JP5339765B2 - High strength hot rolled steel sheet and method for producing the same - Google Patents

Description

  The invention according to the claims relates to a high-strength hot-rolled steel sheet having high workability 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.

When the strength of the steel plate increases, the workability deteriorates, and at the same time, the dimensional accuracy also deteriorates due to the spring back after press forming. It is difficult to apply high-strength steel sheets by cold working methods such as ordinary press forming.
In the hot pressing method, since the hot pressing is performed, the amount of spring back is extremely small and the shape freezing property is good. And the part with very high intensity | strength can be provided with high precision by the hardening effect in the case of a press. However, it is necessary to heat the steel plate before pressing, and an operation to drop the scale is necessary after pressing. Therefore, the work efficiency is very poor. Furthermore, since the mold contacts the heated steel plate, the short life of the mold is also a drawback, which increases the manufacturing cost.
The steel sheet after hot pressing has a small elongation value, and when the member is deformed, it may be broken even if it is slightly deformed. Therefore, it is very difficult to use a hot press part as an important safety part for an automobile or the like.

  One method of improving the dimensional accuracy of a molded product by cold pressing is a method of suppressing the occurrence of springback by providing a bead on the molded product. However, this processing method requires high workability for 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 a low temperature transformation structure is an extremely effective method for producing a steel plate of over 980 MPa, but it cannot be expected to improve the ductility corresponding to 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.
Examples of TRIP steel sheets are shown in Patent Documents 1 and 2. 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.

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 3 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.
JP 60-43425 A JP-A-9-104947 Japanese Patent No. 3247908

  As a conventional technique for improving the elongation characteristics in cold working while having high strength, the composite structure steel plate and the TRIP steel plate can be mentioned.

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.
The TRIP steel sheet of Patent Document 1 is maintained in a temperature range of 450 to 650 ° C. for 4 to 20 seconds in the cooling process after rolling, and after forming ferrite in austenite, it is cooled to 350 ° C. or lower and wound. Produced in the process.
In Patent Document 2, in order to promote the formation of ferrite in the austenite during the cooling process after the end of rolling, mild cooling with Ar3 to Ar1 is performed, or the rolling completion temperature is set to the vicinity of the Ar3 point, and thereafter 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, if C ≦ 0.20%, where spot weldability can be ensured, only a tensile strength of about 800 MPa can be obtained, and it is difficult to produce a steel sheet with a higher strength range that is craving for workability.
Further, after cooling is temporarily stopped in the vicinity of point A1, a step of cooling to 500 ° C. or lower and winding up is required to stabilize martensite and retained austenite structure. Due to heat transfer characteristics, the temperature range of 350 to 500 ° C is an extremely non-uniform region in the cooling of the steel sheet. This causes temperature unevenness on the steel sheet plane, resulting in variations in material characteristics and irregularities in the plate shape. Occurs, and the yield is extremely deteriorated. Furthermore, since the cooling is temporarily stopped in the vicinity of the point A1, it is necessary to increase the length of the cooling equipment 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 after rolling, if the rolling finish temperature is close to the Ar3 point, the formation of fine ferrite can be promoted, but the Ar3 point The material property of the hot rolled steel sheet rolled in the vicinity has a problem of large anisotropy.

Furthermore, the hot-rolled steel sheet described in Patent Document 1 has a low degree of rolling and temporarily stops cooling in the vicinity of the A1 point, and thus exhibits a crystal structure in which coarse ferrite grains and residual austenite grains are adjacent.
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 3 has improved delayed fracture resistance by restricting 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.

  Therefore, the inventors of the present application made the prior austenite grains small in size, and did not perform complicated cooling control, and finely dispersed ferrite, bainite, 1 μm or less residual austenite (5% or more and 20% or less). By obtaining a composite structure, we have developed a new low-alloy / high-strength steel sheet that has both high strength, good workability and delayed fracture resistance, and its manufacturing method.

