JPS63190117A - Production of high tension steel plate having high toughness and less than 90% low yield ratio by direct quenching method - Google Patents

Abstract

PURPOSE:To easily obtain a very thick high tension steel plate having high toughness, low yield ratio and little strength distribution toward plate thickness direction by treating the steel slab specifying component content and regulating carbon equivalent, by the prescribed condition. CONSTITUTION:The steel containing 0.05-0.13wt.% C, 0.05-0.19% Si, 0.70-1.30% Mn, 0.40-1.10% Cr, 0.40-1.10 Mo, 0.01-0.10% V, 0.01-0.10% Al, 0.0005-0.002% B, <=0.045% N and in the range of 0.42-0.65% carbon equivalent Ceq. showd in the equation, is used. The slab of this steel is hot-rolled after heating at 1,000-1,150 deg.C, so that finishing temp. is 880-960 deg.C on the steel plate surface, and the quenching is started within 10-60sec after finishing the above rolling. In this way, the above high tension steel plate having <=90% yield ratio can be produced, and safety of the steel structure of bridge, pressure vessel, etc., can be improved.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a method for producing a high tensile strength steel plate with a yield ratio of 90% or less using a direct quenching method, in particular, increasing the deformability of the steel plate, The aim is to reduce the yield ratio of high-strength steel with the aim of increasing the safety of steel structures, and to achieve advantageous applications in bridges, architecture, penstocks, pressure vessels, etc.

In general, mild steel with a tensile strength of 40 kgf/mxr" class,
Although its yield ratio is low at around 60-70%, the yield ratio tends to increase as the tensile strength of steel increases.
Since the yield ratio of high tensile strength steel of ~110 kgf7wm'' class is usually as high as 90% or more, this is an important consideration when designing buildings.

The yield ratio is considered to indicate the margin from when a steel plate yields until it breaks, and the lower the value, the greater the deformability, and the greater the uniform elongation and total elongation, so it is important for the safety of steel structures. It is clear that it is advantageous.

Furthermore, from the perspective of improving the fatigue properties of steel structures, there is a need for the development of high-strength steel plates with low yield ratios.

(Prior art) In order to manufacture high-strength steel with a tensile strength of 70 to 110 kgf/m, it is necessary to make the structure mainly martensite in order to ensure its strength, but if it is not quenched, it has low toughness and The strength in the thickness direction is uneven.

Therefore, conventionally, tempering treatment at a temperature of about 600°C after quenching has been used to improve the toughness of steel plates and make the strength uniform in the thickness direction, but in this case, the yield ratio of steel plates is 90. It was unavoidable that the value would be as high as %.

As an attempt to solve this problem, it has been considered to omit the tempering process and utilize the low yield ratio of as-quenched steel sheets, but as mentioned above, simply as-quenched steel sheets have low toughness, especially in the thickness direction. Due to non-uniform strength, steel plates that can be put to practical use have not yet been manufactured.

In addition, an attempt to lower the yield ratio by creating a two-phase mixed structure in which ferrite is mixed with martensite using the two-phase region quenching method has been reported for high tensile strength steel treated with 80 kgf/n+m ("low yield ratio"). "Basic properties of 80 kg class high tensile strength steel and its welded parts", Proceedings of the Welding Society of Japan,
3-3.1985 (Reference)). However, in this case as well, the ferrite is soft, so in order to obtain the same strength as conventional hardened and tempered steel sheets, the carbon equivalent must be higher than that of conventional steels, and welding of steel sheets, which is most important when constructing steel structures, is necessary. The disadvantage of increased cracking susceptibility is inevitable.

(Problems to be solved by the invention) It has a carbon equivalent equal to or less than the conventional quenched and tempered steel of 70 to 110) cgf/mm, has the same strength, and has a yield ratio of 90% or less. In addition to making it possible to manufacture high-strength steel with a low yield ratio that can be achieved, it is also necessary to reduce the strength distribution difference in the thickness direction that is observed in conventional as-quenched steel, even in extremely thick steel plates with a thickness of 75 m or more. This is the objective of this invention.

(Means for Solving the Problems) This invention includes C: 0.05 to 0.13 wt% (hereinafter simply expressed as %), Si: 0.05 to 0.19%, Mn: 0.70 to 1.30%, Cr: 0.40-1.10%, Mo: 0.40-1.10%, V: 0.01-0.10 wt%, Al 70
．． 01-0.10 wt%, B: 0.0005-0
．． 002% and N: 0.0045% or less, and has a carbon equivalent Ceq represented by the following formula.

