WO2015198582A1

WO2015198582A1 - High-strength steel sheet

Info

Publication number: WO2015198582A1
Application number: PCT/JP2015/003114
Authority: WO
Inventors: 長谷川　浩平; 義彦小野; 村井　剛; 櫻井　理孝
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2014-06-23
Filing date: 2015-06-22
Publication date: 2015-12-30
Also published as: JP6048623B2; JPWO2015198582A1

Abstract

Provided is a low-cost high-strength steel sheet that has excellent bendability, with a tensile strength of at least 780 MPa, and that is well-suited for the manufacture of structural members for machines such as automobiles. The high-strength steel sheet has a specific component composition. Oxide inclusions present in the high-strength steel sheet within 100 µm of the surface and having a particle major axis with a length of at least 2 µm satisfy specific conditions. The metallographic structure of the high-strength steel sheet, by volume ratio, is 20–65% of a martensite phase, 40–80% of a ferrite phase, less than 5% (including 0%) of a bainite phase, and less than 5% (including 0%) of an austenite phase. The total volume ratio of metallographic structures other than the martensite phase and the ferrite phase is 10% or less (including 0%). The tensile strength of the high-strength steel sheet is at least 780 MPa. Here, "particle major axis" refers to the major axis of particles evaluated in a cross-section containing the rolling direction of the high-strength steel sheet.

Description

High strength steel plate

The present invention relates to a high-strength steel plate having a tensile strength of 780 MPa or more, and particularly to a high-strength steel plate excellent in bending workability suitable for manufacturing machine structural parts such as automobile structural members and reinforcing members.

In recent years, automobile parts have been required to have characteristics of improving fuel efficiency and protecting passengers by reducing weight. In order to satisfy the requirements for these properties, steel sheets used for automobile parts are required to have high strength. On the other hand, a high-strength steel plate is inferior in workability as compared with a soft steel plate, so that it is difficult to form such as press forming. In particular, a steel sheet having a tensile strength of 780 MPa or more is often processed by foam forming mainly in a bending process mode, and therefore bending workability is important among formability.

For this reason, various studies have been made on means for improving the bending workability of high-strength steel sheets. For example, in Patent Document 1, the chemical component is mass%, C: 0.08 to 0.20%, Si: 0.1 to 1.5%, Mn: 1.5 to 2.5%, P: 0.02% or less, S: 0.002% or less, Al: 0.02 to 0.06%, N: 0.0005% or less, Ca: 0.0005% or less, O: 0.0007% or less And the balance is composed of Fe and inevitable impurities, and the structure is composed of a ferrite phase and a low-temperature transformation generation phase. When the size of inclusions in the structure is expressed by the diameter of a circle corresponding to the area, the diameter is 5 μm or more. An ultrahigh strength cold-rolled steel sheet excellent in bending workability is proposed, characterized by having 25 inclusions / mm ² or less and a tensile strength of 780 MPa or more. In Patent Document 1, the cause of cracks caused by bending is oxide inclusions, Ca is 0.0005% or less for forming oxide inclusions, and 0 for easily forming relatively large inclusions is 0. It is disclosed that excellent bending workability is obtained by reducing N to 0.0005% or less to a very low level while reducing the bending workability to an extremely low level of .0007% or less.

In Patent Document 2, a steel layer has a soft layer with a ferrite volume fraction of 90% or more and a thickness of 10 to 100 μm on the surface layer of the steel sheet, the central structure has a tempered martensite volume ratio of 30% or more, and the remainder is a ferrite phase. An ultra-high strength cold-rolled steel sheet having excellent bendability and stretch flangeability is disclosed.

Further, as techniques for improving the material properties of the steel sheet by controlling the amount and shape of inclusions, for example, there are techniques disclosed in Patent Documents 3 and 4.

Patent Document 3 discloses a high-strength cold-rolled steel sheet in which the metal structure and the amount of inclusions are limited for the purpose of improving stretch flangeability. In Patent Document 3, a tempered martensite having a hardness of 380 Hv or less includes an area ratio of 50% or more (including 100%), and the balance has a structure made of ferrite, and is present in the tempered martensite. The number of cementite particles having a diameter of 0.1 μm or more is 2.3 or less per 1 μm ^{2 of the} tempered martensite, and the inclusion having an aspect ratio of 2.0 or more present in the entire structure is 200 or less per 1 mm ^2. A high-strength cold-rolled steel sheet excellent in certain stretch flangeability has been proposed.

In Patent Document 4, the total of one or two of Ce or La is 0.001 to 0.04%, and (Ce + La) / acid-soluble Al ≧ 0.1 on a mass basis. In addition, a high-strength steel sheet having a chemical component having (Ce + La) / S of 0.4 to 50 and excellent in stretch flangeability and fatigue characteristics has been proposed. In Patent Document 4, MnS, TiS, and (Mn, Ti) S are deposited on fine and hard Ce oxide, La oxide, cerium oxysulfide, and lanthanum oxysulfide generated by deoxidation by addition of Ce and La. However, since the deformation of the deposited MnS, TiS, (Mn, Ti) S is difficult to occur during rolling, the stretched coarse MnS is remarkably reduced in the steel sheet. It is disclosed that the MnS-based inclusions are difficult to become crack initiation points and crack propagation paths. Further, in Patent Document 4, Ce and La added to the Al ₂ O ₃ inclusions generated by Al deoxidation are reduced and decomposed by reducing the Ce and La concentrations according to the acid-soluble Al concentration. It is disclosed that fine inclusions are formed and the alumina-based oxide does not cluster and become coarse.

