JPH0319285B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention uses steel with excellent sulfide corrosion cracking resistance.
The present invention relates to a method for manufacturing non-tempered steel using a specific composition and cooling after hot rolling. In general, steel materials used in wet hydrogen sulfide corrosion environments are resistant to HIC (hydrogen-induced cracking) and SSC.
(Sulfide stress corrosion cracking) performance, that is, excellent sulfide corrosion cracking resistance is required. However, non-tempered steel (steel as hot rolled, steel produced by accelerated cooling after hot rolling, normalized steel, etc.)
In this case, a hard low-temperature transformed structure such as martensite or bainite is formed in that part due to segregation during casting. As a result, HIC and SSC occur in segregated areas even in steel that has been treated with ultra-low sulfur and Ca treatment to suppress the occurrence of HIC. Regarding SSC resistance, even in ferrite/pearlite steels that do not contain the above-mentioned low-temperature transformation structure, microcracks are likely to occur in the pearlite phase, and the σth (crack occurrence (critical stress) is low. In general, gas transport pipes have excellent low-temperature toughness, which is evaluated by the BDWTT test as a brittle fracture propagation prevention property, and notch ductility, which is evaluated by the Charpy test, which is an unstable elongated fracture propagation prevention property. required to be present. As already explained, line pipes, tanks, etc. used in wet hydrogen sulfide corrosive environments can be damaged due to blistering, HIC, SSC, etc. The steel used must have excellent resistance to sulfide corrosion cracking. Additionally, in gas transport pipes, BDWTT is generally used as a brittle fracture propagation stopping characteristic.
It is required to have excellent low-temperature toughness, which is evaluated by the test, and notch ductility, which is evaluated by the absorbed energy of the Charpy test, which is a property for inhibiting the propagation of unstable ductile fracture. Among these, blister or HIC is caused by the combination of hydrogen pressure accumulated at the inclusion/steel interface and hydrogen embrittlement of the steel substrate due to hydrogen in the steel (hydrogen that has penetrated into the steel through corrosion reactions). This is known to occur, and as a countermeasure for this, for example,
Publication No. 14606, Japanese Unexamined Patent Publication No. 134155, Japanese Unexamined Patent Publication No. 1987-134155
As disclosed in Publication No. 54-38214 etc.
It is effective to reduce large inclusions (MnS, Al ₂ O ₃ ) and control the shape of MnS, and in steel sheets manufactured by air cooling after hot rolling, P, Mn, Cr, Mo, etc. The amount of impurity elements or alloying elements is concentrated in the segregated part of the cast structure, and this part easily undergoes bainite or martensitic transformation, resulting in the formation of a hard low-temperature transformed structure, and the HIC of this low-temperature transformed structure. The susceptibility is extremely high, and it is difficult to completely prevent the occurrence of HIC even if the shape of inclusions is controlled.As a countermeasure, it is necessary to subject the steel to tempering, quenching, and tempering. Are known. Furthermore, it is reported in Japanese Patent Publication No. 54-38568 and Japanese Patent Publication No. 56-31845 that reducing the amount of Mn and P in the composition is effective.
It has been proposed in the Publication No. On the other hand, SSC is known to occur due to stress (residual stress, external stress), the weight of hydrogen embrittlement of the steel base, or the effects of inclusions superimposed on these.
