JP4646881B2 - Hot-rolled steel sheet with excellent stretch flangeability - Google Patents

Description

  The present invention relates to a hot-rolled steel sheet having a strength class of 980 MPa or more, which is used as a material for parts that require strength and workability, particularly stretch flangeability, in automobiles, various industrial machines, and structures.

In a hot-rolled steel sheet having a strength class of 980 MPa or more, it is effective to increase the uniformity of the structure in order to achieve both strength and good stretch flangeability, and a structure mainly composed of bainite is effective as the structure. It has been known.
As a conventional technique for improving stretch flangeability, Patent Document 1 describes strength-stretch flangeability by increasing the uniformity of the structure in the thickness direction by adding Nb and Ti and optimizing the process, and by making the structure mainly bainite. In Patent Document 2, a structure mainly composed of fine bainite having an average particle diameter of 3.0 μm is added to either Nb or Ti, thereby providing strength-stretch flangeability. It is described that they are compatible. However, in order to ensure a strength of 980 MPa or more, it is necessary to add expensive Nb.

  On the other hand, as a technique for obtaining a bainite structure without adding Nb or the like, which is an expensive hardenability improving element, Patent Document 3 describes that cooling is performed within 2 seconds after hot rolling of C-Si-Mn steel. It is described that both strength and stretch flangeability can be achieved by starting, cooling at a cooling rate exceeding 150 ° C./s, and stopping cooling at 450 to 650 ° C. However, special equipment is required to increase the cooling rate to over 150 ° C./s, and it is difficult to control the stop temperature at such a very high cooling rate, and the stop temperature may vary. Concerned.

JP 2003-119549 A JP 2000-109951 A JP 2003-112204 A

  The present invention has been made in view of the above problems of the prior art, and aims to ensure good stretch flangeability with a hot-rolled steel sheet having a strength class of 980 MPa or more by a relatively simple component system and process. To do.

As a result of various studies conducted by the present inventors to obtain a bainite structure, it was found that the addition balance of Ti and N needs to be controlled in order to exhibit the effect of improving the hardenability by adding B. It was.
Subsequently, as a result of examining the influence of the bainite substructure on the stretch flangeability, in the situation where full bainite is obtained using B, the average particle size of the block that is the bainite substructure is the stretch flangeability. It was strongly influenced, and it was found that the stretch flangeability was improved as the average particle size of the block was finer. The coarsening of the average particle size of the block is contributed by elements of group 4A to 6A such as Nb and Mo which have been added to ensure hardenability in the past. By reducing these elements as much as possible, The stretch flangeability was improved, and good stretch flangeability could be secured with a hot-rolled steel sheet of 980 MPa or more.

  The hot-rolled steel sheet excellent in stretch flangeability according to the present invention is in mass%, C: 0.03% or more, 0.20% or less, Si: more than 1.0%, 2.0% or less, Mn: 1 0.0% or more and 3.0% or less, P: 0.05% or less, S: 0.05% or less, V: 0.010% or less, Zr: 0.010% or less, Nb: 0 0.005% or less, Mo: 0.010% or less, Ta: 0.010% or less, Hf: 0.010% or less, W: 0.010% or less, and the total of these seven elements is 0.02% or less. Yes, N: 0.010% or less, and when Ti mass% is [Ti] and N mass% is [N], [Ti] -3.4 [N] is -0.003%. Or more, 0.03% or less, B: 0.0003% or more, 0.0050% or less, the balance Fe and inevitable impurities, the main structure is The fraction exceeds 90% bainite, wherein the average particle size of bainite blocks is 3μm or less.

  The high-strength hot-rolled steel sheet, if necessary, is further Al: 0.001% or more, 1.0% or less, Ca: 0.0001% or more, 0.1% or less, Mg: 0.0001% or more, 0.1% or less, Ce: 0.0001% or more, including 0.1% or less, or 2 or more types, and / or Cr: 0.01% or more, 5.0% or less, Cu: 0.0. One or more of 01% or more and 5.0% or less, Ni: 0.01% or more and 5.0% or less can be included.

According to the present invention, good stretch flangeability can be secured with a hot-rolled steel sheet of 980 MPa or more by a relatively simple component system and process.
The hot-rolled steel sheet according to the present invention can be suitably used as a material for parts that require strength and workability, particularly stretch flangeability, in automobiles, various industrial machines, and structures.

First, the composition of the hot-rolled steel sheet according to the present invention and the reason for limiting the structure will be described.
C: 0.03% or more and 0.20% or less C is an element effective for securing strength, suppresses ferrite formation, and promotes bainite formation. However, if it exceeds 0.20%, martensite is formed, the strength becomes too high, and the stretch flangeability deteriorates. On the other hand, if it is less than 0.03%, ferrite is formed and the bainite fraction decreases, which is necessary. Strength cannot be obtained. Preferably, they are 0.05% or more and 0.15% or less, More preferably, they are 0.07% or more and 0.12% or less.
Si: more than 1.0%, 2.0% or less Si is an element that contributes to strengthening while suppressing a decrease in stretch flangeability by solid solution strengthening. However, if it exceeds 2.0%, the strength becomes too high and the stretch flangeability deteriorates. On the other hand, if it is 1.0% or less, sufficient strength and stretch flangeability cannot be obtained. Preferably they are 1.1% or more and 1.9% or less, More preferably, they are 1.1% or more and 1.8% or less.

Mn: 1.0% or more, 3.0% or less Mn is an element effective in suppressing the formation of ferrite and promoting the formation of bainite to improve strength and stretch flangeability. However, if it exceeds 3.0%, the hardenability becomes too high and martensite is formed, and the stretch flangeability deteriorates. On the other hand, if it is less than 1.0%, ferrite is formed and the strength cannot be obtained. The stretch flangeability is also deteriorated. Preferably they are 1.2% or more and 2.7% or less, More preferably, they are 1.5% or more and 2.4% or less.
-P: 0.05% or less P segregates at the grain boundary, lowers the grain boundary strength and promotes grain boundary fracture, so the stretch flangeability deteriorates. Therefore, it is limited to 0.05% or less, preferably 0.03% or less, and more preferably 0.02% or less.
S: 0.05% or less,
S forms a sulfide such as MnS, and this sulfide serves as a starting point of fracture during the hole expansion test, so that the stretch flangeability deteriorates. Therefore, it is limited to 0.05% or less, preferably 0.03% or less, and more preferably 0.02% or less.

V: 0.010% or less Zr: 0.010% or less Nb: 0.005% or less Mo: 0.010% or less Ta: 0.010% or less Hf: 0.010% or less W : 0.010% or less, and the total of these 7 elements is 0.02% or less By adding these elements, the formation of ferrite is strongly suppressed, and the action of promoting bainite formation is promoted. However, it suppresses nucleation of ferrite and at the same time suppresses nucleation of bainite, and the average particle size of the block becomes coarse. Therefore, in order to improve stretch flangeability, it is necessary to reduce the addition amount. Moreover, when these elements are added, fine MA (martensite and retained austenite) is formed between the laths of bainite, and this becomes a starting point of breakage during the hole expansion test, thereby reducing stretch flangeability. Also from this viewpoint, it is desirable to reduce the addition amount. In order to obtain good stretch flangeability, it is better not to add. Even when added, V, Zr, Mo, Ta, Hf, and W are individually 0.010% or less, and Nb is 0.005% or less. Therefore, it is necessary to suppress it to 0.02% or less. Preferably, V, Zr, Mo, Ta, Hf, and W are individually controlled to less than 0.005% and Nb to less than 0.003%, and the total is 0.015% or less, and more preferably 0.010%. Keep it below.

B: 0.0003% or more and 0.0050% or less B is an element that strongly suppresses ferrite transformation, and is an extremely effective element for improving the stretch flangeability by forming a bainite-based structure. However, if it exceeds 0.0050%, the effect of addition is saturated, while if it is less than 0.0003%, the hardenability improving effect (ferrite formation suppressing effect) cannot be obtained. Preferably they are 0.0005% or more and 0.0040% or less, More preferably, they are 0.0005% or more and 0.003% or less.
N: 0.010% or less N is an unavoidable element. Since stretch flangeability deteriorates due to strain aging, it is desirable to reduce it. Moreover, when solid solution N exists, since the hardenability improvement effect (ferrite formation inhibitory effect) by B addition is inhibited, it reduces to 0.010% or less. Preferably it is 0.008% or less, More preferably, it is 0.006% or less.

-[Ti] -3.4 [N] is -0.003% or more and 0.03% or less Ti is unavoidably present as TiN, suppressing strain aging, or quenching by adding B It is an element necessary for developing the effect of improving the sexiness. On the other hand, if added excessively, solute Ti contributes to the formation of MA and deteriorates the stretch flangeability. Therefore, it is necessary to adjust the addition amount according to the N amount. When [Ti] -3.4 [N] exceeds 0.03%, MA is formed by remaining solid solution Ti, nucleation of bainite is suppressed, and the average particle size of the block becomes coarse, Stretch flangeability deteriorates. On the other hand, if it is less than −0.003%, N that does not become TiN remains, and the N bonds with B, thereby inhibiting the ferrite formation suppressing effect of B. A preferred range is from -0.001% to 0.02%, and a more preferred range is from 0.0% to 0.01%.

-Al: 0.001% or more, 1.0% or less-Ca: 0.0001% or more, 0.1% or less-Mg: 0.0001% or more, 0.1% or less-Ce: 0.0001% or more 0.1% or less Since these elements have the effect of improving stretch flangeability by refining oxide inclusions, one or more elements are added as necessary. However, if the upper limit is exceeded, the effect is saturated, and if it is less than the lower limit, the effect cannot be obtained.

・ Cr: 0.01% or more, 5.0% or less ・ Cu: 0.01% or more, 5.0% or less ・ Ni: 0.01% or more, 5.0% or less These elements suppress ferrite formation And in order to contribute to bainite structure formation, 1 type (s) or 2 or more types are added as needed. However, if the upper limit is exceeded, the effect is saturated, and if it is less than the lower limit, the effect cannot be obtained.

-Bainite fraction: more than 90% Since bainite has a homogeneous structure, when it becomes a bainite-based structure, the starting point of cracks is reduced and stretch flangeability is improved. If the bainite fraction (area fraction) is 90% or less, the structure becomes non-uniform and the stretch flangeability deteriorates. Preferably it is 92% or more, More preferably, it is 95% or more. Ideally, it should be a 100% bainite structure.
It is desirable that the structure other than the main phase bainite (mainly martensite, residual austenite, or ferrite, and a small amount of pearlite) is small. Since hard martensite and retained austenite become crack generation sites during the hole expansion test and deteriorate stretch flangeability, the area fraction of martensite + retained austenite is preferably 3% or less, more preferably 2% or less, 1% or less is preferable. Also for ferrite or pearlite, strain concentrates at the interface with the hard phase, and the interface becomes the starting point of cracking. Therefore, the area fraction is preferably 5% or less, more preferably 3% or less.

-Average particle diameter of block: 3 μm or less A block refers to a region where crystal orientations are aligned in a substructure in bainite. When the block is miniaturized, the stress concentrated on the block interface is reduced, the stress concentrated on the block interface is reduced, breakage from the block interface is suppressed, and stretch flangeability is improved. When the average particle size of the block is larger than 3 μm, good stretch flangeability cannot be obtained. The preferable range of the average particle size of the block is 2 μm or less, and the more preferable range is 1.5 μm or less. The finer is preferable, but the lower limit of the block size obtained by a practical process is about 0.2 μm.

Next, the manufacturing method of the hot rolled steel sheet according to the present invention will be described.
As shown in FIG. 1, the preferred manufacturing method is hot rolling including finish rolling, rapid cooling after hot rolling, rapid cooling stop, and winding after heating the steel material. Hereinafter, each step will be described.
-Heating before hot rolling Although heating is not limited, it is performed at 1000 ° C or higher and 1250 ° C or lower for 30 minutes or longer and 5 hours or shorter. An austenite single phase is obtained by this heating.
-Hot rolling Hot rolling is performed so that the finishing temperature is in the range of 700 ° C or higher and 800 ° C or lower. By performing hot finish rolling in the non-recrystallized region of the austenite single phase, the nucleation sites of bainite can be increased and the formed blocks can be refined. When the finishing temperature exceeds 800 ° C., the nucleation sites of bainite are insufficient, and the block cannot be sufficiently refined. When the finishing temperature is less than 700 ° C., ferrite transformation occurs before finish rolling, and the structure becomes non-uniform. Stretch flangeability cannot be obtained.

-Rapid cooling after hot rolling Rapid cooling after hot rolling is performed at an average cooling rate of 20 ° C / s or higher until the stop temperature (temperature range where bainite transformation occurs). The formation of ferrite is suppressed by rapidly cooling the temperature range where ferrite transformation occurs. Although it is desirable that this cooling rate is high, control becomes difficult if it is too fast, so it is preferably less than 150 ° C./s, more preferably less than 120 ° C./s.
-Rapid cooling stop and winding The rapid cooling stop temperature should be 300 ° C or higher and 500 ° C or lower. This temperature range is a temperature range where bainite transformation occurs, and a bainite-based structure can be obtained by maintaining in this temperature range. When the stop temperature exceeds 500 ° C., pearlite transformation occurs, the bainite fraction decreases, the strength decreases, and the stretch flangeability deteriorates. When the stop temperature is less than 300 ° C., martensitic transformation occurs, the bainite fraction decreases, and stretch flangeability deteriorates.

  The ingots having the components shown in Tables 1 and 2 are vacuum-melted and made into a 25 mm thick plate by hot rolling. A 25 mm × 200 mm × 120 mm slab is cut out from the slab and heated at 1150 ° C. for 30 minutes. Finish rolling was performed under the conditions shown. After finish rolling, the steel was cooled to a stop temperature of 450 ° C. at an average cooling rate of 50 ° C./s. After cooling to 450 ° C., in order to simulate winding, holding was performed at 450 ° C. for 30 minutes, followed by furnace cooling.

A sample was taken from the obtained hot-rolled steel sheet, and a structure observation, a tensile test, and a hole expansion test were performed as follows.
・ Structural observation 1 (Bainite fraction)
The structure of the TD surface at the center of the steel plate was observed. After polishing the sample to a mirror surface and corroding with 3% nital, observe 5 fields (approximately 30,000 μm ² / field) at 400 times with an optical microscope and point counting method (100 points with uniform mesh for each field) ) To obtain the bainite fraction.

・ Structure observation 2 (average particle size of block)
A block is a region showing the same orientation, and cannot be discriminated by an optical microscope or SEM. In addition, the particle size of bainite observed with an optical microscope or SEM is unknown whether lath, block, or packet is observed, and cannot have a one-to-one correspondence with the particle size of the block. Therefore, it is necessary to measure the crystal orientation such as EBSP (Electron Back Scattering Pattern) for the block measurement. In this example, the block size was measured by setting the large-angle grain boundary where the orientation difference between adjacent ferrites was 15 ° or more as the block boundary, and the particle size of the region surrounded by the block boundary as the block particle size. The apparatus and observation conditions are as follows.
(A) Observation apparatus: Scanning electron microscope (Philips XL30S-FEG)
(B) EBSP system; using 0IM system (ver. 4.0) manufactured by Tecsem Laboratories (c) step interval; 0.25 μm
(D) Mapping the point where the azimuth difference between adjacent ferrites is 15 ° or more as a block boundary
(E) The area of the region surrounded by the block boundary was measured using Image-Pro manufactured by Micromedia, and the equivalent circle diameter (= 2 (A / π) ^1/2 , A: the area of the grain from the area of each grain ) Was averaged.

・ Structure observation 3 (martensite and / or residual austenite fraction)
After corroding with a repeller reagent, observe five fields of view (approximately 5,000 μm ² / field of view) at 1000 times with an optical microscope, analyze the image of white areas as martensite and / or retained austenite, and determine the fraction of the tissue Asked.
In addition, the martensite and / or residual austenite which exist between the laths of a bainite are fine structures, and the bainite fraction obtained by the structure | tissue observation 1 contains this martensite and / or residual austenite. For this reason, when the bainite fraction obtained in the structure observation 1 and the martensite and / or retained austenite fraction obtained in the structure observation 3 are added, there are cases where it exceeds 100%.

-Tensile test The tensile test processed the sample into the No. 5 test piece of JISZ2201, and measured the tensile strength according to the tensile test method of JISZ2241.
-Hole expansion test Stretch flangeability was evaluated by a hole expansion test. The hole expansion test was performed in accordance with Japan Iron and Steel Federation standard JFST1001, and the hole expansion ratio was measured.

The measurement results are shown in Tables 3 and 4. In Tables 3 and 4, the bainite fraction includes fine MA (martensite and / or retained austenite) present between bainite laths.
In Tables 3 and 4, the tensile strength was evaluated as 980 MPa or more as good and 1000 MPa or more as good, and the hole expansion rate was evaluated as 80% or more as good and 90% or more as good.

The measurement results in Table 3 and Table 4 will be briefly described below.
No. 1,2,4,5,7,11,12,21-28,30,32 satisfy the respective requirements of composition, bainite fraction and block average particle size described in the claims, and the content of martensite + retained austenite The rate is 3% or less, and good tensile strength and hole expansion rate are exhibited.

On the other hand, no. No. 3 lacks the C content, ferrite is formed, the bainite fraction is low, and the tensile strength is inferior. No. 6 has an excessive C content, martensite is formed, the bainite fraction is lowered, and the hole expansion rate is inferior.
No. No. 8 has an excessive Si content. No. 9 lacks the Si content, and in all cases, the hole expansion rate is inferior.
No. No. 10 has an insufficient Mn content, ferrite is formed, the bainite fraction is low, the tensile strength and the hole expansion rate are inferior, No. 13 has an excessive Mn content, martensite is formed, the bainite fraction is lowered, and the hole expansion rate is inferior.
No. 14 to 20 are excessive amounts of one or more of V, Zr, Nb, Mo, Ta, Hf, and W, the block average particle size becomes coarse, and fine MA present between bainite laths. The (martensite + residual austenite) fraction also increases and the hole expansion rate is inferior.
No. No. 29 has insufficient B content, so the effect of suppressing ferrite formability is insufficient, the bainite fraction is lowered, and the hole expansion rate is inferior.
No. In No. 31, [Ti] -3.4 [N] is too low, so ferrite is formed, the bainite fraction is lowered, and the hole expansion rate is inferior. In [33], [Ti] -3.4 [N] is too high, so that the average particle size of the block becomes coarse, and the fraction of fine MA (martensite + residual austenite) present between bainite laths is also high. The hole expansion rate is inferior.
No. In 34 and 36, since the finish rolling temperature is too high, the block average particle size becomes coarse and the hole expansion rate is inferior.
No. In 35 and 37, since the finish rolling temperature was too low, ferrite was formed, the bainite fraction was lowered, and the hole expansion rate was inferior.

  The hole expansion ratio and the block size were plotted for those satisfying the steel composition and bainite fraction of the present invention, and those in which the block size (average particle size) was coarsened due to excessive addition of Mo, Nb, Ti, etc. Is FIG. As shown in FIG. 2, when the block size is 3 μm or less, a hole expansion rate of 80% or more is obtained.

It is a figure which shows the manufacturing method of the hot rolled sheet steel of this invention. It is a figure which shows the relationship between a hole expansion rate and block size (average particle diameter).

Claims (3)

C: 0.03% or more, 0.20% or less, Si: more than 1.0%, 2.0% or less, Mn: 1.0% or more, 3.0% or less, P: 0.05% or less, S: 0.05% or less, V: 0.010% or less, Zr: 0.010% or less, Nb: 0.005% or less, Mo: 0.010% or less, Ta: 0.010% or less, Hf: 0.010% or less, W: 0.010% or less, and the total of these seven elements is 0.02% or less, N: 0.010% or less, and the mass of Ti % Is [Ti] and the mass% of N is [N], [Ti] -3.4 [N] is -0.003% or more and 0.03% or less, and B: 0.0003 % And 0.0050% or less, the balance is Fe and inevitable impurities, the main structure is bainite, the fraction exceeds 90%, bainite Hot-rolled steel sheet having an average particle size of the block and excellent stretch flangeability, characterized in that it is 3μm or less. Furthermore, Al: 0.001% or more, 1.0% or less, Ca: 0.0001% or more, 0.1% or less, Mg: 0.0001% or more, 0.1% or less, Ce: 0.0001% or more 1 or 2 types or more of 0.1% or less is included, The hot-rolled steel plate excellent in the stretch flangeability described in Claim 1 characterized by the above-mentioned. Cr: 0.01% or more, 5.0% or less, Cu: 0.01% or more, 5.0% or less, Ni: 0.01% or more, including 5.0% or less The hot-rolled steel sheet having excellent stretch flangeability according to claim 1 or 2.

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