JP4144064B2 - Steel plate manufacturing method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for stably producing a high-strength steel sheet having a uniform hardness distribution in the thickness direction with high productivity.
[0002]
[Prior art]
As a method for improving the strength and toughness of steel materials, the rolling temperature in the hot rolling process is controlled to the low temperature side of the non-recrystallized region of austenite, and after finishing rolling, controlled cooling is performed according to the intended strength. This is well known as a controlled rolling technique and a controlled cooling technique.
[0003]
In normal controlled rolling (CR), it is common to provide a temperature adjustment step between rough rolling and finish rolling in order to lower the rolling temperature and finish rolling temperature. In this case, temperature adjustment is performed by a method of air cooling or a method of forcibly cooling a steel slab during rolling to shorten the temperature adjustment time. For example, in Japanese Patent Application Laid-Open No. 49-120855, in a hot rolling line having two rolling mills, finishing rolling is performed in order to adjust the temperature of the material when rolling the material with the second rolling mill. A method for forcibly cooling a material to be rolled has been disclosed.
[0004]
In this invention, although there is no description about the cooling condition itself of forced cooling, if the cooling capacity in this process is too large, there is a large temperature difference between the thickness center part of the intermediate material and the steel sheet surface layer part. In general, this is caused by uneven cooling such as shower cooling.
[0005]
Therefore, although the cooling rate during temperature adjustment depends on the thickness of the intermediate material to be cooled, it is usually 2 (° C./s) or less, and in the present invention, it is 4.2 (° C./s) at the maximum. Some are shown in the examples. For this reason, although it depends on the thickness of the intermediate material at the time of temperature adjustment, the temperature adjustment process usually takes about 2 to 10 minutes, which is one of the factors hindering the production efficiency of steel sheet production. ing.
[0006]
On the other hand, by performing controlled cooling after finish rolling, a preferable structure can be obtained and the strength of the steel sheet can be increased. In this case, the increase in strength by controlled cooling is desirable from the viewpoints of improving weldability, cost saving, and energy saving because the amount of C and the amount of alloy addition can be suppressed in accordance with the increase in strength. However, even in this process, as the cooling rate is increased, the cooling rate of the steel plate surface portion becomes extremely large compared to the center portion of the plate thickness, so that a hard structure such as a martensite structure is generated in the vicinity of the steel plate surface, and in the plate thickness direction. There is a problem that a steel sheet having an extremely large hardness distribution cannot be avoided.
[0007]
As a method of making the hardness distribution in the plate thickness direction uniform, the rolling finishing temperature is lowered to refine the structure near the steel plate surface, and many deformation bands are introduced to sufficiently secure ferrite nucleation sites. Thus, there is known a method of reducing the relative hardenability near the surface and suppressing quenching near the steel sheet surface during controlled cooling (for example, Steel Research Journal, No. 309 (1982), 18- 34). However, even if the austenite grain size in the vicinity of the steel sheet surface is made fine, depending on the steel composition and the cooling rate of the controlled cooling, it is still difficult to avoid hardening near the surface depending on the steel composition.
[0008]
Further, in Japanese Patent No. 1761584, the controlled cooling start temperature (steel plate surface temperature) after the hot rolling is finished is set to the transformation temperature or lower, and a transformation from austenite to ferrite is caused only in the vicinity of the steel plate surface. A method for suppressing the occurrence of baking on the steel sheet surface is disclosed.
[0009]
However, in the present invention, a standby time for causing the ferrite transformation is required after finishing rolling and before starting controlled cooling, which may reduce productivity. Further, in actual operation, since the controlled cooling start temperature directly affects the organization form of the final product, the organization form tends to vary from product lot to product lot. Therefore, the variation in mechanical properties such as mechanical properties of the steel sheet is also expanded, and it is not necessarily the best method from the viewpoint of manufacturing stability.
[0010]
[Problems to be solved by the invention]
In the present invention, in order to solve these problems of the prior art, first, the temperature adjustment time before finish rolling is greatly shortened to increase the productivity of the steel sheet, and second, the hardness distribution in the thickness direction. It is an object of the present invention to provide a method for producing a high-strength steel sheet with uniform thickness.