It is also known that the susceptibility of the steel base to hydrogen embrittlement is extremely high (easily cracked) when it contains a hard structure such as martensite or bainite. Furthermore, even in ferrite/pearlite steel, which has excellent HIC resistance, microcracks occur in the pearlite phase as shown in Figure 8a and b, so the σth is lower compared to quenched and tempered materials (QT materials). There was a problem. Furthermore, in recent years, in order to improve welding efficiency, it has become common to weld with narrow grooves and small heat input conditions, and as a result, for example, in pipelines, the maximum hardness of the circumferential weld has become high, resulting in a decrease in SSC resistance. A problem is occurring. From the above, it can be concluded that steel materials that have been subjected to quenching and tempering heat treatment at the same time as taking various measures against inclusions have the best HIC resistance and SSC resistance. QT materials have lower productivity and higher manufacturing costs than non-tempered materials, and have limited low-temperature toughness (DWTT test). The present invention has been made in order to improve this problem, and it is possible to produce non-thermal steel with high productivity by limiting the carbon content to less than 0.03% and by specifying hot rolling conditions. The present invention provides a manufacturing method for obtaining HIC resistance and resistance by reducing segregation hardness and homogenizing the structure.
It improves SSC properties, low-temperature toughness, and notch ductility all at the same time. Furthermore, even in welds performed with low heat input, such as on-site welding of pipelines, the maximum hardness is kept low and SSC resistance is improved. The present invention has a carbon content of less than 0.03% and a Si of 0.1% on a weight basis.
~0.5%, Mn1.0% or more but less than 2.0%, Al0.005~0.1
%, and Cu1.0% or less, Ni1.0% or less, Nb0.10
% or less, V 0.15% or less, Mo 0.5% or less, and the balance consists of Fe and unavoidable impurities, after heating the steel to a temperature range of ₃ Ac to 1200℃.
This method involves hot rolling at a cumulative reduction rate of 30% to 90% at 650°C to 900°C, and allowing it to cool after rolling, producing a non-tempered steel with excellent sulfide corrosion cracking resistance. On the other hand, in the invention described in claim 2,
It is manufactured using the above-mentioned steel further containing 0.007% or less of Ca on a weight basis. The present invention is to produce a steel material having the above-mentioned composition by hot rolling and cooling from a large steel ingot of 20 tons or more or a continuous casting slab, and in this process, the carbon content is less than 0.03%. The aim is to obtain a steel material with excellent HIC resistance and SSC resistance of the base metal and welded parts by limiting the The reason for keeping the amount of C low is that by reducing the hardness of the segregated areas where impurity elements and alloying elements are concentrated,
The objective is to improve HIC resistance, homogenize the structure, suppress the occurrence of microcracks, and improve SSC resistance. In the steel welds obtained by the manufacturing method of the present invention, the maximum hardness is all NACE MR-01 even when high efficiency welding is performed with low heat input.
HR _c 22 or less, which is recommended for -75, and SSC resistance
There is no problem in terms of sexuality. Next, the reason why the component composition is limited as described above in the present invention will be explained. The reason for limiting C to less than 0.03% is that if it exceeds this, a hard structure exceeding _Hv 300 will be formed in some of the segregated areas, increasing HIC susceptibility.
In terms of SSC properties, if it exceeds 0.03%, a pearlite phase will appear and microcracks will easily occur in the SSC test, so this is the upper limit. Si is a necessary element for deoxidation and is effective in improving strength and toughness, but if it is less than 0.1%, these effects cannot be obtained, so this is the lower limit, and if it exceeds 0.5%, toughness will deteriorate rapidly. This was set as the upper limit. Mn is an element necessary to ensure strength and toughness, and if it is less than 1.0%, this effect cannot be expected, so this is set as the lower limit, and if it exceeds 2.0%, even if the C content is limited to less than 0.03% as mentioned above, it will not be resistant. This was set as the upper limit because the HIC property deteriorated. Since Al is a necessary element for deoxidation, the lower limit is set as
It shall be 0.005%. However, excessive addition impairs the cleanliness of the steel, so the upper limit was set at 0.1%. Further, Cu, which is added as necessary, is a corrosion-resistant element and is effective in terms of strength, but if it exceeds 1.0%, weldability and toughness deteriorate, so this was set as the upper limit. Ni is an effective element for strength and toughness, and
It is an element that improves the hot workability of Cu-containing steel.
