JP7392904B1 - High strength steel plate and its manufacturing method - Google Patents

Abstract

The object of the present invention is to provide a high-strength steel plate having a tensile strength of 980 MPa or more and excellent fatigue strength, and a method for manufacturing the same. The main phase is an upper bainite phase with a predetermined composition and an area ratio of 75% or more and less than 98.5% in the surface layer region from the steel plate surface to 1/10 of the plate thickness, and an area ratio of 1.5%. A structure consisting of a martensite phase and/or a retained austenite phase of less than 25% is defined as a second phase, and a remaining structure phase other than the upper bainite phase, the martensite phase and/or the retained austenite phase has an area ratio of 2. 0% or less, and the average crystal grain size of all phases in the surface layer region from the steel plate surface to the 1/10th position of the plate thickness is 6.0 μm or less, and the surface layer area from the steel plate surface to the 1/10th position of the plate thickness. A high-strength steel sheet in which the dislocation density of all the phases is 8.0×10 14 /m 2 or more.

Description

The present invention relates to a high-strength steel plate (particularly a hot-rolled steel plate) that is suitable as an automobile member and has particularly improved strength and fatigue resistance, and a method for producing the same.

In recent years, from the perspective of global environmental conservation, reductions in CO ₂ emissions have been required in a global framework. In particular, there is a strong desire to improve the fuel efficiency of automobiles, and there is a trend towards reducing the weight of automobile bodies. Increasing the strength of steel plates used as materials for automobile parts and making them thinner is an effective means for reducing the weight of automobile bodies without reducing their strength. In particular, steel plates with a tensile strength of 980 MPa or more are expected to be a material that can dramatically improve automobile fuel efficiency through thinning.

By the way, in order to ensure durability, which decreases as automobile parts become thinner, it is necessary to improve the fatigue resistance of steel plates. Automotive parts, especially suspension parts such as suspension parts, are subjected to repeated loads from tires. Therefore, if the fatigue strength is low, as the mileage increases, there is a possibility that the durability of the part will fall below what was assumed in the design. However, in general, increasing the strength of a steel plate does not necessarily increase its fatigue strength.

Up to now, various studies have been made in order to improve the fatigue strength while increasing the tensile strength of steel plates (Patent Documents 1 to 3).

Patent Document 1 discloses a high-strength hot-rolled steel sheet with excellent formability and fatigue resistance by controlling the manufacturing conditions of hot rolling, using ferrite as the main phase, and controlling the shape and dispersion form of inclusions. A technology related to this has been disclosed.

Patent Document 2 discloses that by controlling the manufacturing conditions of hot rolling, using bainite as the main phase, dispersing a fine hard second phase, and controlling the amount of solid solution Ti, the hole expandability is improved. Techniques related to high-strength hot-rolled steel sheets with excellent fatigue resistance have been disclosed.

Patent Document 3 discloses that ferrite is used as the main phase, and in addition to the cementite number density within the ferrite grains, the size of the hard second phase and the number density of inclusions are controlled to improve formability, fracture characteristics, and fatigue resistance. Techniques regarding excellent high-strength hot-rolled steel sheets have been disclosed.

Japanese Patent Application Publication No. 2014-31560 Japanese Patent Application Publication No. 2012-012701 Special Publication No. 2021-505759

However, the conventional techniques described in Patent Documents 1 to 3 have the following problems.

With the technique described in Patent Document 1, a tensile strength of 980 MPa or more cannot be obtained.

In the technique described in Patent Document 2, fatigue strength when actually used as an automobile part has not been sufficiently studied.

Although the technique described in Patent Document 3 is said to be able to obtain a high-strength steel plate having excellent fatigue resistance, the specific fatigue resistance is not specified.

As described above, in the prior art, a technology for producing a high-strength hot-rolled steel sheet that has a tensile strength of 980 MPa or more and excellent fatigue resistance has not been established.

The present invention was developed in view of the above circumstances, and an object of the present invention is to provide a high-strength hot-rolled steel sheet having a tensile strength of 980 MPa or more and excellent fatigue strength, and a method for manufacturing the same. do.

In order to achieve the above object, the inventors conducted extensive studies to improve the fatigue resistance of a hot rolled steel sheet while ensuring a tensile strength of 980 MPa or more. As a result, we found the following. In other words, the microstructure is such that the main phase is upper bainite and the hard second phase contains an appropriate amount of martensite and/or retained austenite, and all the phases in the surface layer region from the steel plate surface to 1/10 of the plate thickness are to increase the dislocation density and control the grain size of all the phases. As a result, a steel plate having high strength of 980 MPa or more and excellent fatigue strength can be obtained after heat treatment equivalent to paint baking.

Note that the upper bainite phase referred to here is an aggregate of lath-like ferrite with a misorientation of less than 15°, and means a structure having Fe-based carbide and/or retained austenite phase between the lath-like ferrites. However, this structure also includes cases in which there is no Fe-based carbide and/or retained austenite between lath-shaped ferrites.

Lath-like ferrite differs from lamellar (layered) ferrite and polygonal ferrite in pearlite because it has a lath-like shape and a relatively high dislocation density inside, so both can be easily analyzed using SEM (scanning electron microscopy) and TEM (transmission electron microscopy). They can be distinguished using an electron microscope).

