JPS6324013A - Production of low yielding ratio high-tensile steel plate by direct hardening and tempering method - Google Patents

Abstract

PURPOSE:To produce a steel plate suitable for welded structures having a low yielding ratio and high tensile strength by working a specifically composed steel in an unrecrystallized austenite region at a specific draft, then forcibly cooling thereof, then subjecting the worked steel to direct hardening and tempering treatments. CONSTITUTION:The steel consisting of 0.05-0.2wt% C, 0.03-0.5% Si, 0.5-2.3% Mn, 0.01-0.1% Al, 0.1-0.5% Mo, 0.01-0.05% Nb, and the balance Fe and inevitable impurities is worked in the unrecrystallized austenite region at >=30% draft. The steel is then quickly cooled at >=10 deg.C/sec cooling rate and in succession thereof, the steel is quickly cooled by a direct hardening method and is subjected to the tempering treatment in a 300-700 deg.C range. The low yielding ratio high tensile steel plate having 80-90% yielding ratio and >=70kgf/mm<2> tensile strength is obtd. by the above-mentioned method. >=1 kinds among 0.3-1.5% Ni, 0.3-1.5% Cu and 0.3-1.5% Cr and further 0.01-0.05% Ti and 0.005-0.003% B are incorporated at need into such steel compsn.

Description

[Detailed Description of the Invention] The present invention relates to a method for producing a low yield ratio high tensile strength steel plate by direct quenching and tempering. The present invention relates to a method for manufacturing a low yield ratio high tensile strength steel plate that can realize weight reduction and is therefore suitable as a steel plate for welded structures such as bridge welded structures and bent stocks.

Traditionally, 70-80 kgf/mm'' class thick steel plates for bridges are
After hot rolling, it is cooled to room temperature without forced cooling, and then quenched and tempered, and the yield ratio is 9.
It is about 5%. Recently, after controlled rolling and accelerated cooling, the yield ratio has been increased to 8 by reheating to the ferrite-austenite two-phase region and then quenching and tempering.
A method of manufacturing steel plates with a reduction in the percentage of carbon dioxide to about 5% has been proposed (Sahara et al., Proceedings of the Welding Society, Vol. 3, No. 3, No. 589).
(1985), the present inventors manufactured a steel plate with a low yield ratio comparable to or higher than that of the conventional method by performing controlled cooling, rapid cooling using a direct quenching method, and then performing only a tempering treatment. The present invention has been developed based on this discovery.

In recent years, technological advances in high-strength steel plates for welded structures have been remarkable, and tensile strength up to 100 kgf/mm2 class is being put into practical use. However, in bridge-related applications, conventional 70 kgf7mm" class and 80 kgf/mm2 class steels have a yield ratio that is low. High, after surrender;
Because the load resistance up to failure is small, it is not necessarily satisfactory in terms of hidden safety.Therefore, in bridge applications, the use of high-strength steel plates that can handle 80 kgf/mm has been extremely difficult. However, in the field of bridges as well, there is a strong demand for lighter steel plates, and 70kgf/Low yield ratio, which has excellent hidden safety, has been developed.
There is a demand for high tensile strength steel plates of mm2 class or higher.

In the case of cold-rolled steel sheets and hot-rolled steel sheets, these steel sheets contain ferrite and 5 to 30% martensite, and in some cases,
It is characterized by a structure containing bainite and retained austenite, and it has long been known that the presence of martensite is effective in lowering the yield ratio.

Based on this idea, the present inventors studied the components and manufacturing conditions over a wide range, and found that although it is possible to achieve a low yield ratio, the low transition temperature, high It was discovered that this material had fatal defects in upper shelf energy, hardness distribution of the welded part, toughness, etc. In particular, the present inventors discovered that the cause of toughness deterioration is the coarse dispersion of soft ferrite and hard martensite, and based on this knowledge, we developed a low yield ratio high tensile strength steel sheet with excellent toughness. In order to obtain this, it is effective to homogenize the hardness between each phase and reduce the heating temperature within the range where the desired yield ratio can be obtained as finely as possible. It has been found that the addition of Nb and Mo is effective for forming a fine bainite structure.

