JP2000297350A - High-strength hot-rolled steel sheet excellent in bake hardenability, fatigue resistance, impact resistance and normal temperature aging resistance, and method for producing the same - Google Patents

Abstract

(57) [Summary] [Problem] A baking hardenability suitable as an interior material of an automobile.
Provided is a hot-rolled steel sheet having improved fatigue resistance, impact resistance, and normal-temperature aging resistance, and a method for producing the same. SOLUTION: C: 0.01 to 0.12%, Si: 2.0% or less, M
n: 0.01 to 3.0%, P: 0.2% or less, Al: 0.001 to 0.1
%, N: 0.003 to 0.02%, hot-rolled, and started cooling at a cooling rate of 50 ° C./s or more within 0.5 sec from the end of hot rolling, and the average grain size is 8 μm or less. Of ferrite as the main phase, and the amount of solute N is 0.003 to 0.
01%, average dissolved N concentration N at ferrite grain boundaries
gb and the average dissolved N concentration in ferrite grains Ng
, And Ngb / Ng is set to 100 to 10,000.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled thin steel sheet suitable for use as a structural member of an automobile, a suspension member, and the like, and more particularly, to further improving bake hardenability, fatigue resistance and impact resistance. And to improve the aging resistance at room temperature at the same time. In the present invention, the improvement in bake hardenability means an increase in tensile strength as well as the yield strength after the work-bake coating treatment.

[0002]

2. Description of the Related Art In recent years, steel sheets for automobiles have been required to have higher strength in order to improve fuel efficiency by reducing weight. However, increasing the strength of a steel sheet has a problem that press forming is difficult. In recent years, for the purpose of ensuring the safety of occupants, there is a demand for an improvement in impact resistance evaluated by the amount of deformation energy under a high strain rate such as at the time of a collision.

As a high-strength technique for preventing the deterioration of press formability due to the high strength, a so-called bake hardening property (BH), which is relatively low in strength at the time of molding, is easy to work, and increases the strength by baking at the time of painting. The technology utilizing the above-mentioned properties is known, and cold-rolled steel sheets are widely used (for example, JP-A-6-73498 and JP-A-7-268544). However, the improvement in bake hardenability obtained by these techniques is effective for improving the dent resistance of automobile outer panels because only the yield strength is increased and the tensile strength is not increased. It does not lead to the improvement of the fatigue resistance and impact resistance required for the plate.

On the other hand, Japanese Patent Application Laid-Open No. 1-180917 discloses that C: 0.
030 to 0.100 wt%, N: 0.0015 to 0.0150 wt%, Al: 0.02
A steel containing 5 to 0.100 wt% is heated to 1200 ° C or less,
Finish rolling at a temperature of (Ar ₃ + 30 ° C.) to 950 ° C., rapidly cooling to 500 ° C. or less within 3 seconds after the rolling at a cooling rate of 30 ° C./s or more, winding at 400 to 500 ° C., workability, A method for producing a hot-rolled steel sheet having excellent bake hardenability has been proposed. In the technique described in Japanese Patent Application Laid-Open No. 1-180917, quenching is performed after rolling, and the amount of C, N in the steel sheet is increased,
BH property is improved.

Japanese Patent Application Laid-Open No. 4-74824 discloses that C: 0.
02-0.13wt%, N: 0.0080-0.0250wt%, solAl: 0.10
Reheat steel containing less than wt% to 1100 ° C or more
Perform hot rolling to finish finish rolling at a temperature of ~ 950 ° C,
Then, at a cooling rate of 15 ° C / s or more, a method for producing a hot-rolled steel sheet with excellent baking hardenability and workability is proposed, in which air cooling is interposed or cooled to 350 ° C or less without air cooling in the middle. Have been.

[0006] Japanese Patent Application Laid-Open No. 63-96248 discloses that C: 0.
010 to 0.025 wt%, N: 0.0015 to 0.0030 wt%, Nb: 0.01
A bake-hardenable hot-rolled steel sheet containing -0.05 wt%, solAl: 0.008 wt% or less, and controlling the winding temperature after hot-rolling so that an appropriate amount of solid solution C and solid solution N remain. It is said that the fatigue limit increases after the processing-paint baking treatment. Also,
JP-A-10-183301 discloses that C: 0.01 to 0.12 wt%,
There is disclosed a technique for increasing the BH amount (the amount of increase in yield strength due to baking treatment) by controlling the cooling rate and the winding temperature after hot rolling in steel containing N: 0.0001 to 0.01 wt%.

[0007]

SUMMARY OF THE INVENTION
The hot-rolled steel sheet manufactured by the technique described in JP-A 1-180917 has a problem that the aging resistance at room temperature is deteriorated. Further, the yield strength after the paint baking treatment increases, but at the same time, an increase in tensile strength cannot be expected, so that a remarkable improvement in fatigue resistance and impact resistance cannot be expected. Also, JP-A-4-74
Hot rolled steel sheet manufactured by the technology described in Japanese Patent No. 824,
This is a composite structure mainly composed of ferrite and martensite.Although the tensile strength after processing and painting baking increases, there is no consideration for improving the room-temperature aging resistance. Had left. Also, JP-A-63-9
In the steel sheet described in Japanese Patent No. 6248, the increase in the fatigue limit is small compared to the increase in the yield strength, and the amount of the increase is at most about 25 MPa, and it cannot be said that the fatigue resistance has been significantly improved.

[0008] Further, in a hot-rolled steel sheet manufactured by the technique described in Japanese Patent Application Laid-Open No. 10-183301, it is mainly intended to increase the yield strength after processing and painting baking.
The optimum conditions for fatigue resistance and impact resistance have not always been found. The present invention advantageously solves the above-mentioned problems of the prior art, and in a high-strength hot-rolled steel sheet having a tensile strength exceeding 370 MPa, does not require excessive addition of a solid-solution element, bake hardening, fatigue resistance, An object of the present invention is to provide a hot-rolled steel sheet which is improved in impact resistance and normal-temperature aging resistance and is suitable as an interior material of an automobile, and a method for producing the same. The hot-rolled steel sheet with improved bake hardenability intended in the present invention refers to a hot-rolled steel sheet excellent in bake hardenability in which the yield strength and the tensile strength are simultaneously increased by processing-paint baking.

[0009]

Means for Solving the Problems The present inventors have conducted intensive studies in order to achieve the above-mentioned objects, and as a result, have found that hot-rolling, which has an increased tensile strength after work-paint baking and has excellent aging resistance at room temperature. In order to form a steel sheet, it is effective to control the N present in the solid solution state in the steel sheet and the form of the solute N so that the amount of the solute N existing at the crystal grain boundaries is within an appropriate range. I arrived. Then, the present inventors, after refining the crystal grains and increasing the crystal grain boundaries, control the amount of solute N present in the steel sheet to a certain amount, and furthermore, the amount of solute N existing in the crystal grain boundaries. By adjusting the ratio between Ngb and the amount of dissolved N present in the grains Ng to an appropriate range, there is no deterioration in aging resistance at ordinary temperature, the tensile strength after processing-paint baking treatment increases significantly, and fatigue resistance increases. It was found that both properties and impact resistance were improved.

First, the experimental results on which the present invention is based will be described. 0.065 mass% C-0.005 mass% Si-0.49
mass% Mn-0.01mass% P-0.021 mass% Al-0.015 mass
% N containing steel A1 and 0.07mass% C-0.12mass% Si-
1.2 mass% Mn-0.02 mass% P-0.015 mass% Al-0.015
Production conditions such as hot rolling conditions were adjusted using steel B1 containing mass% N to produce hot-rolled steel sheets in which the amount of solute N and the ferrite grain size were variously changed. First, in Experiment 1, in the hot-rolled steel sheet of steel A1, the amount of solute N was changed in the range of 5 to 100 ppm and the ferrite grain size was changed in the range of 6.0 to 7.9 μm.
In the hot-rolled steel sheet, the amount of solute N was changed in the range of 5 to 100 ppm, and the ferrite grain size was changed in the range of 6.0 to 7.9 μm and 9.0 to 11.9 μm.

For these hot-rolled steel sheets, the amount of solid solution N present in the ferrite crystal grain boundaries and in the grains (hereinafter, referred to as Ngb and Ng, respectively) was measured using a three-dimensional atom probe. This measurement is performed at a temperature of 50 K, and the applied voltage is
7 to 15 kV, and the pulse ratio was 15 to 20%. As a result, all of the hot-rolled steel sheets used had Ngb / Ng in the range of 100 to 10,000. Note that the amount of dissolved N (Ngb) existing at a crystal grain boundary measured using a three-dimensional atom probe is an average concentration of dissolved N existing within a range of ± 5 nm from a crystal grain interface.

Next, JIS No. 5 tensile test pieces were sampled from these hot-rolled steel sheets, subjected to a normal tensile test, subjected to a tensile prestrain of 8%, and then unloaded, and subjected to a heat treatment (painting) at 170 ° C. for 20 minutes. (Equivalent to baking treatment), conduct a tensile test to apply tensile strain again, measure the tensile strength, and apply the processing-paint baking treatment to the tensile strength TS _BH and the hot rolling obtained by the normal tensile test. The difference from the tensile strength TS as it was, ΔTS, was calculated. FIG. 1 shows the relationship between ΔTS and the amount of solute N.

FIG. 1 shows that the ferrite grain size is 6.0 to 7.
By setting the range of 9 μm and the amount of solid solution N to 30 ppm or more, ΔTS becomes 60 MPa or more, and it can be seen that bake hardenability is remarkably improved. On the other hand, the ferrite grain size is
In the range of 9.0 to 11.9 μm, no matter how much the amount of solute N is increased, a remarkable increase of ΔTS of 60 MPa or more cannot be expected. Next, in Experiment 2, using a hot-rolled steel sheet of steel B1, the amount of solute N was 30 to 80 ppm, and the ferrite grain size was 3.0 to 15.0 μm.
m.

For these hot-rolled steel sheets, as in Experiment 1, the solute N
The amounts, Ngb and Ng were measured. Further, for these hot-rolled steel sheets, as in Experiment 1, the difference between the tensile strength TS _BH after being subjected to the work-paint baking treatment and the hot-rolled tensile strength TS obtained by a normal tensile test. , ΔTS. ΔTS
FIG. 2 shows the relationship between and the ferrite crystal grain size.

FIG. 2 shows that by setting the ferrite crystal grain size to 8 μm or less and Ngb / Ng in the range of 100 to 10,000, ΔTS becomes 60 MPa or more, and the bake hardenability is remarkably improved. On the other hand, when Ngb / Ng is less than 100, a remarkable increase of ΔTS of 60 MPa or more cannot be expected regardless of the ferrite crystal grain size. From these hot-rolled steel sheets, a high-speed tensile test specimen was sampled, subjected to a 5% tensile prestrain, then unloaded, subjected to a heat treatment equivalent to a paint baking treatment at 170 ° C. for 20 minutes, and rinsed to obtain a strain rate. A 2 × 10 ³ / s high strain rate tensile test was performed to measure the tensile strength and stress-strain curve. Using the measured stress-strain curve, an integrated value up to a strain amount of 30% was obtained and defined as an absorbed energy E. FIG. 3 shows the relationship between the absorption energy E and the ferrite crystal grain size.

From FIG. 3, it can be seen that by setting the ferrite crystal grain size to 8 μm or less and Ngb / Ng within the range of 100 to 10,000, the absorption energy E becomes 175 MJ / m ³ or more, and the impact resistance is remarkably improved. It is understood that it is done. On the other hand, Ngb /
In Ng is less than 100, regardless of the ferrite grain size, can not be expected significant increase of 175 MJ / m ³ or more absorbed energy E.

Further, as Experiment 3, from the hot-rolled steel sheets used in Experiment 2, the solid solution N was 67 ppm, and the ferrite crystal grain size was 6.
2 μm, Ngb / Ng of 126 and the amount of solute N
12ppm, ferrite grain size 9.6μm, Ngb / Ng 87
And the same experiment as Experiment 1 was performed.
In addition, the tensile prestrain was changed in the range of 2 to 10%. Processing-Difference between tensile strength TS _BH after paint bake treatment and hot-rolled tensile strength TS obtained by ordinary tensile test, △
TS is determined, and the relationship between ΔTS and the amount of predistortion is shown in FIG.

FIG. 4 shows that ΔTS increases with an increase in the amount of pre-strain when the ferrite crystal grain diameter is 6.2 μm, the amount of solute N is 67 ppm, and Ngb / Ng is 126. In addition, a large ΔTS is shown for each of the predistortion amounts. △ TS: 50MPa or more for 5% pre-distortion, 以上 TS: 60M for 8% pre-distortion
Pa or more. The present invention has been made by further study based on the above findings.

That is, in the present invention, C: 0.01% by mass.
~ 0.12%, Si: 2.0% or less, Mn: 0.01 ~ 3.0%, P: 0.
2% or less, Al: 0.001 to 0.1%, N: 0.003 to 0.02%, the balance being a composition consisting of Fe and unavoidable impurities and a main phase of ferrite having an average crystal grain size of 8 μm or less, preferably 6 μm or less. Having the following structure, and in mass%
It has a solid solution N content of 0.003 to 0.01%, preferably 0.005 to 0.01%, and has an average solid solution N concentration Ngb within ± 5 nm from the ferrite crystal grain interface and an average solid solution N existing in the ferrite crystal grain. Ratio of N concentration to Ng, Ngb / Ng is 100 to 10,000
Baking hardenability, fatigue resistance,
It is a high-tensile hot-rolled steel sheet excellent in impact resistance and normal-temperature aging resistance.
It is preferable that the composition contains one or two of Ti: 0.001 to 0.1% and Nb: 0.001 to 0.1%.
Ni: 0.1-1.5%, Cr: 0.1-1.5%, Mo: 0.1-
It is preferable that one or more of 1.5% is contained, and according to the present invention, the tissue comprises, as a second phase,
It is preferable to have a structure containing one or more of pearlite, bainite, martensite, and retained austenite.

Further, the present invention provides a baking hardenability, a fatigue resistance, an impact resistance and a normal temperature aging, wherein a plating layer is formed on the surface of any of the above-mentioned high tensile strength hot rolled steel sheets. High tensile strength hot rolled steel sheet with excellent heat resistance. In addition, the present invention is based on the following: C: 0.01 to 0.12%, Si: 2.0% or less,
n: 0.01 to 3.0%, P: 0.2% or less, Al: 0.001 to 0.1
%, N: 0.003 to 0.02%
Heating to a temperature range of 1300 ° C, preferably 1070-1180 ° C,
After the rough rolling, finish rolling is performed in a final stand rolling reduction of 10% or more, and a final finishing rolling temperature FDT within a temperature range of (Ar ₃ + 100 ° C.) to (Ar ₃ + 10 ° C.).
Cool at a cooling rate of 50 ° C / s or more, and take-up temperature: 600 to 35
The present invention relates to a method for producing a high-strength hot-rolled steel sheet having excellent baking hardenability, fatigue resistance, impact resistance and aging resistance at room temperature, which is characterized in that it is wound in a temperature range of 0 ° C. In addition to the above, in mass%, Ti: 0.001 to 0.1% and N
b: It is preferable that the composition contains one or two of 0.001 to 0.1%, and in the present invention, in addition to each of the above-mentioned compositions, Ni: 0.1 to 1.5% by mass%, Cr: 0.1 to
It is preferable to contain one or more of 1.5% and Mo: 0.1 to 1.5%.

[0021]

First, the reasons for limiting the composition of the hot-rolled steel sheet of the present invention will be described. In addition,% in the following composition
Means mass%. C: 0.01 to 0.12% C is an element that increases the strength of steel, and requires a content of 0.01% or more. On the other hand, if the content exceeds 0.12%, the weldability deteriorates. For this reason, in the present invention, C is 0.01-0.
Limited to 12% range.

Si: 2.0% or less Si is an element that increases the strength of steel by solid solution strengthening, and its content is adjusted according to the desired strength. But 2.0
%, The workability is deteriorated. For this reason, Si
Limited to 2.0% or less. From the viewpoint of securing the strength, it is desirable that Si is contained at 0.003% or more.

Mn: 0.01-3.0% Mn is an element that increases the strength of steel and prevents hot embrittlement due to S. In the present invention, Mn is positively contained. However, if the content exceeds 3.0%, the workability deteriorates. For this reason, Mn was limited to 3.0% or less. In addition, in order to secure desired strength and prevent hot brittleness, the content of 0.01% or more is required.

P: 0.2% or less P is an element that increases the strength of steel, and it is desirable to contain 0.005% or more in order to secure a desired strength. However, when the content exceeds 0.2%, the weldability is degraded, and P may segregate at the grain boundaries to cause grain boundary cracking. Therefore, P is limited to 0.2% or less.

Al: 0.001 to 0.1% Al acts as a deoxidizing agent, and 0.001 to 0.1% for deoxidizing steel.
% Or more is required. On the other hand, if the content exceeds 0.1%, the surface properties deteriorate. For this reason, Al is 0.001 to 0.
Limited to the 1% range. N: 0.003 to 0.02% N is an important element in the present invention, and effectively acts to increase the yield strength, particularly the tensile strength, after forming and solid-dissolution in a steel sheet after processing and coating baking. For this purpose, it is necessary that 0.003% or more, preferably 0.005% or more of solid solution N remains in the steel sheet, and the lower limit of the N content is set to 0.003%. Preferably, it is at least 0.005%. On the other hand, when the content exceeds 0.02%, the moldability deteriorates. Therefore, N is 0.003 to 0.02.
%.

Ti: 0.001 to 0.1% and Nb: 0.001 to 0.1%
One or two of 1% Ti and Nb form carbides, nitrides, and sulfides, and contribute to the improvement of strength and toughness. These effects are 0.00
Although it is recognized at a content of 1% or more, if it exceeds 0.1%, the amount of C and N contributing to bake hardenability decreases, and it becomes impossible to secure desired bake hardenability. Therefore, both Ti and Nb are preferably limited to the range of 0.001 to 0.1%.

Ni: 0.1-1.5%, Cr: 0.1-1.5%, M
o: One or two or more of 0.1 to 1.5% Ni, Cr and Mo are elements that increase the strength of steel by solid solution strengthening, and austenite (γ) during the cooling process after hot rolling. And has the effect of easily forming a two-phase structure. Such an effect is observed at a content of 0.1% or more. On the other hand, if it exceeds 1.5%, the formability, the plating property and the spot weldability deteriorate. Therefore, Ni, Cr,
Mo is preferably in the range of 0.1 to 1.5%.

In the hot-rolled steel sheet of the present invention, the balance other than the above components is Fe and inevitable impurities. As unavoidable impurities, S and O form nonmetallic inclusions and adversely affect the quality. Therefore, it is desirable to reduce S and O to 0.05% or less and 0.01% or less, respectively. The structure of the hot-rolled steel sheet of the present invention having the above-described composition has ferrite as a main phase and consists of only the main phase or the main phase and the second phase. In the present invention, in particular, in order to remarkably enhance the bake hardenability and at the same time to improve the fatigue resistance and the impact resistance, the structure is refined.
Properly adjust the form of presence.

In order to make the structure finer, the average crystal grain size of the main phase ferrite is set to 8 μm or less. The crystal grains are refined, and the crystal grain boundaries as the locations of the solute N are increased.
If the average crystal grain size of the ferrite exceeds 8 μm, as shown in FIG. 2, a remarkable increase in tensile strength after processing-paint baking cannot be obtained, and no remarkable improvement in bake hardenability can be obtained. And, since there is no increase in tensile strength, improvement in fatigue resistance and impact resistance cannot be expected. In addition, from the viewpoint of increasing the tensile strength after processing and baking, it is preferable that the average crystal grain size of ferrite be 6 μm or less. Furthermore, by making the ferrite crystal grains finer, the grain boundary area increases, the ratio of solid solution N present at the grain boundaries increases, and deterioration of aging at room temperature is suppressed. This is because the solute N present in the grain boundaries is stable and cannot be diffused at room temperature. When the grain size of ferrite exceeds 8 μm,
This effect is significantly reduced.

The second phase is one or two of pearlite, bainite, martensite and retained austenite.
It is desirable to have more than species. By the presence of the second phase, the strength can be increased without adding a large amount of expensive additive elements, and the fatigue resistance and collision resistance are improved. The volume ratio of the second phase is preferably 3 to 30% from the viewpoint of workability.

The hot-rolled steel sheet of the present invention contains
0.0030 to 0.01% of solid solution N is left. 0.005
If it is less than 30%, as shown in FIG. 1, the amount of increase in tensile strength after processing-painting baking is small, and remarkable improvement in baking hardenability cannot be obtained. And because there is no increase in tensile strength,
Significant improvement in fatigue resistance and impact resistance cannot be expected. On the other hand, if the amount of solute N exceeds 0.01%, aging at room temperature becomes remarkable, the yield point rises greatly, the yield elongation becomes remarkable, and the total elongation decreases, which is a practical problem. For this reason, the amount of N existing in a solid solution state in the hot-rolled steel sheet is limited to the range of 0.003 to 0.01%, preferably 0.005 to 0.01%. In the present invention,
As the amount of N existing in the solid solution state, a value obtained by subtracting the amount of N compound obtained by the extraction separation method from the amount of N in steel obtained by wet analysis is used.

Ngb / Ng: 100-10000 Ngb is the concentration of solute N present in the ferrite grain boundaries, and Ng is the concentration of solute N present in the ferrite grains, and is a three-dimensional atom probe, It is measured using a microscope, Auger electron spectroscopy, or the like. In the measurement of the amount of solute N, the measurement is started from the inside of the grain and continuously measured through the grain boundary to the inside of the adjacent grain or continuously from the surface of the grain boundary to the inside of the grain. Measurements are one-dimensional, two-dimensional, three-dimensional
It may be dimensional. Detect and analyze ionized atoms, characteristic X-rays, Auger electrons, etc. according to each measuring means, and determine the concentration Ng of solid solution N at a stable part away from the grain boundary, and within ± 5 nm of the grain boundary. The average dissolved N concentration Ngb is determined. This is performed on at least three or more grain boundaries and averaged to be Nb and Ngb, respectively.

When Ngb / Ng is less than 100, the amount of increase in tensile strength after processing-painting baking is small, and remarkable improvements in baking hardenability, fatigue resistance and impact resistance are not observed. On the other hand, Ngb
If / Ng exceeds 10,000, solute N at the crystal grain boundaries precipitates, and the increase in tensile strength after processing-paint baking is reduced.
Therefore, Ngb / Ng is limited to the range of 100 to 10,000.

The reason why the hot-rolled steel sheet has the above-described structure and the tensile strength is remarkably increased after the work-paint baking treatment is not clear at present, but is considered as follows. When a steel sheet having movable dislocations is subjected to a heat treatment such as paint baking treatment, solid-solution N agglomerates around the movable dislocations due to the interaction between the movable dislocations and solid solution N, and fixes the movable dislocations. To increase the yield stress. When the amount of solute N further increases, dislocations are fixed by precipitation of fine nitrides in addition to formation of a Cottrell atmosphere, and the nitrides and the fixed dislocations hinder the movement of movable dislocations, increasing the strength. The source of the movable dislocations is the crystal grain boundary. When the crystal grains are refined and the crystal grain boundaries increase, the mobile dislocations are distributed at a high density and uniformly even if processed by the same amount of strain. The fixed dislocations serving as obstacles of the mobile dislocations are also distributed at a high density, which makes the motion of the mobile dislocations difficult and significantly increases the strength of the steel sheet. Furthermore, Ngb / Ng
In other words, the larger the amount of solute N present in the crystal grain boundary, the larger the amount of solute N in the movable dislocation group deposited near the grain boundary.
Are easily diffused and efficiently fix the movable dislocations. on the other hand,
The solute N present in the grains only contributes to the strengthening of the ferrite ground, and a small percentage contributes to the increase in the tensile strength due to the working-paint baking treatment.

In a steel sheet having an increased tensile strength after processing-paint baking, even when deformed at a high strain rate, fine nitrides and fixed dislocations hinder the movement of dislocations in the same manner as under low strain rate deformation. As a result, the strength increases, the deformation energy required for deformation increases, and the impact resistance improves. Further, even when a repeated load is applied, since the fixed dislocations and the fine nitrides are densely distributed, the resistance to the propagation of fatigue cracks increases, and the fatigue strength increases.

Next, a method for producing the steel sheet of the present invention will be described. First, in the above mass%, C: 0.01 to 0.12%,
Si: 2.0% or less, Mn: 0.01 to 3.0%, P: 0.2% or less,
Al: 0.001 to 0.1%, N: 0.003 to 0.02%, or further Ti: 0.001 to 0.1% and Nb: 0.001 to 0.1
% And / or Ni: 0.1-1.
5%, Cr: 0.1 to 1.5%, Mo: 0.1 to 1.5%
A steel material containing one or more species and the balance being substantially Fe is heated by a generally known device such as a heating furnace. Further, the steel material for rolling is preferably formed into a shape such as a slab by solidifying a molten steel produced by a known melting method by a known continuous casting method or an ingot casting method.

In order to secure the desired solid solution N in the hot-rolled sheet, it is necessary to dissolve the nitride at the time of heating, and in order to make the structure of the hot-rolled sheet fine, the heating temperature must be reduced. It is preferable to lower the temperature to make the austenite grains during heating as fine as possible. Therefore, the heating temperature is 1000
C. to 1300 ° C., more preferably 1070 ° C. to 1180 ° C. If the temperature is lower than 1000 ° C., precipitation of N proceeds, and it becomes difficult to leave N in a solid solution state in the hot-rolled sheet. Also, 13
If the temperature exceeds 00 ° C., it is also difficult to reduce the average crystal grain size of ferrite to 8 μm or less.

The heated rolled material is then hot-rolled. Hot rolling consists of rough rolling and finish rolling. The steel material adjusted to an appropriate thickness by rough rolling is then subjected to finish rolling. In the finish rolling, the final stand rolling reduction is 10% or more, and the final finish rolling temperature FDT is (Ar ₃ +10
0 ° C.) and rolling to a temperature range of _{~ (Ar 3 + 10 ℃)} .

If the FDT exceeds (Ar ₃ + 100 ° C.), the crystal grains cannot be refined and an appropriate amount of solid solution N cannot be secured even if rapid cooling treatment is performed after hot rolling. On the other hand, if the FDT is less than (Ar ₃ + 10 ° C.), the strain distribution in the sheet thickness direction before transformation becomes non-uniform,
The average crystal grain size of ferrite cannot be reduced to 8 μm or less. Therefore, FDT is (Ar ₃ + 100 ° C.)
(Ar ₃ + 10 ° C.).

On the other hand, if the rolling reduction of the final stand is less than 10%, the strain is not sufficiently accumulated before the ferrite transformation, and the refinement of the crystal grains and the control of the form of the solute N become insufficient. For this reason, the rolling reduction of the final stand was set to 10% or more. In addition, it is preferably 30% or less, more preferably 20% or less. Within 0.5 sec after finishing rolling, it is cooled at a cooling rate of 50 ° C./s or more, and is wound up in a temperature range of 600 to 350 ° C.

In the present invention, in order to increase the degree of supercooling in the state where the strain is accumulated, the steel sheet is cooled at a cooling rate of 50 ° C./s or more within 0.5 seconds after the end of rolling. As a result, more ferrite nuclei are generated and ferrite transformation is promoted, and the solute N in γ can be suppressed from diffusing into the ferrite grains, and the amount of solute N present at the ferrite grain boundaries increases, Ng
b / Ng can be increased. If the time until the start of quenching exceeds 0.5 sec or the cooling rate is less than 50 ° C./s, solid solution N precipitates, and the desired amount of solid solution N cannot be secured, and bake hardenability, particularly ΔTS, decreases. Also, the time until the start of rapid cooling is 0.
If the time exceeds 5 seconds or the cooling rate is less than 50 ° C./s, nucleation of ferrite is delayed, and it becomes difficult to efficiently distribute N to grain boundaries. In addition, cooling is delayed and grain growth occurs, making it difficult to reduce the average grain size of ferrite to 8 μm or less.

When the winding temperature exceeds 600 ° C., precipitation of solid solution N occurs after winding, so that the amount of solid solution N required for baking hardening cannot be more than a predetermined value. On the other hand, the winding temperature is 35
If the temperature is lower than 0 ° C., operational problems such as deterioration of the shape of the sheet and deterioration of the sheet passing property occur. For this reason, the winding temperature was limited to the range of 600 to 350 ° C. The above-described hot-rolled steel sheet of the present invention is suitable as an original sheet for various platings, and may be used as various plated steel sheets by forming various plating layers on the surface. Examples of the type of plating include electrogalvanizing, galvanizing, electrotin plating, electrochromic plating, and electronickel plating, all of which are suitable as the plating layer formed on the surface of the hot-rolled steel sheet of the present invention.

[0043]

EXAMPLE Molten steel having the composition shown in Table 1 was melted in a converter and made into a slab by a continuous casting method. Heat these slabs: 1080
After heating to a temperature of 0 ° C. to obtain an appropriate thickness by rough rolling, finish rolling was performed under the conditions shown in Table 2, quenched after rolling, and wound into a coil at a winding temperature shown in Table 2. About these hot rolled steel sheets,
Microstructure test, tensile test, bake hardening test, impact resistance test,
A room temperature aging test and a fatigue test were performed. (I) Microstructure test The microstructure of the cross section of these hot-rolled steel sheets perpendicular to the rolling direction was observed with an optical microscope to identify the microstructure of the hot-rolled steel sheets. Further, the average crystal grain size of the ferrite was measured by a quadrature method, which is a grain size measurement method defined by ASTM, using an optical microscope photograph.

Further, the amount of N in the hot-rolled steel sheet and the amount of N existing as AlN were measured by chemical analysis. The value of {(amount of N in hot-rolled steel sheet) − (amount of N existing as AlN)} was used as the amount of solute N in the hot-rolled steel sheet. Ngb and Ng were measured using a three-dimensional atom probe, and the average value of three or more ferrite grains and grain interfaces was used. (Ii) Tensile test From these hot-rolled steel sheets, JIS 13B tensile test specimens were collected,
A tensile test was performed at a strain rate of 10 ⁻³ / s, and the yield point YS, tensile strength TS, and elongation El were measured. (Iii) Bake hardening test From these hot-rolled steel sheets, JIS 13B tensile test specimens were collected,
After applying a 5% tensile prestrain, unload once and set at 170 ° C x 20
A heat treatment corresponding to the coating baking treatment of min was performed, and then the tensile test was performed again to measure the tensile strength TS _BH . The difference between the tensile strength TS _BH after the heat treatment corresponding to the paint baking treatment and the tensile strength TS as hot rolled, ΔTS = TS _BH −TS, is obtained, and ΔTS is determined.
Is the amount of increase in tensile strength due to processing-paint baking. (Iv) Impact resistance test A high-speed tensile test specimen was collected from these hot-rolled steel sheets and 5%
After applying a tensile prestrain of
Heat treatment equivalent to the paint baking process, followed by a strain rate of 2
A high strain rate tensile test of × 10 ³ / s was performed and the tensile strength TS
_HS and stress-strain curves were measured. Using the measured stress-strain curve, an integrated value up to a strain amount of 30% was obtained and defined as an absorbed energy E. The dimensions and test method for the high strain rate tensile test are described in the Journal of the Society of Material Science.
Japan, vol.47, No.10, p1058 (1998). (V) Fatigue test Fatigue test specimens were taken from these hot-rolled steel sheets, subjected to 5% tensile prestrain, unloaded, subjected to a heat treatment equivalent to a paint baking treatment at 170 ° C. for 20 minutes, and then subjected to JIS. A tensile fatigue test was performed according to the rules of Z2273, and the fatigue limit (1 × 10 ⁷ times) σ _wBH was determined from the SN curve. The same fatigue test was performed on the steel sheet as hot rolled to determine the fatigue limit σ _w . Difference from fatigue limit of hot-rolled steel sheet, Δσ _w = σ
the _{wBH -σ w} was set to improve the amount of fatigue resistance. (Vi) Normal-temperature aging test Samples were taken from these hot-rolled steel sheets, subjected to aging treatment at 50 ° C x 400 hr, and then JIS13B tensile test specimens were taken, a tensile test was performed, and the elongation El _A was measured. . The difference between the elongation El of steel sheets as-rolled, were evaluated for anti-aging property at ΔEl = El-El _A.

Table 3 shows the results.

[0046]

[Table 1]

[0047]

[Table 2]

[0048]

[Table 3]

[0049]

[Table 4]

[0050]

[Table 5]

From Table 3, it can be seen that all of the examples of the present invention show a difference between the tensile strength after the processing-paint baking treatment and the tensile strength of the steel sheet as hot rolled, that is, ΔTS at 5% pre-strain of 40 MPa or more, and high baking. It has hardenability, the difference between the fatigue limit of the steel sheet after baking treatment and the fatigue limit of the steel sheet as hot rolled, Δσ _w is 110 MPa or more, showing significantly improved fatigue resistance, and at high strain rate Absorbed energy E absorbed upon deformation is also 160 MJ / m ³ or more, and has excellent impact resistance. Further, the decrease in elongation due to normal-temperature aging is not remarkable at 0.6 to 1.2%, and the decrease in normal-temperature aging resistance is small. On the other hand, Comparative Examples outside the range of the present invention have little improvement in bake hardenability and fatigue resistance when ΔTS is 9 MPa or less and Δσ _w is 65 MPa or less. Steel plate No.1-6
Since the amount of solute N is out of the range of the present invention and too large, the aging resistance at room temperature is deteriorated.

[0052]

According to the present invention, there is provided a hot-rolled steel sheet which is excellent in bake hardenability, fatigue resistance and impact resistance and has little deterioration in ordinary temperature aging resistance, which is suitable for use as an inner plate part of an automobile. It has a remarkable industrial effect that it can be manufactured stably.

[Brief description of the drawings]

FIG. 1 is a graph showing the difference between the tensile strength of a steel sheet after processing and paint baking treatment, the tensile strength of a steel sheet as hot rolled, and the effect of the amount of solute N on ΔTS.

FIG. 2 is a graph showing the difference between the tensile strength of a steel sheet after processing and paint baking treatment, the tensile strength of a steel sheet as hot rolled, and the effect of ferrite crystal grain size on ΔTS.

FIG. 3
It is a graph which shows the influence of the ferrite crystal grain size on the absorbed energy E of the steel plate after processing-paint baking processing in a high strain rate tensile test.

FIG. 4
5 is a graph showing the relationship between the amount of tensile prestrain and ΔTS.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Osamu Furukun 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba F-term (reference) 4K037 EA01 EA05 EA06 EA15 EA16 EA18 EA23 EA27 EA28 HA01 JA01 JA07

Claims (8)

[Claims] 1. Mass%: C: 0.01 to 0.12%, Si: 2.0% or less, Mn: 0.01 to 3.0%, P: 0.2% or less, Al: 0.001 to 0.1%, N: 0.003 to 0.02% The remainder has a composition of Fe and unavoidable impurities, a structure having a main phase of ferrite having an average crystal grain size of 8 μm or less, and 0.003 to 0.01% by weight of solid solution N.
And the ratio of the average solid solution N concentration Ngb present in the range of ± 5 nm from the ferrite crystal grain interface to the average solid solution N concentration present in the ferrite crystal grain, Ngb / Ng being 100 to 1000
A high-strength hot-rolled steel sheet excellent in bake hardenability, fatigue resistance, impact resistance and aging resistance at ordinary temperature, characterized by being in the range of 0. 2. In addition to the above composition, Ti: 0.00% by mass.
1 to 0.1% and Nb: one or two of 0.001 to 0.1% and / or Ni: 0.1 to 1.5%, Cr: 0.1 to 1.5%
%, Mo: 0.1 to 1.5%, which is excellent in bake hardenability, fatigue resistance, impact resistance and room temperature aging resistance according to claim 1. High tensile hot rolled steel sheet. 3. The ferrite has an average crystal grain size of 6 μm.
And the amount of solute N is 0.005 to 0.01% by mass.
%. 3. A high-tensile hot-rolled steel sheet according to claim 1 or 2, which is excellent in bake hardenability, fatigue resistance, impact resistance and aging resistance at room temperature. 4. The structure according to claim 1, wherein the structure contains at least one of pearlite, bainite, martensite, and retained austenite as a second phase. A high-strength hot-rolled steel sheet excellent in bake hardenability, fatigue resistance, impact resistance and aging resistance at room temperature as described in Crab. 5. A baking hardenability, a fatigue resistance, an impact resistance and a resistance, wherein a plating layer is formed on the surface of the high-tensile hot-rolled steel sheet according to any one of claims 1 to 4. High tension hot rolled steel sheet with excellent aging at room temperature. 6. In mass%, C: 0.01 to 0.12%, Si: 2.0% or less, Mn: 0.01 to 3.0%, P: 0.2% or less, Al: 0.001 to 0.1%, N: 0.003 to 0.02% The steel material having the composition is heated to a temperature range of 1000 to 1300 ° C., and after rough rolling, the final stand rolling reduction is 10% or more, and the final finish rolling temperature FDT is (Ar ₃ + 100 ° C.) to (Ar ₃ + 10 ° C.).
Finish rolling is performed within the temperature range described above, and is cooled at a cooling rate of 50 ° C./s or more within 0.5 sec after the completion of rolling.
A method for producing a high-tensile hot-rolled steel sheet having excellent bake hardenability, fatigue resistance, impact resistance, and aging resistance at room temperature, characterized in that the steel sheet is wound in a temperature range of up to 350 ° C. 7. The composition according to claim 1, further comprising:
i: 0.001 to 0.1% and Nb: one or two of 0.001 to 0.1% and / or Ni: 0.1 to 1.5%, Cr:
0.1 to 1.5%, Mo: one or two of 0.1 to 1.5%
The method for producing a high-tensile hot-rolled steel sheet excellent in bake hardenability, fatigue resistance, impact resistance and normal-temperature aging resistance according to claim 6, comprising at least one kind. 8. The baking hardenability and fatigue resistance according to claim 6, wherein heating is performed in a temperature range of 1070 to 1180 ° C. instead of heating in a temperature range of 1000 to 1300 ° C. For producing high-strength hot-rolled steel sheets with excellent impact resistance and aging resistance at room temperature.