JP3572894B2 - Composite structure hot rolled steel sheet excellent in impact resistance and formability and method for producing the same - Google Patents

Description

【００４４】
【表２】

【００４５】
得られたこれらの結果を表３に示す。また、図１および図２は、強度上昇率と（Ｈ_Ｖ２／３）／（Ｈ_Ｖ１／３＋２０）との関係を示したものである。
【００４６】
【表３】

【００４７】
以上の試験結果から、本発明鋼板は、いずれも、所定の硬さを有するフェライトと第２相とを適正量比で含む複合組織であり、良好な耐衝突特性と成形性を併せ具えていることがわかる。その耐衝突特性は、真歪みが０．１ から０．２５といった広い歪み域でも、高い強度上昇率が得られることがわかる。
【００４８】
【発明の効果】
以上説明したように、本発明によれば、化学組成、金属組織および各相の硬さを適正に制御した複合組織にすることによって、従来よりも一段と優れた、耐衝突特性と成形性を具える熱延鋼板を提供することが可能となる。
また本発明によれば、低歪み域から高歪み域までの広い領域で強度上昇率が高く優れた耐衝突特性を有することがわかる。
したがって、本発明に従う熱延鋼板を自動車用に適用することによって、自動車車体の軽量化と安全性の向上を、一層経済的に達成することが可能になる。
【図面の簡単な説明】
【図１】低歪み域における強度上昇率と（Ｈ_Ｖ２／３）／（Ｈ_Ｖ１／３＋２０）との関係を示すグラフである。
【図２】高歪み域における強度上昇率と（Ｈ_Ｖ２／３）／（Ｈ_Ｖ１／３＋２０）との関係を示すグラフである。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention is mainly used as a material for a part of an automobile, in which high strength and high formability are required, and furthermore, excellent impact resistance (hereinafter simply referred to as "there is a simple" Abbreviated as "collision resistance") and a method for producing the same.
[0002]
[Prior art]
The recent design philosophy of the automobile body is not only to simply increase the strength of the steel sheet, but also in the event of a collision during running, the steel sheet has excellent impact resistance, that is, the deformation resistance when deformed at a high strain rate. The development of large steel sheets has attracted attention as a means of improving the safety of automobiles and effectively contributing to weight reduction of vehicle bodies.
On the other hand, due to the trend toward cost reduction of members in recent years, there is an increasing tendency to adopt a hot-rolled steel sheet having a thickness of 3.0 mm or less in place of a conventionally used cold-rolled steel sheet.
Under these recent circumstances, development of a hot-rolled high-strength steel sheet having excellent collision resistance has been eagerly desired from the viewpoints of improving the safety of automobiles and reducing costs.
[0003]
By the way, conventionally, in the ferrite single phase structure, solid-solution strengthening by adding a substitution element such as Si, Mn, or P, or carbonitride forming element such as Nb or Ti is added in the ferrite single phase structure. In general, a method based on precipitation strengthening was used.
For example, JP-A-56-139654 discloses that ultra-low carbon steel contains Ti and Nb in order to improve workability and aging, and further contains a reinforcing component such as P within a range that does not impair workability. We have proposed a steel sheet with high strength. Further, for example, Japanese Patent Application Laid-Open No. S59-193221 proposes a method of increasing the strength by adding Si to ultra-low carbon steel.
Japanese Patent Application Laid-Open No. 60-52528 proposes a method for producing a high-strength steel sheet having excellent ductility by annealing a low-carbon steel at a high temperature and precipitating a martensite phase after cooling.
[0004]
[Problems to be solved by the invention]
However, in steel sheets that have been strengthened by such a conventionally proposed method, although the thickness of the automobile body can be reduced to some extent, the above-mentioned collision-resistant characteristics are not essentially improved. Did not. The reason is that these proposals require that the yield strength or tensile strength, which is an indicator of the strength of a steel sheet, be changed to a strain rate of 10%.^-3-10^-2(Sec^-1) And extremely slow, so-called static evaluation method only. On the other hand, in the design of the actual automobile body, the strain rate is 10 to 10 in consideration of the safety at the time of collision rather than the static strength.⁴(Sec^-1This is because the strength based on the so-called dynamic evaluation method accompanied by the impact deformation of (1) becomes more important.
Therefore, the conventional proposals described above, which are developed focusing only on the static strength, have a problem that they cannot be used as a fundamental index for reducing the weight of an automobile body.
[0005]
JP-A-7-90482 proposes a two-phase structure steel sheet of martensite and ferrite for the purpose of improving the impact resistance of the steel sheet. However, although the two-phase steel sheet of ferrite and martensite has relatively excellent impact resistance, it has not yet fulfilled the higher level properties required by today's automobile manufacturers. It is the current situation.
[0006]
Therefore, an object of the present invention is to provide a composite structure hot-rolled steel sheet having more excellent impact resistance and formability than conventional hot-rolled steel sheets.
Another object of the present invention is to provide a strain rate of 0.02 sec.^-1Strain rate 2000 sec with respect to strength^-1It is an object of the present invention to provide a hot-rolled steel sheet having excellent impact resistance, wherein the rate of increase in strength expressed by the ratio of the increase in strength is 35% or more at a true strain of 0.1 and 25% or more at a true strain of 0.25. .
[0007]
[Means for Solving the Problems]
The present inventors have conducted detailed studies on the effects of hot rolling conditions, metallographic structure, and the like, based on a two-phase structure steel composed of ferrite and a second phase including martensite, in order to realize the above-mentioned object. As a result, the present invention has been completed. That is, the present invention has the following content as a gist configuration.
[0008]
(1) C: 0.02-0.2 wt%, Si: 0.1-1.5wt%, Mn: 0.5 to 3.0 wt%, S: 0.010 wt% or less, P: 0.01 to 0.15 wt%, Cr: 0.003 to 2.0 wt%, Mo: 0.003 to 2.0 wt% With the balance being Fe and unavoidable impurities and having a microstructure of 60-97% by volume in ferrite phase and the balance being at least one of martensite, bainite and austenite. The hardness of the ferrite phase and the second phase satisfies the relationship of the following formula, and the crystal grain size of the ferrite phase is less than 4 μm.Yes, strain rate 0.02sec ^-1 Rate versus strength of steel 2000sec ^-1 The rate of increase in intensity, expressed as the ratio of the amount of increase in 0.1 so 35 % Or more, true distortion 0.25 so twenty five % Or moreA hot-rolled steel sheet with a composite structure that is excellent in impact resistance and formability.
H_V2/ 3> H_V1/ 3 + 20
Where H_V1: Vickers hardness of ferrite phase
H_V2: Vickers hardness in the second phase region
[0009]
(2) C: 0.02-0.2 wt%, Si: 0.1-1.5wt%, Mn: 0.5 to 3.0 wt%, S: 0.010 wt% or less, P: 0.01 to 0.15 wt%, Cr: 0.003 to 2.0 wt%, Mo: 0.003 to 2.0 wt% And further contains at least one selected from the group consisting of Ti: 0.003 to 1.0 wt%, Nb: 0.003 to 0.5 wt%, and V: 0.003 to 1.0 wt%, with the balance being Fe and unavoidable impurities, and Is composed of a ferrite phase having a volume fraction of 60 to 97% and a second phase region composed of at least one phase of martensite, bainite, and austenite. When the hardness satisfies the relationship of the following formula, and the crystal grain size of the ferrite phase is less than 4 μm,Yes, strain rate 0.02sec ^-1 Rate versus strength of steel 2000sec ^-1 The rate of increase in intensity, expressed as the ratio of the amount of increase in 0.1 so 35 % Or more, true distortion 0.25 so twenty five % Or moreA hot-rolled steel sheet with a composite structure, which is excellent in impact resistance and formability.
H_V2/ 3> H_V1/ 3 + 20
Where H_V1: Vickers hardness of ferrite phase
H_V2: Vickers hardness in the second phase region
[0010]
(3) C: 0.02-0.2 wt%, Si: 0.1-1.5wt%, Mn: 0.5 to 3.0 wt%, S: 0.010 wt% or less, P: 0.01 to 0.15 wt%, Cr: 0.003 to 2.0 wt%, Mo: 0.003 to 2.0 wt% Ni: 0.003 to 3.0 wt%, Cu: 0.003 to 3.0 wt%, B: 0.0005 to 1.0 wt%, N: 0.003 to 0.1 wt%, the balance being Fe and A ferrite phase composed of unavoidable impurities and having a volume fraction of 60 to 97% by volume, and a balance comprising a second phase region composed of one or more phases of martensite, bainite, and austenite; In addition, the hardness of the ferrite phase and the second phase satisfy the relationship of the following formula, and the crystal grain size of the ferrite phase is less than 4 μm.Yes, strain rate 0.02sec ^-1 Rate versus strength of steel 2000sec ^-1 The rate of increase in intensity, expressed as the ratio of the amount of increase in 0.1 so 35 % Or more, true distortion 0.25 so twenty five % Or moreA hot-rolled steel sheet with a composite structure, which is excellent in impact resistance and formability.
H_V2/ 3> H_V1/ 3 + 20
Where H_V1: Vickers hardness of ferrite phase
H_V2: Vickers hardness in the second phase region
[0011]
(4) C: 0.02 to 0.2 wt%, Si: 0.1 to1.5wt%, Mn: 0.5 to 3.0 wt%, S: 0.010 wt% or less, P: 0.01 to 0.15 wt%, Cr: 0.003 to 2.0 wt%, Mo: 0.003 to 2.0 wt% And further contains at least one member selected from the group consisting of Ca: 0.0005 to 1.0 wt%, Zr: 0.0005 to 1.0 wt%, and REM: 0.0005 to 0.5 wt%, with the balance being Fe and unavoidable impurities. Is composed of a ferrite phase having a volume fraction of 60 to 97% and a second phase region composed of at least one phase of martensite, bainite, and austenite. When the hardness satisfies the relationship of the following formula, and the crystal grain size of the ferrite phase is less than 4 μm,Yes, strain rate 0.02sec ^-1 Rate versus strength of steel 2000sec ^-1 The rate of increase in intensity, expressed as the ratio of the amount of increase in 0.1 so 35 % Or more, true distortion 0.25 so twenty five % Or moreA hot-rolled steel sheet with a composite structure, which is excellent in impact resistance and formability.
H_V2/ 3> H_V1/ 3 + 20
Where H_V1: Vickers hardness of ferrite phase
H_V2: Vickers hardness in the second phase region
[0012]
(5) C: 0.02-0.2 wt%, Si: 0.1-1.5wt%, Mn: 0.5 to 3.0 wt%, S: 0.010 wt% or less, P: 0.01 to 0.15 wt%, Cr: 0.003 to 2.0 wt%, Mo: 0.003 to 2.0 wt% And at least one selected from the group consisting of Ti: 0.003 to 1.0 wt%, Nb: 0.003 to 0.5 wt%, V: 0.003 to 1.0 wt%, and Ni: 0.003 to 3.0 wt%, Cu: 0.003 to 3.0 wt%. %, B: 0.0005 to 1.0 wt%, N: at least one selected from 0.003 to 0.1 wt%, the balance being Fe and unavoidable impurities, and having a microstructure of 60 to 97% by volume. The phase and the remainder consist of a second phase region composed of one or more phases of martensite, bainite, and austenite, and the hardness of the ferrite phase and the second phase satisfy the following relationship: And the grain size of the ferrite phase is less than 4 μmYes, strain rate 0.02sec ^-1 Rate versus strength of steel 2000sec ^-1 The rate of increase in intensity, expressed as the ratio of the amount of increase in 0.1 so 35 % Or more, true distortion 0.25 so twenty five % Or moreA hot-rolled steel sheet with a composite structure, which is excellent in impact resistance and formability.
H_V2/ 3> H_V1/ 3 + 20
Where H_V1: Vickers hardness of ferrite phase
H_V2: Vickers hardness in the second phase region
[0013]
(6) C: 0.02-0.2 wt%, Si: 0.1-1.5wt%, Mn: 0.5 to 3.0 wt%, S: 0.010 wt% or less, P: 0.01 to 0.15 wt%, Cr: 0.003 to 2.0 wt%, Mo: 0.003 to 2.0 wt% And at least one selected from the group consisting of Ti: 0.003 to 1.0 wt%, Nb: 0.003 to 0.5 wt%, V: 0.003 to 1.0 wt%, and Ca: 0.0005 to 1.0 wt%, Zr: 0.0005 to 1.0 wt%. %, REM: at least one selected from 0.0005 to 0.5 wt%, the balance being Fe and unavoidable impurities, and the structure being a ferrite phase having a volume fraction of 60 to 97% and a martensite being a balance. A second phase region composed of one or more phases of bainite and austenite, and the ferrite phase and the hardness of the second phase satisfy the relationship of the following formula, and the crystal grain size of the ferrite phase is Less than 4μmYes, strain rate 0.02sec ^-1 Rate versus strength of steel 2000sec ^-1 The rate of increase in intensity, expressed as the ratio of the amount of increase in 0.1 so 35 % Or more, true distortion 0.25 so twenty five % Or moreA hot-rolled steel sheet with a composite structure, which is excellent in impact resistance and formability.
H_V2/ 3> H_V1/ 3 + 20
Where H_V1: Vickers hardness of ferrite phase
H_V2: Vickers hardness in the second phase region
[0014]
(7) C: 0.02-0.2 wt%, Si: 0.1-1.5wt%, Mn: 0.5 to 3.0 wt%, S: 0.010 wt% or less, and at least one selected from P: 0.01 to 0.15 wt%, Cr: 0.003 to 2.0 wt%, Mo: 0.003 to 2.0 wt% Ni: 0.003 to 3.0 wt%, Cu: 0.003 to 3.0 wt%, B: 0.0005 to 1.0 wt%, N: 0.003 to 0.1 wt%, and Ca: 0.0005 to 1.0 wt% %, Zr: 0.0005 to 1.0 wt%, REM: at least one selected from 0.0005 to 0.5 wt%, with the balance being Fe and unavoidable impurities, and the structure being 60 to 97% by volume of ferrite. The phase and the remainder consist of a second phase region composed of one or more phases of martensite, bainite, and austenite, and the hardness of the ferrite phase and the second phase satisfy the following relationship: And the grain size of the ferrite phase is less than 4 μmYes, strain rate 0.02sec ^-1 Rate versus strength of steel 2000sec ^-1 The rate of increase in intensity, expressed as the ratio of the amount of increase in 0.1 so 35 % Or more, true distortion 0.25 so twenty five % Or moreA hot-rolled steel sheet with a composite structure, which is excellent in impact resistance and formability.
H_V2/ 3> H_V1/ 3 + 20
Where H_V1: Vickers hardness of ferrite phase
H_V2: Vickers hardness in the second phase region
[0015]
(8) C: 0.02 to 0.2 wt%, Si: 0.1 to1.5wt%, Mn: 0.5 to 3.0 wt%, S: 0.010 wt% or less, P: 0.01 to 0.15 wt%, Cr: 0.003 to 2.0 wt%, Mo: 0.003 to 2.0 wt% And at least one selected from the group consisting of 0.003 to 1.0 wt% Ti: 0.003 to 0.5 wt%, Nb: 0.003 to 1.0 wt%, Ni: 0.003 to 3.0 wt%, and Cu: 0.003 to 3.0 wt%. , B: 0.0005 to 1.0 wt%, N: at least one selected from 0.003 to 0.1 wt%, and Ca: 0.0005 to 1.0 wt%, Zr: 0.0005 to 1.0 wt%, REM: selected from 0.0005 to 0.5 wt%. Containing at least one type, the balance being Fe and unavoidable impurities, and the structure being a ferrite phase having a volume fraction of 60 to 97%, and the balance being one or more phases of martensite, bainite, and austenite. When the ferrite phase and the hardness of the second phase satisfy the following relationship, and the crystal grain size of the ferrite phase is less than 4 μm,Yes, strain rate 0.02sec ^-1 Rate versus strength of steel 2000sec ^-1 The rate of increase in intensity, expressed as the ratio of the amount of increase in 0.1 so 35 % Or more, true distortion 0.25 so twenty five % Or moreA hot-rolled steel sheet with a composite structure, which is excellent in impact resistance and formability.
H_V2/ 3> H_V1/ 3 + 20
Where H_V1: Vickers hardness of ferrite phase
H_V2: Vickers hardness in the second phase region
[0016]
(9) C: 0.02 to 0.2 wt%, Si: 0.1 to 2.5 wt%,
Mn: 0.5 to 3.0 wt%, S: 0.010 wt% or less
And
P: 0.01 to 0.15 wt%, Cr: 0.003 to 2.0 wt%,
Mo: 0.003 to 2.0 wt%
Steel material containing at least one member selected from the group consisting of (Ar₃Transformation point -50 ° C)-(Ar₃The hot rolling is completed at the transformation point + 150 ° C), and then cooling is started at a rate satisfying the following formula within 0.1 to 5.0 seconds, and cooled to 820 to 620 ° C (first forced cooling). And then air-cooled for 0.5 to 15 seconds, then cooled to 570 to 300 ° C. (secondary forced cooling) at a cooling rate of 30 ° C./sec or more, and wound up. Method for producing a hot rolled steel sheet with a composite structure that is excellent in quality.
v ≧ 1000 × t₁ ^1/2÷ h^1/2÷ t
Here, v: cooling rate (° C./sec)
h: Plate thickness (mm)
t₁: Time from completion of finish rolling to start of primary forced cooling (sec)
t: Air cooling time (sec) from the start of secondary forced cooling to the start of secondary forced cooling
Note that H_V2Is set as the Vickers hardness in the second phase region because, when a plurality of phases are mixed in the second phase, the average hardness is adopted.
[0017]
BEST MODE FOR CARRYING OUT THE INVENTION
Next, in the present invention, the reason why the chemical composition, structure, manufacturing conditions, and the like of steel are limited as in the summary configuration will be described.
C: 0.02 to 0.2 wt%
C is a component necessary for increasing the strength and volume fraction of martensite in the two-phase structure. If the C content is less than 0.02 wt%, a sufficient amount of carbide and second phase mainly composed of martensite cannot be obtained. On the other hand, when the content exceeds 0.2 wt%, solid solution C exists in the ferrite, which impairs the formability. Therefore, the content of C is set to 0.02 to 0.2 wt%.
[0018]
Si: 0.1 ~1.5wt%
Si concentrates the solute C in ferrite in austenite to improve the hardenability of the steel, and also improves the formability of the steel sheet by increasing the purity of the ferrite. This effect appears when 0.1 wt% or more is added,1.5When the content exceeds wt%, the surface properties and surface treatment properties of the hot-rolled sheet are significantly deteriorated. Therefore, the content of Si is 0.1 ~1.5wt%, preferably 0.8 to 1.5 wt%.
[0019]
Mn: 0.5 to 3.0 wt%
Mn is an austenite-stabilizing element. If it is less than 0.5% by weight, hardenability is reduced, and it is difficult to obtain a two-phase structure. On the other hand, when the content exceeds 3.0% by weight, the steel sheet is hardened, and the formability decreases. Therefore, the Mn content is 0.5 to 3.0 wt%, preferably 0.8 to 1.8 wt%.
[0020]
S: 0.010 wt% or less
By reducing the content of S, precipitates in steel are reduced and workability is improved. Such an effect can be obtained by limiting the amount of S to 0.010 wt% or less.
[0021]
P: 0.01 to 0.15 wt%
P has the effect of stabilizing austenite and suppressing the production of pearlite, but this effect appears when added at 0.01 wt% or more. On the other hand, if the content exceeds 0.15 wt%, the formability is reduced due to the hardening of the steel sheet, and the surface treatment property is also deteriorated. Therefore, the content of P is set to 0.01 to 0.15 wt%.
[0022]
Cr: 0.003 to 2.0 wt%
Cr has an effect of suppressing pearlite formation, and its effect appears when added at 0.003 wt% or more. On the other hand, if the content exceeds 2.0 wt%, the effect is saturated and the production cost is increased. Therefore, the content of Cr is set to 0.003 to 2.0 wt%.
[0023]
Mo: 0.003 to 2.0 wt%
Mo has an effect of suppressing the generation of pearlite, and its effect appears when 0.003 wt% or more is added. On the other hand, even if the content exceeds 2.0 wt%, the effect is saturated and the production cost is increased. Therefore, the content of Mo is set to 0.003 to 2.0 wt%.
[0024]
Ti: 0.003 to 1.0 wt%
Ti has the effect of suppressing the coarsening of the crystal grain size when heated to a high temperature, but the effect appears at 0.003 wt% or more and saturates at 1.0 wt%.
[0025]
Nb: 0.003 to 0.5 wt%
Nb has an effect of suppressing coarsening of the crystal grain size when heated to a high temperature, but the effect appears at 0.003 wt% or more and saturates at 0.5 wt%.
[0026]
V: 0.003 to 1.0 wt%
V has the effect of suppressing the coarsening of the crystal grain size when heated to a high temperature, but the effect appears at 0.003 wt% or more and saturates at 1.0 wt%.
[0027]
Ni: 0.003 to 3.0 wt%
Ni has an effect of stabilizing austenite, suppressing pearlite transformation, and easily obtaining a ferrite-martensite or bainite or austenite structure. The effect appears at 0.003 wt% or more, but if it exceeds 3.0 wt%, the production cost is increased. Therefore, it is added in the range of 0.003 to 3.0 wt%.
[0028]
Cu: 0.003 to 3.0 wt%
Cu has an effect of stabilizing austenite, suppressing pearlite transformation, and easily obtaining a ferrite-martensite or bainite or austenite structure. The effect appears at 0.003 wt% or more, but if it exceeds 3.0 wt%, the production cost is increased. Therefore, it is added in the range of 0.003 to 3.0 wt%.
[0029]
B: 0.0005 to 1.0 wt%
B has the effect of suppressing the pearlite transformation and making it easier to obtain a ferrite-martensite or bainite or austenite structure. The effect appears at 0.0005 wt% or more, but if it exceeds 1.0 wt%, the non-uniformity of the structure due to the formation of nitrides increases and the formability decreases. Add in range.
[0030]
N: 0.003 to 0.1 wt%
N has the effect of stabilizing austenite and making it easier to obtain a ferrite-martensite or austenite structure. The effect appears at 0.003 wt% or more, but when it exceeds 0.1 wt%, N₂Is generated, and the surface properties of the steel sheet are deteriorated.
[0031]
Ca: 0.0005 to 1.0 wt%
Ca increases the strength during high-speed deformation by fixing S. The effect appears at 0.0005 wt% or more, but saturates at 1.0 wt%, so it is set to 0.0005 to 1.0 wt%.
[0032]
Zr: 0.0005 to 1.0 wt%
Zr increases the strength during high-speed deformation. The effect appears at 0.0005 wt% or more, but saturates at 1.0 wt%, so it is set to 0.0005 to 1.0 wt%.
[0033]
REM: 0.0005-0.5 wt%
REM increases the strength during high speed deformation. The effect appears at 0.0005 wt% or more, but saturates at 0.5 wt%, so it is set to 0.0005 to 0.5 wt%.
[0034]
・ Steel structure and hardness
As described above, the hot-rolled steel sheet according to the present invention has a composite structure including ferrite and a second phase including at least one phase of martensite, bainite, and austenite, and has a ferrite volume fraction of 60 to 97%. Need to be
When the volume fraction of the ferrite phase is less than 60%, the hard second phase increases, and the formability decreases. On the other hand, when the volume ratio exceeds 97%, the hard second phase decreases, and the collision resistance decreases. Let it.
Furthermore, the Vickers hardness H of the ferrite phase_V1Between the Vickers hardness of the second phase region and H_V2/ 3> H_V1When the relationship of / 3 + 20 is satisfied and the ferrite particle size is less than 4 μm, sufficient collision resistance is obtained. The reason why such a phenomenon appears is not clear, but one of the reasons is that the increase in strength due to non-uniform deformation due to the difference in hardness between ferrite and the second phase increases as the strain rate increases. In addition, it is considered that the strain rate dependence of the strength is increased due to the refinement of the structure (particularly the ferrite grain size).
If the hardness requirement is satisfied, the composition of the second phase is not particularly limited. However, since martensite increases the hardness and bainite has a relatively low hardness, the formation of martensite is promoted more than bainite. Is advantageous.
[0035]
Next, a method for producing a hot-rolled steel sheet with a composite structure according to the present invention will be described. In this manufacturing process, it is particularly important to control the cooling conditions from hot rolling to winding. That is, (Ar₃Transformation point -50 ° C)-(Ar₃The hot rolling is completed at the transformation point + 150 ° C), and then cooling is started at a rate satisfying the following formula for 0.1 to 5.0 seconds, and then cooled to 820 to 620 ° C (“primary forced cooling”). ), Air cooling for 0.5 to 15 seconds, cooling to 570 to 300 ° C. at a cooling rate of 30 ° C./sec or more (referred to as “secondary forced cooling”), and winding. The reason will be described below.
Record
v ≧ 1000 × t₁ ^1/2÷ h^1/2÷ t
Here, v: cooling rate (° C./sec)
h: Plate thickness (mm)
t₁: Time from the end of finish rolling to the start of primary forced cooling (sec)
t: Air cooling time (sec) between primary forced cooling and secondary forced cooling
[0036]
・ Hot rolling end temperature
Hot finishing rolling (Ar₃When the temperature is lower than the transformation point (−50 ° C.), strain is accumulated in the ferrite in the hot-rolled sheet, and the formability is significantly reduced.₃When the finish rolling exceeds the transformation point (+ 150 ° C.), the transformation speed of ferrite decreases, and the proper structure required to satisfy the material properties cannot be obtained. For this reason, hot rolling (Ar₃Transformation point -50 ° C)-(Ar₃(Transformation point + 150 ° C).
[0037]
・ Cooling and winding after hot rolling
During 0.1 to 5.0 seconds after hot rolling, 1000 × t₁ ^1/2÷ h^1/2÷ t (where h: plate thickness (mm), t₁: The time from the end of the finish rolling to the start of the primary cooling (sec), t: The primary forced cooling is started at a speed equal to or longer than the air cooling time (sec) between the primary forced cooling and the secondary forced cooling. The reason is that if the elapsed time from the end of hot rolling to the first forced cooling is less than 0.1 second, it is difficult to control the temperature at the end of rolling, while if it exceeds 5.0 seconds, the austenite grains become coarse. This causes a delay in ferrite transformation, thereby inhibiting carbon concentration in austenite, transforming the second phase into pearlite, and lowering the formability and impact resistance. The cooling rate of the primary forced cooling is 1000 × t₁ ^1/2÷ h^1/2If it is less than Δt, in addition to the same reason, the ferrite grain size becomes coarse, which causes a decrease in impact resistance.
Furthermore, when manufacturing a steel sheet having a thickness of 3.0 mmt or less, it is preferable to increase the rolling speed in order to ensure the productivity and the finish rolling temperature. Therefore, it is preferable to secure the rolling speed by setting the time from the end of finish rolling to cooling to 2 sec or less and the air cooling time to 12 sec or less.
[0038]
This primary forced cooling is performed to 820 to 620 ° C. When the temperature at the end of cooling exceeds 820 ° C., the ferrite transformation rate is low, and a second phase having martensite as the main phase is obtained. This is because moldability deteriorates. Also, if the temperature at the end of cooling is lower than 620 ° C., pearlite transformation starts from austenite, so that the collision resistance deteriorates.
[0039]
Next, the air cooling for 0.5 to 15 seconds is because if the air cooling time is less than 0.5 seconds, the ferrite transformation time is short, so that the amount of ferrite precipitation is small and the formability is deteriorated. This is because, when the ratio exceeds 1, the ferrite grains are coarsened, and the collision resistance is reduced. Air cooling is preferably performed for 2 seconds or more.
[0040]
The secondary forced cooling from 570 to 300 ° C. at a cooling rate of 30 ° C./sec or more with the air cooling interposed is performed when the cooling rate is less than 30 ° C./sec or the cooling stop temperature exceeds 570 ° C. This is because pearlite transformation starts and the moldability deteriorates. Also, if the cooling stop temperature is lower than 300 ° C., solid solution C remains in the ferrite and the formability deteriorates.
[0041]
In the above description, the case where the present invention is exclusively used for an automobile is described. However, the technique according to the present invention is similarly effective for other applications that require strength under a high strain rate. Not even.
[0042]
【Example】
Steel having the chemical composition shown in Table 1 was melted in a converter. These slabs were hot-rolled under the conditions shown in Table 2, cooled, and wound around a coil to produce a hot-rolled steel sheet having a thickness of 1.4 to 2.9 mm.
A test material was sampled from the obtained steel sheet, the constitutional structure was examined by an optical microscope, and the in-phase Vickers hardness of each phase and the volume ratio of the second phase were measured under a load of 3 g. Further, the strain rate is set to 0.02 sec.^-1And 2000 sec^-1The tensile test was performed at two levels of true strain, true stress at true strains of 0.1 and 0.25 was measured, and the strain rate was 2000 sec.^-1, Ie, (2000 sec.)^-1Strength at -0.02 sec^-1Strength) / (0.02 sec)^-1Strength).
In addition, the volume ratio of the second phase is obtained by calculating the number and the average diameter of the first phase and the second phase by image processing, converting the average diameter into a three-dimensional diameter by the following equation, and calculating the first phase and the second phase. The volume ratio was determined from the number and the average three-dimensional diameter.
D = 1.128 L
Where D: average diameter (two-dimensional), L: average three-dimensional diameter
[0043]
[Table 1]

[0044]
[Table 2]

[0045]
Table 3 shows the obtained results. 1 and 2 show the strength increase rate and (H_V2/ 3) / (H_V1/ 3 + 20).
[0046]
[Table 3]

[0047]
From the above test results, each of the steel sheets of the present invention has a composite structure including ferrite having a predetermined hardness and the second phase at an appropriate ratio, and has both good impact resistance and formability. You can see that. It can be seen that the anti-collision property shows that a high strength increase rate can be obtained even in a wide strain range where the true strain is 0.1 to 0.25.
[0048]
【The invention's effect】
As described above, according to the present invention, by providing a composite structure in which the chemical composition, the metal structure, and the hardness of each phase are appropriately controlled, the impact resistance and the formability, which are far superior to those of the related art, are achieved. It is possible to provide a hot rolled steel sheet.
Further, according to the present invention, it can be seen that the strength increase rate is high in a wide region from a low strain region to a high strain region, and the device has excellent collision resistance.
Therefore, by applying the hot-rolled steel sheet according to the present invention to an automobile, it is possible to further reduce the weight and improve the safety of the automobile body more economically.
[Brief description of the drawings]
FIG. 1 is a graph showing a relationship between a rate of increase in intensity in a low strain region and (H_V2/ 3) / (H_V1/ 3 + 20).
FIG. 2 shows the strength increase rate and (H_V2/ 3) / (H_V1/ 3 + 20).

Claims (9)

C: 0.02 to 0.2 wt%, Si: 0.1 to 1.5 wt%, Mn: 0.5 to 3.0 wt%, S: 0.010 wt% or less, P: 0.01 to 0.15 wt%, Cr: 0.003 to 2.0 wt%, Mo: at least one selected from 0.003 to 2.0 wt%, the balance being Fe and unavoidable impurities, and the structure is a ferrite phase having a volume fraction of 60 to 97%, and the remainder is martensite, bainite, A ferrite phase and a second phase composed of at least one phase of austenite, and the hardness of the ferrite phase and the second phase satisfy the following relationship, and the crystal grain size of the ferrite phase is less than 4 μm. Yes , the rate of strength increase expressed as the ratio of the strength increase at a strain rate of 2000 sec ^{-1 to} the strength at a strain rate of 0.02 sec ^-1 is 35 % or more at a true strain of 0.1 , and 25 % or more at a true strain of 0.25. Composite structure hot rolled steel sheet with excellent impact resistance and formability.
H _V2 / 3> H _V1 / 3 + 20
Here, H _V1 : Vickers hardness of ferrite phase H _V2 : Vickers hardness of second phase region C: 0.02 to 0.2 wt%, Si: 0.1 to 1.5 wt%, Mn: 0.5 to 3.0 wt%, S: 0.010 wt% or less, P: 0.01 to 0.15 wt%, Cr: 0.003 to 2.0 wt%, Mo: contains at least one selected from 0.003 to 2.0 wt%, further contains at least one selected from Ti: 0.003 to 1.0 wt%, Nb: 0.003 to 0.5 wt%, and V: 0.003 to 1.0 wt%. The balance is a second phase composed of Fe and unavoidable impurities and having a structure of a ferrite phase having a volume fraction of 60 to 97%, and the remainder being at least one phase of martensite, bainite, and austenite. It consists of a region, yet the hardness of the ferrite phase and the second phase satisfies the following relation, and less than the grain size of the ferrite phase is 4 [mu] m, the strain rate 2000sec to the intensity of the strain rate 0.02 sec ^{-1 - 1} of the intensity increase of the intensity increase rate representing a ratio of 35% or more in true strain 0.1, characterized in that 25% or more in true strain 0.25, crashworthiness characteristics and growth Composite structure hot-rolled steel sheet excellent in resistance.
H _V2 / 3> H _V1 / 3 + 20
Here, H _V1 : Vickers hardness of ferrite phase H _V2 : Vickers hardness of second phase region C: 0.02 to 0.2 wt%, Si: 0.1 to 1.5 wt%, Mn: 0.5 to 3.0 wt%, S: 0.010 wt% or less, P: 0.01 to 0.15 wt%, Cr: 0.003 to 2.0 wt%, Mo: contains at least one selected from 0.003 to 2.0 wt%, Ni: 0.003 to 3.0 wt%, Cu: 0.003 to 3.0 wt%, B: 0.0005 to 1.0 wt%, N: 0.003 to 0.1 wt% One or more selected from the group consisting of Fe and unavoidable impurities, the balance being a ferrite phase having a volume fraction of 60 to 97%, and the balance being at least one of martensite, bainite, and austenite. The ferrite phase and the hardness of the second phase satisfy the following relationship, the crystal grain size of the ferrite phase is less than 4 μm, and the strain rate is 0.02 sec ⁻ intensity increase rate expressed as the ratio of the intensity increase of strain rate 2000 sec ^-1 for the ^first intensity is 35% or more in true strain 0.1, to characterized in that 25% or more in true strain 0.25 , Composite structure hot-rolled steel sheet having excellent formability and crashworthiness characteristics.
H _V2 / 3> H _V1 / 3 + 20
Here, H _V1 : Vickers hardness of ferrite phase H _V2 : Vickers hardness of second phase region C: 0.02 to 0.2 wt%, Si: 0.1 to 1.5 wt%, Mn: 0.5 to 3.0 wt%, S: 0.010 wt% or less, P: 0.01 to 0.15 wt%, Cr: 0.003 to 2.0 wt%, Mo: contains at least one selected from 0.003 to 2.0 wt%, further contains at least one selected from Ca: 0.0005 to 1.0 wt%, Zr: 0.0005 to 1.0 wt%, REM: 0.0005 to 0.5 wt%. The balance is a second phase composed of Fe and unavoidable impurities and having a structure of a ferrite phase having a volume fraction of 60 to 97%, and the remainder being at least one phase of martensite, bainite, and austenite. It consists of a region, yet the hardness of the ferrite phase and the second phase satisfies the following relation, and less than the grain size of the ferrite phase is 4 [mu] m, the strain rate 2000sec to the intensity of the strain rate 0.02 sec ^{-1 - 1} of the intensity increase of the intensity increase rate representing a ratio of 35% or more in true strain 0.1, characterized in that 25% or more in true strain 0.25, and crashworthiness characteristics Composite structure hot-rolled steel sheet excellent in shape retention.
H _V2 / 3> H _V1 / 3 + 20
Here, H _V1 : Vickers hardness of ferrite phase H _V2 : Vickers hardness of second phase region C: 0.02 to 0.2 wt%, Si: 0.1 to 1.5 wt%, Mn: 0.5 to 3.0 wt%, S: 0.010 wt% or less, P: 0.01 to 0.15 wt%, Cr: 0.003 to 2.0 wt%, Mo: contains at least one selected from 0.003 to 2.0 wt%, further contains at least one selected from Ti: 0.003 to 1.0 wt%, Nb: 0.003 to 0.5 wt%, V: 0.003 to 1.0 wt%, and Ni : 0.003 to 3.0 wt%, Cu: 0.003 to 3.0 wt%, B: 0.0005 to 1.0 wt%, N: 0.003 to 0.1 wt%, the balance consisting of Fe and inevitable impurities, In addition, the structure is composed of a ferrite phase having a volume fraction of 60 to 97% and a second phase region composed of at least one phase of martensite, bainite, and austenite. hardness phases satisfy the following relation, and less than 4μm crystal grain size of the ferrite phase, the increase in strength of strain rate 2000 sec ^-1 to the intensity of the strain rate 0.02 sec ^-1 Intensity increase rate representing more than 35% true strain 0.1 in, characterized in that 25% or more in true strain 0.25, composite structure hot-rolled steel sheet having excellent formability and crashworthiness characteristics.
H _V2 / 3> H _V1 / 3 + 20
Here, H _V1 : Vickers hardness of ferrite phase H _V2 : Vickers hardness of second phase region C: 0.02 to 0.2 wt%, Si: 0.1 to 1.5 wt%, Mn: 0.5 to 3.0 wt%, S: 0.010 wt% or less, P: 0.01 to 0.15 wt%, Cr: 0.003 to 2.0 wt%, Mo: contains at least one selected from 0.003 to 2.0 wt%, and further contains at least one selected from Ti: 0.003 to 1.0 wt%, Nb: 0.003 to 0.5 wt%, V: 0.003 to 1.0 wt%, and Ca : 0.0005 to 1.0 wt%, Zr: 0.0005 to 1.0 wt%, REM: one or more selected from 0.0005 to 0.5 wt%, the balance being Fe and unavoidable impurities, and the structure having a volume fraction of 60 9797% of a ferrite phase and a balance of a second phase region composed of at least one phase of martensite, bainite, and austenite, and the hardness of the ferrite phase and the second phase is expressed by the following formula. satisfies the relationship, and the crystal grain size of the ferrite phase is less than 4 [mu] m, the strength increases represented by the intensity increase of the ratio of the strain rate 2000 sec ^-1 to the intensity of the strain rate 0.02 sec ^-1 There 35% or more in true strain 0.1, characterized in that 25% or more in true strain 0.25, composite structure hot-rolled steel sheet having excellent formability and crashworthiness characteristics.
H _V2 / 3> H _V1 / 3 + 20
Here, H _V1 : Vickers hardness of ferrite phase H _V2 : Vickers hardness of second phase region C: 0.02 to 0.2 wt%, Si: 0.1 to 1.5 wt%, Mn: 0.5 to 3.0 wt%, S: 0.010 wt% or less, P: 0.01 to 0.15 wt%, Cr: 0.003 to 2.0 wt%, Mo: contains at least one selected from 0.003 to 2.0 wt%, Ni: 0.003 to 3.0 wt%, Cu: 0.003 to 3.0 wt%, B: 0.0005 to 1.0 wt%, N: 0.003 to 0.1 wt% At least one selected from the group consisting of Ca: 0.0005 to 1.0 wt%, Zr: 0.0005 to 1.0 wt%, and REM: 0.0005 to 0.5 wt%, with the balance being Fe and inevitable impurities; In addition, the structure is composed of a ferrite phase having a volume fraction of 60 to 97% and a second phase region composed of at least one phase of martensite, bainite, and austenite. hardness phases satisfy the following relation, and the crystal grain size of the ferrite phase is less than 4 [mu] m, the strength increases amount of strain rate 2000 sec ^-1 to the intensity of the strain rate 0.02 sec ^-1 Intensity increase rate representing more than 35% true strain 0.1 ratio, characterized in that at least 25% true strain 0.25, composite structure hot-rolled steel sheet having excellent formability and crashworthiness characteristics.
H _V2 / 3> H _V1 / 3 + 20
Here, H _V1 : Vickers hardness of ferrite phase H _V2 : Vickers hardness of second phase region C: 0.02 to 0.2 wt%, Si: 0.1 to 1.5 wt%, Mn: 0.5 to 3.0 wt%, S: 0.010 wt% or less, P: 0.01 to 0.15 wt%, Cr: 0.003 to 2.0 wt%, Mo: contains at least one selected from 0.003 to 2.0 wt%, and further contains at least one selected from Ti: 0.003 to 1.0 wt%, Nb: 0.003 to 0.5 wt%, V: 0.003 to 1.0 wt%, Ni: 0.003 to 3.0 wt%, Cu: 0.003 to 3.0 wt%, B: 0.0005 to 1.0 wt%, N: at least one selected from 0.003 to 0.1 wt%, Ca: 0.0005 to 1.0 wt%, Zr: 0.0005 to 1.0 wt%, REM: at least one selected from 0.0005 to 0.5 wt%, the balance being Fe and unavoidable impurities, and the structure is a ferrite phase with a volume fraction of 60 to 97%, and the remainder is martensite. , A second phase region composed of at least one of bainite and austenite, and the ferrite phase and the hardness of the second phase satisfy the relationship of the following formula, and the crystal grain size of the ferrite phase: But less than [mu] m, the strength increase rate expressed as the ratio of the intensity increase of strain rate 2000 sec ^-1 to the intensity of the strain rate 0.02 sec ^-1 is 35% or more in true strain 0.1, 25% or more in true strain 0.25 Features Hot rolled composite structure steel sheet with excellent impact resistance and formability.
H _V2 / 3> H _V1 / 3 + 20
Here, H _V1 : Vickers hardness of ferrite phase H _V2 : Vickers hardness of second phase region C: 0.02 to 0.2 wt%, Si: 0.1 to 1.5 wt%, Mn: 0.5 to 3.0 wt%, S: 0.010 wt% or less, P: 0.01 to 0.15 wt%, Cr: 0.003 to 2.0 wt%, Hot rolling of a steel material containing at least one selected from Mo: 0.003 to 2.0 wt% at (Ar ₃ transformation point −50 ° C.) to (Ar ₃ transformation point + 150 ° C.), and then 0.1 to 5.0 In seconds, start cooling at a rate that satisfies the following formula, cool to 820-620 ° C, air-cool for 0.5-15 seconds, and then cool to 570-300 ° C at a cooling rate of 30 ° C / sec or more. A method for producing a hot-rolled steel sheet having a composite structure excellent in impact resistance and formability, characterized by winding.
v ≧ 1000 × t ₁ ^1/2 ÷ h ^1/2 ÷ t
Where v: cooling rate (° C / sec)
h: Plate thickness (mm)
t ₁ : Time from the end of finish rolling to the start of first cooling (sec)
t: Air cooling time (sec) from the first cooling to the start of the next cooling

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