JPH0530888B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a method for producing hot-rolled high-strength steel for ERW steel pipes that has excellent workability, and has a tensile strength of 55 Kg・f/mm ² or more and a yield ratio of 62% or less. Regarding high tensile strength steel, used in the field of oil country tubular goods. [Prior art] Steel for ERW oil country tubular goods has excellent weldability, of course, but it is also desirable that it has high strength and a low yield ratio that makes pipe manufacturing easy.
There is API5AK55 steel. In order to obtain a low yield ratio, for example, Jizaimaru et al. showed that a low yield ratio material with a yield ratio of less than 65% could be obtained by changing the cooling pattern after hot rolling to rapid cooling in the second half (R&D Kobe Steel Technical Report vol. 33). , No. 4).
Further, JP-A No. 60-21325 shows that by regulating the chemical components, a low yield ratio material with a tensile strength of 60 Kg·f/mm ² or more and a yield ratio of less than 75% can be obtained. However, in the conventional method, the plate thickness is 8 mm.
It was extremely difficult to obtain a yield ratio of less than 62% on average. This is because as the plate thickness decreases, the total rolling reduction in finish rolling inevitably increases, the γ grains become finer, and the ferrite grains after transformation also become finer. The effect of increasing the yield point as the crystal grains become finer is “Hall−
This is generally known as the ``Petch relationship.'' As a result, thin materials have a high yield ratio, and in practice it has been difficult to obtain a yield ratio of less than 62%. [Problems to be Solved] The purpose of the present invention is to solve the above-mentioned problems of the prior art, and to achieve a tensile strength of 55 Kg・f/mm ² using a material with a plate thickness of 8 mm or less.
The above provides a method for producing hot-rolled high-strength steel that can obtain a yield ratio of 62% or less. [Means and effects for solving the problems] The above objects of the present invention are achieved by the following two inventions. The gist of the first invention is as follows. That is, it contains C: 0.60% or less Si: 0.50% or less Mn: 2.0% or less Total Al: 0.05% or less in weight ratio, and the composition ratio of C, Si, and Mn is 97.6 (% C) + 11.0 ( %Si)+14.5(%Mn)≧52.4, and the remainder is Fe and unavoidable impurities.
The temperature range in the tandem rolling mill after rough rolling is
3 to 800 to 1000℃ and total reduction rate of 30 to 80%
A step of finishing rolling using the stand of No. 5;
After the final rolling, the process is water-cooled and coiled at a temperature range of 550 to 650℃, and the tensile strength is 55Kg.
This is a method for producing hot-rolled high-strength steel for ERW steel pipes having excellent workability and having a thickness of 8 mm or less and having a yield ratio of f/mm ² or more and a yield ratio of 62% or less. The gist of the second invention is that, in addition to the same basic components as the first invention, a steel billet containing Ca: 0.0050% or less, with the balance consisting of Fe and unavoidable impurities, is prepared under the same conditions as the first invention. This is a method for producing hot-rolled high-strength steel for ERW steel pipes, in which finish rolling is performed at The present invention improves workability by limiting the content and composition ratio of the main components C, Si, Mn, Al, and Ca, and by controlling the temperature in finish rolling, the total pressure lower head, the number of stands, and the coil winding temperature. This is a method for producing excellent high-strength hot-rolled steel for oil country tubular goods. Next, the reason for limiting the components of the present invention will be explained. C: The higher the content of C, the more advantageous it is to improving the tensile strength, but if it exceeds 0.60%, cracks in the weld zone are likely to occur due to center segregation, so the upper limit was limited to 0.60%. Si: Since Si is an element that promotes γ→α transformation, the higher the content, the higher the ferrite fraction and the better the ductility and workability. However, on the other hand, as a solid solution strengthening element, it increases the yield point of the ferrite base, so it is not preferable to add an excessively large amount. Also, Si is 0.50%
If the Si content is exceeded, the occurrence of red scale on the surface of the steel sheet will become significant and the value of the product will be lost.
It was set to 0.50%. Mn: Mn is also an element necessary to ensure tensile strength and also to prevent the formation of low melting point FeS during hot rolling, but 2.0%
If the Mn content exceeds the
Limited to 2.0% or less. Al: Al is added for the purpose of deoxidizing and cleaning the steel, but if the amount added is too large, the excess Al present in the steel as substitutional solid solution atoms will combine with oxygen during welding, making it easy to form penetrators. Become. In this way, all-Al is disadvantageous in terms of manufacturing cost, so all-Al
The upper limit was set at 0.05%. Although the above-mentioned limited amounts of C, Si, Mn, and total Al are the basic components of the high-strength steel of the present invention, the purpose of the present invention can be more effectively achieved even when Ca is added in the following limited amounts. It can be added as necessary. The reason for this limitation is as follows. Ca: Ca improves toughness by controlling the form of sulfides, so it is a necessary element when toughness is required, but if it exceeds 0.0050%, it creates CaO and deteriorates the cleanliness of steel. Unfavorable, Ca
The upper limit was set at 0.0050%. Next, the reason why the composition ratios of C, Si, and Mn and the coil winding temperature are limited as described above will be explained. Twelve types of 100Kg vacuum melted steel shown in Table 1 were hot rolled to create steel slabs with a thickness of 50mm, a width of 200mm, and a length of 200mm.
After finish rolling to ~10mm, width 200mm, length lmm,
It is water-cooled to any temperature between 500 and 650℃, and then subjected to heat treatment corresponding to the thermal history after winding into a coil.
We investigated the material and found the tensile strength at 650℃.
We found the following relationship between TS and chemical components. TS (Km・f/mm ² ) = 2.6 + 97.6 (%C) + 11.0 (%Si) + 14.5 (%Mn) ...(1)

【table】

[Table] Generally, the lower the coil winding temperature, the higher the tensile strength, so the tensile strength at 650℃ is 55Kg・f/
For steel with a composition of mm ² or more, the coiling temperature is 550 to 650℃.
If it is within the range, TS≧55Kg・f/ ^mm2 . Therefore, the following equation (2) holds true. 2.6 + 97.6 (%C) + 11.0 (%Si) +14.5 (%Mn) ≧ 55 (Kg・f/mm ² )...(2) Therefore, 97.6 (%C) + 11.0 (%Si) ) +14.5(%Mn)≧52.4...(3) That is, the condition of formula (3) is required to make the tensile strength 55Kg·f/mm ² or more. The tensile strength level is set to 55 from the component range of formula (3) above.
To achieve Kg・f/mm ² or more, the winding temperature is 650℃.
It is necessary to do the following. On the other hand, the winding temperature is 550℃
It has been found that when it is less than 100%, bainite transformation occurs, the strength increases rapidly, and the plate shape becomes disordered. Therefore, the coil winding temperature was limited to a range of 550 to 650°C. In addition, since it is not possible to stably obtain hot rolled steel with a yield ratio of 62% or less based only on the chemical composition and coil winding temperature, we investigated the finishing total rolling reduction ratio at 800 to 1000°C and the number of rolling stands of the finishing tandem rolling mill. did. In other words, steel with the two types of components shown in Table 2 is continuously cast in a converter, hot-rolled to various thicknesses under the conditions shown in Table 2, and then wound into coils at a temperature of around 600°C. Ivy. The materials of the manufactured hot rolled steel sheets were investigated and are also shown in Table 2. The results in Table 2 are summarized in Figure 1. 1st
In the figure, those with a yield ratio of 62% or less are shown with open symbols, those with a yield ratio of over 62% are shown with black symbols, and the number to the right of the symbol indicates the plate thickness (mm). From Figure 1, when material with a thickness of 8 mm or less is wound at a coil winding temperature in the range of 550 to 650°C, the yield ratio is 62%.
It can be seen that in order to obtain the following materials, the hot rolling conditions must be such that the number of finishing rolling stands is at least 3 stands and no more than 5 stands, and the total finishing rolling reduction rate between 800 and 1000°C is 80% or less. .