[0011]
[Means for Solving the Problems]
That is, the first invention is hot rolled to an intermediate thicknessC content: 0.01% by mass or more and 0.2% by mass or less, Si content: 0.6% by mass or less, Mn content: 0.3% by mass or more and 1.8% by mass or less, Al content: 0.005 mass% or more and 0.1 mass% or less, remainder Fe And inevitable impuritiesWhen manufacturing a steel slab into a steel plate having a predetermined thickness, the method includes the following steps. (A) Water density: 1000 liter / m²・ Water cooling at min or more and steel slabThickness directionThe temperature adjustment step of stopping the cooling in the temperature range of Ae3 point + 30 ° C. or lower and 700 ° C. or higher, and (b) the cooled steel slab,Rolling which starts immediately after the temperature adjusting step,steel sheetThickness directionFinish rolling step to finish the rolling at an average temperature of Ae3 point + 30 ° C. or less and 680 ° C. or more, and (c) the steel plate after the finish rolling, water density: 1000 liter / m²-Control cooling process that cools in min.
[0012]
With this invention, it is possible to simultaneously achieve high strength of the steel sheet and uniform hardness distribution in the thickness direction, improve productivity by shortening the temperature adjustment time, operational stability by reducing material variation, Can improve the weldability and reduce the cost by adopting steel with a low content of alloy elements.
[0013]
2nd invention is a manufacturing method of the steel plate characterized by performing the temperature adjustment process of said (a) by the forced cooling installed between the roughing mill and the finishing mill.
According to this invention, the productivity of the steel sheet can be further improved.
[0014]
  The third aspect of the invention relates to the control cooling in the step (c), with the steel plateThickness directionThe steel sheet is stopped at an average temperature of 650 ° C. or lower and 400 ° C. or higher. According to the present invention, residual strain or residual stress in the controlled and cooled steel sheet can be reduced, which contributes to improvement in the shape of the controlled and cooled steel sheet.
[0015]
The fourth invention is a method for producing a steel sheet, characterized in that the steel sheet that has undergone the steps (a), (b), and (c) is further air-cooled to near normal temperature and then tempered at a temperature of 700 ° C. or lower. It is.
According to this invention, the residual stress of the steel sheet can be further reduced.
[0016]
5th invention is the manufacturing method of the steel plate characterized by tempering the steel plate which passed through the process of said (a), (b), (c) further at the temperature of 700 degrees C or less immediately.
According to the present invention, the residual stress of the steel sheet can be further reduced, the productivity of the steel sheet can be improved, and the energy cost for the heat treatment can be reduced.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
As described above, the temperature adjustment step in the controlled rolling usually requires a time of about 2 to 10 minutes, although it depends on the thickness of the steel slab rolled to an intermediate thickness when the temperature adjustment is performed. In order to shorten this temperature adjustment process, powerful water cooling using a device with high cooling capacity, for example, water density: 1000 liter / m²-When water cooling of min or more is performed, the temperature distribution in the plate thickness direction is expanded, and the structure in the plate thickness direction is likely to be uneven based on the temperature distribution.
[0018]
Further, also in the controlled cooling after finish rolling, as described above, even if the structure before the start of controlled cooling is uniform in the plate thickness direction, powerful water cooling that enables high strength, for example, water density: 1000 Liter / m²-When cooling for more than min, the cooling rate in the vicinity of the steel sheet surface becomes extremely high, and as a result, a martensite-based structure is easily formed in the vicinity of the surface. On the other hand, since the ferrite-pearlite structure or ferrite-bainite structure mainly composed of ferrite is formed in the center part of the sheet thickness of the steel sheet, there is a problem that the steel sheet has a very large hardness distribution in the sheet thickness direction.
[0019]
Therefore, the inventors of the present invention have made intensive studies on the steel sheet manufacturing process by the controlled rolling method, from the viewpoint of improving productivity, shortening the time required for temperature adjustment during rolling, and techniques for making materials by controlled cooling after finish rolling. I did it. As a result, the inventors have completed the invention of a method for improving productivity while keeping the hardness distribution in the thickness direction small.