JP 2002-363694 A JP 2005-273002 A JP 2009-215571 A JP 2009-299137 A

However, the technique described in Patent Document 1 requires a great deal of time and cost in the steel making process in order to reduce Ca, N, and O to the ranges described above. Moreover, in the technique described in Patent Document 2, since the surface layer of the steel sheet is made soft, there is a problem that the fatigue characteristics that are significantly affected by the surface layer hardness are remarkably deteriorated.

In addition, the technique described in Patent Document 3 is to improve the stretch flangeability by controlling the form of MnS inclusions, etc., but has suggestions regarding the control of oxide inclusions that greatly affect bending workability. Not give. Moreover, the technique described in Patent Document 4 is not necessarily effective for improving the bending workability to be improved by the present invention. Further, since the addition of special elements such as Ce and La is necessary, the manufacturing cost is remarkably increased.

The present invention has been made as a result of earnest research in view of such circumstances, and has a tensile strength of 780 MPa or more, suitable for the production of automobile structural members, reinforcing members, and all other mechanical structural members, and has bending workability. An object is to provide a high-strength steel sheet excellent in resistance at a low cost.

The present inventors have studied the factors governing the bending workability of high-strength steel sheets. As a result, it was found that the starting point of cracking during processing was an oxide inclusion having a particle major axis of 2 μm or more existing within 100 μm from the steel sheet surface. In order to ensure excellent bending workability, it is effective to reduce the number of inclusions to 100 or less per 100 mm ^2, and to progress of micro cracks generated during bending, It was clarified that the chemical composition (component composition) of the steel sheet and the metal structure of the steel sheet determined by the heat treatment are affected.

Here, it is effective to remove and reduce the oxide inclusions that are the starting point of cracking in the refining process, but this leads to a significant increase in manufacturing cost. For this reason, it was considered that the oxide inclusions were not removed but made harmless to the bending workability. As a result, it has been clarified that by appropriately controlling the composition of oxide inclusions, bending workability can be improved without excessively reducing the inclusions and the impurity elements that cause the inclusions. In addition, the present inventors have completed the present invention by clarifying the appropriate ranges for the chemical composition and metal structure of the steel sheet to obtain a high-strength steel sheet having a tensile strength of 780 MPa or more and excellent bending workability.

That is, the gist of the present invention is as follows.

[1] By mass%, C: 0.05 to 0.18%, Si: 0.8 to 3.0%, Mn: 1.5 to 3.0%, P: 0.02% or less, S: 0.01% or less, Sol. Al: 0.01 to 0.1%, N: 0.0015 to 0.0050%, O: 0.0008 to 0.0020%, with the balance being composed of iron and inevitable impurities , oxide inclusions or more particle diameter 2μm in the steel sheet within 100μm from the surface is not more than 100 per 100 mm ^2, of the total number of oxide-based inclusions, alumina content: 50 wt% or more Yes, silica content: 20% by mass or less, calcia content: 40% by mass or less of the number ratio of the composition is 80% or more, and the metal structure is in volume ratio, martensite phase: 20 to 65%, ferrite phase: 40 to 80%, bainite phase: less than 5% (including 0%), austenite phase: less than 5% (including 0%), less than martensite phase and ferrite phase A high-strength steel sheet having a tensile strength of 780 MPa or more and excellent bending workability, wherein the total volume ratio of the metal structures is 10% or less (including 0%); It is a particle major axis evaluated by a cross section including a rolling direction.

[2] Further, by mass%, one or more of Cr: 0.01 to 1.0%, Mo: 0.01 to 0.5%, B: 0.0001 to 0.0030% are contained. A high-strength steel sheet excellent in bending workability having a tensile strength of 780 MPa or more as described in [1] above.

[3] Further, by mass%, Ti: 0.001 to 0.1%, Nb: 0.001 to 0.1%, V: 0.001 to 0.1%, Zr: 0.001 to 0.00. 1% of 1 type or 2 types or more, The high-strength steel plate excellent in bending workability whose tensile strength as described in said [1] or [2] is 780 Mpa or more.

[4] Further, by mass%, Cu: 0.01 to 0.5%, Ni: 0.01 to 0.5%, Sn: 0.001 to 0.01%, or one or more of them are contained. A high-strength steel sheet excellent in bending workability, wherein the tensile strength according to any one of [1] to [3] is 780 MPa or more.

According to the present invention, it is possible to provide a high-strength steel sheet having a tensile strength of 780 MPa or more and excellent in bending workability, which is optimal for the manufacture of machine structural members such as automobile structural members and reinforcing members, at low cost. Very useful.

First, the reasons for limiting the component composition (chemical component) in the present invention will be described. In addition, although the unit of content of each element is mass%, hereinafter, unless otherwise specified, it is simply represented by%.

C: 0.05 to 0.18%
C is an important element for strengthening the martensite of the quenched structure. If the C content is less than 0.05%, the effect of increasing the strength is insufficient. For this reason, the amount of C is made into 0.05% or more. Preferably, the amount of C is 0.10% or more. On the other hand, if the amount of C exceeds 0.18%, the strength becomes too high and the bending workability is remarkably deteriorated. Further, since the welded portion is broken in the cross tension test in spot welding, the joint strength is significantly reduced. For this reason, the amount of C is made into 0.18% or less. Preferably, the amount of C is 0.15% or less.

