JP2005307339A - High tensile hot rolled steel sheet having excellent strength-ductility balance and method for manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high tensile hot rolled steel sheet of a compound structure type which does not cause the degradation in paintability and corrosion resistance, is excellent in strength-ductility balance of providing tensile strength of ≥590 MPa and strength-ductility balance of ≥19,000 MPa×% in spite of relatively low Si content. <P>SOLUTION: The composite structure having a component composition containing 0.05 to 0.15 mass% C, 0.005 to 0.8 mass% Si, 1.0 to 3.0 mass% Mn, 0.005 to 0.08 mass% P, 0.0002 to 0.005 mass% S, 0.001 to 0.05 mass% Al, and 0.0090 to 0.025 mass% N, and ≥0.0050 mass% N in a solid solution state, and the balance Fe and inevitable impurities and further containing 60 to 94 vol% ferrite phase, 3 to 30 vol% bainite phase and ≥3 vol% retained austenite phase is provided. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a high-tensile hot-rolled steel sheet having a tensile strength of 590 MPa or more, which is mainly suitable for use in automobile body parts and the like, and a method for producing the same. The present invention relates to a rolled steel sheet and a manufacturing method thereof. The high-tensile hot-rolled steel sheet of the present invention is suitable for a wide range of applications from those subjected to relatively light processing such as mild bending and roll forming to those subjected to relatively severe drawing.

  In recent years, there has been a demand for improving the safety of automobile bodies from the viewpoint of improving the fuel efficiency of automobiles and protecting passengers in the event of a vehicle collision for the purpose of protecting the global environment. For this reason, studies for achieving both weight reduction and strengthening of the automobile body are being actively promoted. In order to satisfy this, it is said that it is effective to increase the strength of the material of the parts, and recently, high-tensile steel plates have been actively used for automobile parts. In other words, it is effective to apply a high-tensile steel plate to automobile parts to reduce the thickness of the steel plate to be used.

  However, since many automotive body parts made of steel plates are formed by press working, the high-tensile steel plates used are required to have excellent press formability. Therefore, especially high ductility is calculated | required as a mechanical characteristic of a steel plate. However, generally, when the strength of a steel plate is increased, the ductility (elongation) is lowered and the press formability is deteriorated, so that it is still difficult to achieve both of them.

  A typical example of a high-tensile steel sheet having good press formability is a steel sheet having a composite structure composed of a composite structure of soft ferrite and hard martensite, which is a steel sheet having a low yield stress and a high strength-ductility balance. However, this type of composite structure steel sheet has a yield ratio as low as 70% or less and is excellent in shape freezing property, but the limit of the stable strength-ductility balance (TS x El) is about 19000 MPa ·%. It was. Therefore, the processability under normal conditions is generally good, but there remains a problem with molding under severe conditions.

  Further, a so-called transformation-induced plastic type steel sheet has been proposed in which the structure is a composite structure composed of ferrite and bainite and retained austenite, and the strength-ductility balance is remarkably improved. For example, Patent Document 1, Patent Document 2, Patent Document 3 and Patent Document 4 propose high-strength hot-rolled steel sheets having excellent ductility, utilizing transformation-induced plasticity due to retained austenite.

  These transformation-induced plastic type steel sheets may have a strength-ductility balance (TS x El) exceeding 20000 MPa ·%, but compared with ferrite and martensitic composite steels of the same strength, the content of C and Si Therefore, when C is high, the welded portion becomes brittle and sufficient welding strength, particularly strength in the vertical direction on the welded surface by spot welding, cannot be obtained, resulting in a practical problem. On the other hand, when Si is high, the surface aesthetics are impaired to reduce the paintability and corrosion resistance, and to recover this, it is necessary to perform pickling treatment for a long time, which greatly increases the manufacturing cost. It is inevitable to do.

In recent years, as a steel sheet that can satisfy both good press formability and high strength after forming, it is soft and easy to press form before press forming, and can be hardened by paint baking after press forming to increase the strength of parts. Steel plates have been developed. As an example of such a steel sheet, Patent Document 5 proposes an N-added transformation-induced plastic steel sheet. In this technique, a hot-rolled steel sheet with improved ductility and strain age hardening characteristics is obtained by limiting the solid solution N within a specific range and further controlling the hot-rolling conditions. However, the Si content is as high as 1.0% or more, and also in this case, there is a problem that the cost increase for securing the paintability and beauty is inevitable.
Japanese Patent Publication No. 6-41617 Japanese Patent Publication No. 5-65566 Japanese Patent Publication No. 5-67682 Japanese Patent Laid-Open No. 11-43725 JP 2002-30385 A

  The present invention advantageously solves the above-mentioned problems of the prior art, has a relatively low C content that does not cause a decrease in weld strength, and does not cause a decrease in paintability and corrosion resistance. Manufacturing method of a high strength hot-rolled steel sheet with a composite structure type that has a relatively low Si content and a tensile strength of 590 MPa or more and an excellent strength-ductility balance with a strength-ductility balance of 19000 MPa ·% or more The purpose is to propose together.