A steel slab with a content of 0.42% to 0.65%,
After heating to 1150℃, the rolling finishing temperature is 88℃ on the surface of the steel plate.
Hot rolling is carried out at a temperature of 0 to 960°C, and after the rolling is completed, 10
This is a method for producing a high-toughness, low-yield-ratio, high-strength steel plate with a yield ratio of 90% or less by a direct quenching method, which comprises starting quenching for ~60 seconds.

The basis of the idea of this invention is roughly as follows.

1. By selecting an appropriate rolling finishing temperature where recrystallized austenite is the main component, changes in strength in the sheet thickness direction can be minimized.

2. Effectively utilize the hardenability improvement effect of B by selecting an appropriate time range from the end of rolling to the start of hardening.

3. Find a chemical composition that provides high toughness even when directly quenched, and design the composition based on that.

The inventors melted a large number of steels and made a slab of 950-1
After heating to various temperatures of 250°C, finishing rolling at various temperatures of 750 to 960°C, holding at that temperature for a predetermined time, and then quenching, the finishing temperature of rolling and the time from completion of rolling to quenching can be determined. The influence of this on the strength and toughness of steel sheets was investigated in detail.

As a result, when quenching is performed after rolling in a temperature range where unrecrystallized austenite is the main component, the low quenching temperature reduces hardenability, resulting in insufficient strength at the center of the thickness of the extra-thick steel plate. was not obtained.

On the other hand, if rolling is finished in the recrystallized austenite temperature range and quenched after a predetermined period of time, a steel plate with sufficient toughness as quenched and sufficient strength up to the center of the plate thickness can be obtained. It was done.

The behavior of B greatly contributes to the high strength and toughness achieved here. In other words, when B exists at the austenite grain boundaries, it lowers the grain boundary energy and improves hardenability, but after rolling, B exists at the grain boundaries of the recrystallized austenite grains.
By quenching while B is present, the effect of B is effectively exhibited.

The major feature of this invention is that in order to make the most effective use of this effect, quenching is started while B is present at the grain boundaries after rolling, that is, between 10 and 60 seconds. .

(for production) Next, the reasons for limiting each component will be described.

S- is the most effective component for strengthening martensite. If it is less than 0.05%, the reinforcing effect will be small, and in order to obtain strength, it will be necessary to add large amounts of other alloy components, which is not preferable. On the other hand, if it exceeds 0.13%, martensite becomes brittle, leading to deterioration of toughness. - Needles have a role as an alloying element in addition to acting as a deoxidizing agent, and are an extremely important component in this invention because they influence the precipitation of carbides.

If the Si content is less than 0.05%, no effect as a deoxidizer can be obtained, while if the amount exceeds 0.19%, the effect of improving toughness due to the reduction in Si cannot be expected.

C is required to be 0.70% or more to ensure strength, but
If it exceeds 1.30%, weldability and workability will deteriorate, so the content should be in the range of 0.70 to 1.30%.

If Niji is less than 0.40%, the effect of increasing strength is poor, while if it exceeds 1.10%, carbide will precipitate during direct quenching.
Contributes to toughness deterioration.

Mu is necessary from the viewpoint of improving hardenability and grain size regulating effect, and in order to obtain this effect, 0.40% or more is required. However, if it exceeds 1.10%, the effect decreases, so from the point of view of economy, it is limited to 0.40 to 1.10%.

Y- is an element that increases strength through secondary precipitation hardening during tempering, but if it is less than 0.01%, a sufficient effect cannot be obtained, and if it is added more than 0.10%, it will impair weldability.
It was limited to a range of 0.01 to 0.10%.

B is added for the purpose of exhibiting the effects of deoxidation and addition of B, and fixing N as AIN, but if it is less than 0.01%, the effect of addition is poor, while if it exceeds 0.10%, Since it has an adverse effect, it is limited to a range of 0.01 to 0.10%.

It is an extremely important component because it improves the hardenability of steel sheets even in extremely small amounts. In particular, it can be said to be the most important component in the development of the steel of this invention.

However, if the amount added is less than 0.0005%, B
The effect of improving hardenability cannot be expected by 0.002
If it exceeds %, B precipitates are formed and the effective amount for improving hardenability is actually reduced, and the B precipitates themselves also reduce hardenability, which is not preferable.

It is preferable to reduce L as much as possible since it forms nitrides such as BN and reduces the hardenability improvement effect of B and causes deterioration of toughness. When the content is 0.0045% or less, B can be effectively used without impairing the toughness of the steel sheet.

Next, if the carbon per il mark rainbow shown by the following formula (1) % formula % () is less than 0.42%, 7
It is difficult to simultaneously obtain a tensile strength of 0 kg f/wa'' or more and good toughness, and the weld heat affected zone becomes softened.On the other hand, if Ceq exceeds 0.65%, the weld cracking susceptibility increases and cracking occurs. Preventive preheating temperature becomes high, which is unfavorable from the viewpoint of welding efficiency.

