JPH0132288B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention has a yield point of 80 kg/mm ² or more and a tensile strength of 90 kg/mm 2 or more.
This invention relates to a method for manufacturing high-tensile seamless steel pipes having a strength of mm ² or more. Conventionally, the material for high-strength seamless steel pipes as mentioned above has been Mn- containing about 0.05 to 0.10% Mo.
Mo-based or Mn-Cr-Mo based steels are often used, or Mn is 1.20 to 1.70% and Cr is 0.30 to 1.20%.
Mn-Cr steel containing about 1.9% is also used in some cases. A product is produced by hot rolling (inclined rolling method such as the Mannesmann method) into a seamless tube using steel with such composition as a raw material, followed by quenching and then tempering. However, due to the recent rise in the price of Mo, high-strength steel materials containing Mo have become disadvantageous in terms of cost, and conventional Mn-Cr-based high-strength steels have been shown to have inferior toughness compared to high-strength steels containing Mo. There are some difficulties. The purpose of the present invention is to solve the above-mentioned problems with conventional high-tensile seamless steel pipes, and its aim is to use inexpensive compositional steel that does not contain Mo to replace conventional Mo-containing steel pipes. The present invention provides a method for manufacturing a high-tensile seamless steel pipe having strength and toughness equal to or higher than that of steel. As a result of various studies on ingredients to replace Mo, the inventor found that by increasing Cr content from the conventional content, it is possible to obtain the same strength level as conventional Mo-containing steel, but it is difficult to temper. It has been learned that there are problems such as a decrease in toughness due to the embrittlement phenomenon, and in particular, vertical cracks occur in the steel billet after continuous casting, resulting in deterioration of quality. Therefore, the present inventor first investigated the relationship between the Mn content and Cr content in steel regarding the occurrence of vertical cracks in continuously cast slabs.
As a result of detailed investigation of Mn-Cr steel, the results shown in Figure 1 were obtained. In other words, from Figure 1, as the amount of Cr increases with the amount of Mn, vertical cracking is more likely to occur, and
When the amount exceeds 2.20%, vertical cracks will increase rapidly. The cause of vertical cracking is the formation of band-shaped ferrite at the prior austenite grain boundaries and the precipitation of large carbides. It is thought that thermal deformation during the cooling process of the steel slab concentrates on the ferrite portion, which together with a decrease in ductility due to the precipitation of carbides, causes vertical cracking. It is possible to prevent warping by keeping the Mn + Cr content below 2.20%, but if the Mn + Cr content is below 2.20%, the yield point will be 80 Kg/mm ² or more and the tensile strength will be 90
Strength greater than Kg/mm ² cannot be obtained. Therefore, the present inventor pursued a composition that could compensate for the lack of strength due to the decrease in Mn and Cr and also prevent the occurrence of vertical cracking.The inventors added B to Mn+Cr steel with an Mn+Cr content of 2.20% or less to strengthen it. It has been found that the presence of molten B improves hardenability, and that by quenching and tempering under appropriate conditions, a seamless steel pipe with high strength and excellent toughness can be obtained. However, when the added B combines with N in the steel to form a B--N compound, the amount of solid solution B decreases, and the hardenability improvement effect of B decreases. In order to prevent the formation of this B-N compound and ensure solid solution B that is effective in improving hardenability,
It is necessary to fix N in the steel by adding one or both of Sol.Al or Ti and to make B a solid solution. Furthermore, the present inventor conducted various studies on improving toughness using steel to which B was added, and found that toughness was improved by lowering the P content and lowering the tempering temperature, and that by adding B, the toughness was improved. As a result, the ductile rupture rate was found to be superior to that of conventional Mo-containing steel. Figure 2 shows that the amount of Mn+Cr with different P content is 2.20.
This figure shows the relationship between the tempering temperature and the transition temperature in Mn-Cr steel with a concentration of less than %. In the figure, curve A has a P content of 0.020%, and curve B has a P content of 0.025%.
%, curve C is for the case where the P content is 0.030%. Figure 3 shows Mo corresponding to the material steel used in the method of the present invention.
This figure shows the relationship between the ductile fracture ratio at 0° C. and the P content when the P content in the steel is changed in steel that does not contain Mo and conventional steel that contains Mo. In the figure, curve 2 is a conventional steel that contains Mo, and curve 5 is a steel that does not contain Mo. As is clear from Figures 2 and 3, the transition temperature increases as the P content increases in steel with an Mn+Cr content of 2.20% or less, and when the tempering temperature exceeds 550℃. The transition temperature increases significantly due to temper embrittlement. In addition, the ductile fracture ratio is also P
It decreases as the content increases, but the decrease is smaller in the steel of the present invention containing B than in the conventional steel containing Mo. The present invention, which has been made by integrating the above knowledge, has the gist of the following method for manufacturing a seamless steel pipe.
In addition, in this specification, all % regarding component content means weight %. 〓C: 0.15-0.35%, Si: 0.05-0.50%, P:
0.025% or less, S: 0.025% or less, Mn: 1.00 to 1.60
%, Cr: 0.40~1.00%, B: 0.0008~0.0025%,
Furthermore Sol.Al: 0.02~0.10%, Ti: 0.005~0.025%
Contains one or two of
High tensile strength steel characterized by continuous casting of steel with 2.20% or less into steel billets, hot rolling of the steel billets into seamless tubes, quenching, and then tempering at a temperature of 480 to 550°C. Method for manufacturing seamless steel pipes The reasons for limiting the composition of the steel material used in the method of the present invention will be described below. C is an element necessary to increase the strength of steel, and if the C content is less than 0.15%, the strength after quenching will be low, so the yield point after tempering will be 80 kg/mm ² or more,
In order to obtain a tensile strength of 90 Kg/mm ² or more, the tempering temperature must be lower than 480°C and in the blue brittle region. Tempering in such a low temperature range is not preferable because the elongation value also becomes low. On the other hand, if the C content exceeds 0.35%, the strength after quenching will be sufficiently high, but in order to adjust the strength of the product to an appropriate level, the tempering temperature must be set in the tempering embrittlement region of 550° C. or higher, which is not preferable. Si is an element added as a deoxidizing agent, but Si
If the amount is less than 0.05%, a sufficient deoxidizing effect cannot be obtained, and on the other hand, even if the amount of Si added exceeds 0.50%, the deoxidizing effect becomes saturated and becomes useless. Mn is an element that improves hardenability, but Mn
If the amount is less than 1.00%, the desired hardening effect cannot be obtained;
On the other hand, if the Mn content exceeds 1.60%, vertical cracking tends to occur as shown in FIG. 1, which is undesirable. As shown in FIGS. 2 and 3, when the content of P increases, the transition temperature increases and the ductile fracture ratio deteriorates. In order to keep the transition temperature vTs at 0° C. or below, it is desirable to reduce the P content to 0.025% or below as much as possible. If the amount of S exceeds 0.025%, the precipitation of sulfide inclusions will increase, which will deteriorate the quality of the product steel pipe, which is not preferable. Cr, like Mn, is an element that improves hardenability, but if the Cr content is less than 0.40%, the desired hardening effect cannot be obtained, while if the Cr content exceeds 1.00%, as shown in Figure 1. This is not preferable because vertical cracks are likely to occur. In addition, the total content of this Cr and the above-mentioned Mn is
The reason why it is set to 2.20% or less is because it is necessary to select an area where vertical cracks do not occur as shown in Figure 1. B is an element that improves hardenability, and in order to obtain a solid solution B amount of 0.0005 to 0.0008%, which is effective for improving hardenability, the total B content must be 0.0008% or more. On the other hand, if the amount of B exceeds 0.0025%, carbides will precipitate and cause embrittlement, which is not preferable. Sol.Al is necessary to fix N in the steel and obtain the above-mentioned amount of solid solution B, but if the amount of Sol.Al is less than 0.02%, it will ensure 0.0005% or more of solid solution B, which is effective for hardenability. difficult to do. On the other hand, if it exceeds 0.10%, the effect of fixing N will be saturated, and workability will worsen. Like Sol.Al, Ti fixes N in the steel and is effective in securing the amount of solid solution B. However, if the content is less than 0.005%, 0.0005% is effective for hardenability.
% or more of solid solution B cannot be obtained, and on the other hand, even if it exceeds 0.025%, the effect of fixing N is saturated. The above-mentioned Sol.Al and Ti may be contained alone or both may be used in combination. Material steel having the above composition is melted, continuously cast, and cut into appropriate sizes to form billets. This billet is heated and made into a seamless tube by, for example, Mannesmann hot rolling. Thereafter, quenching is performed either directly from the hot rolling end temperature or by cooling and reheating to the quenching temperature. The quenching conditions may be normal. It is important that the tempering temperature range is 480 to 550°C. If the tempering temperature is lower than 480°C, a sufficient tempering effect cannot be obtained, resulting in an incompletely tempered structure (untempered martensitic structure) and a blue brittle region, which is not preferable. On the other hand, at temperatures above 550℃, as shown in Figure 2,
It becomes a temper embrittlement region, the toughness deteriorates, the strength variation increases, and the ductility also decreases. Example 13 types shown in Table 1 were melted and continuously cast.
After making a billet of 228 mmφ, a seamless steel pipe with a wall thickness of 14 mm and an outer diameter of 273.1 mm was manufactured using the Mannesmann plug mill method. This was quenched under the same heat treatment conditions shown in Table 1, and then tempered. A test piece was taken from the obtained steel pipe, and the yield point, tensile strength, elongation, transition temperature, ductile fracture ratio, and absorbed energy at 0°C were examined. The results are also shown in Table 1.

