JP2003171734A - High strength hot rolled steel sheet having excellent stretch-flanging property and fatigue property, and production method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength hot rolled steel sheet which has excellent stretch-flanging properties and fatigue properties. <P>SOLUTION: The hot rolled steel sheet has a composition containing, by mass, 0.03 to 0.10% C, 0.05 to 1.5% Si, 1.0 to 2.0% Mn, ≤0.05% P, ≤0.01% S, 0.0005 to 0.01% N, and ≤0.004% Al, and further containing either or both of Ti and Nb so as to satisfy -0.05≤äTi+(48/93)×Nb-(48/12)×C-(48/14)⊗N-(48/32)×S}≤0.2, and further containing Ce and O (oxygen) in 0.0003 to 0.012% Ce, and 0.0003 to 0.01% O, also so as to satisfy Ce/O≥0.25, and, if required, containing 0.2 to 2.0% Cu, and 0.1 to 1.0% Ni, and the balance Fe with inevitable impurities. The steel sheet has a structure in which the area ratio of a bainitic-ferritic phase is ≥80%. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength hot-rolled steel sheet excellent in stretch-flangeability and fatigue characteristics and a method for producing the same.

[0002]

2. Description of the Related Art Among automobile parts, high-strength hot-rolled steel sheets are widely used especially for frames and arms called suspension systems. These parts are required to have high fatigue characteristics from the viewpoint of durability against vibration during running. Several steel sheets have been proposed to meet these requirements. For example, Japanese Patent Application Laid-Open No. 11-199973 proposes a steel sheet in which fine Cu precipitates and / or solid solution are dispersed in a steel sheet having a composite structure of a ferrite phase and a martensite phase.

It is known that such a steel sheet generally called a DP steel sheet is excellent in the balance between strength and ductility and fatigue characteristics, but is inferior in stretch flangeability evaluated in a hole expanding test. One of the reasons for this is that the DP steel sheet is a composite of a soft ferrite phase and a hard martensite phase, so that the boundary part between both phases cannot follow the deformation during the hole expanding process and is likely to be the starting point of fracture. Is believed to be.
On the other hand, a hot rolled steel sheet having excellent stretch flangeability and fatigue properties has been proposed. JP 2001-2003
JP-A-31-31 is one example, and the gist is that the structure of the steel sheet is mainly composed of a bainite phase, the difference in hardness from other phases constituting the steel sheet is made small, and further formation of coarse carbides is avoided. Further, in Japanese Patent Laid-Open No. 5-209253, Al
A non-aging steel for cold rolling using an oxide of Ce instead of deoxidation is disclosed, but nothing is described about a hot rolled steel sheet having excellent stretch flange formability and fatigue properties.

[0004]

The hot-rolled steel sheet having a bainite phase structure as the main constituent of the steel sheet and suppressing the formation of coarse carbides certainly exhibits excellent stretch-flange formability, but it is superior to the DP steel sheet containing Cu. Fatigue properties are not always excellent. Further, the cracking cannot be suppressed when a more severe hole expanding process is performed only by suppressing the coarse carbide. According to the research conducted by the present inventors, it has been found that these causes are due to the presence of inclusions mainly composed of oxides in the steel sheet. When subjected to repeated deformation, internal defects are generated around the coarse inclusions that exist in or near the surface layer and propagate as cracks, which deteriorates fatigue properties and also causes coarse inclusions to crack during hole expansion processing. It is considered that this is because it tends to be the starting point of occurrence.
Therefore, it is desirable that the inclusions in the steel be made as fine as possible.

Generally, deoxidation of steel is carried out by using ferrosilicon or aluminum, but aluminum, which is excellent in efficiency and cost, is more versatile. Alumina-based oxides formed as a result of aluminum deoxidation tend to aggregate and remain in the steel as coarse inclusions. It seems that this influences the fatigue characteristics and hole expanding workability (stretch flangeability) as described above, but from the viewpoint of alumina inclusion control, hot rolled steel sheets with excellent stretch flangeability and fatigue characteristics are selected. I can't find the suggested example. An object of the present invention is to provide a high-strength hot-rolled steel sheet excellent in stretch-flangeability and fatigue characteristics and a method for manufacturing the same.

