JP4947176B2 - Manufacturing method of ultra-high strength cold-rolled steel sheet - Google Patents

Description

The present invention relates to a method for producing an ultra-high-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more, which is used for vehicle body structural members such as automobile center pillars and door impact beams, which are mainly produced by pressing or roll forming. It is.

In recent years, in consideration of global warming caused by an increase in the CO ₂ concentration in the atmosphere, in order to reduce CO ₂ emissions from automobiles that are the source of CO ₂ migration, there has been a strong demand for improved automobile fuel consumption. Yes. Lightening the vehicle body is an effective way to improve the fuel efficiency of automobiles. However, since it is also necessary to ensure the safety of the occupant, it is necessary to ensure the collision safety more than before while reducing the weight of the vehicle body. Therefore, in order to achieve both weight reduction of the vehicle body and ensuring of collision safety, thinning is being promoted by application of a high specific strength material. Recently, ultra high strength thin steel sheets with a tensile strength of 980 to 1180 MPa class are centered. 2. Description of the Related Art It has come to be applied to body structural members represented by pillars and door impact beams.

  However, demands for reducing the weight of the vehicle body are increasing, and further weight reduction of the vehicle body has been studied by adopting a thin steel plate having a strength higher than the 1180 MPa class. In general, as a means of increasing the strength of a thin steel sheet, it is effective to include a martensite phase in the metal structure. In order to achieve high strength with a particularly small amount of added alloy components, the metal structure should be martensite. It is effective to use a single phase. This martensitic single-phase steel sheet has a high yield ratio (yield stress / tensile strength) and is excellent in stretch flangeability even though it is an alloy-saving component, and is therefore very promising as a vehicle structural member. ing.

By the way, the martensitic single-phase steel sheet is an upper critical cooling in which a second phase typified by a ferrite phase and a pearlite phase is not formed in a steel sheet having an austenite phase single-phase structure by soaking at a temperature equal to or higher than the Ac ₃ transformation point. It is generally understood that it is manufactured by cooling to the Ms point or less at a cooling rate that is higher than or equal to the rate (hereinafter, this cooling is also referred to as “quenching”).

However, when a martensitic single-phase steel sheet is obtained by the above production method, volume shrinkage due to rapid cooling from a high temperature above the Ac ₃ transformation point and martensitic transformation that occurs when continuously cooled to below the Ms point. The accompanying volume expansion instantly generates non-uniform internal stress in the steel sheet. And when this internal stress exceeds the yield stress of a steel plate, there exists a problem that a steel plate shape deteriorates and remarkable curvature generate | occur | produces especially in a board width direction.

  The deterioration of the shape of the steel sheet due to the above quenching not only affects the operability in the continuous annealing process and the manufacturability in the subsequent process, but also when the steel sheet is processed into a vehicle structural member by press forming or roll forming. However, it causes problems such as operation troubles in the molding line and adverse effects on the dimensional accuracy of the product. Therefore, in order to stably use the martensitic single-phase steel sheet as a material for the structural member of an automobile body, it is also important that the steel sheet has excellent flatness in addition to high strength. The warp height in the width direction of the product steel plate is desired to be 10 mm or less.

Several improvement techniques have been proposed for the problem of deterioration of the steel plate shape. For example, in Patent Document 1, based on the results of investigating the relationship between the warpage height of a steel sheet after continuous annealing with a tensile strength of 1470 MPa to 1960 MPa and the martensite volume ratio in the metal structure, the metal structure of the steel sheet is described. A technique for obtaining predetermined mechanical properties and an excellent steel plate shape by forming a two-phase structure composed of a martensite phase having a volume ratio of 80 to 97% and the balance consisting of a ferrite phase is disclosed.
In Patent Document 2, after continuous annealing to obtain a martensitic single-phase steel sheet having a tensile strength of 1049 to 1240 MPa, temper rolling is performed so that the average roughness Ra of the steel sheet surface is 1.4 μm or more. The technique of obtaining a favorable steel plate shape by applying is disclosed.

Japanese Patent No. 2528387 JP 2009-79255 A

  However, in the technique of Patent Document 1, no consideration is given to the influence of the steel sheet structure on mechanical properties such as stretch flangeability. That is, in the steel sheet of the above strength level, when the main phase is a martensite phase and a metal structure including a small amount of ferrite phase, a large hardness difference occurs between the hard martensite phase and the soft ferrite phase. It is known that stretch flangeability is reduced. Further, hydrogen embrittlement cracking may be promoted starting from the interface between the martensite phase and the ferrite phase.

