JP6828702B2 - Steel plate and its manufacturing method - Google Patents

Description

INDUSTRIAL APPLICABILITY The present invention is suitably used for steel plates having excellent impact resistance, particularly welded structures such as ships, marine structures, bridges, buildings, tanks and construction machines, which are strongly required to have structural safety. The present invention relates to a steel sheet having a high ability to suppress damage at the time of a collision and a manufacturing method thereof.

Strengthening and thinning of steel materials used for structures such as ships, marine structures, bridges, buildings, tanks and construction machines for the purpose of rationalizing the design of the structures and reducing the weight of steel materials used. Long materials have been applied more and more. In addition to being excellent in mechanical properties such as strength and toughness and weldability, such steel materials have excellent impact energy absorption capacity in order to ensure the structural safety of the structure against external forces. May be required.

In particular, the above demands are increasing for ships. That is, if there is a hull damage due to a collision between ships or a grounding, cargo, fuel, etc. will flow out and cause marine pollution. Therefore, as a technology to minimize such damage, although efforts are being made from the structural aspect such as double structuring of the hull, it is not possible to apply such a structure to all the structures of the ship. It is not realistic in terms of workability and manufacturing cost. Therefore, it is desired that the steel plate for the hull itself has an energy absorbing ability to prevent the hull from being destroyed in the event of a ship collision.

Here, Patent Document 1 states that the volume fraction of the ferrite phase is 75% or more in the entire plate thickness direction, the hardness is Hv140 or more and 160 or less, the average crystal grain size is 2 μm or more and 40 μm or less, and ferrite in the central portion in the plate thickness direction. By setting the ratio of the volume fraction of the ferrite phase to the volume fraction of the phase to 0.925 or more and 1.000 or less in the surface layer portion in the plate thickness direction, a steel material having increased uniform elongation and improved collision resistance can be obtained. Have been described.

Further, in Patent Document 2, after rolling with a cumulative reduction ratio of 30 to 98% in the austenite single phase region, accelerated cooling is performed, and then air cooling or tempering treatment is performed, and the area ratio of ferrite is 85% or more. A steel sheet having an improved ability to absorb collision energy, in which the average crystal grain size of ferrite is 5 to 40 μm and the number of cementite particles in the ferrite particles is 50,000 / mm ² or less, is described.

Further, in Patent Document 3, acceleration cooling is performed after final pass rolling (finish rolling), then reheating to a constant temperature and acceleration cooling are performed again to obtain the space factor, average particle size and maximum particle size of ferrite. Further, a steel material in which the size of the second phase is controlled and the collision absorption property is improved is described.

Japanese Patent No. 59535952 Japanese Patent No. 4772932 Japanese Patent No. 4476923

However, the prior art described in Patent Documents 1 to 3 has the following problems.
That is, in the method described in Patent Document 1, a steel sheet is manufactured by an online process of rolling and accelerated cooling control. Therefore, especially in thin long materials having a plate thickness of 25 mm or less, temperature deviations at the tip and tail ends of the steel sheet are likely to occur during hot rolling and accelerated cooling, and stable mechanical properties are maintained over the entire length. I can't get it.

Further, in the methods described in Patent Documents 2 and 3, a steel sheet is manufactured by combining heating, hot rolling, accelerated cooling and heat treatment. However, since the reheating temperature is in the tempering temperature range below the recrystallization temperature, the structure varies due to the cooling deviation after the accelerated cooling following the hot rolling described above, and the structure is stable over the entire length of the steel sheet. It is not possible to obtain excellent mechanical properties. These tendencies are particularly remarkable when the plate thickness is as thin as 25 mm or less.

As described above, conventionally, it has been difficult to provide a steel sheet having impact resistance over the entire length of the steel sheet.
The present invention has been made in view of the above circumstances, and provides a steel sheet having excellent impact resistance over the entire length and a method for manufacturing the same, particularly even when the plate thickness is as thin as 25 mm or less. The purpose.

The present inventors have investigated in detail the effects of chemical components and manufacturing conditions on impact resistance of steel sheets manufactured by hot rolling, and obtained the following findings.
(1) The steel sheet after the hot rolling is completed and cooled has a structure variation due to a cooling deviation, and this structure variation can be eliminated by reheating in the two-phase region.
(2) Even when the plate thickness is thin, the particle size distribution in the plate thickness direction can be controlled by controlling the cooling pattern after the reheating treatment in the two-phase region, and the particle size distribution is high over the entire length. The elongation can be secured and the impact resistance can be guaranteed.