Means for solving the sub-problem The method of manufacturing a low yield ratio high tensile strength steel plate by the direct quenching and tempering method according to the present invention contains 0.05 to 0.2% by weight of C, 0.03 to 0.8% of Si, and Mn. 0.5-2.5%, AAo, 01-0.1%, Mo 0.1-0.5%, Nb 0.01-0.05%, balance iron and unavoidable impurities. After processing at a reduction rate of 30% or more in the austenite region,
It is directly quenched by cooling at a cooling rate of 10°C/second or more, and then tempered at a temperature of 300°C to 7°0°C to achieve a yield ratio of 80-90% and a tensile strength of 70K.
The method is characterized by producing a high tensile strength steel plate with a low yield ratio of 10 gf/mm" or more.

First, the reason for limiting the chemical components in the present invention will be explained.

C is effective in securing strength, and for that purpose it is necessary to add at least 0.05%, but if added in excess of 0.2%, it will cause deterioration of toughness, so the amount of C should be increased. is in the range of 0.05 to 0.2%.

Si promotes the concentration of C into the remaining austenite phase in the austenite transformation process after rolling,
It acts effectively to obtain the required transformed phase during rapid cooling, and is thus effective in controlling the structure. In order to effectively exhibit this effect, it is necessary to add at least 0.03%, but adding too much will lead to deterioration of toughness, so the upper limit of the amount added is 0.8%. do.

Like Si, Mn is also effective in controlling am and requires addition of at least 0.5%, but when added in excess, it forms a band-like structure and reduces the toughness in the C and Z directions. Therefore, the upper limit of the amount added is set at 2.5%.

A7! It is necessary to add 0.01% or more for deoxidizing steel, but even if it is added in excess of 0.1%, the effect as a deoxidizing agent will be saturated.

Therefore, the amount of Aβ added is in the range of 0.01 to 0.1%.

Mo is effective in forming a bainite structure and improving toughness, and in order to effectively obtain such effects, in the present invention, 0.1% or more is added. However, even if added in excess of 0.5%, the above effects will be saturated, so the upper limit of the amount added is set at 0.5%.

Nb has the effect of refining austenite grains, expanding the non-recrystallized rolling region, and improving strength by refining the bainite structure. In order to effectively obtain the darning effect, it is necessary to add at least 0.01%. However, even if you add too much, the effect will be saturated, so
The upper limit of the amount added is 0. Let it be OS%.

In the present invention, the steel contains Ni0 in addition to the above-mentioned elements.
．． 3 to 1.5%, Cu 0.3 to 1.5%, and Cr 0.3 to 1.5%.

Ni and Cu have the effect of improving the weldability and toughness of steel, and in order to effectively obtain this effect, at least 0.3% of Ni and Cu are required.
It is necessary to add However, even if excessively added, the effect of each element is saturated, so the upper limit of the amount added is set to 1.5% for each element.

Cr is effective for the above-mentioned structure control and strength improvement, and for this purpose it is necessary to add 0.3% or more, but 1.5% or more is necessary.
Even if it is added in excess of 1.5%, the effect will be saturated, so the upper limit of the amount added is 1.5%.

Furthermore, in the present invention, the steel together with the above-mentioned elements,
Or independently of them, Ti0.01-0.05% and B 0.0005-0.003% can be contained.

B is effective in controlling structure and improving strength, and in order to effectively obtain this effect, it is necessary to add 0.0005% or more, but even if it is added in excess of 0.003%, Effect power l color harmonizes. .

Ti is an element necessary to fix N in steel and effectively utilize the above-mentioned effects of B, and for this purpose, it is necessary to add at least 0.01%. but,
Even if it is added in excess, the above effect will be saturated, so the upper limit of the amount added is set at 0.05%.