  As a result of intensive studies, the inventors have found that preferable high-strength steel sheets can be obtained by adopting appropriate rolling conditions and component compositions. That is, a slab having an appropriate component range is high in a low alloy composition by finishing post-stage high strain rolling at a high temperature in a hot rolling finish rolling, starting cooling after a few seconds, and winding up at an appropriate temperature. Strength, excellent ductility and delayed fracture resistance can be simultaneously imparted to the steel sheet. Details are shown below.

The high-strength hot-rolled steel sheet described in the claims has a retained austenite structure size of 1 μm or less, a retained austenite structure ratio (volume ratio) of 5% to 20%, and a martensite structure ratio (volume ratio). ) Is 5% or less, and the balance is a high-strength steel sheet characterized by comprising a ferrite structure and a bainite structure.
By obtaining such a texture, a steel sheet having high strength, good workability, and excellent delayed fracture resistance can be obtained.
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 1 μm or less in size and effectively finely dispersed, delayed fracture resistance can be obtained.
At this time, if the prior austenite grain size, which is the previous structure, is 10 μm or less and the average aspect ratio is 2.0 or less, more desirable characteristics can be obtained. In that case, the anisotropy of the material pulled in the rolling direction and the direction perpendicular to the rolling can be reduced, and further workability can be improved.

The component ranges of the high-strength hot-rolled steel sheet described in the claims are: C: 0.10 to 0.25%, Si: 0.5 to 3.0%, Mn: 0.2 to 1.0%, Cr: 1.0 to 2.5%, Ni: 0.02 to 0.50% The remainder is the composition of iron and inevitable impurities.
In particular, the following formula (1) is satisfied, and further, any one or two of the three types of Mo: 0.1 to 1.0%, Ti: 0.02 to 0.20%, and Nb: 0.02 to 0.10% Or it is preferable that it contains 3 types.
0.40 ≦ 0.3−0.06Nieq + 0.13Creq ≦ 0.85 (1)
However, Nieq = Ni + 30C + 0.5Mn
Creq = Cr + Mo + 1.5Si
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.
The alloying elements are mainly made of chromium and silicon, which have a great influence on the ferrite transformation and bainite transformation, in order to obtain the desirable steel sheet structure of the present invention without complicated cooling control in the cooling process after hot rolling. . By adjusting the amount of these elements, it is possible to control to the intended strength by adjusting the ratio of ferrite and bainite while controlling the amount of ferrite phase formed.
“0.3−0.06Nieq + 0.13Creq” used in the equation (1) is the ferrite volume ratio obtained by observation of the structure of steel sheets of various component compositions, Nieq (Ni equivalent), which is the stability index of austenite, This is an empirical formula arranged by Creq (Cr equivalent), which is an index of stability.
If it is 0.85 or more in the formula (1), since the stability of austenite is low, the ferrite transformation proceeds, it becomes difficult to obtain retained austenite, and the balance between strength and elongation decreases.
When the formula (1) is less than 0.40, it is difficult to obtain a ferrite phase. Ferrite has a very low carbon solubility, and the formation of ferrite during the cooling process causes the carbon to concentrate into untransformed austenite. If the carbon concentration in this austenite is sufficient, untransformed austenite can exist as retained austenite even after cooling is completed. However, if the amount of ferrite formed is insufficient, untransformed austenite cannot contain a sufficient amount of carbon, and is transformed into bainite or martensite in the subsequent cooling process. Therefore, when the formula (1) is less than 0.40, it becomes a bainite main phase with a lot of martensite, a sufficient amount of retained austenite cannot be obtained, and the balance between strength and elongation decreases.
If the formula (1) is between 0.40 and 0.85, it has a sufficient amount of retained austenite to obtain a good balance between strength and elongation, and the structure of the present invention mainly composed of ferrite and bainite. Can be obtained.
In addition, the effect | action of each component is mentioned later.

