Steel pipe for airbag and method for producing the same

Abstract

To provide a steel pipe for an airbag and a method for producing the steel pipe that ensure strength and low-temperature toughness without performing quenching and tempering.SOLUTION: A steel pipe for an airbag has a chemical composition comprising C: 0.10 to 0.25%, Si: 0.01 to 0.50%, Mn: 1.00 to 3.00%, P: 0.030% or less, S: 0.020% or less, Cr: 0.01 to 1.00%, Al: 0.010 to 0.060%, N: 0.0020 to 0.0200%, and V: 0.05 to 0.30%, optionally containing Mo: 0.30% or less as an optional element, with the balance being Fe and unavoidable impurities, having a metal structure mainly composed of a ferrite structure and cementite, in which an average crystal grain size of the ferrite structure is 1.5 μm or less, satisfying, with respect to tensile strength TS and elongation at break EL, Formula A: TS≥850 MPa and Formula B: TS×EL≥10000 MPa%, and having a fracture appearance transition temperature vTrs of -50°C or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a steel pipe for an airbag and a method for manufacturing the same. [Background technology]

[0002] For example, the automobile industry is actively promoting the introduction of devices that enhance safety, and a prime example of such devices is the airbag system. Currently, airbags are standard equipment on almost all vehicle models in Japan, the United States, and Europe. In recent years, with the development of regulations in each country, the installation rate of side-impact airbags, such as side airbags and curtain airbags, has increased significantly, and demands for the quality and cost of airbags are growing.

[0003] Seamless steel pipes are used as the gas storage container for the inflator (gas generator), the most important component built into these airbags. The gas generation method for inflators often utilizes a combustion reaction between high-pressure gas pre-filled in the container and gunpowder, which applies stress to the container (steel pipe) at a high strain rate in an extremely short period of time. Therefore, steel pipes used in airbag inflators (steel pipes for airbags) must have high strength and ductility, as well as low-temperature toughness in anticipation of use in cold climates. Furthermore, because the ends of the steel pipes are reduced in diameter by press working and then welded to form the container, they must also be easy to work with and weld. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 10-140250 [Patent Document 2] Japanese Patent Application Laid-Open No. 2004-76034 Summary of the Invention [Problem to be solved by the invention]

[0005] In manufacturing such steel pipes, a commonly used industrial method involves making the pipe through multiple hot working processes, followed by cold working to ensure dimensional accuracy, and then quenching and tempering to obtain the desired properties, as shown in Patent Document 1. However, this method not only involves complicated processes but also many heating steps, which creates problems such as being undesirable in terms of cost and environmental aspects (such as CO2 emissions).

[0006] The present invention has been made in view of the above background, and aims to provide a steel pipe for airbags that can ensure strength and low-temperature toughness even if quenching and tempering are omitted, and a method for manufacturing the same. [Means for solving the problem]

[0007] A first aspect of the present invention has a chemical composition containing, in mass%, C: 0.10 to 0.25%, Si: 0.01 to 0.50%, Mn: 1.00 to 3.00%, P: 0.030% or less, S: 0.020% or less, Cr: 0.01 to 1.00%, Al: 0.010 to 0.060%, N: 0.0020 to 0.0200%, V: 0.05 to 0.30%, and optionally containing Mo: 0.30% or less, with the balance being Fe and unavoidable impurities; The alloy has a metal structure mainly composed of a ferrite structure and cementite, and the average crystal grain size of the ferrite structure is 1.5 μm or less, The tensile strength TS and breaking elongation EI obtained by a tensile test satisfy the following formulas A and B, Formula A: TS≧850MPa, Formula B: TS×EL≧10000MPa%, The steel pipe for airbags has a fracture surface transition temperature vTrs of -50°C or less.

