JPS59104428A - Preparation of hot rolled coil excellent in hydrogen induced cracking resistance - Google Patents

Abstract

PURPOSE:To prepare a hot rolled coil having excellent hydrogen induced cracking resistance in a mass production system, by a method wherein steel prepared by containing C, Si, Mn and Ca in Fe in specific ratios is continuously cast and rolling, heating, rolling and winding-up are successively performed under specific conditions. CONSTITUTION:Steel containing, on a wt. basis, 0.02-0.18% C, 0.01-1.00% Si, 0.4-2.0% Mn and Ca in an amount so as to bring a Ca/S value to 2-8 and, according to necessity, one or more component selected from 0.2-1.0% Cu, 0.1-1.0% Ni, 0.2-1.0% Cr, 0.05-0.1% Mo, 0.01-0.12% Nb, 0.01-0.12% V and 0.01-0.20% Ti and comprising the remainder Fe and inevitable impurities is continuously cast. The cast piece is directly rolled at a draft of 10-70% and a finish temp. of 700 deg.C or more without heating and, after the rolled steel plate is heated to 1,000 deg.C or more, hot rolling is again applied thereto to wind up the same at 400-640 deg.C. By this method, a hot rolled coil excellent in hydrogen induced cracking resistance is obtained economically in good efficiency.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a hot-rolled coil having excellent hydrogen-induced cracking resistance and suitable for manufacturing line pipes for sour oil and sour gas.

In recent years, new oil and gas fields have been actively developed in order to cope with the worsening energy situation. Development attention is now being turned to oil and gas fields located in environments where sour gas (sour gas) exists. However, hydrogen sulfide, etc. in these sour oils and sour gases coexist with water and have an extremely high probability of contributing to corrosion of the steel surfaces that make up line pipes.
However, it has been revealed that the hydrogen generated by the corrosion at that time penetrates into the steel material and causes hydrogen-induced cracking (HIC), and research and development of steel materials with particularly excellent resistance to hydrogen-induced cracking is underway. The current situation is that

By the way, currently, in order to confirm that the steel material is practically free from hydrogen-induced cracking, the NA'CE condition (water + 5
%NaCA -1-0,'5%CH3CO0H H2S
The standard is that no cracking occurs when a test piece without side coating is immersed in a gas-saturated solution for 96 hours, but hot-rolled coil materials generally have better resistance to hydrogen-induced cracking than thick plate rolled materials. However, products manufactured by conventional methods could not give satisfactory results in the above tests, regardless of the chemical composition.

Therefore, conventionally, NAC in H-C (hydrogen-induced cracking) test
To manufacture a hot-rolled coil that does not generate cracks under condition B, the following process is adopted: continuous casting casting → cooling → primary heating → primary rolling → cooling → hot rolling heating → hot rolling. The aim was to compress the cavities that tend to occur in the center of continuously cast slabs, reduce segregation during hot rolling heating, and improve hydrogen-induced cracking resistance.

However, the conventional manufacturing method for hydrogen-induced cracking-resistant hot-rolled coils requires two heating steps: primary heating and hot-rolling heating, which requires a long time to manufacture the coil and increases the energy cost accordingly. However, this was extremely disadvantageous in terms of work efficiency and product cost.

Furthermore, when this conventional method targets steel with strengthening elements such as Nb, V, and Ti, these elements precipitate as large carbonitrides when the continuous casting slab is cooled. The temperature of hot rolling heating to completely dissolve the substances and obtain fine precipitates that are effective in increasing the strength of the final product.
For example, it must be heated to a high temperature of 1250°C or higher, and if these heating conditions are exceeded, coarse precipitates in the form of clusters will remain and become the starting point for cracks, so the HIq under NACE conditions assuming a harsh usage environment The inventors' research has revealed that there are cases in which good results cannot be obtained in the test.