In addition, when there is retained austenite between the laths, only the lath-like ferrite portion is regarded as upper bainite, and is distinguished from retained austenite.

Furthermore, the martensite and/or retained austenite phase has a brighter contrast in the SEM image than the upper bainite phase, the lower bainite phase, and the polygonal ferrite phase. Therefore, martensite phase and/or retained austenite phase can be distinguished from these structures using SEM.

Although the martensite phase and the retained austenite phase have similar contrast in SEM, they can be distinguished from each other by using an electron backscatter diffraction (EBSD) method.

The dislocation density is calculated by irradiating a steel material with X-rays and analyzing the intensity curve (line profile) of the resulting diffracted X-rays with respect to the measurement angle or energy. Analysis of line profiles can be found in “Materials and Processes” Vol. 17 (2004) No. 3, P396-399, and in the present invention, the dislocation density is calculated from the half-width of (110), (211), and (220). do.

The present invention has been made based on the above findings and further studies, and its gist is as follows.
[1] In mass%, C: 0.03 to 0.15%, Si: 0.1 to 3.0%, Mn: 0.8 to 3.0%, P: 0.001 to 0.1% , S: 0.0001 to 0.03%, Al: 0.001 to 2.0%, N: 0.001 to 0.01%, and B: 0.0002 to 0.010%, and further , Ti: 0.01 to 0.30%, and Nb: 0.001 to 0.10%, with the balance consisting of Fe and unavoidable impurities; In the surface layer region from the steel plate surface to 1/10 of the plate thickness, the main phase is an upper bainite phase with an area ratio of 75% or more and less than 98.5%, and an area ratio of 1.5% or more and less than 25%. A structure consisting of a martensite phase and/or a retained austenite phase is used as a second phase, and a residual structure phase other than the upper bainite phase, the martensite phase and/or the retained austenite phase has an area ratio of 2.0% or less. , the average grain size of all phases in the surface layer region from the steel plate surface to the 1/10th position of the plate thickness is 6.0 μm or less, and all of the above in the surface layer region from the steel plate surface to the 1/10th position of the plate thickness. A high-strength steel plate having a phase dislocation density of 8.0×10 ¹⁴ /m ² or more.
[2] The component composition further includes the following groups a to c: group a: Cu: 0.005 to 2.0%, Ni: 0.005 to 2.0%, Cr: 0. Group b: Sb: 0.005-0.2 %, Sn: 0.001 to 0.05%, and group c: Ca: 0.0005 to 0.01%, Mg: 0.0005 to 0.01%, and REM The high-strength steel plate according to [1], further containing at least one group selected from: 0.0005 to 0.01%.
[3] The method for producing a high-strength steel plate according to [1] or [2], which comprises heating a steel material having the above-mentioned composition to a heating temperature of 1150°C or higher, and rough rolling the heated steel material. The total rolling reduction rate in the temperature range of (RC1-150)°C or more and RC1°C or less: 35% or more, and finish rolling end temperature: (RC2-100)°C or more and (RC2+50)°C or less. Finish rolling the steel plate after finish rolling under the following conditions: time from finish rolling to start of cooling: within 2.0 s, average cooling rate at surface: 20°C/s or more, cooling stop temperature: Trs°C or more (Trs + 180 )℃ or less, and the steel plate after cooling is coiled at a coiling temperature of Trs℃ or higher and (Trs+180)℃ or lower, and at an average cooling rate of 1℃/s or lower (Trs-250). A method for producing a high-strength steel sheet, which comprises cooling to below ℃ and skin pass rolling under the conditions of a rolling reduction of 0.1% or more and 5.0% or less.
Note that RC1, RC2, and Trs are defined by the following equations (1), (2), and (3), respectively.
RC1 (℃) = 900 + 120 x C + 100 x N + 10 x Mn + 500 x Ti + 5000 x B + 10 x Cr + 50 x Mo + 1500 x Nb + 150 x V... (1)
RC2 (℃) = 750 + 120 x C + 100 x N + 10 x Mn + 250 x Ti + 5000 x B + 10 x Cr + 50 x Mo + 750 x Nb + 150 x V... (2)
Trs (℃) = 500-450 x C-35 x Mn-15 x Cr-10 x Ni-20 x Mo... (3)
Here, each element symbol in the above formulas (1), (2), and (3) represents the content (mass %) of each element, and is set to 0 if it is not contained.

According to the present invention, it is possible to provide a high-strength steel plate having a tensile strength of 980 MPa or more and excellent fatigue resistance, and a method for manufacturing the same.

When the high-strength steel sheet of the present invention is applied to automobile suspension parts, structural parts, frame parts, and truck frame parts, safety can be ensured and the weight of the automobile body can be reduced, resulting in significant industrial effects. play.

In the present invention, having excellent fatigue resistance means that the ratio of the fatigue strength of 2×10 ⁶ plane bending to the tensile strength (fatigue limit ratio) is 0.50 or more in a complete double-sided plane bending fatigue test. It means something.

FIG. 1 is a schematic diagram showing the shape of a test piece for a plane bending fatigue test in the present invention.

Embodiments of the present invention will be described below. Note that the following description shows examples of public embodiments of the present invention, and the present invention is not limited to the following embodiments.

The steel plate has the following composition. In the following description, "%", which is the unit of content of an element in a component composition, means "% by mass" unless otherwise specified.