It is necessary to use Ti together with B.

The method for producing a high-strength steel sheet with a low yield ratio according to the present invention involves processing a steel having the above-mentioned chemical composition at a reduction rate of 30% or more in the unrecrystallized austenite region, and then cooling it at a cooling rate of 10°C/second or more. directly quenched, then 30
Tempering treatment is performed at a temperature ranging from 0°C to 700°C.

In the method of the present invention, first, during hot rolling of steel, the heating temperature of the steel slab is preferably in the range of 900 to 1) 50°C. In particular, when heating in a low temperature range, austenite grains become fine and a fine structure can be obtained after transformation, thus improving toughness. Therefore, although it depends on the capacity of the rolling mill, it is preferable to heat to a low temperature within a range that ensures the finishing temperature. In addition, the refinement of austenite grains and the introduction of deformation bands within austenite grains are effective in refining the structure after transformation and improving toughness. It is necessary to perform a reduction of

Next, in the present invention, direct hardening is performed.

The plate thickness of steel materials used for bridges is usually about 10 to 50 mm, most often in the range of 15 to 30 mm. Direct quenching equipment in plate factories is designed to provide a wide range of cooling rates through water flow control, etc., but in most cases the cooling rate is determined by the plate thickness. However, since it is advantageous in terms of cost to obtain strength after direct quenching using a system with as few components as possible, it is advantageous to cool as quickly as possible to increase the strength after quenching. For this reason, in the present invention, the cooling rate is set to 10°C.
/second or more.

Then, according to the method of the present invention, the hot rolled steel sheet is
Tempering treatment is performed in the range of 0 to 700°C. When the tempering temperature is lower than 300° C., especially when the yield ratio before the treatment is low, the yield ratio becomes 80% or less after the tempering treatment, making it impossible to obtain the desired properties.

Using Steel B and Steel K, whose chemical compositions are shown in Table 1, they were heated to 1000°C, respectively, and the reduction rate was 50% below 850°C, and the finishing temperature was 780' C1 cooling rate was 40°C.
Fig. 1 shows the mechanical properties of these steel plates obtained by producing thick steel plates with a thickness of 2 On+ at various tempering temperatures. When the tempering temperature is lower than 300°C, the yield ratio is almost the same as that of the base material, and is as low as about 70%.

However, when the tempering temperature is in the range of 300 to 700'C, the target yield ratio of 80 to 90% can be obtained, and elongation at the yield point has also occurred.

On the other hand, when the tempering temperature exceeds 700°C, it enters the ferrite-austenite two-phase region, and depending on the cooling conditions and components, a ferrite-martensitic structure may occur after tempering, resulting in a lower yield point and tensile strength. , or conversely, an increase in these is observed, and furthermore, there are cases in which these do not change, and the properties of the obtained steel sheet are not stable.

As mentioned above, when the tempering temperature is in the range of 300 to 700°C, slight changes are observed depending on the components, but when the tempering temperature is T'C, the amount of decrease in tensile strength is ATS = It can be obtained at 0.35xT, increasing the yield ratio.

Effects of the Invention As described above, according to the method of the present invention, the steel plate can be cooled after controlled cooling without reheating and subsequent quenching and tempering treatment.
By rapidly cooling by direct quenching and then only performing tempering treatment, it is possible to obtain a high-strength steel plate having a yield ratio as low as or higher than that of conventional methods.

EXAMPLES The present invention will be explained below with reference to Examples, but the present invention is not limited to these Examples in any way.

A 100 mm thick slab made of steel having the chemical composition shown in Table 1 was heated to various temperatures ranging from 900° C. to 1)50° C. to produce a steel plate with a thickness of 201° C. The finishing temperature was targeted at 800'C, and in most cases finishing was done around that temperature, but when the slab heating temperature was 900°C, the finishing temperature was about 770°C. Further, in both cases, the rolling reduction ratio at 900° C. or lower was 50% or higher.