From the above-mentioned viewpoint, the present inventors aimed to find a method to economically and efficiently mass-produce hot-rolled coils with excellent hydrogen-induced cracking resistance while minimizing thermal energy consumption. As a result of repeated research, we found that (a) recent continuous casting slabs, which have extremely fewer surface defects due to advances in technology,
Even if the slab is rolled immediately after casting without cooling, no visible defects will appear that would affect the product quality, and if the thickness of the slab is reduced by such direct rolling, the cooling rate will be faster and the precipitates will become coarser. (b) In the conventional method described above, high-temperature heating of 125'O<0>C or higher is sometimes not performed during hot rolling heating, especially due to problems with strength and toughness; In this case, all of the coarse precipitates that precipitate during cooling after casting do not dissolve into solid solution, so in order to reliably achieve the solid solution-precipitation effect, it is necessary to contain a larger amount of reinforcing elements9, which promotes cluster formation. However, if the slab after casting is rolled without being cooled, the amount of solid solution of these reinforcing elements will increase, and this problem will not occur. In addition to the compositional components that achieve the mechanical properties required for steel materials used in environments where induced cracking is likely to occur, we use steel that has Ca added as an essential component to make pentasulfide inclusions spheroidal. Then, by performing primary rolling under specified conditions without cooling immediately after continuous casting, followed by regular hot rolling and then coiling at a specified temperature, a hot-rolled coil with extremely excellent hydrogen cracking resistance is produced. We have come to the knowledge shown in (a) to (C) above.

This invention was made based on the above findings, and includes: C2O: 02 to 0.18% (hereinafter, the amounts of the components are expressed as weight percentages), Si: 0.01 to 1.00
%, Mn: 0.4 to 2.0%, Ca', an amount such that the Ca/S value is 2 to 8, and Cu: 0.
．． 2-1.0%, Ni: 0.1-1.0%, Cr
: 0.2~1.0%, Mo: 0.05~0.1
%, Nb: 0.01-0.12%, V:
0.01-0.12%, Ti: 0.01-0.
After continuously casting a steel having a composition consisting of Fe and two unavoidable impurities, containing one or more of 20'%,
Immediately without heating the slab, rolling reduction: 10-7
0%, by rolling at a finish temperature of near 00℃ or higher, then heating this to a temperature of 1000℃ or higher, hot rolling again, and winding at a temperature range of 400 to 640℃. The present invention is characterized in that a hot-rolled coil having excellent hydrogen-induced cracking resistance can be obtained efficiently and economically.

Next, in the method of the present invention, the reason why the chemical composition of the steel and the conditions for rolling and winding 9 are limited to the above-mentioned conditions will be explained.

A. Chemical component composition ■ C C component has the effect of ensuring the strength of steel, but if its content is less than 0.02%, the desired effect cannot be obtained; on the other hand, 0.18% If the content exceeds 0.02, the weldability and toughness will deteriorate, so the content should be set at 0.02~
It was set at 0.018%.

■ In addition to deoxidizing the steel, the 5I Si component also has the effect of ensuring strength, similar to the C component, but if its content is less than 0.01%, the desired effect cannot be obtained; 1.00%
If the content exceeds 0.2%, it will have an adverse effect on weldability, so the content was set at 0.01% to 1.00%.

■ Mn The Mn component has the effect of improving the strength and toughness of steel, but if the content is less than 0.4%, the desired effect cannot be obtained; on the other hand, if the content exceeds 2.0%, Since this would have an adverse effect on corrosion resistance, its content was set at 0.4 to 2.0%.

■ Ca The shape of sulfide inclusions present in steel has a significant effect on the steel's resistance to hydrogen-induced cracking, and spheroidizing these sulfide inclusions is essential for preventing hydrogen-induced cracking. This is something that cannot be done. Although adding Ca is an extremely effective means for this purpose, if the weight ratio of Ca to the 8% content, that is, the value of Ca/S, is less than 2, the sulfide inclusions become spheroidized. If the desired effect is not obtained in the action of Therefore, the content was determined to be 2 to 8 in Ca/S value.

Ocul N1 r Cr + and MO Both of these components have the effect of increasing strength through solid solution strengthening and transformation strengthening, and from the perspective of corrosion resistance, Cu
The component has the effect of preventing hydrogen absorption under relatively mild acidic conditions, and the Cr and N1 components have the effect of further improving corrosion resistance, and especially the N1 component has the effect of preventing hot embrittlement that occurs when Cu is added. Cu or Cr content is less than 0.2%.

Ni content is less than 0.1% or MO content is 0.
If the content is less than 0.05%, the desired effect cannot be obtained in the above action, and on the other hand, even if the content exceeds 1.0%, no further improvement effect can be obtained.
~ Yu, OS + Hz: O, 1 ~
U, O%, Cr: 0.2-1.0%, and Mo: 0.05-1.0%, respectively.

■Nt), V, and T1 All of these components have the effect of increasing the strength of steel through precipitation hardening, but if the C content is less than 0.01%, the desired effect is not achieved. On the other hand, even if Nb is contained in an amount exceeding 0.01%, it is not possible to obtain any further improvement in the above-mentioned action, and the weld toughness is adversely affected. Nb: 0.01 to 0.12 since the base material toughness deteriorates if the content exceeds 0.02% and T1 exceeds 0.020%.
%, V: 0.01-0.12%, and T1.0,
01 to 0.20%, respectively.

B. Rolling immediately after continuous casting (primary rolling) By minimizing cold rolling for solidification during continuous casting,
Rolling immediately after continuous casting requires no heating.
By compressing the micro defects in the cast slab, it is easy to reduce the segregation that exists in the center, and it has the effect of improving hydrogen-induced cracking resistance, and it also speeds up the cooling of the slab and prevents the coarsening of precipitates. It is effective in preventing

In this case, if the rolling reduction ratio is less than 10%, the above effect cannot be expected; on the other hand, if the rolling reduction ratio exceeds 70%, the rolling ratio during the next hot rolling should be set to 3 or more. Since it is not possible to obtain a hot-rolled coil with the desired strength and toughness, the rolling reduction ratio is
%. It is recommended that the rolling reduction ratio be preferably 20% or more.

Furthermore, the rolling temperature at this time is the thermal efficiency during hot rolling heating,
From the viewpoint of preventing precipitation of precipitation-hardening elements, it is desirable to use a high temperature, and since the above effects cannot be expected when rolling at a temperature below 700°C, the finishing temperature was set at 'i'oo°C or higher.

C0 Hot Rolling If the heating temperature during hot rolling is less than 1000°C, the precipitation-hardening elements that have partially precipitated cannot be solid-dissolved again, making it impossible to obtain the desired strength and toughness, and also decreasing the durability. Since the hydrogen-induced cracking property will also deteriorate, the heating temperature should be set at 100
The temperature was set at 00°C or higher.

Note that heating at high temperatures and for long periods of time is effective in improving hydrogen-induced cracking resistance, but if the temperature exceeds 1250°C, the effect brought about by the method of the present invention will be reduced, so heating is preferably performed at about 1150°C or less for 1 hour or more. Heating is recommended.

Furthermore, for hot rolling heating, it is preferable to charge the slab into the heating furnace while the temperature is maintained at 650°C or higher after rolling immediately after the previous continuous casting, which improves temperature raising efficiency. At the same time, precipitation of precipitation hardening elements is also prevented.

In a hot-rolled coil with a D1 winding temperature, if it is slowly cooled after winding, will the hydrogen sulfide cracking resistance due to self-tempering be improved compared to a thick plate material that does not have a slow cooling process? Winding temperature is 40
If the temperature is lower than 0°C, such an effect cannot be obtained, whereas if the temperature exceeds 640°C, a pearlite layered structure will appear and the hydrogen sulfide cracking resistance will deteriorate. It was set as ℃.

Next, the present invention will be explained using examples and comparing with comparative examples.

Example First, steel having the chemical composition shown in Table 1 was melted by a conventional method, and slabs having the thickness shown in Table 2 were manufactured by continuous casting. Next, before the temperature drops, primary rolling is similarly performed under the conditions shown in Table 2, followed by hot rolling and winding to form a hot rolled coil with a thickness as shown in Table 2. was manufactured.

Next, the mechanical properties and hydrogen-induced cracking resistance of the obtained hot rolled coils were measured, and the results are also shown in Table 2. In addition, the HjC (hydrogen-induced cracking) test was conducted using specimens without side coating (thickness, product thickness -27HL,
After immersing a specimen (width: 20mm, length: 1100M) under NACE conditions for 96 hours, the defect generation area ratio was measured using an ultrasonic flaw detection test (UST).

Note that the * mark in Tables 1 and 2 indicates that the conditions of the method of this invention are not met.

As is clear from the results shown in Table 2, according to methods 1 to 13 of the present invention, hot rolled coils with good mechanical properties and excellent hydrogen-induced cracking resistance can be obtained. Those whose component composition range is outside the range of the method of the present invention (14 to 1
6) and coils (17 to 22) whose rolling and heating conditions are outside the range of the method of the present invention are hot-rolled coils with poor mechanical properties or poor resistance to hydrogen-induced cracking. I could only get it.

As described above, according to the present invention, hot-rolled coils with excellent hydrogen-induced cracking resistance can be mass-produced without the need for primary heating as in conventional methods, resulting in significant savings in energy consumption. Not only that, but it also brings about industrially useful effects such as improved work efficiency.

Applicant: Sumitomo Metal Industries, Ltd.

Claims (4)

[Claims] (1) C: 0.02-0.18%, Si: 0.
01-1.00%, Mn: 0.4-2.0%, Ca: an amount such that the value of Ca/8 is 2-8, and a composition consisting of Fe, unavoidable impurities, and the remainder (the above After continuous casting of steel (weight percentage),
Immediately without heating the slab, rolling reduction: 10-7
0%, Finishing temperature: Rolling at 700°C or higher, then heating to 10'O0°C or higher, hot rolling again, and winding at a temperature range of 400 to 640°C. A method for manufacturing a hot-rolled coil with excellent resistance to hydrogen-induced cracking. (2) C: 0.02-0.18%, Si: 0
．． 01 to 1.00%, Mn: 0.4 to 2.0%, Ca: an amount such that the Ca/S value is 2 to 8,
Further, Cu: 0.2~1.0%, Ni: 0.1~1.0%, Cr: 02~0%, Mo: 0.0
After continuously casting a steel having a composition (the above weight percentage) that also contains 5 to 10% of one or more of the following, and the remainder consisting of Fe and unavoidable impurities,
Immediately without heating the slab, rolling reduction: 10-7
It is characterized by being rolled at a finishing temperature of 700°C or higher, then heated to a temperature of 1000°C or higher, hot rolled again, and wound at a temperature range of 400 to 540°C. A method for manufacturing a hot-rolled coil with excellent hydrogen-induced cracking resistance. (3) C: 0.02-0.18%, Si: 0.0
1 to 1.00%, Mn: 0.4 to 2.0%, and Ca: an amount such that the Ca/S value is 2 to 8. Nb2 0.01 to 0. 12%,
V: O, 2% of C11 to O, Ti: 0.01 to 0.20%, a composition consisting of Fe and unavoidable impurities: remainder (weight percentage) After continuous casting of steel,
Immediately without heating the slab, rolling reduction rate: lO ~ 7
Finishing temperature: Rolled at a temperature of 700°C or higher, then heated to a temperature of 1000°C or higher, hot rolled again, and coiled at a temperature range of 400 to 640°C. A method for manufacturing a hot-rolled coil with excellent hydrogen-induced cracking resistance. (4) C: 0.02-0.18%, Si: 0.0
1 to 1.00%, Mn: 0.4 to 2.0%, Ca: an amount such that the Ca/'S value is 2 to 8, and Cu: 0.2 to 1.0%. , Ni: 0.1-1.0%, Cr: 0.2-1.0%, Mo: 0.05-1.0%, and Nb: O, O2
N2.12%, V: 0.01 to 0.12%, Ti: 0.01 to 0.20%, Composition C consisting of Fe and inevitable impurities: residue. After continuous casting of steel with a weight ratio of
Immediately without heating the slab, rolling reduction: 10-7
0%, rolling at a finish temperature of near 00°C or higher, then heated to a temperature of 1000°C or higher, hot rolling again, and winding at a temperature range of 400 to 640°C. A method for manufacturing a hot-rolled coil with excellent hydrogen-induced cracking resistance.