JP3725367B2 - Ultra-fine ferrite structure high-strength hot-rolled steel sheet excellent in stretch flangeability and manufacturing method thereof - Google Patents

Description

【００３３】
【表２】

【００３４】
表２より、発明例にかかる熱延鋼板は、いずれも強度−伸びバランスに優れ、かつ伸びフランジ性、疲労強度にも優れていることがわかる。
一方、本発明の鋼成分範囲を満足しない鋼（表１鋼種Ａ，Ｈ，Ｉ，Ｊ）を用いた試料No. １，１６〜１８は伸びフランジ性（λ）または／および疲労強度が劣化しており、また発明成分を有する鋼を用いていても、製造条件が発明条件を満足しない試料No. ３、４では、所定の組織、フェライト粒径が得られておらず、伸びフランジ性または疲労強度が劣化している。また、熱延後の冷却速度が１５０℃／ｓ超の試料No. １５では、フェライト粒径が２．０μm 未満と微細になり過ぎて伸びの劣化が著しい。
【００３５】
【発明の効果】
以上述べたように、本発明の熱延鋼板は、所定成分の下、平均粒径２．０〜１０．０μm の超微細フェライト組織を主体とし、特にマルテンサイトや残留オーステナイトを含まない組織としたので、４９０MPa 以上の高強度であっても優れた伸びフランジ性および疲労強度を具備することができる。また、本発明の製造方法によれば、一般的なホット・ストリップ・ミルを用いて、容易に超微細フェライト組織が得られ、延性、伸びフランジ性、疲労強度に優れた高強度鋼板を容易に製造することができる。[0001]
[Technical field to which the invention belongs]
The present invention relates to a high-strength hot-rolled steel sheet having a tensile strength of 490 MPa or more, excellent in fatigue strength having an ultrafine ferrite structure while being hot-rolled, in particular stretch flangeability, and a method for producing the same. It is suitable as a steel plate or structural steel plate.
[0002]
[Prior art]
In order to increase the fatigue strength, especially the stretch flangeability, of steel sheets used for automobiles and structures, it is considered effective to make the crystal structure of the steel sheets ultrafine, and the development of technology for obtaining ultrafine structures has been heretofore Many more have been sought. In particular, in recent years, high strength steel plates are often used to reduce the thickness, reduce weight, and reduce costs. In order to suppress deterioration of workability associated with the increase in strength, The refinement of the microstructure in high-strength steel sheets is an important issue. Moreover, this structure refinement is considered to be an effective means for improving stretch flangeability.
[0003]
As a method for refining the structure, (1) a large rolling reduction method proposed in, for example, JP-A-58-123823, JP-A-58-174523, (2) a controlled rolling method, a controlled cooling method, {Circle around (3)} For example, a low-temperature winding method proposed in Japanese Patent Laid-Open No. 10-8138 has been known.
[0004]
[Problems to be solved by the invention]
The essential point of ultrafine refinement in the microstructure refinement method by the large rolling reduction method is to promote strain-induced transformation from γ (austenite) to α (ferrite) by applying large rolling to austenite grains. However, there is a problem that it is difficult to carry out with a general hot strip mill, for example, it is necessary to set the rolling reduction per pass to 50% or more as rolling conditions.
[0005]
In addition, the controlled rolling method and controlled cooling method are effective for thick plates that have processed ferrite or ferrite at the recovery stage, with significant deterioration in workability, and have the highest priority on the toughness. A thin plate with the highest priority cannot achieve its purpose.
[0006]
Further, in the low temperature winding method described in the above publication, about 5 to 20% of retained austenite which adversely affects stretch flangeability is mixed, and high stretch flangeability cannot be obtained.
[0007]
The present invention has been made in view of the above problems, and can be easily manufactured using a general hot strip mill, and has high fatigue strength, in particular, stretch flangeability, as compared with the prior art. An object is to provide a high-strength hot-rolled steel sheet and a method for producing the same.
[0008]
[Means for Solving the Problems]
As described above, the conventional means for refining ferrite crystal grains is extremely difficult to implement in ordinary rolling equipment, and it is difficult to ensure workability. The present inventors consider that the above-mentioned problems cannot be solved with C-Mn steel or component steel obtained by adding a small amount of Ti or Nb to these elements, and thus the fineness of new crystal grains Investigated ways to improve the stretch flangeability.
