JPH02250941A - Low carbon chromium-molybdenum steel and its manufacture - Google Patents

Abstract

PURPOSE:To improve the weldability, stress relief annealing(SR) embrittlement- resistant properties after welding, and tensile strength after SR treatment in the Cr-Mo steel having specified compsn. by reducing C, improving the hardenability of B by the addition of Ti and specifying the weld cracking sensitivity parameter. CONSTITUTION:A steel contg., by wt.%: 0.03<=C<0.11, 0.10 to 0.90 Si, 0.50 to 2.00 Mn, 0.40 to 2.50 Cr, 0.10 to 1.00 Mo, 0.00015 to 0.0030 B, 0.005 to 0.10 SolAl, 0.002 to 0.06 To, <=0.01 N, <=0.0010 P and <=0.0050 S, contg., at need, one or more kinds among <=0.5% Cu, <=0.5% Ni, <=0.1% Nb and <=0.1% V besides the balance Fe is heated to 1000 to 1250 deg.C and is thereafter subjected to hot working at the Ar3 transformation temp. or above into the prescribed dimension. The steel is furthermore heated to the Ac3 transformation temp. to 1000 deg.C, is thereafter air-cooled and is normalized to transform its main structure into bainite or bainite + martensite. In the steel, >0.21 to 0.35% weld cracking sensitivity parameter PCM and >=53kgf/mm<2> tensile strength after SR treatment are shown.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to a low-carbon chromium-molybdenum steel for pressure vessels with low weld cracking susceptibility, which is suitable for, for example, feed water heaters for nuclear power generation facilities and similar applications, and the like. This relates to a manufacturing method.

<Prior art and its issues> For example, various research and development efforts have been made on materials for pressure vessels for medium and high temperatures used in nuclear power generation facilities, etc.
Many proposals have been made to provide products with better performance.

Under these circumstances, in recent years, low C (carbon) and low PCM
A normalized high-strength steel with a bainite structure as the main structure was proposed (Japanese Patent Publication No. 57-19731) based on B-added steel with a high weld cracking susceptibility index, and has high strength, high toughness, and excellent weldability. In addition to providing stress relief annealing after welding (“
It has attracted attention as a material for pressure vessels that can maintain sufficient toughness and strength even when the material is SR (abbreviated as "SR").

However, the demand for further performance improvement of various equipment is unstoppable, and as a result, the environment in which steel for pressure vessels is used has become even more severe recently, and the SR conditions are becoming increasingly severe. Like PCM≦0.21%
There have been cases in which the "component system" does not necessarily satisfy the required strength after SR.

In other words, since the environment in which pressure vessel steel is used has traditionally been assumed to be in a relatively low-temperature range, it is inevitable that the SR conditions will be
0+1ogt), T: SR hot water temperature K).

t: sR waiting time hr)) is about 18.7 x 10'
Relatively lenient conditions were set. However, recently, with the remarkable progress in numerical simulation technology, it has become possible to easily determine the "accurate temperature distribution or stress distribution during use" of medium- and high-temperature pressure vessels, which have harsh and complex operating environments. Since it has become possible to accurately calculate the safety factor during use, it has become possible to use steel for pressure vessels in harsher high-temperature environments than ever before. Therefore, pressure vessels with high-temperature specifications are being designed, and steel for pressure vessels that satisfies strict specifications such as a tempering parameter of (20 to 21) x 10" is also being sought. This is the translation.

By the way, it goes without saying that steel for pressure vessels is required to have SR embrittlement resistance and also strength characteristics that exhibit sufficiently high values even after SR. However, "excellent SR embrittlement resistance" and "high strength after SR" are contradictory properties.

In other words, lowering C is effective in improving SR embrittlement resistance, but lowering C makes it difficult to ensure strength after SR, and it takes a considerable amount of alloying elements to ensure strength. It is necessary to add However, although the addition of a large amount of such alloying elements is effective in increasing the strength after SR, it becomes a factor that impedes weldability, so it can be said to be a measure that should be avoided in the case of steel for pressure vessels.

Therefore, in order to provide a steel for pressure vessels that has the above-mentioned contradictory properties, we used a low-C B-added chromium-molybdenum steel, and mixed the main structure with [ferrite + bainite] according to the standard and SR of specific tempering parameters. A proposal was made to ensure strength after SR by making it a group rock (
Special Publication No. 63-12935).