[0008] A second aspect of the present invention provides a method for preparing a steel material having a chemical composition containing, in mass%, C: 0.10 to 0.25%, Si: 0.01 to 0.50%, Mn: 1.00 to 3.00%, P: 0.030% or less, S: 0.020% or less, Cr: 0.01 to 1.00%, Al: 0.010 to 0.060%, N: 0.0020 to 0.0200%, V: 0.05 to 0.30%, Mo: 0.30% or less as an optional element, and the balance being Fe and unavoidable impurities; The steel material is subjected to hot working a plurality of times, and in at least the final hot working, the heating temperature is set to 1000°C or higher, and the steel material is cooled under conditions in which the cooling rate to 500°C after the hot working is CR1 (°C / sec) or higher in the following formula 1, to produce an intermediate material; Formula 1: CR1=21-4Cr-8Mn-Si, (Note that the element symbols in Formula 1 represent the content (%) of each element.) The intermediate material is subjected to warm forging under conditions where the forging temperature Tf is 400 ° C to 650 ° C and the total area reduction rate is 75% or more, and then cooled under conditions where the cooling rate to 300 ° C after warm forging is CR2 (° C / sec) or more in the following formula 2, to obtain a steel pipe. Formula 2: CR2=3 / (Cr+Mn)+(Tf-300) / 300, (Note that the element symbols in Formula 2 represent the content (%) of each element.) The manufacturing method of steel pipes for airbags. [Effects of the Invention]

[0009] The steel pipe for airbags of the first aspect can ensure sufficient strength and low-temperature toughness for use as a steel pipe for airbags even when produced without quenching and tempering. Such excellent steel pipe for airbags can be easily produced, for example, by combining hot working and warm forging using the production method for steel pipe for airbags of the second aspect, and optimizing the respective working temperatures and cooling rate conditions after working. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1A is an explanatory diagram showing a flattening test method, and FIG. 1B is an explanatory diagram showing the shape of a pressing tool in Experimental Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0011] In the present invention, in order to ensure strength and low-temperature toughness while omitting the quenching and tempering that was essential in conventional manufacturing methods, a thermomechanical treatment technique using warm working at an appropriate working temperature below the austenite transformation start temperature Ac1 is actively adopted, and a fine ferrite structure obtained by large strain working is utilized.

[0012] Strengthening steels solely through refinement of the ferrite structure not only limits the range of processing conditions under which the desired strength can be achieved, but can also result in a significant deterioration of ductility. In response to this issue, we discovered that adding a precipitation strengthening element (V) to finely precipitate and disperse carbides during the warm-working process or a pre-processing step significantly contributes to improving strength. By utilizing this precipitation strengthening, it is now possible to expand the range of processing conditions and ferrite grain size required to achieve the desired strength and ductility.

[0013] Furthermore, they discovered that in order to fully utilize the effects of precipitation strengthening elements, it is effective to make the structure before warm working a structure in which bainite or martensite is the main phase, and clarified the appropriate manufacturing conditions.

[0014] Furthermore, since recrystallization may occur during the cooling process after warm working, resulting in a decrease in strength, a cooling rate range to suppress this was derived as an equation from the chemical composition and warm working conditions. This will be explained in more detail below.

[0015] <First aspect of steel pipe for airbags>

[0016] First, the reasons for limiting the chemical composition of the steel pipe for airbags will be explained.

[0017] C: 0.10~0.25%; C (carbon) contributes to solid solution strengthening and also contributes to increasing strength after warm forging by bonding with V to form carbides, so the content is set to 0.10% or more. On the other hand, excessive addition of C may lead to deterioration of workability and weldability, so the upper limit of the C content is set to 0.25%.

[0018] Si: 0.01~0.50%; Silicon (Si) is essential as a deoxidizer during steelmaking, so it must be contained at least 0.01%. However, excessive addition of Si may result in a decrease in workability and weldability, so the upper limit of the Si content is set at 0.50%.

[0019] Mn: 1.00~3.00%; Mn (manganese) is contained in an amount of 1.00% or more to ensure hardenability and to obtain the formation of bainite and martensite at a stage before warm forging, which will be described later. On the other hand, excessive addition of Mn may cause a decrease in workability and toughness, so the upper limit of the Mn content is set to 3.00%.

[0020] P: 0.030% or less; P (phosphorus) is contained as an impurity, but if contained in excess, it tends to segregate at grain boundaries, and the segregation of P causes a decrease in toughness, so the upper limit of the P content is set to 0.030%.