The present invention has been made based on the above findings, and the gist structure thereof is as follows.
1. 1. By mass%
C: 0.05 to 0.16%,
Si: 0.10 to 0.50%,
Mn: 0.80-1.60%,
It contains P: 0.05% or less and S: 0.02% or less, and the balance has a component composition of Fe and unavoidable impurities.
The microstructure in the surface layer portion from the surface of the steel sheet to 200 μm in the plate thickness direction contains a ferrite phase having an area ratio of 80% or more.
The microstructure of the intermediate portion other than the surface layer portion contains a ferrite phase having an area ratio of less than 80%, and the balance is composed of one or both of the bainite phase and the pearlite phase.
A steel sheet in which the average crystal grain size SD (μm) in the surface layer portion and the average crystal grain size CD (μm) in the intermediate portion satisfy the following equation (1).
Note 2.0 ≤ SD / CD ... (1)

2. 2. The composition of the components is further increased by mass%.
Cu: 1.0% or less,
Ni: 2.0% or less,
Cr: 1.0% or less,
Mo: 1.0% or less,
Nb: 0.1% or less,
V: 0.1% or less,
Ti: 0.1% or less,
B: 0.005% or less,
The steel sheet according to 1 above, which contains one or more selected from the group of Ca: 0.005% or less and W: 0.05% or less.

3. 3. The steel material having the component composition described in 1 or 2 is heated to 900 to 1200 ° C., and then hot-rolled to obtain a steel sheet having a cumulative reduction rate of 50% or more, and the steel sheet is cooled and then Ac ₁ transformed. The reheated steel sheet is reheated to a temperature range above the point and below the Ac ₃ transformation point, cooled to a cooling stop temperature of 400 ° C. to 600 ° C. at an average cooling rate of 3 to 20 ° C./s, and then hardened. Steel sheet manufacturing method.

According to the present invention, it is possible to provide a steel sheet having high uniform elongation over the entire length and having excellent impact resistance. Since the steel sheet of the present invention has a high energy absorption capacity with respect to an external force, the structure using the steel sheet has increased safety as a structure, which is extremely useful industrially.

Next, the steel sheet of the present invention will be specifically described. The following description shows a preferred embodiment of the present invention, and the present invention is not limited to the following description.
[Ingredient composition]
The steel sheet of the present invention and the steel material used for producing the steel sheet need to have the above-mentioned component composition. The reasons for limiting the amount of each component in the component composition will be described below. In addition, "%" in the following description shall represent "mass%" unless otherwise specified.
C: 0.05 to 0.16%
C is an element having an effect of increasing the hardness of the matrix phase and improving the strength. In order to obtain the above effect, it is necessary to set the C content to 0.05% or more. On the other hand, when the C content exceeds 0.16%, the hardness of the matrix phase increases excessively and the elongation deteriorates. Therefore, the C content is set to 0.16% or less. Preferably, it is 0.07 to 0.15%.

Si: 0.10 to 0.50%
Si is an element that acts as an antacid and dissolves in steel to increase the hardness of the matrix phase by solid solution strengthening. In order to obtain the above effect, the Si content needs to be 0.10% or more. On the other hand, when the Si content exceeds 0.50%, the hardness of the matrix phase is excessively increased, the ductility and toughness are decreased, and the amount of inclusions which are the starting points of voids due to local deformation increases. Therefore, the Si content is set to 0.50% or less. It is preferably 0.25 to 0.40%.

Mn: 0.80-1.60%
Mn is an element having the effect of increasing the hardness of the matrix phase and improving the strength. In order to obtain the above effect, the Mn content needs to be 0.80% or more. On the other hand, when the Mn content exceeds 1.60%, the weldability is lowered and the hardness of the matrix phase is excessively increased. Therefore, the Mn content is set to 1.60% or less. Preferably, it is 1.00 to 1.50%.

P: 0.05% or less P is an element contained in steel as an unavoidable impurity. P is preferably reduced as much as possible because it segregates at the grain boundaries and adversely affects the toughness of the base metal and the welded portion. However, a content of 0.05% or less is acceptable. Therefore, the P content is set to 0.05% or less. On the other hand, although the lower limit of the P content is not limited, the P content is preferably 0.001% or more because an excessive reduction causes an increase in the refining cost.

