JP6417977B2 - Steel plate blank - Google Patents

Description

  The present invention relates to a steel sheet blank, a steel sheet for laser cutting, and a method for manufacturing a steel sheet for laser cutting. More specifically, the present invention relates to a steel plate blank having good elongation and stretch flangeability while having a tensile strength of 440 MPa or more and 780 MPa, a steel plate for laser processing suitable as a material thereof, and a method for manufacturing the same.

From the viewpoint of improving fuel efficiency by reducing the body weight of automobiles for the purpose of reducing CO ₂ emissions and stricter safety standards for collisions, increasing strength of body parts is being promoted. In addition, with the diversification of vehicle body designs, steel sheets that are not only high in strength but also excellent in press formability such as elongation and stretch flangeability have been required from the viewpoint of formability.

  Conventionally, when a press-formed product is manufactured, a steel plate blank, which is a press-forming material, is obtained from a steel plate as a raw material by punching or shearing, and then the required press forming is performed on the steel plate blank. The steel sheet blank obtained by such a shearing process has voids and work hardening in the vicinity of the cut end face, and when subjected to stretch flange molding, it is said that work cracks are likely to occur and propagate easily.

  In recent years, with the development of laser cutting technology, a method of obtaining a steel plate blank having a complicated shape from a steel plate as a material by laser cutting has been proposed. Unlike the steel plate blank obtained by shearing, the steel plate blank obtained by such laser cutting does not cause voids or work hardening at the cut end face, so that an improvement in stretch flangeability can be expected.

  So far, many studies have been reported on the stretch flangeability of steel blanks that have been punched or sheared. However, from the metallurgical point of view, the stretch flangeability of steel blanks that have been subjected to laser cutting has been studied. There are very few examples.

  As an example of reporting the stretch flangeability of a steel sheet blank subjected to laser cutting, Patent Document 1 includes C, Si, Mn, P, S, Cu, Ni, Cr, Mo, Nb, Ti, V, and B, respectively. It is said that excellent stretch flangeability can be obtained by controlling the formula value K determined from the content of 0.09 to 0.5.

  Further, in Patent Document 2, with respect to the martensite in the material structure, by making the martensite with an aspect ratio less than 3.0 more than 95%, it is possible to suppress the plate thickness penetration crack at the initial stage of hole expansion after laser processing. It is said.

JP 61-261462 A JP 2011-225955 A

  However, according to the study of the present inventor, it has been found that the above-described technique has the following problems.

  It was found that excellent stretch flangeability can be exhibited even with a steel sheet exceeding the range of the above-described formula value K defined in Patent Document 1.

  Further, the steel sheet defined in Patent Document 2 is a composite structure steel sheet mainly composed of ferrite and martensite, and does not include a retained austenite phase, and thus does not necessarily satisfy both elongation and stretch flangeability.

  The present invention has been made in view of the above problems, and has a strength of 440 MPa or more and less than 780 MPa while having excellent elongation and stretch flangeability, and laser cutting suitable for such applications. Steel sheet and manufacturing method thereof.

  As a result of intensive studies to solve the above problems, the present inventors optimize the chemical composition and the microstructure of the cut end face part and the base material part for the steel sheet blank having the cut end face that has been laser cut. It has been newly found out that it is possible to have excellent elongation and stretch flangeability.

  Specifically, the total area ratio of martensite and bainite in the cut end face portion having a width of 40 μm from the cut end face to the inside is set to 70% or more. By doing so, the heterogeneous interface that has been the origin of stretch flange cracks is reduced, and the generation and progress of cracks during stretch flange molding is delayed, and as a result, excellent stretch flangeability can be obtained.

  Furthermore, the base material part excluding the cut end face part is a composite structure having a ferrite area ratio of 10 to 90%, a retained austenite area ratio of 1% or more, and the balance being martensite and bainite. Thus, by including a proper amount of austenite, it is possible to obtain excellent elongation due to the TRIP (transformation-induced plasticity) effect.

The present invention has been completed based on the above-mentioned new findings and is as follows.
(1) A steel plate blank having a cut end face subjected to laser cutting,
C: 0.02% to 0.16%, Si: 0.04% to 2%, Mn: 0.4% to 3%, P: 0.05% or less, S: 0 0.02% or less, sol. Al: having a chemical composition of 0.01% or more and 2% or less, the balance being Fe and impurities,
The total area ratio of martensite and bainite in the 40 μm-wide cut end face portion from the cut end face to the inside is 70% or more, the ferrite area ratio in the base material portion excluding the cut end face portion is 10 to 90%, and retained austenite While having a microstructure in which the area ratio is 1% or more and the balance is martensite and bainite,
A steel plate blank having mechanical properties of a tensile strength of 440 MPa or more and less than 780 MPa.