  Moreover, since the method of correcting the steel sheet shape by temper rolling as in the technique of Patent Document 2 is not a technique for suppressing the deterioration of the steel sheet shape that occurs during quenching, it is necessary to improve the operability in the continuous annealing process. I can not connect it. In addition, shape correction by temper rolling requires, for example, a very high rolling load for a high-strength steel sheet having a tensile strength of 1320 MPa or more, and a sufficient shape correction effect cannot be obtained with existing rolling equipment. Furthermore, the increase in the surface roughness of the steel sheet is unsuitable for applications that require surface aesthetics, and there is a problem that the fatigue characteristics may be deteriorated due to the increase in the surface roughness.

Therefore, the present invention has been made in view of the above problems, and its purpose is to suppress the deterioration of the shape of the steel sheet during quenching in continuous annealing itself, thereby achieving ultrahigh strength cold rolling having high flatness. It is to propose an advantageous production method of the steel plate.

  The inventors have intensively studied to solve the above problems of the prior art. As a result, the deterioration of the shape of the martensitic single-phase steel sheet caused by volume shrinkage due to high-speed cooling during quenching and volume expansion due to martensitic transformation is caused by cooling from the soaking temperature to the Ms point during quenching during continuous annealing. It is divided into primary cooling that cools to the vicinity immediately above and secondary cooling that cools to the temperature below 100 ° C. from the vicinity immediately above the Ms point, and during that time, the steel sheet is held at the temperature near the Ms point for a predetermined time to equalize the steel sheet temperature. Was found to be effective, and the present invention was completed.

That is, the present invention includes C: 0.05 to 0.40 mass%, Si: 2.0 mass% or less, P: 0.05 mass% or less, S: 0.02 mass% or less, Al: 0.01 to 0.05 mass. %, N: less than 0.005 mass%, Mn: 1.0 to 3.0 mass%, and the steel sheet after cold rolling having a component composition with the balance consisting of Fe and inevitable impurities is subjected to continuous annealing and tensile strength In the method for producing an ultra-high strength cold-rolled steel sheet of 980 MPa or more, in the above-mentioned continuous annealing, the temperature ranges from the soaking temperature not lower than the Ac ₃ transformation point to the temperature range of Ms point to Ms point + 200 ° C. determined by the following formula (1). Primary cooling is performed at an average cooling rate of ℃ / second or higher, and the secondary cooling is performed at an average cooling rate of 100 ° C / second or higher to 100 ° C or lower after being held in the above temperature range for 0.1 to 60 seconds. Super high A method for producing a high strength cold-rolled steel sheet is proposed.
Ms (° C.) = 550-361 × C-39 × Mn-35 × V-20 × Cr-17 × Ni-10 × Cu-5 × (Mo + W) + 15 × Co + 30 × Al (1)
Here, the element symbol in the above formula represents the content (mass%) of each element.

  Moreover, the manufacturing method of the ultra-high-strength cold-rolled steel sheet of the present invention is characterized in that after secondary cooling, reheating is performed and tempering treatment is performed at 100 to 250 ° C. × 120 to 1800 seconds.

  Moreover, the manufacturing method of the ultra high strength cold-rolled steel sheet of the present invention is characterized in that primary cooling and secondary cooling are performed by water cooling.

  In addition to the above component composition, the steel sheet after cold rolling in the production method of the present invention is further Ti: 0.1 mass% or less, Nb: 0.1 mass% or less, B: 0.0005 to 0.0030 mass%. And Cu: It contains 1 type, or 2 or more types chosen from 0.20 mass% or less, It is characterized by the above-mentioned.

  According to the present invention, it is possible to suppress the deterioration of the shape itself that occurs at the time of quenching the steel sheet in the continuous annealing process, so that not only the improvement in manufacturability in the continuous annealing process etc., but also the shape correction cost by temper rolling etc. It also contributes greatly to reduction. The technology of the present invention can also be applied to ultra-high-strength steel sheets having a tensile strength of 1320 MPa or more, which is considered difficult to correct the shape in temper rolling, etc., so that the use of ultra-high-strength martensitic single-phase steel sheets is expanded. Also contributes. Furthermore, according to the present invention, an ultra-high-strength cold-rolled steel sheet having sufficient flatness can be stably obtained, so that the productivity when manufacturing a structural member for automobiles by press molding or roll molding is improved. And can greatly contribute to quality improvement such as dimensional accuracy.