S: 0.02% or less S is an element contained in steel as an unavoidable impurity. Since S is an element that exists in steel as a sulfide-based inclusion such as MnS and has an adverse effect such as becoming a starting point of fracture, it is preferable to reduce it as much as possible, but the content of 0.02% or less is contained. acceptable. Therefore, the S content is set to 0.02% or less. The S content is preferably 0.01% or less. On the other hand, although the lower limit of the S content is not limited, it is preferable that the S content is 0.0005% or more because an excessive reduction causes an increase in the refining cost.

The component composition of the steel sheet of the present invention contains the above components, and the balance is Fe and unavoidable impurities.
When O (oxygen) and N are contained as unavoidable impurities, it is preferable to suppress the O content to 0.0050% or less and the N content to 0.0050% or less. That is, when the O content exceeds 0.0050%, the abundance ratio of inclusions on the surface of the steel sheet becomes large, so that cracks starting from inclusions are likely to occur, and the elongation may decrease. is there. Similarly, if the content of non-N is more than 0.0050%, the abundance ratio of inclusions on the surface of the steel sheet becomes large, so that cracks starting from inclusions may easily occur.

Further, in the present invention, in addition to the above component composition, Cu: 1.0% or less, Ni: 2.0% or less, Cr: 1.0% or less, Mo: 1.0% or less, Nb, if necessary. : 0.1% or less, V: 0.1% or less, Ti: 0.1% or less, B: 0.005% or less, Ca: 0.005% or less, and W: 0.05% or less. One or more selected from the above can be further optionally contained.

Cu: 1.0% or less Cu is an element having an effect of increasing the hardness of the matrix phase and improving the weather resistance of the steel sheet, and can be arbitrarily added according to desired characteristics. However, if the Cu content exceeds 1.0%, the weldability is impaired and defects are likely to occur during the production of steel materials. Therefore, when Cu is added, the content is 1.0% or less. More preferably, it is 0.01 to 0.8%.

Ni: 2.0% or less Ni is an element that has the effect of improving low temperature toughness and weather resistance, and also improving hot brittleness when Cu is added, and can be arbitrarily added according to the desired properties. Can be done. However, if the Ni content exceeds 2.0%, the weldability is impaired and the steel material cost increases. Therefore, when Ni is added, it should be 2.0% or less. More preferably, it is 0.01 to 1.5%.

Cr: 1.0% or less Cr is an element having an effect of increasing the hardness of the matrix phase and improving the weather resistance, and can be arbitrarily added depending on the desired properties. However, if the Cr content exceeds 1.0%, weldability and toughness are impaired. Therefore, when Cr is added, the content is 1.0% or less. More preferably, it is 0.01 to 0.8%.

Mo: 1.0% or less Mo is an element having an effect of increasing the hardness of the matrix phase, and can be arbitrarily added depending on the desired properties. However, if the Mo content exceeds 1.0%, weldability and toughness are impaired. Therefore, when Mo is added, the content is 1.0% or less. More preferably, it is 0.001 to 0.8%.

Nb: 0.1% or less Nb is an element that has the effect of suppressing recrystallization of austenite during hot rolling to make it finer and precipitating it in the air cooling process after hot rolling to increase its strength. Yes, it can be optionally added depending on the desired properties. However, if the Nb content exceeds 0.1%, a large amount of NbC is precipitated and the toughness is impaired. Therefore, when Nb is added, it should be 0.1% or less. More preferably, it is 0.001 to 0.08%.

V: 0.1% or less V, like Nb, has the effect of suppressing recrystallization of austenite during hot rolling to make it finer, and increasing the strength by precipitating in the air cooling process after hot rolling. It is an element having the above, and can be arbitrarily added depending on the desired properties. However, if the V content exceeds 0.1%, a large amount of VC is precipitated and the toughness is impaired. Therefore, when V is added, it should be 0.1% or less. More preferably, it is 0.001 to 0.08%.