  (2) The chemical composition is selected from the group consisting of Ti: 0.2% or less, Nb: 0.2% or less, and V: 0.2% or less in mass%, instead of a part of the Fe. 1 type or 2 types or more are contained, The steel plate blank as described in (1) term characterized by the above-mentioned.

  (3) The chemical composition is mass% in place of part of the Fe, Cu: 1% or less, Ni: 1% or less, Cr: 1% or less, Mo: 1% or less, and B: 0.1 The steel plate blank according to (1) or (2), which contains one or more selected from the group consisting of% or less.

  (4) The chemical composition is mass% in place of a part of Fe, REM: 0.1% or less, Mg: 0.01% or less, Ca: 0.01% or less, and Bi: 0.1 1 or 2 types or more selected from the group which consists of% or less, The steel plate blank of any one of the term from (1) term to (3) term characterized by the above-mentioned.

  (5) It has the chemical composition described in any one of items (1) to (4), the ferrite area ratio is 10 to 90%, the retained austenite area ratio is 1% or more, and the balance is martensite. And a steel sheet for laser cutting characterized by having a microstructure that is bainite.

(6) The method for producing a steel sheet for laser cutting according to any one of items (1) to (4), comprising the following steps (A) to (C):
(A) Hot rolling is performed on a steel slab having a temperature of 1100 ° C. or higher and 1300 ° C. or lower, and hot rolling is completed at a temperature of 850 ° C. or higher and 950 ° C. or lower to obtain a hot-rolled steel sheet. Rolling process;
(B) A pickling / cold rolling step of subjecting the hot-rolled steel sheet obtained in the step (A) to pickling and cold rolling to form a cold-rolled steel plate; and (C) obtained by the step (B). The cold-rolled steel sheet was heated to a temperature range of {(Ac ₁ point + Ac ₃ point) / 2} or more and (Ac ₃ point + 50 ° C.) or less, and then stayed in the temperature range for 240 seconds or less, and then 2 ° C./second. A continuous annealing step of cooling to a temperature range of 250 ° C. or more and 500 ° C. or less at an average cooling rate of 50 ° C./second or less.

  According to the present invention, a steel plate blank having good elongation and stretch flangeability while having a tensile strength of 440 MPa or more and less than 780 MPa can be obtained. Moreover, the steel plate for laser cutting processing suitable for such a use and its manufacturing method can be obtained. The steel sheet blank and the steel sheet for laser cutting according to the present invention are optimal as materials for structural members used in automobiles and various industrial machines, particularly as structural members represented by automobile seat parts and undercarriage parts.

FIG. 1 is a photograph showing the microstructure in the vicinity of the surface layer of the end face subjected to laser cutting defined in the present invention.

  Below, the manufacturing method of the steel plate blank which concerns on this invention, the steel plate for laser cutting, and the steel plate for laser cutting is demonstrated more concretely. In the following description, “%” regarding the chemical composition is “mass%”.

1. Chemical composition (1) C: 0.02% or more and 0.16% or less C has the effect of increasing the strength of steel and stabilizing austenite. If the C content is less than 0.02%, it is difficult to ensure a tensile strength of 440 MPa or more. Further, the C content in the retained austenite becomes too small, the retained austenite becomes unstable, and the improvement in elongation by TRIP cannot be sufficiently obtained. Therefore, the C content is 0.02% or more. Preferably it is 0.04% or more. On the other hand, if the C content exceeds 0.16%, the laser-processed end face is excessively cured, and the effect of suppressing crack propagation during deformation of the stretch flange becomes small, and the stretch flangeability deteriorates. Therefore, the C content is 0.16% or less. Preferably it is 0.12% or less.

(2) Si: 0.04% or more and 2% or less Si is a ferrite-forming element, promotes ferrite formation in continuous annealing, and promotes concentration of C into austenite in combination with the action of Mn described later. This has the effect of stabilizing austenite. Therefore, it is an effective element for ensuring good ductility by allowing austenite to remain in the base metal part and the steel sheet for laser cutting. Moreover, it has the effect | action which raises the intensity | strength of steel by solid solution strengthening. If the Si content is less than 0.04%, it may be difficult to secure 2% or more retained austenite in the area ratio in the base material portion of the steel sheet blank and the steel sheet for laser cutting. Therefore, the Si content is 0.04% or more. Preferably it is 0.1% or more, More preferably, it is 0.3% or more, Most preferably, it is 0.5% or more.