However, even if such measures are taken, the strength and weldability of the resulting steel will not be sufficiently satisfactory for pressure vessel steel for medium- and high-temperature applications, where the required performance is becoming increasingly strict. I could not say it.

For these reasons, the object of the present invention is to provide a medium- to medium-sized steel that has excellent weldability, excellent SR embrittlement resistance, and exhibits a tensile strength of 53 kgf/- or more even after SR treatment.
The aim was to provide a steel material suitable for use in high temperature pressure vessels.

Means for Solving the Problems> In order to achieve the above object, the present inventors have developed pressure vessel steel with "excellent SR embrittlement resistance", which are contradictory to each other as described above.
As a result of various studies in search of effective measures that can simultaneously achieve both "high strength after SR", we have come to the following knowledge. In other words, it is effective to use chromium-molybdenum steel as a base to ensure the necessary strength for steel for medium- and high-temperature pressure vessels, and to improve its SR embrittlement resistance, first reduce the C content to zero. It is essential to keep the content to less than 11% (hereinafter, percentages representing component proportions are based on weight).

(b) Even if the C content is reduced, the hardenability improving effect of B is utilized and the composition is designed so that P9,4 exceeds 0.21% to form a bainite structure or [martensite in bainite]. If a mixed structure can be realized, there will be no problem in securing strength after SR.

In other words, the strength of low C chromium molybdenum steel is significantly improved by the "hardenability improving effect of B" which can be effectively achieved by fixing free N by adding Ti and securing solid solution B. Normally, it is necessary to reduce PCM to prevent embrittlement after SR, but if PCM is excessively reduced, even with B addition, the strength after SR cannot be ensured, and the strength after SR In order to secure 53kgf/-, the hardenability improvement effect of B should be utilized and at the same time, PC
It is necessary to design the components so that M exceeds 0.21%, and to obtain a mixed structure of bainite vILm or [martensite in bainite] rather than a mixed structure of [ferrite + bainite].

(C) However, if the composition is designed so that PCM exceeds 0.21%, it is expected that negative effects will occur if the SR embrittlement resistance etc. deteriorate and the desired toughness is no longer satisfied. These adverse effects can be effectively suppressed by reducing the S content in the steel to below a specific value and also reducing the P content in the mesh.

In other words, S, which is an impurity element, has the effect of increasing weld cracking susceptibility in addition to reducing the toughness of steel, and P, which is also an impurity element, has the effect of increasing weld cracking susceptibility under severe SR conditions such as in this application. When used, it has the effect of causing significant grain boundary embrittlement during SR and significantly reducing toughness, but when these two types of impurity elements are both reduced to below a specific value, these negative effects are significantly reduced. , pc
, 4 are offset, thereby making it possible to achieve both the required strength and the required 5R1ffl resistance characteristics.

(d) The above-mentioned low C chromium molybdenum steel is normally used after normalizing treatment, but when the plate thickness to be applied becomes thick, normalizing treatment suppresses the formation of ferrite while reducing the thickness to the center of the plate thickness. There is a possibility that a bainite structure or a mixed structure of [bainite and martensite] cannot be obtained. However, if accelerated cooling is performed from a temperature above the Ar3 transformation point by utilizing the retained heat immediately after hot working (e.g. rolling), a bainite structure or a mixed structure of [martensite in bainite] can be formed stably up to the center of the plate thickness. can, SR
It is possible to prevent the strength from decreasing later. As long as the above-mentioned component composition is followed, there will be no problem in realizing the structure.

The present invention has been made based on the above findings, etc. [Chromium molybdenum steel, C: 0.03% or more and less than 0.11%.

Si: 0.10-0.90%, Mn: 0
．． 50-2.00%.

Cr: 0.40-2.50%, Mos 0
．． 10-1.00%.

B: 0.00015-0.0030%.

sol, Al: 0.005-0.10%.

Ti: 0°002~0.06%, N: 0.01
%below.

P: 0.010% or less, S: 0.005
Cu: 0.5% or less, Ni: 0.5% or less.

Nb j 0.1% or less, V: 0.1% or less 1
In addition, the remainder consists of Fe and unavoidable impurities, and the weld cracking susceptibility index PCl4 is 0.21.
% and 0.35% or less, it is characterized by having excellent strength, weldability, and SR embrittlement resistance, and furthermore, rc: 0 .03% or more and less than 0.11%.