Ti: 0.1% or less Ti has a strong tendency to form a nitride and fixes N to reduce solid solution N, so that Ti has an effect of improving the toughness of the base metal and the welded portion. Further, when B is added, by adding Ti together, it is possible to prevent Ti from fixing N and B from precipitating as BN. As a result, the hardenability improving effect of B can be promoted, and the strength can be further improved. Therefore, it can be arbitrarily added according to the desired properties. However, if the Ti content exceeds 0.1%, a large amount of TiC is precipitated and the toughness is impaired. Therefore, when Ti is added, it should be 0.1% or less. More preferably, it is 0.001 to 0.08%.

B: 0.005% or less B is an element having an effect of significantly improving hardenability and increasing strength even when added in a small amount, and can be added according to desired properties. However, when the B content exceeds 0.005%, not only the effect is saturated but also the weldability is lowered. Therefore, when B is added, it should be 0.005% or less. More preferably, it is 0.0001 to 0.004%.

Ca: 0.005% or less Ca binds to S, suppresses the formation of MnS and the like that extend long in the rolling direction, controls the shape so that the sulfide-based inclusions have a spherical shape, and improves the toughness of welds and the like. Can be added according to the desired properties. However, when the Ca content exceeds 0.005%, not only the effect is saturated, but also the cleanliness of the steel is lowered, surface defects occur frequently, and the surface texture is deteriorated. Therefore, when Ca is added, it should be 0.005% or less. More preferably, it is 0.0001 to 0.004%.

W: 0.05% or less W increases the hardness of the matrix phase and improves the weather resistance, so that it can be added according to the desired properties. However, if the W content exceeds 0.05%, the weldability deteriorates or the alloy cost increases. Therefore, when W is added, it should be 0.05% or less. More preferably, it is 0.0001 to 0.03%.

[Micro tissue]
Next, the reason for limiting the microstructure of the steel sheet as described above will be described. In addition, "%" in the description of microstructure shall indicate the area ratio unless otherwise specified. Further, the "tip" of the steel sheet in the following description is defined as a position 500 mm from the tip in the rolling direction of the steel sheet to the tail end side. Similarly, the "tail end" of a steel sheet is defined as a position 500 mm from the tail end in the rolling direction of the steel sheet to the tip end side.

-Surface layer structure The microstructure in the surface layer part (hereinafter, simply referred to as the surface layer part) from the surface of the steel sheet to 200 μm in the plate thickness direction shall contain a ferrite phase having an area ratio of 80% or more. That is, by generating 80% or more of ferrite in the surface layer portion to soften the surface layer of the steel sheet, the elongation characteristics in the entire area in the thickness direction of the steel sheet (hereinafter referred to as the elongation characteristics at the total thickness) can be remarkably improved. it can. This is because if the area ratio of the ferrite phase in the surface layer portion is less than 80%, a large amount of a hard residual structure composed of a bainite phase, a pearlite phase, a martensite phase, or a mixed phase thereof will be present. As a result, the hardness of the surface layer portion increases, and it is not possible to obtain the desired elongation characteristics at the total thickness. In addition, the tensile strength may become excessive, and it is not possible to obtain the desired mechanical properties at the total thickness.

Here, the area ratio of the ferrite phase in the surface layer portion refers to the average value of the area ratio of the ferrite phase in the surface layer portion. Specifically, it can be measured according to the method described later. The area ratio of the ferrite phase in the surface layer portion thus obtained is 80% or more, which means that the microstructure of the surface layer portion at the three positions of the tip, the center and the tail end of the steel sheet is 80% or more, respectively. .. That is, if the microstructures of the surface layer portion at the three positions of the tip, the center and the tail of the steel sheet satisfy the above conditions, the above conditions are satisfied over the entire length of the steel sheet in the rolling direction. In other words, in the steel sheet of the present invention, the area ratio of the ferrite phase in the surface layer portion is 80% or more over the entire length in the rolling direction. Needless to say, the above provisions apply to the surface layer portions on both sides of the steel sheet.

The remainder of the surface layer portion other than the ferrite phase is generally composed of a bainite phase, a pearlite phase, or a mixed layer of a bainite phase and a pearlite phase.

-Intermediate part structure An intermediate part between a position 200 μm in the plate thickness direction from the surface of the steel plate and a position of 1/2 of the plate thickness, that is, an intermediate part other than the surface layer part (hereinafter, simply “intermediate part””. The microstructure in () contains a ferrite phase with an area ratio of less than 80%, and the balance is a bainite phase, a pearlite phase, or a mixed layer of a bainite phase and a pearlite phase. If the microstructure at the center of the plate thickness does not satisfy the above conditions, the desired strength and uniform elongation cannot be obtained.