[0006]

[Means for Solving the Problems] In view of such a situation,
The present inventors have conducted deoxidation not using aluminum, deoxidation using an element whose oxide does not aggregate and do not coarsen, T
Suppresses the decrease in yield due to the oxidation of i and Nb, and
Elucidation of the upper limit of the oxygen concentration that can efficiently secure the Ti amount and the Nb amount necessary for fixing carbon as carbides,
The present invention was completed by further advancing intensive research centering on the four points of elucidating additive elements that do not deteriorate fatigue properties, and further examining chemical components and manufacturing methods.

The gist is as follows. That is, in (1) mass%, C: 0.03 to 0.10%, Si:
0.05-1.5%, Mn: 1.0-2.0%, P:
0.05% or less, S: 0.01% or less, N: 0.000
5 to 0.01%, Al: 0.004% or less, and one or both of Ti and Nb is -0.05.
≦ {Ti + (48/93) × Nb− (48/12) × C
− (48/14) × N− (48/32) × S} ≦ 0.2
To contain Ce and O (oxygen), Ce: 0.0003 to 0.012%, O: 0.0003.
To 0.01% and Ce / O ≧ 0.25, the balance is Fe and inevitable impurities, and the area ratio of bainitic ferrite phase is 80 to 100%. High strength hot rolled steel sheet with excellent stretch flangeability and fatigue properties.

(2) Further, in mass%, Cu: 0.2 to
2.0%, Ni: 0.1-1.0%, The high-strength hot-rolled steel sheet excellent in stretch flangeability and fatigue characteristics as described in (1) above. (3) A method for producing the steel sheet according to (1) or (2) above, which comprises heating a steel material having the chemical composition according to (1) or (2) above to 1150 to 1250 ° C. and performing rough rolling. After that, finish rolling is completed at Ar ₃ points + 50 ° C. to Ar ₃ points + 150 ° C., and further, it is cooled to 300 to 500 ° C. at an average cooling rate of 40 ° C./sec or more and wound at 300 to 500 ° C. It is a method for producing a high-strength hot-rolled steel sheet having excellent stretch-flangeability and fatigue characteristics.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION First, an experiment leading to the completion of the present invention will be described. The present inventors, in mass%,
C: 0.05%, Si: 0.05%, Mn: 1.5%,
P: 0.02%, S: 0.001%, balance F
deoxidizing molten steel which is e using various elements,
A steel ingot was manufactured through measurement of oxygen concentration and addition of Ti and Nb. The obtained steel ingot was hot-rolled to obtain a 4 mm hot rolled steel sheet. The steel sheet was subjected to a hole expanding test and a fatigue test, and chemical composition analysis was performed to investigate the yields of Ti and Nb. As a result, among various elements, the steel sheet deoxidized using Ce has the best hole expandability and fatigue characteristics,
It was also found that the yield of both elements is not problematic if the O concentration before adding Ti and Nb is 0.01% or less.
Based on this, the hot rolling conditions were examined to complete the present invention.

The reasons for limiting the present invention will be described below. First, the reasons for limiting the chemical components will be described. In addition, all the components are shown by mass%. C: 0.03 to 0.10% C is an essential element for ensuring the strength of the steel sheet,
At least 0.03% is required to obtain a high strength steel sheet. However, if it is contained in excess, it becomes unavoidable to generate carbides due to Ti or Nb and to generate a cementite phase which is unfavorable for stretch flangeability, even if the cooling conditions are used, so the content is made 0.10% or less.

Si: 0.05 to 1.5% Si is an element effective for securing the strength without deteriorating the stretch flangeability, and at least 0.05% is necessary, but it is contained excessively. If so, a polygonal ferrite phase unfavorable for stretch flangeability is likely to be formed, so the upper limit is made 1.5%. Mn: 1.0 to 2.0% Mn is an element effective for increasing the strength of the steel sheet together with C and Si, and it is necessary to contain 1.0% or more, but 1.
If the content exceeds 0%, the ductility deteriorates, so the upper limit is made 2.0%.

P: 0.05% or less P is effective as a solid solution strengthening element, but it is necessary to set it to 0.05% or less because there is concern that workability may deteriorate due to segregation. S: 0.01% or less S forms Ti to be contained in order to form inclusions such as MnS to deteriorate stretch flangeability and to make C a carbide.
It has a harmful effect such as binding with and reducing its yield. Therefore, 0.01 should be suppressed as much as possible.
% Or less is acceptable.

N: 0.0005 to 0.01% N combines with Ti contained for the purpose of converting C into a carbide and reduces the yield thereof. Therefore, the amount should be suppressed as much as possible, but 0.01% or less is acceptable. on the other hand,
Since it takes a cost to make it less than 0.0005%,
The lower limit is 0005%. Al: 0.004% or less Al is desirable to be suppressed as much as possible because its oxide easily aggregates and coarsens. However, up to 0.004% can be used as a preliminary deoxidizer.

Ti and Nb serve to improve the workability of the steel sheet by fixing C, S and N as precipitates (so-called scavenging effect). On the other hand, if added more than necessary, they exist in the steel as solid solution Ti and solid solution Nb, raise the recrystallization temperature, and the hot work structure is apt to remain, and the ductility is impaired. Then, the optimum range of the added amount can be appropriately expressed by using the middle side of the following formula described using the chemical equivalent of each element as an index, as shown in the examples. That is, if the value is less than -0.05, the ductility and hole expandability are poor, and if it exceeds 0.2, the ductility is deteriorated. For the above reason, -0.05 ≦ {Ti + (48/93) × Nb−
(48/12) x C- (48/14) x N- (48/3
2) × S} ≦ 0.2 must be satisfied.

Ce: 0.0003 to 0.012% Ce has the effect of increasing the cleanliness of steel by combining with O,
Further, the produced oxide does not adversely affect the stretch-flange formability because it has a property such that it does not easily aggregate and coarsen like alumina-based inclusions. Further, the effect of making inclusions finer is also effective for fatigue characteristics. The effect is that the Ce concentration is 0.0
Since it is expressed at 003% or more, this concentration is the lower limit. On the other hand, even if added over 0.012%, the effect is saturated and the cost of the steel sheet is increased, so 0.012% is made the upper limit. If the concentration is less than 0.25 times the O concentration, the effect cannot be obtained. Therefore, Ce / O ≧ 0.25 is set.

O: 0.0003 to 0.01% Even if the Ce deoxidation of the present invention is used, if the O concentration exceeds 0.01%, good stretch flangeability cannot be obtained, so the upper limit of O is set to 0.
It is set to 01%. Further, if the O concentration exceeds 0.01%, a large amount of both the elements necessary for securing Ti and Nb necessary for fixing C is required, which leads to an increase in cost. This is the reason for setting%. On the other hand, O
If the concentration is less than 0.0003%, a long deoxidizing time is required, which is not preferable in practice. Therefore, this concentration is set as the lower limit.

Cu: 0.2 to 2.0% Cu can be utilized as a solid solution strengthening element or a precipitation strengthening element for increasing the strength of the steel sheet. Further, the addition thereof can further improve the fatigue strength. However, if 0.2% or more is not added, the effect is small and only the cost is increased, so the lower limit is 0.2%. On the other hand, 2.0%
If it is contained in excess of 1.0%, the surface properties of the steel sheet after hot rolling deteriorate, so 2.0% is made the upper limit. Ni: 0.1 to 1.0% Ni has the effect of alleviating the deterioration of the hot rolled surface quality due to the above Cu, and it is desirable to add about half of Cu as a guide. Therefore, the lower limit is 0.1%. On the other hand, 1.
Even if added in excess of 0%, the effect will be saturated and only increase in cost, so 1.0% is made the upper limit. In the present invention, the component other than the above is Fe, but unavoidable impurities mixed from the melting raw material such as scrap are allowed.

Next, heating, rolling, cooling and winding conditions will be described. The heating temperature is required to be 1150 ° C. or higher in order to form a solid solution of TiC, NbC, etc. in the steel. By forming a solid solution of these, the formation of polygonal ferrite is suppressed in the cooling process after rolling, and a structure mainly composed of a bainitic ferrite phase for stretch flangeability can be obtained. On the other hand, if the heating temperature exceeds 1250 ° C, the slab surface will be significantly oxidized,
In particular, wedge-shaped surface defects that are considered to be caused by selective oxidation of grain boundaries remain after descaling and impair the surface quality after rolling, so the upper limit is set to 1250 ° C.

The finish rolling completion temperature is important for controlling the structure of the steel sheet. If the Ar ₃ point is less than + 50 ° C., the crystal grain size of the surface layer becomes coarse, which is not preferable in terms of fatigue characteristics. On the other hand, if the Ar ₃ point exceeds + 150 ° C., a polygonal ferrite phase unfavorable for stretch flangeability tends to be generated, so the upper limit is set to the Ar ₃ point + 150 ° C. Average cooling rate of 40 ° C
/ Sec or more and cooling to 300 to 500 [deg.] C. is for suppressing the formation of polygonal ferrite phase and obtaining a structure mainly composed of bainitic ferrite phase.
If the cooling rate is less than 40 ° C./second, a polygonal ferrite phase is likely to be formed, which is not preferable.

On the other hand, there is no need to set an upper limit on the cooling rate for the purpose of controlling the structure, but an excessively high cooling rate may cause uneven cooling of the steel sheet, and the production of equipment that enables such cooling. Requires a large amount of cost, which may lead to an increase in the price of steel sheets. From such a viewpoint, the upper limit of the cooling rate is preferably 100 ° C./second. Further, when the cooling stop temperature is lower than 300 ° C., a martensite phase which is unfavorable for stretch flangeability is generated, so the lower limit is set to 300 ° C.

The coiling temperature is 300 ° C. in order to suppress the formation of a martensite phase which extremely deteriorates stretch flangeability.
It is necessary to be above. On the other hand, if the temperature exceeds 500 ° C., the formation of polygonal ferrite phase cannot be suppressed, and in the steel containing Cu, Cu may be locally precipitated in the ferrite phase to reduce the fatigue property improving effect. It must be below ℃. By further winding at 500 ° C or lower, TiC and NbC are precipitated in the subsequent cooling process,
It significantly reduces the amount of solute C in the ferrite phase and improves stretch flangeability.

Finally, the structure of the steel sheet will be described. In order to obtain excellent stretch flangeability, it is necessary to have a structure having bainitic ferrite as a main phase, and the desirable area ratio thereof is 80% or more, preferably 90% or more, as shown in Examples. It is more preferably 100%. Further, the balance can contain 20% or less of bainite phase and / or polygonal ferrite phase, and it is desirable to avoid inclusion of martensite phase as much as possible. By virtue of the inclusions being refined in addition to such a steel plate structure, excellent fatigue properties can be obtained at the same time.

[0023]

EXAMPLES Examples of the present invention will be described below together with comparative examples. (Example 1) A steel slab having a chemical composition shown in Table 1 was hot-rolled under the conditions shown in Table 2 to obtain a hot-rolled sheet having a thickness of 3.2 mm. The strength, ductility, hole expandability, sectional structure, and fatigue limit ratio of the steel sheet thus obtained were examined. The results are shown in Table 3 for each combination of steel and conditions. Strength and ductility are
It was determined by a tensile test of a JIS No. 5 test piece taken in parallel with the rolling direction. The hole expandability is that a punched hole having a diameter of 10 mm opened in the center of a steel plate of 150 × 150 mm is spread with a conical punch of 60 °, and the hole diameter D (mm) at the time when a plate thickness through crack occurs is measured, It was evaluated by λ obtained by λ = (D-10) / 10. The fatigue limit ratio is JIS Z 2
2 × 10 ⁶ time strength obtained by the method according to 275,
It was evaluated by a value obtained by dividing (σ _W / σ _B) in the intensity of the sigma _W steel (σ _B). The test piece was the No. 1 test piece prescribed in the same standard, and the one having a parallel portion of 25 mm, the radius of curvature R of 100 mm, and a thickness of 3.0 mm obtained by equally grinding both surfaces of the original plate (hot rolled plate) was used. . As is clear from Table 3, by using the method of the present invention, it is possible to obtain a steel sheet excellent in strength, ductility, hole expandability, and fatigue characteristics.

[0024]

[Table 1]

[0025]

[Table 2]

[0026]

[Table 3]

(Example 2) C: 0.05 in mass%
%, Si: 0.05%, Mn: 1.5%, P: 0.02
%, S: 0.001%, Cu: 1.0%, Ni: 0.5
%, Al: 0.0004%, Ti: 0.3%, Nb:
A steel slab containing 0.03%, different Ce and O contents, and the balance being Fe was produced. Heating these to 125
0 ℃, finish rolling finish temperature 850 ℃, average cooling rate 50 ℃
/ Sec and a winding temperature of 450 ° C, a hot rolled steel sheet of 3.2 mm was prepared. Strength, ductility of the steel sheet thus obtained,
The cross-sectional structure, hole expandability, and fatigue limit ratio were investigated. The evaluation method is the same as in Example 1. As a result, all steels had a strength of 800 MPa or more, a ductility of 20% or more, an area ratio of bainitic / ferrite phase of 80% or more, and a fatigue limit ratio of 0.6.
5 or more, and those properties were hardly affected by the Ce content and the O content, but the hole expandability was strongly affected. The results are shown in FIG. 1 with Ce content and O content as coordinate axes. FIG. 1 is a graph showing the stretch flangeability of a steel sheet with Ce concentration and O concentration as coordinate axes. In this case, all units are ppm by mass, and the inside of the bold line indicates the range of the present invention. As described above, it is apparent that a high-strength hot-rolled steel sheet excellent in stretch flangeability and fatigue characteristics can be obtained within the range of the present invention.

[0028]

According to the method of the present invention, it is possible to obtain a high-strength hot-rolled steel sheet having excellent stretch flangeability and fatigue characteristics.

[Brief description of drawings]

FIG. 1 is a graph showing stretch flangeability of a steel sheet with Ce concentration and O concentration as coordinate axes.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Manabu Takahashi             20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel shares             Company Technology Development Division F-term (reference) 4K037 EA01 EA05 EA13 EA15 EA18                       EA19 EA20 EA22 EA23 EA25                       EA27 EA28 EA31 EA36 FA02                       FA03 FC00 FD04 FE01 HA00

Claims (3)

[Claims] 1. In mass%, C: 0.03 to 0.10%, Si: 0.05 to 1.5%, Mn: 1.0 to 2.0%, P: 0.05% or less , S: 0.01% or less, N: 0.0005 to 0.01%, Al: 0.004% or less, and one or both of Ti and Nb are -0.05 ≦ {Ti + (48 / 93) x Nb- (48 /
12) x C- (48/14) x N- (48/32) x
S and S are contained such that Ce and O (oxygen) are Ce: 0.0003 to 0.012% and O: 0.0003.
To 0.01% and Ce / O ≧ 0.25, the balance is Fe and inevitable impurities, and the area ratio of bainitic ferrite phase is 80.
A high-strength hot-rolled steel sheet excellent in stretch-flangeability and fatigue characteristics, which is characterized by being 100% to 100%. 2. The stretch flangeability according to claim 1, further comprising Cu: 0.2 to 2.0% and Ni: 0.1 to 1.0% in mass%. And high strength hot rolled steel sheet with excellent fatigue properties. 3. A method for producing a steel sheet according to claim 1 or 2, wherein the steel material having the chemical composition according to claim 1 or 2 is heated to 1150 to 1250 ° C. and rough-rolled, and then Ar Finish rolling is completed at ₃ points + 50 ° C to Ar ₃ points + 150 ° C, and further 3 at an average cooling rate of 40 ° C / sec or more.
A method for producing a high-strength hot-rolled steel sheet excellent in stretch-flangeability and fatigue characteristics, which comprises cooling to 00-500 ° C and winding at 300-500 ° C.