However, if added in excess of 1.0%, SSC resistance deteriorates, so this was set as the upper limit. Nb and V are effective elements in terms of toughness and strength of steel, but if added in excess of 0.10% for Nb and 0.15% for V, the toughness deteriorates, so these were set as the upper limits for each. Mo improves organization and increases strength, but 0.5
If it exceeds 0.5%, the toughness deteriorates, so the upper limit was set at 0.5%. On the other hand, in the invention set forth in claim 2, the material to which Ca is added has the effect of controlling the shape of MnS, and is also effective in reducing the number of starting points for HIC. However, if it exceeds 0.007%, CaS clusters will form, making HIC more likely to occur, so this is the upper limit. Next, the reason why the hot rolling conditions are limited as described above in the present invention will be explained. The heating temperature of the steel billet or steel ingot in the pre-rolling process is Ac ₃
However, if this heating temperature is less than Ac ₃ , the structure before rolling will become extremely uneven, and even if the specified rolling is carried out, the structure after transformation will not be the same. This was set as the lower limit because it would be non-uniform and high toughness could not be obtained. Also, the upper limit is 1200℃
The reason for this is that heating at a temperature higher than this causes the austenite grains to become coarse, and high toughness cannot be obtained even if the steel is subsequently rolled to a certain degree. Furthermore, in hot rolling, it is necessary to perform rolling at a cumulative reduction rate of 30 to 90% at 650 to 900°C. Reason for limiting the upper limit temperature to 900°C to obtain the above cumulative reduction rate: The upper limit temperature was set to 900°C in order to apply reduction in the non-recrystallized austenite region to refine the transformed structure. The reason for the lower limit temperature of 650°C to obtain the above-mentioned cumulative reduction ratio: If the temperature is lower than 650°C, rolling difficulties such as increased mill load will significantly increase, making it unproductive. Reason for setting the upper limit of the cumulative reduction rate to 90℃: If rolled at a cumulative reduction rate of over 90%, the steel plate will be cooled during rolling, forming an extended substructure on the steel plate surface, which will deteriorate HIC resistance. Reason for setting the lower limit of the cumulative reduction rate to 30%: If it is less than 30%, the structure after transformation cannot be refined and high toughness cannot be obtained, so the lower limit was set to 30%. Hereinafter, the present invention will be explained based on examples. Table 1 shows the composition of the slab used in the test, the hot rolling conditions, and the characteristic values obtained for each test material.
and as shown in Table 2 (in Table 2, 900℃
The following cumulative rolling reduction ratios were 30% or more for both the inventive material and the conventional material, and the materials were allowed to cool after rolling. For the HIC test, a test piece was created with the sample volume and dimensions shown in Figure 1 A and C (the dimensions are as follows), and the test piece thickness: B = T - 2 mm (maximum 20 mm). 〃 Width: W = 20mm 〃 Length: L = 100mm This specimen was saturated with hydrogen sulfide (5% salt +
3 of each specimen after immersion in an aqueous solution (0.5% acetic acid).
A method of measuring cracks in cross sections was adopted. C in Figure 1 is the CLR (%) and CSR (%) of the HIC test calculated by the following formula: CLR (%) = Σ ai / A × 100 CSR (%) = Σai.Σ bi / AB × 100
This shows how to calculate. Figure 2 shows the amount of ladle Mn, which is considered to correspond to the amount of Mn in the non-segregating part, and the results of the HIC test.
HIC susceptibility increases suddenly when the Mn content exceeds 1.0%, whereas in the steel of the present invention where the C content is limited to less than 0.03, cracking hardly occurs even when the Mn content is 1.0% or more, leading to fracture accidents. It has been shown that step cracking, which is said to be a possibility, does not occur at all (CSR=0%). Figure 3 shows the minute part of the segregated part of the steel plate.
Measure the highest concentration of Mn with EPMA and ladle Mn
This is the result of investigating the ratio to the amount (segregation coefficient). C
Conventional steel containing 0.04% to 0.15% of ladle
The segregation coefficient is as large as 2.0 to 2.5 times, regardless of the Mn content.