The above high-strength hot-rolled steel sheet has the above-described structure, a thickness of 1.0 mm to 3.0 mm, a tensile strength TS (MPa) of 980 MPa or more, and the product TS × EL of TS and elongation value 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 high-strength hot-rolled steel sheet according to the claim is:
1) C: 0.10-0.25%, Si: 0.5-3.0%, Mn: 0.2-1.0%, Cr: 1.0-2.5%, Ni: 0.02-0.50%, the balance is the composition of iron and inevitable impurities, A steel material that satisfies the above formula (1) and further contains one, two, or three of Mo: 0.1 to 1.0%, Ti: 0.02 to 0.20%, and Nb: 0.02 to 0.10%. The
2) Heat to 1200 ° C or higher in a heating furnace, and after rough rolling, a hot rolling mill having a plurality of stands will have a cumulative strain of 0.4 or more, or a reduction ratio of 15% or more in the final stand to be used. Hot rolling so that the rolling end temperature is 940 ° C or higher,
3) It is characterized by starting cooling at a cooling rate of 10 ° C./sec or more after being allowed to cool for 1 second or more and 5 seconds after completion of rolling, and winding at 450 ° C. or more and 650 ° C. or less.
According to this manufacturing method, the cooling control is simple and the temperature management is 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 having both strength and processing characteristics, which are mutually contradictory properties, because residual austenite is mixed in a finely dispersed state in a large amount of ferrite and bainite. It is a steel plate with excellent properties.
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 and delayed fracture resistance while having a tensile strength of 980 MPa or more and a manufacturing method thereof will be described.
As a component system of steel sheet, C: 0.10 to 0.25%, Si: 0.5 to 3.0%, Mn: 0.2 to 1.0%, Cr: 1.0 to 2.5%, Ni: 0.02 to 0.50%, the balance being iron and inevitable impurities (1) is satisfied, and Mo: 0.1 to 1.0%, Ti: 0.02 to 0.20%, and Nb: 0.02 to 0.10% are contained. To do.
0.40 ≦ Vα = 0.3−0.06Nieq + 0.13Creq ≦ 0.85 (1)
However, Nieq = Ni + 30C + 0.5Mn
Creq = Cr + Mo + 1.5Si
The thin steel plate described here is a steel plate having a thickness of 1.0 to 3.0 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 0.10 to 0.25% is required.
Carbon is the most important element for stabilizing retained austenite. If it is less than 0.10%, sufficient stability cannot be obtained, so a carbon amount of 0.10% or more is required. On the other hand, when the carbon content is 0.25% 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 when the carbon content is 0.10 to 0.25%, a composite structure in accordance with the gist of the present invention can be obtained.

  The amount of silicon (Si) is in the range of 0.5 to 3.0%. Silicon is also used to stabilize retained austenite. Silicon also has an effect of improving strength by solid solution strengthening. If the amount of silicon is 0.5% or more, the composite structure and material characteristics of the present invention can be obtained. As the amount of silicon increases, the amount of retained austenite can be increased and at the same time the stability thereof is promoted. However, when the silicon amount is 3.0% or more, the strength-ductility balance characteristic is saturated, so the upper limit of silicon amount is set to 3.0% from the viewpoint of cost reduction.

The amount of chromium (Cr) affects the equation (1). The range is 1.0 to 2.5%.
When the amount of chromium is less than 1.0%, the formula (1) cannot be satisfied, and the amount of retained austenite obtained by this production method is lowered. If it is 1.0% or more, an appropriate amount of retained austenite can be obtained, and the composite structure and material characteristics of the present invention can be obtained.
If the chromium content exceeds 2.5%, the steel sheet strength becomes extremely high, so the upper limit was made 2.5% from the viewpoint of cost reduction.