As a result, in order to make the ferrite crystal grains ultrafine, it is important to make the austenite grains fine and introduce a high dislocation density. As a result of extensive research by the present inventors, Ti and Nb are dissolved in steel in a solid solution state. Therefore, coarsening of the austenite grain size during slab heating is suppressed, and subsequent refinement in hot rolling is facilitated, and these elements release dislocations during hot rolling. It is possible to obtain non-recrystallized austenite grains having a high dislocation density, and it is possible to obtain ultrafine ferrite grains by increasing the frequency of ferrite nucleation with this austenite grains. The invention has been completed.
[0009]
The ultrafine ferrite structure high-strength hot-rolled steel sheet according to the present invention described in claim 1 has a ferrite content of 95% or more in terms of area ratio and an average crystal grain size of ferrite of 2.0 to 10.0 μm. It is a hot-rolled steel sheet that does not contain martensite and retained austenite in the structure and has a tensile strength of 490 MPa or more.
[0010]
According to the high-strength hot-rolled steel sheet of the present invention, the structure is mainly composed of ferrite, and the average crystal grain size is 2.0 to 10.0 μm. Further, the structure does not contain martensite and retained austenite. As is clear from the examples, despite the high strength of 490 MPa or more, the fatigue strength is as high as 300 MPa or more, and the stretch flangeability is 120% or more. can get.
[0011]
The hot-rolled steel sheet of the present invention is basically better as the amount of ferrite is larger, and the area ratio is preferably 98% or more, more preferably a ferrite single phase structure (ferrite amount 100%). As a range that does not hinder the properties of the hot-rolled steel sheet, the second phase may contain 5% or less of pearlite, cementite, or bainite. However, the structure does not include martensite and retained austenite. The presence of the second phase deteriorates the stretch flangeability, but the degree of the deterioration is more remarkable as the hardness difference from the parent phase is larger. For this reason, hard martensite is eliminated. Further, the retained austenite is transformed into martensite during the deformation process, and further transformed into extremely hard martensite because of the high C concentration. Therefore, it is necessary to eliminate the retained austenite as much as possible. For this reason, in the present invention, martensite and retained austenite are not included as the second phase.
[0012]
In addition, the ferrite crystal grain size is improved along with the refinement, when the strength is constant, the stretch flangeability and fatigue strength are improved, but those with an average ferrite grain size of more than 10.0 μm also exist in conventional steel sheets. Therefore, dramatic improvement in stretch flangeability and fatigue strength cannot be expected. Therefore, in the present invention, the ferrite average crystal grain size is 10.0 μm or less, preferably 8.0 μm or less, more preferably 6.0 μm or less. On the other hand, if it is less than 2.0 μm, the yield point and the tensile strength are almost equal, and the decrease in n value and the decrease in uniform elongation become significant. Since it is necessary to ensure a certain degree of uniform elongation for press-molded parts, the lower limit of the ferrite average crystal grain size is set to 2.0 μm, preferably 3.0 μm in the present invention.
[0013]
The ultrafine ferrite structure high-strength hot-rolled steel sheet of the present invention has the following chemical components suitable for obtaining the above structure. That is, the chemical composition is wt%,
C: 0.01 to 0.10%,
Si: 1.5% or less,
Mn: more than 1.0% to 2.5%,
P: 0.15% or less,
S: 0.008% or less,
Al: 0.01 to 0.08%,
Total of one or two of Ti and Nb: 0.32 to 0.60%,
It consists of the balance Fe and inevitable impurities.
[0014]
Hereinafter, the reason for limiting the components of the chemical component (unit: wt%) will be described.
C: 0.01 to 0.10%
C needs to be 0.01% or more in order to obtain the required strength. However, if the amount exceeds 0.10%, the ratio of the second phase such as pearlite, bainite, martensite, etc. increases in the steel type with a small amount of Ti and Nb added, and stretch flangeability deteriorates. For this reason, the lower limit of the C amount is 0.01%, preferably 0.02%, and the upper limit is 0.10%, preferably 0.09%.
[0015]
Si: 1.5% or less Si is an effective element for enhancing strength while improving strength-elongation balance and strength-elongation flangeability balance by solid solution strengthening. The upper limit is set to 1.5%, preferably 1.0%, because it has an adverse effect and increases pickling time and pickling loss. In addition, if added over 0.2%, red scale tends to occur, and depending on the part, the commercial value may be impaired. In such a case, it is preferable to keep the Si amount to 0.2% or less.
