JP5040086B2 - Structural high-strength steel with low strain embrittlement - Google Patents

Description

  The present invention relates to a steel material used for various structures such as ships, offshore structures, low-temperature storage tanks, line pipes, building / civil engineering structures, and a method for manufacturing the same.

  High-tensile steel plates used for large structures such as ships, offshore structures, low-temperature storage tanks, line pipes, and construction / civil engineering structures must have high toughness. Efforts have been made.

  On the other hand, the steel materials used in these structures often have toughness deterioration in the portions once subjected to plastic deformation. For example, it may deteriorate due to plastic deformation due to cold working or plastic deformation due to collision or the like.

  As a steel material that suppresses strain embrittlement or a manufacturing method thereof, Patent Document 1 discloses that Charpy absorbed energy after strain introduction is deteriorated by adding Ca or Mg to steel added with V and N. Techniques for preventing are disclosed. Patent Document 2 discloses a technique for ensuring toughness and ductility after plastic deformation (after processing) by making a metal structure a ferrite-based structure rich in ductility.

In Patent Documents 3 and 4, in direct quenching, a film-like ferrite of several μm or less is generated at the prior austenite grain boundaries, the substantial grain interface area is increased, and cementite is refined. Thus, a technique for suppressing the amount of embrittlement of low temperature toughness is disclosed.

JP 56-127750 A JP 2003-313632 A JP 2001-59131 A JP 2001-59132 A

However, the techniques disclosed in Patent Document 1 and Patent Document 2 are improved low-temperature toughness (Charpy fracture surface transition temperature vTrs) in order to improve characteristics at a certain temperature (use temperature), It does not suppress the embrittlement amount of low temperature toughness due to plastic strain (shift amount of Charpy fracture surface transition temperature vTrs to high temperature). That is, it is not a steel material that suppresses strain embrittlement in a true sense.
In addition, the techniques disclosed in Patent Documents 3 and 4 are limited to a strength level of 60 kg, and are difficult to apply to steel having a strength level exceeding 60 kg.

  In order to solve the above problems, an object of the present invention is to provide a high-strength steel (60 to 100 kilo class) with less strain embrittlement.

  In order to achieve the above-mentioned object, the present inventors perform an appropriate component design, and simultaneously develop the (211) plane and the (100) plane on the rolled surface, so that the strength level is reached at 60 kilometer level or more. Regardless, it has been found that the deterioration of the low temperature toughness after being subjected to plastic deformation (shift amount of vTrs to the high temperature side) can be minimized, and the present invention has been completed. The specific means is as follows.

  1. The first invention is a strain brittleness characterized by having a texture where the X-ray intensity ratio of the (211) plane at the rolled surface is 2.0 or more and the X-ray intensity ratio of the (100) plane is 1.5 or more. It is a high-strength steel material for structural use that is less likely to be used.

  2. The second invention is the strain embrittlement according to the first invention, wherein the X-ray intensity ratio of the (211) plane at the rolled surface is larger than the X-ray intensity ratio of the (100) plane. There are few structural high-tensile steel materials.

  3. According to a third invention, the steel material contains, by mass%, C: 0.15% or less, Si: 0.60% or less, Mn: 0.80 to 1.80%, B: 0.0001 to 0.0050%, and N: 0.0050% or less, and It contains one or two selected from Ti: 0.005-0.20% and Nb: 0.001-0.20%, Cu: 0.1-2.0%, V: 0.005-0.2%, Ni: 2.0% or less, Cr : 0.6% or less, Mo: 0.6% or less, W: 0.5% or less, and Zr: 0.5% or less, one or more selected from Fe, and the balance consisting of Fe and inevitable impurities The structural high-tensile steel material with less strain embrittlement described in the first or second invention.

  4). 4th invention heats the steel materials as described in 3rd invention to the temperature of 950-1350 degreeC, and then the cumulative reduction in the temperature range of 1000-900 degreeC is 15% or more, and the accumulation in less than 900-600 degreeC This is a method for producing a structural high-tensile steel material with less strain embrittlement characterized by performing hot rolling with a rolling reduction of 75% or more and a rolling end temperature of 850 to 600 ° C.

  5). According to a fifth aspect of the present invention, there is provided the structural high-tensile steel material with less strain embrittlement according to the fourth aspect of the present invention, which is cooled to 400 ° C. at a cooling rate of 5 ° C./s or more after completion of hot rolling. It is a manufacturing method.

  According to the present invention, a steel material having small strain embrittlement due to plastic strain can be provided at a wide strength level. The steel material of the present invention can avoid the risk of brittle fracture even when cold working or the steel structure is subjected to a large plastic strain due to an accident such as a collision. Contribute.

  The present inventors investigated in detail about the relationship between the texture inside a plate | board thickness and the low temperature toughness (Charpy fracture surface transition temperature vTrs) before and behind plastic deformation using various steel materials. As a result, it was found that deterioration of the low temperature toughness due to plastic deformation can be suppressed by simultaneously developing the (211) plane and the (100) plane on the rolled surface.

  In this survey, when the X-ray intensity ratio of the (211) plane at the rolled surface is 2.0 or more and the X-ray intensity ratio of the (100) plane is 1.5 or more, indicating the degree of texture development inside the plate thickness Furthermore, it was found that the low temperature toughness deterioration amount (shift amount of Charpy fracture surface transition temperature vTrs to the high temperature side) of various steel materials subjected to plastic deformation with tensile pre-strain (up to 10%) was remarkably reduced. .

  In general, it is known that the fracture surface transition temperature shifts to a higher temperature side by applying tensile prestrain, but the present invention has an X-ray intensity ratio of (211) plane of 2.0 or more and (100) plane. This is based on the new finding that when the X-ray intensity ratio is 1.5 or more, the amount of transition of the fracture surface transition temperature to the high temperature side becomes small. This tendency becomes more remarkable when the X-ray intensity ratio of the (211) plane is larger than the X-ray intensity ratio of the (100) plane. Note that the measurement site of the texture referred to in the present invention may basically be any position in the thickness direction, but the measured value on the outermost surface may not represent the value of the entire steel material. It is preferable to measure at a position excluding the surface.

  The reason why the steel material having a texture in which the (211) plane and the (100) plane are developed simultaneously has a small deterioration in low-temperature toughness due to plastic deformation is not necessarily clear, but the fracture surface of the test piece performed by the present inventors. The following can be considered from the observation results and the measurement results of the texture before and after plastic deformation. That is, in the steel material in which the X-ray intensity ratio of the (211) plane is 2.0 or more and the X-ray intensity ratio of the (100) plane is 1.5 or more, X-rays on the (211) plane and the (100) plane are caused by tensile pre-strain. The strength ratio increases and fine subcracks are likely to occur, and stress relaxation at the crack tip due to the subcracks occurs. As a result, it is considered that the decrease in the toughness of the matrix due to the tensile pre-strain was compensated and the increase in the fracture surface transition temperature could be suppressed.

As described above, in order to reduce strain embrittlement, it is necessary to control to an appropriate texture. Moreover, the structure of the steel material is preferably bainite, martensite, or a mixed structure thereof. For that purpose, the chemical composition of the steel material and the production conditions are preferably set in an appropriate range.
The reasons for these limitations will be described below.

1. Although the reason for limitation of the component composition will be described for the component composition, the content of each element means mass%.

C: 0.15% or less In order to form a bainite single-phase structure, C is preferably limited to 0.03% or less, but in order to obtain a martensite structure, it is necessary to contain more than C. However, when the amount of C exceeds 0.15%, the hardness of martensite tends to increase, leading to deterioration of weldability and toughness. For this reason, the C content is preferably 0.005 to 0.15%. Note that, even if the C content is too low, the above effect does not decrease, but taking into account the ease of steelmaking and the use of the material improvement effect due to precipitation of Nb, V, etc., which will be described later, The content is preferably 0.005% or more.

Si: 0.6% or less Si is added for deoxidation, but if it is too much, there is a tendency to deteriorate toughness, so the upper limit is preferably made 0.6%. In addition, it is preferable to contain 0.02% or more from the viewpoint of deoxidation and ensuring strength.

Mn: 0.8 to 2.0%
Mn is an element effective for suppressing the formation of strain embrittlement by promoting the formation of a bainite structure and a martensite structure and forming a dominant texture in the (211) plane and the (100) plane. In order to obtain such an effect, the content is preferably 0.8% or more. When the content exceeds 2.0%, the hardenability increases, the matrix hardens, and the toughness tends to deteriorate.

B: 0.0001 to 0.0050%
B is an element suitable for suppressing the formation of ferrite from austenite grain boundaries and stably obtaining a bainite structure and a martensite structure at a wide range of cooling rates. In order to obtain such an effect, 0.0001% or more is preferable. However, if the content exceeds 0.0050%, the effect is saturated and economically disadvantageous.

N: 0.0050% or less N is an element that inhibits the above-described effect of B and is disadvantageous for stable formation of a bainite structure and a martensite structure. In addition, in the weld heat affected zone (HAZ), It is also an element that adversely affects toughness by solid solution. For this reason, the N content is preferably limited to 0.0050% or less.

Ti: 0.005 to 0.20%
Ti forms carbide and nitride precipitates, thereby suppressing the growth of austenite grains in the heating process during the manufacture of steel materials and contributing to refinement, while suppressing HAZ crystal grain coarsening, and HAZ It is an element that improves toughness. Moreover, Ti fixes N and promotes the addition effect by said B. Further, Ti promotes bainite transformation and martensitic transformation in a solid solution state. In order to exert these effects, the content is preferably at least 0.005%, but excessive content tends to deteriorate the toughness, so the upper limit is preferably made 0.20%.

Nb: 0.001 to 0.20%
Nb is an element that promotes bainite transformation and martensitic transformation to increase the stability of the bainite structure and martensite structure, and is effective for precipitation strengthening and toughness improvement. Moreover, the recrystallization of austenite is suppressed, and the effect by rolling described later is promoted. In order to obtain these effects, the content is preferably 0.001% or more, but if it exceeds 0.20%, the toughness tends to deteriorate, so 0.20% should be made the upper limit.
The elements described below can be contained as necessary using the above-described elements as basic components.

V: 0.005-0.20%
V is an element having a strengthening action by solid solution and precipitation, but in order to obtain such an effect, the content is preferably 0.005% or more. On the other hand, the content exceeding 0.20% inhibits bainite transformation and martensitic transformation, so the upper limit is made 0.20%.

Cu: 0.1 to 2.0%
Cu is an element having a precipitation strengthening action, and is preferably contained in an amount of 0.1% or more in order to exhibit such an effect. However, if it exceeds 2.0%, precipitation strengthening becomes excessive and toughness deteriorates.

Ni: 2.0% or less
Ni is an element effective in improving strength and toughness and preventing cracking of Cu during hot rolling of the additive. However, even if added excessively, the effect is saturated, and since it is an expensive element, it is preferable to contain it in a range of 2.0% or less. A more preferable content is 0.05% or more.

Cr: 0.6% or less
Cr has the effect of increasing the strength, but if it exceeds 0.6%, the toughness of the welded portion deteriorates, so the Cr content is preferably in the range of 0.6% or less. A more preferable content is 0.05% or more.

Mo: 0.6% or less
Mo has the effect of increasing the strength at normal temperature and high temperature, but if it exceeds 0.6%, weldability deteriorates, so the content is preferably in the range of 0.6% or less. A more preferable content is 0.05% or more.

W: 0.5% or less W has the effect of increasing the high-temperature strength, but if it exceeds 0.5%, it not only deteriorates toughness but is expensive, so it is contained in a range of 0.5% or less. Is preferred. A more preferable content is 0.05% or more.

Zr: 0.5% or less Zr is an element that increases the strength and improves the plating cracking resistance of the galvanized material, but if it exceeds 0.5%, the toughness of the weld deteriorates, so the Zr content Preferably, the upper limit is 0.5%. A more preferable content is 0.05% or more.

2. Manufacturing conditions Steel materials having the above component composition and texture do not need to be limited to specific manufacturing conditions. However, the following manufacturing process is advantageous to ensure even better toughness, especially toughness after plastic deformation. Fits.

Heating temperature: 950-1350 ° C
A heating temperature of 950 ° C. or higher is necessary for homogenization of the material and controlled rolling described later, and a temperature of 1350 ° C. or lower makes surface oxidation noticeable at excessively high temperatures. This is because coarsening is unavoidable due to rapid grain growth derived from low C. In order to improve toughness, the upper limit is preferably set to 1150 ° C.

Cumulative rolling reduction in the temperature range of 1000 to 900 ° C .: 15% or more
Hot rolling is performed so that the cumulative rolling reduction in the temperature range of 1000 to 900 ° C. is 15% or more. By rolling in this temperature range, the austenite grains are partially recrystallized, so that the structure becomes fine and uniform. In the conventional steel, such an action usually does not appear unless it is rolled in a temperature range of 1000 ° C. or higher, but in a steel having a composition suitable for the present invention, an effect appears even at 900 to 1000 ° C., By performing sufficient rolling at a relatively low temperature, the growth of recrystallized grains can be effectively suppressed. Note that rolling at a temperature exceeding 1000 ° C. promotes the growth of austenite grains, and therefore is not preferable for making fine grains. On the other hand, if it is less than 900 ° C., it enters the non-recrystallized region, which is not preferable for making the crystal grains uniform.

Cumulative rolling reduction at less than 900 to 600 ° C .: 75% or more, rolling end temperature: 850 to 600 ° C.
The function of rolling in a temperature range of less than 900 to 600 ° C. is to collect the remaining austenite grains that have not been recrystallized by rolling to further refine the grains and introduce strain into the fine austenite grains. It is to form a structure and achieve strengthening during bainite transformation and inheritance of the texture after bainite transformation or after martensitic transformation. When the rolling is performed at a temperature lower than 600 ° C., the ratio of the two-phase region rolling amount increases, the (100) plane develops excessively, and particularly the strength and toughness in the thickness direction are adversely affected. On the other hand, if rolling is performed at a temperature of 900 ° C. or higher, rolling of non-recrystallized austenite grains is not performed.

  Further, when the cumulative rolling reduction in the temperature range is less than 75% or the rolling end temperature is higher than 850 ° C., sufficient fine graining is not performed and there are many (211) planes and (100) planes. A texture cannot be obtained, and embrittlement due to plastic deformation increases.

Cooling rate: 5 ° C./s or more It is desirable to cool at a cooling rate of 5 ° C./s or more. The reason is that when cooling is performed at a cooling rate of 5 ° C./s or more, inheritance of the texture having a dominant (211) plane is promoted, and strain embrittlement is reduced. When cooled under such conditions, the X-ray intensity of the (211) plane becomes stronger, the generation of subcracks is further promoted, and strain embrittlement is less likely to occur. In the cooling method, a more preferable cooling start temperature is 700 ° C. or higher.

Using steel slabs adjusted to various chemical compositions shown in Table 1, thick steel plates were produced according to the conditions shown in Table 2.
For each thick steel plate obtained, the X-ray intensity ratio between the (211) plane and the (100) plane was measured and the metal structure was observed, and as it was rolled, after applying 10% tensile plastic strain to it. The transition temperature of each Charpy fracture surface was investigated. These X-ray intensity ratios were measured using the inverted pole figure method on the rolling surface at the center of the thickness of the cross section of the steel sheet.
As a result, in the inventive example according to the present invention, it can be seen that the amount of transition to the high temperature side of the fracture surface transition temperature vTrs due to tensile pre-strain is small.

  According to the present invention, a steel material having small strain embrittlement due to plastic strain can be obtained, and therefore it can be widely applied to high-strength hot-rolled steel plates such as thick plates and hot-rolled steel plates.

Claims (2)

In mass%, C: 0.057 to 0.15 %, Si: 0.60% or less, Mn: 0.80 to 1.80%, B: 0.0001 to 0.0050% and N: 0.0050 % Or less, and containing one or two selected from Ti: 0.005 to 0.20% and Nb: 0.001 to 0.20%, and Cu: 0.1 2.0%, V: 0.005 to 0.2%, Ni: 2.0% or less, Cr: 0.6% or less, Mo: 0.6% or less, W: 0.5% or less, and Zr: 1 type or 2 types or more selected from 0.5% or less are contained, the balance consists of Fe and inevitable impurities, and the X-ray intensity ratio of the (211) plane at the rolled surface is 2.0 or more, The X-ray intensity ratio of the (100) plane is 1.5 or more, and the X-ray intensity ratio of the (211) plane at the rolled surface is the X-ray of the (100) plane It has a texture larger than the strength ratio, has a tensile strength of 60 kg or more, and has a fracture surface transition temperature vTrs due to 10% tensile pre-strain of −65 ° C. or less. Tensile steel. The steel material having the component composition according to claim 1 is heated to a temperature of 950 to 1350 ° C, and the cumulative reduction rate in a temperature range of 1000 to 900 ° C is 15 to 20% , and the cumulative reduction rate in less than 900 to 600 ° C. The hot rolling is performed at a rolling end temperature of 850 to 600 ° C., and after the hot rolling is finished, the steel is cooled to 400 ° C. at a cooling rate of 5 ° C./s or more. 2. A method for producing a structural high-tensile steel material with less strain embrittlement according to 1.