JP2010168651A - High strength hot-rolled steel plate and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a new low alloy having no anisotropy and having high strength, good workability, delayed fracture resistance and high speed pressure-fracture resistance, a high strength steel plate and a manufacturing method therefor. <P>SOLUTION: The high strength hot-rolled steel plate is obtained by being subjected to a finish-rolling in two-phase zones having temperature of ≥800°C and ≤Ae3. The steel plate has: ferritic structure having ≤10 μ grain diameter the ratio of which is 5-70%; retained austenitic structure having ≤2 μm grain diameter the ratio of which is 7-20%; and the balance bainitic structure. For example, it is desirable that the steel plate is composed of 0.14-0.30% C, 1.0-3.5% Si, 0.1-0.4% Mn, 0.5-3.0% Cr, 0.03-0.60% Mo and the balance Fe with inevitable impurities. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a high-strength hot-rolled steel sheet having high workability, delayed fracture characteristics, and high-speed crushing characteristics while having high tensile strength, and a method for producing the same.

  The recent demand for high-strength steel sheets with excellent workability will be described by taking the case of automobiles as an example. From the viewpoint of global environmental conservation, it is essential to reduce the amount of exhaust gas such as CO2 in the automobile field. For that purpose, further weight reduction of the automobile body becomes indispensable. In order to reduce the weight of the car body, it is necessary to increase the strength of steel plates used in automobiles and reduce the thickness. At the same time, passengers must ensure the safety of passengers. For this purpose, it is necessary to further increase the strength of the steel sheet.

  Basic methods for increasing the strength include solid solution strengthening, precipitation strengthening, crystal grain refinement, and strengthening by using a low-temperature transformation structure. Only by applying a strengthening mechanism that requires a large amount of alloy addition such as solid solution strengthening or precipitation strengthening, it is impossible to produce a steel sheet that requires extremely high strength. Even if a strengthening mechanism by crystal grain refinement is applied, there is a limit even if the strength can be increased to some extent. Strengthening by using low-temperature transformation structure is an extremely effective method for producing steel plates of over 600MPa, but it cannot be expected to improve ductility to meet the increase in strength.

Generally, when the strength of a steel plate is increased, ductility is reduced and workability is reduced.
Conventional techniques for increasing the ductility of high-strength steel sheets include composite phase (Dual Phase) steel sheets composed of ferrite and martensite, and TRIP (Transformation Induced Plasticity) steel sheets composed of ferrite, bainite and retained austenite structures. .
In the composite steel sheet, hard martensite is finely dispersed in ferrite, and this hard martensite causes large work hardening at the time of deformation, and brings high ductility to the steel sheet.
An example of the TRIP steel sheet is shown in Patent Document 1. This type of steel sheet containing retained austenite has very good ductility and formability due to work-induced transformation, depending on its amount and stability to deformation.
Japanese Patent No. 4002315

Further, when the strength of the steel plate is increased to 980 MPa or more, the problem of delayed fracture occurs. Delayed fracture is a phenomenon in which cracks and breakage do not occur during the processing and assembly of members, but suddenly occur during use.
The high-strength steel sheet shown in Patent Document 2 is good by reducing the soft phase such as ferrite as much as possible to the hard low-temperature transformation phase such as bainite and tempered martensite and limiting the retained austenite to 4% or less. Has established a delayed fracture resistance.
Japanese Patent No. 3247908

Bending and axial high-speed crush tests are performed as an evaluation of automobile crash safety, and the absorbed energy is evaluated. This energy is generally said to increase as the tensile strength of the steel sheet increases. Among them, ferrite and martensite duplex steel sheets of 980 MPa or less have high absorbed energy relative to strength and are folded into a clean accordion shape in the axial crush test. This is thought to be because the soft ferrite phase is a cushioning material. In Patent Documents 3 and 4 below, axial crush tests are reported on a multiphase steel sheet such as ferrite and martensite.
JP-A-9-111396 JP 2008-231480 A