Here, the area ratio of the ferrite phase in the intermediate portion refers to the average value of the area ratio of the ferrite phase in the intermediate portion. Specifically, it is obtained according to the method described in Examples described later. The area ratio of the ferrite phase in the intermediate portion thus obtained is less than 80%, which means that the microstructure of the intermediate portion at each of the three positions of the tip, the center and the tail of the steel sheet is less than 80%. .. That is, if the microstructures of the intermediate portions at the three positions of the tip, the center and the tail of the steel sheet satisfy the above conditions, the above conditions are satisfied over the entire length of the steel sheet in the rolling direction. In other words, in the steel sheet of the present invention, the microstructure in the intermediate portion contains a ferrite phase having an area ratio of less than 80% over the entire length in the rolling direction, and the balance is a bainite phase, a pearlite phase, or a bainite phase and a pearlite phase. It is a mixed layer with.

The microstructure in the surface layer portion and the intermediate portion can be evaluated by the method described in Examples described later.

[Crystal grain size]
In addition to having the above-mentioned composition and microstructure, the steel sheet of the present invention has an average crystal grain size SD (μm) in the surface layer portion and an average crystal grain size CD (μm) in the intermediate portion according to the following (1). It is important to satisfy the formula.
2.0 ≤ SD / CD ... (1)
By satisfying the condition of the above formula (1), the elongation property is improved particularly in a thin material which is required to have excellent elongation property at the total thickness. That is, if the SD / CD is less than 2.0, there are the following problems. For example, when the SD / CD is less than 2.0 as a result of the coarse crystal grains in the central portion of the plate thickness, a region having low brittleness is locally generated in the central portion of the plate thickness where the crystal grains are large, so that the brittleness is low. Voids, which are the starting points of fissures or ductile fractures, are likely to occur. Further, when the SD / CD is less than 2.0 as a result of the crystal grains in the surface layer portion being fine, the crystal grains in the surface layer portion are hardened by miniaturization, so that the elongation in the total thickness tensile test is reduced and the tensile strength is reduced. The strength is greater than desired. Therefore, SD / CD is set to 2.0 or more. It is preferably 2.2 or more.

The average crystal grain size SD in the surface layer portion is preferably 10 to 200 μm. When the average crystal grain size SD in the surface layer portion is less than 10 μm, the tensile strength becomes excessive than desired due to the increase in the yield stress due to the fine graining. Further, when the SD exceeds 200 μm, a region having low brittleness is locally generated, so that a void serving as a brittle crack or a ductile fracture starting point is likely to occur.
Similarly, the average crystal grain size CD in the intermediate portion is preferably 5 to 50 μm.
When the average crystal grain size CD in the intermediate portion is less than 5 μm, the tensile strength becomes excessiveer than desired due to the increase in the yield stress due to the granulation. Further, when the SD exceeds 50 μm, a region having low brittleness is locally generated, so that a void serving as a brittle crack or a ductile fracture starting point is likely to occur.

In the present invention, the condition of the above equation (1) shall be satisfied at all three locations of the tip, the center and the tail of the steel sheet in the rolling direction. That is, if the three points of the tip, the center and the tail of the steel sheet satisfy the condition of the above formula (1), the above condition is satisfied over the entire length of the steel sheet in the rolling direction. The SD and CD can be measured by the method described in Examples described later.

[Tensile strength]
The tensile strength (TS) of the steel sheet is not particularly limited, but is preferably 440 MPa or more. Further, the upper limit of TS is not particularly limited, but for example, in the case of 490 MPa (50 kgf / mm ² ) class in JIS, TS may be 610 MPa or less. Further, in the case of 570 MPa (60 kgf / mm ² ) class in JIS, the upper and lower limits of TS may be 570 MPa and 720 MPa, respectively. In the present invention, it is preferable that the above TS conditions are satisfied at all three locations of the tip, center and tail of the steel sheet in the rolling direction. That is, if the three points of the tip, the center, and the tail of the steel sheet satisfy the above conditions, the above conditions are satisfied over the entire length of the steel sheet in the rolling direction. The TS can be measured by the method described in Examples.

