JPH0140103B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to high tensile strength steels with high COD values. Conventionally, in addition to weldability, the quality characteristics of steel materials have been evaluated primarily by tensile tests and Charpy impact tests, and the conditions of use have often been determined based on these values. However, in recent years, a method of evaluating brittle fracture occurrence characteristics using the COD value of a three-point notch bending test has been considered, and is becoming widely used due to the simplicity of the test method. Furthermore, due to the magnitude of the impact that large structures have on society and the natural environment, we also show the degree of resistance of steel materials to fracture in the presence of notches.
COD testing is increasingly required.
Currently, the materials that require excellent COD values are approximately 50Kg/mm ^2nd grade and 60Kg/mm ^2nd grade high-strength steel sheets. In COD tests, the original thickness is usually used for testing, but since the point of fracture at the crack tip (usually the center of the plate thickness) is subject to strong plastic restraint in the thickness direction, the thickness of the test piece increases. As the thickness increases, the deformation approaches a plane strain state and the COD value decreases. In other words, the thicker the plate, the more difficult it becomes to satisfy a certain required COD value. Looking at the average value, the COD value will also be good if the Charpy impact test is good. Therefore, high COD
The conventional means for obtaining this value include conventionally known means for increasing the Shalpy value. Here we come
Since the COD value uses the minimum value, it has been found that there is no one-to-one correspondence with the Shalpy value, and countermeasures are desired. However, as a means to obtain a high COD value,
-79828 is known. This method is intended to increase the COD value by refining the grains with AlN, but if there are no inclusions, weak structures, or non-uniform material areas due to segregation, etc. in the steel material,
Although it can be an effective means to improve COD values,
Unfortunately, the above-mentioned non-uniform parts of the material always exist in steel materials, and unless these measures are taken, it will not be possible to obtain a consistently high COD value. (Means for solving problems) The present invention has been made to advantageously solve the above problems, and its gist is as follows: (1)
C: 0.15% or less, Si: 0.05 to 0.60% by weight,
Mn: 0.5-2.0%, P: 0.012% or less, S: 0.003%
Below, Nb: 0.02% or less, T, Al: 0.01-0.10%,
Oxygen: 0.003% or less, Cu: 0.5% or less, Ni: 1.0%
Hereinafter, a high tensile strength steel having a high COD value containing 0.1% or less of V, with the remainder consisting of Fe and unavoidable impurities.
(2) Contains Ca: 0.0005 to 0.006% in addition to the ingredients in (1) above, and the balance is Fe and unavoidable impurities, making it a high COD
It concerns high tensile strength steel with a value. In other words, based on the knowledge obtained as a result of numerous experiments, the present inventors have determined that the COD
In order to increase the value, we have found that it is important not only to increase the average value, which is generally considered to be an improvement in base material low-temperature toughness, but also to reduce variations and eliminate extremely low values. Among these, the average value almost corresponds to the Shapey impact test value, so conventional measures to improve the Shapey impact test value can be used. However, the variation, that is, the occurrence of extremely low values, does not correspond to the Charpy impact test values. Therefore, it is necessary to find new and unconventional ways of thinking. The present inventors have clarified that the variation in COD value is caused by local heterogeneity in the steel sheet. this
As a result of a detailed investigation of local factors that affect the COD value, we found that intergranular fracture appears at the tip of the COD crack after stress relief annealing, and that there are inclusions at the tip of the crack that become the starting point of the crack. The synergistic effect of the presence of segregation that reduces ductility results in a low COD value, and even if the average value is high, sometimes low values are obtained, and this value eventually dominates the steel properties. This is what I found out. Based on the above, the inventors found that after stress relief annealing,
Addition of a trace amount of Nb within a range that suppresses precipitation embrittlement to reduce P as a countermeasure against intergranular fracture on COD fracture surfaces, countermeasures against intergranular fracture, and improves low-temperature toughness of the base metal. It is effective in improving COD by obtaining a granular structure [variation within the second largest grain size number].
In addition, various measures are taken to reduce S to reduce distraction inclusions, which are the starting point for crack generation, to lower O to reduce cluster inclusions, and to make distraction inclusions and cluster inclusions harmless by making them spheroidal. It was discovered that by combining these, it is possible to improve the average value and reduce the variation, and the COD value can be greatly improved. In order to further improve the COD value, COD
Since the value is guaranteed to be the lowest value, it is necessary to minimize the variation in the value.