<C: 0.03-0.15%>
C is an element effective in promoting the formation of bainite and improving strength by improving hardenability. If the C content is less than 0.03%, such effects cannot be sufficiently obtained, and a tensile strength of 980 MPa or more cannot be obtained. Therefore, the C content is 0.03% or more, preferably 0.04% or more, and more preferably 0.05% or more. On the other hand, if the C content exceeds 0.15%, martensite and retained austenite increase, making it impossible to obtain sufficient fatigue resistance. Therefore, the C content is 0.15% or less, preferably 0.14% or less, and more preferably 0.13% or less.

<Si: 0.1 to 3.0%>
Si strengthens steel by solid solution and contributes to improving the strength of steel. Therefore, the Si content is 0.1% or more, preferably 0.3% or more, and more preferably 0.5% or more. On the other hand, Si is an element that promotes the formation of ferrite, and when the Si content exceeds 3.0%, ferrite is formed and the fatigue resistance is reduced. Therefore, the Si content is 3.0% or less, preferably 2.5% or less, and more preferably 2.0% or less.

<Mn: 0.8 to 3.0%>
Mn is an element that stabilizes austenite, and is an effective element for suppressing the formation of ferrite and improving strength. If the Mn content is less than 0.8%, these effects cannot be sufficiently obtained, ferrite etc. are generated, and a tensile strength of 980 MPa or more cannot be obtained. Therefore, the Mn content is 0.8% or more, preferably 1.0% or more, and more preferably 1.2% or more. On the other hand, when the Mn content exceeds 3.0%, martensite and retained austenite increase, making it impossible to obtain sufficient fatigue resistance. Therefore, the Mn content is 3.0% or less, preferably 2.8% or less, and more preferably 2.5% or less.

<P: 0.001-0.1%>
Since P deteriorates weldability, it is desirable to reduce its amount as much as possible. In the present invention, a P content of up to 0.1% is allowable. Therefore, the P content is set to 0.1% or less. If the P content is less than 0.001%, production efficiency will decrease, so the lower limit is set to 0.001% or more.

<S: 0.0001-0.03%>
Since S deteriorates weldability, it is preferable to reduce its amount as much as possible. In the present invention, S content up to 0.03% is allowable. Therefore, the S content is set to 0.03% or less. If the content is less than 0.0001%, production efficiency will decrease, so the lower limit is set to 0.0001% or more.

<Al: 0.001-2.0%>
Al is an element that acts as a deoxidizing agent and is effective in improving the cleanliness of steel. If there is too little Al, the effect is not necessarily sufficient. Therefore, the Al content is 0.001% or more, preferably 0.01% or more, and more preferably 0.02% or more. On the other hand, Al is an element that promotes ferrite formation, and when the Al content exceeds 2.0%, ferrite is generated and fatigue strength is reduced. Therefore, the Al content is 2.0% or less, preferably 1.8% or less, and more preferably 1.6% or less.

<N: 0.001 to 0.01%>
N precipitates as a nitride by combining with nitride-forming elements and contributes to refinement of crystal grains. In order to obtain this effect, 0.001% or more is required. However, N easily combines with Ti at high temperatures to form coarse nitrides, and excessive content deteriorates fatigue resistance. Therefore, the N content is 0.01% or less, preferably 0.008% or less, and more preferably 0.006% or less.

<B: 0.0002-0.010%>
B is an element that is effective in promoting the formation of upper bainite and improving the strength of the steel sheet by segregating in prior austenite grain boundaries and suppressing the formation of ferrite. In order to exhibit these effects, the B content needs to be 0.0002% or more. Therefore, the B content is set to 0.0002% or more, preferably 0.0005% or more, and more preferably 0.0007% or more. On the other hand, when the B content exceeds 0.010%, the above effects are saturated. Therefore, the B content is set to 0.010% or less, preferably 0.009% or less, and more preferably 0.008% or less.

<One or more of Ti: 0.01 to 0.30%, Nb: 0.001 to 0.10%>
Ti and Nb are effective elements for forming carbides and improving strength through precipitation strengthening. Therefore, it is necessary to contain at least one of Ti and Nb. The lower limits of the content are Ti: 0.01% or more, Nb: 0.001% or more, preferably Ti: 0.02% or more, Nb: 0.002% or more, Ti: 0.03% or more, Nb: 0.003% or more is more preferable. On the other hand, if the Ti and Nb contents exceed Ti: 0.30% and Nb: 0.10%, respectively, carbides become coarse, hardenability decreases, and the steel structure of the present invention may not be obtained. . For this reason, the upper limits of the Ti and Nb contents are respectively set to Ti: 0.30% or less and Nb: 0.10% or less, preferably Ti: 0.25% or less and Nb: 0.08% or less, and Ti: 0. .20% or less, Nb: 0.05% or less is more preferable.

The remainder is Fe and unavoidable impurities.

The above components are the basic composition of the high strength steel sheet of the present invention. If necessary, the following elements can be further contained.