  On the other hand, if the Si content exceeds 2%, Si-based oxides are generated on the surface, which may cause chemical conversion treatment, non-plating during hot dip galvanizing, or insufficient processing during alloying. There is. Therefore, the Si content is 2% or less. Preferably it is 1.5% or less.

(3) Mn: 0.4% or more and 3% or less Mn is an austenite-forming element, ensures austenite in continuous annealing, and promotes the concentration of C to austenite in combination with the action of Si. This has the effect of stabilizing austenite. Therefore, it is an effective element for ensuring good ductility by allowing austenite to remain in the base metal part and the steel sheet for laser cutting. Moreover, it has the effect | action which raises the intensity | strength of steel by transformation strengthening. If the Mn content is less than 0.4%, it becomes difficult to ensure a tensile strength of 440 MPa or more. Therefore, the Mn content is 0.4% or more. Preferably it is 0.5% or more. On the other hand, if the Mn content exceeds 3%, ferrite formation in continuous annealing is excessively suppressed, and ductility may be reduced. In addition, since the band-like structure develops and the local elongation is significantly reduced, the stretch flangeability may be deteriorated. Therefore, the Mn content is 3% or less. Preferably it is 2.0% or less.

(4) P: 0.05% or less P is an element that is generally contained as an impurity. However, P has an action of increasing the strength of steel by solid solution strengthening, as with Si, and may be actively contained. . However, excessive inclusion causes segregation of P grain boundaries and promotes void formation under deformation of the stretch flange, and thus has the effect of deteriorating stretch flangeability. When the P content exceeds 0.05%, the adverse effect due to the above action becomes significant. Therefore, the P content is 0.05% or less.

(5) S: 0.02% or less S is contained as an impurity, and has an action of forming sulfides in steel and deteriorating stretch flangeability. If the S content exceeds 0.02%, the adverse effect due to the above action becomes significant. Therefore, the S content is 0.02% or less. Preferably it is 0.01% or less.

(6) sol. Al: 0.01% or more and 2% or less Al, like Si, is an element that promotes ferrite transformation in continuous annealing, and has an austenite stabilizing action. sol. If the Al content is less than 0.01%, the above-described effects cannot be obtained. Therefore, sol. Al content shall be 0.01% or more. Preferably it is 0.03% or more. On the other hand, sol. If the Al content exceeds 2%, unrecrystallized ferrite increases and local elongation decreases, so the elongation may deteriorate. Therefore, sol. The Al content is 2% or less. Preferably it is 1% or less.

  The steel plate blank and the steel plate for laser cutting according to the present invention may optionally contain the elements listed below.

(7) One or more selected from the group consisting of Ti: 0.2% or less, Nb: 0.2% or less, and V: 0.2% or less Ti, Nb, and V are C and N It forms carbides and carbonitrides, refines the prior austenite grains, and increases the strength of the steel sheet by precipitation strengthening. In addition, in the vicinity of the cut end face subjected to laser cutting, a heat history of heating and cooling for a short time is received, so that a hardened structure such as martensite and bainite is formed on the cut end face portion. Ti, Nb, and V have the effect of suppressing crack propagation during deformation of the stretch flange in order to refine these hardened structures. Therefore, you may contain 1 type, or 2 or more types of these elements. However, even if these elements are included excessively, the effect of the above action is saturated and uneconomical. Further, since the recrystallization temperature rises and the structure becomes non-uniform, the stretch flangeability may deteriorate. Furthermore, due to the excessive formation of carbides such as TiC and NbC, the C content contained in the austenite becomes too low, and the stability of the austenite is lowered and the elongation may be impaired. Therefore, the content of each element is within the above range. In order to more reliably obtain the effect of the above action, it is preferable to satisfy any of Ti: 0.01% or more, Nb: 0.01% or more, and V: 0.01% or more.