Manganese (Mn) content also affects the equation (1). The range is 0.2 to 1.0%.
If the amount of manganese is less than 0.2%, it becomes difficult to manufacture on steel making, so 0.2% or more.
In order to obtain high strength, it is preferred to add a large amount of manganese. However, if it is too high, martensite is likely to be formed, and the target structure of the present invention cannot be obtained. Therefore, the upper limit of manganese content is set to 1.0%.

  Nickel (Ni) also affects the equation (1). Nickel can improve the strength of steel by solid solution strengthening, but if it is too high, martensite tends to be generated. Furthermore, intentionally adding it causes an increase in cost, so the lower limit was made 0.02% and the upper limit was made 0.5%.

  Molybdenum (Mo), like Cr, can form a bainite structure and improve the strength of the steel, but if added intentionally, the cost increases, so the upper limit was made 1.0%.

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%.

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, hot rolling is performed by a hot rolling mill having a plurality of stands so that the cumulative strain is 0.4 or more, or the rolling reduction in the final stand to be used is 15% 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. In addition, high temperature finish rolling is performed by utilizing the temperature rise of the steel sheet due to processing heat generation.
When the rolling completion temperature becomes low, the prior austenite crystal grains become flat and the average aspect ratio increases. Since the material anisotropy occurs when the aspect ratio of the prior austenite grains increases, the temperature at which the hot rolling is completed is set to 940 ° C. or higher.
Here, “strain” means the difference between the thickness h ₀ of the steel sheet at the entrance side of each stand (each stage) and the thickness h ₁ 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

Complete the hot rolling at 940 ° C or higher, let it cool for 1 second or more and 5 seconds after completion of rolling, then start cooling at a cooling rate of 10 ° C / sec or more and wind it at 450 ° C or higher and 650 ° C or lower. is necessary.
Austenite grains are refined by hot rolling so that the cumulative strain is 0.4 or more, or the rolling reduction in the final stand to be used is 15% or more, and high-temperature finishing is performed, followed by 1 second or more. It is necessary to reduce the dislocation density in the crystal grains in the course of cooling (air cooling) within seconds. By this rolling method, equiaxed and fine prior austenite grains with low dislocation density can be obtained, and the structure obtained in the subsequent cooling (water cooling) process can be made uniform and fine with less directionality. . If the cooling after the end of rolling is too long, pearlite is generated, and the proper structure of the present invention cannot be obtained, so that it is within 5 seconds.
By setting the coiling temperature to a temperature range of 450 to 650 ° C., a structure in which ferrite and bainite are mixed is formed, and austenite also remains. This residual austenite can be dispersed very finely to 1 μm or less if the grain size before transformation is 10 μm or less.
Here, the coiling temperature is set to 450 ° C. or more and 650 ° C. or less. However, in the temperature range of 400 ° C. or less, a lot of martensite structure is generated and delayed fracture is likely to occur. Further, at a temperature of 650 ° C. or higher, ferrite and pearlite structures are formed, and high strength cannot be obtained.
Regarding the coiling temperature, as described above, variation in material characteristics and disturbance of the plate shape due to uneven cooling temperature occur depending on the temperature range. Therefore, the lower limit of the coiling temperature is more preferably 450 ° C. or higher (more preferably 500 ° C. or higher) from the viewpoint of uniform material properties and securing the plate shape.

  FIG. 1 shows the concept of temperature history in hot rolling 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 the relationship between the index (horizontal axis) derived from equation (1), the material properties (TS x EL) of hot-rolled steel sheet manufactured by the manufacturing process of Fig. 1, and the volume fraction Vγ of retained austenite. is there.
In FIG. 2, when the index derived from the equation (1) is less than 0.40, the strength ductility balance (TS × EL) and the retained austenite amount (Vγ) are also low, and both characteristics are between 0.40 and 0.85. Both improve, and at 0.85 or more, the strength ductility balance decreases again.
However, even if the formula (1) is between 0.40 and 0.85, if the amount of carbon (C) is small, sufficient retained austenite cannot be obtained, so that the amount of carbon is less than 0.10 is within the scope of the present invention. Excluded from.