Cr, Ni, Cu, V, and Mo are elements that stabilize austenite, and are effective elements for suppressing the formation of ferrite and improving strength. In order to obtain such effects, it is preferable to include one or more of these. When containing one or more of Cr, Ni, Cu, V, and Mo, the respective contents are Cu: 0.005 to 2.0%, Ni: 0.005 to 2.0%, and Cr: Preferably, the content is 0.005 to 2.5%, V: 0.001 to 0.5%, and Mo: 0.005 to 1.0%. If the contents of Cr, Ni, Cu, V, and Mo each exceed the above-mentioned upper limits, martensite and retained austenite tend to remain, and the steel structure of the present invention may not be obtained. The lower limit of the Cr content is more preferably 0.1% or more. The upper limit of the Cu content is more preferably 0.6% or less. The lower limit of the Ni content is more preferably 0.1% or more. The upper limit of the Ni content is more preferably 0.6% or less. The lower limit of the Cu content is more preferably 0.1% or more. The upper limit of the Cu content is more preferably 0.6% or less. The lower limit of the V content is more preferably 0.005% or more. The upper limit of the V content is more preferably 0.3% or less. The lower limit of the Mo content is more preferably 0.1% or more. The upper limit of the more preferable Mo content is 0.5% or less.

Sb is an element that suppresses element removal from the surface of the steel material when the steel material is heated, and is effective in suppressing a decrease in the strength of the steel. Therefore, when Sb is contained, the content is preferably 0.005 to 0.2%. If the Sb content exceeds the above upper limit, the steel plate may become brittle. The lower limit of the Sb content is more preferably 0.01% or more. A more preferable upper limit of the Sb content is 0.050% or less.

Sn is an element that suppresses the formation of pearlite and is effective in suppressing a decrease in the strength of steel. In order to obtain such an effect, when Sn is contained, the content is preferably 0.001 to 0.05%. If the Sn content exceeds the above upper limit, the steel plate may become brittle. The lower limit of the Sn content is more preferably 0.005% or more. The upper limit of the Sn content is more preferably 0.03% or less.

Ca, Mg, and REM are elements that are effective in improving workability by controlling the morphology of inclusions. In order to obtain such effects, it is preferable to include one or more of these. When containing one or more of Ca, Mg, and REM, the respective contents are Ca: 0.0005 to 0.01%, Mg: 0.0005 to 0.01%, and REM: 0.0005 to 0.01%. It is preferable to set it to 0.01%. On the other hand, if the contents of Ca, Mg, and REM exceed the above upper limits, the amount of inclusions may increase and workability may deteriorate. The lower limit of the Ca content is more preferably 0.001% or more. A more preferable upper limit of Ca content is 0.005% or less. The lower limit of the Mg content is more preferably 0.001% or more. The upper limit of the Mg content is more preferably 0.005% or less. The lower limit of the REM content is more preferably 0.001% or more. A more preferable upper limit of the REM content is 0.005% or less. REM (rare earth elements) is a general term for Sc, Y, and 15 elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, and the REM content here refers to these elements. is the total content of elements.

Note that even if the content of Mo, V, Cr, Ni, Cu, Sb, Sn, Ca, Mg, and REM is less than the above lower limit, the effects of the present invention are not impaired. Therefore, if the content of these components is less than the above lower limit, these elements are treated as unavoidable impurities.

Next, the microstructure of the high-strength steel plate of the present invention will be explained.

The high-strength steel plate of the present invention has the following microstructure in the surface layer region from the surface of the steel plate to a position of 1/10 of the plate thickness. That is, the main phase is an upper bainite phase having an area ratio of 75% or more and less than 98.5%. Further, a structure consisting of a martensite phase and/or a retained austenite phase having an area ratio of 1.5% or more and less than 25% is defined as a second phase. The average grain size of all the phases in the surface layer region is 6.0 μm or less, and the dislocation density of all the phases is 8.0×10 ¹⁴ /m ² or more.

<Upper bainite phase: 75% or more and less than 98.5% in area ratio>
The microstructure of the high-strength steel sheet of the present invention contains upper bainite as a main phase. If the area ratio of upper bainite is less than 75%, excellent fatigue strength cannot be obtained. Therefore, the lower limit of the area ratio of upper bainite is set to 75% or more, preferably 85% or more. On the other hand, if the upper bainite phase is 98.5% or more, the desired dislocation density cannot be obtained. Therefore, the upper limit of the area ratio of upper bainite is less than 98.5%, preferably 97% or less.

<Martensite phase and/or retained austenite phase: 1.5% or more and less than 25% in area ratio>
The microstructure of the high-strength steel sheet of the present invention includes a martensitic phase and/or a retained austenite phase. If the martensite phase and/or retained austenite phase is less than 1.5%, it is impossible to achieve a tensile strength of 980 MPa or more and excellent fatigue resistance. On the other hand, when the area ratio of martensite and/or retained austenite is 25% or more, the interface between martensite and/or retained austenite and upper bainite, which can become a starting point for fatigue crack initiation, increases, leading to a possibility that fatigue resistance properties decrease. There is. For this reason, it is necessary that the total area ratio of martensite and/or retained austenite be less than 25%. Preferably it is 20% or less, more preferably 15% or less. Note that martensite in the present invention means as-quenched martensite.

The effects of the present invention are not impaired as long as the remaining structural phases other than the upper bainite, martensite and/or retained austenite have a maximum area ratio of 2.0% or less. The above-mentioned residual structure includes, for example, known structures such as ferrite and pearlite.

