JP3636112B2 - High-tensile hot-rolled steel sheet and high-tensile plated steel sheet with excellent bake hardenability - Google Patents

Description

【００３０】
【表２】

【００３１】
表２から明らかなように、本発明に従い、所定の成分調整を行った上で、面積率で低温変態フェライト相を50％を超えて含有し、かつ平均結晶粒径が８μm 以下の鋼組織とすることにより、焼付け硬化量が格段に高く、また耐常温時効性も良好な高張力熱延鋼板および高張力めっき鋼板を得ることができた。
【００３２】
【発明の効果】
かくして、本発明によれば、自動車の内板部品に使用して好適な、焼付硬化性に格段に優れ、また耐常温時効性も良好な高張力熱延鋼板および高張力めっき鋼板を安定して得ることができる。
【図面の簡単な説明】
【図１】 製品板の焼付け硬化量（ΔTS）に及ぼす低温変態フェライト相の影響を、鋼板の平均結晶粒径をパラメータとして示した図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-tensile hot-rolled steel sheet and a high-tensile-plated steel sheet that are suitable for use in automobile structural members, leg members, and the like, and in particular, it is possible to further enhance bake hardenability without causing deterioration in normal temperature aging resistance. It is intended to improve.
The improvement in bake hardenability as used in the present invention means not only the yield strength after work-baking coating but also the improvement in tensile strength.
[0002]
[Prior art]
Japanese Laid-Open Patent Publication No. 4-74824 proposes a method for producing a bake-hardening type high-tensile hot-rolled steel sheet comprising making a steel containing a large amount of N a composite structure mainly composed of ferrite and martensite. .
However, the above technique is intended to increase the tensile strength after the processing-paint baking process only by N in the steel, but it is not practical to add too much N, and in this technique, Since no consideration was given to room temperature aging resistance, there was a problem that room temperature aging resistance deteriorated.
[0003]
Japanese Patent Application Laid-Open No. 2000-297350 proposes a hot-rolled steel sheet with improved bake hardenability and room temperature aging resistance by controlling the refinement of crystal grains, the amount of solute N, and the presence form. Yes.
However, in order to further improve the bake hardenability using this technique, it is necessary to further refine crystal grains or further increase the amount of solute N. However, further refinement of crystal grains is a reality. In addition, increasing the solute N causes deterioration of ductility due to normal temperature aging, so there is a limit to the improvement by this technique.
[0004]
[Problems to be solved by the invention]
The present invention advantageously solves the above problems, and further improves the bake hardenability without the need for further refinement of crystal grains or further increase in the amount of dissolved N, and at the same time resistance to room temperature aging. Another object is to propose a high-tensile hot-rolled steel sheet and a high-tensile plated steel sheet.
[0005]
[Means for Solving the Problems]
Now, as a result of intensive studies to achieve the above object, the inventors have not only caused the deterioration of normal temperature aging resistance by appropriately controlling not only the composition of steel but also its structure. The inventors have obtained knowledge that a marked improvement in bake hardenability is achieved.
The present invention is based on the above findings.
[0006]
That is, the gist configuration of the present invention is as follows.
1. C: 0.05 to 0.12% by mass%
Si: 0.5% or less,
Mn: 1.2-3.0%
P: 0.05% or less,
Al: 0.001 to 0.1% and N: 0.005 to 0.02%
The balance is Fe and inevitable impurities, and the steel structure is a composite structure including a low-temperature transformation ferrite phase and a polygonal ferrite phase, and the ratio of the low-temperature transformation ferrite phase to the whole structure is an area ratio. A high-strength hot-rolled steel sheet having excellent bake hardenability, characterized by an average crystal grain size of more than 50% and a low-temperature transformed ferrite phase and a polygonal ferrite phase having an average crystal grain size of 8 μm or less.
[0007]
2. In the above 1, the steel plate is further in mass%.
Cr: 1.0% or less,
Mo: 1.0% or less and
Ni: A high-strength hot-rolled steel sheet with excellent bake hardenability characterized by having a composition containing one or more selected from 1.0% or less.
[0008]
3. In the above 1 or 2, the steel plate is further in mass%.
Ti: 0.1% or less and
Nb: A high-strength hot-rolled steel sheet excellent in bake hardenability characterized by having a composition containing one or two selected from 0.1% or less.
[0009]
4). The high tension plated steel sheet having excellent bake hardenability, wherein a plating layer is formed on the steel sheet surface in any one of the above 1-3.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be specifically described.
In the present invention, the reason why the component composition of the steel sheet is limited to the above range will be described. In addition, unless otherwise indicated, "%" display regarding a component shall mean the mass% (mass%).
C: 0.05-0.12%
C is an element useful not only for increasing the strength of the steel but also for suppressing the coarsening of crystal grains. However, if the content is less than 0.05%, the effect of addition is poor, while 0.12% is added. If it exceeds, the weldability deteriorates, so the C content is limited to the range of 0.05 to 0.12%.
[0011]
Si: 0.5% or less
Si is an element that increases the strength of steel by solid solution strengthening, and the content is appropriately adjusted according to the required strength. However, if the content exceeds 0.5%, not only the workability is deteriorated but also the formation of low-temperature transformation ferrite is inhibited, so the Si content is limited to 0.5% or less.
[0012]
Mn: 1.2-3.0%
Mn is a solid solution strengthening element and a basic constituent element for obtaining a high-strength steel sheet. It also contributes effectively to the production of low-temperature transformation ferrite. However, if the content is less than 1.2%, the effect of addition is poor. On the other hand, if it exceeds 3.0%, not only does the workability deteriorate, but also the weldability is adversely affected, so the Mn content is in the range of 1.2 to 3.0%. Limited to.
[0013]
P: 0.05% or less P is an element that increases the strength of steel, and the content is appropriately adjusted as necessary. However, if the content exceeds 0.05%, the weldability deteriorates, and P may segregate at the grain boundaries to cause intergranular cracking, and further inhibits the formation of low-temperature transformation ferrite. Was limited to 0.05% or less.
[0014]
Al: 0.001 to 0.1%
Al is an element useful as a deoxidizer, and it needs to contain at least 0.001% for deoxidation of steel. However, if it exceeds 0.1%, not only the surface properties deteriorate but also a predetermined amount of solid solution. Since it is difficult to secure N, Al is included in the range of 0.001 to 0.1%.
[0015]
N: 0.005 to 0.02%
N is a particularly important element in the present invention, and effectively acts to increase the yield strength and tensile strength after solid-solution in steel and after the work-paint baking process. For this purpose, it is necessary to contain 0.005% or more of N. However, if it exceeds 0.02%, not only the occurrence rate of internal defects increases, but also slab cracks occur frequently during continuous casting. Therefore, the N content is limited to a range of 0.005 to 0.02%. More preferably, it is 0.007 to 0.02% of range.
[0016]
Although the essential components have been described above, in the present invention, other elements described below can be appropriately contained.
One or more selected from Cr: 1.0% or less, Mo: 1.0% or less, and Ni: 1.0% or less
All of Cr, Mo and Ni not only contribute to increasing the strength of the steel by solid solution strengthening, but also have the effect of facilitating the formation of a low-temperature transformation ferrite phase in hot rolling by stabilizing austenite. . In order to obtain this effect, the Cr, Mo, and Ni contents are each preferably 0.1% or more. However, in any case, if the content exceeds 1.0%, the formation of the low-temperature transformation ferrite phase is inhibited. Therefore, each content is assumed to be 1.0% or less.
[0017]
One or two selected from Ti: 0.1% or less and Nb: 0.1% or less
Ti and Nb contribute to the improvement of strength and toughness by forming carbide and nitride, respectively. In order to obtain this effect, the Ti and Nb contents are each preferably 0.01% or more. However, if the content exceeds 0.1%, the solid solution N is fixed as a nitride, and the bake hardenability is reduced. Therefore, each content is 0.1% or less.
[0018]
As described above, the essential component and the selected component have been described. However, in the present invention, it is not sufficient to adjust the component composition range to the above range, and the structure and particle size thereof must also be specified.
Are complex tissues including low-temperature transformation ferrite phase and polygonal ferrite phase, yet the area ratio to the total organization of the low-temperature transformation ferrite phase V (α _B): 50% greater than the low-temperature transformation ferrite alpha _B here, the ordinary meaning It is distinct from ferrite (polygonal ferrite: α _P ), and is a ferrite formed in a low temperature range (approximately 500 ° C. or lower) and means bainitic ferrite or upper bainite. This structure is particularly important in the present invention, and is responsible for high bake hardenability.
Bake hardening is a phenomenon in which the strength increases when interstitial solid solution elements (C, N) in the steel fix the dislocations in the steel and the resistance to the movement of the dislocations increases. In the low-temperature transformation ferrite structure, the effect is accelerated because the dislocation density is originally high, and the fixed dislocations act as resistance to the movement of dislocations during plastic deformation, so that they exhibit extremely high bake hardenability. become.
[0019]
Steel slabs containing C: 0.08%, Si: 0.2%, Mn: 2.3%, P: 0.01%, Al: 0.015% and N: 0.013%, with the balance being Fe and inevitable impurities. A hot-rolled steel sheet was formed by hot rolling under conditions. From these hot-rolled steel sheets, those having a steel structure containing a low-temperature transformation ferrite phase were selected, and further classified into a group having an average crystal grain size of 8 μm or less and a group having an average crystal grain size of 10-15 μm.
FIG. 1 shows the results of examining the influence of the low temperature transformation ferrite phase on the bake hardening amount for each group. The baking treatment conditions were pre-strain amount: 5%, aging treatment: 170 ° C. × 20 minutes.
As shown in the figure, by including the low-temperature transformation ferrite phase in an area ratio exceeding 50% with respect to the entire structure and making the average crystal grain size 8 μm or less, an excellent bake hardening amount ΔTS of 100 MPa or more can be obtained. Obtained stably.
[0020]
Other than the above-described low-temperature transformation ferrite phase, it is substantially composed of a polygonal ferrite phase, but a part of the martensite phase or pearlite phase may be mixed.
However, if these mixed phases become too large, it is difficult to obtain the desired effect. Therefore, it is preferable to suppress these phases to 10% or less in terms of area ratio. That is, the total area ratio of the two phases of the low-temperature transformation ferrite phase and the polygonal ferrite phase is preferably 90% or more.
[0021]
The average crystal grain size of the two phases of the low temperature transformation ferrite phase and the polygonal ferrite phase is 8 μm or less. The average crystal grain size referred to in the present invention is the two of the low temperature transformation ferrite phase (α _B ) and the polygonal ferrite phase (α _P ). It is the average crystal grain size of the phase, and it is important to limit this average crystal grain size to 8 μm or less.
As shown in FIG. 1, by setting the average crystal grain size to 8 μm or less, the bake hardening amount (ΔTS) of the product plate can be remarkably improved.
In this respect, when the average crystal grain size is 10 to 15 μm exceeding 8 μm, a significant increase in tensile strength cannot be expected.
Here, by making the crystal grains finer, the grain boundary area as the location of the solid solution N increases, but the solid solution N present in the grain boundary is stable at room temperature and cannot be diffused. Deterioration of is suppressed. In this respect, when the average crystal grain size exceeds 8 μm, this effect is remarkably reduced.
[0022]
The reason why high bake hardenability can be obtained by using the above configuration is considered as follows.
Bake hardening occurs because the movable dislocations are fixed by the solid solution N due to the interaction between the movable dislocations and the solid solution N that are produced when pre-processing, and at that time, the crystal grains are refined. When the grain boundaries increase, even when the same amount of strain is processed, movable dislocations are distributed with high density. Further, the low-temperature transformation ferrite structure contains a large amount of movable dislocations before the pre-processing, and the dislocation density after the pre-processing becomes high, so that it is considered that the bake hardenability is exhibited.
[0023]
Next, preferred production conditions for the steel of the present invention will be described.
The steel adjusted to the above-mentioned preferred component composition is melted in a converter or the like and is made into a slab by a continuous casting method or the like. After this steel material is kept in a high temperature state or cooled, it is charged into a heating furnace and then hot rolled to form a hot-rolled sheet, which is then wound around a coil at a predetermined temperature.
Here, the slab heating temperature is preferably about 1000 to 1300 ° C. This is because if the heating temperature is less than 1000 ° C, it is difficult to leave a sufficient amount of N in the hot-rolled sheet in a solid solution state. On the other hand, if the heating temperature exceeds 1300 ° C, the austenite grains during heating become coarse, and the average This is because it becomes difficult to make the crystal grain size 8 μm or less.
[0024]
Next, hot rolling is performed. In this hot rolling, it is preferable to set the finish rolling outlet temperature (denoted as FDT) to about (Ar ₃ + 10 ° C.) to (Ar ₃ + 100 ° C.).
This is because if the FDT is below (Ar ₃ + 10 ° C), the finish rolling temperature becomes too low and the structure becomes non-uniform, and part of the processed structure remains, causing various problems during press forming. This is because the danger increases and, on the other hand, if FDT exceeds (Ar ₃ + 100 ° C.), refinement of crystal grains and the amount of solute N cannot be secured.
[0025]
After the above hot rolling is completed, the subsequent cooling process is strictly controlled to generate a low temperature transformation ferrite phase having an area ratio of more than 50%.
For example, a low temperature transformation ferrite phase having an area ratio of more than 50% can be generated by the method described below.
That is, cooling is started at a rate of 200 ° C / s or more within 0.2 seconds after completion of hot rolling, the cooling is stopped at 400 to 500 ° C, and immediately wound up, and then the temperature range up to 300 ° C is 1 ° C. Slowly cool at / s or less.
[0026]
Since the hot-rolled steel sheet obtained as described above is suitable as various plating original plates, various plating treatments can be performed as necessary.
Here, examples of the types of plating include electrogalvanizing, hot dip galvanizing, electrotin plating, electrochromic plating, and electronickel plating. In the present invention, any plating treatment can be advantageously applied. it can.
[0027]
【Example】
Molten steel having the composition shown in Table 1 was melted in a small melting furnace, and rough rolled to obtain a sheet bar having a thickness of 40 mm. Then, after heating to 1250 ° C, hot rolling of 3 passes under the condition of rolling end temperature: 890 ° C, finishing to a plate thickness: 1.5 mm, followed by gas cooling, a specified range of 600-200 ° C After maintaining at the temperature for 1 h, the hot-rolled steel sheet having various structures was produced by allowing to cool. In addition, a part was hot dip galvanized.
The obtained hot rolled steel sheet and plated steel sheet were subjected to a structure test, a tensile test, a bake hardenability test, and a normal temperature aging test.
[0028]
In addition, the steel structure was investigated by the enlarged image of the corrosion appearance structure by the nital of the cross section of the direction orthogonal to the rolling direction of a hot-rolled steel plate.
In the tensile test, JIS No. 5 tensile test specimens were collected from a direction perpendicular to the rolling direction of the hot-rolled steel sheet, and the tensile test was performed under the condition of strain rate: 10 ⁻³ / s.
In the bake hardenability test, JIS No. 5 tensile test specimens were sampled from the direction perpendicular to the rolling direction of the hot-rolled steel sheet and subjected to aging treatment after pre-straining, and the strain rate was 10 ^-3 / s. Conducted under conditions. The baking treatment conditions were a pre-strain amount of 5% and an aging treatment condition of 170 ° C. × 20 minutes.
The bake hardening amount BH and the increase margin of tensile strength ΔTS are respectively expressed by the following formulas BH = (yield stress after aging) − (predeformation stress before aging treatment)
ΔTS = (Tensile strength after aging) − (Tensile strength as hot rolled)
Sought by.
In the normal temperature aging test, after aging treatment at 50 ° C for 400 h, a JIS No. 5 tensile test piece was taken from the direction perpendicular to the rolling direction, and a tensile test was performed at a strain rate of 10 ^-3 / s. measures the elongation EI _a, the difference between the elongation EI before aging treatment was evaluated by △ El = El-EI _a. If the obtained ΔEl is 2.0% or less, it can be said that there is no problem with room temperature aging.
The obtained results are shown in Table 2.
[0029]
[Table 1]

[0030]
[Table 2]

[0031]
As is apparent from Table 2, the steel structure containing the low-temperature transformation ferrite phase with an area ratio of more than 50% and an average crystal grain size of 8 μm or less after adjusting the predetermined components according to the present invention. By doing so, it was possible to obtain a high-tensile hot-rolled steel sheet and a high-tensile plated steel sheet that have a markedly high bake hardening amount and good room temperature aging resistance.
[0032]
【The invention's effect】
Thus, according to the present invention, it is possible to stably produce a high-tensile hot-rolled steel sheet and a high-tensile plated steel sheet that are suitable for use in automobile inner plate parts, have excellent bake hardenability, and have good room temperature aging resistance. Can be obtained.
[Brief description of the drawings]
FIG. 1 is a diagram showing the influence of a low-temperature transformation ferrite phase on the bake hardening amount (ΔTS) of a product plate, using the average crystal grain size of the steel plate as a parameter.

Claims (4)

C: 0.05 to 0.12% by mass%
Si: 0.5% or less,
Mn: 1.2-3.0%
P: 0.05% or less,
Al: 0.001 to 0.1% and N: 0.005 to 0.02%
The balance is Fe and inevitable impurities, and the steel structure is a composite structure including a low-temperature transformation ferrite phase and a polygonal ferrite phase, and the ratio of the low-temperature transformation ferrite phase to the entire structure is an area ratio. A high-strength hot-rolled steel sheet excellent in bake hardenability, characterized by having an average crystal grain size of more than 50% and a low-temperature transformed ferrite phase and a polygonal ferrite phase having an average crystal grain size of 8 μm or less. In Claim 1, a steel plate is further in mass%.
Cr: 1.0% or less,
Mo: 1.0% or less and
Ni: A high-strength hot-rolled steel sheet with excellent bake hardenability characterized by having a composition containing one or more selected from 1.0% or less. In Claim 1 or 2, a steel plate is further in mass%.
Ti: 0.1% or less and
Nb: A high-strength hot-rolled steel sheet excellent in bake hardenability characterized by having a composition containing one or two selected from 0.1% or less. The high-tensile plated steel sheet excellent in bake hardenability according to any one of claims 1 to 3, wherein a plating layer is formed on the steel sheet surface.

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