It is a figure explaining the method of measuring the maximum curvature height generate | occur | produced in the steel plate.

First, the basic technical idea of the present invention will be described.
The shape deterioration that occurs in a martensitic single-phase steel sheet during quenching in the continuous annealing process is due to the occurrence of non-uniform stress inside the steel sheet due to volume shrinkage associated with high-speed cooling and volume expansion associated with martensitic transformation. . In general, it is considered that the volume shrinkage accompanying high-speed cooling and the stress generated thereby increase in proportion to the temperature difference between the temperature at which cooling starts and the temperature at which cooling ends. On the other hand, the volume expansion accompanying the martensitic transformation is uniform when the metal structure after the final cooling is a martensite single phase structure. Therefore, when the volume shrinkage accompanying cooling and the stress generated thereby are small, the influence on the steel sheet shape due to quenching can be considered to be only a uniform volume expansion accompanying martensitic transformation, and the temperature range below the Ms point. The effect of the cooling rate on the steel plate shape is considered to be small.

Therefore, it is considered that the difference between the cooling start temperature and the cooling end temperature may be reduced in order to reduce the stress generated in the steel sheet due to the volume shrinkage during quenching. Therefore, the present invention maintains the steel plate temperature at the temperature near the Ms point for a predetermined time after the primary cooling in which the quenching of the steel plate in the continuous annealing process is cooled from the soaking temperature not lower than the Ac ₃ transformation point to the temperature just above the Ms point. If the temperature distribution in the steel plate is made uniform and then secondary cooling is performed from the temperature immediately above the Ms point to 100 ° C. or lower to cause martensitic transformation, this occurs with volume shrinkage during quenching. The idea was developed to reduce stress to the maximum.

Next, the reason for limiting the component composition of the ultra high strength cold rolled steel sheet of the present invention will be described.
C: 0.05-0.40 mass%
C is an element that stabilizes the austenite phase and is an element necessary for ensuring the strength of the steel sheet. When C is less than 0.05 mass%, it is difficult to obtain a martensitic single-phase steel sheet having a desired tensile strength (980 MPa or more). On the other hand, if the amount of C exceeds 0.40 mass%, rolling before the continuous annealing process becomes difficult, and the transformation strain and transformation stress accompanying martensitic transformation may remarkably increase to cause firing cracks. This is not preferable in production. Therefore, in the present invention, C is set in the range of 0.05 to 0.40 mass%. Preferably it is the range of 0.15-0 and 30 mass%.

Si: 2.0 mass% or less Si is a substitutional solid solution strengthening element that is effective for increasing the strength without impairing the workability of the steel sheet. However, since Si is also an element that shifts the Ac ₃ transformation point to the high temperature side, excessive addition of Si is not preferable because it causes an increase in annealing temperature and an increase in annealing cost. Moreover, when Si is added excessively, the scale production | generation by hot rolling will become remarkable, the surface defect of a final product will increase, and it is not preferable also on quality. Therefore, Si is set to 2.0 mass% or less. Preferably it is 1.5 mass% or less.

Mn: 1.0 to 3.0 mass%
Mn is an element that stabilizes the austenite phase and makes it easier to obtain a martensite structure. However, if Mn is less than 1.0 mass%, the hardenability of the steel is not sufficient, and the ferrite phase, the pearlite phase, and the bainite phase start to form early during cooling from the soaking temperature during annealing, and the present invention However, it is difficult to stably obtain the intended martensite single-phase structure. On the other hand, if it is added in an amount exceeding 3.0 mass%, segregation may become remarkable or processability may be deteriorated. In addition, the delayed fracture resistance also decreases. Therefore, Mn is set to a range of 1.0 to 3.0 mass%. Preferably it is the range of 1.5-2.5 mass%.

P: 0.05 mass% or less P is also an element that segregates at the grain boundary and promotes grain boundary destruction. Therefore, P is set to 0.05 mass% or less. Preferably it is 0.02 mass% or less, More preferably, it is 0.01 mass% or less. In addition, from a viewpoint of improving weldability, 0.008 mass% or less is desirable.

