JPH07207405A - Hot rolled steel sheet of high-strength composite structure having excellent workability and fatigue resistant characteristic and tensile strength of 45 to 65kgf/mm2 and its production - Google Patents

Abstract

PURPOSE:To provide a high-strength hot rolled steel sheet having excellent workability and fatigue resistant characteristic by limiting the components and microstructure of the steel sheet. CONSTITUTION:This high-strength hot rolled steel sheet has strength of >=45 to <=65kgf/mm<2> and excellent workability and fatigue characteristic and consists, by weight %, of a range of 0.04%<=C<=0.20%, <=2.5% Si, <=2.0% Al and <=2.0% Mn, where the equiv. of C is >=0.11 to <=0.25wt.%, the equiv. of Mn is >=0.6 to <=2.5wt.%, and Si+Al <=0.6wt.%. The final microstructure is a three phases of ferrite+bainite+austenite or four phases including martensite and the occupying rate of the ferrite is >=60%, the occupying rate of the martensite is <=3%, the occupying rate of the austenite/wt.% of C is >=35 to <=110 and the Ms of the residual austenite is <=150 deg.C. This process for production is for such hot rolled steel sheet. Such hot rolled steel sheet contributes to a reduction of the thickness of steel plates for automobiles and reduction of body weights of automobiles.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength hot-rolled composite microstructure steel sheet having excellent workability and fatigue properties, which is used in the industrial fields of automobiles, constructions, electric machines and the like, and a method for producing the same. More specifically, 45 kgf / mm ² or more 65 k
The present invention relates to a high-strength composite hot-rolled steel sheet having a tensile strength of gf / mm ² or less and excellent workability and fatigue characteristics, and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, there has been an increasing demand for vehicle body weight reduction in addition to vehicle comfort and safety. This is a request for energy conservation and environmental issues considered globally, improvement of vehicle fuel efficiency by weight reduction and CO ₂
Its purpose is to reduce harmful exhaust gas. In order to achieve such an object, the strength of the material used for the vehicle body structure is improved and the material thickness is reduced, or
It is necessary to use new low specific gravity materials.

New low specific gravity materials (eg Al, Mg, etc.)
From the viewpoints of price and stable supply, it is premised that the steel sheet is used in the coexistence state with the steel sheet that has been conventionally used as the center of the body material. In this case, the most problematic issue is scrap recycling, and the steel plate scrap mixed with other materials needs to be reused with a large amount of energy and cost for subsequent use. Therefore, in order to maintain the minimum energy and environment of the earth as a whole, excluding special parts, it is very important to take measures to reduce the weight with a single material (that is, steel), and it is expected that the strength of steel will be even higher. Has been done.

In addition to the above-mentioned requirements, the integral molding of vehicle body constituent parts is considered to be an important technical requirement for simplification and continuation of the manufacturing process. Among the steel materials used in such modernizing forming process, especially considering thin steel plate,
Having good formability is the selection criterion for the steel sheet. The formability of thin steel sheet is judged by elongation, Rankford's plastic strain ratio (r value), work hardening index (n value) and yield strength. For integral molding of complex parts, elongation and n value Is a high requirement.

As an example of a steel sheet having a large elongation and a large n value, a conventional dual-phase structure of ferrite and martensite dual P
Hase (DP) steel is known. DP Steel is Japanese Patent Sho5
As shown in Japanese Patent Publication No. 6-18051 and Japanese Patent Publication No. 59-45735, a maximum total elongation of about 30 to 35% can be obtained at 50 to 80 kgf / mm ² . However, it has a relatively low strength (35-45 kgf / mm)
It cannot be said that the strength-ductility balance is sufficient when applied to the parts requiring complicated processing such as the thin steel plate used in ² ).

