JPH0573808B2 - - Google Patents

Description

[Detailed description of the invention]

<Industrial Application Field> The present invention relates to a method for producing heat-treated thick-walled 80 kgf/mm ² or higher grade high-tensile steel, and its purpose is to impart uniform and excellent strength and toughness throughout the entire plate thickness. . <Prior Art> Conventionally, the conventional method for quenching steel was to cool it as quickly as possible, and the surface of the steel plate was cooled with a single amount of water from the start to the end of cooling. However, in such conventional cooling methods, there is a problem that when the thickness of the steel plate to be treated is large, the cooling rate distribution in the thickness direction is not uniform. Non-uniform cooling rate distribution in the thickness direction does not pose a problem with steels that do not contain Ni and have a strength of 80Kgf/mm2 ^or higher, but in order to impart strength or toughness to the steel plate,
In those containing Ni (0.3% or more), deterioration of toughness near the surface layer becomes a problem. Figure 1 shows the relationship between cooling rate, strength, and toughness. The change in toughness is particularly large depending on the cooling rate. Nonuniformity in cooling rate that occurs in the thickness direction is due to nonuniformity in the material in the thickness direction, especially nonuniformity in toughness. It can be seen that this occurs. From this point of view, the following techniques have been proposed in the past in order to make the cooling rate distribution uniform in the plate thickness direction. JP-A No. 57-152430 Japanese Patent Application No. 57-207629 The above methods all attempt to make the hardness distribution in the thickness direction uniform while cooling, and are not methods based on tempering like the present invention. This is a technology whose as-quenched hardness is approximately Hv300 or less at the center of the plate thickness. In order to achieve an expansion force of 80 Kgf/mm ² or more after tempering as in the present invention, as-quenched hardness is required to be 300 Hv or more. Furthermore, although uniformity in the sheet thickness direction can be obtained in the above method, the absolute value of the cooling rate at the center of the sheet thickness decreases, resulting in the problem that it is necessary to increase the amount of alloying elements in order to obtain the desired hardening hardness. be. <Summary of the invention> The present invention has been made to improve the problems of the prior art described above, and uses specific ingredients and cooling methods to produce a sheet with a strength and toughness of 80 Kgf/mm ² or higher, and with uniform strength and toughness in the thickness direction. The aim is to provide steel. First, the ingredients of the steel of the present invention are limited as follows. C: 0.03 to 0.15% If it is less than 0.03%, the necessary hardenability cannot be obtained, and therefore a strength of 80 Kgf/mm ² or more cannot be obtained. Moreover, if it exceeds 0.15%, the weldability of the steel will deteriorate, so it is set within the above range. Si: 0.50% or less Si is an essential element for melting steel, but 0.50% or less
%, weldability deteriorates and base metal toughness also deteriorates, so this is set as the upper limit. Mn: 0.40-1.60% If it is less than 0.40%, hardenability will be insufficient, and if it exceeds 1.60%, not only will weldability deteriorate, but also susceptibility to temper embrittlement will increase. Therefore, it is within the above range. Ni: 0.30-5.0% As mentioned above, the influence of the quenching cooling rate on toughness becomes greater when the Ni content is 0.30% or more. Also 5.0
If it exceeds %, the toughness level will be greatly improved,
Even if there is variation in toughness in the thickness direction, it does not pose a practical problem. Therefore, in the present invention, Ni: 0.30
~5.0% range and target. Sol.Al: 0.005~0.08% Sol.Al is an element necessary for particle size adjustment and N fixation.
It has no effect at less than 0.005%. Moreover, if it exceeds 0.08%, the toughness of the base material deteriorates, so it is set within the above range. In addition to the above components, in order to further improve mechanical properties, one or more of the following components may be added. Cu: 1.0% or less It has an effect on hardenability and strength, but if it exceeds 1.0%, hot brittleness occurs during rolling, so it should be kept at 1.0% or less. Cr: 1.5% or less It is effective for hardenability and strength, but if it exceeds 1.5%, weldability deteriorates, so it should be within this range. Mo, W: 0.8% or less Mo provides hardening and tempering softening resistance, but too much will cause deterioration of weldability and increase in cost, so it is set within the above range. Further, Mo can be replaced with an equivalent amount of W in whole or in part. Nb: 0.08% or less Nb has the effect of increasing hardenability in direct quenching and making the austenite grains finer in reheat quenching, but if it exceeds 0.08%, the effect will be saturated and the toughness of the weld will deteriorate. This is the upper limit. V: 0.15% or less V gives resistance to temper softening, but the effect is saturated even if it is added in excess of 0.15%, so this is the upper limit. Ti: 0.05% or less Ti has the effect of refining grains and fixing N, but if it is too large, large TiN forms and deteriorates the toughness of the base material, so the upper limit is set at 0.05%. Zr: 0.1% or less Similarly, Zr is useful for refining grains and fixing N, but too much Zr causes large nitrides and deteriorates the toughness of the base material. Ca, REM: 0.01% or less Ca and REM improve toughness by controlling the shape of inclusions, but if they are too large, large inclusions will occur and deteriorate the toughness of the base material, so the content should be 0.01% or less. B: 0.004% or less It is effective in improving hardenability, but if it is too large, hardenability deteriorates, so the content should be 0.004% or less. After melting steel consisting of the above components and the balance of Fe and unavoidable impurities to form a slab, the amount of water increases as the steel plate surface temperature decreases, either immediately from the austenite region after hot rolling, or when quenching and cooling after reheating to the austenite region. Cooling is performed to increase the density and make the cooling rate uniform in the thickness direction. Next, tempering is performed to create a mixed structure of 60% or more martensite structure and 5% or more bainite structure at the entire plate thickness position. Now, to explain this using two-stage cooling as an example,
First, the surface layer of the steel sheet is cooled with a low water flow density to a temperature at which a bainite structure with a volume fraction of 5% or more is generated, and then the water flow density is increased and the untransformed austenite becomes a martensitic structure, and the final structure is 60 % or more, resulting in a mixed structure of martensite and bainite. On the other hand, there is a time lag in the cooling of the center of the steel sheet compared to the surface layer, and in the temperature range involved in transformation, it is cooled at a nearly constant rate. The cooling rate in the temperature range involved in this transformation is higher than the weak cooling in the surface layer and lower than the strong cooling, so the center has a mixed structure of martensite and bainite. In the present invention, martensite is limited to 60% or more and bainite is limited to 5% or more because excellent toughness can be obtained within these ranges, as shown in FIG. The optimum cooling rate for strength and toughness is the second
As shown in the figure, it varies depending on the composition of the steel, so it is determined appropriately based on this. Taking two-stage cooling as an example, this cooling rate depends on the cooling time of the first stage, the plate thickness, and the water density of the first and second stages. The cooling rate at the position in the plate thickness direction can be determined as a function of these functions by thermal calculation, and the conditions for obtaining the optimum cooling rate can be determined. Figure 4 shows the cooling rate in the plate thickness direction and the material quality according to the method of the present invention in comparison with the conventional RQ method. The relationship between strength and toughness at the cooling rate is also shown. In the method of the present invention, since the cooling rate in the plate thickness direction is uniform, it is possible to set the cooling rate to obtain the most excellent toughness at all positions in the plate thickness direction. On the other hand, with the RQ method, even if such a cooling rate can be obtained at the center of the plate thickness, the cooling rate increases at the surface layer, resulting in particularly poor toughness. <Example> Next, an example will be shown. After melting and hot rolling steel with the composition shown in Table 1 below,
The product was cooled under the conditions shown in Table 2 and then tempered to obtain a product. Its mechanical properties are shown in the same table.