Si: 0.8 to 3.0%
Si is effective for increasing the ductility of the high strength composite steel sheet. If the Si amount is less than 0.8%, the effect is not sufficient. When the Si content is less than 0.8%, the bending workability improving effect by controlling the composition of oxide inclusions, which is a feature of the present invention, is not recognized. For this reason, the amount of Si shall be 0.8% or more. Preferably, the amount of Si is 1.2% or more. On the other hand, when the amount of Si exceeds 3.0%, a large amount of Si oxide is formed on the surface of the steel sheet in the hot rolling process, and surface defects are generated. For this reason, the amount of Si shall be 3.0% or less. From the viewpoint of chemical conversion properties, the Si content is preferably 2.3% or less.

Mn: 1.5 to 3.0%
Mn is an important element for suppressing the formation of ferrite during cooling to the quenching start temperature in continuous annealing. If the amount of Mn is less than 1.5%, the effect of suppressing such ferrite formation is not sufficient. For this reason, the amount of Mn is 1.5% or more. On the other hand, if the amount of Mn exceeds 3.0%, so-called slab cracking occurs in which the steel piece (slab) is cracked in the continuous casting process, so the amount of Mn is set to 3.0% or less. In order to improve the production stability in the continuous annealing process, the Mn content is preferably 1.8% or more, and preferably 2.5% or less.

P: 0.02% or less P is an impurity in the steel of the present invention, and is desirably removed by a steel making process as much as possible in order to deteriorate spot weldability. Here, when the P content exceeds 0.02%, the deterioration of spot weldability becomes remarkable. For this reason, the amount of P needs to be 0.02% or less. Preferably, the amount of P is 0.01% or less.

S: 0.01% or less S is an impurity in the steel of the present invention, and is desirably removed in the steelmaking process as much as possible in order to deteriorate spot weldability and bending workability. Here, when the amount of S exceeds 0.01%, the spot weldability deteriorates significantly, so the amount of S needs to be 0.01% or less. Preferably, the amount of S is 0.002% or less.

Sol. Al: 0.01 to 0.1%
Al is added to deoxidize and precipitate N as AlN. Sol. If the amount of Al is less than 0.01%, the effect of deoxidation / denitrification is not sufficient. For this reason, Sol. The amount of Al is 0.01% or more. Preferably, Sol. The amount of Al is 0.03% or more. On the other hand, Sol. If the amount of Al exceeds 0.1%, the effect of adding Al becomes saturated and uneconomical. For this reason, Sol. The Al content is 0.1% or less. Preferably, Sol. The amount of Al is 0.06% or less.

Here, Sol. Al is acid-soluble aluminum. The amount of Al is the amount of Al excluding Al existing as an oxide out of the total amount of Al in steel.

N: 0.0015 to 0.0050%
N is an impurity contained in the crude steel, and the N content needs to be 0.0050% or less in order to deteriorate the formability of the steel sheet. Preferably, the N content is 0.0035% or less. On the other hand, if the N content is made less than 0.0015%, the refining cost increases significantly. Further, if the N content is 0.0015% or more, the adverse effect of N on the formability of the steel sheet is small and substantially harmless. For this reason, the N amount is set to 0.0015% or more. Preferably, the N amount is 0.0025% or more.

O: 0.0008 to 0.0020%
O is a metal oxide or the like produced during refining that remains as inclusions in the steel. When the amount of O exceeds 0.0020%, bending workability is remarkably deteriorated. For this reason, the amount of O is made 0.0020% or less. Preferably, the amount of O is 0.0015% or less. On the other hand, if the amount of O is to be less than 0.0008%, the refining cost increases significantly. In the present invention, as will be described later, bending workability can be improved by appropriately controlling the composition of oxide inclusions. Therefore, in order to suppress an increase in refining costs, the O amount is set to 0.0008% or more.

In addition to the chemical components described above, the steel of the present invention may further contain the following chemical components depending on the purpose.

One or more of Cr: 0.01 to 1.0%, Mo: 0.01 to 0.5%, B: 0.0001 to 0.0030% Cr, Mo, and B are continuously annealed. In order to obtain such an effect, one or more of these elements can be contained. Since such effects can be obtained at 0.01% or more, 0.01% or more, and 0.0001% or more, the Cr amount is 0.01% or more, the Mo amount is 0.01% or more, B The amount is 0.0001% or more. Preferably, the Cr amount is 0.1% or more, the Mo amount is 0.05% or more, and the B amount is 0.0003% or more. On the other hand, when Cr, Mo, and B exceed 1.0%, 0.5%, and 0.0030%, respectively, ductility deteriorates. For this reason, the Cr content is 1.0% or less, the Mo content is 0.5% or less, and the B content is 0.0030% or less. Preferably, the Cr content is 0.7% or less, the Mo content is 0.3% or less, and the B content is 0.0020% or less.

One or more of Ti: 0.001 to 0.1%, Nb: 0.001 to 0.1%, V: 0.001 to 0.1%, Zr: 0.001 to 0.1% Ti, Nb, V, and Zr have the effect of improving stretch flangeability by forming carbides and nitrides in steel in the casting and hot rolling processes and suppressing the coarsening of the crystal grain size. In order to obtain an advantageous effect, one or more of these elements can be contained. Such an effect can be acquired by making the content of any additive element 0.001% or more. Therefore, the Ti amount is 0.001% or more, the Nb amount is 0.001% or more, the V amount is 0.001% or more, and the Zr amount is 0.001% or more. On the other hand, if the content exceeds 0.1%, ductility deteriorates due to excessive precipitation strengthening. Therefore, the Ti amount is 0.1% or less, the Nb amount is 0.1% or less, the V amount is 0.1% or less, and the Zr amount is 0.1% or less.