St: 0.10-0.90%, Mn: 0
．． 50-2.00%.

Cr: 0.40-2.50%, Mo: 0
．． 10-1.00%.

B: 0.00015-0.0030%.

Sol, Aj! : 0.005-0.10%.

Ti: 0.002-0.06%, N: 0.
01% or less.

P: 0.010% or less, S: 0.00
Less than 50%.

or further contains Cu: 0.5% or less, Ni: 0.5% or less.

It also contains one or more of Nb: 0.1% or less, V: 0.1% or less, and the remainder consists of Fe and unavoidable impurities, and the weld cracking susceptibility index PcM exceeds 0.21% and is 0.35. % or less of steel with a composition of 1000 to 1
After heating to a temperature range of 250°C, hot working at a temperature range above the Arz transformation point to obtain the specified dimensions, then further heating to a temperature range from the Acl transformation point to 1000°C, air cooling, and skewering to obtain the main m The main structure can be changed to a bainite structure or a mixed structure of [bainitic martensite], or the main structure can be changed to a bainite structure or [ Excellent strength and weldability due to the mixed structure of bainite and martensite.

It is characterized by being able to stably provide a low C chromium molybdenum steel that also has SR embrittlement resistance.

As described above, the present invention utilizes the effect of B, which does not have a large adverse effect on weldability and exhibits the effect of improving hardenability even in a small amount, without adding a large amount of alloying elements. By setting a high value of , suppressing the decrease in strength during SR of low C chromium molybdenum steel as a bainitic structure or a mixed structure of [bainitic martensite], and simultaneously reducing the P and S contents, it is possible to This makes it possible to realize a steel material that also ensures good SR embrittlement resistance, and furthermore, high-strength, low-C chromium-molybdenum steel with these characteristics can be heated by hot working and skewering, or by hot working and acceleration. Although it is possible to stably produce the steel by cooling means, the reason why the composition or processing conditions of the steel are limited as described above in the present invention will be explained in detail below along with their effects.

Effect〉 ^) Composition (a) C C is an element necessary to ensure the desired strength of steel,
If the content is less than 0.03%, sufficient strength will not be obtained, while if the content is 0.11% or more, weld hardenability and weld cracking susceptibility will become higher than the allowable limit.
The C content was determined to be 0.03% or more and less than 0.11%. However, since the C content greatly affects the properties after SR, the strength after SR: 53 kgf while reducing the susceptibility to weld cracking.
0.06 to 0.0.
It is preferable to adjust it to 10%.

Width) 5t St is not only used as a deoxidizing agent during steel manufacturing, but is also a necessary component to ensure strength at room temperature and high temperature, and for that purpose, it must be contained in an amount of 0.10% or more. However, if the Si content exceeds 0.90%, the toughness will be significantly reduced, so the Si content should be 0.10 to 0.9%.
It was set as 0%.

(C) Mn The Mn component has the effect of increasing the strength and toughness of steel, but if the content is less than 0.50%, the desired effect due to the above effect cannot be obtained, while on the other hand, if the content exceeds 2.00%, If Mn is included, the strength and toughness improvement effect is saturated, but the weld cracking susceptibility becomes significantly high, so the Mn content should be 0.50 to
It was set at 2.00%.

(d) Cr The Cr component has the effect of increasing the strength of steel and ensuring corrosion resistance at high temperatures, but if its content is less than 0.40%, the desired effect due to the above effect cannot be obtained;
If the content exceeds 50%, the effect not only becomes saturated but also increases the manufacturing cost.
The r content was determined to be 0.40 to 2.50%.

(el　M.

Since Mo has the effect of increasing hardenability and resistance to temper softening, 0.10% Mo is used to improve strength at high temperatures.
% or more, but if the content exceeds 1.00%, the susceptibility to tempering embrittlement increases and the toughness decreases, so the Mo content was determined to be 0.10 to i,oo.

(') B B is an important component that does not significantly deteriorate the weldability of steel (to ensure strength after SR).

In order to ensure the strength of steel by utilizing the hardenability improvement effect of B, it is necessary to ensure an S content of 0.00015% or more, and on the other hand, B content exceeding 0.0030% is required. Even with low C steel such as the steel of the present invention, the SR
The S content was determined to be 0.00015 to 0.0030% because sometimes carborides are formed and the toughness is significantly reduced.

