: 0.5 to 5.0%,
P: 0.1% or less,
S: 0.005% 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,
In area ratio, ferrite has a structure consisting of tempered martensite of 10% or more and 80% or less, the total of martensite and residual austenite is less than 5% (including 0%), and the balance is hardness 320Hv or more and 450Hv or less,
The ratio of the total area of ferrite having a crystal orientation difference of 5 degrees or less with respect to the adjacent tempered martensite to the total area of all ferrite observed in a 50 μm × 50 μm visual field is 30% or more, A high-strength cold-rolled steel sheet with excellent elongation and stretch flangeability. Ingredient composition further
Cr: 0.01 to 1.0%,
Mo: 0.01 to 1.0%,
Cu: 0.05 to 1.0%,
Ni: 0.05-1.0%
The high-strength cold-rolled steel sheet excellent in elongation and stretch flangeability according to claim 1, comprising one or more of the following. Ingredient composition further
Ca: 0.0005 to 0.01% and / or Mg: 0.0005 to 0.01%
The high-strength cold-rolled steel sheet having excellent elongation and stretch flangeability according to claim 1 or 2.

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