One or more of Cu: 0.01 to 0.5%, Ni: 0.01 to 0.5%, Sn: 0.001 to 0.01% Cu, Ni, and Sn increase the corrosion resistance of the steel sheet. There is an effect, and in order to obtain such an effect, one or more of these elements can be contained. Since such effects can be obtained at 0.01% or more, 0.01% or more, or 0.001% or more, respectively, the Cu content is 0.01% or more, the Ni content is 0.01% or more, Sn The amount is 0.001% or more. On the other hand, if Cu, Ni, and Sn exceed 0.5%, 0.5%, and 0.01%, surface defects are generated due to embrittlement during casting and hot rolling. Therefore, the Cu content is 0.5% or less, the Ni content is 0.5% or less, and the Sn content is 0.01% or less.

In addition, in the steel plate of this invention, components other than the above are Fe and inevitable impurities.

Next, the reason for limitation regarding oxide inclusions will be described.
In the present invention, the number of oxide inclusions having a particle length of 2 μm or more in a steel sheet within 100 μm from the surface of the steel sheet is 100 or less per 100 mm ^2. Of the total number of oxide inclusions, the alumina content is: The number ratio of those having a composition of 50% by mass or more, silica content: 20% by mass or less, and calcia content: 40% by mass or less is 80% or more.

Controlling the form and composition of oxide inclusions within the above ranges is the most important requirement for improving the bending workability of the present invention. Oxide inclusions present on the center side of the plate thickness from the surface of the steel sheet from 100 μm or oxide inclusions having a particle length of less than 2 μm have little influence on the bending workability, and therefore need not be controlled in the present invention. . For this reason, the oxide inclusions within 100 μm from the steel sheet surface and having a particle major axis of 2 μm or more are limited as follows.

When the number of oxide inclusions within 100 μm from the steel plate surface and the particle major axis is 2 μm or more exceeds 100 per 100 mm ² , the bending workability is remarkably deteriorated. For this reason, the number of inclusions is 100 or less per 100 mm ² . In addition, since oxide inclusions are extended by rolling, in the present invention, the size of inclusions is evaluated by a cross section including the rolling direction. In addition, since the distribution of oxide inclusions having a particle major axis of 2 μm or more within a depth direction of 100 μm from the steel sheet surface is generally uniform, the evaluation position may be an arbitrary cross section within 100 μm from the steel sheet surface. However, in the case of non-uniform distribution in the plate thickness direction, the evaluation is made at the depth with the largest number of distributions. Moreover, the evaluation area should just be 100 mm < ² > or more.

In the oxide inclusions, alumina is inevitably contained as a deoxidation product, but alumina alone has a small effect on bending workability. On the other hand, when the alumina content in the oxide inclusions is less than 50% by mass, the oxide has a low melting point, and the oxide inclusions tend to extend during rolling and become a crack starting point during bending. For this reason, the alumina content rate in an oxide inclusion is 50 mass% or more. When silica and calcia coexist with alumina, the oxide has a low melting point, and oxide inclusions extend during rolling and become a starting point of cracking during bending, which deteriorates the bending workability of the steel sheet. . When the content is 20% by mass and exceeds 20% and 40%, respectively, the bending workability is significantly deteriorated. Therefore, the silica content is 20% by mass or less and the calcia content is 40% by mass or less. In addition, as a more preferable inclusion composition, the average composition of oxides in steel in molten steel is mass%, alumina content: 60% or more, silica content: 10% or less, and calcia content: 20%. (The average composition in the molten steel, the average composition of the intermediate product (for example, slab), and the average composition in the high-strength steel sheet are almost the same). At this time, as described above, 80% or more of the total number of oxide inclusions having a particle major diameter of 2 μm or more in the steel plate within 100 μm from the surface of the steel plate to be evaluated satisfies the above composition range. If so, good bending workability can be obtained. For this reason, the number ratio of oxide inclusions satisfying the above composition is 80% or more. That is, the number ratio of oxide inclusions having a composition of alumina content: 50% by mass or more, silica content: 20% by mass or less, and calcia content: 40% by mass or less is 80%. That's it. That is, in the technique of the present invention, the melting point of the oxide inclusions can be increased to 1600 ° C. or higher by controlling the composition of the oxide inclusions as described above. Such high-temperature oxide inclusions do not have extensibility in the hot rolling process in the steel sheet manufacturing process, and therefore the number ratio of oxide inclusions characterized in the present invention is controlled by the above composition control. It can be within the scope of the invention, and further, the bending workability intended by the present invention can be improved. In order to further improve the bending workability, the number ratio is preferably 90% or more. Adjustment of the oxide composition is achieved by adjusting the slag composition of the converter or secondary refining process. That is, when the slag composition is a high basicity slag and the oxide composition is outside the range of the present invention, the Ca concentration in the molten steel tends to be close to within the oxide composition range of the present invention. is there. That is, when the oxide composition is out of the range of the present invention, it tends to be adjusted within the oxide composition range of the present invention by reducing the Ca concentration. Moreover, the average composition of the oxide in steel can be quantitatively determined by cutting out a sample from the slab and using an extraction residue analysis method (for example, Kuraho et al .: Iron and Steel, Vol. 82 (1996), 1017). The upper limit of the alumina content is not particularly limited, but 95% or less is preferable from the viewpoint of melting cost, and the lower limit of the silica content is not particularly limited, but 3% or more is preferable from the viewpoint of melting cost, Although a minimum is not specifically limited, 5% or more is preferable from the point of melting cost.