(Kawa sol.Al 5of, Aj! is used as a deoxidizing agent during steel manufacturing and is added to improve toughness through grain refinement of the structure, but for this purpose, the content of o, oos% or more is required. However, even if the content exceeds 0.10%, the above effect will not only be saturated, but also lead to an increase in cost.
It was set at 0.10%.

(h) Ti The Ti component prevents solid solution B from being lost as BN by capturing free N as TiN, thereby improving the hardenability of the B-added steel according to the present invention and converting the matrix into bainite (+ martensite) organized S
It has the effect of ensuring strength after R, but if the content is 0.
If the Ti content is less than 0.002%, the desired effect by the above action cannot be obtained, while if the Ti content is more than 0.06%, the toughness will be significantly impaired. Therefore, the Ti content should be from 0.002 to
It was set at 0.06%.

(1) N N is so7. Aj! It is an impurity element that is captured as B-t-BN and eliminates the hardenability improvement effect of B when it is contained in an amount exceeding that fixed by Ti, but by suppressing its content to 0.10% or less, the above Since the negative effects can be reduced to an acceptable range, the N content is 0.1.
It was limited to 0% or less.

1 P P is an impurity element that inevitably mixes into steel and causes grain boundary embrittlement, so it is important to reduce it as much as possible. especially,
In the case of steel used under severe SR conditions, significant grain boundary embrittlement occurs during SR, which acts to severely reduce toughness. Therefore, in order to ensure desired properties in steel, it is necessary to suppress the S content to 0.010% or less, and preferably to a range of 0.005% or less.

(k) S S is also an impurity element that inevitably enters steel, which not only reduces the toughness of steel but also has an adverse effect of increasing weld cracking susceptibility. Therefore, in order to avoid such adverse effects, it is necessary to suppress the S content to 0.005% or less, but it is desirable to reduce it to the region of 0.003% or less if possible.

(1) Pew PCM is an index indicating weld cracking susceptibility, and is the value shown by formula 3, %.The lower this value is, the lower the welding preheating temperature can be to weld without causing cracks. becomes. For example, in order to set the welding preheating temperature to 150° C. or lower, it is necessary to set the PCM to 0.35% or lower, and the lower this value is, the better in order to prevent weld cracking. but,
If the PCM is too low, it will not be possible to secure the strength after SR, and in order to ensure strength of 53 kgf/- or more even after S and R, the components must be designed so that the PCM exceeds 0.21%. It is necessary to keep it. Therefore, P was determined to be more than 0.21% and less than 0.35%.

(m) Cu, Ni+ Nb+ and ■These components all have the effect of improving the toughness of steel,
When higher strength is required, one or more types are added as necessary, and the content ratio of each element was determined for the following reasons.

Cu and Ni Both Cu and Ni are particularly effective in improving the high-temperature strength of steel, but if their content exceeds 0.5%, they tend to cause surface cracks and promote weld cracking. Therefore, in order to improve high-temperature strength, the amount of each addition was limited to 0.5%.

Both Nb and V precipitate during SR and are particularly effective in increasing temper softening resistance to prevent strength loss, but if their content exceeds 0.1%, low-temperature toughness decreases. Since this leads to deterioration of weldability, the amount of addition for reinforcement is limited to 0.1% or less.

B) Hot working/heat treatment conditions (al heating temperature The heating temperature prior to hot working (e.g. hot rolling) is 100%
If the temperature is below 0°C, solid solution of carbonitrides such as Nb and V cannot be achieved, making it impossible to utilize their precipitation strengthening. On the other hand, even if heated to a temperature range exceeding 1250"C, the solid solution of carbonitrides such as Nb and V tends to be saturated, so it is not only meaningless, but also leads to coarsening of single grains in the early stage of rolling. Since this would impair the toughness of the material, the heating temperature prior to hot working was set at 1000 to 1250°C.

(bl) Processing in a temperature range lower than Ar3 in the hot processing temperature range means processing in the temperature range where ferrite is generated.In such a case, a clear ferrite band may be formed. When used as a pressure vessel, corrosion progresses relatively quickly due to stress concentration, leading to a reduction in service life. It will be necessary to perform this in a high temperature range.