In addition to inclusions, the cause of variation in COD values is component segregation. The segregated parts have high hardness, and become brittle due to the low-temperature transformation structure, and the ductility decreases, resulting in an extremely low COD value. A commonly used method for evaluating segregation is to cut a slab in steps of 0.5 mm in the slab thickness direction, analyze each step, and examine the segregation status of components in the slab thickness direction. However, when we investigated the relationship between segregation degree and COD value in detail using this method, we found that when the segregation degree (peak value/average value) of C reaches a boundary of 1.5 and exceeds it, COD
It was confirmed that the value decreased significantly. This is shown in FIG. In other words, in order to suppress variations in the COD value, the degree of segregation is reduced to 1.5 or less using the above measurement method, thereby making it possible to further improve the COD value. However, as a means of reducing segregation, in addition to the adoption of wide-cover molds for agglomerated materials and the adoption of electromagnetic stirring for continuous casting materials, soaking can be carried out advantageously for both ingots and continuous casting materials. There are diffusion heat treatment means and the like, and one or more of these methods may be used in combination. The reasons for limiting the ingredients in the present invention are as follows. Although a lower C content is preferable for the purpose of the present invention, it is necessary to have an amount of C that ensures a minimum strength depending on the plate thickness. However, if it exceeds 0.15%, weldability and toughness deteriorate, so there is a limit. Si of 0.05% or more is necessary for deoxidation, but if it exceeds 0.60%, weldability will deteriorate. Mn is a useful element for improving strength without impairing toughness, and therefore it is necessary to have at least 0.5%, but if it exceeds 2.0%, weldability deteriorates. P is an element harmful to COD values. Plate thickness is 50mm
The above thick steel plates are usually subjected to stress relief annealing after welding, but if the amount of P is large, this post-weld heat treatment embrittles the grain boundaries, which is one of the causes of a decrease in the average value of COD. Therefore, P is limited to 0.012% or less. More preferably, it is 0.007% or less. S is a harmful element. When the amount of S is large, Mns-based inclusions extend long in the rolling direction in the rolled material, reducing the average value of COD. In order to reduce distraction inclusions, S is limited to 0.003% or less. Nb is a useful element for increasing tensile strength and improving toughness. Nb helps to regularize the crystal grains (variation within the second grain size number), but if it exceeds 0.02%, precipitation embrittlement occurs due to stress relief annealing applied to thick steel plates, and mixed grains tend to occur. As a result, the average value of COD decreases. Al is an element that deoxidizes steel, forms nitrides, and has the effect of refining the grains, and 0.01% is necessary for deoxidation, but if it increases, inclusions will increase and cleanliness will be impaired. The upper limit was set at 0.10%. Oxygen is a harmful element, and when the amount increases, inclusions increase, impairing cleanliness, increasing variation, and increasing COD.
This causes a decrease in value. For this reason, it is limited to 0.003% or less. The present invention uses the above-mentioned elements as basic components, and contains one or more of the following elements as required. Cu is an element that increases strength and weather resistance when dissolved in solid solution, but if the amount increases, it causes hot brittleness and impairs weldability, so the upper limit was set at 0.5%. Ni is an element that improves low-temperature toughness and prevents the hot embrittlement that occurs when Cu is added, but it is expensive, so the upper limit was set at 1.0%. V is an element that increases strength through precipitation strengthening, but in large amounts it impairs toughness, so it is added at 0.10%.
The following was made. Ca is long in the rolling direction and becomes harmless by spheroidizing distraction inclusions and cluster-like inclusions, resulting in COD
The average value is improved and the variation is reduced at the same time, resulting in improved toughness. In order to work effectively
A content of 0.0005% is necessary, and a content of 0.006% or more increases oxide inclusions, which is undesirable from the viewpoint of toughness and weldability. The steel according to the present invention is suitable for use in large steel structures (for example, offshore structures, nuclear reactor containment vessels, etc.) that require high fracture toughness, and is also suitable for line pipes. The present invention is useful not only for improving the COD value of base metals, but also for improving the COD value of welded parts in particular. This effect is remarkable at a heat input of 50,000 J/cm or less. Further, the steel of the present invention is manufactured by ordinary steel manufacturing methods, such as converter furnaces, electric furnaces, vacuum melting, etc., and it goes without saying that it can be manufactured not only by ordinary ingot making but also by continuous casting. The obtained steel ingot or slab is reduced to a predetermined size by conventionally known rolling or forging, and if necessary, subjected to heat treatment steps such as normalizing, quenching and tempering. Next, examples will be listed together with comparative examples. Examples Table 1 shows the chemical composition, manufacturing conditions, and limits of welded parts of the inventive steel and conventional normalized 50 kg/mm ² grade and tempered 60 kg/mm ² grade high tensile strength steels for comparison.
The COD value was shown. The COD test was conducted using a notched three-point bending test piece according to the method of ASTME399. As a result, the limit is determined by the BS standard DD19.
Asked for COD.