【table】

[Table] As can be seen from the above, in the comparative steels that are not included in the chemical composition range of the present invention, no particular deterioration in surface layer toughness is observed even by the conventional one-step cooling method (No. 1
and No. 9). On the other hand, in Ni-containing steel, deterioration in the toughness of the surface layer is observed in one-step cooling, whereas in the case of steel quenched by the method of the present invention, the toughness of the surface layer and the center portion are the same. In addition, the strength of the center is equivalent to that of one-stage strong cooling (No.
Comparison of Nos. 2 to 5 and Nos. 6 to 8). Also, one-stage cooling with lower water density (No.
10) has a small difference in material quality between the surface layer and the center, but the absolute value of the cooling rate decreases and the volume fraction of the martensitic structure becomes less than 60%, so the strength is lower than that of the method of the present invention (No. 2). Both the toughness and toughness are low.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between cooling rate, volume fraction, fracture surface transition temperature, and tensile strength; Figure 2 is a graph showing the relationship between cooling rate, strength, and toughness depending on the composition;
FIG. 3 is a graph showing the relationship between the cooling rate distribution in the thickness direction and the first stage cooling time based on heat transfer calculations, and FIG. 4 is a graph showing the relationship between the cooling rate in the thickness direction and material variation according to the method of the present invention.

Claims (1)

[Claims] 1 C: 0.03 to 0.15%, Si: 0.50% or less, Mn:
0.40~1.60%, Ni: 0.30~5.0%, Sol.Al: 0.005~
0.08% with the balance consisting of Fe and unavoidable impurities immediately after hot rolling from the austenite region, or after hot rolling the above steel that has been allowed to cool to Ar below ₁ point is reheated to the austenite region and then quenched. During cooling, as the surface temperature of the steel plate decreases, the water volume density is increased to make the cooling rate uniform in the thickness direction, and then tempering is performed to reduce the martensite content to 60% or more and 5% martensite to the entire plate thickness. A method for producing a tempered thick-walled 80Kgf/mm ² or higher grade high tensile strength steel, characterized by forming a mixed structure with the above bainite. 2 C: 0.03-0.15%, Si: 0.50% or less, Mn:
0.40~1.60%, Ni: 0.30~5.0%, Sol, Al: 0.005
Contains ~0.08%, further Cu: 1.0% or less, Cr:
1.5% or less, Mo: 0.8% or less, W: 0.8% or less,
Nb: 0.08% or less, V: 0.15% or less, Ti: 0.05% or less, Zr: 0.1% or less, Ca: 0.01% or less, REM:
0.01% or less, B: 1 or 2 of 0.004% or less
After hot rolling, the steel containing at least 10% of Ar, with the remainder consisting of Fe and unavoidable impurities, is immediately removed from the austenite region, or after hot rolling, the above-mentioned steel, which has been allowed to cool down to ₁ point or less of Ar, is reheated to the austenite region, and then quenched. During cooling, as the surface temperature of the steel sheet decreases, the water flow density is increased to make the cooling rate uniform in the thickness direction, and then tempering is performed to reduce the martensite content to 60% or more and 5% martensite at the entire thickness position. A method for manufacturing a tempered thick-walled 80Kgf/mm ² or higher grade high tensile strength steel, characterized by forming a mixed structure with the above bainite.