Conventionally, in order to form a composite structure composed of ferrite, bainite and retained austenite and to remarkably improve the strength-ductility balance, a large amount of C and Si is required, and the C content is moderate without causing problems in weldability. It has been considered difficult to obtain a high-strength hot-rolled steel sheet that has a composite structure as described above by adding a small amount of Si after the range is reached and has a markedly improved ductility. This is because the addition of a large amount of Si makes it possible to concentrate an amount of C necessary for the formation of retained austenite in the austenite during annealing. That is, Si functions to suppress the formation of Fe ₃ C and to more effectively concentrate C in austenite.

  In order to develop a technique that can improve the balance between strength and ductility even when Si having such a function is reduced, the inventors manufactured various steels by changing the composition of components and manufacturing conditions in various ways. An evaluation experiment was conducted. As a result, it has been found that the strength-ductility balance can be improved by using N, which has not been actively used in fields where high ductility is required, even if Si is used in a very small amount.

Hereinafter, experimental results that led to the present invention will be described.
C: 0.075 mass%, Si: 0.5 mass%, Mn: 1.55 mass%, P: 0.019 mass%, S: 0.002 mass%, and Al: 0.012 mass%, and N is 0.0018-0.0188 mass%. The steel ingots with variously changed compositions were heated to 1250 ° C and soaked for 1 h, then the sheet bar thickness after rough rolling, the cooling start time after finishing rolling, cooling was temporarily stopped, and then cooled again. The presence or absence of air cooling treatment until the start, the winding temperature, and the cooling rate after winding were variously changed to obtain a hot-rolled steel sheet having a thickness of 1.6 mm.

A tensile test was performed on the hot-rolled steel sheet thus obtained. Tensile tests are conducted in accordance with the provisions of JIS Z2241, using JIS No. 5 tensile test pieces with the major axis perpendicular to the rolling direction, and tensile properties (yield strength YS, tensile strength TS, elongation El) Asked.
Further, the amount of solute N and the amount of retained austenite (hereinafter referred to as the amount of residual γ) of each steel sheet were also investigated.
In addition, the analysis method of solid solution N was taken as the value which deducted precipitation N (it calculates | requires with the melt | dissolution method by electrolytic extraction) from the total N amount in steel. Details thereof will be described later.
Furthermore, the amount of residual γ is calculated from the peak intensities of the (211) and (220) planes of the austenite phase and the (200) and (220) planes of the ferrite phase by X-ray diffraction on the plane near the thickness of the steel sheet. The volume fraction of the residual austenite phase was calculated.
The results of these investigations are shown in FIGS.

  First, FIG. 1 shows the relationship between the amount of residual γ and TS × El in comparison with the presence or absence of air cooling during cooling after finishing rolling. As shown in the figure, the residual γ amount and TS × El show a strong correlation, and the relationship is uniquely arranged regardless of the presence or absence of the air cooling process and other manufacturing conditions. That is, it is shown that TS × El of 19000 MPa ·% or more can be obtained by setting the residual γ amount to 3 vol% or more.

  FIG. 2 shows the relationship between the amount of solute N and the amount of residual γ in comparison with the presence or absence of the air cooling treatment. As shown in the figure, when there is an air cooling process, the amount of residual γ increases as the amount of dissolved N increases, and the amount of residual γ can be increased to 3 vol% or more by setting the amount of dissolved N to 50 ppm or more. However, when there is no air cooling treatment, the amount of residual γ is less than 3 vol% even if the amount of dissolved N increases.

  Next, C: 0.075 mass%, Si: 0.5 mass%, Mn: 1.55 mass%, P: 0.019 mass%, S: 0.002 mass% and Al: 0.012 mass% are the basic compositions, and N is 0.0116 mass%. The steel soul having the composition contained in the above was made into a sheet bar having a thickness of 30 to 10 mm by rough rolling, and subsequently subjected to 7-pass finish rolling so that the finish rolling finish temperature was 850 ° C. to a plate thickness of 1.6 mm. After 1.5 seconds after finishing rolling, after cooling to 720 ° C at an average cooling rate of 50 ° C / s, air-cooled for 10s, and then cooled again to an equivalent winding temperature of 600 to 300 ° C at an average cooling rate of 50 ° C / s. As a cooling treatment after the removal, the cooling rate was lowered to 100 ° C. or lower at a constant cooling rate of 1.5 to 10 ° C./min. The amount of solute N at this time was in the range of 80 to 115 ppm.