In addition, if it is used without being heated to a temperature higher than the Ac3 transformation point after processing (for example, if it is cooled at a cooling rate higher than air cooling after processing), the matrix becomes a bainite structure because ferrite has already been formed. Or [bainite + martensite] cannot be organized. Therefore, it was decided that the predetermined dimensions would be finished by hot working in a temperature range equal to or higher than the Ar3 transformation temperature.

Note that Ar=transformation point is calculated by the following formula, where t (m) is the thickness of the steel material.

(C) One of the points of the present invention is to improve the SR embrittlement resistance by lowering the carbon content of the main structure steel, but lowering the carbon content conversely means that the strength decreases significantly after SR. In this case, if ferrite is generated in the steel, the strength after SR will be 53
Since it is impossible to achieve a value of kgf/- or more, the main structure must be a bainite structure or a [bainite + martensite] mixed structure.

(d) Normalizing treatment If the heating temperature during normalizing treatment is below the Ac3 transformation point, the structure of the steel cannot be completely austenitized, so if ferrite is formed during cooling after hot working. Then, the main structure is changed to bainite structure or [
Bainite + martensite] cannot be formed into a mixed structure. On the other hand, when heated to 1000° C. or higher, austenite grains become coarse and toughness deteriorates. Therefore, the heating temperature during the normalizing treatment was limited to the Ac3 transformation point to 1000°C.

le) Cooling conditions after hot working As mentioned above, if the steel is thick, there is a risk that normalizing may not result in a bainitic (M) texture or [bainite + martensite] mixed structure up to the center of the plate thickness. There is,
In order to stably secure high strength after SR in low C chromium molybdenum steel even when the steel size becomes thicker, accelerated cooling under specific conditions is performed immediately instead of normalizing treatment after hot working. is effective.

In this case, ferrite formation cannot be suppressed unless cooling is started from a temperature range above the Ar transformation point after hot working, and the main structure must be cooled to below 600°C at a cooling rate faster than air cooling. Since it is not possible to form a bainite structure or a [bainite + martensite] mixed structure, when accelerated cooling is performed following hot working, the cooling rate is set to air cooling or higher, and accelerated cooling is performed to 600°C or lower. It was decided that

Next, the effects of the present invention will be explained in more detail with reference to Examples.

<Example> First, steels having the compositions shown in Table 1 were melted and subjected to hot rolling and heat treatment under the conditions shown in Table 2 to produce steel plates.

Next, the mechanical properties of each of the obtained steel plates were investigated, and a Y-groove restraint cracking test was also carried out from the viewpoint of weldability evaluation, and the results are also shown in Table 2.

Here, in the Y-groove restraint cracking test, a diagonal Y-groove restraint cracking test piece (plate thickness 25fi) was taken from each steel plate, and the heat input:
Manual welding at 17 kJ/cm (current: 17OA, voltage:
25V.

Speed: 15cm/s+in) “Surface crack” when struck
and by a method of investigating the presence or absence of "root cracks". The criteria for this test is that if the above cracking does not occur even when welding at a preheating temperature of 150°C or less, it is marked as "○", and if the preheating temperature does not exceed 150°C, cracking cannot be suppressed. If the song was sung, it was marked as “×”.

As is clear from the results shown in Table 2, when the obtained steel plate satisfies the conditions specified in the present invention, it has high strength and toughness, and the preheating temperature is 150°C or less. It can be seen that no weld cracks were observed in either case, indicating that both the base metal performance and weldability were good.

On the other hand, in test number 11, A
r, this is an example in which finish rolling could not be performed at a temperature higher than the transformation point. Even in such a case, as specified in the present invention, the ferrite formed during rolling is completely austenitized by normalizing treatment in the temperature range from the Ac3 transformation point to 1000°C, and the structure after normalization is changed to prevent the formation of ferrite. If the bainite structure was suppressed, the same results as Test No. 1 would have been obtained, but due to the normalization treatment at a low temperature of 850°C, ferrite remained and the target strength level was not achieved.

Similar examples are shown in test numbers 12 and 13, but in the former, the heating temperature is higher but the finishing temperature is lower than the Ar+ point.
This is a case where the formation of ferrite could not be suppressed due to the 0°C finishing, and even in the case of such high-temperature heating, the target strength level cannot be satisfied unless the annealing temperature is set to a temperature range of Ac 3 or higher. Furthermore, even if water cooling is performed after finish rolling at 815°C, which is lower than the Arz point, to increase the strength of the structure of ferrite and martensite (partially payinite), the structure in which ferrite remains will have a strength of 20.1 x. If a severe SR treatment of 10" is applied, the strength cannot be ensured, and the target strength of the present invention cannot be ensured.

Test numbers 14 to 18 are examples in which the conditions of the manufacturing process specified by the present invention were satisfied, but the chemical composition conditions of the steel did not satisfy the conditions of the present invention. In this way, even if the manufacturing process conditions are satisfied, if the component composition conditions do not satisfy the provisions of the present invention, the target strength cannot be obtained as in test numbers 14°16 and 17, or PCl4 is 0. .3
Even if the target strength is satisfied, as in test numbers 15 and 18, which exceed 5%, the welding preheating temperature cannot be lowered to 150°C or less, and when assembling into a pressure vessel, etc., this becomes a major obstacle in construction. becomes impossible.

In addition, from Test No. 17, it can be seen that even when PCll is 0.276%, if the S content is high, the welding preheating temperature cannot be lowered to 150° C. or less.

Furthermore, test numbers 16 and 18, which had a high P content, had significantly poor impact properties, indicating that reduction of P is essential.

As described above, the component range in the present invention is not only intended to ensure strength after SR, but also has the meaning of having toughness and weldability, and its importance is confirmed.

(Summary of Effects) As detailed above, the present invention has excellent SR embrittlement resistance and maintains a tensile strength of 53 kgf/- even after SR.
It has become possible to provide the above-mentioned low carbon chromium molybdenum steel, and it will bring extremely useful effects industrially, such as being expected to have a wide range of uses, such as as a material for medium- and high-temperature pressure vessels for nuclear power generation equipment.

Claims (1)

[Claims] (1) In terms of weight percentage, C: 0.03% or more and less than 0.11%, Si: 0.10-0.90%, Mn: 0.50-2.0
0%, Cr: 0.40~2.50%, Mo: 0.10~
1.00%, B: 0.00015-0.0030%, sol. Al: 0.005-0.10%, Ti: 0.002-0.06%, N: 0.01% or less,
Contains P: 0.010% or less, S: 0.0050% or less, and the remainder consists of Fe and inevitable impurities,
A high-strength, low-carbon chromium-molybdenum steel having excellent SR embrittlement resistance, and having a weld cracking susceptibility index P_C_M of more than 0.21% and less than 0.35%. 2) In terms of weight percentage, C: 0.03% or more and less than 0.11%, Si: 0.10-0.90%, Mn: 0.50-2.0
0%, Cr: 0.40~2.50%, Mo: 0.10~
1.00%, B: 0.00015-0.0030%, sol. Al: 0.005-0.10%, Ti: 0.002-0.06%, N: 0.01% or less,
Contains P: 0.010% or less, S: 0.0050% or less, Cu: 0.5% or less, Ni: 0.5% or less, Nb: 0.
1% or less, V: 0.1% or less, the remainder consists of Fe and unavoidable impurities, and the weld cracking susceptibility index P_C_M is 0.
．． A high-strength, low-carbon chromium-molybdenum steel with excellent SR embrittlement resistance, characterized by a content of more than 21% and less than 0.35%. (3) The steel according to claim 1 or claim 2 has a composition of 10
After heating to a temperature range of 00 to 1250°C, hot working in a temperature range of Ar_3 transformation point or higher to obtain the specified dimensions, then further heating to a temperature range of Ac_3 transformation point to 1000°C, air cooling, and normalizing. The main structure is bainitic structure or [
3. The method for producing a high-strength, low-carbon chromium-molybdenum steel according to claim 1 or 2, characterized in that it has a mixed structure of bainite + martensite. (4) The steel according to claim 1 or claim 2 has a composition of 10
After heating to a temperature range of 00 to 1250℃, hot working in a temperature range of Ar_3 transformation point or higher to finish to the specified dimensions, and immediately cooling to a temperature of 600℃ or less at a cooling rate faster than air cooling. The structure can be changed to bainite structure or [
3. The method for producing a high-strength, low-carbon chromium-molybdenum steel according to claim 1 or 2, characterized in that it has a mixed structure of bainite + martensite.