[0016]
Mn: more than 1.0 to 2.5% (1.0 <Mn% ≦ 2.5)
Mn is an element effective for improving the strength, suppressing ferrite transformation, and effective for refining ferrite grains. That is, when the amount of Mn is small, ferrite transformation starts from a high temperature region in the cooling process after hot rolling and grows into coarse grains in a short time, so that desired fine ferrite grains cannot be obtained. If it is 1.0% or less, those effects are too small, and the intended strength and fine particles cannot be obtained. On the other hand, if it exceeds 2.5%, ferrite transformation is significantly delayed, pearlite, bainite and martensite transformation phases are likely to be generated, and stretch flangeability deteriorates. Therefore, the Mn content is more than 1.0%, preferably 1.1% or more, and the upper limit is 2.5%, preferably 2.0%.
[0017]
P: 0.15% or less P has a solid solution strengthening ability and is an element effective for achieving high strength. However, if it exceeds 0.15%, ductility deterioration due to segregation and grain boundary strength are reduced. For this reason, the upper limit of the amount of P is made 0.15%, preferably 0.12%.
[0018]
S: 0.008% or less Since S increases the amount of sulfides such as MnS and degrades stretch flangeability, the lower the better, the upper limit is made 0.008%.
[0019]
Al: 0.01 to 0.08%
Al is an element that is extremely effective for deoxidation at the molten steel stage. However, if it is less than 0.01%, its effect is too small, and if it exceeds 0.08%, coarsening of crystal grains and internal defects due to inclusions are caused. Come to bring. Therefore, the lower limit of the Al amount is 0.01%, and the upper limit is 0.08%.
[0020]
Total of one or two of Ti and Nb: 0.32 to 0.60%
Ti, Nb is a fine unrecrystallized austenite grain with high dislocation density suitable for refining the initial austenite grain in the slab heating stage, suppressing recrystallization in the hot rolling process, and increasing the frequency of ferrite nucleation. It is an essential element for obtaining. Further, the fine initial ferrite obtained in this way is an essential element for preventing the growth and coarsening during the winding process and obtaining the desired fine ferrite structure. In order to effectively exhibit this action, 0.32% or more in total of one or two of Ti and Nb is necessary. On the other hand, the refinement effect increases with the increase of Ti and Nb. However, when the total of one or two of Ti and Nb exceeds 0.60%, the effect becomes saturated. Therefore, the total content of one or two of Ti and Nb is 0.32 to 0.60%, preferably 0.32 to 0.50%.
[0021]
Furthermore, these elements form a strong carbide with C in the winding process, and the amount of dissolved C can be reduced to almost zero. Solid solution C causes dynamic strain aging that lowers local elongation (stretch flangeability), and has a strong adverse effect on stretch flangeability, which can further improve stretch flangeability. it can. In order to exert this effect, the atomic equivalent ratio of the sum of the effective Ti amount (Ti *) and Nb amount obtained by subtracting the Ti consumed as TiN or TiS from the added Ti amount and the C amount to 1 or more However, if it is 0.7 or more, a considerably large effect can be expected. On the other hand, when the atomic equivalent ratio exceeds 3, the effect is saturated. For this reason, as described in claim 3, the lower limit of the atomic equivalent ratio is set to 0.7, preferably 1.0, and the upper limit is set to 3.0, preferably 2.5.
Ti * = total Ti− (48S / 32 + 48N / 14)
Atomic equivalent ratio = (Ti * / 48 + Nb / 93) / (C / 12)
However, the element symbol in the above formula indicates the wt% of the element.
[0022]
The steel component of the hot-rolled steel sheet of the present invention can contain one or more of the following Ca and B, in addition to the above basic components, the balance Fe and unavoidable impurities.
Ca: 0.0005 to 0.0030%
Ca spheroidizes the expanded inclusions such as MnS, thereby controlling the form of the inclusions that are the starting point of the stretch flange cracks, thereby further improving the stretch flangeability. In order to exert this effect effectively, 0.0005% of addition is necessary, while the upper limit is set to 0.0030% of the maximum amount that can be practically added.
[0023]
B: 0.0005 to 0.0030%
It is known that when the solid solution C is 0 to a very small amount, the grain boundary strength becomes weak and the ductility deteriorates. Generally, this problem is improved by making ferrite grains finer, so that it does not lead to a substantial problem. However, B has an action of strengthening grain boundaries, and addition of B further improves this problem. It is valid. In order to exhibit this effect effectively, 0.0005% or more must be added. On the other hand, even if added over 0.0030%, the effect is saturated, so the upper limit is made 0.0030%.