FIG. 3 shows the relationship between the amount of residual γ and the air cooling time in a steel sheet obtained with a total rolling reduction of finish rolling: 95%, coiling temperature: 450 ° C. and cooling rate after winding: 2 ° C./min. It is a thing. From the figure, it can be seen that by controlling the air cooling time to 3 to 30 seconds, a residual γ amount of 3 vol% or more is secured and high ductility can be obtained.
Here, the total rolling reduction of finish rolling (hereinafter also referred to as finishing total rolling reduction)
{(Sheet bar thickness-plate thickness after finish rolling (here 1.6mm)) / sheet bar thickness} x 100%
Is the total rolling reduction during finish rolling calculated by

  FIG. 4 shows the relationship between the amount of residual γ and the total rolling reduction of finish rolling. At this time, the air cooling time was 10 seconds, the winding temperature was 450 ° C., and the cooling rate after winding was 2 ° C./min. From the figure, it can be seen that by controlling the total rolling reduction of finish rolling to 90% or more, a residual γ amount of 3 vol% or more is secured, and high ductility can be obtained.

  FIG. 5 shows the relationship between the amount of residual γ and the coiling temperature. At this time, the final reduction ratio was 95%, the air cooling time was 10 seconds, and the cooling rate after winding was 2 ° C./min. From the figure, it can be seen that by controlling the coiling temperature to 350 to 500 ° C., a residual γ amount of 3 vol% or more is secured and high ductility can be obtained.

  FIG. 6 shows the relationship between the amount of residual γ and the cooling rate after winding. At this time, the final reduction ratio was 95%, the air cooling time was 10 seconds, and the coiling temperature was 450 ° C. From the figure, it can be seen that by controlling the cooling rate after winding to 3 ° C./min or less, a residual γ amount of 3 vol% or more is secured, and high ductility can be obtained.

As described above, when the solid solution N amount is 50 ppm or more, the finishing total rolling reduction is 90% or more, the winding temperature is 350 to 500 ° C., and the cooling rate after winding is 3 ° C./min or less, 3 vol% The above residual γ amount, that is, a strength-ductility balance (TS × El) of 19000 MPa ·% or more can be obtained, and high ductility is realized.
The cause is considered as follows.

  That is, in order to stabilize austenite, it is necessary to concentrate C and N in the austenite phase in the air cooling treatment stage of controlled cooling after hot rolling. In the stabilized austenite phase, the bainite transformation proceeds during cooling after winding, and further enrichment of C and N in the austenite phase, that is, further stabilization of the austenite occurs, and austenite remains even at room temperature. Will do. Therefore, when precipitates such as carbides and nitrides are generated during winding, it becomes difficult to concentrate C and N in the retained austenite phase, that is, to sufficiently stabilize the retained austenite phase.

  Here, N has a lower tendency to form precipitates (nitrides) than C and tends to exist in a solid solution state. For example, when C is used, it is necessary to add a large amount of Si and to concentrate C to the austenite phase by the C discharging action. On the other hand, when N is used, the concentration of N in the retained austenite phase is reduced during the slow cooling after winding by performing the slow cooling after winding while appropriately controlling the winding temperature. Since it is carried out effectively, even if Si is reduced, the amount of C enrichment is insufficient to make up for it, so that austenite can be stabilized. On the other hand, in order to compensate for the decrease in Si as a ferrite-forming element, it is necessary to promote ferrite transformation by increasing the finish reduction ratio.

  By satisfying the above-described conditions derived from the above experimental results, the structure can be a composite structure composed of ferrite and bainite and retained austenite, and the press formability of the steel sheet can be significantly improved. This is what we found. The present invention has been completed by further study based on these findings, and the gist thereof is as follows.

(1) C: 0.05 to 0.15 mass%,
Si: 0.005-0.8 mass%,
Mn: 1.0-3.0 mass%
P: 0.005-0.08 mass%,
S: 0.0002 to 0.005 mass%,
Al: 0.001 to 0.05 mass% and N: 0.0090 to 0.025 mass%
N: 0.0050 mass% or more in a solid solution state, with the balance being a component composition of Fe and inevitable impurities, 60 to 94 vol% ferrite phase, 3 to 30 vol% bainite phase and 3 vol% A high-tensile hot-rolled steel sheet excellent in strength-ductility balance, characterized by having a composite structure containing the above-mentioned residual austenite phase.

(2) The composition further comprises one or more selected from Cr: 0.05 to 1.0 mass%, Mo: 0.05 to 1.0 mass%, and Ni: 0.05 to 1.0 mass%. A high-tensile hot-rolled steel sheet excellent in the strength-ductility balance described in (1) above.

