JP3873579B2 - Manufacturing method of high formability hot-rolled steel sheet - Google Patents

Description

【００３４】
【表２】

【００３５】
【表３】

【００３６】
【発明の効果】
本発明によれば、特殊な成分組成によらず複合組織が微細化され、局部伸びなどの加工性に優れた熱延鋼板を製造することが可能で、産業上、極めて有用である。
【図面の簡単な説明】
【図１】切欠き伸び−強度バランスに及ぼす一次冷却速度の影響を示す図。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a hot-rolled steel sheet, and more particularly to a method for producing a highly workable hot-rolled steel sheet having excellent local elongation.
[0002]
[Prior art]
High-strength hot-rolled steel sheets with a composite structure mainly composed of ferrite and martensite have a good elongation-strength balance and excellent workability, so they are being applied to various structural members and parts such as automobile weight reduction. ing. Recently, with the expansion of the application range, it has been desired to further improve the workability.
[0003]
In order to improve notch elongation characteristics and stretch flangeability in a high strength composite structure, refinement of the composite structure is effective. The composite steel is cooled from the Ar ₃ transformation point or higher to the temperature at which the two phases of ferrite and austenite coexist, and then held at that temperature and ferrite transformation is performed, thereby concentrating C in the austenite phase and then rapidly cooling 2 The austenite phase has a martensitic structure by stage cooling.
[0004]
The refinement of the composite structure is made by such a limitation of the manufacturing process. As prior arts, for example, JP-A-54-65118, JP-A-56-33429, JP-A-60- 121225 gazette etc. are mentioned.
[0005]
JP-A-54-65118 discloses a technique for suppressing the grain growth of ferrite by setting the cooling rate of primary cooling to 80 ° C./second or more, and JP-A-56-33429 discloses a primary cooling start temperature of 720 to 850 ° C. A technology for refining ferrite with a primary cooling rate of 30 to 200 ° C./second, Japanese Patent Application Laid-Open No. 60-121225 performs a cumulative reduction of 45% or more in a temperature range of Ar _{3 to} Ar ₃ + 40 ° C. A technique for finely dispersing martensite is disclosed.
[0006]
[Problems to be solved by the invention]
However, since all of the above-mentioned conventional technologies are based on existing facilities with a small cooling capacity, there is a limit to the refinement of the composite structure and the refinement of the second phase structure, and the recent application range has been expanded. This does not necessarily satisfy the demand for improvement in local elongation.
[0007]
This invention is made | formed in view of the above point, The objective is to provide the manufacturing method of the high strength hot-rolled steel plate excellent in workability, such as local elongation.
[0008]
[Means for Solving the Problems]
In order to achieve the above-mentioned object, the present inventors diligently studied the influence of cooling after finish rolling on the refinement of the composite structure, aiming at the manufacture of the composite structure steel by two-stage cooling. As a result, in the two-stage cooling in the runout cooling after finish rolling, it is effective to set the time until the start of primary cooling within 1.0 second and the primary cooling rate to a high cooling rate exceeding 200 ° C / s. I found.
[0009]
The present invention has been made based on the above findings and further studies. That is, the present invention
1. In mass%, C: 0.04 to 0.2%, Si: 0.25 to 2.0%, Mn: 0.5 to 2.5%, Sol. Al: 0.1% or less , the step of performing rough rolling after continuous casting of steel consisting of Fe and inevitable impurities ,
(B) a step of performing finish rolling including a cumulative reduction of 30% or more at 1050 ° C. or less, with the rolling end temperature being Ar 3 or more and Ar 3 + 60 ° C. or less;
(C) Within 1.0 second after the end of rolling, a step of performing primary cooling at a temperature exceeding 200 ° C./s in a cooling region where the difference between the cooling start temperature and the cooling end temperature is 100 ° C. or more and less than 250 ° C .;
(D) a step of cooling at a temperature range of 720 ° C. or less and over 580 ° C. at 10 ° C./s or less for 2 seconds or more and 20 seconds or less, and then secondary cooling at 30 ° C./s or more;
(E) The strength-notch elongation balance is characterized by comprising a step of winding at a winding temperature of less than 400 ° C. TS (MPa) × N. A method for producing a high workability hot-rolled steel sheet satisfying EI (%) ≧ 7201 (MPa ·%) .
[0011]
2. 2. The high workability according to claim 1, wherein the finishing end temperature is controlled by heating the rough bar by a heating device on the entrance side of the continuous hot finish rolling mill or between the stands of the continuous hot finish rolling mill. A method for producing a hot-rolled steel sheet.
[0012]
3. The high-workability hot-rolled steel sheet according to 1 or 2, further comprising 0.01 to 0.2% of one or more of Ti, Nb, V, and Zr as a steel component by mass%. Manufacturing method.
[0013]
4). The high workability hot-rolling according to any one of 1 to 3, further comprising, as a steel component, one or two of Cr: 1% or less and Mo: 0.5% or less in mass%. A method of manufacturing a steel sheet.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
The limitation on the component composition and production conditions will be described in detail.
[0015]
1． Ingredient composition C
C is added in an amount of 0.04% or more in order to improve the hardenability of austenite and to ensure strength by mixing an appropriate amount of martensite or martensite and bainite in the composite structure. On the other hand, if it exceeds 0.2%, workability and weldability are deteriorated, so 0.04 to 0.2% (0.04% or more and 0.2% or less).
[0016]
Si
Si strengthens ferrite by solid solution strengthening, promotes precipitation of ferrite during slow cooling or cooling after hot rolling, and promotes concentration of C into austenite, so 0.25% or more is added. . On the other hand, if over 2.0%, the weldability and surface properties deteriorate, so 0.25 to 2.0%.