<Average grain size is 6.0 μm or less>
Fatigue crack initiation is said to be caused by sliding deformation within grains in the surface layer. Grain boundaries make it difficult for this sliding deformation to propagate to adjacent grains, and as a result, crack initiation can be delayed. That is, fatigue strength can be improved by grain refinement. Further, by making the crystal grain size finer, it also contributes to improving the strength. Therefore, the average crystal grain size is 6.0 μm or less, preferably 5.0 μm or less. On the other hand, if the average crystal grain size becomes too small, the strength may increase and the elongation may decrease. For this reason, it is preferable that the average crystal grain size is 2.0 μm or more. Note that the average grain size here refers to all phases in the surface layer region from the surface of the steel sheet to a position of 1/10 of the sheet thickness. In addition, when a residual texture phase is included in the surface layer region up to 1/10 of the plate thickness, this residual texture phase is also included in the above-mentioned "all phases".

<Dislocation density is 8.0×10 ¹⁴ /m ² or more>
Most fatigue cracks originate from the surface of a steel plate and enter the fatigue crack propagation stage after growing to a length of several tens of micrometers. In high cycle fatigue, the number of cycles until crack initiation accounts for a large portion of the fatigue life. Therefore, in order to improve the fatigue strength of 2× ¹⁰⁶ cycles, it is necessary to suppress the occurrence of cracks, and it is important to control the dislocation behavior in the surface layer region from the steel plate surface to the 1/10th of the plate thickness. It is. In the high-strength steel sheet of the present invention, dislocations introduced into the structure are pinned by heat treatment in a subsequent process, and become an obstacle to dislocation movement. This prevents the movement and rearrangement of dislocations, delays cyclic softening, and improves fatigue resistance. In order to obtain this effect, the dislocation density is set to 8.0×10 ¹⁴ /m ² or more. It is preferably 1.0×10 ¹⁵ /m ² or more, more preferably 1.2×10 ¹⁵ /m ² or more. Although the upper limit of the dislocation density is not particularly determined, it is preferably 4.0×10 ¹⁵ /m ² or less. Note that it is most important to control the dislocation density of the main phase in the surface layer region from the steel plate surface to the position 1/10 of the plate thickness. However, since it is difficult to measure the dislocation density of only the main phase, the dislocation density of the present invention refers to all phases in the surface layer region up to 1/10 of the plate thickness. In addition, when a residual texture phase is included in the surface layer region up to 1/10 of the plate thickness, this residual texture phase is also included in the above-mentioned "all phases".

The high-strength steel plate of the present invention has both a tensile strength of 980 MPa or more and a fatigue limit ratio of 0.50 or more. Here, the fatigue limit ratio is the ratio of the plane bending fatigue strength of 2×10 ⁶ times to the tensile strength. Therefore, the high-strength steel plate of the present invention has high tensile strength, ensures safety even when thinned, and can be applied to components of trucks and passenger cars.

In the present invention, the area ratio and mechanical properties of each of the above-mentioned structures are determined by the method described in the Examples.

Next, a method for manufacturing a high-strength steel plate according to an embodiment of the present invention will be described. In addition, in the following description, unless otherwise specified, the expression "°C" related to temperature represents the surface temperature of the object (steel material or steel plate).

The high-strength steel plate of the present invention can be manufactured by sequentially subjecting a steel material to the following treatments (1) to (6). Each step will be explained below.
(1) Heating (2) Hot rolling (3) Cooling (first cooling)
(4) Winding (5) Cooling (second cooling)
(6) Temper rolling Note that any steel material can be used as long as it has the above-mentioned composition. The composition of the high-strength steel plate finally obtained is the same as that of the steel material used. As the steel material, for example, a steel slab can be used. Moreover, the manufacturing method of the steel material is not particularly limited. For example, molten steel having the above-mentioned composition can be melted using a known method such as a converter, and a steel material can be obtained using a casting method such as continuous casting. Methods other than the continuous casting method, such as an ingot-blurring rolling method, can also be used. Moreover, scrap may be used as a raw material. After the steel material is manufactured by a method such as a continuous casting method, it may be directly subjected to the next heating process, or the steel material may be cooled to become a hot piece or a cold piece and then subjected to the heating process. good.

(1) Heating First, the steel material is heated to a heating temperature of 1150°C or higher. In the steel material after being cooled to a low temperature, most carbonitride-forming elements such as Ti exist non-uniformly as coarse carbonitrides. The presence of these coarse and non-uniform precipitates causes deterioration of various properties (eg, strength, fatigue resistance, etc.) generally required of high-strength steel sheets for parts for trucks and passenger cars. Therefore, it is necessary to heat the steel material prior to hot rolling to dissolve coarse precipitates into solid solution. For this reason, the heating temperature of the steel material is 1150°C or higher, preferably 1180°C or higher, and more preferably 1200°C or higher. On the other hand, if the heating temperature of the steel material becomes too high, it will cause slab defects and decrease in yield due to scale-off. For this reason, the heating temperature of the steel material is preferably 1350°C or lower, more preferably 1300°C or lower, and even more preferably 1280°C or lower.

In heating, from the viewpoint of uniformizing the temperature of the steel material, it is preferable to heat the steel material to the heating temperature and then maintain it at the heating temperature. The time for holding at the heating temperature (holding time) is not particularly limited, but from the viewpoint of improving the temperature uniformity of the steel material, it is preferably 1800 seconds or more. On the other hand, if the holding time exceeds 10,000 seconds, the amount of scale generated may increase. As a result, scale encroachment is likely to occur during subsequent hot rolling, which may lead to a decrease in yield due to surface flaw defects. Therefore, the holding time is preferably 10,000 seconds or less, more preferably 8,000 seconds or less. Note that, after casting, the steel material before hot rolling may be directly hot rolled (direct rolling) while still at high temperature (that is, while maintaining the temperature within the above heating temperature range).

(2) Hot Rolling Next, the heated (or still hot after casting) steel material is subjected to hot rolling consisting of rough rolling and finish rolling. The conditions for rough rolling are not particularly limited as long as the desired sheet bar dimensions can be ensured. A steel material is roughly rolled to obtain a roughly rolled plate. Before the obtained rough rolled sheet is subjected to finish rolling, descaling (high pressure water descaling) may be performed by injecting high pressure water on the entry side of the finish rolling mill.

Next, in the present invention, in finish rolling, when temperature RC1 and temperature RC2 are defined by the following formulas (1) and (2), the total rolling reduction in the temperature range from (RC1-150)°C to RC1°C is calculated. 35% or more. The residence time in the temperature range is not particularly defined, but may be 3 seconds or more and 20 seconds or less. Further, the finish rolling finishing temperature is set to be at least (RC2-100)°C and at most (RC2+50)°C. RC1 is the austenite 50% recrystallization temperature estimated from the component composition, and RC2 is the austenite recrystallization lower limit temperature estimated from the component composition. If the total rolling reduction in the temperature range from (RC1-150)°C to RC1°C is less than 35%, the average crystal grain size becomes large and the effect of improving fatigue resistance cannot be obtained. Therefore, the total rolling reduction in the temperature range from (RC1-150)°C to RC1°C is 35% or more, preferably 45% or more, and more preferably 60% or more.

Further, hot rolling is carried out under the condition that finish rolling finish temperature is not less than (RC2-100)°C and not more than (RC2+50)°C. If the finish rolling end temperature is less than (RC2-100)°C, ferrite is generated and a tensile strength of 980 MPa or more cannot be obtained. Therefore, the finish rolling finishing temperature is (RC2-100)°C or higher, preferably (RC2-90)°C or higher, and more preferably (RC2-70)°C or higher. On the other hand, when the finish rolling end temperature is higher than (RC2+50)°C, the austenite grains become coarse and the average grain size of upper bainite becomes large, resulting in a decrease in strength. Therefore, the finish rolling end temperature is (RC2+50)°C or lower, preferably (RC2+40)°C or lower, and more preferably (RC2+30)°C or lower. Note that RC1 and RC2 are defined by the following equations (1) and (2).
RC1 (℃) = 900 + 120 x C + 100 x N + 10 x Mn + 500 x Ti + 5000 x B + 10 x Cr + 50 x Mo + 1500 x Nb + 150 x V... (1)
RC2 (℃) = 750 + 120 x C + 100 x N + 10 x Mn + 250 x Ti + 5000 x B + 10 x Cr + 50 x Mo + 750 x Nb + 150 x V... (2)
Here, each element symbol in the above formulas (1) and (2) represents the content (mass %) of each element, and is set to 0 if it is not contained.

(3) Cooling (first cooling)
Next, the obtained hot rolled steel sheet is cooled (first cooling). At that time, the time from the end of hot rolling to the start of cooling (cooling start time) is set within 2.0 seconds after the end of finish rolling. If the cooling start time exceeds 2.0 seconds, grain growth of austenite grains will occur, making it impossible to ensure a tensile strength of 980 MPa or more. Therefore, the cooling start time is within 2.0 seconds, preferably within 1.5 seconds, and more preferably within 1.0 seconds.

In cooling, if the average cooling rate from the finish rolling end temperature to the cooling stop temperature is too slow, ferrite transformation occurs before upper bainite transformation, and an upper bainite phase with a desired area ratio cannot be obtained. Therefore, the average cooling rate is 20°C/s or more, preferably 30°C/s or more, and more preferably 50°C/s or more. On the other hand, the upper limit is not particularly limited, but if it is too fast, it may be difficult to control the cooling stop temperature and obtain the desired microstructure. s or less is more preferable, and 150° C./s or less is even more preferable. Further, in cooling, forced cooling may be performed so as to achieve the above-mentioned average cooling rate. Although the cooling method is not particularly limited, it is preferable to use water cooling, for example.

The cooling stop temperature is at least Trs°C and at most (Trs+180)°C. When the cooling stop temperature is less than Trs°C, the microstructure becomes lower bainite. Although lower bainite has a high-strength structure, its fatigue resistance after heat treatment is low. Therefore, the cooling stop temperature is set to Trs°C or higher. On the other hand, if the cooling stop temperature is higher than (Trs+180)°C, ferrite is generated, so a tensile strength of 980 MPa or more cannot be obtained. Therefore, the cooling stop temperature is set to (Trs+180)°C or lower. Note that Trs is defined by the following equation (3).
Trs (℃) = 500-450 x C-35 x Mn-15 x Cr-10 x Ni-20 x Mo... (3)
Here, each element symbol in the above formula (3) represents the content (mass %) of each element, and is set to 0 if it is not contained.