S: 0.02 mass% or less Since S becomes a sulfide-based inclusion such as MnS and induces a decrease in impact resistance and delayed fracture resistance, it is desirable that S be as low as possible. Therefore, the upper limit of S is 0.02 mass%. Preferably it is 0.002 mass% or less.

Al: 0.01-0.05 mass%
Al is an element added for deoxidation in the steelmaking process, and it is necessary to add 0.01 mass% or more in order to obtain a sufficient deoxidation effect. On the other hand, when it adds excessively, the inclusion in a steel plate will increase and the ductility will fall. Therefore, Al is set to a range of 0.01 to 0.05 mass%.

N: Less than 0.005 mass% N is an element that forms a nitride. In particular, when the content is 0.005 mass% or more, the decrease in ductility at high and low temperatures due to the formation of nitride increases. Therefore, N is limited to less than 0.005 mass%.

In addition to the above essential elements, Nb, Ti, B and Cu can be added to the ultra-high strength cold-rolled steel sheet of the present invention within the following ranges depending on the purpose.
Nb: 0.1 mass% or less, Ti: 0.1 mass% or less Nb and Ti are effective elements for refining crystal grains and increasing the strength of a steel sheet. However, even if Nb and Ti are added in amounts exceeding 0.1 mass%, the effect is saturated, which is not economically preferable. Therefore, when adding Nb and Ti, respectively, it shall be 0.1 mass% or less.

B: 0.0005 to 0.0030 mass%
B is an element effective for enhancing the hardenability and increasing the steel sheet strength. However, if B is less than 0.0005 mass%, the above-mentioned strength increasing effect cannot be expected. On the other hand, if B exceeds 0.0030 mass%, the hot workability deteriorates, which is not preferable for production. Therefore, when adding B, it is set as the range of 0.0005-0.0030 mass%.

Cu: 0.20 mass% or less Cu stabilizes the austenite phase, makes it easy to obtain a martensite single-phase structure, and forms a concentrated layer on the surface of the steel sheet in a corrosive environment. It is an element that has the effect of suppressing penetration and improving delayed fracture resistance. However, when the addition amount exceeds 0.20 mass%, these effects are saturated. Therefore, it is preferable to add Cu with an upper limit of 0.20 mass%.

  In the ultra-high-strength cold-rolled steel sheet of the present invention, the balance other than the above elements is Fe and inevitable impurities. However, addition of other elements is not rejected as long as the effects of the present invention are not impaired.

Next, the metal structure of the ultra high strength cold rolled steel sheet of the present invention will be described.
The ultra-high-strength cold-rolled steel sheet of the present invention requires that the metal structure is a martensite single phase. However, since the martensite phase may not be generated in the range of 10 μm in the thickness direction from the steel sheet surface due to the influence of decarburization or the like in the manufacturing process, it is necessary to exclude this range. An austenite phase may remain in the steel matrix structure (residual austenite phase). If this retained austenite phase is less than 0.5% in volume ratio, it is regarded as a martensite single phase structure. Can be tricked. In addition, carbides, nitrides, and inclusions are inevitably present in the steel sheet structure, but these are not included in the evaluation when determining whether or not these are martensite single phase structures.

  The ultra-high-strength cold-rolled steel sheet of the present invention has a martensitic single phase as-quenched metal structure, but when subjected to a tempering treatment described later after secondary cooling, it becomes a tempered martensite single-phase structure. . However, even in this case, the residual austenite phase needs to be less than 0.5% in volume ratio.

Next, the manufacturing method of the ultra high strength cold-rolled steel sheet of the present invention will be described.
The manufacturing method of the ultra-high strength cold-rolled steel sheet of the present invention is characterized by the continuous annealing process described below. For the processes before that, that is, from the steel making process to the cold rolling process, a conventionally known manufacturing method is adopted. can do. Hereinafter, the reason for limitation of the continuous annealing process which is the feature of the present invention will be described.