(8) Cu: 1% or less, Ni: 1% or less, Cr: 1% or less, Mo: 1% or less, and B: 0.1% or less selected from the group consisting of Cu or Ni , Cr, Mo and B all have the effect of increasing the hardenability of the steel and increasing the strength. Therefore, you may contain 1 type or 2 types of these elements. However, when any one of these elements is contained beyond the above range, the elongation is remarkably lowered and disadvantageous in terms of cost. Therefore, the content of each element is within the above range. In order to more reliably obtain the effect of the above action, Cu: 0.01% or more, Ni: 0.01% or more, Cr: 0.1% or more, Mo: 0.1% or more, and B: 0.001% It is preferable to satisfy any of the above.

(9) REM: 0.1% or less, Mg: 0.01% or less, Ca: 0.01% or less, and Bi: 0.1% or less selected from the group consisting of 0.1% or less REM (rare earth) Element), Mg and Ca all have the effect of improving elongation and stretch flangeability by finely spheroidizing oxides and sulfides, and Bi reducing solidification segregation. Therefore, you may contain 1 type, or 2 or more types of these elements.

  However, if REM (rare earth element), Mg, and Ca are contained beyond the above range, a large amount of oxides and sulfides are formed in the steel, and on the contrary, elongation and stretch flangeability are deteriorated. Moreover, even if Bi is contained exceeding the above range, the effect of improving the stretch flangeability is saturated, and the cost is unnecessarily increased. Therefore, the content of each element is within the above range. In order to more reliably obtain the effect of the above action, it is preferable to contain 0.0005% or more of any of these elements.

Here, REM refers to a total of 17 elements of Sc, Y, and lanthanoid. In the case of lanthanoid, it is added industrially in the form of misch metal. In the present invention, the content of REM refers to the total content of these elements.
The balance is Fe and impurities.

2. Structure of the cut end face part of the steel sheet blank The total area ratio of martensite and bainite in the cut end face part having a width of 40 μm from the cut end face subjected to the laser cutting process to the inside is 70% or more.

  In the present invention, the structure of the cut end surface portion is defined. By making the cut end face part and the base material part excluding the cut end face part into a predetermined structure, it is possible to achieve both excellent elongation and stretch flangeability. Specifically, the total area ratio of martensite and bainite at the cut end face is 70% or more. An example of the microstructure near the surface layer of such a cut end face is shown in FIG.

  When the total area ratio of martensite and bainite at the cut end face is less than 70%, martensite and bainite and other phases (such as ferrite and retained austenite) are formed at the cut end face when subjected to stretch flange deformation. Stress concentration occurs at the heterogeneous interface, and crack generation and propagation are promoted. The total area ratio of martensite and bainite at the cut end face is preferably 80% or more.

  Further, as described later, high elongation can be imparted by satisfying a predetermined condition for the structure of the base material portion excluding the cut end face portion.

3. Steel blank base material and laser cutting steel plate structure The ferrite area ratio in the base material part excluding the cut end face part is 10 to 90%, the retained austenite area ratio is 1% or more, and the balance is martensite and bainite. .

  Ferrite is an important phase contributing to the improvement of elongation. If the area ratio of ferrite is less than 10%, desired elongation cannot be obtained. Therefore, the ferrite area ratio is 10% or more. Preferably it is 30% or more. On the other hand, if the area ratio of ferrite exceeds 90%, the tensile strength decreases, and it becomes difficult to ensure a tensile strength of 440 MPa or more. Therefore, the ferrite area ratio is 90% or less. Preferably it is 85% or less.

  In addition to the above, retained austenite is contained in an area ratio of 1% or more. Residual austenite is transformed into martensite when deformed by molding or the like and exhibits transformation-induced plasticity, so that high strength and good elongation can be achieved at a high level. If the area ratio of retained austenite is less than 1%, the ductility improving action due to transformation-induced plasticity may not be sufficiently obtained. Therefore, the area ratio of retained austenite is 1% or more. Preferably it is more than 3%, more preferably 5% or more. The upper limit of the area ratio of retained austenite is not particularly limited, but if the area ratio of retained austenite exceeds 25%, the area ratio of martensite in the state after molding becomes high, which causes problems in secondary workability and impact resistance. May come. Therefore, the area ratio of retained austenite is preferably 25% or less.

  The balance structure is martensite and bainite. Martensite and bainite are low-temperature transformation phases containing many dislocations in the structure, and are useful phases or structures for increasing the strength of the steel sheet.

4). Mechanical properties of the base material portion of the steel plate blank and the steel plate for laser cutting processing The base material portion of the steel plate blank and the steel plate for laser cutting processing have a tensile strength of 440 MPa or more and less than 780 MPa.