(4) Winding Next, the cooled hot-rolled steel sheet is rolled up at a winding temperature of not less than Trs°C and not more than (Trs+180)°C. If the winding temperature is lower than Trs°C, lower bainite transformation will proceed after winding, and desired martensite and/or retained austenite will not be obtained. Therefore, the winding temperature is Trs°C or higher, preferably (Trs+10)°C or higher, and more preferably (Trs+30)°C or higher. On the other hand, if the winding temperature is higher than (Trs+180)° C., ferrite is generated, so a tensile strength of 980 MPa or more cannot be obtained. Therefore, the winding temperature is (Trs+180)°C or lower, preferably (Trs+150)°C or lower, and more preferably (Trs+120)°C or lower.

(5) Cooling (second cooling)
Next, it is cooled down to (Trs-250)°C or less at an average cooling rate of 1°C/s or less (second cooling). If the average cooling rate from the winding temperature to below (Trs-250°C) exceeds 1°C/s, the progress of bainite transformation will be insufficient, martensite and retained austenite will increase, and the microstructure of the present invention will not be obtained. I won't be able to do it. Therefore, the average cooling rate from the winding temperature to (Trs-250)°C or less is 1°C/s or less, preferably 0.8°C/s or less, and more preferably 0.5°C/s or less. Cooling can be carried out to any temperature below (Trs-250°C), but preferably to about 10 to 30°C. Note that the cooling can be performed in any arbitrary form, for example, it may be performed in the state of a wound coil.

(6) Temper rolling Next, the steel plate after cooling is subjected to temper rolling under a rolling reduction ratio of 0.1% or more and 5.0% or less. When the rolling reduction is less than 0.1%, the dislocation density becomes insufficient and excellent fatigue strength cannot be obtained. Therefore, the rolling reduction ratio is 0.1% or more, preferably 0.3% or more, and more preferably 0.5% or more. On the other hand, temper rolling exceeding 5.0% increases the load on the rolls and increases the number of replacements, resulting in increased manufacturing costs. Therefore, the rolling reduction ratio is 5.0% or less, preferably 4.0% or less, and more preferably 3.0% or less.

The high-strength steel plate of the present invention can be manufactured by the above procedure. Note that after temper rolling, scale formed on the surface may be removed by, for example, pickling according to a conventional method.

Molten steel having the composition shown in Table 1 was melted in a converter, and a steel slab as a steel material was manufactured by a continuous casting method.

得られた鋼素材を、表２に示す加熱温度に加熱し、次いで、加熱後の鋼素材に、粗圧延と仕上げ圧延からなる熱間圧延を施して熱延鋼板とした。熱間圧延における仕上げ圧延終了温度は表２に示したとおりとした。

The obtained steel material was heated to the heating temperature shown in Table 2, and then the heated steel material was subjected to hot rolling consisting of rough rolling and finish rolling to obtain a hot rolled steel plate. The finishing temperature in hot rolling was as shown in Table 2.

Next, the obtained hot rolled steel sheet was cooled under the conditions of the average cooling rate and cooling stop temperature shown in Table 2 (first cooling). The hot-rolled steel sheet after cooling was wound up at the winding temperature shown in Table 2, and the wound steel sheet was cooled at the average cooling rate shown in Table 2 (second cooling) to obtain a high-strength steel sheet. After cooling, temper rolling was performed at the rolling reduction ratio shown in Table 2, and pickling was performed. The pickling was carried out at a temperature of 85° C. using an aqueous hydrochloric acid solution having a concentration of 10% by mass. Next, the steel plate was subjected to heat treatment equivalent to paint baking treatment (170°C, 20 minutes) to produce a high-strength hot rolled steel plate.

得られた高強度鋼板から試験片を採取し、以下に述べる手順でミクロ組織と機械的特性を評価した。

A test piece was taken from the obtained high-strength steel plate, and its microstructure and mechanical properties were evaluated using the procedures described below.

<Microstructure>
A test piece for microstructure observation was taken from the obtained high-strength steel plate so that the cross-section of the plate parallel to the rolling direction was the observation surface. The surface of the obtained test piece was polished and further corroded using a corrosive solution (3% nital solution) to reveal the microstructure. Next, the surface layer from the surface to 1/10 of the plate thickness was photographed using a scanning electron microscope (SEM) at a magnification of 5000 times to obtain an SEM image of the microstructure. The obtained SEM images were analyzed by image processing, and the area ratios of upper bainite (UB), polygonal ferrite (F), and lower bainite (LB) were quantified. Furthermore, it is difficult to distinguish martensite (M) and retained austenite (γ) by SEM. Therefore, the electron beam reflection diffraction (Electron Back Scatter Diffraction Patterns: EBSD) method was used to identify them, and the area ratio and average crystal grain size of each were determined. Table 3 shows the area ratio and average grain size of each microstructure measured. Note that Table 3 also lists the total area ratio (M+γ) of martensite and retained austenite.

<Tensile test>
A JIS No. 5 tensile test piece (JIS Z 2201) was taken from the obtained hot-rolled steel sheet so that the tensile direction was perpendicular to the rolling direction, and the strain rate was set to 10 ^-3 /s, as specified in JIS Z 2241. A tensile test was conducted in accordance with the above, and the tensile strength was determined. In addition, in the present invention, a tensile strength of 980 MPa or more is considered to be passed. The results are shown in Table 3.