In the composite structure steel plate, high strength can be obtained even with a relatively low alloy addition amount, and at the same time, good uniform elongation characteristics can be obtained by work hardening.
TRIP steel sheet exhibits higher ductility and has high deep drawability. Therefore, application to a member that requires high workability with a complicated shape and requires high strength is directed.
In Patent Document 1, in order to promote the formation of ferrite in the austenite during the cooling process after rolling is completed, slow cooling is performed with Ar3 to Ar1, or the rolling completion temperature is set to the vicinity of the Ar3 point, and then 350 to 500 ° C. Manufactured by cooling to range and winding. These TRIP steel sheets have a structure in which martensite, retained austenite and bainite are dispersed in the ferrite matrix, and have excellent strength and elongation characteristics. However, a tensile strength of only about 800 MPa can be obtained, and a steel sheet having a higher strength range of 980 MPa or more is difficult to manufacture from the viewpoint of delayed fracture.
Further, after cooling is temporarily stopped in the vicinity of the A1 point, a process of cooling to 500 ° C. or lower and winding up is necessary to stabilize the martensite and retained austenite structures. It is necessary to lengthen the cooling equipment length after the end of rolling, and the equipment cost increases.
Even in the method of continuously cooling to 500 ° C or less without performing slow cooling in the middle of the end of rolling, the formation of fine ferrite is promoted if the rolling end temperature is close to the Ar3 point in the component system that has been studied so far. However, there is a problem that the material properties of the hot-rolled steel sheet rolled in the vicinity of the Ar3 point have large anisotropy.

Hydrogen dissolved in the steel sheet, which is the cause of delayed fracture, is preferentially trapped in the retained austenite due to the crystal structure. In particular, the interface between martensite and ferrite that has undergone processing-induced transformation is the most dangerous trap site due to the influence of processing.
The coarser the retained austenite grains, the smaller the ratio of the area of martensite-ferrite interface that has undergone work-induced transformation compared to the volume of retained austenite grains, the higher the concentration of trapped hydrogen, and the risk of delayed fracture Increases nature. Furthermore, if martensite and retained austenite coexist in an adjacent state (MA), the propagation of fracture is promoted and the risk is further increased.
The high-strength steel sheet described in Patent Document 2 has improved delayed fracture resistance by limiting the amount of retained austenite. However, in order to obtain excellent workability while having high strength, the utilization of retained austenite is effective, and it is desirable to make it harmless against delayed fracture without providing the restrictions.

  In Patent Documents 3 and 4 described above, although not a high-strength steel sheet, in the axial crush test, a ferrite and martensite two-phase structure steel sheet has a relatively high absorption energy relative to strength, and a soft ferrite phase becomes a cushioning material. Although it has been reported that it is folded into a clean accordion shape, a steel plate having a ferrite phase stably at a strength of 980 MPa or more is desired.

  Therefore, the inventors of the present application have studied various alloy elements, and hot strip mill finish rolling is performed in the two-phase region of high-temperature austenite and ferrite to produce sized ferrite, and without performing complicated cooling control in ROT, After winding, bainite transformation is performed to obtain a composite structure in which retained austenite of 2 μm or less (7% or more and 20% or less) is finely dispersed, so there is no anisotropy, high strength, good workability, and delayed fracture resistance And a new low-alloy, high-strength steel sheet that has both high-speed crushing resistance and its manufacturing method.

  As a result of diligent research, the inventors adopted high-pressure rolling conditions in a high-temperature two-phase region, and added Mn, an alloying element that has always been used in high-strength steel sheets. It was found that preferable high-strength steel sheets can be obtained by limiting the amount and limiting the composition of Si and Mo, which increase the transformation temperature, and Cr with a low transformation point drop, as the strength elements. That is, a slab having an appropriate component range is obtained by finishing high-temperature rolling in the second phase in the finish rolling of hot rolling, and winding it at an appropriate temperature, thereby achieving high strength and excellent strength with a low alloy composition. It is possible to simultaneously impart ductility, delayed fracture resistance and high-speed crush resistance. Details are shown below.