  If the tensile strength is less than 440 MPa, the strength is low in the first place, the elongation and the stretch flangeability are good, and it is easy to obtain the intended strength and formability without depending on the present invention. Accordingly, the tensile strength is set to 440 MPa or more. The tensile strength is preferably 590 MPa or more. Further, when the tensile strength is 780 MPa or more, the uniform elongation is low, and the application to the body member is limited. Therefore, the tensile strength of the base material portion of the steel plate blank and the steel plate for laser cutting is set to less than 780 MPa. .

5. Method for Producing Laser Cutting Steel Plate The laser cutting steel plate according to the present invention can be produced by the following method.

  The steel having the above-mentioned chemical composition is made into a steel ingot by a continuous casting method after being melted by a known means, or a steel slab by a method in which the steel ingot is made by an arbitrary casting method and then cracked and rolled. It is said. In the continuous casting process, in order to suppress the occurrence of surface defects due to inclusions, it is preferable to cause an external additional flow such as electromagnetic stirring in the molten steel in the mold.

  The steel ingot or steel slab may be subjected to hot rolling after re-heating once cooled, or the steel slab in the high temperature state after the continuous casting or in the high temperature state after the continuous casting, Alternatively, it may be kept hot or subjected to auxiliary heating for hot rolling. In the present invention, this steel ingot or steel slab is called a slab.

  Hot rolling is performed on the slab. The manufacturing conditions of the hot rolling process specified in the present invention will be described below.

[Hot rolling process]
(1) Temperature of slab when subjected to hot rolling: 1100 ° C. or higher and 1300 ° C. or lower Temperature of slab when subjected to hot rolling is set to 1100 ° C. or higher and 1300 ° C. or lower. When the temperature of the slab when subjected to hot rolling falls below 1100 ° C., the band-like structure becomes remarkable and the structure becomes non-uniform. Therefore, the total area ratio of martensite and bainite at the cut end face after laser processing is 70% or more. It becomes difficult to secure good stretch flangeability. Moreover, since the rolling load at the time of hot rolling is increased, production trouble may be caused. Therefore, the temperature of the slab at the time of subjecting to hot rolling shall be 1100 degreeC or more. Preferably it is 1150 degreeC or more. On the other hand, if the temperature of the slab when subjected to hot rolling exceeds 1300 ° C., austenite becomes coarse, the structure of the obtained hot rolled sheet becomes non-uniform, and this structure form is maintained even after cold rolling annealing, and laser cutting After processing, it becomes difficult to make the total area ratio of martensite and bainite at the cut end face part 70% or more, and it becomes difficult to ensure good stretch flangeability. Therefore, the temperature of the slab at the time of subjecting to hot rolling shall be 1300 degrees C or less. Preferably it is 1250 degrees C or less.

(2) Hot rolling completion temperature: 850 ° C. or more and 950 ° C. or less The hot rolling completion temperature is 850 ° C. or more and 950 ° C. or less. When the hot rolling completion temperature is lower than 850 ° C., an unrecrystallized structure remains even after cold rolling annealing, and the structure becomes non-uniform, so that the elongation deteriorates. In addition, since the structure of the cut end face after laser processing becomes non-uniform, stretch flangeability also deteriorates. Moreover, since it involves an increase in hot rolling load, there is a concern of causing operational troubles. Therefore, the hot rolling completion temperature is set to 850 ° C. or higher. Preferably it is 870 degreeC or more. On the other hand, when the hot rolling completion temperature exceeds 950 ° C., the austenite becomes coarse, the structure becomes non-uniform, and it becomes difficult to make the total area ratio of martensite and bainite at the cut end face part 70% or more. It is difficult to ensure a stretchable flange. Accordingly, the hot rolling completion temperature is set to 950 ° C. or lower. Preferably it is 900 degrees C or less.

(3) Winding temperature: 650 ° C. or lower The winding temperature is 650 ° C. or lower. When the coiling temperature exceeds 650 ° C., pearlite is generated, the dissolution of carbides is delayed during cold rolling annealing, and a predetermined amount of retained austenite cannot be obtained, resulting in deterioration of elongation. Further, excessive development of the external scale is caused and pickling properties are deteriorated, which is not preferable in production. Therefore, the coiling temperature is 650 ° C. or less. Preferably it is 600 degrees C or less. The lower limit of the coiling temperature is not particularly defined in the present invention, but by setting the coiling temperature to 400 ° C. or higher, excessive martensite generation can be suppressed, and the load reduction in the subsequent cold rolling process can be reduced. This is preferable. Therefore, the winding temperature is preferably 400 ° C. or higher. More preferably, it is 500 degreeC or more.