<Plane bending fatigue test>
A test piece having the dimensions and shape shown in Fig. 1 was taken from the obtained hot-rolled steel sheet so that the longitudinal direction of the test piece was perpendicular to the rolling direction, and a plane bending fatigue test was conducted in accordance with the provisions of JIS Z 2275. was carried out. In the stress loading mode, the stress ratio R=-1 and the frequency f=25 Hz. The load stress amplitude was varied in 6 steps, the stress cycle until fracture was measured, the SN curve was determined, and the fatigue strength (fatigue limit) at 2×10 ⁶ cycles was determined. In the present invention, when the value of the fatigue limit ratio obtained by dividing the fatigue limit by the tensile strength determined by a tensile test is 0.50 or more, it is evaluated as having excellent fatigue resistance properties. The results are shown in Table 3.

発明例は、いずれも９８０ＭＰａ以上の引張強さと優れた耐疲労特性とを有する高強度鋼板である。一方、本発明の範囲を外れる比較例は、９８０ＭＰａ以上の引張強さが得られていないか、優れた耐疲労特性が得られていない。

All of the invention examples are high-strength steel plates having a tensile strength of 980 MPa or more and excellent fatigue resistance. On the other hand, in the comparative examples outside the scope of the present invention, a tensile strength of 980 MPa or more was not obtained, or excellent fatigue resistance properties were not obtained.

Claims (3)

In mass%,
C: 0.03-0.15%,
Si: 0.1 to 3.0%,
Mn: 0.8 to 3.0%,
P: 0.001-0.1%,
S: 0.0001-0.03%,
Al: 0.001-2.0%,
N: 0.001 to 0.01%, and B: 0.0002 to 0.010%
Contains, and furthermore,
Ti: 0.01 to 0.30%, and Nb: 0.001 to 0.10%,
containing at least one selected from the following, having a component composition with the balance consisting of Fe and inevitable impurities,
The microstructure has an upper bainite phase as a main phase with an area ratio of 75% or more and less than 98.5% in the surface layer region from the steel plate surface to 1/10 of the plate thickness,
A structure consisting of a martensite phase and/or a retained austenite phase with an area ratio of 1.5% or more and less than 25% is defined as a second phase,
having a residual structure phase other than the upper bainite phase, the martensite phase and/or the retained austenite phase in an area ratio of 2.0% or less,
The average grain size of all phases in the surface layer region from the steel plate surface to the 1/10th position of the plate thickness is 6.0 μm or less,
A high-strength steel plate, wherein the dislocation density of all the phases in the surface layer region from the steel plate surface to a position of 1/10 of the plate thickness is 8.0×10 ¹⁴ /m ² or more. The component composition further includes the following groups a to c in mass %:
Group a:
Cu: 0.005-2.0%,
Ni: 0.005-2.0%,
Cr: 0.005-2.5%,
V: 0.001 to 0.5%, and Mo: 0.005 to 1.0%,
at least one selected from;
Group b:
Sb: 0.005-0.2%,
Sn: 0.001-0.05%,
at least one selected from;
Group c:
Ca: 0.0005-0.01%,
Mg: 0.0005-0.01%, and REM: 0.0005-0.01%,
The high-strength steel plate according to claim 1, further comprising at least one group selected from: A method for manufacturing a high-strength steel plate according to claim 1 or 2, comprising:
Heating a steel material having the above-mentioned composition to a heating temperature of 1150 ° C. or higher,
After heating, the steel material is roughly rolled into a steel plate,
The steel plate is finish rolled at a temperature range of (RC1-150)°C or higher and RC1°C or lower, with a total rolling reduction of 35% or more, and a finish rolling end temperature of (RC2-100)°C or higher and (RC2+50)°C or lower. death,
The steel plate after finish rolling is subjected to the following conditions: time from finish rolling to start of cooling: within 2.0 s, average cooling rate at surface: 20°C/s or more, cooling stop temperature: Trs°C or higher (Trs+180)°C or lower. cool,
Coiling the cooled steel plate at a coiling temperature of Trs°C or higher and (Trs+180)°C or lower,
Cooling to (Trs-250) °C or less at an average cooling rate of 1 °C/s or less,
A method for manufacturing high-strength steel sheets, which involves skin pass rolling under the conditions of rolling reduction: 0.1% or more and 5.0% or less.
Note that RC1, RC2, and Trs are defined by the following equations (1), (2), and (3), respectively.
RC1 (℃) = 900 + 120 x C + 100 x N + 10 x Mn + 500 x Ti + 5000 x B + 10 x Cr + 50 x Mo + 1500 x Nb + 150 x V... (1)
RC2 (℃) = 750 + 120 x C + 100 x N + 10 x Mn + 250 x Ti + 5000 x B + 10 x Cr + 50 x Mo + 750 x Nb + 150 x V... (2)
Trs (℃) = 500-450 x C-35 x Mn-15 x Cr-10 x Ni-20 x Mo... (3)
Here, each element symbol in the above formulas (1), (2), and (3) represents the content (mass %) of each element, and is set to 0 if it is not contained.

Images