Next, the reason for limiting the metal structure will be explained.

The steel sheet of the present invention needs to have a substantially two-phase structure of ferrite and martensite. Hereinafter, the definition of the metal structure in the present invention will be described.

Martensite volume ratio: 20-65%
By setting the volume ratio of the martensite phase to 20% or more, it becomes easy to stably secure a strength of 780 MPa or more in terms of tensile strength. More preferably, the volume ratio of the martensite phase is 30% or more. on the other hand. When the volume ratio of the martensite phase exceeds 65%, the bending workability may be remarkably lowered. In order to ensure the bending workability stably, the volume ratio of the martensite phase is set to 65% or less. Preferably, the volume ratio of the martensite phase is 60% or less, more preferably 50% or less. In the present invention, the martensite phase includes a tempered martensite phase that has been tempered.

Ferrite phase volume fraction: 40-80%
As described above, the metal structure of the steel sheet of the present invention is preferably substantially a two-phase structure of ferrite and martensite. That is, it is preferable that the remainder other than the martensite phase is substantially the ferrite phase. For this reason, the volume fraction of the ferrite phase is preferably 40% or more, and preferably 80% or less. By making the remainder other than the above-described martensite phase substantially a ferrite phase, it is possible to industrially facilitate the control of the structure ratio and stabilize the mechanical characteristics.

Bainitic phase: less than 5% (including 0%), austenite phase: less than 5% (including 0%), and the total volume fraction of the metal structure other than martensite phase and ferrite phase is 10% or less (including 0%)
As described above, the present invention preferably has a substantially two-phase structure of ferrite and martensite. It is preferable that a phase containing iron other than these two phases as a main constituent element, that is, a bainite phase and an austenite phase are not included in the metal structure. Because there is, it may be included. In particular, the austenite phase is transformed into hard martensite at the time of bending and becomes the starting point of bending cracking. The bainite phase is more preferably 3% or less. Further, a compound phase containing Fe, that is, a cementite phase, etc. may be contained in the ferrite phase, the martensite phase, and their interfaces. Further, a compound phase derived from an additive element such as AlN or MnS or an impurity element is substantially harmless within the chemical component range of the present invention, and may be included in the metal structure.

In the present invention, as described above, the volume fraction of the bainite phase is less than 5% and the volume fraction of the austenite phase is less than 5%, and the bainite phase and the austenite phase are included, except for the martensite phase and the ferrite phase. If the total volume fraction of the metal structure is 10% or less, the influence is small. In this case, it can be said that the metal structure is substantially a two-phase structure of ferrite and martensite. Therefore, the bainite phase is less than 5% (including 0%), the austenite phase is less than 5% (including 0%), and the total volume ratio of the metal structure other than the martensite phase and the ferrite phase is 10% or less (including 0%). ) Is preferable. More preferably, the total volume ratio of the metal structure other than the martensite phase and the ferrite phase is 5% or less.

Next, a preferred method for producing the steel plate of the present invention will be described.

The steel sheet of the present invention has the above-described component composition, and the average composition of oxides in steel is, in mass%, alumina content: 60% or more, silica content: 10% or less, calcia content: 20% or less. The molten steel adjusted to be a steel slab, the steel slab is hot-rolled, cold-rolled at a rolling rate of 60% or more to form a cold-rolled sheet, and then the cold-rolled sheet is heated to 750 to 870 ° C. Hold for 10 sec or more in the temperature range, then cool to a quenching start temperature of 550 to 750 ° C., cool from the quenching start temperature to an quenching stop temperature of 300 ° C. or less at an average cooling rate of more than 100 ° C./sec, and then 150 It is preferable to manufacture by holding at ˜450 ° C. for 100 to 1000 seconds. Hereinafter, preferable production conditions will be described.

The molten steel in which the composition of steel and the average composition of oxides in steel are adjusted as described above is used as a steel slab such as a slab, and the steel slab is subjected to hot rolling. The molten steel is preferably made into a steel piece by continuous casting. What is necessary is just to perform a hot rolling according to a conventional method. Usually, the steel slab once lowered in temperature is heated to a predetermined temperature before hot rolling. The heating temperature before the hot rolling is preferably set to 1190 ° C. or lower in order to suppress the extension due to high-temperature deformation of the oxide inclusions to make it harmless and further improve the bending workability. On the other hand, if the heating temperature becomes too low, the rolling load increases and the production efficiency decreases, so the heating temperature of the steel slab before hot rolling is preferably 1100 ° C. or higher.

Further, the steel slab obtained as described above is hot-rolled, cooled and wound up according to a conventional method to obtain a hot-rolled sheet. At this time, the winding temperature is preferably 550 ° C. or lower. This is to reduce the distribution of solid solution elements such as Mn during cooling after the hot rolling is completed, thereby reducing the unevenness of the structure. On the other hand, when the coiling temperature is less than 450 ° C., it is difficult to control the temperature by water cooling in the cooling after hot rolling, which is normally performed, and the material variation including bending workability increases. For this reason, it is preferable that winding temperature shall be 450 degreeC or more.

Next, cold rolling with a rolling rate of 60% or more is performed to obtain a cold rolled sheet. By performing cold rolling with a rolling rate of 60% or more, the oxide inclusions in the hot-rolled sheet can be discontinuously divided by cold working, and can be made harmless to bending workability. . In order to obtain the form and composition of the oxide inclusions, which is a feature of the present invention, the cold rolling reduction rate is preferably 60% or more. From the viewpoint of bending workability, the higher the cold rolling rate, the better. On the other hand, when the rolling rate of cold rolling is increased, the productivity is remarkably lowered. Therefore, the rolling rate is preferably 80% or less.