“Vα = 0.3−0.06Nieq + 0.13Creq” used in Eq. (1) is an experimental formula obtained by arranging the volume ratio of ferrite obtained by microstructure observation by Nieq (Ni equivalent) and Creq (Cr equivalent). It is.
In equation (1), Nieq (Ni equivalent) is used as an index for austenite stabilization, while Creq (Cr equivalent) is used as an index for ferrite stabilization. Therefore, it can be said that austenite is easy to be stable if the index obtained by introducing the components of the steel material into the formula (1) is low, and ferrite is easy to stabilize if the index is high.
3A, 3B, and 3C show cross-sectional structures of three types of high-strength steel sheets having different ferrite volume ratios.
The ferrite volume ratio observed from the cross-sectional structure of FIG. 3 is (a) <(b) <(c), and the Vα index obtained from the equation (1) is (a) Vα = 0.21, (b) Vα. = 0.6, (c) Vα = 0.92.

The inventors have conducted intensive research on the development of a steel material that can be easily manufactured in a hot rolling process for commercial production, is not accompanied by a deterioration in yield, and has high strength and high ductility. Accordingly, the steel component composition was introduced into the above formula (1), and it was found that the strength ductility balance was improved if the obtained index was between 0.40 and 0.85. In other words, even if complicated cooling control is not performed in the hot rolling process, by adjusting the stability of austenite and the stability of ferrite to appropriate ranges, it has a sufficient amount of retained austenite, mainly ferrite and bainite. It has been found that it is possible to obtain a high-strength and good-workable steel plate having the structure-structured steel plate of the present invention.
Since the manufacturing process of the present invention does not require complicated cooling control, it is easy to manufacture and does not require a long cooling equipment length. In addition, the plate shape is also taken up in the temperature range where cooling begins to become uneven. Good and good yield.

  FIG. 4 shows a structural cross section of the steel of the present invention in which the body-centered cubic ferrite or bainite phase and the face-centered cubic austenite phase are color-coded using the EBSP method. It can be observed that the retained austenite structure shown in white (light color) is finely and uniformly dispersed to 1.0 μm or less.

FIG. 5 is an observation result of prior austenite grains of the steel of the present invention by SEM structure observation. The prior austenite grains have an average grain size of 9.3 μm and a uniform sized structure, and the major axis / minor axis average aspect ratio is 1.7.
The present invention has been developed based on the above findings.

A first embodiment 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. Table 2 shows the hot rolling conditions and the material properties.

  Steel types A, B, C, D, F, G, and H shown in Table 1 are comparative examples, and E, I, J, K, and L are within the scope of the present invention (the optimum range that satisfies all the conditions for chemical components). It is an example.

Steel types A, B, C, and D of the comparative examples mainly improve the strength by manganese (Mn), and the nickel equivalent is high and the chromium equivalent is low, so from the scope of the present invention in terms of formula (1). It's out of place.
Steel type F has a carbon content that is outside the range of the present invention, and even if the formula (1) is within the range of the present invention, a sufficient retained austenite amount of 5% or more cannot be obtained.
Steel type G has a high chromium content but a low silicon content, so that a sufficient chromium equivalent cannot be obtained, and a nickel content is high, so it is out of the range of equation (1).
Steel grade H has a sufficient amount of chromium and silicon that affects the chromium equivalent, but in addition to the low carbon content, the high chromium equivalent makes the range of the upper limit of formula (1). It is a comparative example that exceeded.
Steel types E, I, J, K, and L are examples of the present invention that satisfy the formula (1) without the problem as in the above comparative example.

Nos. 1 to 4 and 8 to 10 in Table 2 are hot-rolled using the steel types of comparative examples that are out of the component range of the present invention.
Steel types A, B, C, and D of No. 1 to 4 deviate from the lower limit of the formula (1), and the austenite is easily stabilized. When the cross-sectional structure of a steel sheet obtained by rolling these steel types at around 550 ° C. after hot rolling is observed, the main phase is upper bainite and the amount of residual austenite observed is small. In addition to upper bainite as the main phase, the amount of retained austenite is small, so the tensile elongation (EL) characteristics are low, and the balance between strength and ductility is low.
No. 8 is the one produced by using steel type F with a low carbon content within the scope of the present invention (1). The amount of retained austenite obtained is not sufficient, and the strength / ductility balance cannot be so high.
No. 9 was manufactured using steel type G, and as described above, the lower limit of the formula (1) was exceeded, and austenite tends to be stabilized. However, unlike the above steel types A to D, the steel type G is high in both the amount of chromium and nickel and has high hardenability, so that the resulting texture has a large amount of martensite. The amount of retained austenite is low as in Nos. 1 to 4, so that the balance between strength and ductility is poor.
No. 10 uses a steel type H that deviates from the upper limit of equation (1). Steel type H has high ferrite stability, and the resulting structure is mainly composed of a ferrite phase. However, since the amount of elements that stabilize retained austenite is small, the amount is also low. As a result, the balance between strength and ductility is low.

Nos. 5 to 7 are examples of rolling using a steel type E that satisfies the formula (1).
No. 5 was rolled under the proper conditions of the present invention (optimum satisfying all the above-mentioned conditions), and a good balance between strength and ductility can be obtained. On the other hand, No. 6 was hot-rolled with the finishing temperature in the vicinity of the Ar3 point, but the aspect ratio of the prior austenite was high, and as a result, the rolling longitudinal (L) direction and the rolling perpendicular (C) direction There is a disparity in tensile elongation characteristics. When rolling is completed in the vicinity of the Ar3 point, grains extending in the longitudinal direction of the rolling are formed, which causes anisotropy in tensile properties.
In No. 7, the hot rolling coiling temperature was set to 450 ° C. or less, and the plate shape was slightly deteriorated. At 450 ° C or lower, uniform cooling is difficult, and it seems to be caused by temperature variation.

Nos. 12 to 15 are examples of rolling using a steel type J that satisfies equation (1).
Nos. 12 and 13 were manufactured under appropriate hot rolling conditions, and good characteristics were obtained. On the other hand, No. 14 was produced under the same rolling conditions as No. 13, but with a slightly reduced reduction in finish rolling. The structure obtained under the rolling conditions of No. 14 has a coarse prior austenite grain size and a coarse residual austenite grain size of about 3 μm. In both conditions, almost the same tensile properties are obtained, but in No. 14, delayed fracture occurs. This is thought to be due to insufficient segregation of diffusible hydrogen, which is the cause of delayed fracture, due to the lack of fine dispersion of the retained austenite structure due to insufficient reduction in finish rolling.
No. 15 has a coiling temperature after hot rolling of 450 ° C. or lower, 5% or more of martensite phase is observed, and shows a structural property in which retained austenite and martensite are mixed. The steel sheet has delayed fracture and is considered to be caused by the fact that the strength is too high. In addition, as in No. 7, this is an example of plate shape disturbance.

Nos. 17 to 19 are examples of rolling using a steel type L that satisfies the formula (1).
No. 17 was obtained by hot rolling under appropriate conditions, and good characteristics were obtained, while No. 18 was completed in the vicinity of Ar 3 as in No. 6. Again, the aspect ratio of the prior austenite grains is high, and there is a difference in the tensile elongation characteristics between the rolling longitudinal (L) direction and the rolling vertical (C) direction.
In No. 19, the coiling temperature after hot rolling was set to 400 ° C. or lower, martensite became the main phase, the amount of retained austenite decreased, and delayed fracture, which is considered to be caused by the coexisting structure, occurred. In addition, as in Nos. 7 and 15, this is an example in which the plate shape is disturbed.

  In addition, Nos. 11 and 16 used steel types I and K that satisfy the formula (1), respectively, and rolling was performed under the conditions of the present invention, and good characteristics such as tension and plate shape were recognized. It is.

The volume fraction of the ferrite grains was measured by using a commercially available image analysis apparatus after polishing a cross section in the rolling direction of the steel sheet, observing with a light microscope after nitral corrosion.
The volume ratio (%) of martensite is obtained by polishing a cross section in the rolling direction of the steel sheet, etching with a mixture of 4% picric alcohol and 2% sodium pyrosulfate in a one-to-one relationship, and the position in the thickness direction 1/4. Was observed with an optical microscope, and martensite etched in white by image analysis processing was measured and determined.
The residual austenite was measured by an X-ray diffraction method using Cu Kα rays. After surface electropolishing finish at 1 / 2t thickness, the integrated strength of the (200) (220) (311) face of the austenite phase and the (200) (211) face of the ferrite phase is measured and calculated from the respective combinations. The average value of the retained austenite volume fraction was used.
The grain size of the retained austenite was measured by image analysis after observing the position in the thickness direction 1/4 with EBSP.
For the prior austenite grains, the position in the thickness direction 1/4 was observed by SEM. After picric acid corrosion, the prior austenite grains were identified by visual discrimination, and the grain size and aspect ratio were measured.
Tensile properties (tensile strength TS, elongation value EL) were measured by tensile testing in the shape of JIS No. 5 test piece.
Delayed fracture resistance is obtained by immersing a sample loaded with a strain equivalent to 800 MPa in 0.5 mol / L sulfuric acid, charging the cathode with a constant current density of 4.0 mA / cm ^2, and resisting cracking within 2 hours. Delayed fracture property was evaluated as “× bad”, and those that did not occur were evaluated as “good”.

Next, a second embodiment of the invention will be described.
In this example, the chemical composition and hot rolling conditions previously shown in Table 1 and Table 2 were changed to those shown in Table 3 and Table 4, respectively, and other manufacturing conditions were the same to manufacture a hot-rolled steel sheet. . Table 4 also shows the structure observation results and material characteristics of the obtained steel sheet measured by the same method as in the first example.

The chemical components of steel types M and N shown in Table 3 are those in which the amount of Si is increased as compared with the steel types shown in Table 1, and both satisfy the component ranges specified by the invention.
Steel plates Nos. 20 and 21 shown in Table 4 were obtained by rolling the above steel types M and N within the scope of the invention, and confirmed good results with respect to any of the properties.
It can be seen that the desired characteristics can be sufficiently obtained if the balance with Cr or the like is taken into consideration even if the Si content is increased to about 3.0% within the range satisfying the formula (1).
FIG. 6 shows a photograph of the structure of the rolled material obtained in this way, and it is confirmed that it has a multiphase structure similar to (b) of FIG.

As described above, the high-strength steel sheets of the examples showing high strength and high ductility characteristics in a low alloy composition are suitable for use as automobile structural members and the like.
For example, the center pillar of an automobile requires the tensile strength necessary to prevent deformation at the time of collision as well as the support of the door, as well as bending, drawing workability for press molding, etc., to form mounting holes for related equipment It can be said that it is a very preferable steel plate as a member that requires a high level of hole expandability and further weldability for joining with other body parts.

The diagram which shows the concept of the temperature history in the hot rolling and annealing in the manufacturing process of embodiment of this invention. (1) Relationship between formula index, strength / ductility balance and retained austenite amount. A typical cross-sectional structure photograph of a material that falls outside the lower limit, within the range, or out of the upper limit of equation (1). The cross-sectional structure | tissue photograph by the EBSP method of the steel plate manufactured in the range of this invention for a component and rolling conditions. The white part of the photograph (light color) is retained austenite. SEM observation of steel sheet cross section manufactured based on the manufacturing method of the present invention and identification results of prior austenite grains It is a typical cross-sectional structure | tissue photograph of the steel plate cross section manufactured based on this invention.

Claims (5)

C: 0.10 to 0.25% by mass, Si: 0.5 to 3.0%, Mn: 0.2 to 1.0%, Cr: 1.0 to 2.5%, Ni: 0.02 to 0.50%, the balance being the composition of iron and inevitable impurities The retained austenite grain size is 1 μm or less, the retained austenite structure ratio is 5% or more and 20% or less, the martensite structure ratio is 5% or less, and the balance is composed of a ferrite structure and a bainite structure,
Satisfy the formula (1) as a component range,
0.40 ≦ 0.3−0.06Nieq + 0.13Creq ≦ 0.85 (1)
However, Nieq = Ni + 30C + 0.5Mn
Creq = Cr + Mo + 1.5Si
The prior austenite grain size is 10 μm or less and the average aspect ratio is 2.0 or less ;
A high-strength hot-rolled steel sheet having a tensile strength of 980 MPa or more and a product of tensile strength and elongation of 20000 (MPa ·%) or more .   The high-strength heat according to claim 1, further comprising one, two, or three of Mo: 0.1 to 1.0%, Ti: 0.02 to 0.20%, and Nb: 0.02 to 0.10% by mass%. Rolled steel sheet. High strength hot rolled steel sheet according to claim 1 or 2, wherein the thickness is 3.0mm from 1.0 mm. A method for producing a high-strength hot-rolled steel sheet according to claim 1,
Contains C: 0.10 to 0.25% by mass, Si: 0.5 to 3.0%, Mn: 0.2 to 1.0%, Cr: 1.0 to 2.5%, Ni: 0.02 to 0.50%, the balance being the composition of iron and inevitable impurities , (1) A steel material having a composition range that satisfies the formula (1) is heated to 1200 ° C. or higher in a heating furnace, and after rough rolling, a hot rolling mill having a plurality of stands has a cumulative strain of 0.4 or more and is used as a final stand. The steel is hot-rolled so that the rolling reduction at 15% or higher and the rolling end temperature is 940 ° C or higher. After the rolling is completed, it is allowed to cool for 1 second or longer and within 5 seconds, and then cooled at a cooling rate of 10 ° C / sec or higher. And a method for producing a high-strength hot-rolled steel sheet, which is wound at 450 ° C. or higher and 650 ° C. or lower.
0.40 ≦ 0.3−0.06Nieq + 0.13Creq ≦ 0.85 (1)
However, Nieq = Ni + 30C + 0.5Mn
Creq = Cr + Mo + 1.5Si A method for producing a high-strength hot-rolled steel sheet according to claim 2,
Contains C: 0.10 to 0.25% by mass, Si: 0.5 to 3.0%, Mn: 0.2 to 1.0%, Cr: 1.0 to 2.5%, Ni: 0.02 to 0.50%, the balance being the composition of iron and inevitable impurities In addition, the range of components which satisfy the formula (1) and further contains one, two or three of Mo: 0.1 to 1.0%, Ti: 0.02 to 0.20%, Nb: 0.02 to 0.10% The steel material is heated to 1200 ° C. or higher in a heating furnace, and after rough rolling, a hot rolling mill having a plurality of stands has a cumulative strain of 0.4 or more and a reduction ratio of 15% or more in the final stand to be used. Hot-rolled so that the rolling end temperature is 940 ° C or higher, and after cooling is completed for 1 second or more and allowed to cool within 5 seconds, starts cooling at a cooling rate of 10 ° C / sec or more, 450 ° C or more, 650 ° C A method for producing a high-strength hot-rolled steel sheet, which is wound up below.
0.40 ≦ 0.3−0.06Nieq + 0.13Creq ≦ 0.85 (1)
However, Nieq = Ni + 30C + 0.5Mn
Creq = Cr + Mo + 1.5Si