[0024]
The production method of the present invention is a suitable production method for the high strength hot-rolled steel sheet having the ultrafine ferrite structure, and after heating the continuous cast slab having the above chemical components to a temperature exceeding 1100 ° C., finish rolling After hot rolling at a temperature of Ar ₃ or higher, cooling is performed at a cooling rate of 10 to 150 ° C./s, and winding is performed at a winding temperature of 500 to 700 ° C.
[0025]
Hereinafter, the hot rolling conditions of the present invention will be described.
In the present invention, first of all, it is an important technical point to maximize the effect of refining initial austenite grains by solute Ti and solute Nb. When the billet heating temperature is 1100 ° C. or less, TiC In other words, the amount of precipitation of NbC increases, in other words, the amount of solid solution Ti and the amount of solid solution Nb decrease, making it difficult to refine the initial austenite grains and secure fine unrecrystallized grains at the end of hot rolling. For this reason, the billet heating temperature (SRT) is over 1100 ° C. (SRT> 1100 ° C.), preferably 1150 ° C. or higher. Although the upper limit temperature is not particularly defined, in reality, if it exceeds 1300 ° C., the heating furnace is damaged and the energy cost is increased, which is not preferable. The most preferable billet heating temperature is 1220 ± 50 ° C. (1170 to 1270 ° C.) as a result of experimental investigation by the inventors.
[0026]
In the present invention, since the rolling reduction during the hot rolling is not defined, the rolling reduction per run is preferably set high within the range of the capability of the rolling mill actually used. In general, the higher the rolling reduction per one, the better results can be obtained, but in reality, there is a limit on the rolling reduction due to the rolling ability of the rolling mill, and it varies depending on the rolling temperature, the chemical composition of the steel and the rolling dimensions, etc. However, a rolling mill capable of performing 20-40% reduction is common. Therefore, hot rolling may be performed at a reduction rate as large as possible within the range.
[0027]
The reason why the finishing temperature at the time of hot rolling is set to Ar ₃ points or more is that if it is less than Ar ₃ points, the processing is performed in the ferrite + austenite region, the processed ferrite grains remain, and the ductility is significantly reduced. . In order to make the obtained ferrite grains ultrafine, the finishing temperature is just above the Ar ₃ point (range of Ar ₃ point to Ar ₃ point + 50 ° C.) so as to maximize the austenite grain refining and the residual dislocation density. It is desirable to make it. As the finishing temperature increases, the effect of refining austenite grains or the degree of residual dislocation density decreases. Therefore, if the upper limit is decided, it is desirable to set Ar ₃ point + 150 ° C.
[0028]
At the stage when finish rolling is finished, the austenite grains have an almost fine non-recrystallized structure. If ferrite transformation is performed as it is, fine ferrite grains begin to be formed by nucleation from both the grain boundaries and the grains. However, at a cooling rate of less than 10 ° C./s, ferrite transformation proceeds due to the growth of ferrite grains occurring in a high temperature region, and the ferrite transformation promoting effect and refining effect are reduced, so the lower limit of the cooling rate is 10 ° C./s, The temperature is preferably 30 ° C./s, more preferably 50 ° C./s. On the other hand, if the cooling rate exceeds 150 ° C./s, the ferrite grain size becomes too fine, the uniform elongation deteriorates, and the flatness of the steel sheet also deteriorates. Is 150 ° C./s, preferably 80 ° C./s.
[0029]
As for the coiling temperature, if the temperature exceeds 700 ° C., the ferrite grains become extremely coarse, and the desired structure and characteristics cannot be obtained. Deterioration of stretch flangeability is caused, and if it is less than 400 ° C., bainite and martensite are generated, and the stretch flangeability is further degraded. Therefore, the lower limit of the coiling temperature is 500 ° C., preferably 550 ° C., and the upper limit is 700 ° C., preferably 650 ° C.
[0030]
【Example】
Steels having chemical components shown in Table 1 were melted, and hot rolled steel sheets having a thickness of 3.5 mm were manufactured under the hot rolling conditions shown in Table 2. Each sample steel plate is ground to 2.5mm thickness by double-side grinding, JIS No. 5 specimen is taken in the rolling direction, and the ferrite average crystal grain size, structure type and amount (area%) are obtained as follows. It was measured. The ferrite average crystal grain size was determined as an average value of 50 average crystal grain sizes in the rolling direction and the plate thickness direction by a cutting method using a 1000 times optical microscope photograph. In addition, the type and amount of the tissue were determined by using a 400-fold optical microscope photograph of the nital-corroded tissue, and the amount was measured by area ratio.