The high-strength hot-rolled steel sheet described in the claims is finish-rolled in a two-phase region of 800 ° C. or higher and Ae3 temperature or lower, and the ferrite structure has a grain size of 10 μm or less and the ratio (volume ratio of the ferrite structure) is 5% or more. The residual austenite structure has a particle size of 2 μm or less, the ratio (volume ratio of the retained austenite structure) is 7% or more and 20% or less, and the balance is a bainite structure.
By obtaining such structural properties, it is possible to obtain a steel sheet having high strength, good workability, and excellent delayed fracture resistance and high-speed impact resistance.
High strength is obtained with the bainite structure, high ductility is provided by the retained austenite, and moderate workability is obtained by the presence of the ferrite structure. Since the martensite structure ratio is reduced and the retained austenite is 2 μm or less and is effectively finely dispersed, delayed fracture resistance can be obtained. It also delays the propagation of cracks and improves local elongation.
Since it is finish-rolled in a two-phase region of 800 ° C. or higher and Ae3 temperature or lower, ferrite recovers and becomes sized, and material anisotropy in the rolling direction and the direction perpendicular to the rolling is reduced. Therefore, the workability can be further improved by this high-strength hot-rolled steel sheet.

The high-strength hot-rolled steel sheet described in the claims is finish-rolled in a two-phase region of 860 ° C. or higher and Ae 3 temperature or lower, the ferrite structure has a grain size of 10 μm or less, and the ratio is 5% or more and 35% or less. The austenite structure has a particle size of 2 μm or less, the ratio is 7% or more and 20% or less, the remainder is a bainite structure, and the component ranges are C: 0.14 to 0.30%, Si: 1.0 to 3.5%, Mn: 0.1 It is preferable to include -0.4%, Cr: 0.5-3.0%, Mo: 0.03-0.60%, with the balance being composed of iron and inevitable impurities.
Further, it is finish-rolled in a two-phase region of 800 ° C. or higher and 860 ° C. or lower (preferably 830 to 860 ° C.) instead of the above temperature, and the ferrite structure ratio is 35% or higher and 70% or lower (preferably 40% to 65%), and instead of Cr and Mo in the above ratio, Cr: 0.5-2.0% (preferably 1.2-1.8%), Mo: 0.03-0.2% (preferably 0.12-0.18%) preferable.
In any of the above cases, it is particularly desirable to satisfy the following formula (1). Further, Ti: 0.02 to 0.20%, Nb: 0.02 to 0.10%, V: 0.02 to 0.20%, B: 0.0001 to 0.0030% Of these, those containing one or two of them are preferred.
Ae3 = 919-266 * C + 38 * Si-28 * Mn-27 * Ni-11 * Cr + 12 * Mo ≧ 890 (1)
If such an appropriate kind and amount of chemical components are included, it is easy to obtain a high-strength steel sheet having the above-described structure and exhibiting desirable mechanical properties. As for alloying elements, it is desirable that the Ar3 transformation temperature is high when performing high-temperature two-phase rolling in order to recover ferrite, so silicon and molybdenum, which increase the Ar3 transformation temperature, are the main elements, and insufficient strength reduces the transformation point. Less chromium was used. Silicon is said to be an effective element for promoting the bainite transformation. If the formula (1) is satisfied, stable two-phase region rolling becomes possible. In addition, the effect | action of each component is mentioned later.

The above high-strength hot-rolled steel sheet has the structure described above, has a thickness of 1.0 mm to 4.0 mm, a tensile strength TS (MPa) of 980 MPa or more, and the product of TS and elongation value EL (%) TS × EL Also preferred are those having an Mn of 20000 (MPa ·%) or more.
This is because such a steel sheet has the above-described structure and has both high strength and good elongation characteristics.