[0017]
Mn
Mn, like C, is added in an amount of 0.5% or more in order to improve the hardenability of untransformed austenite. On the other hand, if it exceeds 2.5%, the effect is saturated and a band-like structure is formed to deteriorate the workability, so the content is made 0.5 to 2.5%.
[0018]
Sol. Al
Al is added to fix N which is contained as a deoxidizing material and inevitable impurities and to improve workability. If it exceeds 0.1%, the effect is saturated, and the cleanliness is deteriorated and the workability is deteriorated.
[0019]
The steel of the present invention contains the above elements as a basic component composition, but may contain other elements as long as its effects can be obtained. For example, one or more of Ti, Nb, V, Zr, Cr, Mo, and Ca can be added according to desired properties such as strength and workability.
[0020]
Ti, Nb, V, Zr
When solid solution C and N are reduced by non-aging by adjusting the strength or forming carbonitride to improve deep drawability, one or more of Ti, Nb, V, and Zr are added in a total amount of 0.01 to Add 0.2%.
[0021]
Cr, Mo
Cr and Mo increase the hardenability of austenite and have the same effect as C and Mn. Therefore, Cr and Mo are added when necessary. Since it is an expensive element, if it is added in a large amount, the material cost increases and the weldability is deteriorated, so Cr: 1% or less, Mo: 0.5% or less.
Ca
When improving workability, Ca is added in a range not exceeding 0.005%.
[0022]
2. Production Conditions The steel of the present invention produces a steel slab by continuous casting, and the steel slab undergoes two-stage cooling including slow cooling after rough rolling and finish rolling. The conditions for rough rolling are not particularly defined, and can be performed directly before finish rolling, after reheating, or after continuous casting.
[0023]
Finishing rolling conditions Finishing rolling has a cumulative rolling reduction of 30% or more at 1050 ° C. or lower in order to promote the formation of ferrite nuclei and refine the structure in the cooling process after finishing rolling by introducing strain. The rolling end temperature is set to Ar3 or higher and Ar3 + 60 ° C or lower in order to refine the crystal grain size of austenite. In order to make the structure more effective, the rolling temperature is precisely controlled by the induction heating device provided on the entrance side or between the stands of the continuous hot finishing mill, and the finishing temperature is just above Ar3. It is preferable.
[0024]
Cooling conditions Primary cooling Primary cooling maintains the deformation band density in the introduced austenite grains by finish rolling, and generates a large number of ferrite nuclei not only from the austenite grain boundaries but also from within the grains. The cooling rate starts within 1.0 second, and the cooling rate is set to exceed 200 ° C./s in order to lower the ferrite transformation start temperature and to slow down the crystal grain growth rate after ferrite nucleation. The faster the cooling rate, the better.
[0025]
The cooling region for primary cooling is a temperature region in which the difference between the cooling start temperature and the cooling end temperature is 100 ° C. or more and less than 250 ° C. in order to ensure refinement of crystal grain size and strength.
[0026]
If the temperature difference is less than 100 ° C., the precipitation of fine ferrite is small and the crystal grains are not sufficiently refined, and if it is 250 ° C. or more, bainite is generated before secondary cooling, and sufficient strength cannot be obtained.
[0027]
After the primary cooling, the secondary cooling is performed after slow cooling. The slow cooling is performed at a temperature range of 720 ° C. or less and 580 ° C. or more and 2 seconds or more and 10 ° C./s or less in order to sufficiently promote the ferrite transformation. If it exceeds 20 sec, pearlite is likely to precipitate, and the workability deteriorates. Note that the slow cooling includes cooling.
[0028]
Secondary cooling The cooling rate of the secondary cooling is set to 30 ° C./s or more in order to stably form austenite with martensite or a structure containing partly bainite in martensite.
[0029]
After passing through the secondary cooling of the winding temperature, the winding is performed. When the winding temperature is 400 ° C. or more, a sufficient amount of martensite cannot be obtained, and the obtained martensite is also tempered and softened in the coil cooling process after winding. Further, the movable dislocation introduced at the ferrite / martensite interface is recovered, and the low yield ratio characteristic of the composite structure steel is lost.
[0030]
In addition, when manufacturing a thin steel plate having a plate thickness of 2.0 mm or less according to the present invention, it is not limited to 2.0 mmt or less, and the finishing temperature narrow range control is effective for structure control. It is preferable to heat the widthwise edge portion of the rough bar with an induction heating device during or before finish rolling, and the effect of the present invention is not impaired. Further, the present invention can also be applied to a continuous hot rolling process in which a rough bar retained by using a coil box or the like is welded.
[0031]
【Example】
Steels having chemical components shown in Table 1 were melted and hot rolled steel sheets having a thickness of 3.2 mm were manufactured by the manufacturing method shown in Table 2. Table 3 shows the mechanical properties of the hot-rolled steel sheets produced. Satisfying the component composition and production conditions of the present invention, sample No. Nos. 1 and 2 have excellent strength-notch elongation balance (TS × N.El) and a low yield ratio. Compared to 3 and 4, it is excellent in workability.
[0032]
FIG. 1 shows the effect of the primary cooling rate on the strength-notch elongation balance (TS × N.E1) according to this example.
[0033]
[Table 1]