[Table] Inventive steels A, B1, and B2 all have chemical components within the range defined by the present invention, and their COD values are extremely high. Furthermore, by comparing B1 and B2, it can be seen that the COD value further increases with the addition of Ca. Next, comparative steel E has a COD of 0.04% Nb.
value is low. Furthermore, since Comparative Steel F does not contain Nb, Comparative Steel G has S higher than the limit range of the present invention, Comparative Steels H1 and H2 have P higher than the limit range of the present invention, and Comparative Steel I has O that is higher than the limit range of the present invention. The COD values are lower because they are higher than the limited range of the invention.
Next, slabs of B1 steel and H1 steel (slab thickness 240mm)
The degree of segregation (peak value/average value) in the slab thickness direction was determined by cutting steps in steps of 0.5 mm in the slab thickness direction and chemically analyzing the slabs subjected to soaking diffusion treatment at 1300℃ x 10 hours and those without soaking diffusion treatment. A COD test was conducted on each slab rolled to a thickness of 100 mm. The results are shown in FIG. From the figure, the degree of segregation of C is. When it becomes 5 or less
It can be seen that the COD value increases dramatically. moreover,
It is clear from FIG. 1 that even if the degree of segregation is within the range of 1.5 or less, if the compositional design of the present invention is not satisfied, the limit COD value is poor.

[Brief explanation of drawings]

Figure 1 shows the degree of segregation of C and −
Explanatory diagram showing the relationship between the limit COD value (mm) of a welded part at 10℃.

Claims (1)

[Claims] 1. In terms of weight ratio, C: 0.15% or less, Si: 0.05~
0.60%, Mn: 0.5-2.0%, P: 0.012% or less,
S: 0.003% or less, Nb: 0.02% or less, T.Al: 0.01
~0.10%, Oxygen: 0.003% or less, Cu: 0.5% or less,
High tensile strength steel with a high COD value, consisting of Ni: 1.0% or less, V: 0.1% or less, and the balance Fe and unavoidable impurities. 2 C: 0.15% or less, Si: 0.05 to 0.60 by weight
%, Mn: 0.5-2.0%, P: 0.012% or less, S:
0.003% or less, Nb: 0.02% or less, T.Al: 0.01~
0.10%, oxygen: 0.003% or less, Cu: 0.5% or less,
Ni: 1.0% or less, V: 0.1% or less, Ca: 0.0005~
High tensile strength steel with a high COD value containing 0.006% and the balance consisting of Fe and unavoidable impurities.