Although the essential components have been described above, in the present invention, Ni can be added in an amount of 4% or less for the purpose of improving toughness and hardenability. The reason why the Ni content is limited to the above range is that the cost is higher than the effect obtained even if it is added in excess of 4%.

Further, in this invention, instead of V, 0.015 to 0.
A similar effect can be obtained by adding 050% Nb or by using Nb and ν in combination.

Furthermore, in order to avoid material variations due to changes in x -7'uA"W, reduce the temperature difference between the surface layer and center of the steel plate during rolling, and produce a steel plate with stable material, it is necessary to limit the slab heating temperature. There is a need to.

In the present invention, if the heating temperature exceeds 1150°C, the austenite grains become coarser, and although strength is obtained, toughness deteriorates. On the other hand, the slab heating temperature is 1000°C
If it is less than 1, the austenite grains become finer and the strength decreases, and the toughness also deteriorates accordingly.

Therefore, in order to produce a steel plate with stable strength and toughness, it is necessary to heat the slab before rolling to a temperature range of 1000 to 1150°C.

The rolling ratio and the roughness also have a large effect on the material quality of the steel plate (the rolling finishing temperature needs to be 880 to 960°C at the surface temperature of the steel plate.

This is because if the steel plate surface temperature is less than 880°C, recrystallization is difficult to occur after rolling, while if it exceeds 960°C, crystal grains become coarse, leading to deterioration of toughness.

Then, after finishing rolling as described above, quenching treatment is performed, and the quenching start time is 10 to 60 minutes after finishing rolling.
It needs to be between seconds. This is because after rolling, it takes at least 10 seconds for B to move to the recrystallized austenite grain boundaries, so even if quenched for less than 10 seconds, sufficient strength cannot be obtained; on the other hand, if it exceeds 60 seconds, BN will This is because hardenability deteriorates due to precipitation.

In addition, in the steel of the present invention, after quenching, if the temperature is at a low temperature of 400° C. or less, no significant change will occur in the strength, toughness, and yield ratio even if the steel is subjected to m days of quenching. - An example is shown in FIG.

(Example) Table 1 shows the slab heating temperature and rolling finishing temperature of a total of three steel ingots, one with a chemical composition according to the present invention and one with a chemical composition outside the preferred range.

Steel plates with a thickness of 100 mm were manufactured at different times from the end of rolling to quenching, and Y, S,
This is the result of examining the (7) 2mmV/ch Charby absorbed energy at 60°C.

Chemical composition, slab heating temperature, rolling finishing temperature.

When the time from the end of rolling to quenching was within the range of the present invention, a high tensile strength steel plate with a low yield ratio and good strength and toughness was obtained at both the plate thickness of 2 and the % position.

On the other hand, if any one of the above manufacturing conditions is lacking, the strength will be low even if the chemical composition is the same, and the toughness at the +A position will be extremely low. In particular, even if the manufacturing conditions were within the range of this invention, when the chemical composition was outside the range, the toughness was low at any of the V, t, and VG t positions.

(Effects of the invention) This invention can easily produce an extra-thick high-strength steel plate with a carbon equivalent composition equivalent to or lower than that of the conventional quenching and tempering method, and having the same strength and high toughness and low yield ratio. Compared to conventional methods, it is not only advantageous in terms of process saving, energy saving, and reduction of added elements, but also has a tensile strength of 7.
Bridges, buildings, and marine structures using high-strength steel of 0 to 110 kgf/lrm" class.

Since the safety of network structures such as penstocks and pressure vessels can be improved, it can be widely applied to these fields.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the influence of tempering temperature on the strength and toughness of a steel plate.

Claims (1)

[Claims] 1. C: 0.05 to 0.13 wt%, Si: 0.05 to 0.19 wt%, Mn: 0.70 to 1.30 wt%, Cr: 0.40 to 1.10 wt% %, Mo: 0.40-1.10 wt%, V: 0.01-0.10 wt%, Al: 0.01-0.10 wt%, B: 0.0005-0.002 wt% and N: 0. 0
045 wt% or less and has a carbon equivalent Ceq.
A steel slab containing 0.42 to 0.65 wt% of
Direct quenching, characterized in that after heating to 0 to 1150°C, hot rolling is carried out so that the rolling finish temperature is 880 to 960°C on the surface of the steel sheet, and quenching is started within 10 to 60 seconds after the end of the rolling. A method for producing a high-toughness, low-yield-ratio, high-strength steel plate with a yield ratio of 90% or less by a method. Ceq. =(C)+1/24(Si)+1/6(Mn)
+1/5(Cr)+1/4(Mo)+1/40(Ni)
+1/14 (V) (Element symbols in the formula are alloy component content [wt%])