Soaking process In order to obtain the martensite single phase structure intended by the present invention, the steel sheet structure before quenching needs to be an austenite single phase, so the soaking temperature in the continuous annealing is set to the Ac ₃ transformation point or higher. There is a need. Here, the Ac ₃ transformation point was described in “Metal Heat Treatment Technology Handbook 3rd Edition” (Metal Heat Treatment Technology Handbook Editorial Committee: Nikkan Kogyo Shimbun, (1966), p. 137) from the chemical composition of the steel sheet. The following formula (2);
Ac ₃ (° C.) = 910−203 × C ^1/2 + 44.7 × Si−30 × Mn−20 × Cu + 700 × P + 400 × Al + 400 × Ti (2)
Here, the element symbol in the above formula represents the content (mass%) of each element.
Can be used to calculate.
The soaking time above the Ac ₃ transformation point is preferably 30 to 1200 seconds, and more preferably 300 to 900 seconds from the viewpoint of suppressing the annealing cost.

Primary cooling process Generally, it is desirable that the cooling stop temperature in the quenching process be as low as possible. However, when the primary cooling stop temperature is less than the Ms point, stress due to volume shrinkage due to rapid cooling and volume expansion unevenness due to martensitic transformation is generated inside the steel sheet, causing shape deterioration. Therefore, in the present invention, in order to reduce stress generated due to volume shrinkage accompanying cooling, a primary cooling step in which the quenching step is cooled from the soaking temperature to a temperature immediately above the Ms point, and from the vicinity immediately above the Ms point to 100. It was decided to control separately from the secondary cooling step of cooling to below ° C.

  Here, the cooling stop temperature in the primary cooling needs to be in the temperature range of Ms point to Ms point + 200 ° C. in the vicinity immediately above the Ms point. At a temperature lower than the Ms point, the martensitic transformation proceeds, and stress due to volume expansion due to the martensitic transformation is generated, so that the effect of suppressing shape deterioration cannot be obtained. On the other hand, if the cooling is stopped at a temperature exceeding the Ms point + 200 ° C., a second phase such as a ferrite phase or a pearlite phase may be generated in the subsequent holding step, and the subsequent secondary cooling start temperature becomes high. This is because volume shrinkage accompanying secondary cooling is increased and the shape is deteriorated.

In addition, Ms point (Martensite transformation start point) is the following (1) formula from the chemical component of a steel plate;
Ms (° C.) = 550-361 × C-39 × Mn-35 × V-20 × Cr-17 × Ni-10 × Cu-5 × (Mo + W) + 15 × Co + 30 × Al (1)
Here, the element symbol in the above formula represents the content (mass%) of each element.
Can be used to calculate.

  The average cooling rate in the primary cooling needs to be 20 ° C./second or more. This is because, at an average cooling rate of less than 20 ° C./second, a second phase such as a ferrite phase and a pearlite phase is generated before the primary cooling stop temperature is reached, and a martensite single phase structure cannot be obtained.

Holding Step The steel plate after the primary cooling described above needs to be held for 0.1 to 60 seconds in a temperature range of Ms point to Ms point + 200 ° C. which is the primary cooling stop temperature in order to make the temperature in the steel plate uniform. When the holding time in this holding process is shorter than 0.1 seconds, temperature unevenness due to the difference in cooling rate in the plate thickness direction or width direction of the steel plate is not sufficiently eliminated, so it is sufficient for reducing the stress in the steel plate. The effect is not obtained. On the other hand, if the holding time is longer than 60 seconds, a ferrite phase, a pearlite phase, and a bainite phase are generated during holding, and a martensite single phase structure cannot be obtained. Therefore, the holding time in the holding step is in the range of 0.1 to 60 seconds. Preferably it is the range of 2 to 30 seconds.

Secondary cooling step After completion of the holding step, secondary cooling is performed from the primary cooling stop temperature (Ms point to Ms point + 200 ° C) to 100 ° C or lower at an average cooling rate of 100 ° C / second or higher in order to obtain a martensite single phase structure. Cooling needs to be done. When the average cooling rate is less than 100 ° C./second, a second phase such as a ferrite phase, a pearlite phase, or a bainite phase is generated during cooling, and a martensite single phase structure cannot be obtained. The stress generated by the volume shrinkage accompanying cooling that occurs in this step and the volume expansion accompanying martensite transformation reduces the temperature difference from the martensite transformation point by the primary cooling, and the volume shrinkage generated in this step Can be suppressed to a minimum by making the temperature in the steel plate uniform in the holding step and reducing the generation of uneven stress in the steel plate width direction.