As a method for further improving this material, a high strength composite structure steel sheet having a microstructure of a mixed structure of ferrite, bainite and austenite (or partly containing martensite) has been proposed. This steel sheet utilizes "transformation-induced plasticity" which shows high ductility by transforming austenite remaining at room temperature into martensite during forming. Steels that utilize transformation-induced plasticity are known as TRIP steels, for example, Zackay et al. (VF Zackay).
Et al: Trans. ASM vol. 60 (1967) 2
52) shows that the maximum is 90 at 70 kgf / mm ² or more.
%, High ductility has been achieved. However, such TRIP steel does not always meet the requirements here, such as the need to add a large amount of expensive alloying elements. As a solution to such a problem, Japanese Unexamined Patent Publication No. 61-15762.
Japanese Unexamined Patent Publication No. 5 discloses a method for manufacturing a thin steel sheet, such as an automobile steel sheet, which is suitable for a low-priced application that is premised on mass production.
The technology described in the invention of this prior application suppresses the precipitation of carbides by adding Si and advances ferrite transformation (bainite transformation) at a low temperature to effectively enrich carbon in untransformed austenite. , Stabilizes austenite. Most of these conventional techniques relate to high-strength steel plates having a tensile strength TS> 65 kgf / mm ² , but when they are used as steel plates for automobiles, a press forming method is generally used. At present, it has not been actively used due to wear of the die of the punch and die, freezing of the shape, load capacity of the press body, and the like.

When such a high-strength steel sheet is used as an undercarriage component of automobiles or a structural steel, not only the good workability described above but also fatigue durability is an important characteristic. It has been said that the above DP steel is the most excellent as a method for simultaneously achieving such good workability and high fatigue strength.

[0008]

At the level of workability and fatigue strength achieved by the conventional DP steel, the weight of the automobile body is reduced by replacing the steel plate of about 35 to 45 kgf / mm ² which has been used so far. It was not enough to measure. In addition, a method of adjusting the addition of Si and Mn to stabilize austenite in the steel material and obtaining good workability including retained austenite in the final microstructure of the steel sheet has been reported, but 45 kgf / mm ² or more 65 kg
Conventional strength of 35 to 45 kgf / mm with strength of f / mm ² or less
^It is still unclear about the steel sheet with high fatigue strength and good workability that can be substituted for about ^two steel sheets and the manufacturing method thereof.

According to the present invention, by properly selecting the alloy addition amount and combination and controlling to an optimum microstructure, it is possible to efficiently achieve a fatigue strength at a level that cannot be achieved with DP steel in the strength range of 45 to 65 kgf / mm ^2. It is an object of the present invention to provide a high-strength composite hot-rolled steel sheet having excellent workability and a method for manufacturing the same.

[0010]

The present inventors have found that C, Si,
For various steels to which Al, Mn, and Cr were added, the effects of the components and manufacturing conditions on fatigue strength and workability were investigated. As a result, they have found that the above objects can be achieved by the present invention having the following gist. (1) In% by weight, C: 0.04% or more and 0.23% or less Si: 2.5% or less Al: 2.0% or less Mn: 2.0% or less Cr: 2.0% or less , Ceq =% C + 0.0635% Si + 0.0247% M
Carbon equivalent Ceq expressed by n + 0.0123% Cr 2 is 0.11% by weight or more and 0.1.
25 wt% or less, the sum of Al and Si is 0.6 wt% or more, and Mn equivalent Mneq represented by Mneq =% Mn + 0.52% Cr is 0.6 wt% or more,
In steel containing 2.5% by weight or less and inevitable impurities, the final microstructure is ferrite, bainite, retained austenite, or three phases, or some phases containing martensite, and the main phase is ferrite. Occupancy rate of 60% or more, martensite occupancy rate of 3% or less,
The value obtained by dividing the space factor of austenite by C weight% is 35 or more and 110 or less, and the martensitic transformation start temperature (Ms) of austenite determined by the chemical components in the retained austenite is Ms ≦ 150 ° C. Excellent tensile workability and fatigue strength 45-65kgf / mm
² High strength composite structure hot rolled steel sheet.

(2) 1% or more of Ca in the range of 0.0005 to 0.01% and REM: 0.005 to 0.05% in weight%. A high-strength composite hot-rolled steel sheet having a tensile strength of 45 to 65 kgf / mm ² , which is excellent in the workability and fatigue properties described.