Next, the obtained cold-rolled sheet is heated to 750 to 870 ° C., held in this temperature range for 10 seconds or more, then cooled to a quenching start temperature of 550 to 750 ° C., and an average cooling rate from the quenching start temperature: Cooling is performed to a quenching stop temperature of 300 ° C. or less at a temperature exceeding 100 ° C./sec, and then heat treatment is performed at 150 to 450 ° C. for 100 to 1000 sec. This heat treatment is preferably continuous annealing performed by a continuous annealing furnace. This heat treatment condition will be described in detail below.

Heat to 750 to 870 ° C. and hold for 10 seconds or more in this temperature range The cold-rolled sheet obtained as described above is heated to 750 to 870 ° C. and held in this temperature range. That is, the heating / holding temperature range is 750 to 870 ° C. Hereinafter, the held temperature is also referred to as a soaking temperature. When the heating / soaking temperature is less than 750 ° C., sufficient austenite is not generated, and it is difficult to obtain a tensile strength of 780 MPa or more. Therefore, the heating / soaking temperature is set to 750 ° C. or higher. Preferably, the heating / soaking temperature is 800 ° C. or higher. On the other hand, when the heating / soaking temperature exceeds 870 ° C., the steel structure becomes austenite single phase and the structure becomes coarse, so that elongation and stretch flangeability deteriorate. Therefore, the heating / soaking temperature is set to 870 ° C. or less. Preferably, the heating / soaking temperature is 840 ° C. or lower. In addition, the holding time in this temperature range is 10 sec or more. Hereinafter, the holding time is also referred to as a soaking time. If the soaking time is less than 10 seconds, austenite is not sufficiently generated, and it is difficult to obtain sufficient strength. Preferably, the soaking time is 30 sec or more. In order not to impair the productivity, the soaking time is preferably set to 1200 sec or less.

Cooling to a quenching start temperature of 550 to 750 ° C. After the heating and soaking described above, it is cooled to a quenching start temperature of 550 to 750 ° C. In this process, an appropriate amount of ferrite is generated to improve ductility and adjust strength. For this reason, it is preferable that the cooling to the start of the rapid cooling is slow cooling. By setting the cooling rate in this process to 20 ° C./sec or less, the stability of the product material is further improved. For this reason, it is preferable that this cooling rate shall be 20 degrees C / sec or less. In addition, if the cooling end temperature, that is, the start temperature of the rapid cooling that follows the cooling is less than 550 ° C., the ferrite volume fraction becomes too high and the strength tends to be insufficient. For this reason, the rapid cooling start temperature is set to 550 ° C. or higher. Preferably, the rapid cooling start temperature is 600 ° C. or higher. On the other hand, when the rapid cooling start temperature exceeds 750 ° C., not only the ductility deteriorates but also the flatness of the steel sheet may deteriorate. For this reason, the rapid cooling start temperature is set to 750 ° C. or lower. Preferably, the quench start temperature is 720 ° C. or lower.

Cooling from the rapid cooling start temperature to an average cooling rate of more than 100 ° C / sec and cooling to a quenching stop temperature of 300 ° C or less The quenching is insufficient when the cooling rate from the rapid cooling start temperature to the rapid cooling stop temperature is 100 ° C / sec or less. And the strength tends to be insufficient. For this reason, the cooling rate from the rapid cooling start temperature to the rapid cooling stop temperature is over 100 ° C./sec. In order to stabilize the product material, the cooling rate is preferably 500 ° C./sec or more. On the other hand, when the quenching stop temperature exceeds 300 ° C., a bainite phase is generated or austenite remains, and stretch flangeability is deteriorated. For this reason, the rapid cooling stop temperature is set to 300 ° C. or lower. In order to stabilize the product material, the quenching stop temperature is preferably 100 ° C. or lower.

Hold at 150 to 450 ° C. for 100 to 1000 sec. Rapidly cool to the quenching stop temperature as described above, and then hold as it is or after reheating at 150 to 450 ° C. for 100 to 1000 sec. By holding at 150 to 450 ° C. in this way, the martensite generated by the previous rapid cooling is tempered and bending workability is improved. If the holding temperature after the rapid cooling stop is less than 150 ° C., such an effect cannot be sufficiently obtained. Therefore, the holding temperature after the rapid cooling stop is set to 150 ° C. or higher. On the other hand, when the holding temperature exceeds 450 ° C., the strength is significantly reduced, and it becomes difficult to obtain a tensile strength of 780 MPa or more. Therefore, the holding temperature after the rapid cooling stop is set to 450 ° C. or less. Further, if the holding time at 150 to 450 ° C. performed after such a rapid cooling stop is less than 100 sec, the above-described effect that martensite is tempered and bending workability is not sufficiently obtained cannot be obtained. Therefore, the holding time at 150 to 450 ° C. is set to 100 sec or more. On the other hand, when the holding time exceeds 1000 sec, the strength is significantly reduced, and it becomes difficult to obtain a tensile strength of 780 MPa or more. Therefore, the holding time at 150 to 450 ° C. is set to 1000 sec or less.

In addition, it is preferable to further perform temper rolling. The temper rolling is preferably performed in the range of 0.1 to 0.7% in order to eliminate yield elongation. The steel sheet of the present invention may be subjected to electroplating or hot dip galvanizing on the surface of the steel sheet, or a solid lubricant may be applied.