[0020]
That is, the inventors of the present invention inclined in advance in the sheet thickness direction so as to cancel the hardness difference caused by the cooling rate difference in the sheet thickness direction accompanying the subsequent controlled cooling, after the finish rolling and before the start of controlled cooling. It has been found that a steel sheet having a uniform hardness distribution in the thickness direction can be produced by forming a texture distribution.
[0021]
This can be realized by controlling the cooling rate and the cooling stop temperature in the temperature adjustment process of the steel slab rolled to an intermediate thickness. Conventionally, when the water cooling is performed strongly, the temperature adjustment process that promoted the non-uniformity of the structure in the thickness direction is taken in the opposite direction, and the structure distribution inclined to the thickness direction on the steel slab rolled to an intermediate thickness. By providing controlled cooling after finish rolling, a uniform structure is obtained in the thickness direction, and the final hardness distribution in the thickness direction is made uniform. In addition, at the same time, the temperature adjustment time before finish rolling can be shortened.
[0022]
That is, the present invention will be described in the metallurgical manner as follows.
When a steel slab rolled to an intermediate thickness is cooled at a high cooling rate, a structure mainly composed of bainite (martensite) is formed only in the vicinity of the surface layer portion, and is maintained in the two-phase region of austenite + ferrite by subsequent reheating. This part is tempered.
[0023]
After being processed in the next finish rolling, it is cooled again at a high cooling rate. However, since the tempered bainite (martensite) is maintained in the vicinity of the surface layer, no structural change occurs. On the other hand, since the inside of the steel sheet is basically processed in the austenite state and then cooled, the structure rises due to controlled cooling and the hardness increases. Thus, a steel plate having a uniform hardness distribution in the plate thickness direction can be obtained.
By this method, the temperature adjustment time before finish rolling can be shortened at the same time.
[0024]
The manufacturing conditions in the present invention will be described below.
First, after heating a steel slab, rough rolling is performed to an intermediate thickness for temperature adjustment. Rough rolling is performed in the same manner as in normal control rolling. For example, taking the example described in page 25, “Controlled rolling / controlled cooling, flow of material creation by rolling” written by Gunji Koji, supervised by the Japan Iron and Steel Institute, Jinshokan (1997)). In a temperature range of 920 ° C. or higher, a predetermined thickness is obtained so that a total cumulative rolling reduction necessary for finish rolling can be secured.
[0025]
Here, the total cumulative rolling reduction required in finish rolling is obtained by the following equation.
Total cumulative reduction rate =
(Finish rolling start thickness−Finish rolling finish thickness) / (Finish rolling start thickness) × 100 (%)
Therefore, for example, if the thickness of the steel sheet is 20 (mm) and the necessary total cumulative rolling reduction is 80%, the finish rolling start thickness is 100 (mm).
[0026]
In the next temperature adjustment step, the water density is 1000 liters / m for the steel slab that has been roughly rolled to an intermediate thickness.²-Cooling is performed at min or more, and cooling is stopped at an average temperature of the steel slab at Ae3 point + 30 ° C or less and 700 ° C or more.
[0027]
The cooling ability by water cooling generally depends on the water density, thereby changing the temperature distribution in the thickness direction of the steel sheet and also the structure. In particular, the water density is 1000 liters / m.²-When it is smaller than min, a vapor film is formed on the surface of the steel sheet, and the cooling rate of the steel sheet is not limited by the heat conduction in the steel sheet. Therefore, the cooling capacity varies directly due to the fluctuation of the water density, which causes variation in the structure of the steel plate and the material.
[0028]
On the other hand, the water density is 1000 liters / m.²In the case of a high water density of min or more, the cooling is rate-controlled by the heat conduction in the steel sheet, so the dependency of the cooling capacity on the water density becomes small. For this reason, as a result of suppressing the change of the structure accompanying the fluctuation of the water amount density, the operational stability is remarkably improved due to the stability of the mechanical properties of the obtained steel sheet and the ease of control of the cooling rate. In the present invention, in the temperature adjustment step, the cooling of the intermediate thickness steel slab is performed under the heat conduction limited cooling condition, that is, 1000 liter / m.²・ Cool at a water density of min or more.