(3) In% by weight, C: 0.04% or more and 0.23% or less Si: 2.5% or less Al: 2.0% or less Mn: 2.0% or less Cr: 2.0% or less In the range of Ceq =% C + 0.0635% Si + 0.0247% M
Carbon equivalent Ceq expressed by n + 0.0123% Cr 2 is 0.11% by weight or more and 0.1.
25 wt% or less, the sum of Al and Si is 0.6 wt% or more, and Mn equivalent Mneq represented by Mneq =% Mn + 0.52% Cr is 0.6 wt% or more,
After casting the steel containing 2.5 wt% or less and inevitable impurities, directly or once cooling and then reheating, the hot rolling is completed within the range of Ar ₃ −50 to Ar ₃ + 140 ° C. Then, after cooling and winding in the range of 350 ° C. to 500 ° C., the final microstructure is made into three phases of ferrite, bainite, retained austenite or four phases containing some martensite, and the main phase of ferrite is A space factor of 60% or more and a martensite space factor of 3
%, The value obtained by dividing the space factor of austenite by C weight% is 35 or more and 110 or less, and the martensitic transformation start temperature (Ms) of austenite determined by the chemical component in the retained austenite is set to Ms ≦ 150 ° C. Tensile strength of 45 to 65 kgf with excellent workability and fatigue characteristics
/ Mm ² High strength composite structure hot rolled steel sheet manufacturing method.

(4) The starting steel further contains, by weight, one or more of Ca within the range of 0.0005 to 0.01% and REM within the range of 0.005 to 0.05%. The method for producing a high-strength composite microstructure hot-rolled steel sheet having a tensile strength of 45 to 65 kgf / mm ² , which is excellent in workability and fatigue characteristics according to the above-mentioned item 3.

(5) The total rolling reduction in the final finishing hot rolling step is set to 80% or more, which is the above item 3 or 4.
Tensile strength 45-65k with excellent workability and fatigue properties
A method for producing a hot rolled steel sheet having a high strength composite structure of gf / mm ² . (6) When hot rolling and cooling a slab adjusted to a predetermined component, it is cooled to a winding temperature after hot rolling at 10 ° C./sec or more, according to any one of the above items 3 to 5. Tensile strength of 45 to 65 kgf with excellent workability and fatigue properties
/ Mm ² High strength composite structure hot rolled steel sheet manufacturing method. (7) When hot rolling and cooling a slab adjusted to have a predetermined composition, the temperature is within the range of Ar ₃ to Ar ₁ up to a temperature T1 of 5 to 2.
Was cooled at 00 ° C. / sec, a temperature ranging from T1 of Ar ₁ T2
Is cooled at 5 to 20 ° C./sec.
Preliminary requirements 3 to 5 characterized by cooling at 0 ° C./second
A method for producing a high-strength composite microstructure hot-rolled steel sheet having a tensile strength of 45 to 65 kgf / mm ² , which is excellent in workability and fatigue characteristics according to any one of 1.

[0015]

The operation of each element of the invention will be described in detail below. First, the reason for defining the component range will be described. C: C is one of the most important elements used in the present invention for stabilizing austenite without using other expensive alloying elements and allowing it to remain at room temperature. Utilizing the transformation from austenite to ferrite by heat treatment to stabilize the austenite by increasing the carbon concentration in the austenite, the average C content is 0.04% by weight.
If the amount is less than the above, the residual austenite space factor finally obtained is at most 2 to 3%, and a sufficient TRIP effect cannot be expected, so this was made the lower limit of C addition. The maximum retained austenite space factor obtained increases as the average C content increases, but the C content is 0.
If it exceeds 23% by weight, it becomes difficult to obtain a strength of 65 kgf / mm ² or less no matter how the other alloying additive elements are adjusted. Therefore, 0.23 wt% was set as the upper limit of C addition.

Further, the strength of the steel sheet 45 kgf / mm ² or more, in order to 65 kgf / mm ² or less is necessary that C equal volume corrected by the alloying element content is in the proper range. That is, only when the formula (1) corrected by the addition amount of alloying element Ceq =% C + 0.0635% Si + 0.0247% Mn + 0.0123% Cr (1) is 0.11% by weight or more and 0.25% by weight or less. Since a steel plate in the above strength range was obtained, this was made the upper and lower limits of the C equivalent.