After the hot rolling was completed, the samples were cooled to room temperature at a cooling rate of 40° C./sec, held at various temperatures for 1 hour, and then air-cooled for tempering.

The mechanical properties of the steel plate thus obtained are shown in Table 2.

Comparative Steels 2 and 13 have chemical components within the range specified by the present invention, but because they are not tempered, both have a yield ratio of about 70%. Comparative steels 19 and 21 are MO
Because it does not contain Mo, its toughness is extremely low (and its tensile strength is also low. Comparative Steel 20 does not contain Mo, but it contains an excessive amount of C, and its toughness is extremely low.

In contrast, the rope according to the invention has a tensile strength of 70 kg
It is clear that it has a low yield ratio of f/mm'' or more and has excellent toughness.

[Brief explanation of the drawing]

Figure 1 shows the tempering temperature and the resulting machine for the steel slab obtained by heating a steel slab having the chemical composition specified in the present invention at a low temperature, cooling it in a controlled manner, and directly quenching it at a cooling rate of 40°C/sec. It is a graph showing the relationship with the physical properties.

Claims (4)

[Claims] (1) In weight%, C 0.05-0.2%, Si 0.03-0.8%, Mn 0.5-2.5%, Al 0.01-0.1%, Mo 0.1 ~0.5%, Nb 0.01~0.05%, balance iron and unavoidable impurities, after processing the steel in the unrecrystallized austenite region at a reduction rate of 30% or more
Yield ratio 80-90%, tensile strength 7, characterized by cooling at a cooling rate of 10°C/second or more, directly quenching, and then tempering in the range of 300°C to 700°C.
A method for producing a high tensile strength steel plate with a low yield ratio of 0 kgf/mm^2 or more. (2) In weight% (a) C 0.05-0.2%, Si 0.03-0.8%, Mn 0.5-2.5%, Al 0.01-0.1%, Mo (b) Ni 0.3-1.5%, Cu 0.3-1.5%, and Cr After processing a steel containing at least one element selected from the group consisting of 0.3 to 1.5% and the balance consisting of iron and unavoidable impurities in the unrecrystallized austenite region at a reduction rate of 30% or more,
Yield ratio 80-90%, tensile strength 7, characterized by cooling at a cooling rate of 10°C/second or more, directly quenching, and then tempering in the range of 300°C to 700°C.
A method for producing a high tensile strength steel plate with a low yield ratio of 0 kgf/mm^2 or more. (3) In weight% (a) C 0.05-0.2%, Si 0.03-0.8%, Mn 0.5-2.5%, Al 0.01-0.1%, Mo 0.1-0.5% and Nb 0.01-0.05%, and further contains (b) Ti 0.01-0.05% and B 0.0005-0.003% However, after processing the steel consisting of residual iron and unavoidable impurities in the unrecrystallized austenite region at a reduction rate of 30% or more,
Yield ratio 80-90%, tensile strength 7, characterized by cooling at a cooling rate of 10°C/second or more, directly quenching, and then tempering in the range of 300°C to 700°C.
A method for producing a high tensile strength steel plate with a low yield ratio of 0 kgf/mm^2 or more. (4) In weight% (a) C 0.05-0.2%, Si 0.03-0.8%, Mn 0.5-2.5%, Al 0.01-0.1%, Mo (b) Ni 0.3-1.5%, Cu 0.3-1.5%, and Cr Contains at least one element selected from the group consisting of 0.3 to 1.5%, (c) Ti 0.01 to 0.05%, and B 0.0005 to 0.003%, the remainder Steel consisting of iron and unavoidable impurities is worked into the unrecrystallized austenite region at a reduction rate of 30% or more, then cooled at a cooling rate of 10°C/sec or more, directly quenched, and then A method for producing a low yield ratio high tensile strength steel plate having a yield ratio of 80 to 90% and a tensile strength of 70 kgf/mm^2 or more, the method comprising performing a tempering treatment at a temperature in the range of 700°C to 700°C.