On the other hand, the segregation coefficient of the inventive steel with C limited to less than 0.03% was shown to be as small as 1.2 to 1.5 times, and compared to the conventional steel, the inventive steel showed less segregation despite the higher Mn content. It turns out to be quite weak. This trend is the same for elements other than Mn; for example, the segregation of P has decreased from 8.0 to 10 times (conventional steel) to about 5.0 times (inventive steel), and the segregation of Mo has decreased by 2.0 to 2.5 times (conventional steel). It was confirmed that the steel) was reduced by 1.3 to 1.6 times (the steel of the present invention). Figure 4 shows the relationship between the amount of Mn in the segregated area and the hardness of the segregated area. If the hardness is already H _v (50g) = 300 or higher, HIC
It has been shown that susceptibility increases [Japan Steel Pipe Technical Report 87 (1980) 61]. In conventional steel, segregation
As the amount of Mn increases, the hardness of the segregated part increases to _Hv 300 or more, and the HIC also increases, whereas in the steel of the present invention, the hardness increases to _Hv 300 when the amount of Mn in the segregated part is 3.0% or less.
The following shows that it is possible to prevent the occurrence of HIC. However, even in steel with less than 0.03% C,
When the amount of Mn in the segregated area exceeds 3.0%, some parts of the segregated area
The incidence of HIC increases as _Hv exceeds 300. Considering Figures 3 and 4 together, the amount of C is 0.03
%, upper limit of Mn content (non-segregating part) < upper limit of Mn content (segregating part) / maximum value of segregation coefficient =
2.0, that is, it was found that the occurrence of HIC can be prevented by reducing the amount of ladle corresponding to the amount of Mn in the non-segregating portion to 2.0% or less, and these results support the results shown in FIG. In addition, in the SSC test, a test piece is prepared according to the test piece collection procedure and dimensions (mm) shown in Figure 5 A and B, and this test piece is tested and measured using the test equipment shown in Figure 5 C. be. The code 1 in the figure C is the test piece.
2 is a test liquid tank, 3 is a test piece fixing chuck, 4 is a load transmission arm, 5 is a load, and the test liquid tank 2
Inside is NACE aqueous solution (5% NaCl + 0.5% CH ₃
(COOH) + H ₂ S saturation). The test method is to first clamp the test piece and place it in the test liquid tank.
In this method, an aqueous solution is added, a predetermined stress is applied, and the test is continued until the test piece breaks or 500 hours have passed. In addition, according to the SSC test results in Tables 2 to 4: 〇 Those that did not break even after 500 hours. ● Items that break within 500 hours. It is. Figure 6 shows the relationship between carbon content and SSC test results, and when the carbon content is 0.03% or less, σth/σYS
If it is less than 0.8, it will not break, and σth/σYS=
σth/σYS is 0.20 compared to conventional steel of 0.55-0.65
It can be seen that there has been some improvement. This is because the structure of the steel of the present invention is homogenized as shown in FIG. 9, and no minute cracks occur in the SSC test. Furthermore, assuming on-site welding of pipelines, which requires highly efficient welding, the groove dimensions shown in A and B in Figure 7 are used.
Table 2 shows the results of welding under the welding conditions shown in the table below. The wire used for welding is Kobe Steel's MG63B (1.2φ) shielding gas (Ar +
20% CO ₂ ) 25/min.