(3) The above component (1) or (2), wherein the component further contains one or more selected from Nb, Ti, V and B within a range satisfying the following formula: A high-tensile hot-rolled steel sheet excellent in the strength-ductility balance described in 1.
N / (Al + Nb + Ti + V + B) ≧ 0.3
Here, N, Al, Nb, Ti, V, B: Content of each element (mass%)

(4) C: 0.05 to 0.15 mass%,
Si: 0.005-0.8 mass%,
Mn: 1.0-3.0 mass%
P: 0.005-0.08 mass%,
S: 0.0002 to 0.005 mass%,
Al: 0.001 to 0.05 mass% and N: 0.0090 to 0.025 mass%
The remainder of the steel slab containing Fe and inevitable impurities is heated to 1100-1300 ° C, and after rough rolling, the finish rolling exit temperature is Ar ₃ to (Ar ₃ +50) ° C and the total reduction rate Is subjected to finish rolling of 90% or more, and then cooling is started at a cooling rate of 20 ° C./s or more within 3 seconds, and the cooling is temporarily stopped within a temperature range of 680 to 760 ° C. for 3 to 30 seconds. After air cooling, cooling at a cooling rate of 20 ° C / s or more is performed again, and after winding at 350 to 500 ° C, the winding temperature is cooled to 300 ° C at a cooling rate of 3 ° C / min or less. A method for producing a high-tensile hot-rolled steel sheet having an excellent strength-ductility balance.

(5) The steel slab further contains one or more selected from Cr: 0.05 to 1.0 mass%, Mo: 0.05 to 1.0 mass%, and Ni: 0.05 to 1.0 mass%. A high-tensile hot-rolled steel sheet excellent in the strength-ductility balance described in (4) above.

(6) The steel slab further contains one or more selected from Nb, Ti, V and B within a range satisfying the following formula (4) or ( A high-tensile hot-rolled steel sheet excellent in the strength-ductility balance described in 5).
N / (Al + Nb + Ti + V + B) ≧ 0.3
Here, N, Al, Nb, Ti, V, B: Content of each element (mass%)

  According to the present invention, since a retained austenite phase necessary for improving ductility is ensured even with a small amount of Si, a strength-ductility balance in which the tensile strength is 590 MPa or more and the strength-ductility balance is 19000 MPa ·% or more. It is possible to provide a composite structure type high-tensile hot-rolled steel sheet.

First, the reason for limiting the composition of the hot-rolled steel sheet of the present invention will be described.
C: 0.05-0.15 mass%
C is an important element from the viewpoint of stabilizing the austenite phase by increasing the strength of the steel sheet or concentrating it into the austenite phase. In the present invention, C is contained in an amount of 0.05 mass% or more to ensure the strength and the desired residual γ amount. Need. On the other hand, the content exceeding 0.15 mass% significantly deteriorates the weldability. For this reason, C amount is limited to the range of 0.05-0.15 mass%. In addition, it is preferable to set it as 0.07-0.12 mass% from a viewpoint of coexistence of extremely high strength-ductility balance and weldability.

Si: 0.005-0.8mass%
Si is a useful strengthening element that can increase the strength of a steel sheet without significantly reducing the ductility of the steel.In addition, it generates carbides when austenite undergoes bainite transformation during cooling after winding. It is an element that has the effect of suppressing and improving the stability of untransformed austenite. Such effects are recognized in the range of 0.005 mass% or more, but the inclusion of Si exceeding 0.8 mass% adversely affects the surface properties and chemical conversion treatment properties, and in order to suppress these adverse effects, the steel sheet surface It is necessary to lengthen the pickling treatment and the like, and a large cost increase is inevitable. In the present invention, even if the content is 0.8 mass% or less, the stability of untransformed austenite is maintained, and the above-described adverse effects of Si can be easily avoided. Therefore, Si is set to a range of 0.8 mass% or less. In applications where more stringent surface properties and chemical conversion properties are required, 0.4 mass% or less is preferable.

Mn: 1.0-3.0mass%
Mn is an element that improves hardenability and greatly contributes to an increase in steel sheet strength. Mn is an effective element for preventing hot cracking due to S, and is preferably added depending on the amount of S contained. Further, Mn concentrates in the austenite phase to improve the hardenability, and also has the effect of stabilizing the retained austenite by concentrating in the austenite phase. Although such an effect is recognized in the range of 1.0 mass% or more, when it exceeds 3.0 mass%, the above-described effect is saturated and the spot weldability is remarkably deteriorated. For this reason, Mn was limited to 1.0 to 3.0 mass%. In applications where better corrosion resistance and formability are required, 2.5 mass% or less is desirable.

P: 0.005-0.08 mass%
P has the effect | action which strengthens steel and can be contained in a required quantity according to desired intensity | strength. Such an effect is recognized in the range of 0.005 mass% or more, but when the P content exceeds 0.08 mass%, press formability and weldability deteriorate. For this reason, P content was limited to 0.08 mass% or less. In addition, when more excellent press formability and weldability are required, the P content is preferably set to 0.05 mass% or less.

S: 0.0002 to 0.005 mass%
Since S is an element that exists as an inclusion in the steel sheet and causes deterioration of the ductility of the steel sheet, it is preferably reduced as much as possible. If the content is reduced to 0.005 mass% or less, the adverse effect on ductility can be ignored. Therefore, the upper limit of the S content in the present invention is 0.005 mass%. In addition, when more excellent ductility is required, the S content is preferably 0.003 mass% or less. On the other hand, since it is industrially difficult to make it less than 0.0002 mass%, this is the lower limit.