[Table] Generally, in ordinary carbon steel, the austenite grain size (γ grain size) on the finish rolling exit side changes greatly depending on the rolling reduction in the temperature range of 800 to 1000°C, and the higher the rolling reduction, the finer the grains become. During the transformation from γ to α, fine grains α tend to come out from fine grains γ, and therefore the yield point becomes high.
That is, the yield point and yield ratio tend to increase as the total rolling reduction ratio increases. On the other hand, when the rolling reduction rate is the same, the more passes, the more
Since the number of recrystallizations of the γ grains increases after each pass, the γ grains tend to become finer, and as a result, the yield ratio also tends to increase. After all, the lower the total rolling reduction and the fewer the number of passes, the lower the yield ratio can be obtained. From the results in Figure 1, the total reduction rate at 800 to 1000℃ is
A material with a yield ratio of 62% or less was obtained by rolling with a yield ratio of 80% or less and 5 stands or less. For this reason, the upper limit of the total reduction rate at 800 to 1000℃ is set to 80%,
Additionally, the upper limit of the number of stands was set at 5. On the other hand, rolling with less than 3 stands would result in excessive load per stand, resulting in poor shape and unstable operation, so the lower limit of the number of stands was set at 3 stands. Regarding the rolling reduction ratio, for the reasons mentioned above, the lower the rolling reduction ratio, the better the yield ratio, but if the rolling reduction ratio is too low, it will interfere with the rolling mill, so the lower limit should be set.
It was set at 30%. Normally, it is thought that the reduction rate in hot rolling has an effect on the material quality at the low temperature side of the γ region of 1000 to 800°C. If this temperature is exceeded, recrystallization and subsequent grain growth will increase and the material will hardly change. Furthermore, if the temperature is lower than 800°C, the rolling load will be large and operation will be difficult. For this reason, we specified the rolling reduction rate in the temperature range of 800 to 1000°C, which is effective for the material and actual operation. Further, the distribution of the rolling reduction rate of each stand may normally be uniform, and is not particularly limited, but from the viewpoint of the shape of the plate, it is desirable that the rolling reduction rate of the final stand be lighter than that of the preceding stage. 〔Example〕

【table】

〔Effect of the invention〕

As is clear from the above examples, the present invention limits the ingredients, and the temperature in finish rolling, the total rolling reduction,
By controlling the number of stands and the coil winding temperature, the tensile strength can be increased to 55Kg・f/mm2 ^or more and the yield ratio can be increased.
We were able to produce hot-rolled high-strength steel for ERW steel pipes with excellent workability of 62% or less.

[Brief explanation of the drawing]

FIG. 1 is a relationship diagram showing the influence of the final rolling reduction ratio between 800 and 1000° C. and the number of finishing rolling stands on the yield ratio, and FIG. 2 is a schematic diagram illustrating the steepness of the steel plate.

Claims (1)

[Claims] 1 Contains C: 0.60% or less, Si: 0.50% or less, Mn: 2.0% or less, total Al: 0.05% or less, and the composition ratio of C, Si, and Mn is 97.6 (% C ) + 11.0 (% Si) + 14.5 (% Mn) ≧ 52.4 A step of heating and rough rolling a steel billet that satisfies the following relational expression, the remainder being Fe and unavoidable impurities;
The temperature range in the tandem rolling mill after rough rolling is
3 to 800 to 1000℃ and total reduction rate of 30 to 80%
A step of finishing rolling using the stand of No. 5;
After the final rolling, the process is water-cooled and coiled at a temperature range of 550 to 650℃, and the tensile strength is 55Kg.
A method for producing hot-rolled high-strength steel for ERW steel pipes having excellent workability and having a thickness of 8 mm or less and having a yield ratio of f/mm ² or more and a yield ratio of 62% or less. 2 Contains C: 0.60% or less, Si: 0.50% or less, Mn: 2.0% or less, total Al: 0.05% or less, and further contains Ca: 0.0050% or less, and the composition ratio of C, Si, and Mn is A step of heating and then rough rolling a steel billet that satisfies the relational expression: 97.6 (%C) + 11.0 (%Si) + 14.5 (%Mn) ≧ 52.4, the balance being Fe and unavoidable impurities;
The temperature range in the tandem rolling mill after rough rolling is
3 to 800 to 1000℃ and total reduction rate of 30 to 80%
A step of finishing rolling using the stand of No. 5;
After the final rolling, the process is water-cooled and coiled at a temperature range of 550 to 650℃, and the tensile strength is 55Kg.
A method for producing hot-rolled high-strength steel for ERW steel pipes having excellent workability and having a thickness of 8 mm or less and having a yield ratio of f/mm ² or more and a yield ratio of 62% or less.