[Pickling / Cold rolling process]
Pickling and cold rolling may be carried out according to conventional methods. The rolling reduction in cold rolling is not particularly limited, but is generally in the range of 30 to 70%.

Hereinafter, the manufacturing conditions in the continuous annealing process after cold rolling will be described.
[Continuous annealing process]
(1) Annealing temperature: {(Ac ₁ + Ac ₃ ) / 2} or more (Ac ₃ + 50 ° C.) or less The cold-rolled steel sheet obtained by the pickling / cold rolling process is {(Ac ₁ point + Ac). ₃ points) / 2} and annealing to a temperature range of (Ac ₃ points + 50 ° C.) or less.

When the annealing temperature is lower than {(Ac ₁ point + Ac ₃ point) / 2}, the processed structure generated in the cold rolling process remains after annealing without undergoing sufficient recovery and recrystallization, and stretch and stretch flangeability It causes a significant decline. Therefore, the annealing temperature is set to {(Ac ₁ point + Ac ₃ point) / 2} or more. From the viewpoint of improving the elongation, the annealing temperature is preferably {(Ac ₁ point + Ac ₃ point) / 2 + 30 ° C.} or higher. On the other hand, when the annealing temperature exceeds (Ac ₃ points + 50 ° C.), austenite becomes coarse, the structure becomes non-uniform, and it becomes difficult to make the total area ratio of martensite and bainite at the cut end face part 70% or more, Stretch flangeability decreases. Therefore, the annealing temperature is (Ac ₃ points + 50 ° C.) or less. From the viewpoint of improving elongation and stretch flangeability, the annealing temperature is preferably (Ac ₃ points + 40 ° C.) or less.

(2) Residence time in the temperature range: 240 seconds or less The residence time in the temperature range is 240 seconds or less. If the stay time exceeds 240 seconds, the austenite becomes coarse, the structure becomes non-uniform, and it becomes difficult to make the total area ratio of martensite and bainite at the cut end face part 70% or more. To do. Therefore, the stay time is 240 seconds or less.

  The lower limit of the stay time is not particularly specified, but if the stay time is too short, the remelting of the carbide formed in the hot rolling stage remains in the product without being completely completed, so there is a concern that the elongation and stretch flangeability may be deteriorated. is there. Therefore, the staying time is preferably 10 seconds or longer, and more preferably 30 seconds or longer.

(3) Average cooling rate: 2 ° C./second or more and 50 ° C./second or less After the above stay, cooling is performed at an average cooling rate of 2 ° C./second or more and 50 ° C./second or less to the following cooling stop temperature. When the average cooling rate is less than 2 ° C./second, pearlite is generated during cooling, a desired austenite amount cannot be obtained, and elongation decreases. Therefore, the average cooling rate is 2 ° C./second or more. Preferably, it is 5 ° C./second or more. On the other hand, if the average cooling rate exceeds 50 ° C./sec, a sufficient amount of ferrite cannot be ensured, resulting in a decrease in elongation. Therefore, the average cooling rate is set to 50 ° C./second or less. Preferably it is 30 degrees C / sec or less.

(4) Cooling stop temperature: 250 to 500 ° C. The cooling stop temperature is 250 to 500 ° C. When the cooling stop temperature is lower than 250 ° C., martensite is excessively generated, and it becomes difficult to make the retained austenite 1% or more in terms of area ratio, and it becomes difficult to ensure good elongation. Therefore, the cooling stop temperature is set to 250 ° C. or higher. Preferably it is 300 degreeC or more, More preferably, it is 350 degreeC or more. On the other hand, when the cooling stop temperature exceeds 500 ° C., pearlite is generated, and the amount of retained austenite is less than 2%, so that the elongation deteriorates. Therefore, the cooling stop temperature is set to 500 ° C. or lower. Preferably it is 450 degrees C or less, More preferably, it is 420 degrees C or less.

Specific examples of the present invention will be described below.
Steel having the chemical composition shown in Table 1 was melted and cast in a laboratory vacuum melting furnace. These steel ingots were made into steel pieces having a thickness of 25 mm by hot forging.