The manufacturing method of the claim for the high-strength hot-rolled steel sheet is obtained by accumulating three stages after finishing by a hot rolling mill having a plurality of stands after roughly rolling a steel material (material slab) having a temperature of 1200 ° C. or higher in the above component range. The final finish rolling is completed in a two-phase region having a strain of 0.45 or more and 860 ° C. or more and Ae3 temperature or less (or 800 ° C. or more and 860 ° C. or less in accordance with the above).
In addition, after completion of finish rolling, after cooling for 2 seconds or more and 5 seconds or less, start cooling at a cooling rate of 10 ° C / sec or more and wind up at Bs or less and Ms + 50 ° C or more. preferable. Bs is expressed by equation (2), and Ms is expressed by equation (3).
Bs = 649-83 * C-19 * Si-26 * Mn-30 * Ni-21 * Cr-29 * Mo (2)
Ms = 539-423 * C-30.4 * Mn-17.7 * Ni-12.1 * Cr-7.5 * Mo (3)
According to this manufacturing method, since it is rolled in a high-temperature two-phase region, the ferrite recovers and becomes sized, material anisotropy in the rolling direction and the direction perpendicular to the rolling can be reduced, and workability can be further improved. .
Moreover, according to this manufacturing method, the temperature management becomes easy. According to the manufacturing test of the inventors, the above-described high-strength steel sheet could be obtained under these conditions as described later.

  The high-strength steel sheet according to the claim is a steel sheet that has both strength and processing characteristics, which are mutually contradictory properties, because ferrite and bainite are mixed in a state in which a large amount of retained austenite is finely dispersed. It is a steel plate with excellent properties and high-speed crush resistance.

  According to the manufacturing method described in the claims, the above-described high-strength steel plate can be manufactured smoothly.

Hereinafter, embodiments of a thin steel plate used for a processed part that requires excellent workability, delayed fracture resistance, and high-speed crush resistance while having a tensile strength of 980 MPa or more and a manufacturing method thereof will be described. .
As the component system of the steel plate, it contains C: 0.14-0.30%, Si: 1.0-3.5%, Mn: 0.1-0.4%, Cr: 0.5-3.0%, Mo: 0.03-0.6%, the balance being iron and inevitable impurities Of the composition. In particular, the following formula (1) is satisfied, and Ti: 0.02 to 0.20%, Nb: 0.02 to 0.10%, V: 0.02 to 0.20%, B: 0.0001 to 0.0030% Or it contains 2 types.
The thin steel plate described here is a steel plate having a thickness of 1.0 to 4.0 mm. The steel sheet to be produced can be used mainly for parts that require high workability and strength, such as automobiles, home appliances, and electronic equipment products. In addition, it can be applied as a material for steel pipes.

First, the components of the steel sheet will be described.
As carbon (C), an amount in the range of 0.14 to 0.30% is required. Carbon is the most important element for stabilizing retained austenite. If it is less than 0.14%, sufficient stability cannot be obtained, so a carbon content of 0.14% or more is necessary. On the other hand, when the carbon content is 0.30% or more, the welded portion is excessively hardened and easily breaks from the welded portion. This is a limitation in use for thin steel plates, so an upper limit was set for the carbon content. And it has been found that a composite structure in accordance with the gist of the present invention can be obtained when the carbon content is 0.14 to 0.30%.

  The amount of silicon (Si) is in the range of 1.0 to 3.5%. Silicon also has an effect of improving strength by solid solution strengthening. Furthermore, Ae3 is used to increase and stabilize retained austenite. If the amount of silicon is 1.0% or more, the composite structure and material characteristics of the present invention can be obtained. The greater the amount of silicon, the higher the Ae3 and the greater the amount of retained austenite, while at the same time promoting its stability. However, when the silicon content is 3.5% or more, the strength-ductility balance characteristic is saturated, so the upper limit of the silicon content is 3.5%.

  The amount of chromium (Cr) is 0.5 to 3.0%. The amount of chromium affects the formula (1), and the strength of Ae3 is relatively small and the strength can be increased. If it is less than 0.5%, the strength cannot be satisfied. When the chromium content exceeds 3.0%, Ae3 is significantly decreased, so the upper limit was made 3.0%.

  Manganese (Mn) content is in the range of 0.1-0.4%. If the amount of manganese is less than 0.1%, it becomes difficult to manufacture on steel making, so 0.1% or more. If the manganese amount is added to 0.4% or more, Ae3 is remarkably lowered and martensite is easily generated, and the target structure of the present invention cannot be obtained.

  Molybdenum (Mo), like Si, can improve Ae3 and improve the strength of steel, but if added intentionally, it causes an increase in cost, so the range was made 0.03-0.6%.

Titanium (Ti) has an effect of refining crystal grains in the hot rolling process.
Titanium is an effective element for finely dispersing ferrite grains and residual austenite grains. If the amount of titanium is less than 0.02%, the effect of suppressing recrystallization and crystal grain growth is lost, so when added, the content is made 0.02% or more. Furthermore, even if the amount exceeds 0.20%, the effect does not increase so much, and it becomes difficult to manufacture on steel making, so the upper limit is made 0.20%.

Niobium (Nb) also has the effect of suppressing recrystallization and crystal grain growth, similar to titanium.
Niobium is an effective element for finely dispersing ferrite grains and residual austenite grains. If the amount of niobium is less than 0.02%, the effect of suppressing recrystallization and crystal grain growth is lost. Moreover, even if the amount of niobium increases beyond 0.10%, the effect does not increase so much, so the upper limit was made 0.10%.

  Vanadium (V) also has the effect of suppressing recrystallization and crystal grain growth, like titanium and niobium. Vanadium is an effective element for finely dispersing ferrite grains and residual austenite grains. If the amount of vanadium is less than 0.02%, the effect of suppressing recrystallization and crystal grain growth is lost, so when added, the content is made 0.02% or more. Further, even if the amount of vanadium is increased from 0.20%, the effect is not so increased, so the upper limit is set to 0.20%.

  Boron (B) also has the effect of suppressing recrystallization and crystal grain growth, like titanium and niobium. Boron is an effective element for finely dispersing ferrite grains and residual austenite grains. When the boron content is less than 0.0001%, the effect of suppressing recrystallization and crystal grain growth is lost, so when added, the content is made 0.0001% or more. In addition, even if the boron content increases from 0.0030%, the effect does not increase so much, so the upper limit was made 0.0030%.

The slab (steel material to be rolled) adjusted to the above-described reference component is either hot-rolled after reheating or hot-rolled immediately after casting. In performing hot rolling, after rough rolling, finish rolling is performed by a hot rolling mill having a plurality of stands so that the cumulative strain becomes 0.45 or more. In order to perform such high-pressure reduction, it is necessary to use a small-diameter roll mill having a work roll diameter of 600 mm or less or a different-diameter roll mill having an average work roll diameter of 600 mm or less for at least a plurality of subsequent stages. preferable. When the rolling completion temperature is low, the flat crystal grains are not recovered and the grain size is not adjusted, so the rolling finishing temperature is set to 860 ° C. or more and Ae 3 temperature or less.
Here, “strain” means the difference between the thickness h0 of the steel sheet at the entrance side of each stand (each stage) and the thickness h1 at the exit side divided by the average thickness of both ε = (h ₀ −h ₁ ) / {(h ₀ + h ₁ ) / 2}
“Cumulative strain” is a weighted integration of strain at each stage of the latter three stands, taking into account the strength of the influence on the metal structure, and the distortion at the last stage and its previous and previous stages. When ε _n , ε _n-1 and ε _n-2 respectively,
ε _c = ε _n + ε _n-1 / 2 + ε _n-2 / 4
Ε _c represented by If the finishing mill has 6 stages,
ε _c ＝ RED ＝ F4 / 4 + F5 / 2 + F6
Can also be expressed.

Hot rolling is completed at a rolling end temperature of 860 ° C or higher and Ae3 temperature or lower (two-phase region). After the rolling is completed, it is allowed to cool for 2 seconds or more and 5 seconds or less, and then cooled at a cooling rate of 10 ° C / sec or more. It is necessary to wind up.
It is necessary to refine the austenite grains by hot rolling so that the cumulative strain becomes 0.45 or more, and to reduce the dislocation density in the grains by high-temperature finishing and the subsequent cooling process for 2 seconds or more. is there. By this rolling system, equiaxed and fine prior austenite grains with a low dislocation density can be obtained, and the structure obtained in the subsequent cooling process can be made into a uniform and fine structure with little directionality. When the coiling temperature is set to a temperature range of Bs or less and Ms + 50 ° C or more, a structure in which ferrite and bainite are mixed is formed, and austenite also remains. This retained austenite can be dispersed very finely at 2 μm or less.
Here, the coiling temperature is set to Bs or lower and Ms + 50 ° C. or higher. However, in the temperature range of Ms + 50 ° C. or lower, a lot of martensite structure is formed and delayed fracture is likely to occur. Further, when the temperature is higher than Bs, the structure becomes coarse and high strength cannot be obtained.

  FIG. 1 shows the concept of temperature history in hot rolling and cooling in the manufacturing process of the embodiment of the present invention, in which the horizontal axis represents time and the vertical axis represents temperature. From the left side of the figure, the range a indicates that a rough rolling process is performed, b indicates a finish rolling process, and c indicates a winding process.

FIG. 2 shows a cross section of the structure of the steel of the present invention in which the ferrite or bainite phase of the body-centered cubic structure and the austenite phase of the face-centered cubic structure are color-coded using the EBSD method. It can be observed that the retained austenite structure shown in white (light color) in (b) is finely and uniformly dispersed to 2.0 μm or less.
Here, (a) is a crystal orientation distribution image (identification of orientation difference of 15 ° or more is identified by a grain boundary), and (b) is a crystal structure distribution image (body-centered cubic face-centered cubic).

  The delayed fracture test method was performed based on the U-bending continuous charge method (reference: “Iron and Steel” vol 83 (1997) No11 P66). That is, the dimensions of the test piece were the original plate thickness, the plate width was 30 mm, the plate length was 100 mm, the end face was cut, the clearance was 15%, and the burrs were arranged on the bent inner surface side. The load stress was 2000 MPa, and the battery was charged in a 1N sulfuric acid aqueous solution for a maximum of 120 minutes. Cracks that occurred within 120 minutes were regarded as cracks, and 120 minutes or more were regarded as no cracks.

FIG. 3 (a) is a drawing of a specimen used in the high-speed axial crushing test, which is a 70 mm × 70 mm prism. FIG. 3B shows the appearance of the specimen after the high speed axial crushing test. As shown in this figure, a product that was folded into a clean accordion shape from the top was regarded as good, and a product that was not folded and cracked was regarded as defective.
The present invention has been developed based on the above findings.

Examples of the invention will be described below.
The molten steel having chemical components shown in Table 1 was used as a slab (rolled material) by a continuous casting method or a forging method. Subsequently, these slabs were reheated and hot-rolled to obtain hot-rolled steel sheets.

  Steel types A, B, C, D, E, F, N, O, P, and Q shown in Table 1 are within the scope of the examples (ranges that satisfy the conditions for chemical composition), and steel types G, HIJ, K, L, M is a comparative example.

Steel types A, B, C, D, E, F, N, O, P, and Q are examples that satisfy the following formula (1).
Ae3 = 919-266 * C + 38 * Si-28 * Mn-27 * Ni-11 * Cr + 12 * Mo ≧ 890 (1)
Steel grade G of the comparative example is within the range of formula (1), but manganese (Mn) is outside the scope of the present invention. Steel type H is within the range of formula (1), but chromium (Cr) is outside the scope of the present invention. Steel type I and steel type J are low in silicon (Si), high in manganese (Mn), and out of the scope of the present invention, and equation (1) is also out of the scope of the invention. As for the steel type K, the formula (1) is out of the scope of the invention. Steel type L and steel type M have a low carbon content (C) and are out of the scope of the present invention.

Table 2 shows the hot rolling conditions and the material properties.

Nos. 1 to 3 in Table 2 are obtained by performing hot rolling within the scope of the invention using steel type A having chemical components within the scope of the present invention. Since two-phase rolling is performed at a high temperature, ferrite is generated, and there is little anisotropy, and tensile strength (TS) * elongation (El) is also good. Delayed fracture characteristics were also good, and high-speed axial crushing characteristics were also good. No. 4 is a comparative example in which steel type A having a chemical composition within the scope of the present invention was used, but the hot rolling temperature was high and no ferrite was formed, so the high speed axial crushing characteristics were poor. No. 5 used a steel type A having a chemical composition within the scope of the present invention, but is a comparative example having a low hot rolling temperature and a large anisotropy. No. 6 is a comparative example in which steel type A having a chemical composition within the range of the present invention was used, but because the cumulative strain of rolling in hot rolling was small, the crystal grains were large and delayed fracture occurred.
No. 7 was obtained by hot rolling within the scope of the invention using steel type B having a chemical composition within the scope of the present invention. There was little anisotropy, tensile strength (TS) * elongation (El) was good, delayed fracture property was good, and high-speed axial crushing property was also good. No. 8 was a comparative example in which steel type B having a chemical composition within the scope of the present invention was used, but the coiling temperature was low, a martensite structure was formed, and delayed fracture occurred.
Nos. 9 to 12 were obtained by hot rolling within the scope of the invention using steel types C, D, E, and F having chemical components within the scope of the present invention. Ferrite is formed, and there is little anisotropy, and tensile strength (TS) * elongation (El) is also good. Delayed fracture characteristics were also good, and high-speed axial crushing characteristics were also good.
Nos. 13 to 20 are hot-rolled using the steel type F of the comparative example that is out of the component range of the present invention. Nos. 13 and 14 were hot rolled within the scope of the invention using the steel types G and H of the comparative examples, but were comparative examples in which a martensitic structure was formed and delayed fracture characteristics were generated.
Nos. 15 to 17 were hot rolled using the steel types I, J, and K of the comparative example, but the Ae3 transformation point was low and the rolling temperature was higher than that, so ferrite did not form and high speed axial crushing occurred. This is a comparative example having poor characteristics. Although it was hot-rolled using the steel type K of No. 18 comparative example, it is a comparative example with a low hot rolling temperature and large anisotropy.
Nos. 19 and 20 were hot rolled within the scope of the invention using the steel types L and M of the comparative example, but the amount of residual γ produced was small, and the tensile strength (TS) * elongation (El) This is a comparative example with low characteristics.
Nos. 21 to 24 were obtained by performing hot rolling within the scope of the invention using steel types N, O, P, and Q having chemical components within the scope of the present invention. Ferrite is formed, and there is little anisotropy, and tensile strength (TS) * elongation (El) is also good. Delayed fracture characteristics were also good, and high-speed axial crushing characteristics were also good.

A second embodiment of the invention will be shown below. In this example, the slab (rolling material) of the steel type R having the chemical composition shown in Table 3 in which the amount of Cr and Mo is reduced from the above example (the mode for carrying out the invention and Example 1) is used as a material (comparative example). Example), and this was reheated and hot-rolled to obtain a hot-rolled steel sheet.

Table 4 shows the conditions of hot rolling using the slab of steel type R as a material and the material properties of the hot-rolled steel sheet obtained thereby. After completion of hot rolling, it is allowed to cool for 2 seconds or more and 5 seconds or less, then starts cooling at a cooling rate of 10 ° C / sec or more, and is wound at Bs or less and Ms + 50 ° C or more as described above. .

The two examples (No. 25 and No. 26) shown in Table 4 were produced by rolling only at the finishing temperature as compared with the examples in Table 2, but good results were obtained for both properties. ing. By adjusting the addition amount of Mo and Cr that suppresses recrystallization of γ grains (residual austenite), a high-strength steel sheet with little anisotropy was obtained even when the rolling temperature was lowered. It is also confirmed that the TS * El balance is further improved by increasing the amount of ferrite.
FIG. 4 is a photomicrograph showing the cross-sectional structure of the rolled material (Example 2). It exhibits a double phase similar to that of FIG.

The diagram which shows the concept of the temperature history in the hot rolling and cooling in the manufacturing process of embodiment of this invention. The cross-sectional structure | tissue photograph by the EBSD method of the steel plate manufactured in the range of this invention for a component and rolling conditions. The white (light color) portion of (b) is the retained austenite. (A) is a crystal orientation distribution image, (b) is a crystal structure distribution image. FIG. 3 (a) is a drawing of the test body used in the high-speed axial crushing test, and FIG. 3 (b) shows the appearance of the test body after the high-speed axial crushing test. It is a cross-sectional structure | tissue photograph shown about the rolling material of Example 2. FIG. (A) is an optical microscope structure, (b) is a crystal orientation difference distribution image, and (c) is a crystal structure distribution image.

Claims (9)

  Finished and rolled in a two-phase region of 800 ° C or more and Ae3 temperature or less, the ferrite structure has a grain size of 10 µm or less and the ratio is 5% or more and 70% or less, and the residual austenite structure has a grain size of 2 µm or less and the ratio is 7 A high-strength hot-rolled steel sheet characterized by having a bainite structure with a balance of not less than 20% and not more than 20%. Finished and rolled in a two-phase region of 860 ° C or more and Ae3 temperature or less, the ferrite structure has a grain size of 10µm or less and the ratio is 5% to 35%, the residual austenite structure is grain size of 2µm or less and the ratio is 7 % To 20%, the balance being a bainite structure,
And C: 0.14-0.30%, Si: 1.0-3.5%, Mn: 0.1-0.4%, Cr: 0.5-3.0%, Mo: 0.03-0.6%, with the balance being the composition of iron and inevitable impurities The high-strength hot-rolled steel sheet according to claim 1, wherein The high-strength hot-rolled steel sheet according to claim 2, wherein the component range satisfies the formula (1).
Ae3 = 919-266 * C + 38 * Si-28 * Mn-27 * Ni-11 * Cr + 12 * Mo ≧ 890 (1)   The high strength hot rolling according to claim 2 or 3, further comprising one or two of Ti: 0.02 to 0.20%, Nb: 0.02 to 0.10%, V: 0.02 to 0.20%, and B: 0.0001 to 0.0030%. steel sheet.   The plate thickness is 1.0 mm or more and 4.0 mm or less, the tensile strength is 980 MPa or more, and the product of the tensile strength and the elongation value is 20,000 (MPa ·%) or more. The high-strength hot-rolled steel sheet described in 1. It is finish-rolled in a two-phase region of 800 ° C. or more and 860 ° C. or less instead of the above temperature, and the ferrite structure ratio is 35% or more and 70% or less instead of the above,
The high strength hot-rolled steel sheet according to any one of claims 2 to 5, wherein Cr: 0.5 to 2.0% and Mo: 0.03 to 0.2% are contained instead of Cr and Mo in the above ratio. A method for producing a high-strength hot-rolled steel sheet according to any one of claims 2 to 5,
After roughly rolling a steel material (material slab) of 1200 ° C or higher in the above-mentioned composition range, the cumulative strain of the final stage 3 stands is 0.45 or higher and 860 ° C or higher and Ae3 temperature or lower by a hot rolling mill having multiple stands. A method for producing a high-strength hot-rolled steel sheet, characterized in that final finishing rolling is completed in a two-phase region. It is a manufacturing method of the high intensity | strength hot-rolled steel plate described in Claim 6,
After roughly rolling a steel material (raw material slab) of 1200 ° C or higher in the above component range, the cumulative strain of the final stage 3 stands is 0.45 or higher and 800 ° C or higher and 860 ° C or lower by a hot rolling mill having multiple stands. A method for producing a high-strength hot-rolled steel sheet, characterized in that final finishing rolling is completed in a two-phase region. 8. After completion of finish rolling, after cooling for 2 seconds to 5 seconds, cooling is started at a cooling rate of 10 ° C./sec or more, and winding is performed at Bs or less and Ms + 50 ° C. or more. Or the manufacturing method of the high intensity | strength hot-rolled steel plate of 8. Bs is expressed by equation (2), and Ms is expressed by equation (3).
Bs = 649-83 * C-19 * Si-26 * Mn-30 * Ni-21 * Cr-29 * Mo (2)
Ms = 539-423 * C-30.4 * Mn-17.7 * Ni-12.1 * Cr-7.5 * Mo (3)