Al: 0.001 ~ 0.05mass%
Al is added as a deoxidizing element for steel, is an element useful for improving the cleanliness of steel, and is also an element desirably added for refining the structure of steel. In the present invention, solid solution N is also used as a stabilizing element or strengthening element of retained austenite, but aluminum killed steel to which aluminum in the appropriate range is added is more suitable than conventional rimmed steel to which aluminum is not added. , Mechanical properties are excellent. When the aluminum content is increased, the surface properties are deteriorated and the solid solution N is remarkably lowered, and it is difficult to ensure an excellent strength-ductility balance at a small amount of Si content, which is an object of the present invention. For this reason, the upper limit was made 0.05 mass% lower than that of the conventional steel. Furthermore, from the viewpoint of material stability, the upper limit is preferably 0.03 mass%, and more preferably 0.015 mass%. In addition, since it is industrially difficult to set it as less than 0.001 mass%, let this be a minimum.

N: 0.0090-0.025mass%
N is an important additive element in expressing an excellent strength-ductility balance. That is, with the appropriate range of N addition and manufacturing conditions, it is possible to achieve the generation and stabilization of the retained austenite phase by securing the necessary and sufficient amount of N concentration in the austenite in the hot rolled product state, The target tensile strength of 590 MPa or more and the strength-ductility balance (TS x El: tensile strength x total elongation) of 19000 MPa ·% or more can be stably obtained. N also has the effect of lowering the transformation point of steel, and its addition is effective in the situation where it is not desired to perform rolling with a thin material that greatly cuts the transformation point. Such an effect can be stably obtained by adding approximately 0.0090 mass% or more. However, if added over 0.025 mass%, the rate of occurrence of internal defects in the steel sheet increases and the occurrence of slab cracking during continuous casting becomes significant, so the upper limit was made 0.025 mass%. The range of 0.0120 to 0.0170 mass% is more preferable from the viewpoint of improving the stability of the material and the yield in consideration of the entire manufacturing process. Even if nitrogen is added, there is no adverse effect on weldability and the like within the scope of the present invention.

N in solid solution: 0.0050 mass% or more In order to stabilize austenite and to secure a sufficient amount of retained austenite to improve the strength-ductility balance, N in the solid solution state is approximately 0.0050 mass% or more. There must be.
Here, the solid solution N is a value obtained by subtracting the precipitation N from the total N amount in the steel, but this precipitation N is preferably obtained by a dissolution method by electrolytic extraction. This is because, although various methods have been examined for the analysis method of the precipitated N, the method of applying the dissolution method by electrolytic extraction using the constant potential electrolysis method employed in the present invention best corresponds to the change in material. In addition, it is preferable to use an acetylacetone system as the electrolytic solution. The residue extracted by the dissolution method by electrolytic extraction using the constant potential electrolysis method was chemically analyzed to determine the amount of N in the residue, and this was defined as the amount of precipitated N. Further, when a greater strength and ductility balance is required, it is effective that the solid solution N is 0.0090 mass% or more.

In the present invention, in addition to the above-described components, the following components can be appropriately contained.
One or more selected from Cr: 0.05-1.0 mass%, Mo: 0.05-1.0 mass% and Ni: 0.05-1.0 mass%
Cr, Mo, and Ni are elements that have the effect of improving hardenability and increasing the strength of the steel sheet as well as making the distribution of retained austenite finely dispersed, and further improving the strength-ductility balance. Such an effect is recognized by each containing 0.05 mass% or more. On the other hand, in the range exceeding 1.0 mass%, moldability is inhibited. For this reason, it is preferable to add Cr, Mo, and Ni in the range of 0.05 to 1.0 mass%, respectively.

One or more selected from Nb, Ti, V and B,
N / (Al + Nb + Ti + V + B) ≧ 0.3 (N, Al, Nb, Ti, V, B: content of each element: mass%)
In a range that satisfies
Nb, Ti and V all have the effect of precipitation strengthening steel, and can be contained in a necessary amount depending on the desired strength. In order to acquire this effect, it is preferable to set it as Nb: 0.002 mass% or more, Ti: 0.002 mass% or more, and V: 0.002 mass% or more, It can select as needed and can contain individually or in combination. However, if the content is too large, the hot deformation resistance increases or hardens to impair ductility. Therefore, the total content of Nb, Ti and V is preferably 0.1 mass% or less.
B has the effect | action which improves the hardenability of steel and increases the fraction of a low-temperature transformation phase, and strengthens steel, and can be contained in a required quantity according to desired intensity | strength. In order to acquire this effect, it is preferable to set it as B: 0.0002 mass% or more. However, if the content is too large, the hot deformability is lowered and the material is hardened to impair ductility. Therefore, B is preferably 0.0030 mass% or less.
However, these Nb, Ti, V, and B form nitrides in the same manner as Al, and if added excessively, there is a risk that the amount of solid solution N is reduced and the improvement of the strength-ductility balance is not achieved. For this reason, when Nb, Ti, V and B are added alone or in combination,
N / (Al + Nb + Ti + V + B) ≧ 0.3
Is added in a range that satisfies the above.
Even when Nb, Ti, V, and B are not added, it is preferable to satisfy N / Al ≧ 0.3 from the viewpoint of securing the amount of dissolved N.

In the present invention, the components other than those described above are not particularly limited, but there is no problem even if Ca, Zr, REM and the like are contained within the range of the normal steel composition.
Further, the balance other than the above components is Fe and inevitable impurities. Inevitable impurities include, for example, Sb, Sn, Zn, Co, etc. The allowable ranges of these contents are Sb: 0.01 mass% or less, Sn: 0.1 mass% or less, Zn: 0.01 mass% or less and Co: The range is 0.1 mass% or less.

Next, the microstructure of the steel sheet of the present invention will be described.
The hot-rolled steel sheet of the present invention has a ferrite phase that is 60 to 94 vol% of the main phase with respect to the entire structure, and 3 to 30 vol% of the bainite phase and 3 vol% or more of the residual phase as the second phase. It shall have a composite structure containing an austenite phase.
In other words, if the ferrite phase, which is the main phase, is less than 60 vol% in terms of the volume ratio relative to the entire structure, it becomes difficult to ensure the high ductility required for automobile steel sheets that require high workability, and press formability is reduced. It tends to decrease. In applications where further good ductility is required, the ferrite phase is preferably 70 vol% or more in terms of the area ratio relative to the entire structure. In order to take advantage of the composite structure, the ferrite phase needs to be 94 vol% or less. For this reason, a ferrite phase shall be 60-94 vol% with respect to the whole structure | tissue.

  Moreover, if a bainite phase is less than 3 vol% with respect to the whole structure | tissue, a high intensity-ductility balance cannot be ensured. When a better strength-ductility balance is required, the bainite phase is preferably 5 vol% or more. When the bainite phase exceeds 30 vol%, the ductility deteriorates remarkably. For this reason, the bainite phase was 3-30 vol%.

  Furthermore, in order to ensure a high strength-ductility balance, the hot-rolled steel sheet of the present invention contains a residual austenite phase of 3 vol% or more, preferably 5 vol% or more. As a result, the strength-ductility balance (TS × El) can be secured to 19000 MPa ·% or more, which is very high as a trace amount of Si-containing steel. The upper limit of the residual austenite phase content is not particularly limited, but is considered to be substantially about 15 vol%.

  In addition to the main phase and the second phase described above, a slight amount (30 vol% or less) of martensite phase or pearlite phase is acceptable.

Finally, the reason for limiting the manufacturing conditions will be described.
First, the slab is desirably produced by a continuous casting method in order to prevent macro segregation of components, but can also be produced by an ingot-making method or a thin slab casting method. In addition to the conventional method in which the slab is manufactured and then cooled to room temperature and then heated again, without being cooled, it is inserted into a heating furnace as it is, or rolled immediately after being kept warm. Energy-saving processes such as direct rolling and direct rolling can also be applied without problems. In particular, direct feed rolling is one of the useful techniques for effectively securing N in a solid solution state.

The hot rolling conditions are specified as follows.
Slab heating temperature: 1100-1300 ° C
The slab heating temperature needs to be 1100 ° C. or higher from the viewpoint of securing N in a solid solution state as an initial state. On the other hand, when the temperature exceeds 1300 ° C, the increase in steel loss with increasing oxidation weight becomes remarkable, and the ferrite transformation is delayed due to the coarsening of austenite grains, so the desired strength-ductility balance (TS x El) It becomes difficult to get.

Finishing rolling temperature during hot rolling: Ar ₃ to (Ar ₃ + 50 ° C)
When the finish rolling temperature is below the Ar ₃ point, the structure of the steel sheet becomes non-uniform, and coarse grains are generated in the surface layer, resulting in a decrease in workability. Therefore, the finish rolling temperature is set to Ar ₃ points or more. On the other hand, when rolling at a temperature higher than (Ar ₃ + 50 ° C.), the grain size of austenite before the ferrite transformation after finish rolling becomes coarse, and the accumulation of strain becomes insufficient and the ferrite transformation is delayed. It becomes difficult to obtain a desired strength-ductility balance (TS × El).

Here, the Ar ₃ point (also referred to as Ar ₃ transformation point) may be measured by the method described below. That is, a steel sample processed into a cylindrical shape having a diameter of 6 mm and a height of 12 mm is heated to 900 ° C., soaked for 5 minutes, and then subjected to a compression process of 50% in the cylindrical axis direction. Immediately after processing, it is cooled at a constant cooling rate of 10 ° C / s. In the cooling process, the length change in the cylindrical axis direction due to thermal expansion and contraction is measured, the point where the contraction changes to expansion is read, and the temperature at this time is measured as the Ar ₃ transformation point.

Total rolling reduction of finish rolling: 90% or more To achieve sufficient ferrite transformation with a small amount of Si addition, strain is accumulated in austenite after finish rolling and before ferrite transformation. Need to increase. In order to exhibit this effect effectively, it is necessary to perform rolling with a reduction ratio of 90% or more in total during finish rolling.
Here, the total rolling reduction in finish rolling is the thickness (sheet bar thickness) before finish rolling is t ₀ , and the thickness after finish rolling is t ₁ .
It is calculated by {(t _o −t ₁ ) / t _o } × 100 (%).

After this finish rolling, cooling at a cooling rate of 20 ° C./s or more is started within 3 seconds.
When the start of cooling exceeds 3 seconds, the ferrite transformation is delayed because the accumulated strain of austenite before transformation is released. A faster cooling rate is preferable from the viewpoint of increasing the driving amount of the ferrite transformation. In order to exhibit this effect effectively, a cooling rate of 20 ° C./s or more is required.

Cooling is stopped within the range of 680 to 760 ° C, and then air cooling is performed for 3 to 30 seconds.
By temporarily stopping the cooling in a temperature range where the ferrite transformation speed is fast and performing air cooling, the concentration of C and N into the austenite phase proceeds during air cooling, and this can be stabilized.
When the cooling stop temperature is lower than 680 ° C. or higher than 760 ° C., the above effect is small because the ferrite transformation rate is low. Moreover, when the air cooling time is less than 3 seconds, the time margin for concentration of C and N is reduced, and the stabilization of austenite becomes insufficient. On the other hand, when it exceeds 30 seconds, the formation of pearlite becomes remarkable, and C and N are precipitated as carbides and nitrides, and it becomes difficult to secure a desired steel structure and solid solution N.

Again, cool at a cooling rate of 20 ° C / s or higher.
When the cooling rate after the air cooling is less than 20 ° C./s, the formation of pearlite becomes remarkable, and C and N are precipitated as carbides and nitrides, and a desired steel structure and solid solution N are secured. It becomes difficult to do.

Hot rolling temperature: 350-500 ° C
In the cooling process after winding, the bainite transformation proceeds from the stabilized austenite, and the concentration of C and N further proceeds to further stabilize the austenite. A phase can be obtained.
That is, when the coiling temperature is lower than 350 ° C. and higher than 500 ° C., the bainite transformation does not effectively occur, and it becomes difficult to obtain a desired residual γ amount.

After winding, the cooling rate from the coiling temperature to 300 ° C is 3 ° C / min or less. When the cooling rate from the coiling temperature to 300 ° C exceeds 3 ° C / min, the above-described bainite transformation occurs effectively. Therefore, it becomes difficult to obtain a desired amount of residual γ.
In order to reduce the cooling rate to 3 ° C./mim or less, for example, the size of the steel strip coil (coil weight) when winding is adjusted or sealed in a container for heat insulation to suppress heat dissipation. And so on.
Further, after hot rolling, pickling and temper rolling are performed to obtain a product, which may be performed by a generally known method.

Molten steel having the composition shown in Table 1 was melted in a converter and made into a slab by a continuous casting method. Subsequently, these steel slabs were made into hot-rolled steel sheets having a thickness of 2.0 mm under the hot-rolling conditions shown in Table 2. Here, the cooling rate after winding (cooling speed after winding) was adjusted by the single coil weight. Subsequently, these hot-rolled steel sheets (also called hot-rolled steel strips) were pickled and temper-rolled with an elongation of 1.5%. Test specimens were collected from the obtained steel plates, and the structure observation, the tensile test, and the evaluation of the surface properties were performed by the following methods.
In addition, Ar ₃ in Table 1 is an Ar ₃ transformation point obtained by the above method using steels having respective component compositions in Table 1 as samples.

(1) Microstructure observation Specimens were taken from the obtained hot-rolled steel strip, and the microscopic structure was imaged using an optical microscope or a scanning electron microscope, and image analysis was performed on the cross section (C cross section) perpendicular to the rolling direction. Using the apparatus, the types of steel structures such as the ferrite phase as the main phase and the bainite and pearlite as the second phase were identified, and their structure fraction (area ratio) was determined. ).
The amount of solid solution N was a value obtained by subtracting the amount of precipitated N measured by the constant potential electrolysis method from the total amount of N obtained by chemical analysis.
The amount of residual γ is calculated from the peak intensities of the (211) and (220) of the austenite phase and the (200) and (220) surfaces of the ferrite phase by X-ray diffractometry on the surface near the plate thickness of 1/4. The volume fraction of the residual austenite phase was calculated.

(2) Tensile test A JIS No. 5 tensile test piece with the long axis perpendicular to the rolling direction was collected from the obtained hot-rolled steel strip, and subjected to a tensile test in accordance with the provisions of JIS Z 2241. (Yield stress (YS), tensile strength (TS), elongation (El)) were determined.