  The steel plate subjected to the above-described quenching treatment has a predetermined strength and sufficient flatness, so that it can be made as it is, but in order to improve toughness and workability, 100 You may perform the tempering process for 120-1800 second at the temperature of -250 degreeC. When the tempering temperature is lower than 100 ° C. or when the tempering time is shorter than 120 seconds, the effect of tempering is not sufficiently obtained. On the other hand, when the tempering temperature is higher than 250 ° C. or the tempering time is longer than 1800 seconds, martensite. This is because the softening of the phase proceeds excessively and the strength is remarkably lowered, and the manufacturing cost is increased. More preferable tempering conditions are in the range of 130 to 220 ° C. × 300 to 1200 seconds. The cooling after the tempering treatment is not particularly limited and may be either air cooling or water cooling. In addition, it is preferable to perform this tempering process using the overaging zone of a continuous annealing line.

As a cooling method in continuous annealing, it is desirable to use water cooling in order to achieve uniform cooling and a high cooling rate, but roll cooling, gas cooling, mist cooling (air-water cooling), or the like may be used. In addition, as a method of maintaining the steel sheet temperature in the temperature range of Ms point to Ms point + 200 ° C., it may be a method of immersing in a salt bath or metal bath in which the temperature is adjusted to the primary cooling stop temperature range, in combination with primary cooling. Or you may use the method of reheating to a primary cooling stop temperature range using an induction heating apparatus after a primary cooling stop.
In the present invention, the steel sheet after continuous annealing does not need to be subjected to temper rolling for the purpose of shape correction, but from the viewpoint of surface roughness adjustment and material adjustment of the steel sheet, temper rolling is appropriately performed. Also good.

Thickness of steel of grades A to S having the composition shown in Table 1 is made into a slab, the slab is heated to 1250 ° C., and then finished at a finish rolling temperature of 900 ° C. to obtain a sheet thickness of 2. An 8 mm hot-rolled steel sheet was taken up at a winding temperature of 650 ° C. Then, after pickling the said hot-rolled steel plate and removing a surface scale, it cold-rolled and it was set as the cold-rolled steel plate of board thickness 1.0mm x board width 800-1400mm. Next, the cold-rolled steel sheet is soaked under the conditions described in Table 2 and then subjected to continuous annealing that is quenched through primary cooling, holding, and secondary cooling, or further subjected to tempering treatment. A rolled steel sheet was obtained. In Table 1, the Ms point and Ac ₃ transformation point obtained from the above-described formulas (1) and (2) from the chemical components of each steel type are also shown.

About the various cold-rolled steel plates obtained as described above, the maximum warp height in the width direction was measured by the method shown in FIG. Specifically, the steel plate was placed on a surface plate, and the distance from the surface plate to the lower surface of the steel plate at the position where the height of the steel plate was the highest was measured.
Moreover, the test piece was extract | collected from the said steel plate, and metal structure, a tensile characteristic, and the stretch flange characteristic were evaluated as follows.
(1) Observation of metal structure A test piece is taken from each of the above-mentioned cold-rolled steel sheets, a cross section parallel to the rolling direction is mirror-polished, and a metal structure is revealed by performing a nital etching, and an optical microscope or scanning electron By observing a fine metal structure using a microscope, identifying the types of constituent phases such as martensite phase, tempered martensite phase, and ferrite phase, and binarizing the photographed structure photograph with an image analyzer, The volume ratio of the martensite phase and the second phase was determined. In addition, since there is a possibility that the retained austenite phase exists in the cold-rolled steel sheet, for the steel sheet of the inventive example, an attempt was made to measure the volume ratio of the retained austenite phase by X-ray (Mo-Kα ray) measurement. The abundance was less than 0.5%, and could be regarded as a martensite single phase structure or a tempered martensite single phase structure.
(2) Tensile test JIS No. 5 tensile test specimens were taken from each of the above cold-rolled steel sheets in a direction perpendicular to the rolling direction, and subjected to a tensile test in accordance with JIS Z2241, 0.2% proof stress (PS), tensile strength. (TS) and elongation at break (El) were measured.
(3) Stretch flange characteristics Stretch flange characteristics were evaluated by performing a hole expansion test in accordance with the provisions of the Japan Iron and Steel Federation Standard JFST1001. That is, a 10 mmφ punch hole is made in the test piece taken from each cold-rolled steel sheet, and a crack penetrating the plate thickness is generated using a conical punch with an apex angle of 60 ° so that the burr is on the outside. Hole expansion was performed until the hole expansion rate λ was obtained using the following formula.
λ (%) = {(d−d ₀ ) / d ₀ } × 100
Where d ₀ : initial hole inner diameter (mm), d: hole inner diameter (mm) when cracking occurs