[Table] As is clear from Table 2, conventional steel has H _v 235 ~
350, which far exceeds the upper limit of hardness HRC22 ( _Hv 248) recommended by NACEMR-01-75. On the other hand, in the steel of the present invention, _Hv is considerably lower in the range of 195 to 235 than _Hv 248 for all steel types, and it is clear that there is no problem with the SSC resistance of the circumferential weld. Ta. In addition, SSC resistance
The difference between the upper limit of hardness for properties (H _v 248) and the upper limit of hardness for HIC resistance (H _v 300) is due to the difference in stress state. In other words, the former calculates the critical hardness assuming a high stress load corresponding to actual usage conditions, whereas the latter determines the critical hardness at which cracking occurs when no external force is applied. by. Next, low temperature toughness will be explained as a general material property. Table 3 shows various properties when the slabs of steel type J shown in Table 1 were hot rolled under different conditions (J-2 is the same as in Table 2). As is clear from this table, there is no difference between the three (J-2, J-5, J-6) in terms of HIC resistance and SSC resistance, but the cumulative reduction rate below 900℃ is 30
%, J-6 does not have sufficient low temperature toughness. Table 4 shows various properties of steel plate K-1 obtained by hot rolling the steel K slab shown in Table 1 (see Table 2) and steel plate K-3 which was subjected to QT heat treatment after hot rolling. It is something. As is clear from this table, steel material K-3 as well as J-6 shown in Table 3 cannot obtain sufficient low-temperature toughness. When compared with these, it can be seen that the low-temperature toughness of steel types J to L shown in Table 2 is very excellent. Furthermore, notch ductility will be explained. All of the conventional steels shown in Table 2 have a ferrite-pearlite structure, and the brittle pearlite part becomes the starting point of ductile _fracture. E _p is extremely low. As is clear from the above examples of the present invention, the steel produced by the method of the present invention has excellent HIC resistance and
It has excellent SSC properties, low-temperature toughness, and notch ductility.

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【table】 [Brief explanation of the drawing]

Figure 1 A, B, and C are explanatory diagrams showing the collection capacity, dimensions, and shape of HIC test pieces, Figure 2 is a graph diagram showing the relationship between ladle Mn content and HIC, and Figure 3 is a graph diagram showing the segregation tendency of Mn. Figure 4 is a graph showing the relationship between the amount of Mn in the segregated area and microhardness, Figure 5 A, B, and C are explanatory diagrams showing the SSC test piece and testing equipment, and Figure 6
Figure 7 is a graph showing the relationship between C content and SSC test results, Figures 7A and 7B are explanatory diagrams showing the dimensions and shape of the groove when contact is required, and Figure 8 is a microstructure microscope showing SSC of ferrite/pearlite steel. In the photograph, a is (x50) and b is the part surrounded by the mouth (x200). Figure 9 is a microscopic photograph (x200) of the structure of steel obtained by the method of the present invention.
It is.

Claims (1)

[Claims] 1. Less than 0.03% C, 0.1 to 0.5% Si, on a weight basis
Mn 1.0% or more and less than 2.0%, Al 0.005-0.1%, and
Cu1.0% or less, Ni1.0% or less, Nb0.10% or less,
Steel containing one or more of V0.15% or less and Mo0.5% or less, with the balance consisting of Fe and unavoidable impurities.
Ac ₃ points ~ 650℃ ~ 900℃ after heating in the temperature range of 1200℃
1. A method for producing non-tempered steel with excellent sulfide corrosion cracking resistance, which comprises performing hot rolling at a cumulative reduction rate of 30% to 90%, and cooling after completion of rolling. 2 Based on weight, C less than 0.03%, Si 0.1-0.5%,
Mn1.0% or more and less than 2.0%, Al0.005~0.1%,
Contains one or more of Ca0.007% or less, Cu1.0% or less, Ni1.0% or less, Nb0.10% or less, V0.15% or less, Mo0.5% or less, the remainder being Fe and unavoidable impurities. After heating the steel to a temperature range of Ac ₃ points to 1200℃, the cumulative reduction rate at 650℃ to 900℃ is 30%.
A method for producing non-tempered steel with excellent sulfide corrosion cracking resistance, which comprises performing hot rolling to a temperature of ~90% and cooling after completion of rolling.