(3) Surface texture The surface texture was evaluated by visually observing the surface of the steel sheet and evaluating the degree of the surface pattern in five stages as follows. If the evaluation value is 4 or more, it is a level with no practical problem.
5: No pattern 4: Small pattern 3: Small pattern 2: Medium pattern 1: Large pattern

(4) Welding characteristics As for the welding characteristics, the shear tensile strength and the cross tensile strength at the spot weld point were measured in accordance with JIS Z3136 and Z3137. Spot welding is performed in two layers, the electrode is a DR type with a tip diameter of 8 mmΦ, the applied pressure is 5.0 kN, the welding current value is 10 kA, the welding time is 19 cycles (50 Hz), and the pressure holding time after energization is 5 The cycle (50 Hz) was used. From this, the ductility ratio (cross tensile strength / shear tensile strength) was calculated, and the welding characteristics were evaluated. Since this value decreases as the strength in the direction perpendicular to the weld surface decreases, the higher the value, the better the welding characteristics.

  The above measurement results are shown in Table 3. Each of the examples of the present invention has a high tensile strength (TS) of 590 MPa or more and a residual γ amount of 3 vol% or more, so a high strength-ductility balance of 19000 MPa ·% or more (TS × El ). Further, the scores of the surface properties are all good at 4 or more, and the welding characteristics are good at a ductility ratio of 0.5 or more.

It is a figure which shows the relationship between residual gamma amount and TSxEl. It is a figure which shows the relationship between the amount of solid solution N, and the amount of residual (gamma). It is a figure which shows the relationship between air cooling time and residual gamma amount. It is a figure which shows the relationship between the total rolling reduction of finish rolling, and residual γ amount. It is a figure which shows the relationship between coiling temperature and residual gamma amount. It is a figure which shows the relationship between the cooling rate after winding, and residual γ amount.

Claims (6)

C: 0.05-0.15 mass%,
Si: 0.005-0.8 mass%,
Mn: 1.0-3.0 mass%
P: 0.005-0.08 mass%,
S: 0.0002 to 0.005 mass%,
Al: 0.001 to 0.05 mass% and N: 0.0090 to 0.025 mass%
N: 0.0050 mass% or more in a solid solution state, with the balance being a component composition of Fe and inevitable impurities, 60 to 94 vol% ferrite phase, 3 to 30 vol% bainite phase and 3 vol% A high-tensile hot-rolled steel sheet excellent in strength-ductility balance, characterized by having a composite structure containing the above-mentioned residual austenite phase.   The composition further comprises one or more selected from Cr: 0.05 to 1.0 mass%, Mo: 0.05 to 1.0 mass%, and Ni: 0.05 to 1.0 mass%. A high-tensile hot-rolled steel sheet excellent in the strength-ductility balance described in 1. The strength-ductility according to claim 1 or 2, wherein the component further contains one or more selected from Nb, Ti, V and B within a range satisfying the following formula. High-tensile hot-rolled steel sheet with excellent balance.
N / (Al + Nb + Ti + V + B) ≧ 0.3
Here, N, Al, Nb, Ti, V, B: Content of each element (mass%) C: 0.05-0.15 mass%,
Si: 0.005-0.8 mass%,
Mn: 1.0-3.0 mass%
P: 0.005-0.08 mass%,
S: 0.0002 to 0.005 mass%,
Al: 0.001 to 0.05 mass% and N: 0.0090 to 0.025 mass%
The remainder of the steel slab containing Fe and inevitable impurities is heated to 1100-1300 ° C, and after rough rolling, the finish rolling exit temperature is Ar ₃ to (Ar ₃ +50) ° C and the total reduction rate Is subjected to finish rolling of 90% or more, and then cooling is started at a cooling rate of 20 ° C./s or more within 3 seconds, and the cooling is temporarily stopped within a temperature range of 680 to 760 ° C. for 3 to 30 seconds. After air cooling, cooling at a cooling rate of 20 ° C / s or more is performed again, and after winding at 350 to 500 ° C, the winding temperature is cooled to 300 ° C at a cooling rate of 3 ° C / min or less. A method for producing a high-tensile hot-rolled steel sheet having an excellent strength-ductility balance.   The steel slab further contains one or more selected from Cr: 0.05 to 1.0 mass%, Mo: 0.05 to 1.0 mass%, and Ni: 0.05 to 1.0 mass%. Item 5. A high-tensile hot-rolled steel sheet excellent in the strength-ductility balance according to Item 4. The strength according to claim 4 or 5, wherein the steel slab further contains one or more selected from Nb, Ti, V and B in a range satisfying the following formula. -A high-tensile hot-rolled steel sheet with excellent ductility balance.
N / (Al + Nb + Ti + V + B) ≧ 0.3
Here, N, Al, Nb, Ti, V, B: Content of each element (mass%)

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