Al, Si: Al and Si are important additive elements for carbon concentration so that austenite is stable even at room temperature. Although the core of the technique of the present invention is to concentrate carbon in austenite by heating the steel sheet to the ferrite / austenite two-phase region and advancing the ferrite transformation at the time of cooling, as the ferrite transformation progresses (therefore, in the austenite). Carbide formation (with increasing carbon concentration), pearlite is formed at high temperature and upper bainite is formed at low temperature, which reduces the total carbon content in austenite and consequently the retained austenite content. Becomes As is well known, Al and Si are carbides (here, cementite)
Since it does not form a solid solution, it has the function of significantly delaying the formation of carbides. This enables efficient carbon enrichment to austenite without wasting carbon atoms in the form of carbides. For this purpose, it is indispensable that the total amount of Al and Si added is 0.6% by weight or more. Therefore, 0.6% by weight was set as the lower limit of the total amount of Al and Si added.

At this time, Si dissolves in ferrite to form a solid solution and strengthens the ferrite. Therefore, addition of an unnecessarily large amount brings about an unnecessary increase in strength of the steel sheet and deterioration of workability and toughness. Therefore, the amount added is limited to 2.5% or less. Further, in the case of Al as well, if an unnecessarily large amount is added, the workability and toughness are deteriorated, so the upper limit of the addition amount is limited to 2.0% by weight.

Since Al hardly increases the strength of the steel sheet, it is not included in Ceq of the equation (1), but Si increases the strength of the steel sheet, and therefore satisfies the equation (1) in relation to other additive elements. You need to limit the amount. Mn, Cr: Mn and Cr are also additive elements that contribute to the retention of austenite because they have a function of delaying the formation of carbides like Si and Al. In addition to this, the addition of Mn and Cr lowers the martensitic transformation start temperature of austenite. In order to stabilize austenite at room temperature, it is necessary to suppress the precipitation of carbides and increase the carbon concentration in austenite as described above, but at the same time, it is important to lower the martensitic transformation start temperature of the austenite. If the martensite transformation temperature is higher than room temperature, a portion of austenite inevitably transforms to martensite, which increases the strength of the steel sheet and deteriorates ductility. Mneq =% Mn + 0.52% Cr
When the Mn equivalent expressed by is less than 0.6% by weight, the amount of martensite produced cannot be suppressed to 3% or less while securing a sufficient amount of retained austenite.
6% by weight was the lower limit of Mn equivalent. On the other hand, the Mn equivalent is 2.
If it exceeds 5% by weight, it is difficult to reduce the strength of the steel sheet to 65 kgf / mm ² or less, so 2.5% by weight was made the upper limit of the Mn equivalent amount. Cr is an element that can be easily utilized for the purpose of the present invention because it has a smaller strengthening ability than Mn. However, when it is added in an amount of more than 2.0% by weight, the effect of leaving austenite is saturated and 2.0% by weight was set as the upper limit of Cr addition, because it is economically disadvantageous and suppresses the formation of ferrite as the main phase. Further, when Mn is also added in an amount of more than 2.0% by weight, the formation of ferrite is unnecessarily suppressed and the strength of the steel sheet is increased.

Ca, REM: Ca and REM combine with S to make inclusions spherical and improve cold workability and fatigue properties. However, when the addition amount is Ca, 0.0005
If it is less than 0.005% by weight and in the case of REM less than 0.005% by weight, the effect is not sufficient. Therefore, Ca: 0.
0005 weight% and REM: 0.005 weight% are the lower limits of the addition amount of both. Further, even if these are added excessively, not only the effect is saturated, but also the weld defects are increased, so the upper limit of the addition amount is 0.01% by weight in the case of Ca, RE
In the case of M, it is 0.05% by weight.

Next, details of the action of each component other than the components will be described. Microstructure: The steel sheet of the present invention has a strength of 45 to 65 kgf /
Since the target is a relatively low strength TRIP steel of mm ² , it is premised that soft ferrite is used as the main phase. In order to retain austenite in the final microstructure, bainite transformation is used because sufficient C concentration cannot be achieved by ferrite transformation alone. Therefore, it is desirable that the final microstructure is a mixed structure of three phases of ferrite + bainite + austenite. However, it may be difficult to set the martensite transformation temperature of austenite to room temperature or lower. In that case, in order to prevent deterioration of workability in the strength range of the present invention, the space factor of martensite is controlled to 3% or less. 3% is set as the upper limit of the martensite space factor. Further, when the space factor of soft ferrite is 60% or less, the workability of the steel sheet is significantly deteriorated, so 60% was made the lower limit of the ferrite space factor. The amount of retained austenite contained in the final structure greatly affects the workability of the steel sheet, but at the same time, the workability of austenite is one of the factors that govern the workability of the steel sheet. The working stability of austenite can be expressed by the Ms temperature of austenite, and the lower the Ms, the more stable the austenite, which works effectively in the latter stage of working and improves the ductility of the steel sheet. To reduce the Ms of austenite, Mn
It is important to increase the equivalent amount, but it is also important to increase the C concentration in austenite to a certain amount or more. In the actual manufacturing process, some of the C contained in steel is dissolved in ferrite or at grain boundaries as solid solution C, and some as carbides such as cementite, and in solid solution in martensite formed during cooling. Added C because it is wasted as C
Not all can be thickened to austenite. However, the maximum residual austenite amount finally obtained increases with an increase in the average C concentration of the steel sheet. At this time, if more austenite than necessary is left, the average C concentration in the austenite becomes low, and the stability of the austenite decreases. When the value obtained by dividing the amount of retained austenite by the amount of C exceeds 110, the workability of austenite is lowered and the workability of the steel sheet is significantly deteriorated. Therefore, 110 is defined as the upper limit of (austenite space factor) / (% C). did. Experiments show that the C concentration in austenite cannot be increased without limit. It has been confirmed that the workability of the steel sheet is better as the C concentration in austenite is higher in the range where the steel can be concentrated. However, when the above-mentioned index (austenite space factor) / (% C) becomes less than 35 and the retained austenite space factor decreases, the amount of hard products such as martensite and cementite in addition to ferrite, bainite and austenite. Increases, and as a result, the workability and fatigue properties of the steel sheet are significantly deteriorated.
5 was set as the lower limit of the above index.

Martensitic transformation start temperature (Ms): Ms of retained austenite determines the workability of retained austenite, and is therefore an important factor in determining workability and fatigue properties of steel sheet. The Ms of the retained austenite is determined by the substitutional alloying element of the steel sheet and the C concentration and N concentration experimentally obtained by X-ray analysis and Moessbauer spectroscopy. Since the N concentration in the steel sheet is very small compared to the C concentration,
As the interstitial element, it is preferable to mainly consider the C concentration. C
The concentration is determined, for example, by X-ray analysis using Kα rays of Mo, using the integrated reflection intensities of the (200) plane, (211) plane of ferrite and the (200) plane, (220) plane, and (311) plane of austenite. , Journal of T
he Iron and Steel Institute
te, 206 (1968) p60. The Ms of the retained austenite using these element concentrations is, for example, Ms (° C.) = 561−325 ×% C-33 ×% Mn−
17x% Ni-17x% Cr-21x% Mo-20x
It can be calculated using% Cu.

When the steel sheet is applied to structural parts for automobiles, it is generally used after being processed by press forming or roll forming. At this time, it is important from the viewpoint of workability that the retained austenite is effectively utilized during plastic deformation, but on the other hand, from the viewpoint of fatigue strength in the actual use stage, it is possible that the retained austenite remains effective after working. is necessary. The effect of the Ms temperature on the workability is as shown in FIG. 2. Good workability can be obtained by setting Ms ≦ 150 ° C. The relationship between fatigue strength and Ms of retained austenite is as shown in FIG.
High fatigue strength is achieved by setting the temperature to 50 ° C. Further, the fatigue strength after 10% pre-working in the range of uniform elongation as the amount of plastic working does not deteriorate if Ms ≦ 150 ° C. before pre-working. Therefore, in the present invention, the Ms of retained austenite is
Is limited to 150 ° C or lower.

Hot rolling conditions: When the steel sheet of the present invention is produced by hot rolling, a slab adjusted to have a predetermined composition is heated as it is or after being once cooled and then hot-rolled. At this time, the hot rolling completion temperature is determined by the chemical composition of the steel material Ar
_{3 If the} transformation temperature is lower than -50 ° C, a work ferrite layer is formed in the surface layer of the steel sheet and in the vicinity thereof, and workability and fatigue strength are significantly deteriorated. Therefore, this is the lower limit of the hot rolling completion temperature. The hot rolling completion temperature is Ar ₃ +140.
If the temperature exceeds ℃, the proportion of low-temperature generation layers other than ferrite will increase, not only will the strength of the steel plate increase more than necessary, but
Coarsening of ferrite grains occurs and deteriorates the fatigue strength of the steel sheet. Further, when hot rolling is completed at such a high temperature, the surface roughness of the steel sheet increases, and the fatigue strength of the steel sheet deteriorates. Therefore, this is the upper limit of the hot rolling completion temperature.

Final finishing of hot rolling The total rolling reduction in the hot rolling process is 80
If it is less than%, the coarsening of ferrite, which is the main phase of the steel sheet, becomes severe and the fatigue strength of the steel sheet deteriorates, so this was made the lower limit. After the hot rolling is completed, the material is cooled and wound up, but if the winding temperature exceeds 500 ° C, retained austenite cannot be obtained, and if it is less than 350 ° C, martensite is excessively formed to improve workability. Spoil. Therefore, these are set as the upper and lower limits of the winding temperature after hot rolling. In order to optimize the balance between the workability and the fatigue strength of the final steel sheet, it is desirable that the winding temperature be 350 ° C or higher and 450 ° C or lower.

Cooling to the coiling temperature after hot rolling is an important factor to secure the amount of ferrite in the final structure of the steel sheet and to avoid waste of C used for stabilizing austenite as much as possible. When cooling after hot rolling in a single stage from the hot rolling completion temperature to the coiling temperature, if the cooling rate is less than 10 ° C./sec, pearlite transformation proceeds during cooling, and C is wasted. Since it deteriorates the workability and fatigue characteristics of a typical steel sheet, this was made the lower limit of the cooling rate in the first-stage cooling. On the other hand, the method of gradually cooling the ferrite transformation temperature range in order to efficiently progress the ferrite transformation during cooling is effective for improving the workability and fatigue characteristics of the final product. At this time, after hot rolling, Ar ₃ and Ar ₁
If the cooling rate to the temperature T1 during the period is less than 5 ° C./sec, the ferrite grain size becomes coarse and the fatigue properties of the steel sheet deteriorate, so this was made the lower limit. A cooling rate of more than 200 ° C./sec is difficult to achieve in the actual manufacturing process, so this was made the upper limit. Next, from T1 to T to accelerate the progress of ferrite transformation
Gradually cool to a temperature T2 between 1 and Ar ₁ . At this time, if the cooling rate is less than 5 ° C./sec, the ferrite grain size becomes coarse and the fatigue properties of the steel sheet deteriorate, so this was made the lower limit. If the temperature exceeds 20 ° C / sec, the transformation is not sufficiently promoted,
This is set as the upper limit because it deteriorates the workability of the finally obtained steel sheet. When cooling below T2 is less than 20 ° C./sec, pearlite transformation proceeds during cooling, C is wasted, and the workability and fatigue properties of the final product are deteriorated, so this was made the lower limit. A cooling rate of more than 200 ° C./sec is difficult to achieve in the actual manufacturing process, so this was made the upper limit. The above cooling is started immediately after the hot rolling is completed, but 0.1 to 4.0 after the hot rolling is completed due to the structure of the hot rolling apparatus.
Although air cooling may be performed for a second, if the subsequent cooling is within the scope of the present invention, the intended material of the steel sheet can be obtained. Similarly, air-cooling is performed from the end of the cooling zone where cooling is performed after hot rolling to the time of actual winding, but the temperature drop here is small and can be ignored.

[0027]

EXAMPLES For each steel type shown in Table 1 and Table 2 (continued from Table 1), Table 3, Table 4 (continued from Table 3-1), Table 5 (continued from Table 3-2), Table 6 ( Continuation of Table 3-3), Table 7 (Table 3
-4) and hot rolling under various conditions shown in Table 8 (Continued-5 of Table 3) to obtain a hot rolled steel sheet having a thickness of 2.5 mm, mechanical property investigation and determination of residual austenite were performed. I was broken. The cooling conditions after hot rolling are as shown in FIG. The thickness of the partially finished plate was also changed for the purpose of changing the cooling rate after hot rolling. Hot rolling completion temperature (FT ° C), total rolling reduction (%), T1 and T2 temperatures (° C), cooling rate from the end of hot rolling to T1 (CR1 ° C / sec), T1 to T2
Cooling rate (CR2 ° C / sec), cooling rate from T2 to winding (CR3 ° C / sec) and winding temperature (CT)
(° C) is shown in Table 2. In the table, Vf%, Vg%, and Vm% are ferrite, retained austenite, and martensite space factor in the steel sheet, Ceq and Mneq are C equivalent and Mn equivalent shown in claim 1, and the toughness is particularly high. In the case of deterioration, x is shown in that column, and in the case of being equivalent to the conventional material, o is shown. Cγ
(%) Is the measured carbon concentration in retained austenite,
Ms is a martensitic transformation start temperature (° C) calculated from Cγ and the amount of other alloying elements added (average value of steel). Further, the fatigue strength of the steel sheet is the fatigue strength σW (kgf / m) at 2 × 10 ⁶ times obtained by the plane bending fatigue test of full swing.
The value (fatigue limit ratio) obtained by dividing m ² ) by the strength of the steel sheet was evaluated. Regarding the fatigue strength after pre-working, the case where σW after 10% tensile pre-strain is equal to or more than the case without pre-strain is ○, and the deterioration margin is over 5% of σW ×
And In the column of Cγ and Ms? Indicates that the carbon concentration in the retained austenite could not be measured because the amount of retained austenite was small or the retained austenite was not contained.

From the table, the steel sheet satisfying the conditions of the present invention (indicated by the present invention steel in the table) has strength in the range of 45 to 65 kgf / mm ² , has excellent breaking elongation, and has strength and breaking strength. It can be seen that good workability and strength balance of elongation product TS × El of 2200 kgf / mm ² ×% or more are achieved and at the same time high fatigue strength of fatigue limit ratio of 0.5 or more is achieved. .

[0029]

[Table 1]

[0030]

[Table 2]

[0031]

[Table 3]

[0032]

[Table 4]

[0033]

[Table 5]

[0034]

[Table 6]

[0035]

[Table 7]

[0036]

[Table 8]

[0037]

As described above, according to the present invention, 45
It becomes possible to manufacture a high-strength steel sheet having excellent workability of up to 65 kgf / mm ² and fatigue characteristics, and when applied to automobile parts, it can greatly contribute to weight reduction of automobile bodies.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a fatigue limit ratio of a hot-rolled steel sheet having a tensile strength of 45 to 65 kfg / mm ² and an Ms temperature of retained austenite.

FIG. 2 is a diagram showing the relationship between the fracture elongation of hot-rolled steel sheets having a tensile strength of 45 to 65 kgf / mm ² and the Ms temperature of retained austenite.

FIG. 3 is a conceptual diagram of cooling conditions after hot rolling, where the cooling pattern is for cooling to the coiling temperature by one-stage cooling after hot rolling, and the cooling pattern is for gradually cooling the ferrite transformation temperature range. Show.

Claims (7)

[Claims] 1. C .: 0.04% or more and 0.23% or less Si: 2.5% or less Al: 2.0% or less Mn: 2.0% or less Cr: 2.0% or less In the range, Ceq =% C + 0.0635% Si + 0.0247% M
Carbon equivalent Ceq expressed by n + 0.0123% Cr 2 is 0.11% by weight or more and 0.1.
25 wt% or less, the sum of Al and Si is 0.6 wt% or more, and Mn equivalent Mneq represented by Mneq =% Mn + 0.52% Cr is 0.6 wt% or more,
In steel containing 2.5% by weight or less and inevitable impurities, the final microstructure is ferrite, bainite, retained austenite, or three phases, or some phases containing martensite, and the main phase is ferrite. Occupancy rate of 60% or more, martensite occupancy rate of 3% or less,
The value obtained by dividing the space factor of austenite by C weight% is 35 or more and 110 or less, and the martensitic transformation start temperature (Ms) of austenite determined by the chemical components in the retained austenite is Ms ≦ 150 ° C. Excellent tensile workability and fatigue strength 45-65kgf / mm
² High strength composite structure hot rolled steel sheet. 2. The content of Ca in the range of 0.0005 to 0.01% and REM in the range of 0.005 to 0.05% by weight, and one or more of them are contained. A high-strength composite hot-rolled steel sheet having a tensile strength of 45 to 65 kgf / mm ² , which is excellent in the workability and fatigue properties described. 3. By weight%, C: 0.04% or more and 0.23% or less Si: 2.5% or less Al: 2.0% or less Mn: 2.0% or less Cr: 2.0% or less In the range, Ceq =% C + 0.0635% Si + 0.0247% M
Carbon equivalent Ceq expressed by n + 0.0123% Cr 2 is 0.11% by weight or more and 0.1.
25 wt% or less, the sum of Al and Si is 0.6 wt% or more, and Mn equivalent Mneq represented by Mneq =% Mn + 0.52% Cr is 0.6 wt% or more,
After casting the steel containing 2.5 wt% or less and inevitable impurities, directly or once cooling and then reheating, the hot rolling is completed within the range of Ar ₃ −50 to Ar ₃ + 140 ° C. Then, after cooling and winding in the range of 350 ° C. to 500 ° C., the final microstructure is made into three phases of ferrite, bainite, retained austenite or four phases containing some martensite, and the main phase of ferrite is A space factor of 60% or more and a martensite space factor of 3
%, The value obtained by dividing the space factor of austenite by C weight% is 35 or more and 110 or less, and the martensitic transformation start temperature (Ms) of austenite determined by the chemical component in the retained austenite is set to Ms ≦ 150 ° C. Tensile strength of 45 to 65 kgf with excellent workability and fatigue characteristics
/ Mm ² High strength composite structure hot rolled steel sheet manufacturing method. 4. The starting steel further contains one or more of these in the range of Ca: 0.0005 to 0.01% and REM: 0.005 to 0.05% by weight. The method for producing a high-strength composite hot-rolled steel sheet having a tensile strength of 45 to 65 kgf / mm ² excellent in workability and fatigue properties according to claim 3. 5. The total rolling reduction in the final hot rolling process is 80% or more, and the tensile strength of 45 to 65 kg is excellent in workability and fatigue properties according to claim 3 or 4.
A method for producing a hot rolled steel sheet having a high strength composite structure of f / mm ² . 6. The hot rolling of a slab adjusted to a predetermined composition and cooling is performed at a temperature of 10 ° C./sec or more up to the coiling temperature after hot rolling. Tensile strength of 45-excellent in workability and fatigue characteristics described in item 1
A method for producing a high strength composite structure hot rolled steel sheet of 65 kgf / mm ² . 7. A hot rolled slab adjusted to a predetermined composition
Ar when cooling ₃To Ar₁Up to temperature T1
Is cooled at 5 to 200 ° C / sec, and from T1 to Ar ₁Range of
Cool down to temperature T2 at 5 to 20 ° C./sec, and lower than 2
4. Cooling at 0 to 200 [deg.] C./sec.
Excellent in workability and fatigue properties according to any one of 1 to 5
Tensile strength 45-65kgf / mm²High strength composite set
Manufacturing method of woven hot rolled steel sheet.