[0021] S: 0.020% or less; S (sulfur) is contained as an impurity, but if contained in excess, it increases sulfide-based nonmetallic inclusions, causing a decrease in toughness, so the upper limit of the S content is set to 0.020%. In addition, since it also leads to a decrease in ductility in the direction perpendicular to the rolling direction (longitudinal direction), the upper limit of the S content is preferably set to 0.010%.

[0022] Cr: 0.01~1.00%; Cr (chromium) is contained in an amount of 0.01% or more to ensure hardenability and corrosion resistance. However, excessive Cr addition may increase material costs and reduce workability, so the upper limit of the Cr content is set to 1.00%.

[0023] Al: 0.010~0.060%; Aluminum (Al) is essential as a deoxidizer during steelmaking, and to achieve this effect, it must be contained in an amount of 0.010% or more. However, excessive addition of Al may increase the amount of oxide-based nonmetallic inclusions, which may result in a decrease in toughness, so the upper limit of the Al content is set at 0.060%.

[0024] N: 0.0020~0.0200%; N (nitrogen) is contained in amounts of 0.0020% or more as an impurity during the steelmaking process. Excessive N content reduces workability due to solid solution N and causes strength and toughness to decrease due to the formation of coarse nitrides with V, so the upper limit of N content is set at 0.0200%.

[0025] V: 0.05~0.30%; Vanadium (V) combines with C to disperse finely as carbides in the steel, contributing to increased strength, and is added at 0.05% to achieve this effect. However, excessive addition of V reduces workability and reduces strength and toughness due to coarsening of carbides, so the upper limit of the V content is set at 0.30%.

[0026] Mo as an optional element: not more than 0.30%; Mo (molybdenum) is an optional element and does not necessarily need to be contained, and a content of 0% is acceptable. However, when manufacturing by melting scrap in an electric furnace, it may be contained as an unavoidable impurity. Furthermore, Mo is an element that is effective in improving hardenability when contained in an amount of 0.01% or more, so it can be added as needed. On the other hand, if the Mo content is too high, it leads to increased material costs and reduced workability, so it is limited to 0.30% or less.

[0027] Next, the steel pipe for airbags has a metallographic structure mainly composed of ferrite and cementite, and the ferrite has an average crystal grain size of 1.5 μm or less. This metallographic structure can be realized by employing the manufacturing method of the second aspect described below. By providing this metallographic structure, it is possible to ensure strength and low-temperature toughness.

[0028] A metal structure mainly composed of ferrite and cementite is a metal structure in which the proportions of ferrite and cementite are higher than those of other types of structures (the total area ratio of ferrite and cementite is 90% or more, preferably 95% or more). Given this structure, it is important that the ferrite structure is refined. When a metal structure is not mainly composed of ferrite and cementite, or when the average grain size of the ferrite structure exceeds 1.5 μm even in this structure, it becomes difficult to ensure strength and low-temperature toughness.

[0029] Here, the method for confirming the metal structure and the method for measuring the average crystal grain size of the ferrite structure will be explained in detail in the experimental examples described later.

[0030] Next, the tensile strength TS and breaking elongation EI of the steel pipe for airbags obtained by a tensile test satisfy the following formulas A and B. The test specimens for the tensile test, the method for collecting them, and the tensile test conditions will be described in detail in the experimental examples described later.

[0031] Formula A: TS≧850MPa; The tensile strength (TS) must be 850 MPa or more.

[0032] Formula B: TS×EL≧10000MPa%; The TS×EL, which indicates the balance between strength and elongation, must be 10,000 MPa or more.

[0033] Next, when considering use in cold climates, the steel pipe for airbags needs to have a fracture appearance transition temperature vTrs of −50° C. or less. The detailed test method for determining the fracture appearance transition temperature vTrs will be explained in detail in the experimental examples described later.

[0034] Next, there is a second embodiment of the method for manufacturing the steel pipe for an airbag.

[0035] <Manufacturing method of the second embodiment>

[0036] In this manufacturing method, a steel material having the above-mentioned chemical composition is prepared, and the steel material is subjected to multiple hot working processes, and in at least the final hot working process, the heating temperature is set to 1000°C or higher, and the steel material is cooled under conditions where the cooling rate to 500°C after the hot working is equal to or higher than CR1 (°C / sec) in the following formula 1, to produce an intermediate material.

[0037] The hot working may be a combination of multiple types of hot working, such as subjecting a cast slab to hot working such as hot rolling to produce a slab, and then subjecting the slab to final hot working such as hot rolling or hot forging.

[0038] Furthermore, the final hot working can be hot forging to form a solid shape into a hollow shape. In this case, it is possible to proceed directly to the subsequent warm forging without adding machining or other processing to change the shape.

[0039] The final hot working is hot working in which a solid shape is worked as is, and a forming step of forming the solid shape into a hollow shape by machining or cold or warm working can be added after the hot working. In this case, the final hot working can be, for example, hot rolling or hot forging.

[0040] In any case, in the hot working step, at least the final hot working must satisfy the heating temperature and cooling rate conditions after hot working. By setting the heating temperature to 1000°C or higher, V carbides can be solid-dissolved once, and fine V carbides can be precipitated during subsequent warm forging, etc., thereby improving strength and ductility. Note that the final hot working refers to one or more stages of hot working after heating for the final hot working, and multiple stages of working are also acceptable.

[0041] Furthermore, the cooling after the hot working is performed under conditions where the cooling rate to 500°C is equal to or greater than CR1 in Equation 1. This allows the metallographic structure of the intermediate material after the final hot working to be a structure mainly composed of martensite, or a structure mainly composed of bainite and martensite. By achieving this metallographic structure, fine precipitation and uniform dispersion of V carbides are possible during the subsequent warm forging, and the final ferrite structure can be refined. Note that a structure mainly composed of martensite and a metallographic structure mainly composed of bainite and martensite refer to a metallographic structure in which the proportion of martensite, or the proportion of bainite and martensite, respectively, is higher than that of other types of structure (preferably, the total area ratio of bainite and martensite is 80% or more).

[0042] Next, after the final hot working, the intermediate material that has been hollowed out by the hot working, or the intermediate material that has been hollowed out by a forming process other than the hot working after the hot working, is subjected to a warm forging process. That is, the intermediate material is warm forged one or more times under the conditions of a forging temperature Tf of 400°C to 650°C and a total reduction of area of ​​75% or more. Furthermore, the steel pipe is obtained by cooling the intermediate material after warm forging under the conditions that the cooling rate to 300°C is equal to or greater than CR2 in the following formula 2. Formula 2: CR2=3 / (Cr+Mn)+(Tf-300) / 300, (Note that the element symbols in Formula 2 represent the content (%) of each element.)

[0043] The warm forging process is carried out under conditions in which the forging temperature Tf is in the range of 400°C to 650°C. The forging temperature Tf here refers to the heating temperature by a heating device immediately before forging. If the forging temperature Tf is less than 400°C, the metal structure is simply elongated, and no refinement of the structure by dynamic recrystallization occurs, resulting in a decrease in workability. On the other hand, if the forging temperature Tf exceeds 650°C, static recrystallization and reverse transformation to the austenite phase during processing occur, resulting in coarsening of the structure and also coarsening of V carbides, which may result in a decrease in strength and toughness.

[0044] In addition, the total area reduction rate in the warm forging process must be 75% or more. Warm forging can be performed in one or more stages, but the total area reduction rate must be 75% or more. This allows for the amount of strain necessary to obtain a fine equiaxed ferrite structure. On the other hand, if the total area reduction rate is less than 75%, the metal structure will only elongate and no refinement of the structure by dynamic recrystallization will occur, which may result in a decrease in workability.

[0045] Here, the total area reduction rate in this application is defined as (cross-sectional area A - cross-sectional area B) / cross-sectional area A x 100%, where B is the cross-sectional area of ​​the circular cross section perpendicular to the axial direction of the final steel pipe, and A is the cross-sectional area of ​​the round bar or hollow round bar of that portion before warm forging.

[0046] In addition, the cooling rate to 300°C after warm forging must be equal to or greater than CR2 in Equation 2. If the cooling rate is slower than CR2, static recrystallization may cause abnormal grain growth in the local structure during cooling, resulting in a decrease in strength.

[0047] By carrying out warm forging under conditions that satisfy all of the above-mentioned requirements, the metal structure of the resulting steel pipe will have a metal structure that is mainly composed of ferrite and cementite, and the average crystal grain size of the ferrite structure will be 1.5 μm or less, making it possible to exhibit excellent material properties.

[0048] Furthermore, it is preferable to add a pre-forging heat treatment in which the intermediate material is heated to a temperature of 350°C or higher and the austenite transformation start temperature Ac1 or lower before carrying out the warm forging. Although this pre-forging heat treatment is not essential, adding it can improve warm forgeability and suppress shortening of the life of the warm forging die due to wear, etc., and breakage of the forming tool due to high applied load during forming.

[0049] The heating temperature conditions are 350°C or higher and the austenite transformation start temperature Ac1 or lower. This improves warm forgeability without affecting the final properties of the steel pipe. As long as the steel pipe is held at this heating temperature for a certain period of time (30 minutes or more), the subsequent cooling method is not important.

[0050] Furthermore, by adding this pre-forging heat treatment, it is preferable that the surface hardness of the intermediate material be 250 HV or less in Vickers hardness, which can more reliably improve warm forgeability. [Example]

[0051] (Experimental Example 1) The present invention will now be described with reference to examples of steel pipes for airbags and methods for manufacturing the same. In these examples, 10 types of steel (steel types A to J) were prepared, as shown in Table 1. Mo, an optional element, was not actively added to the steel types E, F, and H because it was mixed in as an impurity from scrap. Therefore, the analytical values ​​shown are those for the Mo content as an impurity. Steel type I was SMnC420, a comparative steel. Test specimens corresponding to steel pipes, which were warm-forged products, were prepared using these steels under 14 different manufacturing conditions (Examples 1 to 8 and Comparative Examples 9 to 14), as shown in Table 2, and various properties were evaluated.

[0052] [Table 1]

[0053] [Table 2]

[0054] <Preparation of test specimens> Each steel material was melted and cast in an electric furnace to produce a slab, which was then subjected to the first hot processing step of forging to produce a round bar with an outer diameter of 40 mm. Next, the round bar was subjected to the final hot processing step of hot forging to produce a ring-shaped (hollow) intermediate material (raw material) with an outer diameter of 70 mm, an inner diameter of 15 mm, and a height of 50 mm.

[0055] The heating temperature in the final hot working was set under the conditions shown in Table 2. In addition, the value of CR1 in the above-mentioned formula 1 and the cooling method actually adopted for each example and comparative example are shown in Table 2. From the results of measurements using a radiation thermometer in previous experiments, it was determined that the cooling rate to 500°C after the final hot working was approximately 70°C / sec for "water cooling" and approximately 1.5°C / sec for "natural cooling."

[0056] Next, a pre-forging heat treatment was added to the intermediate material only in Example 7. As the austenite transformation start temperature Ac1 of steel type G in Example 7 is 625°C, the heating conditions adopted were that the intermediate material was heated to 600°C within the range of 300°C to 625°C and held at that temperature for 1 hour.

[0057] Next, the intermediate material was subjected to warm forging, and the samples were finished into steel pipes with an outer diameter of 40 mm and a wall thickness of 5 mm so that the total area reduction rate was approximately 85% except for Comparative Example 11. Comparative Example 11 was finished into a steel pipe with an outer diameter of 50 mm and a wall thickness of 8.3 mm so that the total area reduction rate was approximately 70%. The area reduction rate was calculated as follows: Area Reduction Rate = ((Cross-sectional area before processing) - (Cross-sectional area after processing)) / (Cross-sectional area before processing) x 100. Here, in this example, the cross-sectional area before processing was the cross-sectional area of ​​the cross section perpendicular to the axial direction of the round bar, and the cross-sectional area after processing was the cross-sectional area of ​​the cross section perpendicular to the axial direction of the steel pipe.