[Plate thickness]
The term "steel plate" in the present invention refers to a steel plate having a thickness of 6 mm or more according to the usual definition in the present art. On the other hand, the upper limit of the plate thickness of the steel plate in the present invention is not particularly limited and can be any value. However, as described above, the effect of the present invention is particularly remarkable in a thin material in which the temperature deviation at the tip and tail ends of the steel sheet tends to be large and excellent elongation characteristics at the total thickness are required. Therefore, the thickness of the steel sheet is preferably 25 mm or less, and more preferably 20 mm or less.

[Production method]
In one embodiment of the present invention, a steel material having the above-mentioned composition is sequentially subjected to the following treatments to obtain a steel sheet.
(1) Heating (2) Hot rolling (3) Cooling (4) Reheating (5) Cooling (6) Quenching

[Steel material]
As the steel material, any material having the above-mentioned composition and capable of hot rolling can be used, but usually a steel slab may be used. For example, molten steel having the above component composition can be melted by means such as a converter and used as a steel material such as a slab by a casting method such as a continuous casting method. Further, a steel material such as a slab can be used by the ingot-decomposition rolling method.

[heating]
The steel material having the above composition is heated to 900 to 1200 ° C. If the heating temperature is less than 900 ° C., the deformation resistance of the steel material in the next hot rolling step increases, the load on the hot rolling mill increases, and hot rolling becomes difficult. Therefore, the heating temperature is set to 900 ° C. or higher. The heating temperature is preferably 950 ° C. or higher. On the other hand, when the heating temperature exceeds 1200 ° C., not only the crystal grains in the middle portion of the steel sheet become coarse and the toughness decreases, but also the slab surface is significantly oxidized and the unevenness of the base iron-scale interface becomes sharp. , Surface irregularities tend to remain even after the product. Such surface irregularities may become the starting point of ductile fracture due to stress concentration. Therefore, the heating temperature is set to 1200 ° C. or lower. The temperature is preferably 1150 ° C. or lower.

When a steel material (slab) is produced by a method such as continuous casting, the slab may be directly subjected to the heating step without being cooled, or may be subjected to the heating step after being cooled. The heating method is not particularly limited, but for example, heating can be performed in a heating furnace according to a conventional method.

[Hot rolling]
Next, the heated steel material is hot-rolled to obtain a steel sheet. At that time, in order to secure the toughness which is the basic performance of the product steel sheet, it is necessary to miniaturize the ferrite grains through the miniaturization of the austenite grains in the intermediate portion of the steel sheet. Therefore, the cumulative rolling reduction in hot rolling is set to 50% or more. That is, when the cumulative reduction rate is less than 50%, the ferrite grains in the intermediate portion of the steel sheet are not miniaturized, regions with low brittleness are locally generated, and brittle cracks are likely to occur. Other conditions relating to the hot rolling process are not particularly limited.

[cooling]
Next, the steel sheet after the completion of hot rolling is cooled (first cooling step). In the cooling step, it is preferable to cool to room temperature. The cooling can be performed by any method, for example, air cooling or accelerated cooling.

[Reheat]
Next, the cooled steel sheet is reheated to a temperature (reheating temperature) equal to or higher than the Ac ₁ transformation point and lower than the Ac ₃ transformation point. By heating in the two-phase region of ferrite and austenite in this way, the variation in mechanical properties due to the cooling deviation introduced over the entire length of the steel sheet in hot rolling without damaging the hot-rolled sheet structure before heating. It can be resolved. When the reheating temperature is Ac ₃ or higher, austenite in the middle part of the hot-rolled plate structure grows and coarsens, resulting in the formation of locally low toughness regions, which can be used as a brittle crack or ductile fracture starting point. Voids are likely to occur. On the other hand, when the reheating temperature is less than ₁ point of Ac, the surface layer of the hot-rolled plate structure is not appropriately coarsened, and the effect of improving the total thickness elongation by softening the surface layer portion cannot be obtained.

The Ac ₁ transformation point can be obtained by the following equation (2).
Ac ₁ (° C.) = 723 + 22 × Si-14 × Mn-14.4 × Ni + 23.3 × Cr… (2)
The Ac ₃ transformation point can be obtained by the following equation (3).
Ac ₃ (° C.) = 912.0-230.5 x C + 31.6 x Si-20.4 x Mn-39.8 x Cu-18.1 x Ni-14.8 x Cr + 16.8 x Mo ... (3) )
Here, the element symbol in the above equations (2) and (3) means the content (mass%) of each element, and is set to zero when the element is not contained.