[0031]
In addition, specimens were collected from each of the sample steel plates, and the tensile properties, the hole expansion test according to the iron standard (JFST1001), and the fatigue properties by the double swing plane bending test method were investigated. The hole expansion test was performed until a 10 mmφ punched hole (initial hole: hole diameter d0 = 10 mm) was drilled in the test piece, and a crack penetrating the plate thickness with a conical punch with an apex angle of 60 degrees occurred with the burr up. A hole is expanded and a hole diameter d1 mm at the time of occurrence of a crack is measured, and a critical hole expansion rate λ (%) is obtained by the following formula. These results are also shown in Table 2. The remaining structures other than ferrite and bainite described in Table 2 are pearlite and cementite.
λ = (d1−d0) / d0 × 100
[0032]
[Table 1]

[0033]
[Table 2]

[0034]
From Table 2, it can be seen that all of the hot-rolled steel sheets according to the invention examples are excellent in strength-elongation balance, and are excellent in stretch flangeability and fatigue strength.
On the other hand, Sample Nos. 1 and 16 to 18 using steels that do not satisfy the steel component range of the present invention (Table 1 steel types A, H, I, and J) have deteriorated stretch flangeability (λ) and / or fatigue strength. In Sample Nos. 3 and 4 where the manufacturing conditions do not satisfy the invention conditions even when steel having the invention components is used, a predetermined structure and ferrite grain size are not obtained, and stretch flangeability or fatigue The strength has deteriorated. Further, in sample No. 15 where the cooling rate after hot rolling exceeds 150 ° C./s, the ferrite grain size becomes too small as less than 2.0 μm, and the elongation is remarkably deteriorated.
[0035]
【The invention's effect】
As described above, the hot-rolled steel sheet of the present invention is mainly composed of an ultrafine ferrite structure having an average grain size of 2.0 to 10.0 μm under a predetermined component , and particularly has a structure that does not contain martensite or retained austenite. Therefore, even with a high strength of 490 MPa or more, excellent stretch flangeability and fatigue strength can be provided. Further, according to the production method of the present invention, an ultrafine ferrite structure can be easily obtained using a general hot strip mill, and a high-strength steel sheet excellent in ductility, stretch flangeability and fatigue strength can be easily obtained. Can be manufactured.

Claims (5)

The ferrite content is 95% or more by area ratio, the average crystal grain size of ferrite is 2.0 to 10.0 μm, the structure does not contain martensite and residual austenite , the chemical composition is wt %,
C: 0.01 to 0.10%,
Si: 1.5% or less,
Mn: more than 1.0% to 2.5%,
P: 0.15% or less,
S: 0.008% or less,
Al: 0.01 to 0.08%,
Total of one or two of Ti and Nb: 0.32 to 0.60%,
A high-strength hot-rolled steel sheet with an ultrafine ferrite structure excellent in stretch flangeability, which is composed of the remaining Fe and inevitable impurities and has a tensile strength of 490 MPa or more. The ultrafine ferrite structure high-strength hot-rolled steel sheet according to claim 1 , wherein the addition amounts of Ti and Nb satisfy the following conditions.
(Ti * / 48 + Nb / 93) / (C / 12): 0.7 to 3.0
Ti * = total Ti− (48S / 32 + 48N / 14)
However, the element symbol in each formula shows the content (wt%) of the element. Chemical component is further Ca: 0.0005 to 0.0030%
The ultra-fine ferrite structure high-strength hot-rolled steel sheet according to claim 1 or 2 , comprising: Chemical component is further B: 0.0005 to 0.0030%
The ultrafine ferrite structure high-strength hot-rolled steel sheet according to claim 1, 2 or 3 . After heating the continuous cast slab having the chemical component according to any one of claims 1 to 4 to a temperature of more than 1100 ° C, hot rolling at a finish rolling temperature of ₃ points or more, then 10 to 150 ° C. A method for producing a high-strength hot-rolled steel sheet with an ultrafine ferrite structure excellent in stretch flangeability that is cooled at a cooling rate of / s and wound at a coiling temperature of 500 to 700 ° C.