The results are shown in Table 3. No. suitable for the present invention. Each of the steel sheets of Invention Examples 1 to 13 has a martensite single-phase structure (tempered martensite single-phase structure), and the maximum warp height of the warp generated in the steel sheet is 6 mm or less, which is highly flat. In contrast to the No. 1 in which the conventional quenching method was carried out. In Comparative Example 14, the martensite single-phase structure intended by the present invention is obtained, but the maximum warp height of warpage is as large as 23 mm, and sufficient flatness is not obtained. Furthermore, the steel sheet of the present invention has a hole expansion ratio λ, which is an index of tensile characteristics and stretch flange characteristics, having a value equivalent to that of a martensite single phase steel sheet (No. 14) manufactured by a conventional method. Yes.
On the other hand, the cooling rate of the primary cooling step is lower than the range of the present invention. In No. 15, since all austenite phases were transformed into ferrite phases or pearlite phases during primary cooling, no martensite single phase structure was obtained. Similarly, No. 1 in which the primary cooling stop temperature is higher than the range of the present invention. In No. 16, no pearlite phase was generated, but most of the austenite phase was transformed into a ferrite phase, and a predetermined metal structure was not obtained. Moreover, the holding time in the holding step is longer than the range of the present invention. In No. 17, since a large amount of ferrite phase and pearlite phase are generated during the holding step, a predetermined metal structure is not obtained. Moreover, the cooling rate in the secondary cooling step was less than the cooling rate of the present invention. In No. 18, since a ferrite phase and a pearlite phase were generated during cooling from the primary cooling stop temperature to the Ms point, a martensite single phase structure was not obtained.
From the above results, the martensitic single-phase steel sheet according to the present invention can achieve excellent flatness while having the same strength and processing characteristics as the martensitic single-phase steel sheet manufactured by the conventional method. It was confirmed that it was possible.

  The ultra-high-strength martensitic single-phase steel sheet obtained by the present invention produces automotive structural members such as automobile door impact beams and center pillars formed by press molding or roll molding with high productivity and dimensional accuracy. It can make a big contribution.

Claims (4)

C: 0.05-0.40 mass%, Si: 2.0 mass% or less, P: 0.05 mass% or less, S: 0.02 mass% or less, Al: 0.01-0.05 mass%, N: 0.00. Less than 005 mass%, Mn: 1.0-3.0 mass%, ultra-high strength with a tensile strength of 980 MPa or more by continuously annealing a steel sheet after cold rolling having a component composition consisting of Fe and inevitable impurities as the balance In the method for producing a cold-rolled steel sheet, in the above-described continuous annealing, an average cooling of 20 ° C./second or more from a soaking temperature not lower than the Ac3 transformation point to a temperature range of Ms point to Ms point + 200 ° C. obtained by the following formula (1) An ultra-high strength cold-rolled steel sheet characterized by primary cooling at a rate and holding in the above temperature range for 0.1 to 60 seconds, followed by secondary cooling to 100 ° C. or less at an average cooling rate of 100 ° C./s or more. Manufacturing method .
Ms (° C.) = 550-361 × C-39 × Mn-35 × V-20 × Cr-17 × Ni-10 × Cu-5 × (Mo + W) + 15 × Co + 30 × Al (1)
Here, the element symbol in the above formula represents the content (mass%) of each element. The method for producing an ultra-high-strength cold-rolled steel sheet according to claim 1 , wherein after the secondary cooling, reheating is performed and a tempering treatment is performed at 100 to 250 ° C. × 120 to 1800 seconds. The method for producing an ultra high strength cold-rolled steel sheet according to claim 1 or 2 , wherein the primary cooling and the secondary cooling are performed by water cooling. In addition to the above component composition, the steel sheet after cold rolling is further Ti: 0.1 mass% or less, Nb: 0.1 mass% or less, B: 0.0005 to 0.0030 mass%, and Cu: 0.20 mass%. The manufacturing method of the ultra-high-strength cold-rolled steel sheet according to any one of claims 1 to 3 , comprising one or more selected from the following.

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