[0058] The forging temperature Tf for warm forging is the temperature measured on the side surface of the steel pipe at the axial center position using a radiation thermometer, and is shown in Table 2. Regarding the cooling conditions after warm forging, the value of CR2 in the above-mentioned formula 2 and the cooling method actually adopted are also shown in Table 2. From the results of measurements using a radiation thermometer in previous experiments, it has been determined that the cooling rate to 300°C after warm forging is approximately 50°C / sec for "water cooling" and approximately 1.1°C / sec for "natural cooling."

[0059] <Evaluation of intermediate materials> The metal structure of the intermediate material after the final hot working was observed. Specifically, the cross section parallel to the height direction of the intermediate material and perpendicular to the radial direction was used as the observation surface, and after mirror polishing the observation surface, it was etched with nital solution. Microstructure photographs were taken at the radial center position using an optical microscope with 10 fields of view, each field being 0.4 mm x 0.3 mm (magnification: 400x), and the area ratio of each structure was measured using image analysis software from Quick Grain inotech.

[0060] When the area fraction of martensite was 80% or more, the structure was deemed to be almost entirely martensite, and was indicated in Table 2 as "M." When the area fractions of bainite and martensite were 5% or more but less than 80%, and the combined area fraction was 80% or more, the structure was deemed to be a bainite + martensite structure, and was indicated in Table 2 as "B + M." When the area fraction of ferrite was more than 20%, and bainite accounted for more than half of the area fraction of the remainder excluding ferrite, the structure was deemed to be a ferrite + bainite structure, and was indicated in Table 2 as "F + B." When the area fractions of ferrite and pearlite were 5% or more but less than 80%, and the combined area fraction was 80% or more, the structure was deemed to be a ferrite + pearlite structure, and was indicated in Table 2 as "F + P." Table 2 also lists the combined area fractions of bainite and martensite.

[0061] <Steel pipe evaluation> (microstructure) The cross section parallel to the height direction of the steel pipe and perpendicular to the radial direction was used as the observation surface. The observation surface was mirror-polished and then etched with nital solution. Microstructure photographs were taken at the radial center position using an optical microscope, with each field measuring 0.4 mm × 0.3 mm (magnification: 400x), across 10 fields. The area fraction of each structure was measured using Quick Grain Inotech image analysis software. When the area fraction of each of the ferrite and cementite structures was 5% or more and the total area fraction was 95% or more, the structure was determined to be a ferrite + cementite structure, and this was indicated in Table 2 as "F + C." When the area fraction of each of the ferrite and pearlite structures was 5% or more but less than 80% and the total area fraction was 80% or more, the structure was determined to be a ferrite + pearlite structure, and this was indicated in Table 2 as "F + P." Table 2 also shows the total area fraction of F + C.

[0062] (Average grain size of ferrite structure (α)) A specimen was taken for observation so that the cross section parallel to the axial direction of the steel pipe and perpendicular to the radial direction served as the observation surface. The observation surface was mirror-polished and etched with nital solution, and photographs were taken of five fields of view using a scanning electron microscope (magnification: 15,000x). The average particle size of the ferrite particles was measured for the obtained photographs using image analysis software from Quick Grain Inotech. The average particle size was defined as the ferrite area ratio per crystal grain converted to the equivalent circle diameter and then averaged.

[0063] (Tensile test) A certain length was cut out from the steel pipe, and a JIS 2A test piece was taken in accordance with JIS Z2241 so that the pipe axis direction was the tensile direction. A tensile test was then carried out to measure the tensile strength (TS) and elongation at break (EI).

[0064] (Fracture transition temperature measurement) A 2.5 mm wide sub-size V-notch Charpy test specimen was taken from the steel pipe parallel to the pipe axis in accordance with JIS Z2241, and a Charpy impact test was carried out. The lowest temperature at which a ductile fracture ratio of 50% could be obtained was defined as the fracture transition temperature, and this value was determined in the same manner as in JIS Z2241.

[0065] (Flatness test) As shown in Figure 1, a 50 mm long test piece 1 was taken from a steel pipe parallel to the pipe axis. The test piece 1 was clamped using a pair of pressing tools 5, each consisting of a V-block (edge ​​angle 60°) with a tip radius of 12 mm, and flattened so that the height (h) of the maximum flattened part was H × 1 / 2 times the original height. The workability was evaluated based on the presence or absence of cracks. A sample with no cracks was marked as "pass" (◯), and a sample with cracks was marked as "fail" (×). The results of each of the above tests are shown in Table 2.

[0066] As can be seen from Table 2, for Examples 1 to 8, the desired state was achieved in all evaluation items, and it can be seen that even if quenching and tempering are omitted, excellent strength and low-temperature toughness can be ensured, making them suitable as steel pipes for airbags.

[0067] Comparative Example 9 is conventional steel SMnC420, but since it does not contain V, precipitation strengthening is not sufficient, the tensile strength TS and TS×EI values ​​are low, and sufficient strength characteristics are not obtained.

[0068] In Comparative Example 10, the forging temperature Tf of the warm forging was too high, which failed to achieve refinement of ferrite grains, resulting in low tensile strength TS and TS × EI values, and insufficient strength characteristics.Furthermore, it was found that the fracture transition temperature was high and low-temperature toughness could not be ensured.

[0069] In Comparative Example 11, the area reduction rate in warm forging was too low, and the ferrite grains could not be refined by dynamic recrystallization, resulting in low tensile strengths (TS) and (TS × EI), and sufficient strength characteristics and flattening workability were not obtained. Furthermore, the fracture transition temperature was high, and it was found that low-temperature toughness could not be ensured.

[0070] In Comparative Example 12, the cooling rate after the final hot working was too slow compared to CR1, so the metal structure of the intermediate material did not become a structure mainly composed of bainite and martensite. Even if the subsequent warm forging was performed appropriately, the tensile strength TS was low and sufficient strength characteristics were not obtained.

[0071] In Comparative Example 13, the cooling rate after warm forging was slower than that of CR2, which is thought to have caused localized abnormal grain growth in the structure due to static recrystallization during cooling, preventing refinement of the ferrite grain size, resulting in a low tensile strength (TS) and insufficient strength characteristics.Furthermore, it was found that the fracture transition temperature was high and low-temperature toughness could not be ensured.

[0072] In Comparative Example 14, the heating temperature in the final hot working was too low, which resulted in insufficient solid solution of V carbide. This resulted in the subsequent coarsening of the V carbide and ferrite structures, resulting in a total area ratio of bainite and martensite of 20%, and the predominant structure being ferrite + pearlite. Under these conditions, warm forging was performed at an even lower temperature, resulting in a structure after warm forging with a total area ratio of ferrite + pearlite of 80% or more. This resulted in insufficient refinement of the ferrite grain size by dynamic recrystallization, a low TS × EI value, and insufficient strength and flattening workability. Furthermore, the fracture transition temperature was high, and low-temperature toughness could not be ensured.

[0073] In addition, the table shows that the total area ratio of F+C in Comparative Example 14 is 85%, but this is because the majority of the structure is a ferrite structure, and the remainder is a structure in which pearlite and cementite account for half and half.

[0074] Example 2 In this example, the effect of the cooling rate after warm forging on the microstructural state after warm forging was investigated in more detail. Steel types B, F, and H from Experimental Example 1 were used to produce ring-shaped (hollow) intermediate materials (raw materials) with an outer diameter of 70 mm, an inner diameter of 15 mm, and a height of 50 mm, as prepared in the same manner as in Experimental Example 1. The final hot processing conditions for producing these intermediate materials were a heating temperature of 1100°C and a cooling method of water cooling, as shown in Table 3. The metallographic structures of these intermediate materials were all primarily martensite. In this example, test pieces with a diameter of 5 mm and a height (longitudinal direction) of 10 mm were produced from these intermediate materials by machining.

[0075] Each test specimen was subjected to a process equivalent to warm forging. Specifically, using a thermomechanical processing simulator "Thermec Master" manufactured by Fuji Electric Industrial Co., Ltd., the specimens were heated to the forging temperature Tf listed in Table 3, and then upset at a compression ratio (corresponding to the area reduction ratio) of 85%. The compression ratio in this example was calculated using the formula: Compression ratio = height before processing - height after processing / height before processing × 100. After upsetting, the specimens were cooled to 300°C at the cooling rate listed in Table 3, and the ferrite grain size was measured using the same method as in Experimental Example 1. The cooling rate was automatically adjusted by adjusting the flow rate of nitrogen gas used for cooling by inputting the conditions into the testing machine in advance. The temperature of the test specimen was measured using a thermocouple welded to the side of the test specimen. The measurement results are shown in Table 3.

[0076] [Table 3]

[0077] As can be seen from Table 3, when a cooling rate faster than the value of CR2 is used, the average grain size of the ferrite structure becomes sufficiently small and can be refined. On the other hand, when a cooling rate slower than CR2 is used, the average grain size of the ferrite structure becomes larger than 1.5 μm, making refinement difficult. [Explanation of symbols]

[0078] 1 test piece 5 Push tool

Claims

1. It has a chemical composition containing, in mass%, C: 0.10 to 0.25%, Si: 0.01 to 0.50%, Mn: 1.00 to 3.00%, P: 0.030% or less, S: 0.020% or less, Cr: 0.01 to 1.00%, Al: 0.010 to 0.060%, N: 0.0020 to 0.0200%, V: 0.05 to 0.30%, and optionally containing Mo: 0.30% or less, with the balance being Fe and unavoidable impurities; The steel has a metal structure mainly composed of a ferrite structure and cementite, and the average crystal grain size of the ferrite structure is 1.5 μm or less, The tensile strength TS and breaking elongation EI obtained by a tensile test satisfy the following formulas A and B, Formula A: TS≧850MPa, Formula B: TS×EL≧10000MPa%, A steel pipe for airbags having a fracture surface transition temperature vTrs of -50°C or lower.

2. A steel material is prepared which has a chemical composition containing, in mass%, C: 0.10 to 0.25%, Si: 0.01 to 0.50%, Mn: 1.00 to 3.00%, P: 0.030% or less, S: 0.020% or less, Cr: 0.01 to 1.00%, Al: 0.010 to 0.060%, N: 0.0020 to 0.0200%, V: 0.05 to 0.30%, Mo: 0.30% or less as an optional element, and the balance being Fe and unavoidable impurities; The steel material is subjected to hot working a plurality of times, and in at least the final hot working, the heating temperature is set to 1000°C or higher, and the steel material is cooled under conditions in which the cooling rate to 500°C after the hot working is equal to or higher than CR1 (°C / sec) in the following formula 1, to produce an intermediate material; Formula 1: CR1=21-4Cr-8Mn-Si, (Note that the element symbols in Formula 1 represent the content (%) of each element.) The intermediate material is subjected to warm forging under conditions where the forging temperature Tf is 400 ° C. to 650 ° C. and the total area reduction rate is 75% or more, and then cooled under conditions where the cooling rate to 300 ° C. after warm forging is CR2 (° C. / sec) or more in the following formula 2 to obtain a steel pipe. Formula 2: CR2=3 / (Cr+Mn)+(Tf-300) / 300, (Note that the element symbols in Formula 2 represent the content (%) of each element.) Manufacturing method for steel pipes for airbags.

3. 2. The method for manufacturing a steel pipe for an airbag according to claim 1, wherein the final hot working is hot forging for forming a solid pipe into a hollow pipe.

4. 2. A method for manufacturing a steel pipe for an airbag according to claim 1, wherein the final hot working is hot working in which a solid shape is worked while remaining in its solid form, and after the hot working, a forming step is added in which the solid shape is formed into a hollow shape by mechanical working or cold or warm working.

5. 2. The method for manufacturing a steel pipe for an airbag according to claim 1, further comprising the step of: before the warm forging, performing a pre-forging heat treatment in which the intermediate material is heated to a temperature of 350°C or higher and not higher than the austenite transformation start point Ac1 temperature.

Images