JP3248118B2 - High strength composite structure hot rolled steel sheet having a tensile strength of 45 to 65 kgf / mm2 excellent in workability and fatigue properties, and a method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled steel sheet having a high-strength composite structure and excellent in workability and fatigue properties, which is used in the industrial fields such as automobiles, construction and electric machines, and a method for producing the same. More specifically, 45 kgf / mm ² or more and 65 k
The present invention relates to a high-strength composite structure hot-rolled steel sheet having tensile strength of not more than gf / mm ² and excellent in workability and fatigue properties, and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, there has been an increasing demand for lighter vehicle bodies in addition to the comfort and safety of automobiles. 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 a purpose, it is necessary to improve the strength of the material used for the body structure and reduce the thickness of the material,
It is necessary to use a new material having a low specific gravity.

[0003] New low specific gravity materials (eg Al, Mg, etc.)
In the case of using steel, it is considered that it is premised that the steel is used in the coexistence state with the steel sheet which has been conventionally used as the main component of the vehicle body material from the viewpoint of price and stable supply. The most problematic in this case is the recycling of the scrap, and the steel sheet scrap mixed with other materials needs to be reused at the expense of a lot of energy and cost. Therefore, it is very important to reduce the weight of a single material (ie, steel) except for special parts in order to minimize energy and maintain the environment as a whole, and it is expected that steel will have even higher strength. Have been.

[0004] In addition to the above requirements, integral molding of vehicle body components is considered to be an important technical requirement for simplification and continuity of the manufacturing process. Among steel materials used in such a modern forming process, especially considering thin steel sheets,
Having good formability is a criterion for selecting the steel sheet. The formability of thin steel sheets is judged by elongation, Rankford's plastic strain ratio (r-value), work hardening index (n-value) and yield strength, and elongation and n-value for integral molding of complex parts. Is one requirement.

[0005] As an example of a steel sheet having a large elongation or n value, there is a dual ferrite and martensite dual-phase structure of Dual P
Hase (DP) steel is known. DP steel is No. 5
As disclosed in JP-A-6-18051 and JP-B-59-45735, a total elongation of up to about 30 to 35% can be obtained at 50 to 80 kgf / mm ² . However, conventionally relatively low strength (35-45 kgf / mm
It is hard to say that a sufficient strength-ductility balance is obtained when the method is applied to a part requiring complicated processing such as the use of a thin steel sheet of ² ).

As a method for further improving this material, a high-strength composite steel sheet having a microstructure of a mixed structure of ferrite, bainite and austenite (or partially including martensite) has recently 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 utilizing 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 a maximum of 90 kgf / mm ² or more
% Ductility has been achieved. However, such a TRIP steel does not always meet the requirements here, for example, it is necessary to add a large amount of expensive alloying elements. Japanese Patent Application Laid-Open No. 61-15762 discloses a solution to such a problem.
No. 5 discloses a method for producing a thin steel sheet suitable for an inexpensive use which is premised on mass production such as a steel sheet for an automobile.
The technique described in the prior application invention suppresses the precipitation of carbides by adding Si and promotes ferrite transformation (bainite transformation) at a low temperature, thereby effectively enriching carbon in untransformed austenite. , To stabilize austenite. Most of these prior arts relate to high-strength steel sheets having a tensile strength TS> 65 kgf / mm ² , but when they are used as steel sheets for automobiles, a press forming method is generally used. At present, it has not yet been actively used due to the abrasion of the punch and die dies, the shape freezing property, the load capacity of the press machine body, and the like.

When such a high-strength steel sheet is used as an undercarriage part of an automobile or structural steel, not only the above-mentioned good workability but also fatigue durability are important characteristics. As a method for simultaneously achieving such good workability and high fatigue strength, the above-mentioned DP steel has been conventionally said to be the most excellent.

[0008]

The level of workability and fatigue strength achieved by conventional DP steel is to reduce the weight of automobile bodies by substituting steel plates of about 35 to 45 kgf / mm ² which have been used so far. It was not enough to measure. Also, a method of adjusting the addition of Si and Mn to stabilize austenite in a steel material and obtaining good workability including residual austenite in a microstructure of a final steel sheet has been reported. mm ² or more 65kg
Conventional 35~45kgf / mm at f / mm ² or less of the intensity
It is still unclear steel sheet and a manufacturing method thereof provided with a high fatigue strength that can replace the ² order of steel plate and good processability.

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

[0010]

Means for Solving the Problems The present inventors have proposed C, Si,
For various steels to which Al, Mn, and Cr were added, effects of 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) 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
The carbon equivalent Ceq expressed by n + 0.0123% Cr 2 is 0.11% by weight or more.
25% by weight or less, and the sum of Al and Si is 0.6% by weight or more, and the Mn equivalent Mneq expressed by Mneq =% Mn + 0.52% Cr is 0.6% by weight or more;
In a steel containing 2.5% by weight or less and further containing unavoidable impurities, the final microstructure is made of three phases of ferrite, bainite, retained austenite or four phases partially 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 austenite martensitic transformation start temperature (Ms) determined by the chemical component in the retained austenite is Ms ≦ 150 ° C. Tensile strength 45-65kgf / mm with excellent workability and fatigue properties
^2. High strength composite structure hot rolled steel sheet.

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

(3) In 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
The carbon equivalent Ceq expressed by n + 0.0123% Cr 2 is 0.11% by weight or more.
25% by weight or less, and the sum of Al and Si is 0.6% by weight or more, and the Mn equivalent Mneq expressed by Mneq =% Mn + 0.52% Cr is 0.6% by weight or more;
2.5 or less wt%, after casting the steel containing more unavoidable impurities, is heated again after the direct or once cooled, hot rolled completed within the Ar ₃ -50~Ar ₃ + 140 ℃ Then, after cooling and winding in the range of 350 ° C. to 500 ° C., the final microstructure becomes three phases of ferrite, bainite, and retained austenite or four phases partially containing martensite. Space factor over 60%, Martensite space factor 3
%, The value obtained by dividing the space factor of austenite by C weight% is 35 or more and 110 or less, and the austenite martensitic transformation start temperature (Ms) determined by the chemical components in the retained austenite is Ms ≦ 150 ° C. Excellent tensile strength 45-65kgf 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 a range of Ca: 0.0005 to 0.01% and REM: 0.005 to 0.05% by weight. 3. A method for producing a hot-rolled steel sheet with a high-strength composite structure having a tensile strength of 45 to 65 kgf / mm ² , which is excellent in workability and fatigue characteristics according to item 3 above.

(5) The total rolling reduction in the final finishing hot rolling step is set to 80% or more.
Excellent tensile strength 45-65k with the described workability and fatigue properties
A method for producing a hot-rolled steel sheet having a gf / mm ² high-strength composite structure. (6) The method according to any one of (3) to (5) above, wherein when the slab adjusted to a predetermined component is hot-rolled and cooled, the slab is cooled at a rate of 10 ° C./sec or more to a winding temperature after hot rolling. Tensile strength 45-65 kgf with excellent workability and fatigue properties as described
/ Mm ² High strength composite structure hot rolled steel sheet manufacturing method. (7) when the slab is adjusted to a predetermined component thermally cast cooling, to a temperature T1 in the range from Ar ₃ to Ar ₁ is 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.
Claims 3 to 5 characterized by cooling at 0 ° C / sec.
The method for producing a hot-rolled steel sheet with a high-strength composite structure having a tensile strength of 45 to 65 kgf / mm ² and excellent in workability and fatigue properties according to any one of the above items.

[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 in the present invention used for stabilizing austenite without using other expensive alloying elements and remaining at room temperature. Austenite is stabilized by increasing the carbon concentration in austenite by utilizing the transformation of austenite to ferrite by heat treatment, but the average C content is 0.04% by weight.
If it is less than 3, the finally obtained retained austenite space factor is at most 2 to 3%, and a sufficient TRIP effect cannot be expected. As the average C content increases, the maximum retained austenite space factor obtained increases, but the hardenability of the steel sheet also increases, so that the C content is reduced to 0.1%.
If it exceeds 23% by weight, it is difficult to obtain a strength of 65 kgf / mm ² or less, no matter how the other alloying elements are adjusted. Therefore, the upper limit of C addition was 0.23% by weight.

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 equation (1) corrected by the addition amount of the alloy element is 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 sheet having the above-mentioned strength range was obtained, these were set as the upper and lower limits of the C equivalent.

Al, Si: Al and Si are important addition elements for concentrating carbon so that austenite is stable even at room temperature. Heating a steel sheet to the ferrite / austenite two-phase region and enriching carbon in austenite by allowing the ferrite transformation to proceed during cooling is central to the technology of the present invention. Increasing the formation of carbides (as the carbon concentration of the steel increases), producing pearlite at higher temperatures and upper bainite at lower temperatures, reducing the total carbon content in austenite and consequently reducing the amount of retained austenite Becomes As is well known, Al and Si are carbides (here, cementite).
Since it does not form a solid solution with carbon, it has a function of significantly delaying the formation of carbides. This allows efficient carbon enrichment to austenite without wasting carbon atoms in the form of carbides. For this function, it is essential that the total amount of addition of Al and Si is 0.6% by weight or more, so 0.6% by weight was set as the lower limit of the total amount of addition of Al and Si.

At this time, Si dissolves in the ferrite and strengthens the ferrite, so that an unnecessarily large addition causes an unnecessary increase in the strength of the steel sheet and a deterioration in workability and toughness. Therefore, the amount of addition was limited to 2.5% or less. Also, in the case of Al, if an unnecessarily large amount is added, the workability and toughness are deteriorated, so the upper limit of the amount added is limited to 2.0% by weight.

Al does not substantially increase the strength of the steel sheet and thus is not included in the Ceq of the equation (1), but Si increases the strength of the steel sheet and satisfies the equation (1) in relation to other added elements. Need to limit to quantity. Mn, Cr: Mn and Cr are elements that contribute to the retention of austenite because they have a function of delaying the formation of carbides, similarly to Si and Al. In addition, the addition of Mn and Cr lowers the martensitic transformation start temperature of austenite. 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. At the same time, it is also important to lower the martensitic transformation start temperature of the austenite. If the martensite transformation temperature is higher than room temperature, a part of austenite is inevitably transformed into martensite, increasing the strength of the steel sheet and deteriorating the ductility. Mneq =% Mn + 0.52% Cr
When the Mn equivalent represented by the formula is less than 0.6% by weight, the amount of martensite cannot be suppressed to 3% or less while securing a sufficient amount of retained austenite.
6 wt% was the lower limit of the 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. Therefore, 2.5% by weight was set as the upper limit of the Mn equivalent. Cr is an element that is easy to use for the purpose of the present invention because it has a smaller strengthening ability than Mn, but when added in excess of 2.0% by weight, the effect of retaining austenite is not only saturated, but also 2.0% by weight was set as the upper limit of the addition of Cr because of economic disadvantages and suppression of ferrite as a main phase. Further, when Mn is added in excess of 2.0% by weight, the formation of ferrite is unnecessarily suppressed and the strength of the steel sheet is increased. Therefore, 2.0% by weight is set as the upper limit of Mn addition.

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 the content is less than 0.005% by weight in the case of REM, the effect is not sufficient. Therefore, Ca: 0.
0005% by weight and REM: 0.005% by weight are the lower limits of the addition amount of both. Even if these are added excessively, the effect is not only saturated, but also the weld defect is increased.
In the case of M, it is 0.05% by weight.

Next, the operation of each component other than the components will be described in detail. Microstructure: The steel sheet of the present invention has a strength of 45 to 65 kgf /
Since it is intended for a relatively low-strength TRIP steel of mm ² , it is assumed that soft ferrite is used as a main phase. In order to leave austenite in the final microstructure, bainite transformation is used because sufficient ferrite transformation alone cannot achieve sufficient C enrichment. Therefore, it is desirable that the final microstructure be a three-phase mixed structure of ferrite + bainite + austenite. However, in some cases, it is difficult to reduce the martensite transformation temperature of austenite to room temperature or lower. In such a case, the space factor of martensite is controlled to 3% or less in order to prevent the workability from deteriorating within the strength range of the present invention. Therefore, the upper limit of the martensite space factor is set to 3%. When the space factor of soft ferrite is 60% or less, the workability of the steel sheet is significantly deteriorated. Therefore, the lower limit of the space factor of ferrite is set to 60%. The amount of retained austenite contained in the final structure greatly affects the workability of the steel sheet, and at the same time, the work stability 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, and it works effectively in the later stage of working to improve the ductility of the steel sheet. To lower 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, part of the C contained in the steel is dissolved as solid solution in ferrite or at grain boundaries, part is converted into carbide such as cementite, and further dissolved in martensite generated during cooling. Since C is wasted, the added C
Not all can be concentrated to austenite. However, the ultimately obtained maximum retained austenite amount increases with an increase in the average C concentration of the steel sheet. At this time, if more than necessary austenite remains, the average C concentration in the austenite decreases, and the stability of austenite decreases. If the value obtained by dividing the amount of retained austenite by the amount of C exceeds 110, the work stability of austenite is reduced and the workability of the steel sheet is significantly deteriorated. Therefore, 110 is set to the upper limit of (austenite space factor) / (% C). did. Experiments have shown that the C concentration in austenite cannot be increased without limit. It has been confirmed that the higher the C concentration in austenite is, the better the workability of the steel sheet is in the range in which enrichment is possible. However, when the residual austenite space factor decreases as the above index (austenite space factor) / (% C) becomes less than 35, the amount of hard products such as martensite and cementite besides ferrite, bainite and austenite is reduced. Increase, and as a result, significantly deteriorate the workability and fatigue properties of the steel sheet,
5 was set as the lower limit of the above index.

Martensitic transformation onset temperature (Ms): Ms of retained austenite is an important factor that determines the work stability of retained austenite and therefore determines the workability and fatigue properties of a steel sheet. The Ms of the retained austenite is determined by the substitutional alloy element of the steel sheet and the C and N concentrations experimentally determined by X-ray diffraction and Mossbauer spectroscopy. Since the N concentration in the steel sheet is smaller than the C concentration,
As the interstitial element, it is good to mainly consider the C concentration. C
The concentration is determined by, for example, the integrated reflection intensity of the (200) plane, (211) plane of ferrite and the (200) plane, (220) plane, and (311) plane of austenite by X-ray analysis using Mo's Kα ray. , Journal of T
he Iron and Steel Institute
te, 206 (1968) p60. Ms of retained austenite is expressed by using these element concentrations, for example, when expressing the alloy concentration by weight%, Ms (° C.) = 561-325 ×% C-33 ×% Mn-
17x% Ni-17x% Cr-21x% Mo-20x
% Can be calculated.

When a 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 used during plastic deformation.On the other hand, in terms of fatigue strength in the actual use stage, it is necessary that effective retained austenite remains after processing. 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.
By setting the temperature at 50 ° C., high fatigue strength is achieved. In addition, the fatigue strength after 10% pre-working within 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, Ms of retained austenite
To 150 ° C. or less.

Hot-rolling conditions: When producing the steel sheet of the present invention while hot-rolling, a slab adjusted to a predetermined component is heated again after casting or once cooled and then hot-rolled. At this time, the hot rolling completion temperature is determined by the chemical composition of the steel material.
_{(3) When the} transformation temperature is lower than −50 ° C., a processed ferrite layer is formed on the surface layer of the steel sheet and in the vicinity thereof, and the workability and the fatigue strength are significantly deteriorated. Therefore, this is set as the lower limit of the hot rolling completion temperature. The hot rolling completion temperature is Ar ₃ +140.
If the temperature exceeds ℃, the ratio of the low-temperature generation layer other than ferrite increases, and not only does the strength of the steel sheet unnecessarily increase,
Ferrite grains become coarse and deteriorate 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 set as the upper limit of the hot rolling completion temperature.

Final Rolling of Hot Rolling The total rolling reduction in the hot rolling step is 80.
%, The ferrite, which is the main phase of the steel sheet, becomes so coarse that the fatigue strength of the steel sheet deteriorates. After the completion of hot rolling, cooling is performed and a winding process is performed. If the winding temperature is higher than 500 ° C, retained austenite is not obtained. If the winding temperature is lower than 350 ° C, martensite is excessively generated and workability is increased. Spoil. Therefore, these are 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 more and 450 ° C or less.

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 minimize waste of carbon used for stabilizing austenite. In the case of cooling after hot rolling in a single stage from the hot rolling completion temperature to the winding temperature, if the cooling rate is less than 10 ° C./sec, pearlite transformation proceeds during cooling, and C is wasted. This deteriorates the workability and fatigue characteristics of a typical steel sheet, and this is set as the lower limit of the cooling rate in the single-stage cooling. On the other hand, a method of gradually cooling the ferrite transformation temperature range in order to promote the ferrite transformation efficiently during cooling is effective for improving the workability and fatigue properties of the final product. At this time, Ar ₃ and Ar ₁
If the cooling rate up to the temperature T1 during this 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 set as the lower limit. Since a cooling rate exceeding 200 ° C./sec is difficult to achieve in the actual manufacturing process, the cooling rate is set as the upper limit. Next, from T1 to T to promote the progress of ferrite transformation
Slow cooling to a temperature T2 of between 1 and Ar _1. At this time, when 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.
Since the workability of the finally obtained steel sheet deteriorates, this is set as the upper limit. If the cooling at T2 or less is less than 20 ° C./sec, the pearlite transformation proceeds during cooling and wastes C, and the workability and fatigue properties of the final product are deteriorated. Since a cooling rate exceeding 200 ° C./sec is difficult to achieve in the actual manufacturing process, the cooling rate is set as the upper limit. The above cooling is started immediately after the completion of hot rolling, but 0.1 to 4.0 after the completion of hot rolling 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 desired steel plate material can be obtained. Similarly, air cooling is performed from the end point of the cooling zone in which cooling after hot rolling is performed to the time point when winding is actually performed, but the temperature drop here is small and can be ignored.

[0027]

EXAMPLES For each steel type shown in Tables 1 and 2 (continued in Table 1), Tables 3 and 4 (continued in Table 3-1), Table 5 (continued in Table 3-2), and Table 6 (continued in Table 3) Table 3 (continuation-3), Table 7 (Table 3)
Hot rolling was performed under the various conditions shown in Table 4 (Continued in Table 4) and Table 8 (Continued in Table 3-5) to obtain a hot-rolled steel sheet having a thickness of 2.5 mm. Was done. The cooling conditions after hot rolling are as shown in FIG. In order to change the cooling rate after hot rolling, the thickness of the final finished plate was also partially changed. Hot rolling completion temperature (FT ° C), finish rolling total reduction (%), T1 and T2 temperatures (° C), cooling rate from 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, martensite occupancy, Ceq, and Mneq in the steel sheet are C equivalent and Mn equivalent shown in claim 1, respectively. In the case of deterioration, "X" was shown in that column, and in the case of equivalent to the conventional material, "O" was shown. Cγ
(%) Is the measured carbon concentration in retained austenite,
Ms is the martensite 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 was determined by a 2 × 10 ⁶ fatigue strength σW (kgf / m
m ² ) was divided by the strength of the steel sheet (fatigue limit ratio). Regarding the fatigue strength after pre-processing, ○ W after 10% tensile pre-strain is given is equal to or greater than the case without pre-strain, and x is indicated when the deterioration allowance exceeds 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 did not include the retained austenite.

According to the table, the steel sheet satisfying the conditions of the present invention (indicated as the present invention steel in the table) has a strength in the range of 45 to 65 kgf / mm ² , has excellent breaking elongation, It can be seen that a good balance between workability and strength with a product of elongation TS × El of 2200 kgf / mm ² ×% or more is achieved, and at the same time, high fatigue strength with a 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 is possible to manufacture a high-strength steel sheet excellent in workability and fatigue properties of up to 65 kgf / mm ² , and when applied to automobile parts, it can greatly contribute to weight reduction of an automobile body.

[Brief description of the drawings]

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

FIG. 2 is a graph showing the relationship between the breaking elongation of a hot-rolled steel sheet 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, in which a cooling pattern is a case of cooling to a winding temperature by one stage cooling after hot rolling, and a cooling pattern is a case of gradually cooling a ferrite transformation temperature region. Show.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-62-174322 (JP, A) JP-A-55-44582 (JP, A) JP-A-4-329848 (JP, A) JP-A Sho 56-174848 105422 (JP, A) JP-A-59-41424 (JP, A) JP-A-62-93006 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00-38 / 60 C21D 8/02 C21D 9/46

Claims (7)

(57) [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
The carbon equivalent Ceq expressed by n + 0.0123% Cr 2 is 0.11% by weight or more.
25% by weight or less, and the sum of Al and Si is 0.6% by weight or more, and the Mn equivalent Mneq expressed by Mneq =% Mn + 0.52% Cr is 0.6% by weight or more;
In a steel containing 2.5% by weight or less and further containing unavoidable impurities, the final microstructure is made of three phases of ferrite, bainite, retained austenite or four phases partially 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 austenite martensitic transformation start temperature (Ms) determined by the chemical component in the retained austenite is Ms ≦ 150 ° C. Tensile strength 45-65kgf / mm with excellent workability and fatigue properties
^2. High strength composite structure hot rolled steel sheet. 2. The composition according to claim 1, wherein one or more of these are contained in a range of 0.0005 to 0.01% REM: 0.005 to 0.05% by weight%. A high-strength composite structure hot-rolled steel sheet having a tensile strength of 45 to 65 kgf / mm ² and excellent in the described workability and fatigue properties. 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, Ceq =% C + 0.0635% Si + 0.0247% M
The carbon equivalent Ceq expressed by n + 0.0123% Cr 2 is 0.11% by weight or more.
25% by weight or less, and the sum of Al and Si is 0.6% by weight or more, and the Mn equivalent Mneq expressed by Mneq =% Mn + 0.52% Cr is 0.6% by weight or more;
2.5 or less wt%, after casting the steel containing more unavoidable impurities, is heated again after the direct or once cooled, hot rolled completed within the Ar ₃ -50~Ar ₃ + 140 ℃ Then, after cooling and winding in the range of 350 ° C. to 500 ° C., the final microstructure becomes three phases of ferrite, bainite, and retained austenite or four phases partially containing martensite. Space factor over 60%, Martensite space factor 3
%, The value obtained by dividing the space factor of austenite by C weight% is 35 or more and 110 or less, and the austenite martensitic transformation start temperature (Ms) determined by the chemical components in the retained austenite is Ms ≦ 150 ° C. Excellent tensile strength 45-65kgf 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 a range of Ca: 0.0005 to 0.01% and REM: 0.005 to 0.05% by weight. 4. The method for producing a hot-rolled steel sheet with a high-strength composite structure having a tensile strength of 45 to 65 kgf / mm ² and excellent in workability and fatigue properties according to claim 3. 5. A tensile strength of 45 to 65 kg having excellent workability and fatigue characteristics according to claim 3 or 4, wherein the total draft in the final finishing hot rolling step is 80% or more.
A method for producing a hot-rolled steel sheet with a high strength composite structure of f / mm ² . 6. The hot-rolling and cooling of a slab adjusted to a predetermined component is cooled to a winding temperature of 10 ° C./sec or more after hot-rolling. 4. Tensile strength 45-excellent in workability and fatigue properties according to item 1.
A method for producing a hot-rolled steel sheet having a high-strength composite structure of 65 kgf / mm ² . 7. A slab adjusted to a predetermined component is hot-rolled.
When cooling, Ar _ThreeTo Ar₁Up to temperature T1 in the range
Is cooled at 5-200 ° C./sec. ₁Range of
Cool down to the temperature T2 at 5 to 20 ° C./sec.
4. Cooling at 0 to 200 [deg.] C./sec.
Excellent in workability and fatigue characteristics described in any one of
Tensile strength 45-65kgf / mm^TwoHigh-strength composite set
Manufacturing method of woven hot rolled steel sheet.