[0034]
[Table 2]

[0035]
[Table 3]

[0036]
【The invention's effect】
According to the present invention, it is possible to produce a hot-rolled steel sheet having a finer composite structure and excellent workability such as local elongation regardless of a special component composition, which is extremely useful industrially.
[Brief description of the drawings]
FIG. 1 is a graph showing the effect of primary cooling rate on notch elongation-strength balance.

Claims (4)

In mass%, C: 0.04 to 0.2%, Si: 0.25 to 2.0%, Mn: 0.5 to 2.5%, Sol. Al: 0.1% or less , the step of performing rough rolling after continuous casting of steel consisting of Fe and inevitable impurities ,
(B) a step of performing finish rolling including a cumulative reduction of 30% or more at 1050 ° C. or less, with the rolling end temperature being Ar 3 or more and Ar 3 + 60 ° C. or less;
(C) Within 1.0 second after the end of rolling, a step of performing primary cooling at a temperature exceeding 200 ° C./s in a cooling region where the difference between the cooling start temperature and the cooling end temperature is 100 ° C. or more and less than 250 ° C .;
(D) a step of cooling at a temperature range of 720 ° C. or less and over 580 ° C. at 10 ° C./s or less for 2 seconds or more and 20 seconds or less, and then secondary cooling at 30 ° C./s or more;
(E) The strength-notch elongation balance is characterized by comprising a step of winding at a winding temperature of less than 400 ° C. TS (MPa) × N. A method for producing a high workability hot-rolled steel sheet having EI (%) ≧ 7201 ( MPa ·%) . 2. The high workability according to claim 1, wherein the finishing end temperature is controlled by heating the rough bar by a heating device on the entrance side of the continuous hot finish rolling mill or between the stands of the continuous hot finish rolling mill. A method for producing a hot-rolled steel sheet.   The high workability according to claim 1 or 2, further comprising 0.01 to 0.2% of one or more of Ti, Nb, V, and Zr as a steel component by mass%. A method for producing a hot-rolled steel sheet.   The high workability according to any one of claims 1 to 3, further comprising, as a steel component, one or two of Cr: 1% or less and Mo: 0.5% or less in mass%. A method for producing a hot-rolled steel sheet.

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