Steel ingots having steel components shown in Table 1 were melted and cast. The average composition of oxides in the steel in the molten steel was mass%, and the alumina content was 75 to 85%, the silica content was 3 to 7%, and the calcia content was 10 to 20%. This steel ingot (steel piece) was heated to 1180 ° C. and hot-rolled to a thickness of 2.6 mm. The final pass outlet temperature in hot rolling (end temperature of hot rolling) was about 860 ° C. After hot rolling, cooling at 20 ° C./sec, simulating winding at 540 ° C., holding in a furnace at 540 ° C. for 1 hour, furnace cooling to obtain a hot rolled sheet. Next, cold rolling was performed to obtain a sheet thickness of 1.0 mm (cold rolling rate: 61.5%), and further heat treatment was performed simulating continuous annealing. In the heat treatment simulating this continuous annealing, it was heated at 780 to 830 ° C. at a heating rate of 20 ° C./sec and held for 300 sec. Subsequently, it was cooled to a quenching start temperature of 600 to 700 ° C. at 10 ° C./sec, and then rapidly cooled to a water temperature in jet water having a water temperature of 20 ° C. That is, the quenching stop temperature was the water temperature (20 ° C.). The cooling rate (average cooling rate) at this time was 2000 ° C./sec. Next, it was reheated to 200 to 300 ° C., tempered by holding at 200 to 300 ° C. for 300 to 600 seconds, and after cooling, 0.2% temper rolling was performed.

About the cold-rolled steel sheet obtained as described above, the oxide inclusions are investigated and evaluated as shown below, and the metal structure (structure fraction), tensile properties, bending workability, and chemical conversion properties are evaluated. Investigated and evaluated.

Evaluation of Oxide Inclusions in Steel Sheet A surface parallel to a 50 μm deep plate surface from the steel sheet surface was observed in a range of 10 mm × 10 mm, and the number of inclusion particles having a particle major axis of 2 μm or more was investigated. The plane parallel to the plate surface is a cross section including the rolling direction. In addition, for inclusion particles having a particle length of 2 μm or more, all are subjected to SEM-EDX analysis, and the composition is quantitatively analyzed. The alumina content is 50% by mass or more and the silica content is 20% by mass or less. Yes, calcia content: The number of inclusion particles having a composition of 40% by mass or less (the number corresponding to the composition) was determined. Further, the ratio of the number corresponding to the composition to the total number of inclusion particles having a particle long diameter of 2 μm or more ((composition corresponding number) / (total number of inclusion particles having a particle long diameter of 2 μm or more)) obtained by the above observation was obtained and the composition was determined. Applicable ratio.

Metal structure (structure fraction)
The cross section in the rolling direction was examined by observing a surface at a half position of the plate thickness with a scanning electron microscope (SEM). Observation is carried out at N = 5 (5 observation fields), and the area occupied by each phase existing in a 50 μm × 50 μm square area arbitrarily set by image analysis using a cross-sectional structure photograph of magnification 2000 times. Was obtained and averaged to obtain the volume fraction of each phase. Here, other than the ferrite phase and the pearlite phase, a structure that has a relatively smooth surface and is observed in an island shape as a granular, acicular, and massive shape is determined as a martensite phase or a retained austenite phase, and other. When the remainder was observed, this remainder was made into the bainite phase. Next, the amount of retained austenite phase was determined by X-ray diffraction using Mo Kα rays. That is, using a test piece having a surface near a thickness of 1/4 of the steel sheet as a measurement surface, the peaks of the (211) surface and the (220) surface of the austenite phase and the (200) surface and (220) surface of the ferrite phase The volume fraction of the retained austenite phase was calculated from the strength and used as the volume fraction value. Next, the difference between the volume fraction of the retained austenite phase and the volume fraction of the retained austenite phase was determined as the volume fraction of martensite from the volume fraction of the structure regarded as the martensite phase or the retained austenite phase.

Tensile properties JIS No. 5 test piece (JISZ2201) was sampled with the direction perpendicular to the rolling direction as the longitudinal direction and subjected to a tensile test according to JISZ2241, yield strength (YS), tensile strength (TS), total elongation (El). Asked.

Bending workability Using a JIS No. 3 test piece with the coil width direction as the longitudinal direction, the bending test V block method according to JISZ2248 (the tip angle of the metal fitting: 90 °, the tip radius R: changed from 0.5 mm to 0.5 mm pitch) ) To obtain the limit bending radius (R), and R / t, which is a value divided by the plate thickness (t), was used as an index.

Chemical conversion property Chemical conversion treatment was performed by the following method using a chemical conversion treatment liquid (Palbond L3080 (registered trademark)) manufactured by Nihon Parkerizing. After degreasing with a degreasing liquid Fine Cleaner (registered trademark) manufactured by Nihon Parkerizing Co., Ltd., washing with water, and then adjusting the surface for 30 seconds with surface conditioning liquid preparen Z (registered trademark) manufactured by Nihon Parkerizing Co. After immersing in a liquid (Palbond L3080) for 120 seconds, it was washed with water and dried with warm air. The chemical conversion film thus obtained was randomly observed with a scanning electron microscope (SEM) at a magnification of 500 times, and the scale area ratio of the chemical conversion film was measured by image processing. A scale area ratio of 5% or less was regarded as acceptable (◯), and the chemical conversion treatment property was considered good. In addition, a scale area ratio of more than 5% was determined to be rejected (x).