[0029]
Cooling in the temperature adjustment step needs to be stopped at an average temperature of the steel slab at Ae3 point + 30 ° C. or lower and 700 ° C. or higher. When the stop temperature is lower than 700 ° C., the rolling temperature in the finish rolling, which is the next step, becomes too low, and the rolling load becomes excessive. If the stop temperature exceeds the Ae3 point + 30 ° C., the amount of reverse transformed austenite near the surface of the intermediate material increases due to subsequent recuperation.
[0030]
In the controlled cooling after finish rolling, when the cooling rate is high, a martensitic structure is formed in the vicinity of the steel sheet surface, and the uniform hardness distribution in the thickness direction intended by the present invention cannot be obtained. Therefore, the cooling stop temperature in the temperature adjusting step is regulated to Ae3 point + 30 ° C. or lower and 700 ° C. or higher as the average temperature of the steel slab as an intermediate material.
[0031]
Here, the average temperature T (° C.) of the steel slab under the heat conduction-controlled cooling condition can be expressed by the following equation. Therefore, in actual operation, the cooling time: Δt is selected such that the average temperature of the steel slab is Ae3 point + 30 ° C. or lower and 700 ° C. or higher.
T = TRS− (2q / ρ · Cp · H) xΔt
TRS: Cooling start temperature (° C)
q: Heat flux (J / m²・ S)
ρ: density of steel slab (kg / m^Three)
Cp: Specific heat of steel slab (J / kg · ° C)
H: Thickness of steel slab (mm)
Δt: Cooling time (s)
[0032]
The Ae3 point refers to the boundary temperature between the γ single-phase region and the γ + α two-phase region in the equilibrium state of steel. Basically, it is determined by the steel composition, but it is the A3 temperature in the equilibrium state, and can be determined using computational thermodynamic software such as Thermocalc, for example. Here, the Ae3 point is used as a control factor. The Ar3 point changes depending on the processing amount and cooling rate received by the steel plate, and the cooling rate is continuously increased in the plate thickness direction like the cooling of the intermediate material. This is because it is not suitable as a control factor when it changes.
[0033]
In addition, the cooling device used for this temperature adjustment process is a material to be rolled by installing it between the roughing mill and the finishing mill when the roughing mill and the finishing mill are installed separately. Therefore, the logistics can be simplified and the production efficiency can be further improved.
[0034]
In the next step, finish rolling, the steel slab rolled to an intermediate thickness is (b) immediately after the temperature adjustment step, and After heating, it can basically start at any point. In the finish rolling, the above-described predetermined cumulative reduction rate, for example, a total rolling reduction rate of 60 to 80% is added in the finish rolling to finish to a predetermined product sheet thickness.
[0035]
Moreover, finish rolling needs to complete | finish at the average temperature of a steel plate at Ae3 point +30 degrees C or less 680 degrees C or more. When the rolling temperature is lower than 680 ° C., the rolling load becomes excessive. On the other hand, when the temperature exceeds Ae3 point + 30 ° C., the amount of reverse-transformed austenite increases in the vicinity of the steel sheet surface layer. The distribution of hardness in the thickness direction of the sheet is increased.
[0036]
Finally, controlled cooling is performed. Controlled cooling, water density: 1000 liters / m²・ Continuous or intermittent at min or more. Controlled cooling, water density: 1000 liters / m²・ The reason for setting it to min or more is that, as with the cooling in the temperature adjustment step, the water density density dependence of the cooling capacity is achieved while achieving high strength by adopting the water density range in which the cooling becomes the rate of heat conduction in the steel sheet. This is because the purpose is to reduce the fluctuation of the organization and to stabilize the operation.
[0037]
In addition, the residual stress resulting from a cooling thermal stress may generate | occur | produce in a steel plate by control cooling. For this, by setting the stop temperature of the controlled cooling to 650 ° C. or lower and 400 ° C. or higher, the effect of tempering by reheating after cooling stop, the so-called self-tempering effect is given to the steel plate after the controlled cooling, Residual residual strain or residual stress can be reduced. In this case, when the cooling stop temperature exceeds 650 ° C., the effect of increasing the strength by the controlled cooling is reduced, and when it is lower than 400 ° C., the effect of reducing the residual stress is small.
[0038]
In order to further reduce the residual stress generated in the steel sheet by controlled cooling, it is also effective to perform tempering on the controlled cooled steel sheet at a temperature of 700 ° C. or lower. If the tempering temperature is higher than 700 ° C., the effect of improving the residual stress is great, but the strength is greatly reduced, and is not suitable for the production of the high strength steel sheet of the present invention. In this case, after the controlled cooling is stopped at a temperature of 650 ° C. or lower and 400 ° C. or higher, the steel is air-cooled to near room temperature and then tempered, or the steel plate whose controlled cooling is stopped is immediately tempered. Any of the methods (direct tempering) is effective in reducing the residual stress.
[0039]
The present invention can be applied without any problem as long as it is a steel in the range of general low carbon low alloy steel as a chemical component of steel. That is, even if the transformation point (Ae3 point, Ar3 point) changes depending on the alloying element content of steel, the effects of the present invention can be widely obtained. In addition, the low carbon low alloy steel here means steel having the following component ranges (hereinafter, “%” represents weight percentage).
[0040]
C content shall be 0.01% or more and 0.2% or less. Since C is an element indispensable for strengthening the steel sheet, if it is less than 0.01%, sufficient strengthening cannot be obtained. Further, since the steel sheet produced according to the present invention is often used for welded structures, the C content is preferably 0.2% or less from the viewpoint of weldability.
[0041]
Si content shall be 0.6% or less. Si is originally a deoxidizing element, but an action as a strengthening element of the steel sheet is also recognized. However, if it exceeds 0.6%, not only the ductility is lowered but also the weldability is lowered.
[0042]
The Mn content is 0.3% or more and 1.8% or less. Mn is a strengthening element of the steel sheet, but if it is contained in a large amount, the weldability of the steel sheet is impaired, so this range is desirable.
[0043]
Al content shall be 0.005% or more and 0.1% or less. Al is a deoxidizing element and combines with N in steel to form AlN, which has effects such as adjustment of crystal grains during rolling and heating. In order to obtain the deoxidation effect, 0.005% or more is necessary. On the other hand, if it exceeds 0.1%, the cleanliness of the steel is impaired and the generation of flaws at the slab manufacturing stage is promoted.
[0044]
In addition, elements such as Ni, Cr, Mo, Cu, V, Nb, Ti, Zr, B, Ca, and REM can be contained for the purpose of ensuring improvement in strength, toughness, and high temperature strength.
[0045]
Pcm is set to 0.30% or less. It is assumed that the present invention is applied as a steel sheet for welded structure having a tensile strength of 400 to 780 MPa. Therefore, it is necessary to have a weld crack sensitivity corresponding to the strength level. If Pcm exceeds 0.3%, the weldability deteriorates, and the weld crack prevention preheating temperature becomes such a high temperature that it cannot be adopted in the work, so the upper limit is made 0.30%.
Pcm is expressed by the following formula.
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20
+ Mo / 15 + V / 10 + 5B
[0046]
Moreover, there is no problem if the impurity elements such as P, S, N, and O are in amounts that are inevitably contained in the steel.
[0047]
【Example】
Below, the Example of the steel plate manufactured by the method of this invention is described. Table 1 shows the chemical composition of the test steel as FIG. In addition, the result of having calculated | required Ae3 point of each steel by thermodynamic calculation is combined with Table 1, and is shown.
Example 1
Using a continuous casting slab having a thickness of 220 (mm) having the chemical composition of steel D in Table 1, a sheet thickness of 40 (mm) is obtained by the conventional method (A) and the method of the present invention (B) described below. A steel plate was produced.
[0048]
Conventional method (A)
Slab heating temperature: 1150 ° C,
Rough rolling conditions: Rolling to 100 (mm) at a rough rolling end temperature of 1050 ° C.,
Temperature adjustment conditions: Shower cooling (Water density: 100 liters / m²· Min) stops cooling at an average temperature of 800 ° C,
Finish rolling conditions: Rolling to a sheet thickness of 40 (mm) at a finish rolling temperature of 750 ° C.,
Control cooling condition: Water density: 2000 l / m²・ Cooling is stopped at 500 ° C for min and then air cooling.
[0049]
Method (B) of the present invention
Slab heating temperature: 1150 ° C,
Rough rolling conditions: Rolling to 100 (mm) at a rough rolling end temperature of 1050 ° C.,
Temperature adjustment conditions: Slit nozzle method (Water density: 2000 liters / m²-Min stops cooling at an average temperature of 800 ° C,
Finish rolling conditions: Rolling to a sheet thickness of 40 (mm) at a finish rolling temperature of 750 ° C.,
Control cooling condition: Water density: 2000 l / m²・ Cooling is stopped at 500 ° C for min and then air cooling.
[0050]
FIG. 1 shows the hardness distribution in the plate thickness direction of the steel plates produced by the conventional method (A) and the method (B) of the present invention. Here, the hardness was measured using a micro Vickers hardness tester with a load of 98N. In the steel plate manufactured by the conventional method (A), a marked hardened layer is recognized in the vicinity of the surface, whereas in the steel plate manufactured by the method of the present invention (B), such a hardened region in the vicinity of the surface is not recognized, A uniform hardness distribution is shown.
[0051]
In the conventional method (A), it took a little less than 6 minutes to adjust the temperature, and it took a total of more than 10 minutes from the heating furnace to the completion of finish rolling after extraction. On the other hand, in the method (B) of the present invention, the time required for the temperature adjustment is only 30 seconds, the time from the start of temperature adjustment to the start of finish rolling is less than 2 minutes, the time required from the extraction from the heating furnace to the end of finish rolling. Was less than 6 minutes. Therefore, when the method of the present invention was used, the rolling production efficiency was improved by shortening the time of the temperature adjustment step, the strength of the steel plate was increased, and the hardness distribution in the thickness direction was made uniform.
[0052]
(Example 2)
In the temperature adjustment process using the forced cooling device installed between the rough rolling mill and the finishing mill using the steel D in Table 1, the water density is 500, 1200, 1800, 2400 liters / m.²-It changed with min and it cooled, and cooling was stopped at 800 degreeC with the average temperature of steel slab. Other conditions are the same as the conditions of the method (B) of the present invention shown in (Example 1).
[0053]
FIG. 2 shows the measurement results of the hardness distribution in the plate thickness direction. Water density: 500 l / m²・ A steel sheet whose temperature has been adjusted in min has a clear hardened area in the vicinity of the surface layer of the steel sheet, whereas the water density of the present invention is 1200, 1800, 2400 liters / m.²-When cooling is performed at min, generation of a hardened region near the surface is suppressed in all cases. Therefore, in the temperature adjustment step, the water density for cooling the steel slab is 1000 liters / m.²-It became clear that generation | occurrence | production of a hardening area | region can be prevented when it becomes more than min.
[0054]
(Example 3)
Using steel D in Table 1, the steel slab was heated to 1150 ° C., and then roughly rolled to an intermediate thickness of 62.5 (mm) or 100 (mm). The temperature is adjusted using a shower type forced cooling device or slit nozzle type forced cooling device installed in the middle, and then rolled to a thickness of 25 (mm) or 40 (mm), respectively, for final controlled cooling. Carried out.
[0055]
Table 2 shown as FIG. 4 shows temperature adjustment conditions for the intermediate material, finish rolling conditions from the intermediate material, and accelerated cooling conditions after rolling. In addition, for these steel plates, the Vickers hardness of the plate thickness center portion and the steel plate surface vicinity portion (surface layer position at a depth of 1/20 in the steel plate thickness direction) is also shown. In addition, about some steel plates, the tempering process was performed on the conditions shown in Table 2.
[0056]
In the case of plate numbers 25-1 to 25-8 (plate thickness 25 mm) which is an example of the present invention, the hardness of the center portion of the plate thickness is approximately Hv 170 or more, and the difference in hardness between the center portion of the plate thickness and the steel plate surface layer vicinity is It is within ΔHv15, and both high strength and uniform hardness distribution in the thickness direction are achieved.
[0057]
On the other hand, the plate number 25-9 is an example in which the temperature adjustment stop temperature exceeded the Ar3 point although the water amount density at the time of temperature adjustment was appropriate, and the plate number 25-10 was used for temperature adjustment in a conventional shower. Performed by cooling, 2000 liters / m after finish rolling²-This is an example of controlled cooling with a water density of min. All of these steel plates have a large hardness difference of about 60 or more between the center portion of the plate thickness and the vicinity of the surface layer of the steel plate.
[0058]
In addition, the plate number 25-11 is an example in which the temperature adjustment condition is appropriate but the cooling stop temperature of the control cooling is high, and the plate number 25-12 was air-cooled after finishing rolling by performing temperature adjustment in a conventional shower. It is an example of what is called a normal control rolling material. Although these steel plates have a small difference in hardness between the central portion of the plate thickness and the vicinity of the surface layer of the steel plate, the hardness of the steel plate itself is small and high strength is not achieved.
[0059]
Similarly, in plate numbers 40-1 and 40-2 (plate thickness 40 mm) manufactured under appropriate conditions within the scope of the present invention, the hardness of the plate thickness center portion is 160 or more, the plate thickness center portion and the steel plate surface layer. The height difference in the vicinity is ΔHv15 or less. On the other hand, the plate number 40-3 in which the temperature adjustment stop temperature was high, and the plate number 40-4 in which shower cooling with a small cooling capacity was used for temperature adjustment had the same hardness as that of the present invention material. However, the hardness difference between the central portion of the plate thickness and the vicinity of the surface layer of the steel plate is a very large hardness difference of ΔHv of about 80. In addition, plate number 40-5, which had a high controlled cooling stop temperature after finish rolling, has a small hardness difference between the central portion of the plate thickness and the vicinity of the surface layer of the steel plate, but the strength of the steel plate itself is small and high strength is achieved. Not.
[0060]
Example 4
A steel slab having a thickness of 220 (mm) having the composition of steels A, B, and C shown in Table 1 is heated to 1150 ° C. and then roughly rolled to 100 (mm) to produce a steel plate having a thickness of 40 (mm). did. Temperature adjustment and finish rolling are performed under the conditions shown in Table 3 shown in FIG. 5, and the water density after finishing rolling is 2000 liters / m.²-After performing the controlled cooling at min, the controlled cooling was stopped at the temperature shown in Table 3. Thereafter, tempering treatment was performed on some steel plates under the conditions shown in Table 3. In the same manner as in Examples 1 to 3, the Vickers hardness of the plate thickness center portion and the steel plate surface vicinity portion (surface layer position at a depth of 1/20 in the steel plate thickness direction) was measured.
[0061]
The plate numbers A-1 to A-4, B-1 to B-3, C-1, and C-2 manufactured under appropriate conditions within the scope of the present invention are all the plate thickness center portion and the steel plate surface layer. The hardness difference in the vicinity is within ΔHv15, and the steel sheet has a small hardness distribution in the thickness direction. The plate numbers A-4 and B-1 are examples (tempering condition 2 shown in Table 3) in which tempering was immediately performed at 600 ° C. after the completion of controlled cooling at 500 ° C. and 550 ° C. Compared with A-1 which is an example of the invention which did not carry out, it has sufficient hardness and uniform hardness distribution.
[0062]
In addition, after the control cooling is completed, the steel sheet is air-cooled to near room temperature, and then re-heated and tempered, compared with the tempering condition 1 (plate numbers A-2 and A-3), and tempering is performed immediately after the control cooling is completed. In the example of the tempering condition 2 (plate number A-4), the hardness of the steel plate surface layer portion is approximately the same, but the time required for production is greatly shortened.
[0063]
The effect of the present invention is sufficiently recognized even in steel C having a low C and Mn content. When manufactured under appropriate temperature control conditions and controlled cooling conditions (plate numbers C-1 and C-2), C and plates manufactured under conditions outside the scope of the present invention using steel B having a high Mn content No. B-6, a higher strength and uniform hardness distribution can be obtained, that is, according to the present invention, even if the alloy element content is small, the hardness distribution in the thickness direction is maintained uniformly, Strength can be achieved, and not only the weldability can be improved, but also alloy saving and cost reduction can be achieved.
[0064]
On the other hand, the temperature adjustment is performed by conventional shower cooling, and the cooling rate is small plate numbers A-6, B-5, C-4, and the water amount density at the time of temperature adjustment is sufficient, but the temperature adjustment stop temperature is Ae3 point + 30 ° C. In plate numbers A-5, B-4, and C-3, the hardness difference of ΔHv50 or more is generated between the plate thickness center portion and the steel plate surface layer vicinity portion. Further, plate numbers A-7 and B-6, which have a high controlled cooling stop temperature and an insufficient cooling rate, have a small hardness difference in the plate thickness direction, but have a low absolute value of hardness and have not achieved high strength. .
[0065]
【The invention's effect】
In the present invention, when controlled cooling is performed after finishing rolling, when controlled cooling is performed by strong water cooling that achieves high strength, the vicinity of the surface layer portion of the steel plate that has been caused by the difference in cooling rate in the thickness direction This eliminates the hardened part. For this reason, in order to suppress the hardening in the vicinity of the surface layer portion, a structure distribution having an inclination in the plate thickness direction is formed in advance at an intermediate stage of rolling. Such a structure distribution is basically achieved by powerful water cooling with a high water density in the temperature adjustment stage of the intermediate material before finish rolling.
[0066]
By this method, it is possible to simultaneously achieve high strength of the steel sheet and uniform hardness distribution in the thickness direction, improve productivity by drastically shortening the temperature adjustment time, stable operability by reducing material variation, Has achieved weldability improvement and cost reduction by adopting steel with low alloying element content. Further, in the present invention, thick plate rolling is mainly premised, but the idea of the present invention can also be implemented in shape steel rolling and hot strip rolling, and its industrial value is great.
[Brief description of the drawings]
FIG. 1 is a view showing the hardness distribution in the thickness direction of a steel sheet produced by a conventional method (A) and a method (B) of the present invention in steel D.
FIG. 2 is a diagram showing a water amount density and a hardness distribution in the thickness direction in cooling of an intermediate material in a temperature adjustment step of steel D.
FIG. 3 shows the chemical composition of the test steel shown in the examples as Table 1.
FIG. 4 shows the production conditions of the test steel shown in Example 3 (temperature adjustment conditions for the intermediate material, finish rolling conditions from the intermediate material and accelerated cooling conditions after rolling), the thickness center of the test steel sheet, and the steel sheet surface. FIG. 3 is a diagram showing Vickers hardness in the vicinity (surface layer position at a depth of 1/20 in the steel plate thickness direction) as Table 2.
FIG. 5 shows the production conditions of the test steel shown in Example 4 (temperature adjustment conditions for the intermediate material, finish rolling conditions from the intermediate material and accelerated cooling conditions after rolling), the center of the thickness of the test steel sheet, and the steel sheet surface. It is a figure which shows the Vickers hardness of the vicinity (surface layer position of depth 1/20 of the steel plate thickness direction) as Table 3.

Claims (5)

Hot rolled to an intermediate thickness , C content: 0.01% to 0.2% by mass, Si content: 0.6% by mass or less, Mn content: 0.3% to 1.8% by mass When manufacturing a steel slab composed of 0.005% by mass to 0.1% by mass, the balance Fe and unavoidable impurities into a steel plate having a predetermined thickness, the following steps are involved. A method for producing a steel sheet.
(A) The steel slab is water-cooled at a water density of 1000 liters / m ² · min or more, and the cooling is stopped when the average temperature in the steel slab thickness direction is Ae3 point + 30 ° C. or lower and 700 ° C. or higher. And (b) rolling the cooled steel slab immediately after passing through the temperature adjusting step, wherein the average temperature in the steel sheet thickness direction is Ae3 point + 30 ° C. or lower and 680 ° C. or higher. A finish rolling step to finish the steel plate, and (c) a controlled cooling step of cooling the steel plate after the finish rolling at a water density of 1000 liter / m ² · min or more.  The method of manufacturing a steel sheet according to claim 1, wherein the step (a) is performed by forced cooling between a roughing mill and a finishing mill. The method for producing a steel sheet according to claim 1 or 2, wherein the controlled cooling in the step (c) is stopped at an average temperature in the thickness direction of the steel sheet of 650 ° C or lower and 400 ° C or higher.  The steel sheet that has undergone the steps (a), (b), and (c) is further air-cooled to near normal temperature, and then tempered at a temperature of 700 ° C or lower. Steel plate manufacturing method.  The method for producing a steel sheet according to any one of claims 1 to 3, wherein the steel sheet that has undergone the steps (a), (b), and (c) is further immediately tempered at a temperature of 700 ° C or lower.

Images