[Table] As is clear from Table 1, even if the component system does not contain Mo, by appropriately selecting the composition as described above and properly performing heat treatment after pipe manufacturing, Mo High tensile strength seamless steel pipes have been obtained that have mechanical properties that are equivalent to or even superior to steel pipes made from conventional steels. Incidentally, no vertical cracks were observed in the continuously cast billet. According to the method of the present invention, since steel that does not contain Mo can be used as a material, the cost of manufacturing steel pipes can be significantly reduced.

[Brief explanation of drawings]

Figure 1 shows the Mn and
FIG. 3 is a diagram showing the relationship between the amount of Cr and the occurrence of vertical cracks in a continuously cast steel billet. FIG. 2 is a diagram showing the relationship between the tempering temperature and the transition temperature of Mn-Cr steels with different P contents and an Mn+Cr content of 2.20% or less.
Curve A...P content: 0.020%, Curve B...P content: 0.025%, Curve C...P content: 0.030%. FIG. 3 is a diagram showing the relationship between the P content and the ductile fracture ratio at 0° C. of a steel containing Mo and a steel corresponding to the material steel of the method of the present invention. Curve...
Mo-containing steel, curved line ho...Material steel for the method of the present invention.

Claims (1)

[Claims] 1 C: 0.15-0.35%, Si: 0.05-0.50%, P:
0.025% or less, S: 0.025% or less, Mn: 1.00 to 1.60
%, Cr: 0.40~1.00%, B: 0.0008~0.0025%,
Furthermore Sol.Al: 0.02~0.10%, Ti: 0.005~0.025%
Contains one or two of
High tensile strength steel characterized by continuous casting of steel with 2.20% or less into steel billets, hot rolling of the steel billets into seamless tubes, quenching, and then tempering at a temperature of 480 to 550°C. A method for manufacturing seamless steel pipes.