Table 2 shows the evaluation results. As is apparent from the results, the examples of the present invention have a tensile strength TS ≧ 780 MPa, TS × E1 ≧ 15000 MPa ·%, and a limit bending radius R / t ≦ 1.5, which is excellent in mechanical properties and bending workability. Excellent. Moreover, chemical conversion property was also favorable. On the other hand, the comparative example is inferior in any of the characteristics. For example, the steel plate symbol 1C has a C amount that is too low, and a tensile strength of 780 MPa or more is not obtained. Steel plate symbol 1D is inferior in TS × El and limit bending radius because the amount of C is too high. Since the steel plate symbol 1E has a high ratio of the composition of oxide inclusions outside the range of the present invention, the bending workability deteriorated. Since the steel sheet symbol 1F has an excessively large amount of O, the number of oxide inclusions is larger than the range of the present invention, and the bending workability deteriorates. Since the steel sheet symbol 1H has an excessively low Si content, TS × El decreased, and accordingly, the bending workability deteriorated. Since the steel plate symbol 1I has too much Si, the bendability deteriorated and the chemical conversion property deteriorated. Since the steel plate symbols 1K, 1N, 1O, and 1S have low ratios in which the composition of oxide inclusions falls within the scope of the present invention, bending workability was deteriorated in all cases. Since the steel sheet symbol 1L has an Mn amount that is too low, a tensile strength of 780 MPa or more is not obtained. Since the steel plate symbol 1M has an excessively high Mn content, TS × El decreased, and accordingly, the bending workability deteriorated.

Steel sheets were manufactured using steel slabs (steel pieces) whose chemical components are shown in Table 1. The average composition of the oxide in the slab was determined by an extraction residue analysis method (Kurabo et al .: Iron and Steel, Vol. 82 (1996), 1017). Here, AlN was solution-treated at 1200 ° C. Extraction of oxide inclusions (oxides in steel in the slab) was performed by a bromine-methanol method to obtain Al, Si, Ca and composite oxides, and the composition ratio of each oxide was calculated.

Table 3 shows the steel number of the steel used in Example 2 and the average composition of oxides in the slab. The slabs shown in Table 3 were hot-rolled, cold-rolled, and continuously annealed under the conditions shown in the same table, and further subjected to temper rolling of 0.1 to 0.5% to obtain a cold sheet having a thickness of 1.0 mm. A rolled steel sheet was obtained. The soaking time was 120 to 480 sec. The cold-rolled steel sheet produced in this way was evaluated by investigating oxide inclusions in the same manner as in Example 1, and the metal structure (structure fraction), tensile properties, bending workability, and chemical conversion treatment properties. Was also investigated and evaluated in the same manner as in Example 1. The results are shown in Table 4.

As is apparent from the results shown in Table 4, the steel sheets of the examples of the present invention have a tensile strength TS ≧ 780 MPa, TS × E1 ≧ 15000 MPa ·%, limit bending radius: R / t ≦ 1.5, and tensile properties. Excellent bending workability. Moreover, chemical conversion processability is also pass and it is excellent also in chemical conversion processability.

On the other hand, the steel plate of the comparative example is inferior in either characteristic. For example, steel plate symbols 2A, 2C, and 2F have a low volume ratio of the martensite phase, do not satisfy the metal structure defined in the present invention, and do not have a tensile strength of 780 MPa or more. Steel plate symbol 2D has a high volume ratio of martensite, does not satisfy the metal structure defined in the present invention, TS × E1 decreases, and bending workability deteriorates accordingly. In steel plate symbol 2J, the composition inclusion ratio of oxide inclusions is out of the scope of the present invention, and the composition ratio of oxide inclusions in the product falls within the scope of the present invention, so the bending workability deteriorates. did.

Claims

By mass%, C: 0.05 to 0.18%, Si: 0.8 to 3.0%, Mn: 1.5 to 3.0%, P: 0.02% or less, S: 0.01 % Or less, Sol. Al: 0.01 to 0.1%, N: 0.0015 to 0.0050%, O: 0.0008 to 0.0020%, with the balance being composed of iron and inevitable impurities The number of oxide inclusions having a particle length of 2 μm or more in a steel sheet within 100 μm from the surface is 100 or less per 100 mm 2. Of the total number of oxide inclusions, the alumina content is 50% by mass or more. Yes, silica content: 20% by mass or less, calcia content: 40% by mass or less of the number ratio of the composition is 80% or more, and the metal structure is in volume ratio, martensite phase: 20 to 65%, ferrite phase: 40 to 80%, bainite phase: less than 5% (including 0%), austenite phase: less than 5% (including 0%), less than martensite phase and ferrite phase A high-strength steel plate having a total volume ratio of 10% or less (including 0%) and a tensile strength of 780 MPa or more; where the particle major axis is a particle evaluated by a cross section including the rolling direction of the steel plate The major axis.
Further, it contains one or more of Cr: 0.01 to 1.0%, Mo: 0.01 to 0.5%, and B: 0.0001 to 0.0030% by mass%. The high-strength steel sheet according to claim 1, wherein the steel sheet has high strength.
Further, by mass%, Ti: 0.001 to 0.1%, Nb: 0.001 to 0.1%, V: 0.001 to 0.1%, Zr: 0.001 to 0.1% The high-strength steel sheet according to claim 1 or 2, comprising one or more kinds.
Furthermore, it contains one or more of Cu: 0.01 to 0.5%, Ni: 0.01 to 0.5%, Sn: 0.001 to 0.01% by mass%. The high-strength steel sheet according to any one of claims 1 to 3, characterized in that: