JPH111747A - High tensile strength hot rolled steel plate having superfine grain and excellent in ductility, toughness, fatigue resistance, and balance between strength and elongation, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably obtain a high tensile strength hot rolled steel plate excellent not only in ductility but also in toughness, fatigue characteristic, and balance between strength and elongation (in concrete, TS×El>=26000 MPa.%, FL/TS>=0.58, and TS×λ>=3000 MPa.%) in an as-hot-rolled state. SOLUTION: This steel plate has a composition consisting of, by weight, 0.05-0.30% C, 0.30-2.0% Si, 1.0-2.5% Mn, 0.003-0.100% Al, 0.05-0.50% Nb, and the balance Fe with inevitable impurities and also has a steel structure consisting of 5-20 vol.% retained austenite and the balance essentially polygonal ferrite. Further, among the grains of polygonal ferrite, >=85%, by ratio by number, of the grains are superfine grains having <=8 μm grain size and also having <=5 μm average average grain size.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a hot-rolled
Suitable for applications such as automotive materials, structural materials and pipe materials, ductile, tough, and fatigue-resistant properties with ultrafine grains,
The present invention relates to a high-tensile hot-rolled steel sheet excellent in strength-elongation balance and a method for producing the same.

[0002]

2. Description of the Related Art As means for enhancing the mechanical properties of steel,
It is already well known that microstructuring is effective. Therefore, many methods for obtaining a microstructure have been conventionally sought. In particular, recently, high-strength steel sheets have been widely used for cost reduction, and in order to suppress deterioration of ductility, workability, toughness, etc. due to the increase in tensile strength, the microstructure of high-tensile steel has been refined. It is an important issue.

Hitherto, as a means for refining the structure, a controlled rolling method, a controlled cooling method, a large rolling reduction method and the like are known. Of these, Nb and Ti have been widely used as methods for simultaneously achieving high tensile strength and fine structure.
This is a controlled rolling method applied to a high-tensile hot-rolled steel sheet containing.
The background of the widespread use of this method is that not only high tensile strength can be easily achieved by the precipitation strengthening action of Nb and Ti contained in these steel sheets, but also the recrystallization control action of austenite grains of Nb and Ti, Γ → α when low temperature rolling is applied
Since the strain-induced transformation into the ferrite is promoted, the refinement of ferrite grains is also achieved.

However, a drawback of the high-strength steel sheet manufactured by this method is that the mechanical properties are highly anisotropic. For example, in a steel sheet for an automobile for press forming, the forming limit is determined by a characteristic level in a direction in which ductility is inferior. Therefore, it is difficult to secure high press formability in a steel sheet having a large anisotropy. Also, in structural materials or pipe materials, a large anisotropy such as toughness and fatigue strength leads to problems in design and use.

On the other hand, a structure refinement method by a large rolling reduction method is disclosed in, for example, JP-A-58-123823 and JP-A-59-2294.
No. 13 discloses this. The key point of the refining mechanism in this method is to promote the strain-induced transformation from γ to α by applying a large pressure to the austenite grains. The difference from the controlled rolling method applied to the high-strength hot-rolled steel sheet containing Nb and Ti described above is that the controlled rolling method uses the effect of suppressing the recrystallization of austenite grains of Nb and Ti, while the large rolling reduction In the method, the crystal grains can be refined without adding Nb and Ti. However, it is difficult to use a general hot strip mill, for example, it is necessary to reduce the rolling reduction per pass to 40% or more.

[0006] In addition, as a high strength steel sheet having both strength and workability, TRI of retained austenite is considered.
A steel sheet utilizing the P effect (Transformation Induced Plasticity) has been proposed. For example, JP-A-60-43425 discloses that after hot rolling,
There has been proposed a method for producing a steel sheet having retained austenite, which comprises maintaining the temperature in a temperature range of 4 ° C. for 4 to 20 seconds and then winding it at 350 ° C. or less. However, in this method, a special cooling control for maintaining the temperature at a predetermined temperature is required, so that a problem remains in that it is difficult to obtain a stable and uniform material.

[0007] In this regard, the present inventors previously solved the above-mentioned problem by using TiC or (TiC + NbC) to refine the initial austenite grains before the start of hot rolling, that is, at the slab heating stage. As a result, even in the product plate, the average crystal grain size: 75 vo ferrite grains of less than 10 μm
A technique for realizing an ultrafine structure of l% or more has been developed and disclosed in Japanese Patent Application Laid-Open No. 9-87798. With the development of this technology, ductility, toughness, fatigue resistance, strength-elongation balance, and the like have been remarkably improved as compared with the prior art, and great expectations are placed on applications such as automotive materials.

[0008]

SUMMARY OF THE INVENTION The present invention relates to an improvement in the above-mentioned finer technology, and particularly relates to a strength-elongation balance,
The durability ratio and hole expandability were further improved (specifically, TS × El ≧ 26000MPa ·%, FL / TS ≧ 0.58, TS ×
λ ≧ 4500 MPa ·%).

[0009]

As described above, conventionally, by using TiC or (TiC + NbC) as a carbide, the initial austenite grains in the slab heating stage are refined, and then the product is subjected to rolling and dynamic recrystallization. The crystal grains in the plate were refined. The inventors have further studied in order to obtain an ultrafine structure one rank higher than the above-described fine grains and to further improve the characteristics. As a result, TiC and (TiC + Nb
By using NbC instead of C), the initial austenite grains in the slab heating step, and thus the crystal grains in the product plate, can be further refined as compared with the conventional one, and thus the strength-elongation is much better than before. The balance, durability ratio, hole expanding workability, etc. could be obtained. The present invention is based on the above findings.

That is, the gist of the present invention is as follows. 1. C: 0.05 to 0.30 wt%, Si: 0.30 to 2.0 wt%, Mn:
1.0 to 2.5 wt%, Al: 0.003 to 0.100 wt%, Nb: 0.05 to
0.50 wt%, the balance is composed of Fe and unavoidable impurities, and retained austenite is 5 to 20 vol%.
The remainder has a steel structure mainly composed of polygonal ferrite, and among the polygonal ferrite grains, fine grains having a grain size of 8 μm or less account for 85% or more of the total in number ratio, and the average grain size is 5 μm or less. A high-strength hot-rolled steel sheet having ultrafine grains and excellent in ductility, toughness, fatigue resistance properties, and strength-elongation balance.

2. C: 0.05 to 0.30 wt%, Si: 0.30 to 2.
0 wt%, Mn: 1.0 to 2.5 wt%, Al: 0.003 to 0.100 wt
%, Nb: 0.05 to 0.50 wt%, Ni: 2.5 wt% or less, Cr: 2.5 wt% or less, Mo: 2.5 wt% or less, Cu: 2.5 wt%
% Or one or more selected from below,
The remainder has a composition of Fe and unavoidable impurities, has a residual austenite content of 5 to 20 vol%, and the remainder has a steel structure mainly composed of polygonal ferrite. The following fine grains occupy 85% or more of the total in number ratio, and have an average grain size of 5 μm or less, and have excellent ductility, toughness, fatigue resistance and strength-elongation balance having ultrafine grains. High tensile hot rolled steel sheet.

3. C: 0.05 to 0.30 wt%, Si: 0.30 to 2.
0 wt%, Mn: 1.0 to 2.5 wt%, Al: 0.003 to 0.100 wt
%, Nb: heating a steel slab having a composition containing 0.05 to 0.50 wt% to a temperature of 950 ° C or higher and 1100 ° C or lower, and then performing rough rolling at least twice in which the rolling reduction per pass is 20% or higher. At the same time, after finishing the hot finish rolling under the condition of the finishing temperature: Ar ₃ transformation point or higher, it is cooled at a cooling rate of 20 ° C./sec or more, and wound up in a temperature range of 350 to 550 ° C. A method for producing a high-strength hot-rolled steel sheet having fine grains and excellent in ductility, toughness, fatigue resistance, strength-elongation balance.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the reason why the composition of a steel sheet in the present invention is limited to the above range will be described. C: 0.05 to 0.30 wt% C needs to be at least 0.05 wt% in order to obtain necessary strength and to secure a sufficient amount of NbC in a heating step which is important for refining the structure. On the other hand, if it exceeds 0.30 wt%, the ratio of the pearlite phase increases, and not only the ductility and toughness deteriorate, but also the weldability deteriorates. Therefore, the C content was limited to the range of 0.05 to 0.30 wt%. A particularly preferred range is 0.05 to 0.20 wt%.

Si: 0.30 to 2.0 wt% Si is a useful element that not only increases strength without drastic reduction in elongation due to solid solution strengthening, but also promotes ferrite transformation. It also has the effect of facilitating obtaining a structure containing retained austenite by promoting ferrite transformation. In order to fully exhibit such effects, 0.30wt
% Or more is required, while large amounts of Si
Scales with poor descaling properties are formed during hot rolling, which adversely affects the surface properties of the product. In the present invention, since the heating temperature is set to a low temperature range in order to obtain an ultrafine structure, the upper limit of the amount of Si that deteriorates the surface state can be set higher than usual, but if it exceeds 2.0 wt%, the adverse effect is exceeded. Since it becomes obvious, the upper limit was set to 2.0 wt%. In terms of promoting ferrite transformation, 0.50 to 2.0 wt% is more preferable.

Mn: 1.0 to 2.5 wt% Mn is an element effective for improving the strength. For this purpose, Mn alone needs to be added in an amount of 1.0 wt% or more. However, when the content exceeds 2.5 wt%, ferrite transformation is significantly delayed, and it is difficult to obtain a desired ferrite structure in the present invention. Therefore, the Mn content is limited to the range of 1.0 to 2.5 wt%.

Al: 0.003 to 0.100 wt% Al is an element that acts very effectively on deoxidation.
If it is less than 3 wt%, the effect cannot be obtained, while if it exceeds 0.100 wt%, the crystal grains become coarse and internal defects due to inclusions are caused. Therefore, the Al content is limited to the range of 0.003 to 0.100 wt%. 0.010-0.060 wt for grain refinement
% Is preferred.

Nb: 0.05 to 0.50 wt% Nb is an indispensable element as NbC in order to refine initial austenite grains in a slab heating stage and to cause dynamic recrystallization in a rolling process. In order to obtain NbC necessary for austenite grain refinement, at least 0.05%
%, But if it exceeds 0.50%, the refining effect of NbC reaches saturation, so the Nb content is 0.05 to 0.50 wt%.
Limited to the range.

Although the basic components have been described above, in the present invention, Ni, Cr, Mo and Cu are further added as strength improving components.
Etc. can be appropriately added in the following ranges. Ni: 2.5 wt% or less, Cr: 2.5 wt% or less, Mo: 2.5 wt% or less, Cu: 2.5 wt% or less Ni, Cr, Mo and Cu are all useful elements for improving the strength. In addition to strength, it also effectively contributes to improvements in corrosion resistance and weldability. The amount of addition can be selected depending on the strength and required characteristics. However, if any of the elements exceeds 2.5 wt%, ferrite transformation is significantly delayed, and it becomes difficult to obtain a desired amount of ferrite. It was taken. Preferably, in order not to significantly delay the ferrite transformation,
The content of Ni, Cr, Mo and Cu is desirably 1.5 wt% or less.

As unavoidable impurities, P, S, N
The smaller the number, the better. However, considering economics, O:
0.008 wt% or less, N: 0.006 wt% or less, P: 0.020 wt%
Hereinafter, it is desirable to set S: 0.010 wt% or less.

Next, the steel structure and polygonal ferrite
The reason for limiting the particle size of the above to the above range will be described. Retained austenite amount: 5 to 20 vol% Retained austenite is retained in the range of 5 to 20 vol%.
Need to be Figure 1 shows the amount of retained austenite and char
Fracture transition temperature in P impact test ( _VT_rs), Swing
The ratio between the fatigue limit and tensile strength in the plane bending fatigue test (FL
/ TS) and strength-elongation balance (TS × El)
The results of the investigation are shown in the figure.
On the other hand, when the amount of retained austenite is less than 5 vol%, toughness,
Both fatigue resistance and strength-elongation balance are low, while 20
 It is difficult to secure retained austenite exceeding vol%
The amount of retained austenite is in the range of 5 to 20 vol%
And One of the main requirements of the present invention, residual aus
The stable presence of tenite indicates that the
Stability, toughness, fatigue resistance and strength-elongation balance
This is one of the factors that can be secured.

Other than the above-mentioned retained austenite, polygonal ferrite is preferably used (preferably 80 vol.
%). This is because polygonal ferrite is extremely useful for achieving high tensile strength while securing ductility and toughness.In this regard, strength increases as bainite and martensite other than polygonal ferrite increase. However, ductility and toughness deteriorate. Therefore, it is desirable that the structure be ferrite or polygonal ferrite as much as possible. Figure 2 shows the polygonal ferrite quantity, _v T _rs, the results of examining the relationship between FL / TS and TS × El. As shown in the figure, the higher the amount of polygonal ferrite, the higher the toughness, the higher the ratio between fatigue limit and the tensile strength, and the higher the strength-elongation balance, particularly in the range of 80 vol% or more. The polygonal ferrite herein refers to a ferrite having a grain size ratio in a direction perpendicular to the rolling direction of crystal grains in a range of 1.0 to 1.3.

Polygonal ferrite grain size: the number ratio of fine grains of 8 μm or less is 85% or more, and the average crystal grain size is 5
μm or less It is indispensable to obtain the above-mentioned ultrafine structure in order to obtain various characteristics that are much better than before. Polygonal ferrite particle size: The number ratio of fine particles of 8 μm or less is 85%. If the average grain size is less than 5 μm, the ductility, toughness,
This is because fatigue resistance and strength-elongation balance cannot be obtained. In addition, such an ultrafine structure can be obtained only by utilizing NbC as a carbide according to the present invention in the existing rolling equipment, and in order to obtain a finer structure than this, a significant modification of the hot rolling equipment is required. is necessary.

FIG. 3 shows the results of a study on the relationship between the average particle size of polygonal ferrite and _v T _rs , FL / TS and TS × El. As shown in the figure, the smaller the average particle size, the higher the toughness, the higher the fatigue resistance limit and the tensile strength ratio, and the higher the strength-elongation balance, particularly in the range of the average particle size ≦ 5 μm.

Next, a specific manufacturing process according to the present invention will be described. The smelting method in the present invention may be an ordinary method and is not particularly limited. It is melted in a converter or an electric furnace, subjected to ladle refining, degassing, etc., and converted into a slab or a steel ingot by continuous casting or ingot making. Rolled into a hot-rolled steel sheet. The slab may be reduced in width before hot rolling.

Next, the hot rolling conditions will be described. Slab heating temperature: 950 to 1100 ° C. The most important point of the present invention resides in the use of NbC precipitation for refining the initial austenite grains. For this reason, the heating temperature of the slab was limited to the range of 950 to 1100 ° C. If the temperature is lower than 950 ° C, it becomes difficult to finish the finish rolling in the austenite region. Therefore, it is necessary to obtain the desired polygonal ferrite structure and mechanical properties such as ductility, toughness, fatigue resistance, and strength-elongation balance. This is because it becomes difficult to secure the ductility, especially ductility. Further, if the temperature exceeds 1100 ° C., the dissolution of NbC increases, and the effect of NbC to refine the austenite grains is lost, and the increase in solid solution Nb makes it difficult for dynamic recrystallization during hot rolling to occur. This is because it becomes difficult to obtain an ultrafine polygonal ferrite structure containing retained austenite.

Reduction rate per pass: at least two reductions of 20% or more Another important point of the present invention is to refine austenite grains by dynamic recrystallization. In order for the austenite grains to undergo dynamic recrystallization, it is necessary to refine the above-described initial austenite grains and to further appropriately set rolling conditions. As rolling conditions, it is necessary to set the rolling reduction per pass at least twice or more to 20% or more.
The reason is that if it is less than 20%, miniaturization by dynamic recrystallization hardly occurs. From the viewpoint of miniaturization of austenite, it is preferable that the rolling reduction is large. However, in practice, there is a limit in terms of the rolling mill capacity and productivity, so it is preferable to set the rolling reduction to about 20% to 50%. The number of passes in which the rolling reduction per pass is 20% or more requires at least two passes.
The reason for this is that the number of passes is important, because the more the number of times of causing dynamic recrystallization increases, the more miniaturization proceeds. If the number of passes is less than two, an ultrafine ferrite having an average particle size of 5 μm or less, which is the object of the present invention, cannot be obtained. In the present invention, the austenite grains are refined to substantially equiaxed grains at the stage after finish rolling. Γ → α
Upon completion of the transformation, fine polygonal ferrite grains are obtained.

The hot rolling finishing temperature: Ar ₃ why the transformation point or more hot rolling finishing temperature was than the Ar ₃ transformation point is for stably obtain a sufficient amount of polygonal ferrite, Ar ₃ transformation point If the rolling is performed at less than 10%, good workability (ductility, stretch flangeability) due to polygonal ferrite cannot be obtained.

Cooling rate after rolling: 20 ° C./sec or more If the cooling rate is less than 20 ° C./sec, the growth of ferrite grains generated at a high temperature proceeds, making it difficult to form fine polygonal ferrite grains. Cooling after rolling was performed at a rate of 20 ° C./second or more. From the point of ferrite grain refinement, the higher the cooling rate after rolling, the better. The upper limit of the cooling rate is not particularly specified, but it is about 100 ° C./sec or less in order to maintain good flatness of the steel sheet. Is preferred.

Winding temperature: 350 to 550 ° C. After rolling and cooling, the film is wound into a coil. The winding temperature needs to be in the range of 350 to 550 ° C. This is because, when the temperature exceeds 550 ° C, the formation of residual austenite decreases, and undesired results such as the growth of ferrite grains due to self-annealing after winding occur.
If it is less than 1, the amount of martensite increases, the amount of retained austenite decreases, and the property balance deteriorates and the flatness of the steel sheet decreases.

[0030]

EXAMPLES Steel having the composition shown in Table 1 was melted by a converter-continuous casting method to form a slab having a thickness of 220 mm. Next, a hot-rolled steel sheet having a thickness of 3.0 mm was obtained under the rolling conditions shown in Table 2. For each of the obtained hot-rolled steel sheets, the volume fraction of polygonal ferrite, 8
The number ratio of fine particles of μm or less and the average crystal grain size were measured using an image processing apparatus. Ferrite grain size is
Measurements were made at 10 points in a visual field of 0.1 × 0.1 mm, and the average value was displayed. The amount of retained austenite was measured by X-ray diffraction. The obtained results are also shown in Table 2. Furthermore, characteristic tensile by JIS 5 test piece No., Frequency: fatigue limit by Reversed Plane Bending Test Method 20 Hz, ductility by 2mmV notch Charpy impact test piece - investigating brittle transition velocity _(v T _rs), display the result 3 is shown.

[0031]

[Table 1]

[0032]

[Table 2]

[0033]

[Table 3]

Further, the obtained data is organized and crystal
Relationship between grain size and TS × λ, crystal grain size and _VT_rsRelationship with
Relationship between grain size and FL / TS, relationship between grain size and TS × El
And the results summarized in the relationship between TS × El and FL / TS.
4, 5, 6, 7 and 8.

As shown in FIGS. 4 to 7, according to the present invention, the crystal grain size of polygonal ferrite grains is 5 μm or less (d
If it is ^-1/2 , it is 14.1 or more).
Starting with, _V T _rs, is remarkably improved as FL / TS and TS × El, thus according to the present invention, as shown in FIG. 8, the TS × in El is 26000 MPa ·% or more and FL / TS 0.58
The above excellent strength-elongation balance and the ratio between the fatigue limit and the tensile strength in the fatigue test could be obtained.

[0036]

Thus, according to the present invention, by using rolling and dynamic recrystallization in the presence of NbC, ultrafine polygonal ferrite grains can be obtained, and an appropriate amount of retained austenite can be retained. A high-strength hot-rolled steel sheet excellent in ductility, toughness, fatigue resistance, and strength-elongation balance can be stably obtained. When a product is manufactured using the steel sheet of the present invention, improvements in workability, productivity, yield, and the like are expected.

[Brief description of the drawings]

Fig. 1 Retained austenite and _V T _rs , FL / TS and TS
It is a graph which showed the relationship with xEl.

FIG. 2 is a graph showing the relationship between the amount of polygonal ferrite and _v T _rs , FL / TS and TS × El.

Fig. 3 Average particle size of polygonal ferrite and _v T _rs , FL
5 is a graph showing the relationship between / TS and TS × El.

FIG. 4 is a graph showing the relationship between the crystal grain size and TS × λ.

5 is a graph showing the relation between the grain size and _V T _rs.

FIG. 6 is a graph showing the relationship between the crystal grain size and FL / TS.

FIG. 7 is a graph showing the relationship between the crystal grain size and TS × El.

FIG. 8 is a graph showing the relationship between TS × El and FL / TS.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI C22C 38/58 C22C 38/58

Claims (3)

[Claims] (1) C: 0.05 to 0.30 wt%, Si: 0.30 to 2.0 wt%, Mn: 1.0 to 2.5 wt%, Al: 0.003 to 0.100 wt%, Nb: 0.05 to 0.50 wt%, and the balance is Fe and inevitable impurities, the residual austenite is 5 to 20 vol%, and the remainder has a steel structure mainly composed of polygonal ferrite. Of the polygonal ferrite grains, the particle size is 8 μm or less. High tensile strength with excellent ductility, toughness, fatigue resistance and strength-elongation balance with ultra-fine grains characterized by fine grains occupying 85% or more of the whole in number ratio and having an average grain size of 5 μm or less. Hot rolled steel sheet. 2. C: 0.05 to 0.30 wt%, Si: 0.30 to 2.0 wt%, Mn: 1.0 to 2.5 wt%, Al: 0.003 to 0.100 wt%, Nb: 0.05 to 0.50 wt%, and Ni: 2.5 wt% or less, Cr: 2.5 wt% or less, Mo: 2.5 wt% or less, Cu: 2.5 wt% or less.
Fe and inevitable impurities, the residual austenite is 5 to 20 vol%, and the remainder has a steel structure mainly composed of polygonal ferrite. Of the polygonal ferrite grains, the particle size is 8 μm or less. Fine grains occupy 85% or more of the total in number ratio and have an average grain size of 5 μm or less. High tensile strength with excellent ductility, toughness, fatigue resistance and strength-elongation balance with ultra-fine grains. Hot rolled steel sheet. 3. A composition containing 0.05 to 0.30% by weight of C, 0.30 to 2.0% by weight of Si, 1.0 to 2.5% by weight of Mn, 0.003 to 0.100% by weight of Al, and 0.05 to 0.50% by weight of Nb. After heating the steel slab to a temperature of 950 ° C or higher and 1100 ° C or lower, rolling is performed at least twice so that the rolling reduction per pass is 20% or higher, and the finishing temperature is Ar _3.
After finishing hot finish rolling under the condition of transformation point or higher, 20 ℃
High-strength hot-rolled steel sheet with ultra-fine grains characterized by being cooled at a cooling rate of / sec or more and winding in a temperature range of 350 to 550 ° C, and having excellent balance of ductility, toughness, fatigue resistance, and strength-elongation Manufacturing method.