In the reheating treatment, it is preferable to heat the temperature to the reheating temperature and then maintain the temperature. At that time, if the holding time is less than 10 minutes, the transformation to the austenite phase is not started over the entire length of the steel sheet, and the hardenability may be significantly lowered in a part of the region. Therefore, the holding time is preferably 10 minutes or more.

[cooling]
The steel sheet reheated in the reheating step is cooled to a cooling stop temperature of 400 ° C. to 600 ° C. (second cooling step). At that time, the average cooling rate is set to 3 to 20 ° C./s. That is, when the average cooling rate is less than 3 ° C./s, pearlite is formed in a band shape, and ductile cracks along the band structure are likely to occur, so that the elongation is reduced. Further, if the average cooling rate is less than 3 ° C./s, ferrite is excessively generated in the intermediate portion of the steel sheet, the entire steel sheet is softened, and desired mechanical properties cannot be obtained. On the other hand, when the average cooling rate exceeds 20 ° C./s, the crystal grains on the surface layer of the steel sheet become finer and harden, so that the elongation in the total thickness tensile test decreases. Further, when the cooling stop temperature is less than 400 ° C., ferrite is excessively generated in the central portion of the plate thickness, so that the entire steel sheet becomes soft and the desired mechanical properties cannot be obtained. On the other hand, when the cooling stop temperature exceeds 600 ° C., the crystal grains on the surface layer of the steel sheet are refined and hardened in the subsequent quenching process, or hard bainite and martensite are excessively generated, so that the elongation in the total thickness tensile test is increased. descend.

[Quenching]
The steel sheet cooled to the cooling stop temperature is hardened. When the interval from the cooling stop to the quenching start is widened, the hardness of the second phase other than the ferrite phase is lowered. Therefore, the time from the cooling stop to the quenching start is preferably 60 seconds or less. More preferably, it is within 30 seconds. Quenching can be performed under arbitrary conditions without particular limitation. For example, the quenching temperature is in the range of 400 to 600 ° C., and water cooling is performed to a temperature below the Mf point, preferably 200 ° C. or lower.

The Mf point can be obtained by, for example, the following equation (4).
Mf (° C.) = 410.5-407.3 x C-7.3 x Si-37.8 x Mn-20.5 x Cu-19.5 x Ni-19.8 x Cr-4.5 x Mo … (4)
Here, the element symbol in the above formula (4) means the content (mass%) of each element, and is set to zero when the element is not contained.

Hereinafter, the effects of the present invention will be specifically described based on examples, but the present invention is not limited to these examples.
The molten steel having the composition shown in Table 1 was melted and used as a steel material (slab). The values of Ac ₁ , Ac _3, and Mf shown in Table 1 are the values obtained according to the above-mentioned equations (2), (3), and (4), respectively.

Next, the obtained slab was heated and hot-rolled under the conditions shown in Table 2 to obtain a steel plate having a total length of 20 m and a plate thickness shown in Table 2. Then, the steel sheet was cooled to room temperature, reheated to the reheating temperature shown in Table 2, and held for 30 minutes. Next, cooling water was sprayed on both sides of the steel sheet, cooled to the cooling stop temperature at the average cooling rate shown in Table 2, and then subjected to quenching treatment. In the quenching treatment, water cooling was performed from the cooling stop temperature to 200 to 150 ° C.
For comparison, in some comparative examples (No. 20 in Table 2), quenching was performed immediately after reheating without cooling to satisfy the conditions of the present invention. The quenching conditions in the comparative example were an average cooling rate of 44.0 ° C./s and a cooling stop temperature of 110 ° C.

Tensile tests and microstructure observations were carried out on the obtained steel sheets. In order to evaluate the characteristics of the total length of the steel sheet, test pieces were taken from the tip, center and tail of the steel sheet in the rolling direction. The test method is as follows. The test pieces at the tip and the tail end were taken from a position 500 mm from the rolling direction end of the steel sheet.
(1) Tensile test A tensile test was carried out using a JIS Z 2201 1B full-thickness test piece collected so that the plate width direction coincided with the tensile direction from the center of the width of the steel sheet, and the tensile strength (TS) and tensile strength (TS) and We asked for total thickness growth. Tensile strength of 440 MPa or more was accepted. Elongation characteristics of 20% or more were accepted.

(2) Observation of microstructure The microstructure was observed by the following procedure, and the area ratio of the ferrite phase in the surface layer portion, the area ratio of the ferrite phase in the intermediate portion, and the residual structure other than the ferrite phase in the intermediate portion were evaluated.
First, from the obtained steel sheet, a test piece for structure observation is taken so that the observation surface has a cross section perpendicular to the rolling direction (cross section in the thickness direction), polished until it becomes a mirror surface, and then a corrosive liquid (methanol nitrate solution). ), And an optical microscope (magnification: 400 times) was used to observe from the surface of the steel sheet to the center position (1/2 position) of the sheet thickness in the plate thickness direction, and images were taken so that the screen was continuous. Using the obtained microstructure photograph, the phase was identified by image analysis, and the area ratio of the ferrite phase was calculated. As the area ratio of the ferrite phase, both the average value of the area ratio of the ferrite phase in the surface layer portion of the steel sheet and the average value of the area ratio of the ferrite phase in the intermediate portion were obtained.

Further, from the microstructure photograph obtained in the above microstructure observation, the average crystal grain size SD (μm) in the surface layer portion and the average crystal grain size CD (μm) in the intermediate portion were calculated by using the line segment method. Using the obtained SD and CD, the ratio SD / CD of crystal grain size was calculated.

The results obtained are shown in Table 3. The remaining structure other than the ferrite phase in the central portion of the plate thickness is described collectively because it was the same at the tip, center and tail end in the rolling direction of the steel sheet.

In the invention example satisfying the condition of the present invention, a steel sheet having high uniform elongation over the entire length of the steel sheet and having excellent impact resistance was obtained. On the other hand, in the comparative example not satisfying the conditions of the present invention, the elongation characteristics at the total thickness were inferior at at least one position of the tip, the center and the tail of the steel sheet.

Claims (3)

By mass%
C: 0.05 to 0.16%,
Si: 0.10 to 0.50%,
Mn: 0.80-1.60%,
It contains P: 0.05% or less and S: 0.02% or less, and the balance has a component composition of Fe and unavoidable impurities.
The microstructure in the surface layer portion from the surface of the steel sheet to 200 μm in the plate thickness direction contains a ferrite phase having an area ratio of 80% or more.
The microstructure of the intermediate portion other than the surface layer portion contains a ferrite phase having an area ratio of less than 80%, and the balance is composed of one or both of the bainite phase and the pearlite phase.
A steel sheet in which the average crystal grain size SD (μm) in the surface layer portion and the average crystal grain size CD (μm) in the intermediate portion satisfy the following equation (1).
Note 2.0 ≤ SD / CD ... (1) The composition of the components is further increased by mass%.
Cu: 1.0% or less,
Ni: 2.0% or less,
Cr: 1.0% or less,
Mo: 1.0% or less,
Nb: 0.1% or less,
V: 0.1% or less,
Ti: 0.1% or less,
B: 0.005% or less,
The steel sheet according to claim 1, which contains one or more selected from the group of Ca: 0.005% or less and W: 0.05% or less. The steel material having the component composition according to claim 1 or 2 is heated to 900 to 1200 ° C., and then hot-rolled to obtain a steel sheet having a cumulative reduction ratio of 50% or more, and the steel sheet is cooled and then Ac1 transformed. The reheated steel sheet is reheated to a temperature range above the point and below the Ac3 transformation point, cooled to a cooling stop temperature of 400 ° C. to 600 ° C. at an average cooling rate of 3 to 20 ° C./s, and then hardened. The microstructure in the surface layer portion from the surface of the steel sheet to 200 μm in the plate thickness direction contains a ferrite phase having an area ratio of 80% or more, and the microstructure in the intermediate portion other than the surface layer portion is ferrite having an area ratio of less than 80%. The balance is composed of one or both of the bainite phase and the pearlite phase, and the average crystal grain size SD (μm) in the surface layer portion and the average crystal grain size CD (μm) in the intermediate portion are as follows (1). A method for manufacturing a steel sheet that satisfies the formula .
Note 2.0 ≤ SD / CD ... (1)