  The obtained steel slab was hot-rolled under the conditions shown in Table 2 to obtain a hot-rolled steel sheet having a thickness of 4 mm. The hot-rolled steel sheet was made into a cold-rolling base material having a thickness of 2.8 mm by machining, and then cold-rolled at a reduction rate of 50% to obtain a cold-rolled steel sheet having a thickness of 1.4 mm.

The cold-rolled steel sheet thus obtained was heated to 650 ° C. at a heating rate of 10 ° C./second using a continuous annealing simulator, and then heated to various temperatures shown in Table 2 at a heating rate of 1 ° C./second. After that, soaking was maintained at that temperature. Then, after cooling to various cooling stop temperatures at various cooling rates shown in Table 2, holding at that temperature for 300 seconds, annealing was performed to cool to room temperature. The measurement of Ac ₁ point and Ac ₃ point was performed by heating the above-described cold-rolled steel sheet to 650 ° C. at a heating rate of 10 ° C./sec using a Formaster tester, and then at 950 at a heating rate of 1 ° C./sec. It annealed to ° C and measured Ac ₁ point and Ac ₃ point from the thermal expansion curve during heating.

The following test was implemented with respect to the obtained high strength steel plate. The test results are summarized in Table 3.
(1) Evaluation of steel structure The type of steel structure of the steel sheet was determined by corroding the cross section parallel to the rolling direction of the steel sheet with the Nital reagent, and identifying each structure using an optical microscope and SEM at 1/4 position of the plate thickness. did. The area ratio of ferrite, bainite, and martensite was measured by a point counting method from an image obtained with an optical microscope or SEM. Martensite and bainite were distinguished from each other based on the result of thermal expansion during cooling by Formaster. Regarding the area ratio of retained austenite, 1/4 was obtained from the steel sheet surface layer after chemical polishing, X-ray diffraction was performed, and diffraction intensity was calculated from specific lattice plane peak values of austenite and ferrite to obtain a value.

(2) Microstructure evaluation of the cut end face after laser processing The structure in the cut end face having a width of 40 μm from the cut end face to the inside after laser processing has a hole cross section parallel to the rolling direction from the hole end face after laser processing. The total area ratio of martensite and bainite was measured by a point counting method with respect to the structure of a region consisting of the entire plate thickness and a distance of 40 μm from the end face.

(3) Evaluation of mechanical properties Using the obtained steel sheet, the following tests were performed to evaluate tensile properties and stretch flangeability.

(3-1) Evaluation of tensile properties A JIS No. 5 B tensile test was taken from the rolling parallel direction of each steel plate. The test method was performed according to JIS Z2241, and the yield point YP, tensile strength TS, and elongation El were measured.

(3-2) Evaluation of Stretch Flange Property A 100 mm square base plate was cut out from each steel plate, and a 10 mmφ hole was cut into the center portion of the base plate by laser. Laser cutting conditions were as follows: laser model (Komatsu NTC TLV-510), oscillator (FANUC C2000-Model E), output 1400 W, pulse 2000 Hz, duty 90%, assist gas oxygen, assist gas pressure 0.07 MPa The holes were drilled at a processing speed of 3000 mm / min. A hole expansion test was performed on the base plate. The critical hole expansion ratio (HEL: Hole Expansion Limit) obtained by the hole expansion test was used as an evaluation index of stretch flangeability. The hole expansion test was performed by a method according to the Japan Iron and Steel Federation standard (JFST1001-1996), and each sample was measured three times under the same conditions, and the average value was defined as HEL.

  Specimen No. which is the present invention. Nos. 1 to 11 have a high tensile strength of 440 MPa or more, an excellent elongation of TS × El of 13000 MPa ·% or more, a critical hole expansion ratio (HEL) with respect to the initial hole after laser processing, and TS. TS × HEL, which is the product of No. 1, had excellent stretch flangeability of 40000 MPa ·% or more.

  On the other hand, the test material No. No. 12, since the slab heating temperature was higher than the temperature specified in the present invention, the prior austenite grains became coarse, the total area ratio of martensite and bainite at the cut end face after laser processing was less than 70%, and TS × HEL was less than 40000 MPa ·%, and stretch flangeability was inferior.

  Specimen No. No. 13, because the slab heating temperature was lower than the temperature specified in the present invention, the band-like structure developed and the structure became non-uniform. As a result, the total area ratio of martensite and bainite at the cut end face was 70%. The stretch flangeability was inferior.

  Specimen No. No. 14, since the hot rolling completion temperature was higher than the specified range of the present invention, the prior austenite grains had a coarse and non-uniform structure, and the total area ratio of martensite and bainite at the cut end face after laser processing As a result, the stretch flangeability was inferior.

  Specimen No. No. 15 is because the hot-rolling completion temperature was lower than the specified range of the present invention, so that the unrecrystallized structure remained in the structure and the amount of retained austenite specified in the present invention could not be satisfied. And the stretch flangeability was inferior.

  Specimen No. No. 16, the coiling temperature of hot rolling is higher than the specified range in the present invention, and pearlite is formed, so that re-dissolution of carbides in the cold rolling annealing process is suppressed, and C concentration in the retained austenite does not proceed. Since the amount of retained austenite specified in the present invention could not be satisfied, the elongation was inferior.

  Specimen No. No. 17, since the soaking temperature in the continuous annealing process was higher than the specified temperature in the present invention, the austenite grains became coarse and the structure became non-uniform, so the total area ratio of martensite and bainite at the cut end face was 70%. The stretch flangeability was inferior.

  Specimen No. No. 18, since the soaking temperature in the continuous annealing process was lower than the specified temperature in the present invention, the cold-rolled work structure remained after cold rolling annealing, and the total area ratio of martensite and bainite at the cut end face Was less than 70%, and the elongation and stretch flangeability were inferior.

  Specimen No. No. 19, the soaking time of the continuous annealing process is longer than the specified range of the present invention, the austenite becomes coarse and the structure becomes nonuniform, so that the total area ratio of martensite and bainite at the cut end face becomes less than 70%, Flangeability was inferior.

  Specimen No. No. 20, the average cooling rate up to the cooling stop temperature after holding the soaking in the cold rolling annealing process was larger than the specified range of the present invention, the ferrite amount specified in the present invention could not be secured, and the elongation was inferior.

  Specimen No. No. 21 has an average cooling rate up to the cooling stop temperature after the soaking in the cold rolling annealing process is smaller than the specified range of the present invention, and pearlite formed during cooling, so that the amount of retained austenite specified in the present invention can be obtained. The growth was inferior.

  Specimen No. In No. 22, since the cooling stop temperature in the cold rolling annealing process was lower than the specified range of the present invention, excessive martensite was formed, and the residual γ amount specified in the present invention could not be obtained, and the elongation was inferior.

  Specimen No. In No. 23, since the cooling stop temperature in the cold rolling annealing process was higher than the specified range of the present invention, pearlite was generated, and the amount of retained austenite specified in the present invention could not be obtained, so the elongation was inferior.

  Furthermore, the test material No. No. 24 was not in the steel component range defined in the present invention, so it did not satisfy the tensile strength of 440 MPa or more and less than 780 MPa. In addition, the end face hardness increased, and the stretch flangeability was inferior. It was.

Claims (4)

A steel plate blank having a cut end face that has been laser cut,
C: 0.02% to 0.16%, Si: 0.04% to 2%, Mn: 0.4% to 3%, P: 0.05% or less, S: 0 0.02% or less, sol. Al: having a chemical composition of 0.01% or more and 2% or less, the balance being Fe and impurities,
The total area ratio of martensite and bainite in the 40 μm-wide cut end face portion from the cut end face to the inside is 70% or more, the ferrite area ratio in the base material portion excluding the cut end face portion is 10 to 90%, and retained austenite While having a microstructure in which the area ratio is 1% or more and the balance is martensite and bainite,
A steel plate blank having mechanical properties of a tensile strength of 440 MPa or more and less than 780 MPa. The chemical composition is one selected from the group consisting of Ti: 0.2% or less, Nb: 0.2% or less, and V: 0.2% or less in mass% instead of a part of the Fe. Or the steel plate blank of Claim 1 containing 2 or more types .   Instead of a part of the Fe, the chemical composition is, in mass%, Cu: 1% or less, Ni: 1% or less, Cr: 1% or less, Mo: 1% or less, and B: 0.1% or less. The steel plate blank according to claim 1 or 2, comprising one or more selected from the group consisting of:   Instead of a part of the Fe, the chemical composition is, in mass%, REM: 0.1% or less, Mg: 0.01% or less, Ca: 0.01% or less, and Bi: 0.1% or less. The steel plate blank according to any one of claims 1 to 3, comprising one or more selected from the group consisting of: